Defect Reduction In Plastic Packaging Industry

Defect reduction in plastic packaging industry

Jutatip Thaprasop1 Natcha Thawesaengskulthai2 2,3 Department of Industrial Engineering, Faculty of Engineering, Chulalongkorn University, Pathumwan, Bangkok, 10330 E-mail: [email protected], [email protected]

Abstract The aim of this study is to identifying root causes of the defect problem and generating a solution to reduce the percentage of defect by using quality techniques and continuous improvement. The methodology used in this study composed of five steps: quality problem definition, root causes identification, problem-solving generation, selection of alternative application, and evaluation. Along with these steps, the employed quality tools were brainstorming, process flowchart, graph, scatter diagram, cause-and-effect diagram, questionnaire, pareto diagram, affinity diagram, tree diagram, design of experiment, and control chart. Key performances indicators of this study were the defect quantity. The result indicated that the applied quality improvement process together with quality techniques could increase the effectiveness of gravure printing process which reduced the quantity of defective products and processing time. Moreover, the result from this study developed a system of continuous improvement for the company including a holistic process flowchart, a detail work instruction for defect reduction, a set of quality team, and a criteria standard to control the product quality.

Keywords: defect reduction, plastic packaging, continuous improvement, design of experiment

1. INTRODUCTION Plastic packaging is an important product in a plastic industry which contributes to almost forty percent of domestic packaging markets. (Siam City Bank Public, 2550) It plays some major roles in everyday life because of its superior properties over other packaging such as toughness, light weight, water resistance, chemicals durability, and a wide range of its application. However, there are some barriers to the growth of this industry due to the high cost of poor quality. Consequently the entrepreneurs have to adjust their business for surviving. The quality of plastic packaging depends on many factors including the quality of raw materials, types of production process, specific or flexible machine, and the efficiency of human labor and machine. Most of the Thai manufactures as well as a plastic packaging production are labor intensive which results in low throughputs. In addition, the highly efficient machines are expensive which the industry cans hardly effort to buy. Hence, continuous improvement is vital to solve these two dilemmas of the Thai plastic packaging industry. Packaging is the largest segment of the gravure printing industry. It will continue to be a growth area into the future, because of the needs for packaging, and the cost effectiveness of gravure packaging. (Gravure Education Foundation and Gravure Association of America, 2003) This research was carried out in a flexible packaging manufacturer which produces various plastic packaging and sanitary bag. The preliminary survey indicated that the printing process, named the gravure printing process, had the most impact to company product quality. The main problem of gravure printing process was its high percentage of defect. The main purpose of this study is to develop a quality improvement approach in plastic packaging factory. The study focuses on identifying root causes of the defect problem in gravure printing and generating a solution to reduce the percentage of defect by using quality techniques and continuous improvement.

1.1 Gravure Printing Process Gravure printing (or rotogravure printing) is a type of intaglio printing process. It works by engraving the image onto a copper cylinder. On the surface of a gravure cylinder, the engraved image is composed of small recessed cells that act as tiny wells. Depth and size of each cell control the amount of ink that is transferred to the plastic film via processes of pressure, osmosis, and electrostatic pull. (Wikipedia, 2008) Each gravure printing machine consists of four parts: an unwinder, a press, a dryer, and a rewinder (see figure 1). First, a web is handled from the unwinding reel through the first printing unit on a press. The press has one printing unit for each color. The number of units varies depending on what colors are required to produce the final image. There are four basic components in each color unit: an engraved cylinder (whose circumference can change according to the layout of the job), an ink fountain, a doctor blade, and an impression roller (see figure 2). While the press is in operation, the engraved cylinder is partially immersed in the ink fountain, filling the recessed cells. As the cylinder rotates, it draws ink out of the fountain with it. Acting as a squeegee, the doctor blade scrapes the cylinder before it makes contact with the substrate, removing ink from the non-printing (non-recessed) areas. Next, the substrate gets sandwiched between the impression roller and the gravure cylinder. This is where the ink gets transferred from the recessed cells to the substrate. Then the substrate goes through a dryer because it must be completely dry before going through the next color unit and absorbing another coat of ink. (Wikipedia, 2008) Finally, the web is handled again from the last printing unit to the rewinding reel until finish process. Dryer

