Assignment Ref PART B – Case Study Module Name Design for Quality – Distance Learning

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Assignment Ref PART B – Case Study
Module Name Design for Quality – Distance Learning
Module Ref No. MECH60656
Weighting 50% +20% Journaling = 70% overall module
Author / Tutor Pete Jones
Reference Text Reliability Engineering – Kapur and Pecht
Case Study – Six Sigma DMAIC project to improve the performance of an aluminium die
casting operation
JONES Aluminium Window Systems Ltd (JAWS Ltd.), a UK Midlands based manufacturing company
that specialises in the design and production of systems for doors and windows, particularly for
aluminium profiles. JAWS has implemented ISO 9001 and is certified since 2006 and uses both Lean
and Six Sigma for their continual improvement. Projects are selected using a matrix that rates them
in function of different parameters linked to JAWS Ltd. strategy such as: environmental, safety, time
reduction, savings, and payback in case of an investment is required.
Your task as a Quality consultant is to review these report data and provide additional insight and
analysis of the issues and how you might address them. Also, the internal project team at JAWS ltd
are struggling with the interpretation of some of their experimentation and require some support.
Read through the case and provide supporting documentation cross-referenced against the various
headings of this report. There are 16 sub tasks related to the case study.
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Project selection
A window handle model, shown opposite, manufactured using a gravity die
casting process operation has shown to have a very high rejection rate.
Today the quality and aesthetics expectations require significant process
improvements. The components are produced in low silicon aluminium and
finished with an anti-corrosion anodising process that supports low
maintenance. Many of the defects are only detected at the end of the
manufacturing process. The problems of late detection and high rejections
to lead to significant waste and cost factors, including:
1. Waste of time and unnecessary consumption of resources in processing defective items.
2. Extensive rework prevents other products to be manufactured, creating two additional
• To satisfy the specified service levels to the customer, the levels of safety stocks
need to be higher, which increases the holding costs.
• The company has more difficulties in complying with the necessary lead time to
deliver on-time the order demanded by the customer.
3. The waste factors result in additional labour, materials, and energy consumption costs,
among others.
4. Non-quality costs resulting from the lack of capability of the process in meeting the product
5. Intangible costs, including image and reputation costs.
6. The sales of anodized aluminium architectural hardware represent 5% of the company’s
turnover, and it is growing.
7. Depending on operators experience and other unknown factors occasionally sometimes the
production yield was higher.
The company decided to select this as a Six Sigma project using the DMAIC approach.
Project planning
Planning is a prior stage that takes place before the DMAIC roadmap is initiated. It intends to reach a
clear understanding of the problem and detailed project planning tasks. JAWS ltd needs support to
analyse the preliminary data and enable the team to develop the project charter and communicate
the relevant information about the Six Sigma project.
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Complete the project scoping / schedule document in the table below.
Project area Description
Project statement
Description of the problem or opportunity
Project scope
Importance of the project
Main goals of the project
Main resources for the project
Table 1: Project schedule (partial)
Provide the JAWS team with a list of project documents that will help guide their actions. E.g. The
project schedule and a “responsible for, accountable for, consulted, informed” matrix to assign and
communicate the roles and responsibilities within the team, must be developed as part of the
project planning.
Define phase
The initial efforts of this phase targeted a deep understanding of the manufacturing process. Process
mapping tasks were carried out using a SPIOC diagram, Table 2.
Suppliers Inputs Process Outputs Customers
Ingots supplier Aluminium alloy
Rejected casted
Fusion time
Fusion Melted
aluminium alloy
“Die casting”
Volume of
aluminium alloy
Temperature of
the melted alloy
Temperature of
the mould Type
of die coat
applied Rotation
speed of machine
Die casting cycle
Die Casting Solidified casted
“Separation of
the rigging”
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“Die casting”
Solidified and
cooled casted
during cut Type
of saw
Segregation of
the rigging
Rigging system
separated from
the window
“Separation of
the extractor
pin” operation
“Separation of
the rigging”
Window handles
with an extractor
Cutting force
Positioning of the
Segregation of
the extractor pin
Extractor pin
separated from
the window
“Separation of
the extractor
pin” operation
Handles with
Type of ribbon
Manual Grinding
and filing
Surface of the
smoothed in the
feed region
Handles to be
Types of ribbon
Strength of the
robot’s claw at
the end of the
Grinding (robot)
Surface of the
except the region
of the ellipse
except in the
ellipse region
Type of ribbon
Manual Grinding
(final smoothing)
Surface of the
Polishing time
Type of brushes
Polishing Surface of the
Composition of
the bath
Temperature of
the bath Polished
handles Amount
of electric charge
Anodizing Handles anodized Packaging and
Table 2: SPIOC Diagram
The team has gathered and analysed data from the manufacturing process, by inspecting handles
that were rejected in the previous two weeks of production.
