ABSTRACT DEVELOPMENT of a METHOD to EVALUATE WRINKLING TENDENCY of INK-JET PAPERS by Brahmananda Reddy Mulaka This Thesis Presen

ABSTRACT

DEVELOPMENT OF A METHOD TO EVALUATE WRINKLING TENDENCY OF INK-JET PAPERS

by Brahmananda Reddy Mulaka

This thesis presents the development of an apparatus and method to characterize the propensity of papers to form wrinkles in commercial ink-jet printing. An apparatus was developed to apply ink to the paper and then capture the dynamic features of wrinkle development using Shadow Moiré fringe technique and quantify the severity of wrinkling. The developed techniques enable the study of structural and manufacturing variables of the ink-jet papers that affect the severity of wrinkling.

A parametric study was carried out to evaluate the influence of specimen dimensions, width of inked region, and tensile load on wrinkling severity. From the fringe images, a set of wrinkle parameters were determined. Finally, twelve different commercial ink-jet papers were studied using the test conditions determined from the parametric study. The proposed method provides a reasonable laboratory technique to evaluate wrinkle tendency of papers. DEVELOPMENT OF A METHOD TO EVALUATE WRINKLING TENDENCY OF INK-JET PAPERS

A thesis

Submitted to the Faculty of Miami University

in partial fulfillment of

the degree requirements for the degree of

Master of Science

Department of Paper Science and Engineering

Brahmananda Reddy Mulaka

Miami University

Oxford, Ohio

2005

Advisor ______Dr. Douglas W. Coffin

Reader ______Dr. Robert C. Peterson

Reader ______Dr. Martin D. Sikora TABLE OF CONTENTS

1 INTRODUCTION ...... 1 2 THEORETICAL BACKGROUND ...... 5 2.1 INK-JET PRINTING ...... 5 2.2 PAPER-INK INTERACTIONS ...... 10 2.3 THEORY BEHIND WRINKLE FORMATION ...... 12 2.3.1 Definition of Wrinkles...... 13 2.3.2 Wetting of paper during the ink application:...... 13 2.3.3 Desorption during drying ...... 15 2.3.4 Irreversibility of Hygroexpansive strain...... 15 2.3.5 Equilibrium state of the printed sheet...... 17 2.4 WRINKLE DEVELOPMENT DURING WETTING AND DRYING PHASES ...... 17 2.5 SURFACE TOPOGRAPHY BY INTERFEROMETRY ...... 19 3 PROJECT DEFINITION AND OBJECTIVES...... 25 4 EXPERIMENTAL METHODS ...... 26 4.1 METHOD FOR EVALUATING WRINKLES...... 26 4.2 DEVELOPMENT OF INK-JET WRINKLE ANALYZER ...... 28 4.2.1 Ink Application system...... 28 4.2.1.1 The Development pathway ...... 32 4.2.2 Paper Holder mechanism ...... 43 4.2.2.1 The Development Pathway ...... 46 4.2.3 Load and Heat Application...... 49 4.2.4 Shadow Moiré Station...... 50 4.2.4.1 The Development Pathway ...... 52 5 PARAMETRIC EVALUATION...... 60 5.1 OVERVIEW OF THE EXPERIMENT ...... 60 5.1.1 Sample Preparation ...... 60 5.1.2 Preparation of the apparatus...... 61 5.1.3 Running a test ...... 62 5.2 EVALUATION OF TESTING PARAMETERS ...... 63 5.3 RESULTS AND DISCUSSION...... 63 5.3.1 Quantification of the Ink Application ...... 63 5.3.2 Image analysis ...... 67 5.3.3 Data Analysis...... 69 5.3.3.1 Wet Image...... 70 5.3.3.2 Dry Image ...... 71 5.3.4 Effect of the Variables...... 76 5.3.4.1 Effect of Ink width...... 76 5.3.4.2 Effect of Sample dimensions ...... 80 5.3.4.3 Effect of MD Tension on the paper...... 87 5.3.4.4 Effect of Temperature:...... 92 6 EVALUATION OF INK-JET PAPERS ...... 95

ii 6.1 RANK BASED ANALYSIS...... 100 6.2 COMPARISON WITH COORDINATE MEASUREMENT RESULTS ...... 110 6.3 GENERAL OBSERVATIONS ...... 114 7 CONCLUSIONS AND RECOMMENDATIONS...... 119 8 REFERENCES ...... 122 9 APPENDICES...... 128 APPENDIX A...... 129 APPENDIX B...... 132 APPENDIX C...... 133 APPENDIX D ...... 134 APPENDIX E...... 135 APPENDIX F...... 141

iii LIST OF FIGURES

Figure 1. Ink-Jet Printing Technology map...... 6 Figure 2. Drop on Demand Ink-Jet Printing. a) Drop formation in a thermal bubble jet b) Drop formation in a piezoelectric ink-jet...... 7 Figure 3. Typical depiction of a continuous ink-jet printing system...... 9 Figure 4. Different types of wrinkles formed in the Paper under different conditions a) Wrinkles: Non-Uniform wetting/Drying of the sheet. b) Wrinkles: Wetting on one edge of the paper. c) Cockle: Non-Uniform sheet properties. d) Curl: Two- Sidedness...... 11 Figure 5. . Irreversibility of hygroexpansive strain. a) Loss in the hygroexpansive strain in a restraint dried sheet and b) hygroexpansive strain in a freely dried sheet [16]...... 16 Figure 6. Forces due to non uniform expansion and shrinkage. a) Shows the action of Tensile forces in the wetting state and. b shows during the drying state...... 18 Figure 7. Schematic arrangement of shadow Moiré setup. a) Light source and observer are placed parallel to the plane of the grid forming straight contour lines. b) The setup where the contour lines are cylindrical...... 20 Figure 8. Formation of shadow Moiré fringes at Infinite camera and observer position..22 Figure 9. Ink/water application onto the substrate...... 26 Figure 10. Formation of wrinkles and change of wrinkle location as the ink is dried. a) Wrinkles in the wet state b) Development of new wrinkles in the adjacent area to the wetted area after drying...... 27 Figure 11. Ink application system and its components. a) Schematic diagram of the ink application system b) Image of the ink application part on the wrinkle analyzer.31 Figure 12. Application of water by wet blotting paper method. A) Liquid application by using a soaked blotting paper B) wrinkles in the wetted area C) wrinkles after the sheet is dried...... 35 Figure 13. Liquid application device using dryer fabric ...... 36 Figure 14. Spray nozzle application system to apply liquid by the pendulum movement of the ink tube...... 37 Figure 15. Teejet’s Flat spray nozzle with spray at different operating pressures. The left side of the spray is with low operating pressure and the right side of the spray at high operating pressure...... 38 Figure 16. Ink application system with added ink-tank ...... 41 Figure 17. Ink application system with L-shaped felt holder arrangement to trim the excessive spray ...... 42 Figure 18. Sheet holder mechanism a) Schematic diagram of the mechanism b) Image of the sheet holder mechanism fitted on the linear rail below the grating...... 45

iv Figure 19. Paper holder mechanism before modification of the paper clamping system .47 Figure 20. The modified sample mounting frame with corrected alignment and sample mounting using adhesive tape ...... 48 Figure 21.Schematic Diagram of the load application mechanism mounted on the paper holder...... 49 Figure 22. Schematic diagram of Shadow Moiré station...... 51 Figure 23. Shadow Moiré station before modification of the Grating holder and the light source ...... 53 Figure 24. Shadow Moiré fringes on a wet sample with the initial shadow Moiré station setup...... 55 Figure 25. Shadow Moiré setup after modification of the light stand...... 57 Figure 26. Modified double frame floating grating holder...... 58 Figure 27. Fringes on a flat aluminum plate with an inclination of 0.1" on one end...... 59 Figure 28. Applied ink quantity Vs. nozzle height at different nozzle speeds...... 65 Figure 29. Shadow Moiré fringes in the inked area on a sample while it is wet. A) Image after performing FFT filter with ImajeJ B)Image before FFT filtering. ....68 Figure 30. Parameters used to quantify the wrinkles in the un-dried sample at T0 time ..70 Figure 31. Parameters used to quantify the fringes in the post dried samples...... 72 Figure 32. Comparison of wrinkle formation before drying at different ink widths...... 77 Figure 33. Effect of change in ink width on the number of wrinkles after drying...... 78 Figure 34. Effect of change in ink width on the Co-planarity and the wrinkle height in the Wrinkles after drying...... 79 Figure 35. Effect MD sample dimensions on the Approximated wrinkle width before drying...... 81 Figure 36. Effect of MD sample dimensions on the number of wrinkles after drying...... 82 Figure 37. Effect of MD sample dimensions on the Co-planarity of the wrinkles after drying...... 83 Figure 38. Effect of MD sample dimensions on the wrinkle height after drying...... 83 Figure 39. Effect of MD sample dimensions on the edge wrinkles' angle...... 84 Figure 40. Effect of CD sample dimensions on the maximum flatness of the inked area after drying ...... 86 Figure 41. Effect of MD tension on the wrinkle height before drying the sample...... 88 Figure 42. Effect of MD tension on the Wrinkle Wavelength before drying the sample .88 Figure 43. Effect of MD tension on the wrinkle wavelength after drying ...... 89 Figure 44. Effect of MD tension on the Number of wrinkles after drying...... 90 Figure 45. Effect of MD tension on the Flatness of inked region after drying ...... 91 Figure 46. Effect of MD Tension on the Edge wrinkle angles after drying...... 91 Figure 47. Effect of heating on the wrinkle formation in the post dried samples...... 92 Figure 48. Predicted rank versus the actual Miami rank for different ink-jet papers under test condition C3...... 105 Figure 49. Residual Versus fitted Value for the Miami ranking under test condition C3106

v Figure 50. Normal Probability plot of the Residuals for Miami-Ranking regression model under test condition C3...... 106 Figure 51. Predicted versus Actual Kodak Ranking under test condition C3 ...... 107 Figure 52. Residuals versus the fitted values of Kodak Ranking under test condition C3 ...... 108 Figure 53. Normal Probability plot of the residuals for Kodak ranking under Test condition C3 ...... 109 Figure 54. Predicted versus Actual ImageXpert wrinkle number under test condition C3...... 112 Figure 55. Residuals versus the Fitted values for ImageXpert value based regression model under test condition C3...... 113 Figure 56. Normal Probability plot of the Residuals for the regression model to predict ImageXpert values under test Condition C3 ...... 113 Figure 57. Fringe image after drying. The fringe pattern shows higher coplanarity at the edges compared to the center along the CD of the sample...... 115 Figure 58. Change in number of wrinkles with time. Time stamp on each frame shows the elapsed time after ink application...... 117 Figure 59. Spray nozzle volume median diameter versus pressure curves...... 130 Figure 60. Drop size versus accumulated volume percentage for Teejet Spray nozzles 131 Figure 61. Ink Application drive motor calibration curve for converting the dial gaue setting into actual speed...... 132 Figure 62. FFT filtering tool and the parameters used to process the images to filter out noise...... 134

vi LIST OF TABLES

Table 1. Operating Parameters of the Ink Application Station ...... 32 Table 2. Variables tested on the Pendulum-spray nozzle system...... 39 Table 3. Repeatability of the ink application at a given set of parameters on two isolated runs...... 43 Table 4. Operating parameters of the paper holder mechanism...... 46 Table 5. Test settings of nozzle speed and nozzle height for ink quantification...... 64 Table 6. Summery of the Parameters used to characterize the wrinkles ...... 75 Table 7. Effect of Wrinkle analyzer variable on the wrinkle characteristics ...... 94 Table 8. Basic properties of the 12 different commercial Ink-jet papers used for testing on the wrinkler...... 96 Table 9. Test conditions used for evaluation of commercial Ink-jet papers ...... 97 Table 10. Data table showing how the overall variation of a parameter was determined. The data included in table is for test condition C3...... 99 Table 11. Multicolinearity based filtering of the independent variables. Independent Rsquare matrix for the variables measured under test condition C3...... 100 Table 12. Sample data to demonstrate the calculation of the rank based on the sign of the correlation coefficient...... 102 Table 13. Consolidated rank based regression analysis results for each test condition ..103 Table 14. Regression variables that explain the variation in the ranking at different test conditions ...... 104 Table 15. Consolidated Regression Analysis results for each test condition - regression carried out for comparing ImageXpert wrinkle results...... 110 Table 16. Best fit regression coefficients for each test condition - Regression carried out for comparing ImageXpert wrinkle results...... 111 Table 17. Results of Wrinkle parameters measured for different ink-jet papers under test condition C1...... 141 Table 18. Results of Wrinkle parameters measured for different ink-jet papers under test condition C2...... 142

vii ACKNOWLEDGEMENTS

I would like to express my sincere thanks everyone involved and helped during this project. First, I would like to thank my advisor Dr. Douglas W. Coffin for his patient guidance throughout this endeavor without which the project would not have been in this great shape. I would also like to convey my appreciation to Dr. Robert C. Peterson and Dr. Martin D. Sikora for being on my thesis reading committee and for their invaluable advises. I would also like to thank the M/s Kodak Versamark team for their technical and financial support of the project.

I would like to express my appreciation to Mr. Rodney J. Kolb, Mr. Douglas W. Hart and Mr. Barry D. Landrum for their help during the fabrication of the system. Also, I would like to thank the staff at Instrumentation and mechanical workshops for their help. And a special thanks to the staff of PS&E department Mrs. Trish Otto and Mrs. Laurie Guest for their continuous support throughout this course.

Finally, I would like to acknowledge my parents and my sister for their continuous support and encouragement. My deepest appreciation goes to my friends and former colleagues such as Madhu, Anu, Hari, vishy, Jagan , Nagahari, Dr.Bhima and many others who supported and encouraged my idea of pursuing masters degree. The management of ITC Ltd. Encouraged my pursuit by granting a study leave.

viii 1 INTRODUCTION

Formation of wrinkles in and around areas of heavy ink application is a common problem in printing on paper. This is especially the case in water-based ink-jet printing. These wrinkles are esthetically unpleasing and can lead to problems in converting and handling. The wrinkles arise because of the inherent dimensional instability of paper, but it is expected that some papers will exhibit less wrinkling than others and there is a need to be able to distinguish the wrinkle tendency of different papers. This thesis focuses on the development of a laboratory method to evaluate the propensity of papers to wrinkle. The developed method is to be used to evaluate papers used for web-fed ink-jet printing.

Wrinkling, cockle, and curl are all out-of-plane deformations of paper associated with non-uniform shrinkage and swelling of the sheet of paper undergoing a change in moisture and/or temperature. These types of defects have previously been studied. For example the work of K.H. Paik and W.S. Nam[1] deals with the effect of localized moisture variations and basis weight variations on cockle development. They tried to correlate cockle locations with the tensile index to determine the impact of cockle on the web-breaks on a paper machine. They concluded that cockling is highly dependent on the local basis weight variations and the drying rate. Drying rate on the paper machine changes the shrinkage of the paper web and thus affects the dimensional stability. Although this work relates to cockle formation during the initial drying of the sheet some of the methods they used are important to the present study. They used a simple method to quantify the cockles by marking the hills and valleys on the paper and then counting the number of cockles. This method does not provide any detailed characterization of the cockle other than number of cockles and cockle distance. Even this minimal quantification is only possible as long as the cockle formation is uniform. Related work was completed on the fluting phenomena in offset printed papers [2]. Fluting describes wrinkles that are typically aligned in the Machine Direction (MD) in heavily inked regions of web-fed offset printed sheets. Coffin [2] utilizes both analytic and finite element results that show that the fluting phenomenon is caused by differential moisture between inked and non-inked regions. This paper also concludes that the flute wavelength is dominated by MD tension on the web and the MD length of the inked area. The paper points out hygroexpansion, CD bending stiffness and CD modulus as the key sheet parameters for the development of the flutes.

The analysis presented in reference [2] extends the previous work of Habeger[3] who proposed that the fluting phenomena were tension wrinkles. and the proposal of Fujiwara et al.[4] who hypothesized that fluting was due to the fact that the moisture levels in the image area remain high during printing and drying stages. Because of this the image and non image areas undergoes relatively more shrinkage causing compression on the image areas. The conclusion of their work suggests that compressive forces and the image area size determine the flute wavelength.

No previous analysis of wrinkle formation in ink-jet printed papers was found, but several patented techniques were found that suggest methods to reduce the wrinkle tendency of the ink jet printed papers[5-8]. However these patents approach the problem not from the standpoint of the base sheet, but by modifying the printing machine configuration[6, 8], changing ink formulation[5], and applying coatings and treating the paper with anti-cockle agents[7] . All these techniques target high end application of the ink-jet printing. None of these techniques deal with uncoated/untreated commercial substrates.

Very little work is done in this area, and most of that deals with cockle and the localized variations in the sheet that causes these cockles. Except Coffin’s work [2] on the fluting in web-offset printing, no other literature is found that deals with the global

2 out of plane deformation in the image and non-image area. However there has been little work done in the area of wrinkle development in the ink-jet printed paper. And there is no reference of any work available on the untreated and uncoated ink-jet paper wrinkle problems.

