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9-1-1986 The effect of the relief plate shoulder angle on flexographic dot gain Bruce Brier

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Recommended Citation Brier, Bruce, "The effect of the relief plate shoulder angle on flexographic dot gain" (1986). Thesis. Rochester Institute of Technology. Accessed from

This Thesis is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. THE EFFECT OF THE

RELIEF PLATE SHOULDER ANGLE

ON FLEXOGRAPHIC DOT GAIN

by

Bruce Brier

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the School of Printing in the College of Graphic Arts and Photography of the Rochester Institute of Technology April 1986

Thesis Advisor: Professor Walter Home 1.sed sample statement for granting or denying permission to

)roduce an RIT thesis.

;le of Thesis ~~~k~E_f~~_eG_t~o_~~~~! ~!_R_e~I_;e_f_- ~p~J_a~~~· ~~~ <;/'o",ld.e..r ~t~~ 6" Flexog y¥h ~ L '001- CcJ;",

~~_B_r_u_c~e_B~r_ie_r~~~~~~~~~~~ hereby (grant, lY) permission to the Wallace Memorial Library, of R.I.T., to )roduce my thesis in whole or in part. Any reproduction will ; be for commercial- use or profit.

Or

------prefer to be Itacted each time a request for reproduction is made. I can be

~ ~ed at the following address. e ABSTRACT

The mechanism of image- formation in photopolymer

flexographic relief printing plates is reviewed. In

addition to the identified factors, it was hypothesized

that the level of exposure could influence the plate

shoulder angle. It was further hypothesized that an optimization of shoulder angle with respect to halftone dot area may reduce the dot gain of the printing process,

An experiment was conducted using a novel method of plate exposure. Continuous -tone masks were placed in contact with the plate back and the halftone negative used on the plate face. The resulting shoulder angles were measured and analyzed for significant fit to a lin ear mathematical model. For many dot sizes the face expo

sure level was significant in determining shoulder angle.

The plates were printed on a flexographic press, and prints were analyzed to identify any benefits in dot gain reduction. Regression analysis showed very inconsistent correlations between shoulder angle and dot gain for indiv

idual dot sizes printed at an optimized level of impression,

11 Certificate of Approval -- Master's Thesis

School of Printing Rochester Institute of Techo1ogy Rochester, New York

CERTIFICATE OF APPROVAL

MASTER'S THESIS

This is to certify that the Master's Thesis of

Bruce Brier

With a major in Printing Technology has been approved by the Thesis Committee as satisfactory for the thesis requirement for the Master of Science degree at the convocation of

September 1986

Thesis Committee: Walter Horne Thesis Advisor Joseph L. Noga Graduate Program Coordinator Miles Southworth Director or Designate TABLE OF CONTENTS

Page

ABSTRACT.. O9O*OO49OOOOO099OO 11

INTRODUCTION

Effect of Pressure on Flexographic Printing Plates. 3 Consistency of Tonality in Halftone

Process Flexographic Printing...... ,...... ,,. .4

Elastomeric Printing Plates ,..,..,,,..,....,.,,,... 6

Light Theory ...... 10

Light Propagation Model ...... 11

Refraction. oooaooooooevoooo ..12

DispersionLooo0oo*ooeo99o ..13

Particle Properties of Light o o o o o ..15

Introduction to Photopolymerization. . . ..16

' Unsaturated Organic Compounds o o o oaoooo* O 9 9 o o ...17

Photopolymerization, ...... ooaaooo ...18

Dot Gain...... o o o o ooo*oooooo ...25

REVIEW OF LITERATURE

Bennett and Webster.. o o o o o ...26

urawrorQo ooo.oooo.o ...28

Plambeck and Crawford ...31

~ Halpern. oooooooooooooo a o o o o o 34<-S

Moore...... ooooooooo OOO 9 O O O 37o' '

Moore oooooocoooooooooo oooocooooa ...38

Newberry oooooooooooooo oo*ooa*oooo**o ...40

Smith O O 0 O oooooooooooooo*o 0 0 0 0 0*0 ...43

HYPOTHESIS

Hypothesis O0000000O000O0O0OQ0000OO90OOOOO**0O* 46

~ The Problem Proper o*oooooaooo*o*oooooo o o 47r

METHODOLOGY

Test Object. . ooooooooo ..48

Exposure. o*o*ooaooo*o ..50

Plate Processing. o o o o o o o oooooooo ?ooo ..51

Determination of Shoulder Angle. oooooooooooa ..51

Press Operation...... oooa*soooo ...52

Measurement of Prints ...... o**o*ooooaoo o o 53>J +J

111 TABLE OF CONTENTS (cont.)

Page

RESULTS ...... jj

L1 \J -LjO*o*ao0000*00*0***0**000*0*1**9a'3***0**<'9***^''*M

BIBLIOGRAPHY. ....,.,.,..,,..,.. 64

njTrLDUJ i^V*O00O9*00O0904*9**409?a?9?a*09?')'3*'^'' Dafa A^Cl L* Ct> OO0O*O*'OaOO9OO****Oao*O*O*BO99O*9'3O9O9*9O'71

oo**ooo*' UlUbbdiy n oocoo oosoooooo i o o o

IV LIST OF FIGURES

Page

1 The Shoulder Angle 9 8

2 Characteristic Dot Profiles 8

3 Refraction of Light 13

4 An Example of Addition Polymerization 20

5 Critical Angle of Exposure 22

6 Sidewall Exposure in Shadow Tones 24

7 Characteristic Relief Profile of Laser- Engraved Printing Element 27

8 Compound Curve Relief Profile. ..,,..... 32

9 The Cross-Section of Layer During Exposure. .. .35

10 Characteristic Dot Profile of the Hercules Capped Plate 41

11 Taking Measurements of a Halftone or Line Image in a Relief Plate 52

12 Prints Showing Kiss Impression. 88

v LIST OF TABLES

Page

1 Dot Gain Curve for Average Plate...... ,.., 60

2 Dot Gain Curves for Individual Inks ...... 61

3 Face Exposure vs. Theta for 6.87, dot on film... ,71

4 Face Exposure vs. Theta for 16.17o dot on film.. .72

5 Face Exposure vs. Theta for 27.2% dot on film... 73

6 Face Exposure vs. Theta for 35,2% dot on film.. .74

7 Face Exposure vs. Theta for 51,3% dot on film... 75

8 Face Exposure vs. Theta for 56.3% dot on film.. .76

9 Face Exposure vs. Theta for 68.87, dot on film... 77

10 Face Exposure vs. Theta for 82.5% dot on film... 78

11 Face Exposure vs. Theta for 91.2% dot on film. ..79

12 Back Exposure vs. Theta for 6.8% dot on film.... 80

13 Back Exposure vs. Theta for 16.1% dot on film... 81

14 Back Exposure vs. Theta for 27.4% dot on film... 82

15 Back Exposure vs. Theta for 35.27, dot on film., .83

16 Back Exposure vs. Theta for 51.3% dot on film. ..84

17 Back Exposure vs. Theta for 68.87, dot on film... 85

18 Back Exposure vs. Theta for 82.5% dot on film. ..86

19 Back Exposure vs. Theta for 91.2% dot on film. ..87

20 Regressions Categorized by Printing Ink.. 89

21 Experimental Exposure Levels . 90

Theta- 22 Film Dot Area vs. -Summary...... 91

vi Chapter One

INTRODUCTION

Flexography

Flexography is a form of relief printing in which the resiliant element of the press configuration is the plate itself. The inks used in flexography are low viscosity fluid inks. The ink metering roll, called an anilox roll, is essentially a gravure cylinder which prints an overall solid. The anilox roll consists of a copper cylinder engraved with a fine mesh of intaglio cells, then heavily chrome plated.

Consistent metering of the amount of ink available to the plate is accomplished by covering the surface with ink, and then shearing the ink at the extreme surface. The excess ink is then ducted away (doctored) leaving the cells full of ink.

The shearing function is performed by either a dif ferentially rotating rubber-covered fountain roll or by

rubber- a reverse-angle doctor blade. At higher speeds the covered fountain roll may deflect with the high. hydro static pressure in the shearing nip. The reverse-angle doctor blade, however, is held more tightly to the anilox roll as speed increases.

"reverse-angle" The term stems from the fact that its angle to the roll is the reverse of the gravure doctor blade angle. The reverse-angle doctor blade contacts the anilox roll at an obtuse angle to a tangent drawn from the point of contact toward the ink fountain rather than an acute angle as in gravure.

The amount of ink transferred to the plate is deter mined by the following process variables: the volume of the cells per unit area on the anilox roll, the surface energy of the chromium, the visco-elastic cohesive strength of the outer phase of the ink, the surface energy of the plate, and the pressure in the nip. If the surface energy constants are ordered properly the ink will preferentially wet the plate rather than remain in the anilox roll cells.

Anilox rolls are specified according to cell count,

volume, and shape. Cell shape is typically either an

inverted pyramid or an inverted truncated pyramid called

cell" a "quad (quadrangular) configuration.

Cell volumes can be computed using standard formulas for

solid geometrical volume:

02d = Volume of a pyramid -j

JO r\

= Volume of a truncated pyramid -^(0 + OB + B ) where d = the cell depth

0 = the side of the square forming the opening at

the top of the cell

B = the side of the square forming the floor of

the cell2

Effect of Pressure on Flexographic Printing Plates

When printing with an elastomeric plate, any excess pressure will cause a physical distortion of the plate

surface. To print the image perfectly, the plate would

the substrate with good con have to meet tangentially ,

tact but no pressure. This condition is the "kiss impres

sion"; uniform ink transfer at kiss impression is not possible in the flexographic process. This is due to the

fact that the mounted plate has variations of radius re

sulting from the plate cylinder indicator runout (a mea

plate caliper variations sure of cylinder concentricity) , ,

"stickyback" and caliper variations. The press cylinders

must be brought together so that the lowest point on the

plate cylinder prints at kiss impression, even though higher points will be printing with some excess pressure.

"squeeze," Printing impression, also called is the

distance the two cylinders are brought together beyond

the kiss impression. Because the plate and the substrate

cannot occupy the same space at the same time, the elasto

a local meric plate is displaced, causing decrease in

plate volume. The incompressibility of the elastomer causes a general increase of plate volume in unstressed

areas to a degree determined by the area's proximity to

the impression nip and the structural continuity of the re

lief structure involved.

Consistency of Tonality

in Halftone Process Flexographic Printing

The reflectance of a halftone print can be expressed

as the sum of the products of the reflectances of the

image and non-image areas, times their respective fraction

al areas. This relationship in equation form is:

R=CR, + (1 - C )R .ad a p

where R = halftone reflectance

C = dot area coverage (<1) a

R, = microreflectance of the dot (including the a

paper beneath the dot) 3 R = reflectance of the paper P

A change in the fractional dot area printed or a change in

the reflectance of the ink will affect the tonality of

the print.

Changes in the printed dot area will occur if

variations occur in the amount of impression, the sub

strate caliper, the slur conditions, the surface energy

of the substrate, or the visco-elastic properties of the ink. In addition, plate wear may contribute to a change

in relative dot area.

Changes in the ink's reflectance will occur if variations occur in the ink film thickness transferred

to the substrate, or in the relative content of solids

in the ink. The ink film thickness is determined by the

anilox geometry, and the film splitting characteristics

in both the inking nip and the impression nip. The film

splitting characteristic, like the previously described

ink transfer to the plate is a function of the tack of

the ink, the surface energy of the plate and printing

substrate, and the speed and pressure in the nip.