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Describe how an affinity diagram may be constructed to group similar types of defects into a same
class/category of defect. Please provide examples and a template for the team.
Data showing the number and percentage of defects that occurred in both weeks for each category
of defects, is confusing the JAWS team.
Make a detailed analysis of this data. You may need to research gravity die casting techniques and
problem areas. Are there any actions to be taken regarding operator skills and training and such
issues as contamination?
Data also unveiled that most of the grinding defects are likely to be generated in the manual
grinding operations, which are a result of the filling process of the die.
The team are aware of the importance of documenting critical to quality (CTQ) characteristics and of
their operational definitions.
Please critically appraise the teams efforts to create a clear, unambiguous, and observable standard
of acceptance. Suggest a set of rules for creating CTQC documents and where possible provide
examples or a template.
Group of defects Number of occurrences % Number of occurrences %
Solidification shrinkages 156 3.7 91 15.5
Pores 89 21.2 58 9.9
Filling errors 50 11.9 146 24.9
Scoria and impurities 52 12.4 12 2
Cutting defects 43 10.3 46 7.8
Grinding errors 17 4.1 169 28.8
Polishing errors 2 0.5 0 0
Anodizing defects 0 0 0 0
Other types of defects 10 2.4 65 11.1
Wk 1 Wk 2
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Filling Errors
5 subtypes of defect
Die Casting
Visual detection
The handle will conform if:
An edge in the functional region of the handle presents a
clear rounded edge
The handle will conform if:
The edge delimiting the region indicated does not present a
rounded edge
Subtype of defect 2. Rounded functional edge
Operational Definition
Description of the requirement
Type of defect (CTQC)
Number of opportunities for defect
Origin of the defect
Subtype of defect Rounded edge in the border of the functional and asthetic
Description of the requirement Visual detection
Operational Definition
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Table 3:CTCQ Defect chart
Measure phase
Production tests were conducted to assess the baseline performance of the gravity die casting
operation. In these tests, all key controllable input variables remained constant, at the levels usually
used during production.
The results of these tests per category of defect are shown in the chart below. You are required to
Subtype of defect 5. Hole in the square hole
Operational Definition
Description of the requirement Visual detection
The handle will conform if:
The hole does not have any hole
4. Filling error on the top surface of the hole
Operational Definition
Description of the requirement Visual detection
The handle will conform if:
The square hole is correctly centred
There is no lack of material around the square hole
The surface is flat
Subtype of defect 3. Imcomplete filling in the lateral surface of the square block
Operational Definition
The handle will conform if:
The block presents a perfect square shape
All the four faces of the square block are flatand absent of
any cavity and do not have a lack of material
Subtype of defect
Description of the requirement Visual detection
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• to create a PARETO chart of this data
• estimate the DPMO
• identify the most relevant problems
• estimate a value for sigma (Z)
A total of 108 handles were produced, and 78 of them were labelled as defective; thus, the resulting
proportion of defectives was around 72 percent, which is a very high value.
Analyse phase
The first activity of this phase was the development of an interrelationship digraph, Table 4, to study
and understand the chain of causes-and-effects that will ultimately create the different types of
defects in the aluminium gravity die casting operation.
Determination of the significant factors that contribute to the most relevant types of defects. The
objective of the project team was then to learn as much as possible from the production process by
making a set of cost-effective experiments.