In the past decade, Ink-jet printing has earned a considerable market share in both commercial and desktop/office printing. By the year 2000, market share for nonimpact printing was at 12% with 12% growth rate[9]. This rapid growth has increased the demand for inkjet paper in terms of volumes and the functional performance. The paper available in the market today is not able to meet the growth speed of the ink-jet printing technology[10]. Ink-jet papers used for various printing methods1 can be categorized into three different types:

1. Uncoated commodity grade

2. uncoated surface treated

3. coated papers

The performance of any of these papers is determined by numerous papermaking as well as printing conditions. Apart from these isolated conditions the interaction of the printing ink and the substrate play an important role in the final print quality. When the print quality is mentioned the dimensional stability of the printed sheet is as important as the matter/figures printed on it. The lack of dimensional stability/flatness in the printed sheet leads to problems related to runnability of the finishing machines as well the final product aesthetics. For high-end printing applications, wrinkles issues have been addressed to a reasonable level. For example modifications of the printing inks, machine configuration or surface treatment of paper are used to alleviate the

1 Detailed description on various ink-jet processes is given in section 2.1 “Ink-jet Printing”

3 wrinkling. However, commercial printers demand low cost paper and can not utilize the previously stated options. These low end applications are typically printed on uncoated commodity grade papers. In this case, the basic paper construction components such as wood species, nature of the pigments or fillers, degree of pulp refining, sheet formation, porosity[10] and paper machine conditions dictate the flatness after printing.

Kodak Versamark2 is a commercial ink-jet printer manufacturing company located at Dayton, OH. Miami University has a research project funded by Kodak Versamark to study the wrinkle problem with uncoated / untreated ink-jet papers. The overall objective of the project is to establish the material and structural characteristics of the paper that influence sheet flatness. To achieve this object a method to create and characterize the wrinkle propensity is required. The objective of the research project reported in the following is the development of a method to evaluate wrinkles in uncoated inkjet paper used for commercial printing.

2 Kodak Versamark is formerly named M/S Scitex

4 2 THEORETICAL BACKGROUND

2.1 Ink-jet Printing

Ink-jet printing, often synonymously termed as digital printing is the most predominant non-contact printing technology both in domestic and commercial printing markets. The basic principle of ink-jet printing technology is breaking a continuous stream of fluid into droplets, which was explained by Lord Rayleigh back early in 1878. The break-through in producing low-cost ink-jet printers is achieved when both Cannon and Hewlett-Packard simultaneously invented the ‘bubble jet’ and ‘thermal ink-jet’ respectively. Both the techniques works with a similar principle of ejecting drops from the nozzle by the growth and collapse of a water vapor bubble by use of a heater near the nozzle[11].

Ink-jet printers can be divided into two major categories [12, 13]

1. Drop-on-Demand technique (DOD)

2. Continuous Jet technique

There are several different variants in each of these two systems. Figure 1[11] shows the basic deviations in the designs of ink-jet printing technology.

Figure 1. Ink-Jet Printing Technology map

The basic difference in the DOD and continuous jet printer technology is the timing of drop ejection by the print head. In the DOD system the ink drop is ejected only at appropriate position on the image. While in the continuous jet the ink drops are formed continuously and the drops are distinguished by charging them based on their positioning requirement.

As seen in Figure 1, the DOD can be divided into four major categories based on the mechanism used to form the drop. In a typical thermal jet (Figure 2a) a heater is used

6 behind the orifice to form vapor bubble when ever the orifice is required to fire an ink drop. The vapor bubble pushes the drop through the orifice and then collapses to allow refilling the ink for next drop. Several orifices can be fitted to the pressure chamber allowing a faster printing. In a piezoelectric ink-jet [Figure 2b] the piezoelectric is fitted in the ink nozzle and when ever an electrical pulse is applied the piezoelectric piece deforms temporarily. The piezoelectric is placed inside the nozzle in such a way that the deformation pushes the ink drop out of the nozzle. There are four different variants of piezoelectric ink-jet technologies based on the piezoelectric deformation mode (Figure 1).

a. b.

Figure 2. Drop on Demand Ink-Jet Printing. a) Drop formation in a thermal bubble jet b) Drop formation in a piezoelectric ink-jet

7 Thermal and piezoelectric are the most widely used techniques. Both the Electrostatic and Acoustic methods are still under development [11].

In the Continuous-jet application3 (Figure 3) the ink-drops are generated by using either thermal or piezoelectric technique. The drops are then passed through a charging electrode. The drops can be charged in two different ways. In the first method the drops are either charged or not charged (binary deflection). In the second method the drops are charged at various levels (multiple deflections). In either case the charging of the drops is done according to their placement on the substrate and is controlled digitally. The charged drops are then passed through a high voltage deflector which directs the droplets according to their charge. The uncharged drops are collected in a gutter for recycling [11-13]]. Hertz’s method uses a unique technique of applying a burst of small droplets to obtain the grayscale[11]. This concept is used in Iris’s Realistic for graphic arts market and Scitex’s Digital Press for the high-speed on demand printing market.

3 This is the technique used by most commercial ink-jet printers

Figure 3. Typical depiction of a continuous ink-jet printing system

In DOD technique the drop generation speed is limited by the drop collapse and reformation in case of thermal jet and the piezo deformation time lapse. This limits the speed of printing of DOD printers. This is overcome in continuous jets by forcing the ink through a small nozzle[12]. The continuous spray systems in commercial printing generate drops up to 1000kHz compared to a maximum of 12 kHz in the DOD systems[13]. This provided opportunity for the continuous jet printers for commercial on-demand printing. However when we look in the point of view of paper, higher speeds means more ink volume applied in a given amount of time. Most of the commercial printers use water based inks with typical components like water, solvent, pigment, surfactant, biocide, buffer and other additives. The molar volume of the water can be as high as 90% [11]. This means proportionately large amounts of fiber swelling in the wetted/inked area and eventually the paper shrinks non-uniformly

9 during drying process4. All these demand for a high dimensional stability of the substrate.

2.2 Paper-Ink Interactions

Paper is a hydrophilic substrate which readily absorbs and releases water upon wetting and drying respectively. The absorption and release of water by paper is mainly attributed to the fundamental structure of the cellulose fiber. When moisture enters the paper the sheet swells. The amount of swelling is directly related to the amount of water absorbed by the fibers. An increase of temperature at constant moisture would also cause swelling, but the application of heat drives off moisture so the sheet shrinks[14].

Often paper is dimensionally unstable, and problems like curling, cockling, misregister, waviness of the edges occur. Curl develops in paper due to the differential hygroexpansivity between the top and bottom sides of the sheet. Cockling arises because of variation of swelling within the plane of the sheet. Misregistration occurs because of an irreversible swelling and shrinkage.

In inkjet printing, the paper is wetted wherever ink is placed on the sheet. In areas of solid print, the sheet will expand. The entire sheet is heated and the wet areas dry. The swelling and shrinkage is non-uniform and results in wrinkles in either the printed or adjacent areas. The mechanism involved in the formation of these wrinkle is discussed in detail in Section 2.3. Figure 4 shows different types of wrinkles that may form in a paper substrate under different conditions of wetting and drying.

4 The interactions of paper and ink during different stages of printing are explained in section 2.3-“Theory behind Wrinkle formation

a b

c d

Figure 4. Different types of wrinkles formed in the Paper under different conditions a) Wrinkles: Non-

Uniform wetting/Drying of the sheet. b) Wrinkles: Wetting on one edge of the paper. c) Cockle: Non-

Uniform sheet properties. d) Curl: Two-Sidedness.

Figure 4 .a shows the formation of wrinkles. Wrinkles form when the sheet is non- uniformly wetted or dried in an interior region of the sheet. This non-uniform moisture change causes non-uniform expansion or shrinkage respectively within that area of the sheet. When the interior region is under compression out of plane deformation can occur. This deformation pattern is regular through out the effected area. Figure 4. b shows the formation of wrinkles in similar conditions as in Figure 4.a. however this kind of wrinkle happens when the wetting or drying is at the edge of the sheet. Figure 4.c depicts an out-of-plane deformation called cockle. As it can be seen

11 in the picture, unlike in the first two cases here the deformation is random. This deformation is caused by non uniformities within the sheet like basis weight variation, moisture variation, density variation etc. These non-uniformities result in non- uniform swelling and shrinkage of the sheet and the sheet buckles locally forming the cockle. Figure 4.d depicts typical curling phenomena. Instead of several localized out of plane deformations, the whole sheet curls to one side. The reason for this kind of behavior is two sidedness of the sheet resulting in differential expansion or shrinkage from one side to the other of the sheet.

The terms cockle and wrinkle are often used interchangeably. Both phenomena are caused by the out of plane deformation in the sheet. However, for this discussion these two are differentiated; Cockle is a random pattern developed within the sheet, the bucking that occurs in the cockle depends on the localized non-uniformities developed in the sheet during the paper making process. This is analogous to the phenomenon in Figure 4c. Hence the pattern of cockles is random. While wrinkles phenomena is global to the sheet with a specific pattern that is governed by the whole sheet properties rather than the localized variations. Wrinkles are the phenomenon as shown in Figure 4a and Figure 4b.

In inkjet printing, the pattern shown in Figure 4.a is more common especially at locations where there is solid print or pictures. This type of wrinkle is the subject of the work discussed here.

2.3 Theory behind Wrinkle formation

In the inkjet printing process, the paper undergoes several changes in terms of fiber swelling and shrinkage. The process can be divided into following components.

1. Wetting of the paper by ink 2. Desorption during drying 3. Equilibrium state of the printed sheet

In the following sections, the interactions between ink and water in each stage and the mechanical and hygrothermal changes that occur in the sheet that finally result in the wrinkle formation are discussed in detail. First, a clear demarcation of wrinkle and its close relatives like and cockles is made.

2.3.1 Definition of Wrinkles

Wrinkle is defined as “the out of plane deformation of a sheet resulting from a planar non-uniformity in the potential for free hygrothermal expansion of sheet”[15]. .

2.3.2 Wetting of paper during the ink application:

When the ink droplets are forced onto the paper substrate through the ink nozzles of the printing cartridge the colorant and the other additives are retained on the surface while the water present in the ink is absorbed into the sheet. The amount of water absorbed and the depth of water penetration depends on the printing speed, time span between the print head and the dryer, water absorbtivity of the sheet, and the temperature of the sheet and ink. For the simplicity of discussion, it is assumed that the water is absorbed though out the thickness of the sheet. Where the water is absorbed, the sheet will swell. This swelling can be expressed as a hygroexpansive or

13 hydroexpansive strain that is positive during the ink application stage and negative during subsequent drying.

During the papermaking process, often the web is dried under restraint. This restricts the natural shrinkage the fibers would experience during drying. As a result, internal strains are built into the sheet and these are called ‘dried-in-strains’. The magnitude of dried-in-strain is related to the degree of restraint during the drying process. When the paper is rewet during printing, the absorption of moisture softens the material and releases these strains. This release of dried-in-strain can be represented as a negative strain during printing. In most cases, the magnitude of this shrinkage is much smaller than the hygroexpansion of the sheet.

During the web fed printing process the paper web travels under tension. This tension is a result of differential velocities placed on drive rolls. The differential velocity causes a mechanical strain in the sheet and leads to the tension. During the printing process, areas that are wetted with the ink solution will experience a drop in modulus and a corresponding drop in tension.

Therefore, one must consider at least three strains when looking at the total strain in the sheet during wetting and drying. The change in dimensions due to wetting can be quantified as:

εwet= εhygro+ εmech- εD (1)

Where

εwet = Overall strain during wetting

εhygro = Hygroexpansive strain due to change in moisture (β Δm)

εmech = Mechanical strain in CD due to MD tension (-γT/EA)

14 εD = Strain due to release of dried in restraints

2.3.3 Desorption during drying

The printed sheet will undergo desorption during the drying process. Desorption causes the sheet to shrink; the magnitude of this shrinkage depends on the amount of moisture lost during the drying process. Apart from the shrinkage due to loss in moisture during drying, the dimensional change may also be influenced by the web tensions on the printing machine.

εdrying= εShr+ εmech (2)

Where εShr = shrinkage due to change in moisture (β Δm)

εmech = mechanical strain in CD due to web tension

2.3.4 Irreversibility of Hygroexpansive strain

The Hygroexpansive strain is defined as the incremental change in the dimensions when the moisture content changes. The hygroexpansive strain is a linear and reversible function of the moisture content, provided the moisture content does not become too high (<15%) [1]. As stated earlier (Section 2.3.2) the internal stresses of the paper release when moisture content increases sufficiently. As a result, a sheet undergoing a moisture cycle may experience irreversible shrinkage. The amount of the irreversibility depends on the amount of released dried-in-strains. Figure 5 shows a comparison of Hyroexpansive strain between a sheet dried under restraint and one allowed to dry freely. From Figure 5a. it can be noticed that the first moisture cycle shows the maximum irreversible shrinkage as explained by the loss in the Hygroexpansive strain..

a b

Figure 5. . Irreversibility of hygroexpansive strain. a) Loss in the hygroexpansive strain in a restraint dried sheet and b) hygroexpansive strain in a freely dried sheet [16].

After a complete cycle of wetting with the ink followed by drying of the printed sheet, the final strain in the printed sheet is the difference between Equations (1) and (2).

εcycle= Δεhygro/shr-εD (3)

The parts of the sheet that were not wet with ink will undergo a different cycle of strain. The main difference will be that the dry regions will not have released the dried-in strains. This differential swelling and shrinkage of strains between wetted and not wetted areas results in stresses in the sheet. These stresses is compressive may result in the development of wrinkles in the sheet.

2.3.5 Equilibrium state of the printed sheet

From Equation (3), it can be seen that the factor contributing to the final wrinkle development is the irreversible hygroexpansive strain in the sheet. This implies that the freely dried sheets should have no wrinkle visible once the sheet is brought to equilibrium state. However, this likely will never happen. There are several reasons why wrinkles will persist, those include hysteresis in moisture, relative humidity curves and stress relaxation. At a given humidity level, the amount of moisture the paper comes in equilibrium depends on the previous state from which the paper came. In the present case the wetted area and the non-wetted areas have different moisture history. Thus, even upon conditioning the printed sheet, both the printed and non- printed areas will absorb different amounts of moisture resulting in non uniform stress throughout the sheet that may keep the wrinkles present. In addition, during time that the sheet is wetted, stress-relaxation may occur. This would result in a loss of potential for shrinkage. Thus, wrinkles in the sheet still remain.

2.4 Wrinkle development during wetting and drying phases

While the sheet is wet by the printing ink, the wet portion of the sheet swells and the total dimension in the Cross Direction (CD) of the sheet increases. As shown in Figure 6.a the expanded wet area causes tensile stress on the adjacent non wet area. The non-

17 wetted areas restrict expansion of wetted area by creating compressive forces while the wetted area pulls on the adjacent dry area creating tensile forces. The compression in the wetted area can lead to out of plane direction. The frequency of the wrinkles and the magnitude of the out of plane deformation depend on the bending stiffness of the sheet.

During the drying process, the wet areas shrink more than its original dimensions due to the reasons explained previously. Figure 6.b shows the directional forces during this process. The excessively shrunk wet areas exert compressive forces on the dry area while the adjacent dry area exerts tensile forces on the wetted area. This results in the formation of wrinkles in the non wet areas adjacent to the wetted area. The amount of compressive force on the non-wetted area is proportional to the distance from the wetted area.

Under Tensile Under Under Under Tensile

Forces Compression Compression forces

a during wetting b during drying

Figure 6. Forces due to non uniform expansion and shrinkage. a) Shows the action of Tensile forces in the wetting state and. b shows during the drying state.

The previous sections explain the non uniform wetting and drying cycles during the printing process causes the out of plane deformation in the printed paper. This

18 phenomenon is more predominant with inks that use higher water proportions. To understand the dynamics and the characteristics of the wrinkle formation during the printing process and to understand their fate after the drying process it is necessary to develop a method that can closely replicate the printing process: ink/water application, drying, conditioning. The method should then be complemented with a way to quantify the key characteristics of the wrinkles such as frequency, height and width. This enables us to understand the formation of wrinkles and to understand the key furnish and papermaking factors influencing wrinkle formation.

2.5 Surface topography by Interferometry

As seen in the previous sections the wrinkle development from the time of ink application till the drying is a continuous changing process. This calls for a dynamic- full field quantification system for evaluating the wrinkles. A literature review of different techniques [16-45] showed that the shadow Moiré technique provides a simple method that can be used to obtain real-time and full-field measurements. Another advantage with the shadow Moiré method is that the preliminary analysis can be completed without too much complexity.

The basis of the Shadow Moiré system is the use of reference gratings to superimpose on its shadow resulting in the formation of Moiré fringes[18]. The gratings used in shadow Moiré interference are linear amplitude, binary transmittance type[30]. In a shadow Moiré system a grating of known pitch5 is placed above the object under inspection. The object is illuminated at from a known distance and angle to the center of the grating. The shadows formed by the grating are in turn viewed through the grid itself. Depending on the shape of the object under observation Moiré fringes are

5 Pitch of a grating is the distance between the centers of two adjacent lines of similar transmittance

19 formed. The direction of observation and the angle of illumination play a critical role in the sharpness and the shape of the thus formed fringes. The angle of illumination is generally at 45° and the angle of observation is the usually symmetrical to the grating normal[30].

The formation of the shadow Moiré fringes can be explained by simple geometric analysis of the setup. Figure 7[30] shows two different typical shadow Moiré setups based on which the formation of the fringes can be deduced.