The reflectance of a dry ink film will vary if

the proportion of solids in the ink varies even if the

thickness of wet ink metered to the substrate remains

constant. The main causes for a variation in ink solids

content during a press run is evaporation of volatile

solvents. Evaporation of the solvent or solvents used

in the highly fluid flexographic inks is aggravated by

hot air drying of the web in close proximity to the ink

reservoirs. To eliminate variation in this parameter

there are automatic viscosity control devices available

for maintaining a constant proportion of solids and ve

hicle in the ink.

Automatic viscosity control devices typically

consist of two paddles immersed in the ink reservoir, one being power driven and the other being a sensing de

vice which is free to rotate. The speed of the undriven

paddle gives an indication of the ink's ability to trans

fer kinetic energy. A servo control adds a metered

quantity of a mixture of solvents (in a proportion cor

rected for unequal rates of evaporation ) to the ink

reservoir when excessive viscosity causes the sensing

paddle's speed to drop below the set point.

Elastomeric Printing Plates

Elastomers are rubber-like resiliant materials.

Having no crystalline structure, elastomers are not solid but are rather like extremely high viscosity liquids.

As such, elastomers are not compressible. They deform easily under pressure, but do not change their total volume. A force on one portion of an elastomer causes a local decrease in volume which is equal to the cor responding expansion in volume in adjacent unstressed regions. The extent of the expansion is along the lines of least resistance and is functionally related to the modulus of elasticity of the elastomer and the cross- sectional area of the deforming member.

A distinction must be made between elastomers and

elastomeric foams which contain gas-filled voids which can compress in volume.

Relief plates generally are described in terms of their top, side wall, bottom, and floor. The top is the

printing surface which transfers an ink image to the

substrate. The bottom is the area of maximum uniform

relief, while the floor is the thickness in the bottom

region. The sidewall is the surface which connects the

top with the bottom along all interfacial edges.

Relief images must fill the dual requirements of

both sharp printing and good durability. Sharp print

ing means reproducing the exact shapes and areas of all

image elements. Durability includes abrasion resistance,

chemical stability, and adhesion of the printing elements

to the base. Optimization of the two parameters will

produce a plate which is a compromise, because the pre

ferred structure for each consideration is in conflict.

Optimum conditions for the compromise indicate the use of

a sloped image wall which meets the floor at an obtuse

angle. This configuration increases the area of contact

between a printing element and the base, improving its

lateral strength but adding two detrimental effects:

1) As plates wear at the printing surface, the

image area will increase.

2) As pressure in the nip increases, so does the

plate area in contact.

The quantitative description for the degree of sidewall slope is the shoulder angle, 6. The shoulder angle lies within the relief image element volume and can be visualized as the angle between the sidewall and a normal extending from the interfacial edge down to the

base.

top l*A

sidewall

bottom s/ ] floor T

Figure 1. The Shoulder Angle 9

A shoulder angle of zero degrees indicates a per

pendicular sidewall, while a negative shoulder angle in

dicates undercutting.

r\ r\ r\ A 3

straight curved compound curve

Figure 2. Characteristic Dot Profiles

In addition to the straight sidewall of constant

angle, the sidewall shape may be either a convex or con

cave curve, or a compound curve consisting of both concave

and convex curve segments, the concave portion of the side- wall close to the top of the plate and the convex portion at the base. This configuration is reported as yielding greater strength with less image gain than straight side-

walls . Some platemaking processes have been designed to 4,5,6.7,8,9 u j ,-u -a 11 s yield a compound curve in the sidewall surface.

Process details will be reviewed later.

Elongation

Flexographic plates which are made flat and mount

ed on a cylinder undergo a surface distortion due to

stress. The bending force creates a compression of the

base surface and an elongation or stretch of the printing

surface. (At some point between, there is a neutral plane

of curvature in which no distortion occurs.)

Any printing plate material, including metal, will when curved, distort from a neutral center line in the form of elongation on the convex surface and contraction on the concave surface, in ^ the direction of the curve.

In uncontrolled plates the neutral plane of curvature

lies at the midpoint of the local plate height, taking into consideration the relief area as a reduction in height,

More dimensionally stable plates incorporate a flexible layer of very low elastic modulus material such as poly ester sheeting laminated to the base of the plate, or at some point within it. This level is the neutral plane of curvature for these plates.

The degree of elongation, or stretch, can be quanti fied as the ratio of any distance, measured in the web direction on the print, and the corresponding distance on the copy negative. Stretch can be predicted using a 10

formula based on the fact that stretch is directly pro portional to the thickness of the plate which expands, and inversely proportional to the cylinder diameter.

S = ^

where S = stretch

= t thickness of the plate strata above the neutral

plane of curvature

1 = press cylinder repeat length

Light Theory

In order to understand photochemical reactions and optical systems utilized by the reprographic trade, a re view of the properties of light is a prerequisite. Be cause light can affect the heating of matter, it is de fined as a form of energy. This energy is transferable at great speeds over great distances. Experiments have shown that in a vacuum, the velocity of light equals that of radio transmission. This holds true for light of any color and electromagnetic radiation of any wavelength.

This relationship places both types of energy at different points on a continuous spectrum of radiant energy.

Electromagnetic radiation has been found to have properties which are at once both wave-like and particle like. The electric and magnetic wave-like properties are 11

those such as frequency. There is an electric field and

a magnetic field associated with a beam of electromagnetic

radiation, both varying in intensity in a sinusoidal

fashion at the same frequency.

Given that there is a velocity and a frequency to

the radiation, at any moment, the locations of maximum

field strength within a medium must be separated by a

distance A. The relationship is shown by

X = v

where c is the speed of the radiation and v is the

frequency of the phase shifts with respect to time.

The frequency of radiation is determined by the emitter and is independent of the medium through which

12 the radiation passes. The speed cf, however, is functionally related to the electroconductivity e, and magnetic permeability u of the medium.

c' =

ep

Light Propagation Model

An emitter radiates spherical waves whose radii increase at the velocity of light. Perpendicular to any tangent plane on any sphere is a ray. If the total 12

luminous flux of the emitter is radiated equally to all

points on a sphere of radius R, then

the illumination E, falling on a unit area of that sphere is given by the ratio of the luminous flux F to the area A of the sphere (which is equal to 4ttR2).

F _ 4ttI I _ t? ~ " * ~ A VirtJ R*

For concentric spheres the luminous

flux F is constant , thus

F F = =7- = E i -i and E? ->

The amount of light per unit area arriving from a point source varies inversely as the square of the dis tance from the source.^

Refraction

Refraction occurs when a ray of light traveling

through one transparent medium encounters the interface of another transparent medium which has different electro- conductivity and magnetic permiability. The speed, direc

tion, and wavelength will change, while the frequency remains the same. Other effects are dispersion and re flection.

If the angle at which the ray meets the media

interface is perpendicular, there is no change in ray direction.

If a ray of light as described enters the second medium at some angle other than the normal to the interface

there will be a bending of the ray called refraction. 13

Figure 3 shows how the angle of refraction differs from

the angle of incidence when the speed c2 is less than c, .

The degree of change is described by the refractive index n

which is the ratio of the speeds in the two media. Accord

ing to Snell's Law:

ci Sin9i 14 n21 = ~2 Sine

incident raj

interface : phase one

/'Tiase two refracted ray J$ normai

Figure 3. Refraction of Light

Dispersion

Light usually contains many frequencies within one

beam which behaves as a single unit. When such a beam

enters a transparent medium under the conditions which 14

produce refraction, the degree to which refraction takes place will vary depending on the frequency of the com ponent ray. That is, lower frequencies refract less and higher frequencies refract more. It is sometimes necessary to specify the index of refraction in terms of the frequency of the light used in the determination (or

the wavelength of the light in air) .

In addition to refraction and dispersion in the optical system described, there is also reflection, even though both media are transparent. Reflection occurs at an angle 93 opposite but equal to the angle of incidence

6i. The intensity of the reflected ray which results when a beam of unpolarized light is perpendicularly inci dent can be determined by Fresnel's formula: 15 I n2i - 1 F- (5irTT)2

O where I = intensity of the reflected ray

I = intensity of the perpendicularly incident

ray of unpolarized light

n2i = refractive index where light passes from

medium 1 to medium 2.

A perfectly smooth surface illuminated by an emitter of luminous intensity I, located at a distance R from the point of incidence at an angle of incidence 9 will have an illumination E such that:

16

_ Icos9 E R' 15

Particle Properties of Light

Analysis of the photoelectric intensity effects

of radiation emitted by a black body at high temperature

has led to the theory that there are particle-like proper

ties to radiant energy. It was postulated by Planck in

1901 that the energy emitted was not variable continu

ously but could take only discrete values as if the energy

was delivered in tiny packages. The energy contained in

each package, or quantum, varies directly with the fre

quency of the radiation; thus for the energy E of a

quantum

** E = hz; =

10"27 erg_sec where h = Plancks constant = 6.6256 x /quantum

For radio waves the energy per mole of quanta (or einstein) is extremely small and is nearly undetectable except for its influence on the electrons in conductive metals. The order of magnitude for this band is

-8 10 kcal/einstein. The quanta produced at microwave and infrared wavelengths have somewhat higher energy and can effect the raising of temperature in materials which absorb them. The order of magnitude of microwave energy

_3 is 10 kcal/einstein; for infrared the range is from

10 101 to kcal/einstein. 16

= Quanta associated with radiation of A 720 nm

have a potential energy of 40 kcal/einstein. Smaller wave

lengths have increasingly greater potential energy. Most

chemical bonds are of the order of magnitude of 40 kcal/ mole, and some compounds have a bond energy of 30 kcal/

i 18 mole.

The first law of photochemistry postulated by

Grotthus and Draper states: Only the light which is ab

sorbed by a molecule can produce a photochemical change.

The second law of photochemistry postulated by Stark,

Einstein, and Bodenstein states that in the primary

photoreaction each molecule absorbs one quantum of the

radiation responsible for that reaction and that the quan

tum yield is always equal to unity.

Secondary reactions such as thermal chain reactions

6 can produce very high quantum yields up to 10 molecules

reacted from a single one-quantum primary reaction.

Introduction to Photopolymerization

Flexographic relief images can be made by the

traditional method of molded rubber platemaking. This

the of an process, briefly described, entails making

original metal photoengraving, the subsequent molding

the of a phenolic matrix from it, and finally molding of vulcanized rubber duplicate plates from the matrix.

Another usable platemaking process makes use of

photochemical reactions of organic compounds to achieve 17

image transfer to three dimensions. Imagewise exposure

of the organic compounds causes photopolymerization, and produces a change in the physicochemical properties of

the reactants, usually their solubility in a solvent.

The differential in solubility between exposed and un

exposed areas comprises the latent image. Subsequent processing makes the image more discernable.

Unsaturated Organic Compounds

Organic compounds are those containing carbon as one of the components.

Atoms within organic molecules are generally non- ionic and are linked by covalent bonds. Each covalent bond consists of a pair of shared electrons. When ad jacent carbon atoms are joined by only a single pair of shared electrons, the compound is said to be saturated.

If any two adjacent carbon atoms are linked by two or more pairs of shared electrons, the compound is un- saturated.

Examples :

Ethane Ethylene Acetylene

H H H H II II H-C-C-H H-C = C-H H-CeC-H I I HH

Unsaturated compounds are more active than saturated compounds due to their instability. Saturated compounds generally react by substitution, while unsaturated com- 18

addition and produce saturated pounds generally react by

addition compounds as products. The mechanism of

reactions entails an opening of the double or triple bond permitting additional atoms to join the chain. Photopoly merization begins with the "cleaving of the unsaturated bond of a molecule due to the absorption of light energy.

The resultant radical species can cleave the unsaturated bond of a monomer forming a saturated bond at that site and a radical species at the opposite end of the monomer molecule. This radical repeats the addition reaction.