Press Rewinding Unwinding

Figure 1 gravure printing process

Figure 2 Gravure process in a printing unit (Wikipedia, 2008)

1.2 Continuous Improvement Continuous improvement is the never ending process to the improvements of quality committed by everybody in an organization. A quality team composed of management members, managers, and operators is formed with an aim to coordinate in continuous improvement activities. The complexity of problem solving in the continuous improvement requires use of quality tools to assist in the organization and analysis of information and data surrounding the concern. A proposed classification scheme for problem-solving tools allows the user to identify the correct tool at the proper time in the problem-solving process. This may assist the problem solver to efficiently and effectively work toward problem solution. (Hagemeyer and Gershenson, 2006) The most popular technique for continuous improvement is Shewhart Cycle of Deming Cycle, known as PDCA cycle. Using the quality tools such as seven QC tools and seven management tools (7 new QC tools) will help to improve the quality and productivity of a company. The seven QC tools consist of histogram, check sheet, Pareto chart, cause-and-effect diagram, control chart, scatter diagram, and graph, are mainly used to obtain and analyze data for situations when the objectives are known and rely on visual displays. The seven management tools consist of affinity diagram, relationship diagram, tree diagram, matrix chart, matrix data analysis chart, arrow diagram, and process decision program chart, can be used by managers to understand the causes of the problems and opportunities that exist in improvement of operations and processes. (Chandra, 1993)

1.3 Design of Experiment To be able to base decisions on facts and to perform quality improvements it is necessary to collect and treat data systematically. However, the facts naturally accumulated during product and process operation are not enough. Knowledge accumulation has to begin earlier and it has to be accelerated. For that purpose experiments also have to be planned and performed early in product and process development Design of experiment (DOE) therefore is a very important stage in quality improvement. (Bergman, B. and Klefsjö, B., 1994) It deals with product characteristics (parameters) and process variable settings that result in product performance with minimum variation while in use. (Chandra, 1993) It has been a very useful tool to design and analyze complicated industrial design problems. It helps us to understand process characteristics and to investigate how inputs affect responses based on statistical backgrounds. (Park and Ahn, 2004) The objectives of the experiment in this research include the determining which variables are influential on the number of defective and where to set them.

2. RESEARCH METHODOLOGY The methodology used in this study composed of five steps: quality problem definition, root causes identification, problem-solving generation, selection of alternative application, and evaluation. Along with these steps, the employed quality tools shown in table 1. Table 1 Tools or techniques used in the research steps

Research Methodology Tools/Techniques Use Phase I: Quality problem definition Process flowchart To provide a diagrammatic picture of the focused process Graph To show the quantitative data Brainstorming To summarize the focused problem Phase II: Root causes identification Cause-and-effect diagram To determine and break down the potential causes of the problem: material, machine, method, people, etc. Questionnaire To relate the important priorities of problem causes by individual importance ranking Pareto diagram To focus efforts on the root causes of problem from importance ranking Brainstorming To summarize the root causes of the problem Phase III: Problem-solving generation Brainstorming To find the ranking of possibility causes of the focused problem from members in the team Tree diagram To generate the problem-solving alternatives Affinity diagram To allow the team to summarize groupings among problem-solving alternatives Design of Experiment To estimate the main effects and interactions of factors which affect the focused problem Phase IV: Selection of alternative application Brainstorming To develop the action plan and implement Phase V: Evaluation Brainstorming To evaluate and summarize the results

3. QUALITY PROBLEM DEFINITION PHASE This phase describes the quality improvement team members, the production process and the result of choosing a problem solving case.

3.1 Quality improvement team During this study, a quality improvement team had been set and worked closely with this quality improvement project. The members of the team included the printing manager, the production planning leader, the printing leader, the gravure cylinder leader, and the printing machine leader.