Figure 7. Schematic arrangement of shadow Moiré setup. a) Light source and observer are placed parallel to the plane of the grid forming straight contour lines. b) The setup where the contour lines are cylindrical

The formation of the Moiré fringes can be explained by using Figure 7. In the Figure S is the illumination point and the O denotes the observer. A grating G of pitch d is

20 placed above a reference plane. When the rays from S pass through the grating they intersect the object surface at the object contour lines. The superposition of the reference grating and the shadows on the object result in Moiré fringes. Lines N1, N2, etc are the lines of intersection of constant contour. The shape of the lines for a given reference plane depend on the positioning of the light source S and the camera O. the setup in Figure 7b form cylindrical lines. The setup in Figure 7a forms fringes parallel to the grating plane. An elementary geometric analysis of the fringes in Figure 7b gives the equation of the Nth order cylindrical contour surface:

2 N osoN os )]()([)( lNdlllNdllxblzNdbz os =−−−−++− 0 (4)

Where N=0, 1, 2 and d is the grid period and N is the absolute Moiré fringe order.

Assuming d/b <

l =Δ dz (5) b

In the case of S and O placed at an infinite distance then Δz (resolution) can be calculated by using

d z =Δ (6) +TanTan βα

Where α and β are the angle of illumination and angle of observation respectively. The development of this equation by geometric analysis is shown with the help of Figure 8.

Figure 8. Formation of shadow Moiré fringes at Infinite camera and observer position

This type of shadow Moiré setup can be achieved in two different ways. The camera and the light source are positioned at infinite distance beyond which the change of distance will not effect the fringe formation. On the other hand this can also be achieved by using a collimator to generate parallel light beams and the camera system is telecentric. The light source and the camera are positioned at 45° and 0° respectively with reference to the center of the reference grating. The dotted parallelograms indicate the illuminated areas that can be viewed by the camera

22 through the reference grating. The size of these parallelograms depends on the pitch of the grating. These adjacent parallelograms join together to form shadow Moiré fringes. The distance between two adjacent fringes is explained by the imaginary triangle shown on the Figure 8.

By applying Equation (6), for a given fringe order N the height of the object from the reference plane can be calculated by,

*dN h = (7) Tan + tan βα

Where: N is the absolute fringe order.

Researchers developed several methods to determine the absolute fringe order of shadow Moiré fringes: adding a vertical line to the grating, the two-frequency grating method, the moving-light source method, the two differently colored light sources, crossed-grating method, flexible reference gratings etc[21, 44]. However all these methods require automatic fringe analysis which requires the use of complicated image analysis routines[35].

The resolution of a given shadow Moiré setup can be increased by determining the sub-order of the fringes. The approaches for sub-order resolution are: Phase shifted Moiré[19, 22], phase stepping Moiré[26] and phase unwrapping[46]. All these techniques require automated image processing routines. Phase sifted Moiré utilizes information from multiple frames to extract phase information of each fringe. Phase measurement can be either “temporal phase measurement” or “spatial phase measurement”[19, 22, 39, 40]. Different frames or captured while moving the reference grating horizontally by a fraction of the pitch of the grating. The phase

23 stepping is done by moving the grating perpendicular to the plane of the initial grating position. All these resolution enhancement techniques needs very precise movement of the reference grating followed by complicated image analysis algorithms. In the presented technique however only the basic shadow Moiré method is used and the fringes are then analyzed by manually counting the fringes with reference to a known surface.

24 3 PROJECT DEFINITION AND OBJECTIVES

Currently no method to evaluate the tendency for a given base sheet to wrinkle during ink-jet printing exists short of conducting a printing trial. To evaluate the effect of sheet properties on wrinkling propensity, conducting many trials on printing presses would be costly of both time and money. In addition, only commercial and rolled webs could be tested. Therefore, there is a need to develop a laboratory method that can be used to evaluate wrinkles developed in simulated ink-jet printing conditions. This is the objective of the proposed study and is formally stated as:

“Develop a method to evaluate the wrinkling tendency of ink-jet papers in a manner similar to that experienced in a commercial web-fed process”.

Scope: For this project, the focus is on uncoated and untreated ink-jet papers with wrinkles occurring in the cross direction.

4 EXPERIMENTAL METHODS

4.1 Method for Evaluating Wrinkles The primary target in developing the laboratory method was to capture the fundamental aspects of the ink jet printing process with a simple method that can be replicated. In essence, the method consists of taking a sample of paper, wetting it in a specific manner and observing the subsequent wrinkling that occurs during the wetting and drying. Secondly, the entire width of the sheet is wet to reduce the complexity of the resulting stress state in the sheet. In this method, a predefined amount of water or ink is applied to a specified region of the paper. Figure 9 shows the proposed geometry for wetting the substrate.

Wetted area

Figure 9. Ink/water application onto the substrate.

Once the sheet is wetted wrinkles will form in the wet region as explained earlier. The number of wrinkles and the size depends on the various characteristics of the sheet

26 like grammage, caliper, tensile modulus, and the degree of sizing. In addition, the width of the wetted region in the MD direction and any applied MD tension will affect the wrinkle characteristics. Once the sheet is dried the wetted area will shrink more than the non-wetted area, thus transferring the wrinkles into the adjacent areas to the wetted region. Figure 10 shows the formation of the wrinkles in the wet state and after the drying process.

wrinkles

b) wrinkles

Figure 10. Formation of wrinkles and change of wrinkle location as the ink is dried. a) Wrinkles in the wet state b) Development of new wrinkles in the adjacent area to the wetted area after drying.

4.2 Development of Ink-jet Wrinkle Analyzer

The first step in the development of the method to evaluate the wrinkle formation in ink-jet printing papers was to construct a bench top wrinkle analyzer. The major objective of this involved the development of an evaluating station that is equipped to mimic the web-fed ink-jet printing process and then complemented with a sheet topography quantification system. The various components involved in the developed wrinkle analyzer Moiré consist of four different systems:

1. Ink application system

2. Paper holder mechanism

3. Load and Heat application

4. Shadow Moiré station

Each of these systems is explained in the following sections with appropriate details and operating methods.

4.2.1 Ink Application system

The first module of the wrinkle analyzer is the ink application station. The purpose of the ink application station is to apply a desired amount of ink on to any given substrate. Based on the explanation given in the previous sections on the wrinkle

28 formation, it was decided to use a solid print in the cross direction of the sample. This was achieved by use of a pressurized spray nozzle6 that can apply ink by a uniform spray on the sample. This spray nozzle is moved back and forth to cover the entire width. The dimensions of the inked area were controlled by covering the entire sample with a blotting paper having an opening of required dimensions. Thus even though the spray from the nozzle is always same, the inked area changes by changing the opening in the blotting paper.

The ink application station (Figure 11) consists of a ‘flat jet’ spray nozzle N, which is mounted at the end of a shallow cylindrical tube T which supplies ink to the nozzle from the supply Tank S. The supply tank is fitted with a screw cap C to allow refilling the ink. A gate valve G is fitted below the supply tank for refilling the supply tube. Right above the gate valve and below the supply tank the tube T is fitted with air supply tube for controlling the pressure inside the tube and thus to the spray nozzle. Just before the nozzle the ink tube is fitted with a solenoid valve V to control the supply of the ink to the nozzle. This entire ink-unit is mounted on a motorized rack and pinion mechanism R. The forward motion of the ink unit is driven by the motor and the repositioning of the ink unit is done by pulling the handle manually. The speed of the motor is controlled by a dial gauge with the speed with 10 dial settings7. The motor and the solenoid valve are connected to a controller for operator control. The motor is set off by using an ON/OFF switch and the movement is terminated by the limit switch fitted at the end of the rack and pinion moving channel. The moving channel is fitted on the far end with a stopper for avoiding damage in case the sensor fails to stop the motor.

6 The technical details of the spray nozzle are given in APPENDIX A 7 The calibration curve for the motor speed at different dial gauge settings is given in APPENDIX B

30 Ink tank

Ink flow valve Compressed air Ink-tube mount

Solenoid valve

Nozzle Air pressure regulator Rack and pinion arrangement

Figure 11. Ink application system and its components. a) Schematic diagram of the ink application system b) Image of the ink application part on the wrinkle analyzer

Table 1 gives the various specifications about the Ink application station.

Table 1. Operating Parameters of the Ink Application Station

S.NO Parameter Value

1 Motor operating speed 6.63 – 42.6 in/sec

2 Equivalent orifice Diameter 0.026”

3 Nozzle operating pressure 15-60psi

4 Ink supply tank volume 500ml

5 Motor Acceleration Distance 2 inch

6 Total movement span of the ink- 15 inch unit at set speed

4.2.1.1 The Development pathway

Ink application system is the most crucial part of the development of the wrinkle analyzer since the configuration of all other components is based around this mechanism. The principal function of the ink application system is to apply a

32 controlled amount of any liquid on to the paper in any given dimensions. Various options were considered to perform this function such as: applying a wet blotting paper, desktop Ink-jet Printer, using a woven fabric/sponge with controlled squeezing, Gravure cylinder, spray nozzle, Cobb type pond8. Each of these options was reviewed for their practical suitability for the kind of setup needed as well as the repeatability of the liquid application.

As the proposed method targets at dynamic quantification technique, the ink application time and the time lag between the application and measurement needed to be minimized. This factor eliminated the possibility of using a desktop ink-jet printer for the reasons that to print a solid print, any regular desktop ink-jet printer takes several seconds depending on the area of the print before the printed sheet can be ejected and can be laid flat to measure the surface topography. This eliminates the possibility to quantify the initial wrinkles in the inked area which is crucial to understand the ink-paper interactions. Another idea that was debated was the use of a cobb type pond that holds water on the surface of the paper for a specified time period. However, it was apparent that the amount of water applied on the paper can not be controlled by this method because the liquid application is completely dependent on the absorbtivity of the paper.

Use of a gravure cylinder to hold ink gives a very controlled ink transfer. The amount of ink can be controlled by varying the cell size on the gravure cylinder. However for each change in the liquid quantity or the dimensions of the print a different gravure cylinder is needed which are highly expensive. Even then the flexibility of the application is limited by the availability of a suitable gravure cylinder. Apart from this drawback the gravure cylinder printability is limited by the surface roughness and requires relatively higher printing pressure. These higher loads may introduce an

8 Name of the test used for water absorbency of paper, which uses a metal frame clamped tightly over a sample of paper to hold water.

33 unknown variable into the sheet. For these reasons the gravure cylinder method was not used.

The first method used practically on the paper was to apply a wet strip of blotting paper of desired size and gently squeeze the water onto the paper surface. This method as shown in Figure 12 a blotting paper cut into required dimensions and then soaked in water for a specified time is placed on the sample paper along the CD of the paper. The blotting paper strip is then pressed gently against the sample to squeeze water onto the paper before the blotting paper is removed. This method produced uniform wrinkles in the wetted area. However the repeatability of this method is not encouraging. For a sample size of 20, the Coefficient of Variance of the ink quantity applied is 28%. This variability could be due to the different factors like non- uniformity in the absorbing characteristics of the blotting paper and variation in the applied pressure.

Wet blotter strip Paper sample

B C

Figure 12. Application of water by wet blotting paper method. A) Liquid application by using a soaked blotting paper B) wrinkles in the wetted area C) wrinkles after the sheet is dried.

Since the blotting paper method posed repeatability problems, it was decided to replace the blotting paper with a paper machine dryer fabric which is supposed to have a controlled absorption and release of the liquid. From the experience of the blotting paper method, a device to apply uniform pressure and controlled water absorption to the dryer fabric was developed (Figure 13). This device consisted of a metal shaft with vertical movement, supported at both the ends over compression springs. The bottom of the shaft holds dryer felt of any specified size with the help of Velcro. The shaft can then be lowered with pressure into a small well, so that the felt absorbs a controlled amount of liquid. The PVC well then replaced by paper sample and the felt was lowered once again to squeeze the liquid onto the paper sample. This method provided controlled amount of liquid application and uniform loading across the length of the application.

35 Loading beam

Dryer felt

Liquid well

Figure 13. Liquid application device using dryer fabric

However the uniformity of the liquid applied by using this device varied across the wetted area within the sample, leaving dry spots in the wetted area. This problem may be due to the high compressibility of the dryer fabric.

The final option available was the use of a spray nozzle. This method uses application of liquid through a pressurized spray nozzle. Unlike the previous methods this one involves the application of ink by non contact process. The wetted area dimensions were indirectly controlled by masking the non-print sample surface with a blotting paper. The nozzle is then moved across the blotting paper window to apply liquid onto the paper sample. This principle was transformed into an operating mechanism as shown in Figure 14. The nozzle was mounted at the tip of a steel tube that works as a container for the liquid to be applied. The other end of the steel tube was connected to a compressed air supply through a pressure regulator. The flow of the liquid was controlled by a solenoid valve fitted right above the nozzle. The solenoid valve was

36 controlled by an on/off switch. This whole tube setup was fitted onto a pillow block to allow the whole setup work as a pendulum. This pendulum movement imparts a repeatable velocity to the nozzle each time it is dropped off a constant position. For ink application, a paper sample with the masking blotting paper was placed in such a way that the center of the length of the ‘to be inked area’ is at the center of the pendulum movement. Releasing the pendulum from a preset elevation while turning on the spray valve simultaneously, sprayed the liquid through the nozzle over the blotting paper resulting in printing the unmasked area of the sample.

Pressurized air

Solenoid Valve Pillow block

Steel tube/liquid tank

Spray nozzle

Figure 14. Spray nozzle application system to apply liquid by the pendulum movement of the ink tube.

The nozzle setup consisted of a TEEJET flat spray nozzle and a strainer of 100 mesh size (60micron). The strainer helps to filter the liquid and thus avoid the plugging of

37 the fine orifice of the nozzle. The solenoid valve was set to a minimum operating pressure of 5PSI. The minimum operating pressure setting helps in avoiding any dripping of the nozzle due to gravity or due to the static pressure inside the steel bar.

Figure 15. Teejet’s Flat spray nozzle with spray at different operating pressures. The left side of the spray is with low operating pressure and the right side of the spray at high operating pressure.

A flat spray nozzle has a uniform spray distribution across the spray area. This type of spray pattern is required to minimize any variation within the wetted area.

After satisfactory initial testing, this setup appeared to be consistent in ink application. To establish the operating parameters a series of different tests were performed on a copier paper. The variables tested were the nozzle orifice size, operating pressure, spray height and ink surface tension. Table 2 shows the variables tested for establishing the ink application parameters.

38 Table 2. Variables tested on the Pendulum-spray nozzle system

S.No Variable Settings

1 Air pressure (PSI) 20 30 40 50

2 Nozzle flow capacity (l/min) High Medium Low

(0.57- (0.34- (0.23- 1.14) 0.68 0.45)

3 Height (inches) 1 2 3 4 5

4 Speed (relative speed by changing S1 S2 S3 S4 S5 the height of pendulum drop)

5 Ink surface tension 49 30

A full parametric evaluation of the process variability was performed with each of the settings that are shown in Table 2. The paper samples thus sprayed with ink were analyzed visually for uniformity of the ink application and for establishing the ink application parameters. At the end of this exercise the various parameters for ink application were established. As shown in Figure 15 the drop size increases with lower pressure for a given nozzle type. Application of high surface tension ink with ‘low’ capacity nozzle at 30PSI pressure gave optimum results. The pendulum speed for this sample setting was then calculated with the help of a video camera to determine the time taken for pendulum to complete a full cycle. The speed was calculated as 1.2ft/sec. at a height of 6inch between the nozzle and the paper sample.

39 The variability of the ink quantity applied by this pendulum driven nozzle system is lower compared to the previous methods. For a sample size of 10, the coefficient of variance was found to be at 6%. However from the visual analysis of the samples, it was clearly seen that the ink application was not uniform across the length of the application. The ink in the middle of the sample where it corresponds to the lowest point of the pendulum was relatively sparse compared to the ends. This can be attributed to the variation in the pendulum speed across its travel path. Because of the consistency in the ink application among different samples, it was decided to use the spray nozzle system for ink application. However to eliminate the speed variation caused by the pendulum arrangement, it was decided to mount the nozzle on a rack and pinion arrangement driven by a motor. The use of motor with parallel movement of the nozzle along the plane of the paper was expected to increase the repeatability of the ink application.

The motorized ink application system thus developed is the final version of the ink application system shown in Figure 11b. The ink tank as shown in Figure 16 was added later on to increase the ink holding capacity there by reducing the need to refilling the ink-tube for each test. A gate valve separates the ink-tube from the ink tank. This gate valve enables the refilling of the ink-tube as well as it isolates the ink tank from the air pressure supplied to apply the ink. For refilling the ink tank both control valve and the input valve of the air supply need to be turned off. However for safety reasons the ink tank was made to withstand up to 60 psi pressure.

Figure 16. Ink application system with added ink-tank

The ink spray from the spray nozzle is in inverted cone shape. The spray comes out of nozzle at 80° and 15 psi, so the greater the distance between the spray nozzle and the sample the wider the jet is. Since only the center of the ink-jet is used for wetting the sample, the excessive edges of the spray were trimmed to avoid spraying on the other parts of the equipment. For this purpose a vertically movable ‘L-shaped’ holder was fitted on the guard plate of the ink system. An absorbing felt with an opening wide enough for the ink application area is fixed on this L-shaped holder. This helped to cut off all the excessive ink.