Photopolymerization

According to the classical definition, a polymer is a large molecule built up by linear or branched repetition of small units, called monomers. Poly merization can proceed only when an initiator is present in the system. An initiator can be either a monomer mole cule that has received enough energy to combine with another molecule or, in most instances, a different molecule, one more easily activated than the monomer, is added. The activation takes place through the absorption of energy, accompanied by the formation of short-lived intermediates. These number of intermediates have an odd .q electrons and are known as free radicals.

In addition to dissociation by the absorption of

through light quanta, radical formation may be the phe

electron- nomena of pyrolytic decomposition, transfer, or thermal dissociation. A quantum of light carries a quantity of energy inversely proportional to its wave length. A mole of light quanta whose wavelength is 300 nm has 95.5 kcal of energy, while at 455 nm the energy is 19

63 kcal/mole, and at 590 nm the energy is 48.5 kcal/mole.

photoinitia- The energy associated with covalent bonds in

tors are of this order of magnitude, so if a quanta of

light is absorbed by a photoinitiator molecule, the bond

ruptures, thus forming a pair of free radicals. hv

= -- Example: CH2 CHX .CH2 + CHX =

In order to be effective, the initiator must absorb light

of a wavelength which has sufficient energy to break the

double bond. The double bonds of photoinitiators have

sufficiently low energy of dissociation and good absorb-

tance of wavelengths which can produce dissociation.

Photoinitiators are more efficient in forming radicals

than the actual monomers which undergo addition polymeri

zation.

Upon formation, free radicals initiate polymeriza

tion, or chaining, by an addition reaction to the double

bond of the monomer which has two electron pairs . One

electron bonds with the free radical's unpaired electron, while the other pair from the double bond forms a new free

radical. This radical, in turn, will react to the double 20 bond of another monomer.

The polymerization of thousands of monomers can occur through the absorp tion of a single quanta of actinic radiation especially if crosslinking between two or more polymeric chains

occurs . 21

The Photomultiplication Effect refers to the mul

tiplicity of molecules which react as a result of the ab- 20

sorption of a single quantum.

If two growing chains meet, the free radicals will react yielding no unpaired electrons :

...the activity of the two radicals will be mutually annihilated and a "dead" 22 molecule will be formed.

All light sensitive systems function as a result of

the ability of the material to absorb certain wavelengths

of electromagnetic radiation and causing it to react

chemically. Some of those compounds which react chemic

ally as a result of this absorbed energy have been applied

to photographic and photomechanical processes.

Kosar reports on the extensive research done by the Dupont

Company on benzoin initiators for the photopolymerization

of vinyl monomers useful in the preparation of relief

printing plates.

R. * c-c -> R-c-c H X H *

H H HH M H M , ii i i i i * R-C-C* -f cc n-c-c-c-c I I II > I I I H x MX HXHX

VH x/n N x x H V.x /

Figure 4. An Example of Addition Polymerization

from J. Kosar Light Sensitive Systems p. 159 21

L. Plambeck, Jr. explains the formation of print

ing elements profile in these excerpts from U.S. Patent

#2.760,863, column lines 56 3, to 71, which apparently

refers to the popular photopolymer relief plate exposure

unit which uses a multitude of parallel fluorescent lamps

placed in close proximity to the exposure plane.

By using a broad beam light source, relatively close to the image-bearing transparency, the light rays passing through the clear areas of the trans parency will enter as divergent beams into the photopolymerizable layer, and will thus irradiate a continually divergent area in the photopolymeriz

able layer. . .resulting in the forma tion of a polymeric relief which is at its greatest width at the bottom surface..., the top surface of the relief being the dimensions of the clear area. These reliefs are in the form of conical and pyramidal frustrums... The cross section of such a relief image, obtained by passing a plane through the surface of the relief and meeting the base... at right angles, will be [an isoceles]

trapezoid. ..

The author claims that preferred line-work shoulder angles

14 26.6 are between and and that preferred halftone

0 shoulder angles are between and 35.

The degree of taper of the relief image below its printing surface can be con

trolled. . .within limits, by the geometry of the light source and can be calculated optically.

[The maximum shoulder angle obtainable] is determined by the refractive indices of the image-bearing transparency 22

support and of the photopolymerizable layer, and is related to the critical angle at which total reflection takes

place in the system. .. This is the theoretical limit and is unattainable in practice since it would require that the incident ray be in the plane of the negative. *

interface. critical antf/e

normal

Figure 5. Critical Angle of Exposure 23

With the use of the short distance, broad beam

emitter, intensity is increased substantially allowing the

use of slow speed photosensitive materials, short expo

sures, and lower power consumption. Rays from all points

of a broad beam emitter can reach each point of the expo

sure plane. It can be inferred that there is a distribution

of ray intensities and directions coming to each point of

the exposure plane and passing into the layer of photo

sensitive material.

The low angle rays have lower intensity, having come from more remote portions of the light source, hence they are lower in intensity and do not ordinarily effect polymerization. If the duration of exposure increases, so would the resulting shoulder angle due to the poly merization by lower angle rays.

If two image elements are quite close to one another, then low angle low intensity rays may be addi tive in intensity under small non-image areas of the process negative and may result in a shoulder angle greater than that formed in areas where image elements are remote. This change of shoulder angle is postulated to be prevalent in shadow areas of halftone images.

three- For the case of image-wise exposure of a dimensional photopolymer relief plate, the shoulder angle requirements determine the design parameters of the emitter's dimensions and proximity to the exposure plane.

When the photosensitive material is three-dimensional, 24

the angle and intensity of incident rays determines the

direction and extent of the photochemical reaction.

Figure 6. Sidewall Exposure in Tones

A photopolymer plate material, exposed by a broad light

source, has three-dimensional thickness such that side-

wall regions of dots would be subject to unwanted ex

posure from divergent light entering the layer in adja

cent image areas. Such light is limited to angles closer

to the normal than the critical angle. For polymerizable

60 media of refractive index 1.5, the critical angle is

from the normal. Because the sidewall regions of High

light dots are more isolated they are less susceptible

to this undesirable effect. Conversely, the shadow wells 25

receive unwanted exposure on all sides from the clear

image areas, increasing their shoulder angle.

Dot Gain

Some factors contributing to dot gain in the flexo

graphic process non- are inking of image shoulder areas,

distortion of the relief resulting from the impression

squeeze, the ink flow during impression and the ink flow

after impression in the heat drying step.

Fill-in and spread are related to such things as press settings, plate accur acy, ink film thickness, ink viscosity, ink metering ability of the equipment, beading of ink on the plates, and sub strate characteristics. 24

One of the most important characteris tics of the plate is its relative in- compressibility. Flexographic plates can be deformed by pressure... As the interference or amount of squeeze is increased, sideward displacement of the rubber plate causes an increase in width of the print.25

Magnitude of the shoulder angle and relative plate hardness are also factors in determining the degree of

dot gain. Pressure in the ink metering nip must be optimized so that ink transfer to the plate is maximized while inadvertant inking of the sidewalls of the plate due to excessive pressure is minimized. Some inadvert ant sidewall inking may be present due to ink flow from an anilox roll cell situated partially on an image area edge. 26

Chapter Two

REVIEW OF LITERATURE

A review follows for D. Bennet and J. Webster's paper given at the 1979 SPOI Laser Printing symposium and published in the Proceedings of the Society of Photo-

Optical Instrumentation Engineers, Volume 169, pp. 48-55.

A limitedly successful application in compound shoulder angle flexo platemaking is demonstrated in the

Zedco-Coherent flexographic rubber roller engraving machine. The machine scans the original mechanical with a Helium-Neon read laser and photoreceptor, the signal of which is used to control a Carbon Dioxide laser to engrave the non-image areas by ablation of the rubber.

The software utilizes an array of electronic shift regis ters which define a support area or perimeter of variable width around all image areas. These areas are engraved to a shallower depth than open areas well away from image elements. The software directs the output of the write laser to one of two levels as determined by electronic data contained in the image registers and shift registers

of the computer system .

The relief depth produced in the support areas

can be controlled to one of two levels, 0 . 5 mm (.020 in.) and 1.0 mm (.039 in.). Relief depth in non-image areas outside the support areas can be controlled between 1 mm 27

(.039 in.) and 3 mm (.118 in.). The width of the support

area can be controlled to one of three levels, zero, 0.1

mm (.004 in.) and 0.2 mm (.008 in.). The Gaussian distri

bution of the energy in the write beam ablates the rubber

12 to a shoulder angle.

_TV

Figure 7 . Characteristic Relief Profile

of Laser-Engraved Printing Element

The Zedco-Coherent process is excellent for con tinuous design rolls, line work, step-and-repeat work, and coarse halftones, especially shadow tones. In high light areas, fine screens and fine positive-printing type, the resolution is poor. The process is improved for these problem areas through the use of the dual level engraving control offered by the system. 28

A review follows for the paper given by H. E.

Crawford at the 1960 Meeting of TAGA and published in

Proceedings of the 1960 Meeting of the Technical Associ

ation for the Graphic Arts, p. 193, 193A-193H entitled

'Dycril' "The Photopolymer Printing Plate".

The characteristics desired in a plastic relief

plate are specified as insolubility in most solvents.

insensitivity to changes in temperature or pressure,

dimensional stability, wear resistance, good shelf life,

adequate photosensitivity, optical clarity, reproduci bility or results and proper image structure.

The components of the Dupont Dycril ppm relief plate and an explanation of the image conversion process

from the proper negative film image are described. The

three basic components of this image carrier are a photopolymerizable layer, a non- sensitive bonding layer and a metal support.

The photopolymerizable layer is composed of a

cross- linkable monomer, a photoinitiator , and an inhibi tor. The layer is gas permeable to some extent and there is a diffusion of gases within the layer in proportion to the partial pressures in image and non- image regions.

The cross -linkable monomer provides the mechanism of a change of solubility after an image-forming reaction.

The resultant crosslinked structure is reported to be long molecular chains which are periodically joined to 29

adjacent chains. This structure is responsible for the

physical characteristics of the plate. The monomer un

dergoes a polymerization reaction in which thousands of

monomer molecules combine to form the crosslinked struc

ture: large molecular chains which are periodically

joined across two adjacent chains. It is this structure

which is chiefly responsible for the physical character

istics of insolubility in solvents, resistance to wear by

abrasion, deformation and recovery from mechanical stress

and clarity to image forming light.

The photoinitiator provides a rate of polymeriza

tion several orders of magnitude higher than the monomer

alone, the photoinitiator determines the composition's

spectral sensitivity and the overall photosensitivity.

Changes in initiator concentration affect this property.

The band of radiation most strongly absorbed by Dycril

is 310-400 nm. This fact facilitates safelighting and

selection of imaging light sources.

In order to provide good shelf life, an inhibitor

is used to prevent spontaneous polymerization. Storage

of the plate in an oxygen-containing atmosphere will also

increase shelf life by inhibiting thermal polymerization.

However, at the time of exposure, it is desirable to purge

the oxygen. The Dupont system includes a special cabinet

for conditioning a plate over the 24 hours prior to ex posure. The atmosphere inside the cabinet is maintained 30

with less than 1% oxygen to ensure a high photospeed but

not less than 0.3% oxygen to inhibit spontaneous polymer

ization during the conditioning period.

For imagewise exposure a right reading negative with a matte-surfaced emulsion is placed in intimate con

tact with the polymerizable layer.