Table 4: Chain of causes-and effects that lead to the occurrence of die casting defects
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To carry out the DOE in the analyse phase, the following sequence was adopted:
• selection of the response variables;
• choice of the controllable factors (input variables);
• preparation of the DOE
• conduction of the DOE
• analysis of the results
• identification of possible interactions.
Selection of the response variables.
A DOE strategy was developed to investigate which controllable input factors of the gravity die
casting operation have a significant effect on the CTQ characteristics of the handle model under
study. Three specific CTQs were considered:
• Filling errors (CTQ1)
• Solidification shrinkages (CTQ2)
• Pores (CTQ3)
The variables inherent to these three CTQs are not continuous, since the detection of any of these
defects in a handle is only possible by visual inspection. Choice of the controllable factors. Using the
information from the interrelationship diagram, the audit of the production process, several
brainstorming meetings were held with the shop floor people and their supervisors to identify which
factors could influence the CTQs (output variables). The project team agreed to screen the following
four controllable input factors:
• volume of molten alloy poured into the mould
• temperature of the molten alloy
• rotation speed of the gravity die casting machine
• type of die coat.
To investigate their individual and combined influence on the CTQs, two distinct levels were defined
for each factor, as shown below. A standard sample size (120) was suggested but in practice this
varied slightly for each run.
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Table 5: Input controllable factors and their low and high levels
Preparation of the DOE.
Suggest how a ½ factorial experiment would be constructed for this data. The defectives for each
CTQ and the sample sizes are shown below:
Table 6:Experimental results of the screening DOE
Outputs from MINITAB in the form of “Analysis of Variance” are show below for each CTQ
characteristic. Discuss the significant factors for each CTQC.
Replicate Run Order A B C D Sample size CTQ1 CTQ2 CTQ3
1 1 -1 -1 -1 -1 112 40 5 16
2 9 -1 -1 -1 -1 132 18 21 27
1 7 1 -1 -1 1 116 62 8 13
2 14 1 -1 -1 1 122 65 44 2
1 6 -1 1 -1 1 108 63 14 18
2 13 -1 1 -1 1 120 50 29 32
1 3 1 1 -1 -1 106 27 5 13
2 12 1 1 -1 -1 123 5 12 19
1 5 -1 -1 1 1 114 92 45 12
2 15 -1 -1 1 1 120 91 27 7
1 4 1 -1 1 -1 122 73 0 7
2 11 1 -1 1 -1 128 25 7 32
1 2 -1 1 1 -1 112 9 10 18
2 10 -1 1 1 -1 122 37 18 24
1 8 1 1 1 1 100 86 41 27
2 16 1 1 1 1 124 59 16 24
Defectives for each CTQ
Controllable input factor Levels used High and low levels
Volume of alloy (A) Small die casting spoon
Large die casting spoon
Temperature of the alloy (B) 730°C
Rotation speed (C) 42.5 Hz
50.0 Hz
Type of die coat (D) KS201
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Factorial Regression: CTQ1 Filling Errors
Factorial Regression: CTQ2 Solidification Shrinkages
Factorial Regression: CTQ3 Pores
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The project team also noticed that filling errors tended to appear in the same region of the handle,
namely, in the edge that separates the functional region from the aesthetical region. Two possible
causes for this situation were identified: it could be due to an accumulation of air or to a rapid
cooling in that specific region. Tests were performed and based on their results the team concluded
that the main cause was the rapid cooling of the alloy temperature.
Experimental data from the DOE should enable the team to compute a set of important metrics. The
table below summarizes the values for the rate of defective handles, the average number of defects
per handle (defects per unit (DPU)), the number of DPMO and the Sigma Level.
However, the team need your support to compute these important metrics. Please complete the
table for them.