41 Guard plate

Paper sample

Masking blotter

Absorbing felt

L-shaped holder

Figure 17. Ink application system with L-shaped felt holder arrangement to trim the excessive spray

The coefficient of variance in the ink quantity applied at these final settings was 4.5% for a sample size of 8. The repeatability of the ink application was tested at 5” nozzle height and 39.2 in/Sec nozzle speed. For this the first set of ink application was done at the specified parameters after which all the valves were shut down and motor speed set to zero and the whole setup was restarted at the specified parameters to perform the second set of ink applications. The two-sample T-test at 99% confidence level assuming unequal variance verifies that there is no difference between the mean ink quantities of these two samples.

42 Table 3. Repeatability of the ink application at a given set of parameters on two isolated runs.

t-Test: Two-Sample Assuming Unequal Variances Ink Quantity (g) Set 1 Set 2 Mean 0.08619 0.08654 Variance 1.5E-05 4.2E-06 Observations 8 8 Hypothesized Mean 0.00035 Difference Df 11 t Stat -0.4489 P(T<=t) one-tail 0.33111 t Critical one-tail 2.71808 P(T<=t) two-tail 0.66221 t Critical two-tail 3.10581

4.2.2 Paper Holder mechanism

The paper holder mechanism holds the paper specimen that will be evaluated. The sample needs to be transferred from the ink-application station to the shadow-Moiré station. This mechanism (Figure 18 ) was built using two horizontal bars on which the sample can be mounted. The left hand side bar A is fixed while the right hand side bar B is free to slide in one direction. The retractable bar B was mounted on two linear slides L1 and L2, which allows the bar to move back and forth. On each side the linear bars were fixed to two horizontal bars, the left end bar making one end of the paper

43 mount system and the bar on the other end of the liner slides was the mounting bar C for the whole paper holder system on to a linear slide S. The support bar was mounted on the shuttle H with the support of a mounting plate P. The linear slide S enables the movement of the paper holder mechanism back and forth between the ink application station and the shadow Moiré station. The paper sample can be mounted on the paper mount system in two different ways depending on the gap required between the paper sample and the shadow Moiré station. The first method was to fix the sample between clamping rods provided with each of the paper mounting bars. This provided the sample positioning relatively lower with respect to the shadow Moiré grating9. In the second method, the sample is mounted using a double sided adhesive tape on both mounting bars A and B. The second method provided a closer positioning of the sample with reference to the grating.

P H L1

Retractable bar S B A C L2

9 The positioning and purpose of this grating is explained in detail in section 4.2.4

44 Grating

Paper mounting bars

Linear rail Mounting plate

Fixture to set MD length

Figure 18. Sheet holder mechanism a) Schematic diagram of the mechanism b) Image of the sheet holder mechanism fitted on the linear rail below the grating.

To fix the sample length in machine direction a wire loop with adjustable length was used by strapping it around the hooks provided on the top of paper mounting bars. The measuring scales provided on both sides of the frame helped in positioning the sample.

Table 4 gives the operating parameters of the paper holder mechanism.

45 Table 4. Operating parameters of the paper holder mechanism

S.No Parameter Value

1 Max. sample width 15 inch

2 Max. Sample length 17 inch

3 Maximum allowed movement 26 inch of linear slide S

From these maximum sample width and length it can be observed that, the design well accommodates the testing of a full A4 size sample.

4.2.2.1 The Development Pathway

The development process of the sheet holder mechanism was not significantly changed. The original requirements for this component were:

• It needed to accommodate different sample sizes with the maximum limit of an A4 size sheet and the mounted sample.

• The paper mounting frame should be able to accommodate a provision for applying tension along the machine direction.

46 • The entire frame should move from the ink application point to the shadow Moiré setup.

The main modification came in developing the clamping of the specimens. The original clamping device comprised of a groove and a rod. The paper was aligned in the groove and the rods were pulled back with clamps to hold the paper. The paper made a quarter wrap around the rod. Figure 19 shows this set-up.

Sliding bar

Steel rod

Clamp

Fixed bars

Figure 19. Paper holder mechanism before modification of the paper clamping system

The clamping of the paper sample against the hinges on the bars proved to be extremely difficult. It was found that very rarely could the paper be aligned with both clamps. Typically, the paper had slack at some point along the clamped edges. This problem was corrected by avoiding the use of clamping rods. Instead the paper was fixed on both the bars by using two-sided adhesive tape (Figure 20).

47 The paper sample mounted using adhesive tape was tested for any tape slippage or deformation. This was done by taping down the sample on both the ends to the frame. A thick line was drawn at the edge of the tape and at the edge of the sample. The paper with the lines was then allowed to stand under load for a regular test time of one hour. After one hour, the lines did not show any deformation at the interface confirming that neither the paper nor the tape had deformed or slipped.

Measuring scale

Support strip for sample alignment

Double sided adhesive tape

Figure 20. The modified sample mounting frame with corrected alignment and sample mounting using adhesive tape

In the absence of the hinges in the modified frame, a support plate was fixed on the outer end of the sliding bar to align the sample mounting. Two measuring scales were fixed in both horizontal and vertical directions of the frame to assist in the positioning of the sample.

48 4.2.3 Load and Heat Application

To apply external variables in the measurement system, two mechanisms were installed on the wrinkle analyzer. Those two variables are application of machine direction load and heated drying to simulate the web-fed ink-jet printing.

The load application components (Figure 21) were mounted onto the paper holder mechanism. The load application string was connected at the center of the retractable bar of the sheet mounting system. A string was carried over a series of three pulleys, fixed on the support bar providing additional elevation to suspend any desired load D.

P H

Pulleys Retractable bar

S Exhaust fan Weight B Hood A C L2

Heater

Figure 21.Schematic Diagram of the load application mechanism mounted on the paper holder

A low elevation laboratory (Figure 21) hotplate is mounted below the sample holder to provide heating. The heater was fixed directly below the shadow Moiré station. The distance between the sample when mounted by adhesive tape and the heater surface is 2.2”. The heater was provided with an air hood with an exhaust fan to facilitate air circulation. The air hood helped to provide a relatively uniform heating and also

49 eliminates the fogging of the grating by the evaporating vapors from the ink. The temperature of the heater was controlled by the dial knob and the front of the heater.

4.2.4 Shadow Moiré Station

The central part of the equipment is the shadow Moiré setup, which involves intricate alignments while accommodating a flexible setup to allow free movement of the sample holder to and away from the shadow Moiré measuring system.

The shadow Moiré setup (Figure 22) consists of: A binary transmittance grating, an incoherent light source and a camera. The binary transmittance grating commercially named as a Ronchi-ruled plate is a glass plate with equidistant and alternating black and white lines etched in. The Ronchi plate was fitted into a holder which provided alignment along any axis. The grating holder was mounted on two linear slides providing horizontal and vertical movement. The grating holder was made up of two plates. The top (aluminum) plate was fixed to the linear slide while the bottom (PVC) plate, which holds the grating floats below the aluminum plate with the help of four thumb screws on four corners. This accommodated the alignment of the grating holder in the direction perpendicular to the plane of the grating.

Figure 22. Schematic diagram of Shadow Moiré station.

The whole grating can be moved in both perpendicular and parallel directions to the paper holder movement. However the parallel movement position was fixed at a position such that the center of the grating makes a 45° angle with the light source.

The second component of the shadow Moiré system is the camera. The camera used was a commercial Sony camcorder10 with streaming11 capability. It recorded the images at 640*480 pixels. The 20X optical zoom of the camera allowed zooming in on the sample. The camera is mounted using a three point pivoting arm that was fixed at a height of 21” above the grating. As explained in Section 2.5 this provided enough distance above which the fringes do not distort. The three point arm provided the movement of the camera in all three directions for alignment purposes.

10 Camera used is Sony DCR HC 21, for technical specifications see APPENDIX C 11 Video Streaming is the ability to send the live image/video over a cable to a computer 51 A fiber optic light source was used to provide illumination. The fiber optic light source consisted of two parts, a base unit which contains the light control electronics and the actual bulb. The light from this source is then carried by a fiber optic cable and the far end of the cable provides the actual illumination. A 100W Halogen light bulb was used in the light source. This provides incoherent illumination. The fiber optic light source was mounted on the camera arm at its mounting edge. The fiber optic cable then carried the light and the illuminating end of the cable was mounted over a light stand. The light stand can move in all the three directions to adjust the distance and height between the grating and the light for achieving the required illumination angle. The illuminating tip of the fiber optic cable was fitted with an aperture mechanism that provided rectangular aperture adjustment to illuminate only the grating area. The use of aperture eliminated the reflections from the excessive light falling on any reflective surfaces, which will reduce the sharpness of the fringes. The light source was mounted at 35” away and 35” high (Figure 25) from the center of the grating providing the light at 45° angle. The near and the far end of the grating made 47.6° and 42.5° respectively angle with the illumination point.

4.2.4.1 The Development Pathway

The functional objective of the shadow Moiré station is to provide the ability of dynamic quantification of the surface topography. From section 2.5 it can be seen that the shadow Moiré station is composed of three basic components: grating, light source and camera. In the initially developed setup (Figure 23), a grating holder made of PVC was mounted on two linear rails providing X and Y movement of the grating with respect to the sample holder.

52 Light tip holder

Camera

Light Source Grating holder aperture Support for grating holder

Grating

Sloped frame for the grating Fringes

Grating Fixtures

Grating

Figure 23. Shadow Moiré station before modification of the Grating holder and the light source

53 This entire grating holder was fixed to two vertical bars. The positioning of the grating holder on these bars can be changed to move the grating vertically for changing the distance between the grating and the paper sample. In the initial setup as shown in Figure 23 the distance between the grating and the paper surface was 0.66 inches. This gap is critical, since the lower gap forms sharper fringes for a given grating pitch. A sloped frame was cut into the grating holder to position the grating at the bottom of the grating holder, thus minimizing the gap between the paper sample and grating. The grating was then placed into this frame and held firmly with the help of the fixtures on four sides. A 50 LPI (lines per inch) grating was used in the initial setup.

The light source for the shadow Moiré setup can be either a coherent or an incoherent source. After evaluating various options like collimated fiber optic lighting, florescent light tube, halogen gas discharge tubes, etc. it was decided to use a fiber optic point light source [30]. The light source consisted of a base unit where the light source exists and a fiber optic cable connects at one end to the base unit and the other end works as a virtual light source. A 100W halogen photo optic lamp was used as an incoherent light source. The lighting tip of the fiber optic cable was mounted into a stand. This stand was mounted with a rotating camera holder with adjustable aperture. The axis of rotation of this holder was perpendicular to the stand. The camera holder which can be moved vertically was used to fix the fiber optic light tip. The aperture facilitated the control to focus the light onto the grating. The light stand was positioned in between the ink application system and the grating with the light holder fixed at 45° angle to the grating.

The camera was mounted directly above the center of the grating with three point pivoting arm. The base of the arm was fixed on top of one of the grating holder support bar. The three pivoting points of the arm as well as the single point fixture of the camera to this arm enabled the camera movement virtually in any direction.

54 The shadow Moiré fringes obtained with this initial setup however proved that there were several adjustments needed. This was revealed by the shadow Moiré fringes obtained with this setup. Figure 24 shows a deviation from the expected pattern. The fringes patterns shown here are for a sample immediately after wetting, in which case the wrinkles are like waves in the inked region. As the each shadow Moiré fringe represents regions of constant contour, the fringes formed on the wrinkles in the inked region should be in elliptical shape. (The fringes seen in the un-inked area are due to the misalignment in the clamping system which was modified later as explained in Section 4.2.2.1).

Figure 24. Shadow Moiré fringes on a wet sample with the initial shadow Moiré station setup

After a brief analysis of the setup it was understood that the deviation in the fringe pattern shape was due to the light source being positioned too close to the grating. This setup was similar to the one shown in Figure 7b. This kind of setup forms cylindrical contour lines for a plain reference surface. This is due to the relatively high

55 deviation of the angle of illumination across the grating. Even though theoretically it is possible to use this setup, it requires complicated software that can compensate for the deviation in the angle throughout the image. Another reason for the problem was the grating holder being not parallel to the plane of the paper sample. This was due to the linear rail that was used for mounting the grating holder onto the vertical bars. This linear rail being old was worn out and caused vertical misalignment of the grating.

To correct the problem, the light source was moved to the outer side of the wrinkler. A new light stand was made with a three dimensional flexibility. The new light stand was made of an aluminum bar of 5ft height mounted perpendicularly on a linear rail. A small cross bar was fitted to the vertical bar with a screw clamp so that it can slide across the length of the 5ft. The light tip of the fiber optic cable was mounted in a holder with square aperture to trim the excess light. The holder was then clamped to the cross bar in such a way that the light holder angle can be adjusted with the cross bar as the axis of the angular movement. To get a reasonably good geometry of the illumination point and the grating, it is calculated that the light tip should be at 35” high and 35” away from the center of the grating. This reduced the variation of the illumination angle to 2.6° and 2.4° respectively at the near and far ends from the center of the grating. Since the fiber optic cable length was limited, the light source unit was moved onto the camera arm end where it was fixed to the vertical bar.

The problematic linear rail of the grating holder was replaced with a linear guide system ‘Dryln’ provided by IGUS Inc12 (http://www.igus.com/show_dt.asp). The shuttle of this linear guide can be adjusted for fine alignment adjustments and it can be locked at any required position for more stability.

12 This linear guide is donated by IGUS Inc. under their Young engineer Support (Y.E.S) program

56 Light Light tip Camera Source

35”

19”

35”

Grating holder

Figure 25. Shadow Moiré setup after modification of the light stand

With the modification of the light stand and the grating slide mounted on the new linear guide, the shadow Moiré can be aligned with more precision. The shadow Moiré setup alignment was fine tuned by using a flat aluminum plate as a reference plane on the sample holder. With a perfect alignment, on a flat plate the shadow Moiré setup is not supposed to produce any fringes. The modified setup of the shadow Moiré system was aligned based on this principle. However it was then found out that grating positioned in the sloped frame was not stable. This grating holder also did not provide any facility for alignment in the direction perpendicular to the plane of the paper sample. To address this problem a new double frame grating holder was designed.

57 Aluminum Frame Thumb screw PVC frame

Figure 26. Modified double frame floating grating holder

This new holder consisted of an aluminum frame and a thin PVC frame. Both the frames were attached together with the help of thumb screws on the four corners. The aluminum frame was mounted on the linear rail while the PVC frame floats under the aluminum frame with the support of the four thumb screws on the corners. The PVC frame on the bottom is machined so that it tightly holds the grating at its bottom edge. This floating arrangement provided increased alignment precision. The top aluminum plate is painted black to reduce any reflection of the light.

The fine tuned design of the shadow Moiré station proved to give more control of the alignment of any part of the setup. In the initial setup a 50LPI grating was used. However it was decided to use a 100 LPI grating to increase the measurement sensitivity of the shadow Moiré system. But the use of higher frequency grid needed either a more intense light source or the distance between the grating and the paper sample needed to be reduced to half of the previous setting. However the reduced elevation of the grating holder interfered with the movement of the paper sample holder. The bolts used to mount the sample holder on the linear slide shuttle interfered with the lowered grating holder. To avoid this problem the paper holder mounting plate was redesigned, so that it could be mounted using flat head screws. After this modification the grating was successfully positioned at 0.33” from the paper sample

58 surface. In this setup the shadow Moiré system was aligned using a flat aluminum plate as reference.

To cross check the shadow Moiré system with the Equation (7) an aluminum plate with a 0.1” inclination on one side was used to generate fringes.

Figure 27. Fringes on a flat aluminum plate with an inclination of 0.1" on one end.

From Figure 27, it can be seen that there are 11 full fringes formed on the aluminum plate. By using Equation (6) the distance between two adjacent fringe centers is 0.01”. Thus the 11 fringes equal to a height of 0.11” (equation 7). The deviation of 10% between the calculated and actual heights may be due to the arrangement of the elements used to provide the elevation. Figure 27, shows that the fringes formed are not perfect straight lines. This is because of the small variation in the angle of illumination across the grating. However this small curvature would not affect the results when we are analyzing the fringes manually. This resulted in the fine tuning of the setup which is an improvement of the initial design.

5 PARAMETRIC EVALUATION

5.1 Overview of the Experiment

Once the wrinkle analyzer was developed, testing of papers could be carried out. This section provides an overview of the experimental procedure followed for testing the paper samples.

5.1.1 Sample Preparation

A variation in room humidity has shown significant variation in the number of wrinkles. During the ink application with the pendulum setup, it was noticed that the number of wrinkles during the wet stage has changed without change in any ink- application variables. The only change observed was the change in room humidity. For this reason, paper samples to be tested were conditioned for 24 hours in the same controlled environment in which the samples were tested. The testing conditions used were: 23°C temperature and 50% RH. Paper samples were then cut into required dimensions with adequate care to maintain the squareness. The size of the sample in machine direction was maintained 2” longer than the required sample size to allow for the adhesive tape of one inch on each end of the sample.

A blotting paper having dimensions one inch larger than the dimensions of the paper sample was prepared. A rectangular cut-out was made in the blotting paper. The dimensions of this cut-out depended on the sample size and the required inked area

60 dimensions. However the CD dimension of this cut-out must be at least 0.2” more than the paper sample CD dimension.