3.2 Production process The process flowchart of plastic packaging production was drawn from the observation and interview with production leaders and operators. It was broken down in seven processes including the material inspection, film blowing, slitting, gravure printing, conversion, quality inspection, and packing. Figure 3 shows the flowchart of overall plastic packaging production processes in the case study.

Material Blowing Supplier inspection

Slitting Gravure Conversion Printing

Quality Packing Inspection Customer

Figure 3 Overall plastic packaging production processes

3.3 Quality problem identification The historical data of the case study was summarized and transformed into graphical illustrations shown in figure 4 and 5. Figure 4 Pareto chart of defect weight in each process

Figure 5 Pareto chart of defect type and defect weight

Figure 4 and 5 show a Pareto chart of defect weight in each process and defect type respectively ranked by frequency of occurrences. The process which effected to the highest percentage of defect was the printing process and the defect type occurred between production process was the defect code P2 (the overlapping printing defect). Using Pareto chart with “80-20 rule” as a tool to identify and select the quality problem and brainstorming could summarize that the quality problem focused in this study was the overlapping printing problem from gravure printing process.

3.4 Problem definition After the existence of quality problem in the production process was revealed from the former section, a target to reduce the percentage of overlapping printing defect was set. Figure 6 shows the process flowchart of gravure printing from investigation in the case study. It enabled all employees in the gravure printing process a better understanding in the consequences of overall processes. There are various types of plastic film used in the gravure printing of the case study include LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), HDPE (High Density Polyethylene, PP (Polypropylene), OPP (Oriented Polypropylene), PET (Polyethylene Terephthalate), and PVC (Polyvinyl Chloride). For the PE plastic film will be purchased from supplier as PE resin. It will be passed through blowing process to convert to plastic film roll. Another plastic film was purchased as plastic film roll which could bring to printing process immediately. Web inspection

Gravure cylinder setting

Machine Problem solving Ink formulation Web loading Printing operation Quality inspection preparation between operation

Impression roll setting

Machine and equipment checking Figure 6 Gravure printing process

4. ROOT CAUSES IDENTIFICATION PHASE The improvement team which the printing manager was the leader constructed a cause-and-effect or fishbone diagram for overlapping printing problem. The goal of this tool was to reduce the percentage of overlapping printing defect. After brainstorming, the causes were broken down in any sources of problem including material, machine, method, people, and other shown in figure 7.

Machine Material Thickness variation (C1) Shaft rotating to control Nonsmooth of film surface (C2) Lack of preventive maintenance (C6) gravure cylinder (C9) Bad quality web Failure Width variation (C4) Impression roll (C8) Too tight web winding in a roll (C3) Old (C5) Doctor blade (C7)

Overlapping printing defect

Machine stop due to Preemption between printing (C16) Force of doctor blade on gravure cylinder (C13) any problems (C20) Degree of doctor blade (C14) Lack of motivation to work (C18) Lack of experience Improper parameter setting Lack of training (C17) Web loading between operating (C15) Roller force on gravure cylinder (C12) Other Lack of concern about print Temperature on header of quality control (C19) print unit (C11) Web tension (C10) People Method

Figure 7 Cause-and-effect diagram of overlapping printing defect

After all the possible causes of overlapping printing defect had been generate by brainstorming, the questionnaire was conducted within an improvement team of 5 members to voting. The score of each item was between 0-10, which “10” was the most effect to the overlapping printing defect and “0” was no effect to that problem. The result from questionnaire could be constructed as a Pareto chart shown in figure 8. Figure 8 Pareto chart of overlapping printing defect causes

The criterion which set from team members to identify the root causes was the voting score equal to or above 25 points. Therefore the top nine causes were pointed out as the root causes of the overlapping printing defect problem were (1) the improper temperature level on heater of the printing unit, (2) the improper web tension, (3) the lack of adequate training for operators, (4) the lack of preventive maintenance, (5) the lack of concern about print quality control, (6) too tight web winding in a roll, (7) the nonuniformity of film thickness in a roll, (8) the lack of motivation to work, and (9) the nonsmooth of film surface.