Determination of the root causes for the occurrence of structural abnormalities in
the castings
In addition to filling errors, solidification shrinkages, and pores, when inspecting the handles
produced in the experiments, team members realized that many of them had structural
abnormalities, such as fissures. Structural abnormalities tend to appear when the transition of the
alloy from its liquid state to the solid state does not occur properly, which might happen if the
mechanical properties of the alloy are jeopardized due to mixing with other types of alloys or
because it is contaminated with scoria and impurities. To investigate the influence of alloy mixes in
the quality of the manufactured handles, some samples of defective handles containing structural
abnormalities were sent to an accredited laboratory for analysis. The laboratory found high contents
of silicon in the holes and fissures of the handles, which is a typical element in other types of alloys,
Structural Abnormailites Filling Errors Solidification shrinkages Pores Buried Extractor Scoria and impurities All Other
824 735 293 288 251 207 13
Number of handles produced (P) 1777
Total defective handles (D) 1408
Total number of defects 2611
Number of opportunities for defect (O) 7
Rate of defectives (%) 79.2
DPU 1.26
Sigma Level (Z) 1.27
DPMO 209,904
Mean of defects 373.00
SD of defects 294.42
Die Casting Experiment Results
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not in the AG4Z alloy that used to cast this type of handle. The laboratory report thus confirmed that
alloy mixings is a significant cause for the occurrence of structural abnormalities.
How would you suggest this laboratory report may be used to improve quality at JAWS ltd.
Table 7: Factory plant Factory plant layout, areas where operations of the handle Ho-2AG take place, and point where
problematic issues were identified in the audit
An initial investigation by the team has found problems within the layout of the shop. Can you
suggest why these areas are at risk of contamination. However, it is known that robots drop
defective shapes and they get left on the floor for recycling and that most of the manufacturing
personnel working on the operations downstream are not able to distinguish the different types of
alloys in which the products were cast. Throughout the process, work-in-process boxes with items to
be processed are accompanied by a coloured card labelling the type of alloy in which those items
were casted; those label cards are sometimes times absent or missed.
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Improve and control phases
After having determined the root causes for the high rejection rate of the manufactured handles in
the previous phase of the DMAIC approach, the project team now needs to consider the
development of improvement actions to be implemented. Define 4 critical improvement steps
making use of the data that follows. Include a reference on how a control plan may be integrated.
DoE Main effect plots for filling errors, solidification shrinkages, and pores:
Using the main effects plots above suggest the optimum settings for the die casting operation.
Selecting a lower temperature for the molten alloy might increase the chance of filling errors to
occur in the functional region of the handle. The temperature in the functional region of the handle
should be high during the gravity die casting operation, while at the same time the temperature in
the aesthetical region of the handle should be low. A torch, was used and experiments
demonstrated the effectiveness of this solution.
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Table 8: Illustration of the physical contradiction that needed to be solved using the separation principles of TRIZ and
Illustration of the solution that was adopted based on the separation in space principles from the TRIZ theory
The interaction plots from the experiments would indicate that an on-the-job training program
would be indicated to increase the knowledge and the technical skills of the personnel. Suggest
three core topics for that training based upon the interaction charts.
Table 9: Plot of the interaction that has a significant effect on pores (left) and plot of the interaction that has an important
effect on solidification shrinkages (right)s
An initial study of the shop layout concluded that alloy mixings are more likely to occur at
workplaces located downstream in the manufacturing process. It happens not only due to lack of
knowledge, but also because an effective procedure to properly sort rejected items according to
their type of alloys does not exist. To solve this problem, a simple but effective preventive system,
comprising a set of procedures, was implemented. Briefly analyse the modified plant layout and
discuss the role of procedures and layout in context of quality improvement.
• There are two central points in the factory plant, where the rejected items can be deposited
in specific boxes according to the alloy in which those items were cast.
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• To each alloy there is a corresponding colour, so each box is labelled according to the colour
of the alloy.
• The central point located more downstream is used by the workstations within region “A,”
namely, the following: anodizing, lacquering, and liquid painting.
• The central point in the middle of the plant is used by the workstations within regions “B”
and “C,” namely, the following: automatic grinding, polishing, and polishing by vibration.
• The area labelled as “D” contains all the materials that are to be fused in the fusion furnace.
It includes ingots, rigging systems that were separated from the casted part, as well as
rejected items coming from the two central points.
• There are specific places to store these materials, and corridors to manage the flow of the
rigging systems per type of alloy.
Table 10: System to prevent the mixing of alloys
How can a 5S program, be developed to promote organization and discipline, as well to prevent alloy
TASK 16 conclusions
Briefly describe the benefits and key elements in each of the DMIAC approach deployed at JAWS Ltd.