5.1.2 Preparation of the apparatus

For the tests, the motor speed of the ink application system was set at 39.2 in/sec by placing the dial gauge at 8. The nozzle height was set at 4” and the air pressure to the spray nozzle was set at 30 psi (Section 5.3.1). After ensuring that the ink supply tank filled, the ink flow valve was opened until the ink tube was filled. Then the valve was closed to isolate the ink supply tank from the compressed air.

One must pull the handle of the ink application system to position the spray nozzle in ‘Run’ position. To apply MD tension weights were placed on the loading hook. The sliding bar of the sample holder was held at required sample MD length by using the adjustable length wire loop. A double sided adhesive tape (3M brand) of 1” width was stuck on both of the sample holder bars. The tape was pressed firmly against the bars to ensure uniform and firm adhesion. The grating was positioned in the grating holder with the lines perpendicular to the incident light. The grating holder alignment was then cross checked by using a water bubble. This allowed each end of the paper sample to be fixed on the adhesive tape ensuring that any of the edges is not slack. By using the tape roll moderately high pressure was applied while fixing the sample to the tape to ensure full bonding. The sample was positioned so that its CD center coincided with the shadow Moiré setup. This was done with the help of the measuring scale on the fixed bar of the sample holder. The 6” mark on this scale corresponds to the center of the grating in its operating position. Now the blotting paper with cut-out was placed on top of the sample in such a way that the cut-out was extending just a little over the CD size of the sample on both ends. Since the blotting paper is of same size as the

61 sample MD length, the cut-out in the blotting paper automatically aligned at the MD center of the sample.

5.1.3 Running a test

Now everything was ready to start the test. To identify the sample a small tag was placed on the far end of the grating holder. The wire loop hook holding the bars was removed to apply tension. Then the sample holder was moved under the ink application nozzle. The paper sample was positioned in such a way that the cut-out in the blotter centered below the spray nozzle. At this point the fiber optic light source was switched on to maximum intensity setting and lights in the room were switched off to avoid any external illumination (This ensures maximum intensity of the fringes). The video camera recording was started just before pressing the ‘Launch’ button to operate the spray nozzle motor. The sample was then moved to the ‘measuring’ position immediately after spraying the ink without any delay. The center of the sprayed ink strip should be aligned with the center mark on the grating holder. The masking blotting paper was removed from the sample and then the grating was moved under the camera. Now the wrinkles can be seen on the LCD of the camera. Recording was continued for a desired period to capture the wet state wrinkles. After allowing the sample in this condition for sixty minutes to dry (for room temperature drying), the image of the fringes was recorded for capturing dry state wrinkle information. This concludes the testing phase.

Once the test was completed the initial (wet) and final (dry) still images were extracted from the recorded video for image analysis and manual characterization of the sample based on the shadow Moiré fringe formation principles.

62 5.2 Evaluation of testing parameters The wrinkling apparatus has several parameters that can be varied in a test. These include

• Drying temperature

• Sample dimensions

o Machine direction

o Cross direction

• Inked area dimensions

• MD Tension

• Nozzle speed, pressure, and height (for changing Ink quantity)

Each of these parameters was evaluated individually for its effect on the wrinkle formation with the exception of ink quantity. Ink quantity was tested for its effect on the uniformity of the ink application which is very critical for getting consistent results.

5.3 Results and Discussion

5.3.1 Quantification of the Ink Application

The ink quantity applied on the paper sample was mainly dependent on three operating parameters of the ink application system: nozzle height from the sample, nozzle speed,

63 nozzle operating pressure. From the initial spray nozzle system evaluation (section 4.2.1.1) it was established that the right drop size is achieved at a pressure of 30psi. Hence this pressure setting remained unchanged. The ink application was tested at following combinations of the remaining two parameters.

Table 5. Test settings of nozzle speed and nozzle height for ink quantification.

Nozzle Speed - in/sec (dial gauge) Height (in) 16.3(6) 27.2(7) 39.2(8) 42(9) 2" NA13 NA 9 9 3" NA 9 9 9 4" NA 9 9 9 5" NA 9 9 9 6" 9 9 9 9

A plain 60GSM copier paper was used for all these tests. The sample dimension used for this test is 5.0” * 5.0 “. The Ink used for all the tests listed from now onwards is Kodak’s High surface tension (49) yellow-2014. This ink quantification test was performed before modification of the sample holder. i.e. the samples are clamped to the bars.

Ink quantification was done by measuring differential weight of the paper sample before and after ink application. Three repetitions were performed for each setting to establish variability.

13 These are the conditions in which the ink applied ink is unacceptable by visual observation.

64 0.25

The error bars are +/- Standard 0.2 deviation

0.15

0.1 Ink quantity (g) quantity Ink

0.05 Sample: 60 GSMCopier paper Nozzle height (inch) Sample dimensions:5"*5" 2 3 4 5 6 Ink width: 1" Ink: Kodak 2014 yellow-49ST 0 15 21 27 33 39 45 Nozzle Speed(in/sec)

Figure 28. Applied ink quantity Vs. nozzle height at different nozzle speeds

From Figure 28, it is clear that the applied ink quantity is inversely proportional to both the variables: Nozzle speed and Nozzle height. The change in slope above 39 in/sec speed is due to the fact the motor speed of the ink application system flattens in this region and the motor calibration curve in this region is little erratic. Hence this behavior may be due to the error in the motor speed reading.

To quantify the applied ink at any given combination of nozzle speed and height, an inverse fit equation was developed using the ink application data from the above analysis by the method of sum of least squares.

65 A W = (8) *VH

Where W : Ink quantity applied in Grams

A: Inverse proportionality constant=14.98

H: Nozzle height in inches

V: Speed of the nozzle in in/sec

Even though this equation gives the quantity of the ink applied it does not give any information about the uniformity of the ink application on the paper sample. Before choosing any specific combination of the nozzle settings, the acceptability of ink uniformity on the applied sample should be checked visually. From this visual observation it was noticed that among all the application combinations, nozzle height of 4” at 39.2 in/sec gave relatively better uniformity compared to other combinations. From the visual observation of the ink application at different settings, it was found out that as the nozzle height or the application speed increases the ink uniformity reduces. On the other hand either at the lower nozzle height or the lower application speed the sample was flooded with ink and the excessive ink dripped into the valleys of the wrinkles formed in the inked area. In Table 5, the data points marked with NA are all the combinations at which the ink application was too heavy for the paper to carry, thus the paper breaks.

66 5.3.2 Image analysis

In the previous sections it was explained that how the shadow Moiré fringes can be used to quantify the surface topography of the paper samples. Thus in turn, by analyzing these fringes we can characterize the wrinkles formed on the paper before and after drying the ink. There are various computer aided Moiré techniques[22, 26, 35, 39, 47] available to automate the analysis of these Moiré fringes. In the present case, the fringe analysis is done manually. The manual analysis of the Moiré fringes is completely qualitative, thus to introduce some basic quantification techniques, few basic image analysis software were used.

The initial step in the image analysis process was capturing still images from the recorded video frames. The Video recorded on the camera was then converted into a digital video file by using ‘Picture Package software provided with the camera. Then “Windows Movie maker – V2.114” was used to capture the still images from the video file. The video file carries the time tag, which helped to capture images at any time frame. All the still images of a specific sample were stored in a separate directory named as the sample code.

To perform basic image analysis on the still images, a open-source software named “ImageJ-V1.33u” was used. This software is developed and distributed as a freeware by “National Institutes of Health, USA15”. By the virtue of Open source programming, a wide variety of plug-ins is available to perform various image enhancement and image analyses. Two such plug-ins along with the functionalities of the base software was found useful for the fringe analysis.

14 This is a default tool that comes with any windows operating system. 15 The source code and the software are available at http://rsb.info.nih.gov/ij

67 The fringes recording in a video camera introduces noise into the images[30, 47], making it difficult to analyze the fringes without the help of any image enhancement technique. Apart from this the contrast of the fringes in the wet state of the sample was not high enough in most cases. There are two reasons that could be attributed for this problem: the wet ink strip intensity was too high, and the focusing of the camera on a per frame basis was not sufficient enough. To resolve this problem a Fourier filter plug-in of the ImajeJ was used to reduce the noise. The image needed to be converted to an 8-bit grey scale format from its original RGB format in order to perform the FFT filtering16. The filtered image (Figure 29) was relatively better for visual analysis.

A B

Figure 29. Shadow Moiré fringes in the inked area on a sample while it is wet. A) Image after performing FFT filter with ImajeJ B)Image before FFT filtering.

The FFT filtering was done basically by using lower and upper threshold limits to filter out large and small structures. These limits used in this case were 2 and 20. These parameters were chosen based on several trials at different setting and by

16 A snapshot of the ImajeJ tool along with the FFT filter settings used is given in APPENDIX D.

68 visually observing the fringes in the wrinkled area for any apparent change or distortion in the fringe patterns.

Another plug-in we widely used in the image analysis of the Moiré fringes is called the ‘measure’. The use of this tool is explained in the following sections. This tool basically gives the length in pixels of a line drawn on the picture. However by using the ‘set-scale’ functionality in the analysis options, we can set a known length of any units. Once we set the scale the software uses this scale for any further measurements made on this picture and gives the results in the units that we set instead of in pixels. The grating holder’s inner frame dimensions (6.01”) were used to set the scale, since this is constant irrespective of sample dimension we used. Apart from the linear length measurement the ‘measure’ tool also calculates the angle at which a line is drawn. This option was liberally used in the post-dried samples, for calculating the angle of the wrinkles at the CD - edges of the samples.

5.3.3 Data Analysis

The data obtained from the image analysis of the fringes was divided into two sets. The data collected for the wrinkles while the samples were wet and the data for wrinkles after the samples were dried. From here onwards ‘before drying’ or ‘wet state’ is annotated as ‘T0’ and ‘after drying’ or dry state’ is annotated as ‘Tf’. The T0 was typically observed to be between 6-9 seconds after the ink application. This time was established by observing the time stamps of the recorded video during the testing phase. Tf is 30 minutes for the samples dried at 45°C with the heater switched-on and 60 minutes for the samples air dried at room temperature (23°). For the T0 samples the data was collected for parameters like: number of wrinkles, the length of the wrinkles, Wavelength, fringe order, wrinkle height and the approximated width of the

69 wrinkles. Each of these parameters is defined below as they were used for measurement.

Width of wrinkles

Length of wrinkles

Figure 30. Parameters used to quantify the wrinkles in the un-dried sample at T0 time

5.3.3.1 Wet Image

The Parameters used specifically for the wet images:

Number of wrinkles: It is half the total number of fully formed elliptical fringe groups in the inked region counted based on the edges of each elliptical fringe group. The fringes that are not full (at the edges) were omitted from counting. In some situations there are wrinkles that are very thin and when the fringes are looked at the out side of the inked area it looks like two smaller ellipses of the fringes are contained in one bigger ellipse. A detailed discussion on the formation of these wrinkles and the impact

70 of (not)counting them is given in Section 6.3. In such cases these two wrinkles are considered as one wrinkle.

Length of wrinkles: It is the total length in which all the fully formed wrinkles exist measured by using the ImajeJ.

T0-Wavelength: It is the average Wavelength of the T0 wrinkles calculated by dividing the Length of wrinkles by the total number wrinkles in the inked region.

Approximated width of the wrinkles: This is the distance between two lines drawn on either side of the MD width of the wrinkles. These lines are drawn so that it passes through the first order fringe of maximum number of wrinkles. This parameter is measured to define how wide the wrinkles extend during the wet state.

5.3.3.2 Dry Image

The parameters used for the fringe analysis of the samples after drying are somewhat different from those explained above. The fringes formed in the Tf images were much more prominent and allowed for more data to quantify. The parameters used in this case are: fringe order, co-planarity order, angle of the edge wrinkles, number of wrinkles, wavelength and maximum flatness.

71 Machine Direction Angles

Maximum flatness

Wavelength No. of fringes= co-planarity order

Figure 31. Parameters used to quantify the fringes in the post dried samples.

Fringe order: fringe order is counted by using a reference plane which is assumed to be at the original undistorted plane of the sample. This parameter is used in analyzing both the T0 and Tf images. In case of T0 images the white area away from the inked region is assumed as the reference plane. This assumption was based on the fact that during the wet state the wrinkle were observed only in the inked region and most of the un-inked area remained flat. Thus the region is mostly free of fringes.

72 In case of Tf images the non-inked region has wrinkles and there is no ideal region in the sample that can be considered as the reference plane. However physical examination of the dried samples showed that inked region becomes relatively flat. Thus the white areas in the shadow Moiré images of this region were assumed as reference plane. As explained in the shadow Moiré fringe formation theory, the distance between centers of two alternate fringes is counted as one fringe order. Hence the distance between centers of an adjacent light and dark fringe is equal to half the fringe order.

Wrinkle height:

The fringe order can be used to calculate the height or depth of a wrinkle with the help of equation (7). However height and depth can not be differentiated by the basic nature of classical shadow moiré fringe analysis as explained in Section 2.5. For this reason from now onward this parameter is referred only as height. The wrinkle height in both cases is the average height calculated based on the number of observations within a sample.

Number of wrinkles: Number of wrinkles in the Tf images is half the total number of fully formed fringe groups on either side of the inked region.

Co-planarity: Co-planarity is the height difference between a hill and valley. Since a wrinkle is a combination of one hill and valley, the height difference between the lowest point and the highest point in a wrinkle is the co-planarity of that wrinkle. To measure the co-planarity order the number of fringes between the lowest point and the highest point are counted, which is named as co-planarity order. For counting the fringes the same technique used in the fringe order counting was used. For e.g. in the above figure the co-planarity order for the marked wrinkle is 5. Co-planarity was then calculated by using equation (7).

73 Wavelength: The wrinkles formed after drying the samples have wide variations in the width within a sample. In general the wrinkles formed at the edges seemed to have squeezed together tightly because of the edge effects. For this reason to avoid the variability of the edge effect the wavelength of the dried wrinkles was measured differently than the wet wrinkles. As shown in the above figure the wavelength was measured as the distance between two adjacent highest or lowest fringes in the center of the sample. In this way the wavelength of wrinkles on both sides of the inked region was measured and the average of these two readings is termed as Tf-Wavelength.

Angle of the edge wrinkles: The wrinkles at the edges are always oriented off the axis to the machine direction of the sample. To measure the angle of edge wrinkles, a line was drawn through the center of the wrinkle and then the angle of this line with respect to the MD of the sample was measured by using the measure tool of ImajeJ software. Like wise the angle of all four edge wrinkles was measured

Maximum flatness: Once the sample is dried, the wrinkles in the inked area subside. This was quantified by using the ‘maximum flatness’ parameter. Maximum flatness is the length of a single largest fringe across inked region. The fringe can be either black or white, considering the fact that each single fringe represents a contour of uniform elevation/depression. In the above figure, the black fringe that has the maximum width is measured for its length along the inked region. This length is the ‘Maximum flatness’

These parameters were quantified manually by using the measurement tools in the ImajeJ software, and the measurements are somewhat subjective. All the quantification was done based on the still images extracted from the video except the fringe order at T0. Because of the poor contrast between the fringes due to the wetness of the ink, the videos of these samples were used to count the fringes. The videos provided a better contrast because of more number of frames were viewed per second.

74 A summery of the parameters used for quantification of the fringe images is presented in Table 6.

Table 6. Summery of the Parameters used to characterize the wrinkles

Parameter Description Before drying T0-Wavelength This is Wrinkle length/T0-# wrinkles T0- width (inch) Approximated average width of the wet wrinkles

T0-Height (mm) Average Height of the wrinkles. Product of fringe order and the resolution of shadow Moiré setup (0.254mm) After Drying Tf-Wavelength Average distance between the peaks of two adjacent hills/valleys (inch) Tf- # Wrinkles Half the total number of fringe groups on either side of the inked region. Co-planarity Average height difference between each hill and valley. Product of (mm) Co-planarity order and the Shadow Moiré resolution.

Flatness (inch) Length of single longest fringe in the inked region.

Angle of edge Average angle with the MD axis of the fringes on four corners of fringes the sample

Tf- Height (mm) Average height of the wrinkles with reference to the inked region.

75 5.3.4 Effect of the Variables

5.3.4.1 Effect of Ink width

The ink dimension in the machine direction was varied to determine the effect on the wrinkle formation. All the samples were tested at constant MD and CD dimensions, constant MD tension and air dried for one hour. Three MD widths (0.6, 1.0, and 1,5 inches) of the inked region were tested.

Of all the measured parameters only the height of wrinkles showed a significant change with the change in the ink dimensions before drying. However this is based only on the difference between the 1” and 1.5” ink width samples, for these samples the wrinkle heights are observed to be 0.58mm and 0.83 mm. For the 0.6” ink width the wrinkle height was not measurable for reasons explained below. With the increase in ink width the wrinkle height has increased. The increased ink width means the span of the wet region is wider. Thus the buckling loads at the center of this region are lower allowing larger out of plane deformation, i.e. more wrinkle height. However the wrinkles before drying have shown difference in the wrinkle structure. The fringes in the samples with 0.6” ink width were unclear. This is because the fringes are too close to each other in that small inked region, thus the distance between each fringe is too low. This made it difficult to do any image analysis. On the other hand the wrinkles in the 1.5” width ink are very non uniform. The wrinkles within the inked area are distorted. Only in the 1” inked region the samples have shown reasonable wrinkle formation. Because of this reason the 1” ink width was selected as the optimum ink width for testing the wrinkles on the present setup.

