Defect reduction in plastic packaging industry

Defect reduction in plastic packaging industry

Plastic packaging is an important product in a plastic industry which contributes to almost forty percent of domestic packaging   markets. 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.

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

     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 Finally, the web is handled again from the last printing unit to the rewinding reel until finish process.

Unwinding

Rewinding

Press

Dryer

Figure 1 gravure printing process

Figure 2 Gravure process in a printing unit  

1.       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.

Customer

Quality

Inspection

Packing

Slitting

Gravure Printing

Conversion

Supplier

Material inspection

Blowing

Figure 3 Overall plastic packaging production processes

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.

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.

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 non uniformity 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

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.

               

Set the suitable temperature level of printing unit no.7

Set the suitable web tension

Design of experiment

Set the suitable temperature level of printing unit no.8

Problem solving for overlapping printing defect

Training the right method for operators

Construct the preventive maintenance implementing in the process

Work instructions development

Figure 10 Affinity diagram of problem solving for overlapping printing defect

5.1 Design of experiment

               

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 experiments

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.

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.

 

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