Exposure to the proper amount and kind of ultra violet radiation through the negative will produce long- wearing high quality relief characters. The intensity of

light reaching the base is only 10% of that at the surface

The cross-sectional profile of relief characters is deter mined by the angle of entrance of radiation into the poly mer. Highly collimated radiation yields a shoulder angle

0 20 of while a broad source yields a shoulder angle,

The rate of polymerization is dependent on the temperature which may vary due to infrared radiation present from the emitter plus the exothermic heat of the polymerization reaction. Process exposure equipment is designed to mini mize this variation by providing airflow over the lamps and the polymerizable material. 31

A review follows for Dupont assigned U.S. Patent

2,964,401 L. by Plambeck, Jr., December 13. 1960, and

Dupont assigned U.S. Patent 2,993.789 by H. E. Crawford, July 25, 1961.

The patents describe a process for manufacturing

photopolymer relief plates embodying a compound shoulder

angle, which results from chemical formulation differen

tials between upper and lower strata within the layer.

Light-induced addition polymerization reactions will pro

ceed with different reaction rates at the different

depths.

It is inferred from the experimental results that

the local concentration of polymerization inhibitor de

termines the characteristic shoulder angle. The mechanism

can be explained as follows: A high concentration of in

hibitor causes a failure of the law of reciprocity at low

exposure levels. This introduces a point of increased

contrast gradient in the characteristic curve for the

polymerizable material.

Photosensitivity requirements for lower layers of

the plate are higher due to the absorption of actinic

light by upper layers. In addition, relief profile char

acteristics at the base of a printing element optimize with a broad shoulder angle for good adhesion to the base.

The photosensitivity and contrast gradient of the

lower layers is increased by increasing the concentration 32

of polymerization initiator and decreasing the concentra

tion of polymerization inhibitor. Because a broad shoulder

angle is undesirable at the printing surface and because

the radiation is most intense in the upper layers, the

photosensitivity and contrast gradient of the extreme

upper layers is decreased by decreasing the concentration

of polymerization initiator and increasing the concentra

tion, of polymerization inhibitor.

Figure 8. Compound Curve Relief Profile

Using a single exposure, the process can automatic

ally establish a compound shoulder angle consisting of a nearly perpendicular shoulder in the extreme upper layers near the printing surface and a broad shoulder angle jutting out from the printing element sidewall and angled obtusely toward the base. Decreasing the concentration of inhibitor in the lower strata produced a rounded buttress for the sidewall in the lower strata. The 33

process has the added advantage of using a single exposure for both line and halftone images. This structure yields sharp printing, good retention of image area shape during plate surface wear, and good adhesion of the printing element to the support. 34

Dupont-assigned U.S. Patent 3,249,436, issued May

3, 1966 by B. R. Halpern, describes a modification of the

exposure process for photopolymer relief plates imaged

by a single face exposure. The process employs a con

tinuous-tone mask in contact with the process negative

to modulate the exposure intensity above different halftone

values contained in the image.

To determine optimum exposure levels, plates were

made at various levels of exposure using a test object.

The image consisted of three lines of varying thickness

and two halftone tints of percent dot areas 30% and 70%

respectively.

After a short uniform exposure, the polymerized

image elements do not reach rather the base , but they

extend varying distances toward the base depending upon

the line width or the percent dot area, respectively. With

a uniform increase of exposure time to all areas it is seen

that finer lines and highlight dots reach the base, but

simultaneously the non-image wells in the shadow tone

become progressively shallower. Figure 12 shows that the

preferred exposure level for highlight areas differs from

that for shadow areas.

The author next explains an experimental procedure for determining the preferred exposure level for five

tonal bands from 10%-90%: The minimum exposure time to polymerize all the way down to the base is recorded for 35

in

Figure 9. The Cross-Section of Layer During Exposure 36

each band. A proportion is made between every tonal level's required exposure time and the required exposure time for the highlight tone. Taking each proportion separately, the common logarithm times negative one equals the required density for the mask in that tonal band.

The required mask is made from the halftone negative

negative- by contact printing the image onto continuous -tone working masking film with a spacer film between the object film and the unexposed masking film. The author reports that using diffusing film as the spacer, suitable results may be obtained with a point source. With a diffused light source, a clear spacer is substituted.

Examples cited in the patent show that relief depths in shadow areas can be increased from 0.00635 mm (0.25 mils) to 0.0305 mm (1.20 mils) and in midtones from 0.0305 mm

(1.20 mils) to 0.0457 mm (1.85 mils). The resultant plate reportedly obtained sharper printing and a longer plate life. 37

A review follows for N. L. Moore's article, "The

Photopolymer Challenge in Flexo", published in the 1978/

1979 Penrose Annual, volume 71, pp. 99-104.

The Dupont Cyrel photopolymer plate was introduced

in 1974 as the first elastomeric photopolymer printing

plate. The plates hold a distinct advantage over conven

tional molded rubber plates because each is made directly

from a negative without the intermediate image carriers of

photoengraved metal master, or phenolic matrix. These

image conversions in the conventional process introduce

errors which are cumulative in the final rubber plate,

thus limiting its quality. The principal errors are a

lack of uniformity of thickness and a change in overall

dimensions due to the cooling of materials thermoset or

vulcanized at high temperatures.

In addition, photopolymer plates have better ink

transfer characteristics than rubber. This fact plus the

im- greater uniformity of caliper allow the use of lower

pression settings than with rubber plates. This allows for

improved printing of fine halftone screens rulings and

Universal Product Codes.

Photopolymer plates offer a more intimate fit into

today's graphic arts materials mix due to greater compati

bility with computerized phototypesetting; however, Mr.

Moore concludes that there will always be a need for molded

rubber plates in flexography and that these may improve due

to the pressure of the newer technology. 38

A review follows for "The Chemistry of Photoelasto-

Plates" mers for Flexographic Printing by N. L. Moore,

published in Professional Printer, volume 22, number 5,

September 1978, pp. 9-13.

Mr. Moore probes the chemistry of elastomeric

photopolymerizable materials as disclosed in the patent

literature. (Product information from the manufacturers

is proprietary and well- guarded. )

The mechanism of photopolymerization for photo-

elastomeric plates is the same as their predecessors, the rigid photopolymer relief plates such as Dupont Dycril and BASF Nyloprint. These plates have been established

to contain a photoinitiator, an unsaturated vinyl monomer, particles of pre-formed polymers, a cross-linking agent, and a thermal inhibitor. The relative rigidity of the plate formulation is determined by the pre-formed poly mer and the extent to which it is cross- linked.

The photoelastomer family utilizes synthetic rubber compounds as the pre-formed polymer. The degree of cross- linking is of the order of magnitude of once every 50-100 monomer units in a linear chain. This provides the characteristics of flexibility and quick return to shape from a deforming stress.

In the Dupont Cyrel photopolymer flexo plate, the elastomeric chains are of one butadiene species which has been entrapped by polymeric chains of an acrylate 39

which propagates as a result of actinic radiation expo

sure. The polymerization is specifically induced by

the radical species formed by the dissociation of the

photoinitiator molecules when they absorb quanta.

For the Dupont Cyrel plate the pre-formed poly mer is probably poly(2-chloro-l ,3-butadiene) :

CI I CH0 - C = CH - CH0 2 2 n

The crosslinking agent is probably 1 ,1 ,1-trimethylolpropane triacrylate. The vinyl monomer in the Cyrel composition is probably the same as that in the Dycril composition, namely, polyethylene glycol dimethacrylate, and the photoinitiator is one of a group of widely used photo initiators, namely, the anthraquinone class. These consist of a carbonyl group bonded to an aromatic ring of six

carbon atoms . 40

In his article "Capped Plates - Strength/Quality",

from Paper, Film and Foil Converter, volume 54, no. 4,

April 1980, pp. 61-64, David Newberry describes his firm's

product, the Hercules Merigraph capped plate. This

material consists of two layers of slightly different

properties which can produce a relief image by photo

polymerization of an unmixed liquid monomeric composition

using imagewise exposure to actinic radiation.

The thickness and elastic moduli of the two layers

differ such that the upper layer, when polymerized, is

harder, more inelastic, and quite thin compared to the

lower layer. On the press the easily deformable lower

layer absorbs mechanical stress while the surface layer maintains the exact geometry of the image during impres

sion.

The photosensitivities of the two layers also

differ such that the intensity required to polymerize the

lower layer is less than the intensity required to poly merize the upper layer. As a result, the characteristic

shoulder angles for each layer differ. The higher photo

sensitivity of the lower layer permits polymerization by weaker rays present at angles closer to the plane of ex posure. Conversely, the lower photosensitivity of the upper layer permits polymerization only by the strongest rays which are present at angles closer to the normal to the plane of exposure. 41

Figure 10. Characteristic Dot Profile

of the Hercules Capped Plate

The author illustrates the shoulder angle variation which results from using two different photopolymers of

different photosensitivity imaged with identical expo

sures. The dot profiles produced differ in shoulder angle and relief depth. One photopolymer produced a

30 shoulder angle of and good depth of relief; the other

60 photopolymer produced a shoulder angle of and con siderably less depth of relief. This second formulation is more suitable for the lower strata where good adhe sion to the base is preferable to steep sided relief

forms . 42

With the photopolymer layers used in the Hercules capped-plate, the characteristic printing element profile has a segmented shoulder of two slopes, steep at the printing surface abruptly changing to a broad shoulder for the thicker sublayer.

The exposure unit casts two layers of liquid poly merizable materials on a support base of thick polyester

film. A release film is placed between the top and the film image and the machine provides uniform pressure to ensure contact and uniform caliper. A grid of cold

cathode light sources provide the broadly diffused expo

sure source.

The author claims less inadvertant printing of the

shoulder within highlight tones and in fine line positive printing areas and less fill-in within shadow tones and fine line reverse printing. 43

A review follows for "A Mathematical Model of

Photometric Oxygen Consumption in Photopolymer" by D. K.

Smith of Dupont, published in Photographic Science and

Engineering, vol. 12, no. 5, September/October 1968, pp. 263-266.

Photoinitiator-induced free radical polymerization

of monomer is inhibited by the presence of oxygen. The

rate of reaction between free radicals and oxygen is

far greater than that of the reaction between free radi

cals and monomer. As oxygen is consumed, it also diffus

es into the system at a rate determined by the partial pressures inside and outside the polymerizable layer, and

a diffusion constant. In order to achieve polymerization,

the rate of consumption of oxygen must be greater than

the rate of diffusion of oxygen into the layer. The period of time during which the oxygen is depleted is the

induction period.

During chain propagation the rate of reaction may

non- vary due to the presence of oxygen in image areas and the subsequent diffusion of it into the polymerizing region. The Dycril letterpress plate system utilizes two procedures to reduce the presence of oxygen. The plate is conditioned in a carbon dioxide atmosphere for 24 hours prior to subsequent processing. Next, the plate is ir radiated by light of a spectral distribution for which it

sensitive. The lack of absorption is only slightly strong 44

"bump" of this exposure by upper layers results in better exposure of lower layers in relation to upper layers.

The bump exposure is currently practiced using an optic ally dense filter transmitting primarily green light, a weakly absorbed band. The bump exposure is of an inten sity and duration such that it falls within the induction period. Oxygen is consumed but no polymerization results,

Simplifying, the bump exposure results in a change in the photosensitivity of the composition.

The author describes a mathematical model for the reactions as postulated:

1) Absorption of quanta by Initiator I forming

activated initiator I*

hv I*

2) Abstraction of hydrogen from monomer MH result

ing in two radical species

I* + MH - IH + M

3) The reaction of monomer radicals with oxygen

yielding relatively stable monomer-peroxy

radicals

M + 02 - M02 45

4) Simultaneous with this is the reaction of in

itiator radicals and oxygen yielding regener

ated initiator and peroxy radicals

IH + 02 - I + H02

5) The reaction of peroxy radicals with themselves

yielding hydrogen peroxide and oxygen

2H02 - H202 + 02

Differential equations are formulated for the reac

tion mechanism using the individual reaction rates, the

absorbance by the initiator of the spectral distribution

of intensities emitted, the diffusion rate, and the thick

ness of the layer.