76 0.6” 1.0” 1.5”

Figure 32. Comparison of wrinkle formation before drying at different ink widths. .

In case of the wrinkles after drying, the ink width appears to influence the number of wrinkles, co-planarity and wrinkle height. The number of wrinkles decreased with the increase in the ink width.

12 R2 = 0.8071 Number of wrinkles of Number 11

8 0.4 0.6 0.8 1 1.2 1.4 1.6 Ink width (inches)

Figure 33. Effect of change in ink width on the number of wrinkles after drying

78 1.400

1.200 R2 = 0.435

1.000

0.800

0.600 Height (mm) Height R2 = 0.6881

0.400 Wrinkle Height Coplanarity 0.200 Linear (Wrinkle Height) Linear (Coplanarity)

0.000 0.4 0.6 0.8 1 1.2 1.4 1.6 ink width (inch)

Figure 34. Effect of change in ink width on the Co-planarity and the wrinkle height in the Wrinkles after drying.

The increase in the ink width resulted in higher co-planarity and wrinkle height, thus higher out of plane deformation. Even though the change in the Co-planarity or the height of the wrinkles is not very significant, the trend in these parameters is apparent. The increase in these two parameters could be attributed to the same reasons explained above for the effect of ink width in the wet stage. In both the Co-planarity and the Wrinkle height the deviation in case of the 1” ink width is observed to be higher than the other two widths.

79 5.3.4.2 Effect of Sample dimensions

To test the effect of sample dimensions on the wrinkle formation before and after drying the ink, different MD and CD dimensions were tested. The length in each direction was tested while keeping the other direction at constant size.

5.3.4.2.1 CHANGE IN MD DIMENSIONS: The samples were tested at different MD lengths keeping the CD at 7.5 inch and MD tension at 52.5g/cm.

BEFORE DRYING The change in the MD length did not show any significant effect on the wavelength and the fringe height of the T0 wrinkles, however the width (this is the approximated wrinkle width, from now onwards referred only as width) of the wrinkles increased with increase in the sample size in machine direction.

80 1.48

1.46

1.44 R2 = 0.4989 1.42

1.4

1.38

1.36 Wrinkle width (inch)

1.34

1.32

1.3

1.28 23456789 MD length (inch)

Figure 35. Effect MD sample dimensions on the Approximated wrinkle width before drying.

A linear fit has given best R squared value for the relationship between the wrinkle width and the MD sample size. However from the above figure it is apparent that variation in the measured parameter is not significant between the two extremes of the MD length. This variation becomes more negligible considering the fact that the variation within a single sample size is as high as 55% of the total variation for all the measurements. This implies that the change in the width of wrinkles before drying is not a significant evaluation parameter.

AFTER DRYING The number of wrinkles after drying the samples for one hour showed an inverse relation with the MD length (Figure 36).

81 10

R2 = 0.6976 No. of wrinkles

4 23456789 MD length (inch)

Figure 36. Effect of MD sample dimensions on the number of wrinkles after drying.

The relationship was best described by an inverse power equation with R squared value of 0.7, showing that there is a strong dependency of number of wrinkles with increase in the sample MD size.

Unlike the wrinkles at T0, majority of the evaluated parameters of wrinkles after drying have shown a correlation with change in machine direction length. All these parameters also have shown a significant overall variation between the extremes of the test conditions. The parameters that have shown a strong correlation are number of wrinkles, Co-planarity, wrinkle height and the edge wrinkles’ angle while the other parameters like wavelength and maximum flatness have relatively lower correlation.

The effect of MD length on these parameters of the wrinkles in the dried samples is shown in Figures 38-40.

82 1.800

1.600

1.400

1.200

1.000 R2 = 0.7927

0.800 Coplanarity (mm) Coplanarity 0.600

0.400

Average Coplanarity 0.200 Linear (Average Coplanarity)

0.000 3456789 MD length (inch)

Figure 37. Effect of MD sample dimensions on the Co-planarity of the wrinkles after drying

0.9

0.8

0.7 R2 = 0.6346 0.6

0.5

0.4

Wrinkle Height (mm) 0.3

0.2 Wrinkle height 0.1 Linear (Wrinkle height)

0 3456789 MD sample size(inch)

Figure 38. Effect of MD sample dimensions on the wrinkle height after drying.

83 26

R2 = 0.461 18

angle 16

12 Angle of edge wrinkles 10 Power (Angle of edge wrinkles)

8 3456789 MD length(inch)

Figure 39. Effect of MD sample dimensions on the edge wrinkles' angle

From Figure 37, Figure 38 and Figure 39 it is evident that the co-planarity, wrinkle height and the wrinkle angle are affected by the MD sample dimensions. This signifies that the change in MD length of sample has a reasonable effect on the measured parameters. This variability shown by MD dimension can be used in the evaluation of the “commercial Ink-jet papers”. To evaluate the ink-jet papers two extreme MD lengths can be used. However for the low MD length 5” MD length is a better option to increase the available measuring area. In the 4” samples, on either side of the inked region, there is only 1.5” non-inked region, which restricts the free formation of the wrinkles.

The deviation of the wrinkles at the edges from the sample machine direction may be caused by two different factors. Since there is no restraining factor at the center of the

84 samples, while drying the inked area shrinks more than the un-inked area. And also for the same reason the CD width of the sample at the center less than that of the fixed ends on the bars. These two factors cause the wrinkles to spread away from the machine direction. It was also observed that the closer the wrinkle to the center the lower the angle. Increase in the sample length in the machine direction reduced this angle. Thus, because the difference in CD width between the inked area and the fixed edges is same irrespective of the sample length, thus the increase in MD length reduces the steepness of this variation in the CD width.

The number of wrinkles is reducing with the increase in MD length while the coplanarity and the wrinkle height is increasing. This implies that the z-deformation of the wrinkles is higher with increased sample length.

5.3.4.2.2 CHANGE IN CD DIMENSIONS:

For testing the effect of CD dimensions on the wrinkle properties, different CD dimensions were tested at 8.5” MD sample length and keeping the MD tension constant at 52.5g/cm +/- 1.5%. The MD length of 8.5” is chosen because of the fact that the edge effect (edge wrinkle angles) on the wrinkles is minimal at this size.

BEFORE DRYING: None of the wrinkle parameters have shown any noticeable dependency on the change in CD dimensions before drying.

AFTER DRYING: As in the un-dried wrinkles, the CD dimension has no effect on any of the wrinkle parameters after drying. The only parameter that is affected by the CD dimension

85 change is the ‘maximum flatness’ length in the inked region of the dried sample Figure 40. This relationship has a strong correlation with an R squared value of 0.8 for a power curve fit.

Maimum flatness in the inked area Vs CD width after Drying

1.8

2 1.6 R = 0.7971

1.4

1.2

0.8

Maximum flat inked length 0.6

0.4 Maximum Flatness 0.2 Power (Maximum Flatness)

0 3456789 CD width

Figure 40. Effect of CD sample dimensions on the maximum flatness of the inked area after drying

The increase in the flatness of the inked area with the CD length suggests for a higher CD sample size to get the wrinkle formation behavior as explained in the conceptual method to evaluate wrinkles (section 4.1). Based on these results the sample dimensions for the rest of the tests is decided as 8.5” and 7.5” respectively in MD and CD. The 7.5” in CD is chosen to avoid the use of a square sample and also for the ease of fixing the samples on the sample holder and the lower CD widths are avoided so that the edge effects can not be seen in the 6” wide grating area.

86 5.3.4.3 Effect of MD Tension on the paper

The MD tension was used to simulate web tension that exists in a continuous printing operation. For this test the sample size of 8.5”*7.5” in MD*CD is chosen based on the above results (see Sections 5.3.4.2.1 & 5.3.4.2.2). Five different tensions between the extreme low and extreme high were chosen. The low tension of 12.1g/cm can be considered as the test with no tension. At this point the tension was just adequate to compensate the friction of the sliding bar. A maximum tension of 157.5g/cm was also tested. This is considered as maximum, because above this level the inked area of sample could not withstand the tension.

BEFORE DRYING: Both the wrinkle height and wavelength decreased with increase in tension (Figure 41,Figure 42), indicating higher number of wrinkles per unit length at higher tension. However the increased tension resulted in reduced height of the wrinkles.

87 0.900

0.800

0.700

0.600

0.500

R2 = 0.4839 0.400

Wrinkle Height (mm) Height Wrinkle 0.300

0.200 wrinkle height Power (wrinkle height) 0.100

0.000 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 MD tension (g/cm)

Figure 41. Effect of MD tension on the wrinkle height before drying the sample

1.400

1.200

1.000

0.800

0.600 R2 = 0.8339 Wavelength (inch)

0.400

0.200 wavelength(inch) Power (wavelength(inch)) 0.000 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 Load in MD (g/cm)

Figure 42. Effect of MD tension on the Wrinkle Wavelength before drying the sample

The tension is applied in the plane of the sample and it restrains the out of plane deformation. Hence the higher the tension the more it restrains the z-deformation, thus lowers height of the wrinkles but the number of wrinkles increases.

AFTER DRYING:

Similar to the wrinkles in the un-dried samples, the number of wrinkles increased with the MD tension after drying. However unlike in the before drying scenario, MD tension has not effected neither the height of the wrinkles nor the co-planarity. This indicates that the MD tension only increases the severity of the wrinkles in terms of the total number of the wrinkles.

3.000

2.500

2.000

R2 = 0.4977

1.500 Wavelength (inch) Wavelength 1.000

0.500 Wrinkle Wavelength Power (Wrinkle Wavelength)

0.000 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 MD-load(g/cm)

Figure 43. Effect of MD tension on the wrinkle wavelength after drying

89 10

8 R2 = 0.8449

6 No. of wrinkles

4 no. of wrinkles Linear (no. of wrinkles)

3 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 MD tension (g/cm)

Figure 44. Effect of MD tension on the Number of wrinkles after drying

The flatness in the inked region has increased with tension and the edge wrinkles’ angle decreased. The end result of the increased tension is flatter inked region and increased wrinkles outside the inked region after the sample is dried.

90 3.5

2.5

R2 = 0.7339 2

1.5

Max. length of flat area (inch) area flat of length Max. 1

0.5 Maximum flatness Linear (Maximum flatness)

0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 MD-Tension(g/cm)

Figure 45. Effect of MD tension on the Flatness of inked region after drying

15 R2 = 0.593 Angle

5 Angle of the edge wrinkles Linear (Angle of the edge wrinkles)

0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 MD-tension(g/cm)

Figure 46. Effect of MD Tension on the Edge wrinkle angles after drying

91 5.3.4.4 Effect of Temperature:

The used heater to dry the samples simulated the ink-jet printing process conditions and reduced the testing time. The heater used for this is set at 45°C temperature (temperature of the air at the sample surface). All the samples were dried at same temperature for 30 minutes.

The wrinkles formed at Tf time were not uniform across the CD of the sample for all the samples that were dried with heater turned on. Most of the tests with the heating have shown a region in the center of the sample with no wrinkles (Figure 47). Only the magnitude of this region changed with change in sample dimensions. This region appeared to have a large scale out of plane deformation compared to the rest of the sample. The same was noticed by visual observation of these samples.

Center region

Drying at 45°C(7”*7”) for 30min Air dried (7”*7.5”-CD) for 1Hr

Figure 47. Effect of heating on the wrinkle formation in the post dried samples.

As shown above, unlike the air dried samples, the samples dried with the heater switched on developed non-uniform distribution in the wrinkles after drying. This non-uniformity was reduced with increase in MD length and it increased with increase in CD length of the sample. This was due to the fact that the heater provided was only a 6”*5” are in the CD*MD of the sample. Apart from the smaller heater size, the temperature above the heater varied by about 5°C from the center to the edges due to the cooler air interface at the edges of the heater. This would have caused non-uniform drying along the inked region resulting in the non-uniform distribution of the wrinkles. Because of this reason it was decided to conduct the tests without using the heater, rather the samples were allowed for air drying for one hour.

Out of all the wrinkle analyzer variables the MD-tension showed maximum impact on the measured wrinkle parameters. Table 7 shows the relative effect of each of these variables on the wrinkle characteristics. CD sample dimension has the least impact of all the variables. The change in MD dimension and the MD tension has affected more number of parameters while the effect of MD tension is strong on all these parameters compared to any other variable.

93 Table 7. Effect of Wrinkle analyzer variable on the wrinkle characteristics

Measured Parameter Variables’ Effect

MD sample CD Sample MD Tension Ink size size Width T0-Wavelength 8 8 999 8 T0-Wrinkle Width 8 8 8 8 T0-Wrinkle height 8 8 9 8 Tf- Number of 9 8 999 999 wrinkles Tf-Wavelength 8 8 99 8 Co-planarity 999 8 8 9 Maximum Flatness 8 999 999 8 Edge wrinkle angles 9 8 99 8

Tf-Wrinkle height 9 8 8 99

9 : Little effect

99 : Moderate effect

999 : Large effect

8 : No effect

6 EVALUATION OF INK-JET PAPERS

A set of 12 different ink-jet papers used for commercial ink-jet printers were provided by ‘Kodak Versamark’. These samples were used for testing the validity of the test method established so far and to investigate the usefulness of the wrinkle characterization parameters. These 12 samples represent various grades of ink-jet paper available in the commercial ink-jet paper market. They included both uncoated and surface treated papers. The fundamental characteristics of these papers are listed in Table 8. The characterization of these papers was done by the ‘contract research’ section of the Paper Science and Engineering Department at Miami University.

There are two different wrinkle rankings given in the Table below. Both the rankings were given for a 5”*5” printed sample. Two sets of these samples were printed under similar conditions with the pendulum driven ink application system (Section 4.2.1.1). One set of these samples were ranked by Kodak Versamark and the other set was ranked at Miami University. The Kodak ranking was based on the results of their coordinate measurement system. The laser based coordinate measurement system (ImageXpert) measures the out-of-plane deformation with respect to the X,Y coordinates of a sample and gives a consolidated wrinkle quantification number. The lower the number the less severe the wrinkles are. The Kodak wrinkle ranking was based on these values. Miami Ranking was based on the visual observation of the wrinkles and ranking them qualitatively. In both the cases the lower the rank of a sample the less severe the wrinkles are.

95 Table 8. Basic properties of the 12 different commercial Ink-jet papers used for testing on the wrinkler

ImageXpert wrinkle Paper/Substrates HST ASH quantification Caliper value Sample ID Grammage Grammage Paper Type Paper Type Wrinkle Ranking Kodak Miami (g/m2) mils sec % @ 525 C Ranking Ranking K1 Uncoated/US Glatfelter Pixelle® Engr'g Bond SX 83.2 3.91 2.2 22.9 9 9 1235.5 K2 Uncoated/US IP Data Speed™ Form Bond 77.4 3.85 0.7 14.6 6 8 1157 K3 Uncoated/Europe M-Real Galerie One™ Silk 82.3 3.18 11.4 30.6 7 11 1188.5 K4 Treated/US IP ImageGrip™ 75.6 3.73 2.5 15.1 10 10 1305 K5 Treated/US IP ImageGrip™ 91.1 4.56 5.7 14.5 11 6 1305.5 K6 Uncoated/US Willamette CI-2000 90.4 3.84 8.1 17.5 5 5 1154.5 K7 Treated/Europe IP Jetset Colour™ 88.8 4.74 0.5 15.1 4 7 1152.5 Kishu K8 Treated/Japan Paper IJ <70> DX 82.0 4.74 460.0 8.4 1 1 734.5 K9 Treated/Japan Oji OKH-J [90] 82.6 3.78 0.2 11.9 3 3 1075 K10 Treated/Europe Ziegler Z-Plot Marke™ 650/90 Color 90.5 3.99 1.3 16.5 8 4 1215 K11 Uncoated/Austral Australian Glopaque Laser 84.2 3.95 59.4 18.3 12 1357.5 K12 Mitsubishi IJ FORM 81.7 3.76 0.3 11.4 2 2 1011.5

These 12 different samples were tested at three different conditions based on the results discussed in Section 5.3.4. The different test conditions used for testing these 12 samples are listed in Table 9. Each sample was tested twice under each of these conditions.

Table 9. Test conditions used for evaluation of commercial Ink-jet papers

Test MD(inch) CD(inch) MD tension Ink-width(inch) Drying (°C)

C1 8.5 7.5 52.5g/cm 1” 23

C2 5.0 7.5 52.5g/cm 1” 23

C3 8.5 7.5 24.1g/cm 1” 23

The test of the “effect of variables” only showed which wrinkle analyzer variable affects the wrinkle characterization parameter. To evaluate the usefulness of the wrinkle analyzer parameters the observations made in testing the variables were used. As seen in Section 5.3.4, MD tension and MD sample size were the most critical test variables that effect the wrinkle formation. For this reason the extremes of these parameters were chosen as the test conditions for evaluating the ink-jet papers.

The images (Appendix E) of the before and after drying of all these samples were analyzed as explained in the Section 5.3.3. The fringe parameters collected for before drying (T0) the samples were: wrinkle height, wrinkle wavelength, and the approximated width of the wrinkles. For the post-drying (Tf) wrinkles: Wavelength,

97 Co-planarity, wrinkle height, maximum flatness and the standard deviation of the coplanarity were used to characterize the wrinkles. The standard deviation of the coplanarity is the variation co-planarity within a sample; it was used to quantify the variation of the deformation within a sample.

