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: i_love_tips@hotmail.com, natcha.t@chula.ac.th
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
Phase I: Quality problem definition
Tools/Techniques
Process flowchart
Graph
Brainstorming
Cause-and-effect diagram
Phase II: Root causes identification
Questionnaire
Pareto diagram
Brainstorming
Brainstorming
Phase III: Problem-solving generation
Tree diagram
Affinity diagram
Design of Experiment
Phase IV: Selection of alternative application
Phase V: Evaluation
Brainstorming
Brainstorming
Use
To provide a diagrammatic picture of the focused process
To show the quantitative data
To summarize the focused problem
To determine and break down the potential causes of the
problem: material, machine, method, people, etc.
To relate the important priorities of problem causes by
individual importance ranking
To focus efforts on the root causes of problem from
importance ranking
To summarize the root causes of the problem
To find the ranking of possibility causes of the focused
problem from members in the team
To generate the problem-solving alternatives
To allow the team to summarize groupings among
problem-solving alternatives
To estimate the main effects and interactions of factors
which affect the focused problem
To develop the action plan and implement
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.
Supplier
Material
inspection
Blowing
Slitting
Gravure
Printing
Conversion
Quality
Inspection
Packing
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
Ink formulation
Machine
preparation
Web loading
Problem solving
between operation
Printing operation
Quality inspection
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
gravure cylinder (C9)
Lack of preventive maintenance (C6)
Nonsmooth of film surface (C2)
Bad quality web
Failure
Width variation (C4)
Impression roll (C8)
Old (C5)
Too tight web winding in a roll (C3)
Doctor blade (C7)
Overlapping printing defect
Preemption between printing (C16)
Machine stop due to
any problems (C20)
Lack of motivation to work (C18)
Lack of experience
Lack of training (C17)
Web loading between operating (C15)
Improper parameter setting
Roller force on gravure
cylinder (C12)
Temperature on header of
print unit (C11)
Lack of concern about print
quality control (C19)
Other
Force of doctor blade on gravure cylinder (C13)
Degree of doctor blade (C14)
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
on the heater of printing
unit
Set the suitable level of
temperature
Improper web tension
Set the suitable level of web
tension
Lack of adequate training
for operators
Training the right method
for operators
Lack of preventive
maintenance
Construct the preventive
maintenance implementing
in the process
Overlapping
printing defect
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
level of printing unit no.7
Training the right method for
operators
Set the suitable temperature
level of printing unit no.8
Construct the preventive
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
A
B
C
Factor
Temperature of printing unit no.7
Temperature of printing unit no.8
Web tension
Level
Low (-1)
50
50
15
Unit
High (+1)
70
70
20
°C
°C
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
Run
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
A
(Temp1)
-1
-1
-1
1
-1
-1
-1
1
1
-1
-1
-1
1
1
1
-1
1
1
-1
1
1
-1
1
1
B
(Temp2)
1
-1
-1
1
-1
1
-1
-1
1
1
1
1
-1
1
-1
1
-1
1
-1
1
1
-1
-1
-1
C
(Tension)
-1
1
1
-1
-1
1
-1
-1
1
-1
1
1
-1
1
1
-1
1
-1
-1
-1
1
1
1
-1
Number of defect
11
8
10
13
9
15
7
11
18
13
16
12
13
17
12
13
15
12
5
10
14
9
13
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
Main Effects
2-Way Interactions
3-Way Interactions
Residual Error
Pure Error
Total
DF
3
3
1
16
16
23
Seq SS
157.458
23.792
2.042
47.333
47.333
230.625
Adj SS
157.458
23.792
2.042
47.333
47.333
Adj MS
52.486
7.931
2.042
2.958
2.958
Figure 11 Normal probability plot of standardized effects
F
17.74
2.68
0.69
P
0.000
0.082
0.418
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
and sequence of the work instruction
incorrect
Printing machine leader and
gravure cylinder leader revise
correct
Printing manager approve
no
yes
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.
1.
2.
3.
4.
Task description
Printing step training
Reconsider the overall steps of each operator
Perform along with the established work instruction
Performance evaluation
Responsible person
Printing leader
Printing machine leader
Printing machine leader
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
1) Average number of overlapping printing defect (points/roll/month)
2) Average percentage of overlapping printing defect (%/month)
3) Average processing time of printing (minutes/roll)
Before
35.14
27.60
37.26
After
8.02
12.66
28.39
Difference
27.12
14.94
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
Process flowchart
Graph
Brainstorming
Pareto diagram
Cause-and-effect diagram
Questionnaire
Tree diagram
Affinity diagram
(Design of experiment
Ease of use
Suitability
Easy
Easy
Easy
Not easy
Easy
Not easy
Easy
Easy
Difficult
Suitable
Suitable
Suitable
Suitable
Suitable
Suitable
Suitable
Probably suitable
Suitable
Implementation probability
after study
Probable
Probable
Probable
Probable
Probable
Probable
Probable
Probable
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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Montgomery, D. C. (2005), Introduction to statistical quality control, 5th edition, John Wiley & Sons, New Jersey.
Park, K, and Ahn, J. H. (2004), “Design of experiment considering two-way interactions and its application to injection molding
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Wikipedia. (2008), Rotogravure, Available at: http://en.wikipedia.org/wiki/Rotogravure