Summing the differential equations led to contour

diagrams of the response surface for the yield variable,

oxygen concentration, with respect to the input variables

time and distance below the photopolymer surface. Two

contour diagrams are illustrated, one for light weakly

absorbed and one for light strongly absorbed by the photo

polymer layer. The plot for weakly absorbed radiation

showed more uniform consumption of oxygen at all depths

after a 2.3 minute bump exposure. The empirical data

obtained in the laboratory showed the same result at a bump exposure level of 2.4 minutes. 46

Chapter Three

HYPOTHESIS

It is hypothesized that the index of refraction of

a photopolymeric flexographic printing plate is the prim

ary factor in the determination of the plate shoulder angle.

It is further hypothesized that variations in the levels

of face exposure and back exposure may have limited influ ence in that determination.

It is further hypothesized that various regions of

the halftone printing scale will show a significant loss of undesirable dot gain if the shoulder angle is optimized for the particular tone or dot size. This hypothesis will lead to a change in the platemaking workflow in that a cont inuous-tone masking image will be superposed above the half tone negative during face exposure, or it may be used in a registered (well-alligned) position on the back of the plate during back exposure, or separate masks might be used in both positions,

A method for testing hypotheses is proposed whereby the exposures are varied in a controlled fashion using silver masks of known densities, and the resulting plates are measured for shoulder angle, placed on a press, and

"squeeze," printed with ever-decreasing

"kiss" The impression shall then be selected and 47

measured for its Effective Dot Area, from which the origi nal dot area on film will be subtracted to calculate the

dot gain for that plate and shoulder angle.

Regression analysis and analysis of variance can then

determine if shoulder angle is a significant factor in

determining a plate's characteristic dot gain. Tests of

significance should use an alpha value of 0.05 so that a

95%, confidence is placed on the results.

The Problem Proper

The aim of this research is to determine whether a

silver mask is a viable control of plate geometry in half

tone flexographic plates. If a linear relationship exists between face exposure and/or back exposure levels for par ticular dot sizes, the utilization of image masks may be further researched.

A further aim is to identify the relationship between shoulder angle and dot gain for particular dot sizes, so as to lessen the detrimental effects of dot gain, and ren der flexography a much-needed improvement in halftone and four-color process printing. 48

Chapter Four

METHODOLOGY

The first phase of the experiment involved the making of the films and plates used, and their measurement. The

second phase was the press run of those plates and the measurement of the printing. Following the selection of

sample prints, analysis of all measurements could proceed.

Test Object

The test image consisted of a lattice of solid print

1.125" 12.3125" ing which measured by and contained 12 patches of halftone tint of uniform value. Each tint

0.375" measured square and was centered between two equal

"bearers" 0.3125" measuring in width. Between each tint

0.625" was a solid measuring in length and contiguous with all other afoementioned bearers. The tints placed in each test patch were made by taking commercially available tint images of 100- line screen ruling, and contact printing with

exposure variations to produce a variety of tint values .

Tints were placed into the lattice using a zero-degree

screen angle. Letters coding the patches were placed out

side the lattice.

Distortion of the test object was implemented by

Nashua Control, Inc. of Nashua, NH on a Flexo-Repro

film printer using Dupont CRR matte-emulsion film. 49

"shrink" The elongation compensation factor, or was 0.966 as recommended by the plate manufacturer for a elate cali

0.112" 20.420" per of used on a press cylinder of repeat

length.

After distortion the film was measured for dot diame

ter using an American Optical gravurescope. Measuring

the dots in the undistorted direction and calculating the dot area from the area of a circle formula as a ratio with

the area of a halftone unit one hundreth of an inch squared

After the reproportioning, the film dot area was

determined. The film was measured using an American

Optical gravurescope with 600x optics and an engraved ret

0.0004" icle accurate to the nearest to determine the

dot diameters in the undistorted direction. Area CalCUla-

area tion utilized the formula for a circle A=TJr , and dot

followed by creating a ratio between the area of one dot

and the area of one halftone unit, one one-hundreth square.

Note that the use of the undistorted diameter, the circle

formula, and the square screen unit are consistent, and will calculate an identical result to one using an ellipse

in ratio to the rectangular screen unit.

The dot areas obtained were as follows, expressed in percent: 6.8. 16.1, 27.4, 35.2, 51.3, 56.6, 68.8, 70.8,

82.5, 91.2, and 94.9%.

The continuous tone masks consisted of three sheets

of uniformly toned photographic film developed in a con- 50

tinuous tone Dektol developer. The films containing transmission densities of 0.30 and 0.60 were selected for use and stipped into registration with duplicate test films made with Dupont CRR matte-emulsion film.

The matte surface ic recommended to allow rapid bleed ing of air during vacuum drawdown using the extremely smooth photopolymer plates.

All films and plates were punched for registration purposes. This allowed faster handling of the films and plates during the exposure sequence.

Exposure

(0.112" The plate material, Dupont Cyrel II FR112 thickness) was placed between the halftone test image and the back exposure contone mask. Then the face exposure contone mask was added above the halftone negative, and all materials were pinned together. The exposure pack was placed in the Cyrel 3040 exposure unit face down, cov ered with Kreen diffusion sheeting, and pressed together in a vacuum. Back exposure was made, vacuum released, the

and vacuum re-applied. exposure pack quickly inverted,

The face exposure followed adequate drawdown. The proced

as possible. Except for the ure was conducted as rapidly

proceedure was to recommendations silver masks, the according

Exposures used are listed in the Appendix in Table 21. 51

Plate Processing

Plate processing was entirely according to the

manufacturer's recommendations, using a washout solvent

of 75% perchloroethane and 25% butyl alcohol. The

processing followed this with infrared drying, immersion

in de-tack solution and a standard post-exposure step.

Determination of Shoulder Angle

A gravurescope manufactured by American Optical of

Buffalo, NY was used to determine the shoulder angle of

plates. This instrument has a 200x objective and an en

graved reticle with a limit of five microns of accuracy.

A distance scale on the focussing knob allows for depth

readings by taking the difference between the settings

where the top of the plate is in focus and where the

lowest depth is in focus.

Width measurements of halftone dots were taken in

the undistorted direction. Since a 100-line screen has

a screen pitch of 254 microns, the distance between any

two dots, 2y in Figure 11, is calculated as 254 microns minus the measured dot diameter in microns.

Depth measurements were taken by focussing on the

knob and then top, marking the focus position, focussing

on the depth at point F in Figure 11, the midpoint be

tween two dots. The difference between the two focus knob

positions, distance x, was read from the instrument. 52

The shoulder angle can be determined using:

^ 0 = arc tan I I x

where y = one-half DE

x = the relief depth at point F

VAl

VEB DIRECTION*

i i

Figure 11. Taking Measurements of a Halftone or Line Image

in a Relief Plate

Press Operation

It was of utmost importance to the experiment to

effects inherent the avoid excess impression in flexo

represents graphic process. The following methodology

the effects of cylinder run one way to separate out

variations in plates and out, and caliper mounting

tapes (stickyback) . 53

Printing was done at Union Industries, Providence,

60" RI on a Windmoller and Hoelscher six-color reverse-

angle doctor bladed central impression cylinder flexo

graphic printing press. The printing substrate was

white polyethylene, and the inks were polyamide process

inks in the process colors, cyan, magenta, and yellow.

Viscosity of the inks was 24 seconds in a #2 Zahn cup.

Automatic viscosity controllers were used.

The pressure in the inking nip (anilox to plate) was

kept constant. The press was run at 200 feet per minute while impression between the plate cylinder and impression

cylinder was slowly and evenly reduced. In so doing, indiv

idual plates came out of contact when the local radius of

"kiss" the plate cylinder was insufficient to print a im

pression. The resulting web of printed substrate thus con

tained a record of each plate's dot gain at all printing

pressures .

Measurement of Prints

By searching through the progression of prints from

high impression levels to low levels, it was possible to

select the kiss impression as the impression which immed

contact between the plate and iately preceded a loss of

most useful substrate. This criteria is for assigning

fixed and uniform impression levels to plates of varying

calipers mounted on imperfect plate cylinders. The 100% 54

solid printing bearer provided an easy image by which incom plete contact could be judged.

One print was selected from each plate, based on the described criteria. On each of these two densitometric readings were taken using a Macbeth RD-918 reflection den sitometer with SPI narrow-band sensitivity. One reading was taken in the solid bearer, one in the halftone patch.

Effective Dot Area was calculated using the Yule-Nielson equation and an assumed n-factor of 1.65. The equation is:

10(-Dt/n)

E.D.A =

10(-Vn)

where E.D.A.= effective dot area D = density of tint D = density of solid n = Yule-Nielson factor assumed to be 1.65

For each plate, the dot gain was calculated

as the difference between the dot are on film and the

Yule-Nielson Effective Dot Area just calculated. 55

Chapter Five

RESULTS

Regression analysis was carried out to determine the

plate shoulder angle as a function of face exposure, back

exposure, and the dot area of the image film, To en

hance any variation due to the levels of the two expo

sures , the exposure time in minutes was converted to its

equivalent logarithm to the base two. For example, an

exposure of 16 minutes converts to four exposure units.

A multiple linear regression was performed with the

following results:

Y = 33.54 + 1.68 ((2)X]-)

where Y = shoulder angle in degrees

xx = face exposure in minutes

Y = 37.06 + 0.30 ((2)Xz)

where x2 = back exposure in minutes

Y = 24.5 + 0.26 x3

where x~ = percent dot area on film

The square of the correlation coefficients for these three

regressions are:

(rx)2 0.232 = = 0.053

(r2)2 0.0392 = = 0.0015

(r3)2 0.502 = = 0.25

shoulder angle is Thus the primary factor in determining 56

the dot size, with some effect possible due to face expo

sure.

There is an average shoulder angle which will be

formed at any given dot size, but some variation can be produced by varying the face exposure. To further study

this effect, the face exposure data for each dot size was individually analyzed with the corresponding shoulder angle data to determine what relationship, if any, exists. Twelve regression analyses were performed, one on each of the

twelve dot sizes. There were between 20 and 32 samples in each regression calculation. An analysis of variance, or

ANOVA, was carried out to test whether the factor had a

95% probability of producing the linear relationship de

scribed by the regression formula, and testing the signif icance of the slope term in producing the observed varia tion in shoulder angle.

with the results of The data scatter diagrams , along regression and ANOVA are contained in Tables 3 to 11 in

the Appendix. An F-test was performed by comparing the mean square for lack of fit to the mean square for error at a 95% confidence level. The significance of the fit

Ho" of the data to the linear model is indicated by "ACC indicating the acceptance of a null hypothesis that the

of fit to a linear model variance resulting from lack is equal to or less than the variance due to error. This

test was rejected for only the data taken on the 6.87, dots. 57

A second F-test was performed to verify the signifi

cance of the b-^ term, or slope of the regression line.

This test indicates whether the dependent variable, shoul der angle is genuinely related to the independent variable, face exposure. In six out of the acceDtable 11 ANOVAs , significance was accepted for the regression term.

The b-j_ terms ranged from 0.36 to 1.04 with an

average of about 0.7, inferring that the shoulder angle

might change about 0.7 degree per minute of face exposure

variation.

A similar set of calculations was performed on back expo

sure vs. theta data, with results reported in Tables 12 to

19 in the Appendix. Here only two dot sizes produced a

95% confidence that the regression slope is a significant

term in the relationship. The slopes were 0.71 for the

27% dot on film and 1.10 for the 35% dot on film.