After consolidating each of these fringe parameters before and after drying the samples, each of them was filtered for their statistical validity in explaining the wrinkles. For this two different techniques were used. First to test the range variability of a parameter between the highest and lowest ranked samples, the % variation is calculated (It is the range divided by the average of the total observations). All the parameters that has at least 70% overall variation were considered to show a reasonable change in the value to be useful in explaining the variation between the paper samples.

98 Table 10. Data table showing how the overall variation of a parameter was determined. The data included in table is for test condition C3

T0 Tf Sample T0-WL Width T0_height # Wrinkles Tf-WL Co-planarity Tf-Height Flatness SD- Coplanarity K1 0.757 1.232 0.483 6.5 1.215 1.435 0.857 1.485 1.226 K2 0.920 1.218 0.540 6.0 2.277 1.765 0.667 1.999 0.550 K3 0.908 1.259 0.677 6.0 2.180 1.664 0.699 1.902 0.497 K4 0.842 1.246 0.651 4.0 2.873 2.392 1.143 1.735 0.585 K5 0.993 1.259 0.607 5.0 2.610 2.413 0.889 0.847 0.968 K6 0.810 1.191 0.671 5.5 2.499 1.510 0.730 2.027 0.882 K7 0.979 1.246 0.861 5.5 2.589 1.552 0.603 1.138 0.651 K8 1.502 1.287 0.490 5.5 2.152 0.648 0.286 1.208 0.725 K9 1.153 1.259 0.841 5.0 2.158 1.468 0.730 1.180 1.277 K10 1.073 1.259 0.847 5.0 2.535 1.705 0.635 1.210 0.699 K11 0.917 1.273 0.621 5.0 2.375 2.016 0.931 1.774 2.008 K12 0.981 1.218 0.798 6.0 1.837 1.168 0.540 1.980 1.243 %Variation 75.5 7.7 56.1 46.2 72.9 107.3 118.1 76.6 160.3

As shown in Table 10 the variation was calculated for each parameter and then those parameters that showed less than 70% variation were eliminated from further data processing. The data for other test conditions (C1 & C2) is attached in Appendix F.

Sometimes two or three parameters are correlated. This is tested by calculating the multicolinearity between these parameters. If the multicolinearity is high between any two variables, one of them is omitted from further data analysis based on the subjectivity of its measurement. For example if there was high multicolinearity between Co-planarity and the Tf-Wrinkle height Table 11, the Tf wrinkle height is omitted since measurement of Co-planarity is relatively more accurate as explained in Section 5.3.3.

Table 11. Multicolinearity based filtering of the independent variables. Independent R-square matrix for the variables measured under test condition C3.

Tests for Multicolinearity between Independent Variables Independent R-Square Matrix

T0-Wavelength 100% 40% 0% 2% 0% 33% 50% 25% 1% T0-width 40% 100% 0% 12% 2% 0% 1% 35% 2% T0-height 0% 0% 100% 7% 11% 0% 1% 3% 0% # wrinkles 2% 12% 7% 100% 63% 31% 23% 8% 0% Tf-Wavelength 0% 2% 11% 63% 100% 30% 6% 2% 10% Coplanarity 33% 0% 0% 31% 30% 100% 75% 0% 0% Tf-Height 50% 1% 1% 23% 6% 75% 100% 1% 6% Flatness 25% 35% 3% 8% 2% 0% 1% 100% 0% SD-Coplanarity 1% 2% 0% 0% 10% 0% 6% 0% 100%

Flatness T0-Width Tf-Height T0-Height # Wrinkles Coplanarity Tf-Wavelength T0-Wavelength SD-Coplanarity

6.1 Rank based analysis

After each of these independent variables (the wrinkle characterization parameters explained above) was filtered, the remaining variables’ values were converted to ranks. For a given variable each paper’s corresponding value was converted into a relative rank for that variable.

Each variable was ranked 1-12, thus ranking each of the samples based on that variable. A variable value is converted into corresponding rank based on its correlation

100 coefficient with the wrinkle rank. If there was a positive correlation between the variable and the wrinkle rank, that variable’s ranking increases with increase in the variable value. On the other hand variable’s ranking decreases with increasing variable value for negative correlation.

The dependency of each of the wrinkle rankings on each of these independent variables’ ranking was then calculated by calculating the R-squared value of each of the variables. The independent variables were screened one more time based on this R-square values. Any variable with less than 0.1 R-square value was omitted from the final data analysis. After this the remaining independent variables were used to calculate their interaction effect on the wrinkle rank. All the first degree interaction values of the remaining variables are calculated and their corresponding ranks were calculated.

101 Table 12. Sample data to demonstrate the calculation of the rank based on the sign of the correlation coefficient.

Miami T0-WL Tf-WL Sample Ranking (inch) (Inch) K1 9 0.757 1.215 K2 8 0.920 2.113 K3 11 0.908 2.180 K4 10 0.842 2.873 K5 6 0.993 2.610 K6 5 0.810 2.499 K7 7 0.979 2.589 K8 1 1.502 2.152 K9 3 1.153 2.158 K10 4 1.073 2.535 K12 2 0.981 1.837 Correlation -0.717 0.047 Miami T0-WL Tf-WL Sample Ranking (Rank) (Rank) K1 9 11 1 K2 8 7 3 K3 11 8 6 K4 10 9 11 K5 6 4 10 K6 5 10 7 K7 7 6 9 K8 1 1 4 K9 3 2 5 K10 4 3 8 K12 2 5 2 R-square 0.556 0.056

As shown in Table 12, the correlation coefficient for the T0-WL is negative, thus the corresponding ranking for this parameter decreases with increase in the value of the parameter. The ranking is done in the proportionate manner for the Tf-WL. After the ranking the R-square value for each of these parameters with respect to the Miami

102 ranking is calculated. Since this value for Tf-WL is negligible, this parameter is omitted in the next stages of analysis.

After the initial validity tests and the conversion of each independent variable and their interacting variable data into corresponding rank data, a series of multiple regression routines were carried out for each possible combination of these independent variables and their interacting variables. From the regression analysis the best fit equations are calculated. The R-squared and adjusted R-squared values of the multiple regression statistics are used to choose the best combination of the independent variables and their interaction variables that best explain the variability in each of the wrinkle ranking. The consolidated results of the regression analysis are shown in the Table 13. Table 14 shows the corresponding regression variables and their equations.

Table 13. Consolidated rank based regression analysis results for each test condition

Test Adjusted Standard confidence Condition Rank R-Square R-Square Error level % Kodak No significant correlation C1 Miami 0.72 0.60 2.10 98.60 Kodak 0.66 0.43 2.50 92.20 C2 Miami 0.68 0.52 2.11 95.80 Kodak 0.88 0.83 1.48 99.90 C3 Miami 0.81 0.73 1.71 99.70

103 Table 14. Regression variables that explain the variation in the ranking at different test conditions

Test Condition - C1 Kodak Miami Intercept 0.298 Co-planarity 1.054 No significant relationship Flatness 0.325 Co-planarity* SD-Co-planarity -0.429 Test Condition – C2 Kodak Miami Intercept 0.253 Intercept -0.637 Co-planarity 1.551 T0-Wavelength 0.867 SD-Co-planarity -0.985 SD-Co-planarity 1.003 Co-planarity/T0-Wavelength -0.918 Co-planarity/T0-Wavelength -0.754 SD-Co-planarity 1.310 Test Condition – C3 Kodak Miami Intercept -0.625 Intercept 2.298 T0-Wavelength 0.658 T0-Wavelength 0.614 Tf-Wavelength/T0-Wavelength -0.888 Co-planarity 0.302 Tf-Wavelength*Co-planarity 1.326 SD-Co-planarity 0.300

From Table 13 & Table 14, it can be seen that the Co-planarity is the single most influencing parameter. This parameter being the least subjective measurement in the entire set of fringe data collected assures the least error. From the Table 13 it is also clear that the variables under test condition C3 explain both the Kodak and Miami wrinkle rankings with the best R-squared value compared to the other two test conditions.

The test condition C3 is the test with minimum MD-tension on the samples while testing. Since the Kodak measurements and the Miami visual ranking were performed under no tension, the results of the wrinkle analyzer seem reasonable.

104 At this test conditions 81% of the variation in Miami Ranking is explained by the three primary independent variables. The use of interacting variables has not improved the R-squared value in this case. Figure 48 compares the actual ranking with the predicted ranking. The predicted ranking is calculated by using the corresponding regression coefficients given in Table 14; these calculated values are then ranked to get the predicted ranking.

1 9 1

5 Predicted value Ranking

Rank of the predicted value

1 1357911 Actual Ranking

Figure 48. Predicted rank versus the actual Miami rank for different ink-jet papers under test condition C3

From the above figure it is clear that the predicted ranking is significantly comparable to the actual Miami ranking. Figure 49 & Figure 50 also verifies the validity of the fitted model for predicting the Miami-rank under test conditions C3.

105 3

0 Residual -1

-2

-3 (response is Miami-Ranking) 0 2 4 6 8 10 Fit t e d V a lue

Figure 49. Residual Versus fitted Value for the Miami ranking under test condition C3

70 60 50 40 Percent 30 20

(response is Miami-Ranking) 1 -4 -3 -2 -1 0 1 2 3 4 Residual

Figure 50. Normal Probability plot of the Residuals for Miami-Ranking regression model under test condition C3

106 Considering the small sample size of 12, the residuals show a reasonably good normal distribution.

The best model of all however, was for predicting the Kodak Ranking at C3 test conditions. This model explains 83% of the variability in the Kodak wrinkle ranking by using the fringe data before and after drying the samples at 99.9% confidence level. Figure 51 shows the actual Kodak ranking versus the predicted value and the predicted value rank. In the lower half of the ranking the predicted value ranking coincides with the actual Kodak ranking while the upper half of the rankings are deviating slightly from the actual ranking.

1 1 9

Predicted value Rank value Predicted 5

Predicted Value Ranking

1 1357911 Actual Kodak Ranking

Figure 51. Predicted versus Actual Kodak Ranking under test condition C3

107 The established model can be validated from Figure 52 & Figure 53. The random distribution of residuals on both positive and negative sides and the linearity of the probability distribution of the residuals prove that the model is adequate.

0 Residual -1

-2

-3 (response is Kodak Ranking) 0 2 4 6 8 10 12 Fit t e d V a lue

Figure 52. Residuals versus the fitted values of Kodak Ranking under test condition C3

108 99

70 60 50 40 Percent 30 20

(response is Kodak Ranking) 1 -3 -2 -1 0 1 2 3 Residual

Figure 53. Normal Probability plot of the residuals for Kodak ranking under Test condition C3

These two models suggest the possible use of the wrinkle analyzer parameters to evaluate the ink-jet papers. The applicability of these two models for the intended evaluation however, is limited by the relevance of the wrinkle ranking of the papers to the performance on the commercial ink-jet printing operations.

Co-planarity and the T0-Wavelength were established as the parameters that are useful in explaining both the Kodak and Miami ranking. In addition to these two parameters, the Tf-Wavelength is used for predicting Kodak ranking while Standard deviation of Co-planarity is used for predicting Miami ranking.

109 6.2 Comparison with Coordinate measurement results

In the previous section the wrinkle quantification parameters are compared with the ranking given based on the ImageXpert results and the visual ranking of the printed samples. This established that the lower tension test condition gives a better quantification of ranking system. In this section the wrinkle quantification numbers obtained from the ImageXpert are compared with the actual values of the different parameters derived from the image analysis of the wrinkler results. For this comparison the same preliminary data filtering techniques used for the rank based analysis are used.

With this technique the best fit multiple regressions for each test condition are better than the rank based analysis (Table 15).

Table 15. Consolidated Regression Analysis results for each test condition - regression carried out for comparing ImageXpert wrinkle results.

Test Adjusted Standard confidence Condition R-Square R-Square Error level % C1 0.54 0.37 131.59 93.0 C2 0.84 0.77 84.07 99.8 C3 0.91 0.89 55.55 99.9

110 Table 16. Best fit regression coefficients for each test condition - Regression carried out for comparing

ImageXpert wrinkle results.

Test condition C1 Test condition C2 Test condition C3 Intercept 329.4 Intercept 855.5 Intercept 1,134.9 Coplanarity 589.6 T0-Wavelength -280.1 T0-Wavelength -347.4

Flatness 156.4 Coplanarity 518.3 Coplanarity 222.2 Coplanarity*SD-Coplanarity -189.3 SD-Coplanarity 180.0

Under the same test conditions, the variation ImageXpert results were explained at higher percentage than the rank based analysis. The highest adjusted R-square value for the rank based analysis was 0.83 against the 0.89 of the actual value based regression.

Table 16 shows that just like in the rank based analysis, coplanarity is the single most important parameter in explaining the variation in the dependent variable. From this Table it can also be noticed that, the best fit models for both C2 and C3 conditions where significant regression results are achieved, the models did not include any interacting parameters.

However, in both the types of analysis, the wrinkler parameters under test condition C3 proved to have better correlation with the wrinkle quantification.

The best regression model for the actual value analysis explains 89% of the variation in the imageXpert results with the help of the independent variables obtained from the image analysis of wrinkle analyzer. The significance of this model holds at 99.9%.

111 Figure 54 shows that the predicted values are quite close to the actual ImageXpert values.

1500

1400

1300

1200

1100

1000

Predicted value 900 1

800

700

600

500 500 600 700 800 900 1000 1100 1200 1300 1400 1500 ImageXpert result

Figure 54. Predicted versus Actual ImageXpert wrinkle number under test condition C3.

Figure 55 shows the randomly scattered residuals on either side of 0 with out any specific trend. This validates the regression model developed to predict ImageXpert values. The regression model can be further validated by using the normal probability plot of the residuals (Figure 56).

112 100

0 Residual

-50

(response is ImageXpert value) 700 800 900 1000 1100 1200 1300 1400 Fit t e d V a lue

Figure 55. Residuals versus the Fitted values for ImageXpert value based regression model under test condition C3.

70 60 50 40 Percent 30 20

(response is ImageXpert value) 1 -150 -100 -50 0 50 100 Residual

Figure 56. Normal Probability plot of the Residuals for the regression model to predict ImageXpert values under test Condition C3

113 From the regression model results it is obvious that the wrinkle analyzer parameters can explain the wrinkle tendency of a paper either in terms of a relative ranking or to quantify the sample paper to any absolute value.

6.3 General Observations

In all the tests conducted, the Co-planarity of the dried samples was observed to be higher at the edges and it is decreasing as the wrinkles move toward the center. This could be due to the fact that while the paper is under tension during drying, the edges are not restrained in the CD direction. Thus enabling the edges of the inked area to shrink much more freely compared to the center. This free shrinkage of the inked region and the lack of any restraint on the overall sample at the edges, allows the sample to deform higher than the corresponding center region. Figure 57 shows a typical example of this phenomenon. From the figure it can also be noticed that even in the inked region, the frequency of the fringes increases towards the edges.

114

Figure 57. Fringe image after drying. The fringe pattern shows higher coplanarity at the edges compared to the center along the CD of the sample

Another observation that is of importance to the whole test procedure and the rate of change of the wrinkles while it is wet is, the rate of change in the shape and number of wrinkles formed in the initial few seconds is very drastic. This rate of change was observed only because of the Sample K8 in the ‘Kishu Paper’ because of its very low absorbency levels (HST of 460 seconds Table 8). The low absorbency of the sample results in a slower penetration of ink into the sheet thus, slowing the process of wrinkle formation. While testing this sample it was noticed that immediately after the ink application the number of wrinkles formed was less and with increase in the

115 amount of absorption with time the number of wrinkles increased. This observation provided information about how critical is the initial time gap between the ink application and the start of the measurement. Figure 58 shows how the wrinkles split within to form higher number of wrinkles.

116 2 2 2

1 1 1

9 Sec 13 Sec 18 Sec

Figure 58. Change in number of wrinkles with time. Time stamp on each frame shows the elapsed time after ink application .

The three frames shown in the above Figure are of the sample test on sample K8. It can be noticed that the wrinkle numbers 1 & 2 split within to form individual wrinkles. The same phenomenon is observed in all the three test conditions. This phenomenon suggests the importance of initial few seconds after the ink application. However all the samples tested for the wrinkle analyzer variables and some of the commercial ink- jet papers tested did not have the low absorption rates as sample K8. This explains the reason for the T0-Wavelength not showing any appreciable change with the change in the wrinkle analyzer variables.

118 7 CONCLUSIONS AND RECOMMENDATIONS

The developed apparatus can be utilized to study the propensity of papers to form wrinkles during ink application and subsequent drying of the ink. The ink application station appears to function properly and allows for repeatable and uniform application of the ink to the sheet. The sample holder allows for a range of sample sizes and the application of tension in one direction. The wrinkle characterization station allows for a full-field and real-time capture of Shadow Moiré fringes that correspond to contours of constant topography. Using these fringe pattern images, it was demonstrated that the severity of wrinkles could be characterized.

The analysis of the data from the parametric study showed that changes in the MD- tension had the most significant impact on the wrinkle parameters. Changing the sample length was the second most important parameter in altering the wrinkle parameters. Change in CD sample length had very little effect on the overall wrinkle characteristics. It is concluded the equipment possesses enough versatility so that the set-up can be adapted to obtain results that will correlate to wrinkling induced in a given printing method.