5. PROBLEM-SOLVING GENERATION PHASE In this phase the four causes out of the nine root causes, included the improper temperature level on heater of the printing unit, the improper web tension, the lack of adequate training for operators and the lack of preventive maintenance, were selected to implement first. The brainstorming session could generate the solutions for those four root causes shown in figure 9.

Problem Causes Solutions Improper temperature level Set the suitable level of on the heater of printing temperature unit

Set the suitable level of web Improper web tension tension

Overlapping printing defect Lack of adequate training Training the right method for operators for operators

Construct the preventive Lack of preventive maintenance implementing maintenance in the process

Figure 9 Tree diagram of overlapping printing defect solutions

Employed the affinity diagram to grouping all the solutions together could summarize in two alternatives shown in figure 10. These alternatives were the design of experiment on three factors (temperature on print unit no.7 and 8, and web tension) and develop the work instruction to control the variation of printing operation. Problem solving for overlapping printing defect

Design of experiment Work instructions development

Set the suitable temperature Training the right method for level of printing unit no.7 operators

Set the suitable temperature Construct the preventive level of printing unit no.8 maintenance implementing in the process Set the suitable web tension

Figure 10 Affinity diagram of problem solving for overlapping printing defect

5.1 Design of experiment A factorial design with three factors, each at two levels is employed because it has a greatly simplified analysis (Montgomery, 2005) and suits for the limitations of this research. The objective of this design was to determine where to set the influential so that variability in the number of overlapping printing defect was small.

Factor: There were three factors of interest in this experiment included the temperature level on header of printing and web tension. The temperature level was only set in the unit no.7 and 8 by adjust at the heater control board of printing machine. The outcomes of that setting were the customer requirement to the high gloss of color of image printed on those printing unit. The web tension was controlled by tension control board. It was the important parameter in the gravure printing to help the process operation. Two levels were set for each factor. For each factor a low and a high value were chosen (see table 2).

Table 2 Factors and levels of factors in 23 factorial experiment

Symbol Factor Level Unit Low (-1) High (+1) A Temperature of printing unit no.7 50 70 °C B Temperature of printing unit no.8 50 70 °C C Web tension 15 20 N/mm.

Response: The parameter observed in this experiment was the number of overlapping printing defect. It was not the percentage of the defect because the defect inspection of the case study would finish after packing process. Therefore it was not detected in a real time, but it could be detected the number of defect in every roll after finished print instead by using the specific inspection machine which works by using the image processing principle.

Experiment Plan: The experiment was done under the randomization by using MINITAB program to generate the random sequence such as table shown in table 3. The sample size was three replicated due to the limited plastic film roll of the case study, therefore the total runs of this experiment was equal to 3x23 = 24. The sequence and output of each run shown in the figure 3.

Table 3 The 23 design for the overlapping printing defect experiment A B C Run Number of defect (Temp1) (Temp2) (Tension) 1 -1 1 -1 11 2 -1 -1 1 8 3 -1 -1 1 10 4 1 1 -1 13 5 -1 -1 -1 9 6 -1 1 1 15 7 -1 -1 -1 7 8 1 -1 -1 11 9 1 1 1 18 10 -1 1 -1 13 11 -1 1 1 16 12 -1 1 1 12 13 1 -1 -1 13 14 1 1 1 17 15 1 -1 1 12 16 -1 1 -1 13 17 1 -1 1 15 18 1 1 -1 12 19 -1 -1 -1 5 20 1 1 -1 10 21 1 1 1 14 22 -1 -1 1 9 23 1 -1 1 13 24 1 -1 -1 9

Data analysis: The data in table 3 was analyzed by using the ANOVA in MINITAB processing shown in the table 4. From the result of ANOVA could interpret as shown in the figure 11 that the main effects included the temperature level on the heater of printing unit no.7, the temperature level on the heater of printing unit no.8 and the web tension, and the interaction between the temperature level on the heater of printing unit no.7 and 8 affected to the number of defect.