It was hypothesized that some relationship exists

between the plate shoulder angle and the dot gain

characteristic encountered under printing. Regres

sion analyses were carried out on print data to det

ermine if a linear relationship exists. Because

three inks were used, the regression was done for

each ink and each dot area on film as constants, with

shoulder angle, theta, as the independent variable, and 58

the difference between the Yule-Nielson effective dot

area and the dot area on the film (dot gain) as the

dependent variable. Therefore, 36 regression calculations

were carried out, the results of which are shown in

Table 20.

By examining the results an observation can be made

that shoulder angle seems to have no relationship to dot

gain. The basis for this judgement is the fact that

the slope of the line of regression is inconsistent be

tween different calculations. One would expect at least

some similarity amongst the slopes, labeled Bl in Table 20.

Even the differences in tack of the three inks would not

seriously alter the slope of this regression line for

any given dot size on film. Further support to the

rejection of this hypothesis is suggested by observing

that adjacent levels of dot area on film for a constant

ink color may have much different regression line slopes.

Amongst the possible sources of error in this phase

of the experiment is the fact that plates chosen at random were printed using different inks. Each of these inks may

have had different amounts of tack, even though their

viscosities were quite similar. In addition, the average

shoulder angle of plates assigned to each ink was differ

ent. The average for yellow plates was about 45 degrees,

for magenta, about 39 degrees, and for cyan, about 35 de

grees. 59

The overiding source for error in the experiment is impression variations. As logical as the method for as signing a standard level of impression appears, there is no assurance that impression was uniform between plates,

Judgemnt of impression in halftone areas was done on the basis of ink transfer from adjacent solid printing areas.

The relative calipers of the plate at those adjacent zones is not guaranteed to be the same. This effect was noticed in the observation of the impression series, where some halftones printed after the solid ceased printing, while other halftones ceased printing before the solid did.

In addition, the supposition that uniform impression levels can allow the study of lesser effects is a clearly academic activity, in view of the known strength of the relationship between impression and dot gain in flexo graphic printing.

The graph in Table 1 represents the optimum dot gain characteristic acheived with the particular plates, press, and ink with every plate location printing at kiss impres sion. This was not acheived with a single press setting.

Actual printing optimizes only the location of the smal lest plate radius at the expense of all locations of great er radius. Future research should should focus on tech niques which reduce the variations at the printing plate surface, particularly compressible mounting materials. 60

DOT GAIN

"H

O

>

m > <

a o H

O >

Table 1. Dot Gain Curve for Average Plate 61

DOT GAIN

-^N>ON)^-O>D0ON-^O>00ON)^a> 1 1 1 o - 1 1 1 1 1 I i i i i

> D O ? "H o > a o < IS) > ~ > on O

a -o a o o t> a o

A _ 2^1 O > F rn m

> > > O ? m < > O if)

> u > oa > o ? o m r

00 o > OQ >

0 > D

* D

(1 -

Table 2. Dot Gain Curves for Individual Inks 62

Notes

F. Dick, "Surface Energy - Ink/Metering Systems Interplay," Report of the Proceedings of the 21st Annual Meeting and Technical Forum"! Flexographic Technical Association, 1979, 52-56.

2 Rolls," J. P. Trungale, "Update on Anilox Paper, Film, & Foil Converter v. 54, no. 4 (April 1980) : 57-60.

30. G. Hauser & F. R. Ruckdeschel, "Yule-Nielsen Effect in Printing: A Physical Analysis," Applied Optics v. 17, no. 21 (November 1, 1978): 3377.

a Strength/Quality." D. A. Newberry, "Capped Plates - Paper, Film, & Foil Converter v. 54, no. 4 (April 1980): 61-64.

D. Bennett & J. Webster, "The Zedco Rubber Roller Machine," Photo- Engraving Proceedings of the Society of

- Optical Instrumentation Engineers Laser Printing v. T6~9 . Los Angeles, CA, 1979: 48-55.

J. S. Hamlin, "Processing of Photopolymer Printing Plates," Dupont assigned patent U.S. 3,146,106 (August 25, 1964).

H. E. Crawford, "Photopolymerizable Elements, Their Use," Preparation and Dupont assigned patent U.S. 2,993,789 (July 25, 1961).

L. Plambeck, Jr., Dupont assigned patent U.S. 2,964,401 (December 13, 1960).

9R. B. Kitson, "Process for Preparing Printing Photopolymerization," Elements by Two-Step Dupont assigned patent U.S. 3,129,098 (April 14, 1964).

1QFlexography. Principles and Practices 2nd ed. Brooklyn, NY: Flexographic Technical Association, Inc., 1970, 61.

11Ibid. , 61-62. 63

1 2 J. Calvert & J. Pitts, Photochemistry. New York: John Wiley and Sons, Inc., 1966, 3.

13 A. W. Smith & J. N. Cooper, Elements of Physics. New York: McGraw-Hill Book Co., 1972, 380.

A. Joseph, K. Pomeranz, J. Prince, & D. Sacher. Physics for Engineering Technology. New York: John Wiley and Sons, Inc., 1966, 673.

J. & cit. Calvert J. Pitts, op. , 5.

W. Smith & op. cit. 16A. J. N. Cooper, , 380.

J. Calvert & J. Pitts, op. cit., 17.

18 J. Kosar, Light Sensitive Systems. New York: John

and . Wiley Sons, Ine , 1965, 159.

19Ibid. , 158.

20Ibid. , 159. Example: from V. C. Chambers, A. B. Cohen, & D. W. Woodward, "Image-Forming Systems Based on Photopolymerization," Photographic Science and Engineering v. 7 (November/December 1963) : 360-368.

21 Kosar, op. cit. , 160.

22Ibid. , 159.

23 L. Plambeck, Jr., Dupont assigned patent U.S. 2,760,863 (1956).

Flexography: Principles and Practices, op. cit. , 174.

25 Ibid. , 175. 64

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Alles, F. P., & Smith, C. W. "Flexible Photopolymeriz Element." able Dupont assigned U.S. Patent 3,186,844. June 1, 1965.

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Halpern, B. R. "Photographic Masking Layer Used to Con Exposure." trol Dupont assigned patent U.S. 3,249,436

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APPENDIX 71

FACE EXPOSURE VS. SHOULDER ANGLE 6.8X DOT AREA ON FILM

UJ X

FACE EXPOSURE (minutes)

n : 21 X-BAR:

1 1 .3333 Y-BAR: 26.2857 bl :

0 .58525 b0: 19.6528

dot area 6.87. ANOYA -face exposure KIS, theta SOURCE SS MS F TABLE SIG? CRUDE 16498.24 21 SS b8 14509.71 1 14509.71 TOT SS 1988.53 20 SS bl 658.33 1 650.33 13.97 19' RES ID* 1338.19 70.43 SS ERROR 745.81 16 46.56 L.O.F. 593.19 3 197.73 4.25 3.24 REJ Ho

. vs . Theta For 6 8% Dot on Film Table 3 . Face Exposure 72

FACE EXPOSURE VS. SHOULDER ANGLE 16.1% DOT AREA ON FILM

> V

9 V "O %? y o z < tr UJ

3 O X

T r 8 12 16 20 24 32 FACE EXPOSURE (minutes)

n : 29 X-BAR: 8.65517 Y-BAR: 26.0172 bl :

1 .94133 b8: 17.0843

ANOUA dot area 16. IX face exp ws theta SOURCE SS v MS TABLE SI6? CRUDE 24890.53 29 SS b0 19630.01 1 19630.01 TOT SS 5260.52 28 SS bl 2652.97 1 2652.97 32.8 7 4.28 YES RESID 2607.55 27 96.58 SS ERROR 1902.91 23 82.74 L.O.F. 704.64 4 176.16 2.13 2.80 ACC H0

. Theta For 16 . 1% Dot on Film Table 4 . Face Exposure vs 73

FACE EXPOSURE VS SHOULDER ANGLE 27X DOT AREA ON FILM

(/> UJ UJ tr o UJ o

FACE EXPOSURE (MINUTES)

n : 30 X-BAR: 8.38333 Y-BAR:

31 .1166 bl : 0.36336 b0: 28.0704

dot area 27. 2V. ANOVA FOR FACE EXPOSURE VS THETA

SOURCE SS V MS TABLE SIG? CRUDE 30105.05 30 SS b0 29047.41 1 29047.41 TOT SS 1057.64 29

.40 SS bl 331 .52 1 331 .52 11 4.28 YES RESID 726.12 28 25.93 SS ERROR 669.15 23 29.09

L.O.F. 56.97 5 11 .39 0.39 2.64 ACC Ho Tabl . Theta For 27 . Tabie 5 ^ Face Exposure vs 4% Dot on Film 74

FACE EXPOSURE VS SHOULDER ANGLE 35% DOT AREA ON FILM

to UJ UJ tr

UJ Q v *

UJ r

20 -+ r i 8 12 16 20 24 28 32 FACE EXPOSURE (MINUTES) n : 31 X-BAR: 8.14516 Y-BAR: 34.0870 bl : 0.12549 b0: 33.0649

dot area 35. 2'/.

ANOVA FOR FACE EXPOSURE VS THETA

SOURCE SS V MS F TABLE SIG? CRUDE 37703.31 31 SS b0 36019.84 1 36019.84 TOT SS 1683.47 30 SS bl 40.37 1 40.37 0.64 4.26 NO RESID 1643.10 29 56.66

SS ERROR 1521 .64 24 63.40 L.O.F- 121 .46 5 24.29 0.38 2.62

Table 6 . Face Exposure vs . Theta For 35.2% Dot on Film 75

FACE EXPOSURE VS SHOULDER ANGLE 51 2% DOT AREA ON FILM

a 70 -

65 -

60 - a t UJ - UJ 55 IT O 0 50 - a a

ti a ,

- UJ 45 X a a 40 - B a a a B 35 - Q a a a a a a 30 -

B a

- 25 -i i 1i i i i i i i i i i i i i 8 12 16 20 24 28 32 FACE EXPOSURE (MINUTES)

31 X-BAR: 8. 14516 Y-BAR: 40.0709

bl : 0.67188 b0: 34.5983

DOT AREA ON FILM 51. 3Y. ANOVA FOR FACE EXPOSURE VS THETA SOURCE SS v MS TABLE SIG? CRUDE 52873.80 31 SS b0 49776. 16 1 49776.16 TOT SS 3097.64 30 SS bl 1157.27 1 1157.27 17.02 4.26 YES RESID 1940.38 29 66.91 SS ERROR 1632.20 24 68.01

L.O.F. 303.18 61 .64 0.91 2.62 ACC Ho

Table 7 . Face Exposure vs . Theta For 51.3% Dot on Film 76

FACE EXPOSURE VS SHOULDER ANGLE 56.3% DOT AREA ON FILM 7*^/ D

a 70 -

65 -

60 - D [] UJ - UJ 55 OL O UJ - Q 50 a a

a '1

- UJ 45 X 0 o 40 - a a o a = 35 - a a a a D a 30 -

B a

- 25 ~r i i i i i i i i i i i i i 12 16 20 24 28 32

FACE EXPOSURE (MINUTES)

n : 31 X-BAR: 8.14516 Y-BAR: 40.8677 bl :

-0.0454 b0:

41 .2382

DOT AREA ON FILM 56 . 3'/. ANOVA FOR FACE EXPOSURE VS THETA

SOURCE SS V MS TABLE SIG? CRUDE 57840. 93 31

SS b0 51775 .34 1 51775 .34

TOT SS 6065 .59 30

SS bl 5 .30 1 5 .30 0.02 4.26 NO

RESID 6d6d 28 29 208 .98

SS ERROR 5702 .19 24 237 .59

L.O.F. 358 .09 5 71 .62 0.30 2.62 ACC Ho

Table 8. Face Exposure vs. Theta For 56.3% Dot on Film 77

FACE EXPOSURE VS SHOULDER ANGLE 68.8% DOT AREA ON FILM

70 xr

65 -

60 -

55 -

UJ a UJ D 50 ? UJ Q a 4-5 -

UJ X 40 - a a

? a a a 35 - a B a 30

r~ - 25 ~\ 1 I I i 4 12 16 20 24 28 32 FACE EXPOSURE (MINUTES)

n : 32 X-BAR: 3.89062 Y-BAR: 43.6093

bl : 9. 65997 b0: 37.7417

DOT AREA ON FILM 68 . BY. ANOVA FOR FACE EXPOSURE VS THETA

SOURCE SS V MS TABLE SIG? CRUDE 65733 29 32

SS b0 60856 .88 1 60856 88 TOT SS 4876 41 31 SS bl 1356 73 1 1356 73 10.62 4.24 YES RESID 3519 68 30 117 32 SS ERROR 3194 47 25 127 78

L.O.F. 325 .21 5 65 .04 0.51 2.60 ACC Ho

Table 9 . Face Exposure vs . Theta For 68 . 8% Dot on Film 78

FACE EXPOSURE VS SHOULDER ANGLE 82% DOT AREA ON FILM

(/> UJ UJ ( e> UJ a

UJ X

FACE EXPOSURE (MINUTES)

N: 32 X-BAR: 8.89062 Y-BAR: 42.4812 bl : 0.51870 b0: 37.8696

DOT AREA ON FILM 82. 5X ANOVA FOR FACE EXPOSURE VS THETA

SOURCE SS v MS TABLE SIG? CRUDE 62388.98 32 SS b0 57749.01 1 57749.01 TOT SS 4639.97 31 SS bl 838.06 1 838.06 6.10 4.24 YES 126.73 RESID 3801 .91 30 SS ERROR 3436.75 25 137.47 L.O.F. 365.16 5 73.03 0.53 2.60 ACC Ho

Exposure vs . Theta For 82 . on Table 10 . Face 5% Dot Film 79

FACE EXPOSURE VS SHOULDER ANGLE 91% DOT AREA ON FILM ftrt n | U "

80 -

- 70 a

UJ UJ 60 - a a o UJ B a a D D 50 - a UJ X a a a a 40 - qO ? a a [1 a a a a a 30 - a a a

!- r- 20 H i i i i 1 I T i i i i i 8 12 16 20 24 28 32

FACE EXPOSURE (MINUTES) n : 29 X-BAR: 6.5 Y-BAR: 45.0310

bl :

-0.3591 b0: 47.3653

DOT AREA ON FILM 91.2'/. ANOVA FOR FACE EXPOSURE VS THETA

SOURCE SS V MS TABLE SIG? CRUDE 63936 61 29

SS b0 58806 .03 1 58806 03 TOT SS 5130 58 28 SS bl 173 73 1 173 73 Q.96 4.30 NO RESID 4956 85 27 183. 59 SS ERROR 3984 30 22 181 10 L.O.F. 972 56 5 194. 51 1.07 2.66 ACC Ho

Table 11. Face Exposure vs. Theta For 91.2% Dot on Film 80

BACK EXPOSURE VS. SHOULDER ANGLE 6.8% DOT AREA ON FILM oo -

a 50 -

a

- 40 II

G a

a

UJ a a X a 20 -

10 -

T" o -+ 2 4 6

BACK EXPOSURE (minut)

n : 20 X-BAR: 3.575 DOT AREA ON FILM 6. ay. Y-BAR: ANOUA FOR BACK EXPOSURE VS THETA 27.6 SIG^ bl : SOURCE SS v MS F TABLE

-9.065 CRUDE 16498.2 20 b0: SS b0 15235.2 1 15235.2 27.835 TOT SS 1263.04 19 R: SS bl 0.70562 1 0.70562 0.810286 4.60 NO

-0.023 RESID 1262.33 18 70.1296 RA2: SS Error 960.35 14 68.5964

1 . 100583 3.11 ACC Ho 0.0005 L.O.F. 301 .984 4 75.4960

. on Table 12 . Back Exposure vs . Theta For 6 8% Dot Film 81

BACK EXPOSURE VS. SHOULDER ANGLE 16.1% DOT AREA ON FILM

v e 9 V

o z < oc UJ a

_i

O I 00

BACK EXPOSURE (minutes)

n : 27 X-BAR: 3.02 DOT AREA ON FILM 16.1 '/ Y-BAR: ANOVA FOR BACK EXPOSURE VS. THETA 27.94 bl : SOURCE SS v MS TABLE SIG?

1 .2963 CRUDE 24890.53 27 b0: SS b0 21084.08 1 21084.08 24.0314 TOT SS 3806.45 26

R: SS bl 319.48 1 319.48 1 .95 4.32 NO 0.2897 RESID 3486.96 25 139.48 R*2: SS Error 3447.57 21 164.17 0.0839 L.O.F. 39.39 4 9.85 0.06 3.47 ACC Ho

Table 13 . Back Exposure vs . Theta For 16 . 1% Dot on Film 82

BACK EXPOSURE VS SHOULDER ANGLE 27% DOT AREA ON FILM ou - ii

B

- 40 ii a u a ? a D UJ - S a UJ 30 B II a: o UJ a a o

- UJ 20 X

10 -

- ...... 0 9 i , ,

BACK EXPOSURE (MINUTES)

n : 36 X-BAR: 2.79 DOT AREA ON FILM 27,4 X Y-BAR: ANOVA FOR BACK EXPOSURE VS. THETA 30.83 bl : SOURCE SS u Mb TABLE SIG?

0.7156 CRUDE 35341 .67 36 b0: SS b0 34206.50 1 34206 50 28.8273 TOT SS 1135.17 35 R: SS bl 125.93 1 125 93 4.18 4.17 YES 0.3331 RESID 1009.23 34 29 68 R*2: SS Error 904.28 30 30 14 0. 1 109 L.O.F. 104.95 4 26 24 0 .87 i9 ACC Ho

Exposure vs . Theta For 27 . 4% Dot on Film Table 14 . Back 83

BACK EXPOSURE VS SHOULDER ANGLE 35% DOT AREA ON FILM

in UJ UJ oc o UJ a

UJ x

BACK EXPOSURE (MINUTES)

n : 31 X-BAR: 3.03 DOT AREA ON FILM 35 . 2X EXPOSURE VS. THETA Y-BAR: ANOi'A FOR BACK 34.09 SIG"5 y Mb TABLE bl : SOURCE SS CRUDE 37703 31 31 1 . 1042 1 36019 34 b0: SS b0 36019 84 30 30.6854 TOT SS 1683 47 1 273 02 5.36 4 . 24 YES R: SS bl 273 02 29 48 64 0.4027 RESID 1410 45 25 50 93 R'2: SS Error 1273 21 L.O.F. 137 25 4 34 31 2.7o CC Ho 8 . 1622

Exposure vs . Theta For 35 .2% Dot on Film Table 15 . Back 84

BACK EXPOSURE VS SHOULDER ANGLE 51 % DOT AREA ON FILM

m UJ UJ tr o UJ a

UJ x

BACK EXPOSURE (MINUTES)

n : 31 X-BhP: 3.08 DOT AREA ON FILM 51.3* THETA Y-BAR: ANOVA FOR BACK EXPOSURE VS.

40 . 87 Mb TABLE SIG? bl : SOURCE SS V CRUDE 52873.80 31 1 . 1492 49776 16 b9: SS b0 49776. 16 1 36.5308 TOT SS 3097.64 30 70 2.89 4.24 R: SS bl 295.70 1 295 NO 62 0.3090 RESID 2801 .94 29 96 102 44 R*2: SS Error 2561 .07 25 240.87 4 *0 22 0.59 2.76 ACC Ho 0 .0955 L.O.F.

Table 16. Back Exposure vs. Theta For 51.3% Dot on Film 85

BACK EXPOSURE VS SHOULDER ANGLE 68.8% DOT AREA ON FILM

UJ UJ C O UJ a

UJ X

BACK EXPOSURE (MINUTES)

n ; 32 X-BAR: 3.23 DOT AREA ON FILM 68 .QV. Y-BAR: ANOVA FOR BACK EXPOSURE VS. THETA 43.61 V MS TABLE bl : SOURCE SS SIG? 0.6931 CRUDE 65733.29 32 b0: SS b0 60856.88 1 60856.88 TOT SS 4876.41 31 41 .3676 Rs SS bl 118.84 1 1 18.84 0 .74 4.24 NO 158.59 0.1561 RESID 4757.57 30 R"2: SS Error 4022.22 25 160.89 0.0244 L.O.F. 735.35 5 147.07 0.91 2.60 REJ Ho

vs . . Table 17 . Back Exposure Theta For 68 8% Dot on Film 86

BACK EXPOSURE VS SHOULDER ANGLE 82% DOT AREA ON FILM

in UJ UJ oc o UJ Q

UJ X

BACK EXPOSURE (MINUTES)

n : 32 X-BAR: 3.23 DOT AREA ON FILM 82.5% Y-8AR: ANOVA FOR BACK EXPOSURE VS. THETA 42.48 bl : SOURCE SS y MS F TABLE SIG? 62388 98 32 1 .1 124 CRUDE b0: SS b0 57749 01 1 57749 01 38.8834 TOT SS 4639 97 31

R: SS bl 306 09 1 306 09 .05 4.23 NO 0.2568 RESID 4333 88 30 144 46 R*2: S Error 3890 99 26 149 65 L.O.F. 442 89 4 110 72 2.74 0 .Q66Q ACC Ho

vs . . on Table 18 . Back Exposure Theta For 82 5% Dot Film 87

BACK EXPOSURE VS SHOULDER ANGLE 91% DOT AREA ON FILM 90 -

80 -

70 - (]

m UJ UJ tr 60 - o il a UJ a Q

DQ a

50 - a UJ X a a a a

- 40 a o

a a (i a a a 30 - a

20 - i i 1 1 T i 1

BACK EXPOSURE (MINUTES)

29 X-BAR:

2 . 38 DOT AREA ON FILM 91 .27. Y-BAR: ANOVA FOR BACK EXPOSURE VS. THETA 45.03 bl : SOURCE SS y MS TABLE SIG? 0.7846 CRUDE 63936.61 29 b0: SS b0 58806.03 1 58806.03 42.7721 TOT SS 5130.58 28

R: SS bl 121 .69 1 121.69 Z.66 4.28 NO 0.1540 RESID 5008.89 27 185.51 R-2: SS Error 4226.00 23 133.74

0 .0237 L.O.F. 782.89 4 195.72 1 .07 2.80 ACC Ho

Table 19. Back Exposure vs. Theta For 91.2% Dot on Film 88

Figure 12. Prints Showing Kiss Impression 89

REGRESSION CATEGORIZED BY PRINTING INK

CYAN

94.9 V. DOT AREA ON FILM>>>> 4.8 14.1 19 27.4 33.2 31.3 34.3 48.8 78.8 82.3 91 .2

Nl Ni Ni N: N: Nl N: N: N: N: N: N: No OF SAMPLES>>>>>>>>> 4 8 9 3 ? 9 9 18 1 1 1 1 1 1 1 1

XBAR: XBARi XBAR: XBAR: XBAR: XBARl XBAR: XBAR: XBARl XBAR: XBAR: XBARl 48.82 44.48 AUG THETA>>>>>>>>>>>>> 27.33 24. 3S 23.52 38.79 32.73 33.29 39.12 37 . ?4 39.27 38.53