The high correlation of measurements made on a group of ink-jet printing papers to both a subjective ranking and an independent laser-based characterization of topography demonstrates that the developed apparatus and characterization techniques can be used to evaluate wrinkle tendency of papers. Several measurable parameters were obtained from the fringe images, including T0-Wavelength, Tf-Wavelength, Co- planarity, Maximum flatness and Standard deviation of Co-planarity. Co-planarity was the only parameter that had a significant impact in every regression model developed as part of this thesis. Co-planarity was also the most robust measurement of

119 all, and the measurement error in the final regression model was minimal. These parameters then can be used to characterize the wrinkle propensity of various papers. A few of the measured parameters such as T0-width, T0-height, and number of wrinkles after drying did not show any significant variation between samples.

The current evaluation method compares the results of samples printed using a method very similar to the one implemented in the wrinkle analyzer. For establishing a relationship with the end-use performance of the ink-jet papers in terms of wrinkles, it is suggested to carry out a similar analysis in comparison with results from a controlled commercial ink-jet printing trial. Since the commercial printing is done under tension, the lab evaluation should be completed under multiple tensions. If a correlation can be established with a particular set-up of the apparatus, many papers can then be evaluated with the newly developed analyzer.

The measurements in the wet state were confounded by the time needed to move the sample from the ink-station to the evaluation station. Automation of this movement would reduce the variability of the time interval between the ink application and capturing the wet wrinkle images. The time interval can be reduced by moving the shadow moiré station closer to the ink application system. This also reduces the overall footprint of the wrinkle analyzer as the tailing light stand moves closer to the base.

The fringe sharpness during the wet state improves to a reasonable level by using a higher resolution camera. Fringe sharpness can be further improved by using a higher intensity light source. The whole system could be enclosed in a dark chamber to reduce the external light falling on the grating, thus helps in improving the fringe intensity.

It is suggested that the holder for the Ronchi-ruled plate be considered for re-design. Although it allows for ease of use, it experiences vibrations, which could interfere

120 with the fringe patterns. Although, this was not observed for the present measurements, a redesign would make the equipment more robust.

The sample holder system currently used induces constant MD elongation across the width of the sample. An alternative clamping method would be to allow the sample to pivot to account for any differences in tension across the width of the sample. The effect of this change on wrinkle formation would need to be analyzed.

The developed wrinkle analyzer could be retrofitted with a commercial automated fringe enhancement and fringe analysis techniques to enhance the surface topography information of the papers. However, the presented analysis techniques provide adequate information to quantify the wrinkles formed on ink-jet papers.

121 8 REFERENCES

1. K.H. Paik, W.S.Nam, "Cockle depending on drying conditions and local basis weight distribution". Journal of Pulp and Paper Science, 27(5) (2001).

2. Coffin, D.W. "A Buckling analysis corresponding to the Fluting of Lightweight Caoted webs" in Proceedings of the 2003 International Paper Physics conference, Victoria BC, Canada (2003).

3. HABEGER, C.C., "Tension Wrinkling and the Fluting of Light-Weight Coated Papers in Web-Offset Printing". Journal of Pulp and Paper Science, 19(5), 214-218 (1993).

4. FUJIWARA, S., T. FUKUI, and H. T. "Factors of the Fluting of Coated Paper in Web-offset Printing". in Pan-Pacific and International Printing and Graphic Arts Conference (1998).

5. Adamic, R. and G.E. Radke, "Low water content inks for minimizing wet cockle in thermal ink-jet inks", U.S.P. Office, Editor, Hewlett-Packard Corporation: USA (1995).

6. Chen, et al., "Wet colorant hard copy apparatus media handling to reduce cockle", U.S.P. office, Editor, Hewlett-Packard (Ft. Collins, CO): USA (2000).

7. Moffat, et al., "Formulation for control of paper cockle in thermal ink-jet printing", U.S.P. office, Editor: USA (1993).

8. Wotton, et al., "Method and apparatus for ink-jet print zone drying", U.S.P. office, Editor, Hewlett-Packard Company (Palo Alto, CA): USA (2002).

122 9. Retulainen, E., et al. "Nonimpact printability of condensing belt dried and conventional fine paper". in Progress in Paper Physics Seminar (2002).

10. Rahman, L., "Factors affecting the performance of inkjet papers", in TAPPI Spring Technical Conference and Exhibit (2003).

11. Le, H.P., "Progress and trends in ink-jet printing technology". Journal Of Imaging Science And Technology, 42(1), 49-62 (1998).

12. Hawkes, P.W., "Ink-Jet Printing", in Advances in Electronics and Electron Physics, P.W. Hawkes and B. Kazan, Editors, Academic Press, Inc.: London (1985).

13. Arthur S. Diamond, D.S.W., "Hand book of Imaging Materials", 2nd Ed. (2002).

14. Niskanen, K., "Paper Physics". Papermaking Science and Technology, ed. J. Gullichsen and H. Paulapuro, 16, Helsinki: Fapet Oy. 324 (1998).

15. Coffin, D.W., "Literature review of Cockle and a proposed mechanism".

16. Stout, K.J. and L.Blunt, "Development of Surface Characterization", in Three-Dimensional Surface Topography, Jessica Kingsley Publishers Ltd.: London (1994).

17. Hojjatie, B. and D. Coffin. "Measurement and Analysis of Sheet Topography Using Shadow Moiré System". in Progress in Paper Physics Seminar, Finger Lakes / Syracuse, New York, (2002).

18. D’Acquisto, L., L. Fratini, and A.M. Siddiolo, "A modified Moiré technique for three-dimensional surface topography". Measurement Science and Technology, 13, 613-622 (2002).

123 19. Degrieck, J., W.V. Paepegem, and P. Boone "Application of digital phase- shift shadow Moiré to micro deformation measurements of curved surfaces". Optics and Lasers in Engineering, 29-40 (2001).

20. Bartl, J., R. Fíra, and M. Hain. "Inspection of Surface by the Moiré Method". Measurement Science Review, Measurement Science Review, 1, 29-32 (2001).

21. Martínez-Antón, J.C., H. Canabal, and E.B. J. A. Quiroga, "Enhancement of surface inspection by Moiré interferometry using flexible reference gratings". OPTICS EXPRESS, 8(12), 649-654 (2001).

22. Cordero, R.R., "Uncertainty analysis of displacements measured by phase- shifting Moiré interferometry", Optics Communications, 237(1-3),25-36 (2004).

23. Chang, C.-W., D.-C. Su, and J.-T. Chang, "Moiré Fringes by Two Spiral Gratings and Its Applications on Collimation Tests". Chinese Journal of Physics, 33(4), 439-449 (1995).

24. Chiang, C., "Moiré Patterns from the surface and their application in measuring Topography". British Journal of Applied Physics, 2(2), 287-292 (1969).

25. Heredia-Ortiz, M. and E.A. Patterson, "On the Industrial Applications of Moiré´ and Fringe Projection Techniques". Strain, 95-100 (2003).

26. Dirckx, J.J.J. and W.F. Decraemer, "Phase shift Moiré apparatus for automatic 3D surface measurement ". Review of Scientific Instruments, 60(12),3698-3701 (1989).

27. Moore, D.T. and B.E. Truax, "Phase-locked Moiré fringe analysis for automated contouring of diffuse surfaces". Applied Optics, 18(1), 91-96 (1979).

124 28. Andrews, D.R., "Shadow Moiré Contouring of impact craters". Optical Engineering, 21(4), 650-654 (1982).

29. Roger, R.E. and J. Buitrago, "On the Interpretation of Shadow-Moiré Fringes", Experimental Mechanics (1978).

30. Patroski, K., "The Moiré Fringe Technique", Amsterdam: Elsevier Science Publishers, (1993).

31. Quan, C., Y. Fu, and C.J. Tay, "Determination of surface contour by temporal analysis of shadow moiré fringes". Optics Communications. 230(1- 3), 23-33 (2003).

32. Moreno Yeras, A., "Moiré topography in odontology". Optics & Lasers in Engineering, 40(1), 143 (2003).

33. Ahrens, F., B. Hojjatie, and D. Coffin, "Influence of drying variables in development of cockle: An experimental study", in International Paper Physics Conference, Proceedings, Inst. of Paper Sci. and Technology, Atlanta, GA 30318, United States (2003).

34. D' Acquisto, L., L. Fratini, and A.M. Siddiolo, "A modified Moiré technique for three-dimensional surfae topography". Measurement Science and Technology, 613-622 (2002).

34. D' Acquisto, L., L. Fratini, and A.M. Siddiolo, "A modified Moiré technique for three-dimensional surfae topography". Measurement Science and Technology, 13, 613-622 (2002).

35. Asundi, A., "Computer Aided Moiré Methods". Optics and Lasers in Engineering, 18, 213-238 (1993).

125 36. Burrows, J.M. and B.J. Griffiths, "A Vector Modelling Technique for the Representation of 3-Dimensional Surface Topography". International Journal of Mechanical Tools Manufacture, 38(5-6), 637-542 (1998).

37. Xie, X.C., D.R. Chen, and X.M. Kong, "An analysis of the three- dimensional surface topography of textured cold-rolled steel sheets". Tribology International, 32, 83-87 (1999).

38. Sherrington, I. and G.W. Howarth, "Approximate Numerical Models of 3-D Surface Topography Generated Using Sparse Frequency Domain Descriptions". International Journal of Mechanical Tools Manufacture, 38(5- 6), 599-606 (1998).

39. Quan, C., Y. Fu, and C.J. Tay, "Determination of surface contour by temporal analysis of shadow moir e fringes". Optics Communications, 230, 23-33 (2003)

40. Chen, Chao-Chang.A. and N.A. Duffie, "Development of An Automated Surface Finishing System (ASFS) With In-process Surface Topography Inspection", Journal of Materials Processing Technology, 62, 427-430 (1996).

41. Kokidko, D., et al., "Method for Measuring Transient Out-of-Plane Deformation during Impact". International Journal of Impact Engineering, 19(2),127-133 (1997).

42. WENDT, U., K. STIEBE-LANGE, and M. SMID, "On the influence of imaging conditions and algorithms on the quantification of surface topography". Journal of Microscopy, 207, 169-179 (2002).

43. Xia, C.C., et al., "Quantifying topography and closure deformation of rock joints". International Journal of Rock Mechanics and Mining Sciences, 40(2), 197-220 (2003).

126 44. Yuk, K.C., J.H. Jo, and S. Chang, "Crossed-grating method of determining the absolute order of shadow moiré fringes", http://Moiré.kongju.ac.kr/Moiré/hwp/shadow%20Moiré.htm

45. Hojjatie, B. and D. Coffin. "Measurement and Analysis of Sheet Topography Using Shadow Moiré System". in Progress in Paper Physics Seminar, Finger Lakes / Syracuse, New York, (2002).

46. Xie, X.J., et al., "Phase-unwrapping algorithm in the presence of discontinuities using a system with crossed grating". Optics And Lasers In Engineering, 29(1), 49-59, (1998).

47. Kafri, O., "The physics of Moiré metrology / Oded Kafri, ILana Glatt", ed. I. Glatt. New York: Wiley (1990).

127

9 APPENDICES

128 APPENDIX A

Spray nozzle Description : XR TEEJET Extended range flat spay tip.

Spray nozzle code : XR 8001 VS

Equivalent Orifice Diameter: 0.026”

129

Figure 59. Spray nozzle volume median diameter versus pressure curves.17

17 Source : TEEJET website and electronic communication with the company. http://www.teejet.com/APPS/MS/PDFS/11825-43.pdf

130

Figure 60. Drop size versus accumulated volume percentage for Teejet Spray nozzles18

18 http://www.teejet.com/APPS/MS/PDFS/12135-107M.pdf

131

APPENDIX B

20 Speed in/sec

0 4567891011 Dial gauge setting

Figure 61. Ink Application drive motor calibration curve for converting the dial gaue setting into actual speed.

132

APPENDIX C

Technical Specifications of Sony DCR HC21 Camcorder used in the shadow moiré system

Low Light Capability : NightShot Plus Viewfinder : Black and White, 123K Pixels Imaging Device : 1/6" 680K Gross Pixel CCD Minimum Illumination : 5 Lux (0 Lux with NightShot Infrared System) Lens Type : Carl Zeiss® Lens: Vario-Tessar® Zoom : 20X, 800X (Digital) Focus : Full Range Auto/Manual (Touch Panel) 35mm Conversion : 44-880mm (4:3); 48-960mm (16:9) Focal Distance : 2.3-46mm Aperture : 1.8 - 3.1 Shutter Speed : 1/60-1/4000 (AE Mode) Exposure : Yes, Touch Panel (24 steps) Power Consumption :1.9W/2.4W/2.5W (VF/LCD/VF LCD) Limited Warranty : 1 Year Parts; 90 Days Labor Video Actual : 340K Pixels Filter Diameter : 25mm Weight : 14 oz (397 g) w/out Tape and Battery Dimensions (WHD) : 2 1/5" x 1 2/9" x 3 15/16" (56 x 31 x 100mm)

133 APPENDIX D

Figure 62. FFT filtering tool and the parameters used to process the images to filter out noise.

134 APPENDIX E

K1 K2 K3 K4

K5 K6 K7 K8

K9 K10 K11 K12 Fringe images Before Drying – Test conditions(C1): MD: 8.5”;CD:7.5”;Tension:52.5g/cm K1 K2 K3 K4

K5 K6 K7 K8

K9 K10 K11 K12 Fringe images After Drying – Test conditions(C1): MD: 8.5”;CD:7.5”;Tension:52.5g/cm K1 K2 K3 K4

K5 K6 K7 K8

K9 K10 K11 K12 Fringe images Before Drying – Test conditions(C2): MD: 5.0”;CD:7.5”;Tension:52.5g/cm K1 K2 K3 K4

K5 K6 K7 K8

K9 K10 K11 K12 Fringe images After Drying – Test conditions(C2): MD: 5.0”;CD:7.5”;Tension:52.5g/cm K1 K2 K3 K4

K5 K6 K7 K8

K9 K10 K11 K12 Fringe images Before Drying – Test conditions(C3): MD: 8.5”;CD:7.5”;Tension:24.1g/cm K1 K2 K3 K4

K5 K6 K7 K8

K9 K10 K11 K12 Fringe images After Drying – Test conditions(C1): MD: 8.5”;CD:7.5”;Tension:24.1g/cm

APPENDIX F

Table 17. Results of Wrinkle parameters measured for different ink-jet papers under test condition C1.

T0 Tf Sample T0-WL(inch) Width (inch) T0_height(mm) # Wrinkles Tf-WL (inch) Co-planarity (mm) (mm) Tf-Height (inch) Flatness SD-Coplanarity K1 0.683 1.166 0.584 6.5 1.733 1.732 0.667 2.453 1.146 K2 0.759 1.214 0.582 6.5 1.347 1.454 0.603 1.640 0.927 K3 0.917 1.255 0.712 7.25 1.898 1.342 0.572 1.592 0.542 K4 0.641 1.161 0.536 7.5 1.529 1.424 0.702 1.652 0.985 K5 1.130 1.223 0.651 7.5 1.578 0.918 0.445 3.359 1.121 K6 0.730 1.261 0.642 6 2.294 1.302 0.556 1.556 0.859 K7 0.830 1.230 0.586 6 2.090 1.422 0.699 2.336 1.287 K8 1.146 1.375 0.475 5.75 1.862 0.592 0.286 1.587 0.636 K9 0.776 1.262 0.576 6.75 1.968 1.253 0.603 2.314 0.814 K10 0.787 1.193 0.557 6 1.822 1.300 0.683 1.401 0.919 K11 0.802 1.326 0.482 6.5 1.795 1.183 0.508 2.061 0.708 K12 0.816 1.242 0.587 6 1.953 1.070 0.508 1.538 0.672 %Variation 60.4 17.3 40.7 26.8 52.0 91.3 73.1 100.1 84.2

141

Table 18. Results of Wrinkle parameters measured for different ink-jet papers under test condition C2.

T0 Tf Sample T0-WL(inch) Width (inch) T0_height(mm) # Wrinkles Tf-WL (inch) Co-planarity (mm) (mm) Tf-Height (inch) Flatness SD-Coplanarity K1 0.695 1.164 0.536 8.5 1.309 0.911 0.413 4.484 0.834 K2 0.777 1.197 0.466 9.0 1.220 0.753 0.381 4.211 0.634 K3 0.831 1.186 0.524 10.5 1.025 0.722 0.318 4.180 0.783 K4 0.991 1.164 0.572 9.0 1.367 1.024 0.413 3.386 0.876 K5 0.800 1.145 0.562 8.0 1.358 0.931 0.406 2.757 0.862 K6 0.938 1.227 0.587 8.0 1.486 0.987 0.540 2.932 0.961 K8 1.428 1.295 0.490 6.5 1.697 0.434 0.381 3.012 0.257 K9 0.818 1.270 0.663 7.5 1.581 0.838 0.381 3.392 0.368 K10 0.839 1.177 0.550 9.0 1.250 0.818 0.381 3.347 0.774 K11 0.926 1.287 0.572 9.0 1.141 0.948 0.356 2.992 0.799 K12 1.014 1.202 0.810 9.0 1.305 0.740 0.381 2.882 0.775 %Variation 80.2 12.4 59.8 46.8 50.2 71.3 56.2 50.6 97.6

142