SPC

1
Injection Moulding Technology
Part 3
Quality
2
Session aim
To improve the delegates understanding of quality
issues, relating to injection moulding and how
the process can be optimised and monitored.
3
Session objectives
By the end of the session you will be able to

• State 3 Quality Improvement tools.

• Explain how weight can be used to monitor
• the process.

• Calculate Cm and Cmk values.

4
Quality control - Detection systems
The manufacturing process
Methods
People
Material
Output
Environment
Equipment
5
Quality assurance - Prevention systems
The manufacturing process
Method
People
Material
Output
Decisions taken
Environment
Equipment
SPC
Information on quality
Improve Designs
FMEA
Update Performance
DoE
6
Quality tools
7
Failure Mode Effects Analysis
1. Process function Capability study on m/c.
2. Potential failure modes Zero cushion
position 3. Potential effects of the failure
High scrap rate 4. Potential causes of the
failure Worn/damaged check ring 5. Current
process controls None 6. Recommended actions
a) Barrel tolerances /- 100C.
b) Monitor cushion position c) Visual
check every 6 months (Abrasive
polymers) PC GF grades
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SPC - More detail
Collecting, representing and analysing
data, developing and understanding patterns.
A sequence of operations, not only the machine
cycle.
Measuring performance, taking action on the data.
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Terminology
Total Tolerance
x x xx x x x x x
Target
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Case study
Target piston diameter 60 mm (/- 1mm)
Target
60.0
61.2
60.8
60.6
60.4
60.2
61.0
58.8
59.8
59.4
59.2
59.0
59.6
Measured sizes
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Normal distribution curves
12
Normal distribution curves
13
Why choose 6 sigma?
1 sigma 691,462 DPM or 30.9 Defect free 2
sigma 308,538 DPM or 69.1 Defect free 3 sigma
66,807 DPM or 93.3 Defect free 4 sigma
6,210 DPM or 99.4 Defect free 6 sigma 3.4 DPM
or 99.9997 Defect free
14
Machine capability
Cm Measure of the variation present, in
relation to the available tolerance.
15
High capability
Curve fits into tolerance 3 times.
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Low capability
Capability Cm Total tolerance 1.2 0.75
6 x Sigma 1.6
Curve does not fit inside the tolerance.
17
Minimum capability
Cm 1.67 or greater
LSL
USL
18
Minimum capability
Cm is still 1.67
but some samples are outside limits.
Target
LSL
USL
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Targeting
Cmk Measure of the variation present in
relation to the available tolerance,
combined targeting of the set-up.
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Worked example
0.215
Cmk Difference between Avg. and nearest
Limit

2.04

3 x 0.035 (Sigma)
0.105
Cmk 1.67 or greater
LSL
USL
TARGET
Avg.(Mean)
21
Injection Moulding Technology
Part 3
Quality