Table 4 Analysis of variance for overlapping printing defect in coded units

Source DF Seq SS Adj SS Adj MS F P Main Effects 3 157.458 157.458 52.486 17.74 0.000 2-Way Interactions 3 23.792 23.792 7.931 2.68 0.082 3-Way Interactions 1 2.042 2.042 2.042 0.69 0.418 Residual Error 16 47.333 47.333 2.958 Pure Error 16 47.333 47.333 2.958 Total 23 230.625

Figure 11 Normal probability plot of standardized effects Consider the main effects and interaction plot in figure 12, the optimal level of each factor were 50 °C of the printing unit no.7, 50 °C of the printing unit no.8, and 15 N/mm. of the web tension. These levels of factors would be set after the work instruction development.

Figure 12 Main effects plot and interaction plot for overlapping printing defect

5.2 Work instructions development Work instructions were developed from the standard steps of printing process shown in figure 6 include the web inspection process, the gravure cylinder setting process, the ink formulation process, impression roll setting process, machine and equipment checking process, web loading process, machine preparation process, printing operation process, problem solving between operation process, and quality inspection process. Figure 13 provides an approach to establish the work instruction step.

Printing machine leader and gravure cylinder leader construct the work instructions in details

Printing leader check the description Printing machine leader and incorrect and sequence of the work instruction gravure cylinder leader revise

correct

Printing manager approve no

Conduct the work instruction to implement in the printing process

Figure 13 Work instruction development steps

6. SELECTION OF ALTERNATIVE APPLICATION PHASE This phase describes the generation of action plan for problem solving shown in table 6. The action plan was used to assign each action to the responsible persons, who were mainly in the printing process in the case study.

Table 5 Action plan for solving the overlapping printing defect

No. Task description Responsible person 1. Printing step training Printing leader 2. Reconsider the overall steps of each operator Printing machine leader 3. Perform along with the established work instruction Printing machine leader 4. Performance evaluation Team members

7. EVALUATION PHASE The results from the problem solving application include the parameter setting of the temperature level and web tension, and the work instructions performance will be described in this phase.

7.1 Result After finished due date of the problem solving implementation, the outcome was revealed that the average number, the average percentage of the overlapping printing defect, and the average processing time of printing operation were reduced 27.12 points per roll per month, 14.94 percent per month, and 8.87 minutes per roll respectively (see table 6).

Table 6 Result from problem solving implementation

Outcome Before After Difference 1) Average number of overlapping printing defect (points/roll/month) 35.14 8.02 27.12 2) Average percentage of overlapping printing defect (%/month) 27.60 12.66 14.94 3) Average processing time of printing (minutes/roll) 37.26 28.39 8.87

7.2 Application of tools Through this study, the quality tools were employed for different objectives. Table 7 summarizes the use of tools in three aspects: ease of use, suitability and implementation probability after study.

Table 7 Evaluation of tools in three aspects

Tools Ease of use Suitability Implementation probability after study Process flowchart Easy Suitable Probable Graph Easy Suitable Probable Brainstorming Easy Suitable Probable Pareto diagram Not easy Suitable Probable Cause-and-effect diagram Easy Suitable Probable Questionnaire Not easy Suitable Probable Tree diagram Easy Suitable Probable Affinity diagram Easy Probably suitable Probable (Design of experiment Difficult Suitable Not probable due to statistical analysis requirment

8. CONCLUSION In this study the problem solving alternatives of overlapping printing defect in gravure printing process of plastic packaging were generated including the design of experiment and the work instruction development. From the design of experiment found that the temperature level on the heater of printing unit no.7, temperature level on the heater of printing unit no.8, web tension level, and interaction between the temperature level of printing unit no.7 and 8 affect to the number of overlapping printing defect, and the suitable level of those factors were all in low level. After the work instruction development and implementation revealed the average number of defect and the average percentage of defect were decreased. The unexpected outcome was the reduction of the average processing time. However, there are some limitations due to this research scope of the focused process (gravure printing) and the focused machine (only one gravure printing machine was observed).

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