YBAR: YBAR: YBAR: YBAR: YBAR: YBAR: YBAR: YBAR: YBAR: YBAR: Y8AR: YBAR: AUG PRINT Y. OOT GAIN>> 14.48 17.84 24.49 18.71 28.44 23.23 18.99 13.77 13.71 7.71 3.34 9 .44

B9: B8 t BB: B8: S3: 88: 88: B8: 88: B8l B8: B8:

-1 .89 >>>>>>>>>>>>>>>>>>>>>> 21 ,71 -5.4S -1 .18 1 .43 23.44 11 .87 32.33 8.48 2.47 -8.49 -2.22 REGRESSION

COEFFICIENTS SI : 81 : Bl : Bl : Bl : 81 i Bl : 81 : Bl : BH Bl l 81 :

.84 >>>>>>>>>>>>>>>>>>>>>> -8.27 8.38 1.81 8 .34 -8.89 8.32 -8.34 8.14 9.33 8.21 8 . 14 8

r : r : r : r : r : r : r : r i r : r : r : r :

>>>>>>>>>>>)>>>>>>>>>> -8.31 8.77 8.28 8.78 -9.8 7 9.38 -8.41 8.17 8.41 8. 38 8.37 8.12 CORRELATION

COEFFICIENTS r-2l r"2: r'!l r"2l r"2: r-2: r"2: r"2: p"2l r"2: r"2l r"2:

>>>>>>>>>>>>>>>>>>>>>>e1.8949 8.5888 8.8481 8.4924 8.8849 8.2388 8.1728 8.8388 8.1713 8.1433 8.1344 9 .9144

MAGENTA

'/. OOT AREA ONFILM>>>> 4.3 14.1 33.2 31.3 34.3 78.8 82.3 91 .2 94.9

N: N: Nl N: Nl N: Nl Nl Nl Nl Nl No OF SAMPLES) > >>>>>>> 4 9 9 18 9 11 18 9 9 18 18

XBAR: XBAR: XBARl XBARl XBARl XBARl XBARl XBARl XBARl XBARl XBARl XBARl AUG THETA>>>>>>>>>>>>> 21.37 23.33 24.49 38.98 32.49 37.29 44.91 44.9? 42.23 41.41 44.14 34.47

YBAR 1 YBAR 1 YBAR 1 YBAR 1 YBAR 1 YBAR 1 YBAR: YBAR I YBAR 1 YBAR 1 YBAR I YBAR 1 AUG OOT GAIN>> PRINT V. 9.27 28.97 22.48 21.83 21.78 23.93 14.32 13.13 11.88 4. 88 -1.88 -3.83

B81 88 1 88: 88 1 88 1 881 B9: 88 I 88 I B8 I 88 1 B81 >>>>>>>>>>>>>>>>>>>>>> -17.24 48.13 9.23 -8.7? 23.81 13.88 13.37 18.21 17.48 7.43 -2.17 -4.2? REGRESSION COEFFICIENTS 81: Bl 1 Bl 1 81: Bit Bl: Bl : 81 1 Bl 1 Bli an Bl 1 >>>>>>>>>>>>>>>>>>>>>> 1.24 -1.84 8.31 8.79 -8.19 8.21 3.84 8.87 -8.14 -8.82 8.82

>>>>>>>>>>>>>>>>>>>>>> 9.83 8.43 9.43 -8.19 8.37 9.14 -9.39 -8.84 8.82 8.13 CORRELATION

COEFFICIENTS r"2: r'2l r"2: r"2l r'2: r-2: r-2: r'2: r"2: r"2l r'2: r "2l

>>>>>>>>>>>>>>>>>>>>>> 9. 7243 .2177 .1888 8.1828 .8343 .1389 8.9347 9.9238 8.1523 8.8832 8.8984 8.8223

YELLOW

Y. OOT AREA ON FIU1>>>> 4.8 14.1 19 27.4 51.3 54.3 48.8 91 .2

Nl Nl Nl Nl Nl N: Nl Nl Nl Nl Nl Nl No OF SAMPLES) >> >>>>>> 9 ? 9 7 7 7 3 9 9 a 8 3

XBARi XBARl XBARl XBARi XBARi XBARl XBARi XBARl XBARi XBARl XBARl XBARl AUG THETA>>>>>>>>>>>>> 32.34 34.31 39.22 33.77 3?. 13 47.41 39.48 58.38 48.83 58.33 37.13 44.94

YBAR I YBAR I YBARl YBARl YBARl YBARl YBARl YBARl YBARi YBARl YBARl YBARl 13.34 AUG PRINT /. DOT GAIN>> 12.24 14.38 19.84 13.83 13.47 14.1? 8.78 4.21 1.93 2.29 8.34

88: 88 1 B81 B8 : B81 88 1 B8 1 881 set B81 881 B81 >>>>>>>>>>>>>>>>>>>>>> 18.18 17.38 14.12 22.77 13.18 22.23 9.73 1.33 -5.88 11.8? -1 .91 -3.34 REGRESSION COEFFICIENTS Bl 1 Bl : Bl 1 81 1 Bl 1 Bl 1 Bl 1 Bt 1 81 1 BIl Bl 1 Bl : 9.14 >>>>>>>>>>>>>>>>>>>>>> 8.87 -9.83 8.13 -8.1? 8.81 -8.17 e.13 8.21 -9.28 9.97 8.9?

r 1 r 1 r 1

>>>>>>>>>>>>>>>>>>>>>> 9.1? -8.95 9.24 -8.27 )1 -8.48 8.38 8.4? -8.78 8.32 8 .49 CORRELATION COEFFICIENTS r"2i r"2i p"2i r'2i r"2i p-2i r-2: r'2: r-2: r'2l r-2: r"2:

.232? .2314 >>>>>>>>>>>>>>>>>>>>>>9. 8343 .8823 8.8378 8.8748 8.9992 1.9334 9.2444 8.4843 8.1888 8.3448

Table 20 . Regressions Categorized by Printing Ink 90

EXPOSURE TIMES IN MINUTES

FACEXP BACKEXP REPS FACEXP BACKEXP REPS

8 .5 3.00 8.0 0.25

1 .0 1 .00 * 8.0 0.50

1 .0 1 .00 * 8.0 2.00

1 .0 2.00 8.0 4.00

* 1 .0 4.00 3.0 3.00

1 .0 4.00 # 3.0 3.00 * 2.0 0 . 50 16.0 2.00 2.0 0.50 * 16.0 4.00

2.0 1 .00 16.0 4.00 2.0 2.00 * 16.0 3.00 2.0 2.00 * 32.0 2.00 2.0 2.00 * 32.0 4.00 4.0 0.25 32.0 8.00 4.0 0.50

4.9 1 .00 *

4.0 1 .00 * 4.0 2.00 4.0 4.00 4.0 8.0 0

Tab].e 21. EExperimental;xp< Exposure Levels 91

FILM DOT AREA VS SHOULDER ANGLE Avg 6c std exposure data 3h - a 52 -

s 50 - S s

48 - ' a * 46 -

44 - a ^

x "a a 42 - a m LU LU 40 - or a- o o o 38 - UJ 0 Q y - 0 36 S* o s o

- "a 34 ,

32 - ^'u S

^r& 30 - a 28 -

0 26 - 0

94. " 1 1 1 "T i i i i i c; 20 40 60 80 1 C

FILM DOT AREA a AVG FOR ENTIRE TEST o STD CONDITIONS : 2 m.rv. back e p-

8 *.n. -fd-ee e*p REGRESSION FOR DOT AREA ON FILM VS THETA

bl : 0.26 N: b8: 356 24.50 X-BAR: r : 53.10 0.52

Y-BAR: r*2: 38.18 0.26578

SUMMARY ANOVA FOR DOT AREA ON FILM VS THETA

SOURCE SS V MS TABLE SIG? CRUDE 590824.5? 356 SS be 519054.05 1 519054. 85 TOT SS 71770.54 355 SS bl 19075.39 1 19975. 39 127.86 <3.9 YES RESID 52695.15 354 148 86 SS ERROR 51320.84 344 149. 19

L.O.F. 1374.31 18 137 43 0.92 <1 .91 ACC Ho

Table 22. Film Dot Area vs. Theta Summary 92

GLOSSARY

anamorphic distortion- the result of resizing an image using a different factor in each of two directions at right angles to one another

anilox roll- an engraved cylinderwhich meters ink to the flexographic printing plate

cell- a tiny geometric volume removed from a uniform surface

count- cell the number of cells ner linear unit along the line of the nearest adjacency of two cells

continuous tone- a classification of imagery describing the capacity to absorb light at continuous levels

copy- the original image to be reproduced

doctor- to shear and duct a liouid

doctor blade- a flexible blade used to wipe the surface of an engraved cylinder

dot- one of the tiny elements which make up a halftone

area- dot the fraction of the local area covered by image areas in a halftone image

dot gain- the increase in the dot area of a halftone caused by the printing process

dot gain curve- the plot of the dot area of the film vs. the dot gain

rubber- elastomer- any resiliant, like material not con taining voids

foam- elastomeric any resiliant, rubber-like material containing voids

flexography- relief printing using elastomeric plates, ink, and an anilox roll

floor- the portion of a relief plate where the relief area is uniformly flat

fountain roll- the press cylinder which has a rubber surface used to wipe an anilox roll 93

gravure- the printing process which uses an engraved cell or groove as the image-forming element

halftone image- an image which has been transformed into an array of tiny dots from a continuous tone image

image- any picture, or geometric object recorded in two dimensions

area- image the locations in an image where ink is printed

imagewise exposure- the transfer of an image using photographic means

impression- the amount of decrease in distance between the plate cylinder's axis of rotation and that of the impression cylinder beyond the point of coin cidental tangents

impression cylinder- the press cylinder which supports the print substrate during ink transfer kiss- the condition of coincidental tangents; the point at which a plate prints with minimum pressure

monomer- unreacted polymer

Mylar- duPont tradename for polyester sheeting

areas are low negative- an image in which image density

and non- image areas are high density

curvature- plane at which a neutral plane of the unstrained stressed material is

area- an image where ink non- image the locations in is not printed

to photopolymer- reacted an organic chemical which has of quanta form molecular chains upon absorption

which transfers ink to the plate- the surface printing

substrate

cylinder- the press cylinder which supports the plate

plate

cells- shaped an inverted truncated quad cells like pyramid 94

relief area- the non-image area of a relief plate

relief plate- a printing plate of uniform thickness at all image areas, and lowered (relieved) in non- image areas

reverse angle doctor blade- a doctor blade used at an angle which is a mirror- image of the angle used in gravure

sets screen- (1) a fine mesh usually consisting of two of parallel lines at right angles to one another; (2) the structure of a halftone image

screen angle- the angle between the horizontal and the line which joins a halftone dot center to that of its nearest adjacent dot

screen ruling- the number of halftone dots per linear unit measured along a line joining a dot center to that of its nearest adjacent dot

shoulder- the sloped portion of a relief plate immed iately adjacent to an image area

shoulder angle- the angle which describes the sidewall s deviation from vertical shrink- the reduction factor in anamorphic distortion

tint- a halftone image of uniform dot area

tone reproduction curve- a plot of the densities of the original vs. the densities of the reproduction

top- the printing surface of a relief plate

transfer, ink- the movement of ink from one surface to another viscosity- the resistance of a liquid to flow visco-elastic cohesion- a manifestation of inter- molecular forces occurring at the surface of a liquid

Zahn cup- a vessel used to measure viscosity by timing the draining of a standard volume of the sample liquid through a standard- sized hole