processing | Moulding masterclass
Figure 1: Data from typical process monitoring page. The tolerance band for each process parameter would be selected from a PWC or DoE exercise.
Tool – deviation n [1000] 1000
Reset deviation £ Cycle time [s]
Inject time [s] Dosing time [s]
Screw stop [mm] Melt cushion [mm]
Barrel zone 3 [˚C] Cav pr 1 –max
Cav pr 1 – integral p-inj - max
p-inj –integral Allowable Actual
23.55 0.86
0.44
29.11 9.9
295 0
0
763 90
23.40 0.84
0.40 28.9
9.8
290 0
0
755 85
23.79 0.88
0.46 29.2
10.0 300
0 0
770 95
2 n 1 n
2 n 3 n
5 n 1 £
0 £ 0 £
3 n 3 n
LL 0
0 0
0 0
0 0
0 0
0 Actual
Actual Lower Upper deviation deviation deviation Output (%)
UL 0
0 0
0 0
0 0
0 0
0
providing an ineffective control of the process and typically resulting in non-compliant mouldings being undetected during the production run;
l Lack of awareness that a monitoring facility exists within the injection moulding machine;
l Inadequate understanding of how to effectively use the available monitoring facility. For those moulding companies that are manufactur-
ing components in accordance with strict guidelines imposed by regulatory bodies, then derivation of the upper and lower values for particular process variables becomes part of their moulding protocol. Furthermore, such procedures may stipulate that only designated process variables can be altered within the specified range and that the remainder are to be untouched. To derive such boundary values a range of predictive
techniques can be used, the most popular being Design of Experimentation (DoE). DoE principles are employed to ascertain the extent of variability that may inherently occur through process condition changes and to identify those variables that are more pertinent. Opinions are somewhat divided regarding the use of certain DoE packages and, for this reason, a universally accepted approach is not evident within the moulding industry. Most moulders, therefore, employ their own
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34 INJECTION WORLD | April 2013
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approach and analysis package. However, when attempting to identify the extent of variability each process parameter induces it is important to ensure that the analysis does not include too much data. For example, the inclusion of the entire range of process variables used to produce the moulding as well as other influences such as raw material batch changes, different colours, different percentages of moisture within the material, different types of drying equipment, the use of gravimetric or volumetric additive addition or use of reground material. Such an extensive range of data inclusion is too
exhaustive for the analysis package to effectively assess and analyse, leading to confusion and contradiction rather than clarity. More importantly, not only is the outcome inconclusive but the time utilised to undertake such experimentation is wasted. G&A Moulding Technology advises that when a moulding process has been suitably optimised and its stability verified using the Price 32-shot technique, the next step is to create a simple framework upon which critical process variables are increased and decreased around the optimised value. Such critical process variables will often include holding pressure, holding pressure time, cooling time, mould temperature, melt temperature, etc. The actual selection of process variables used is dependent upon either the nature of the polymeric material being processed (semi-crystalline or amor- phous) or the quality attributes that the moulding component is expected to meet in accordance with the physical dimensions. Therefore, the high and low boundary values used for a Window Study are deter- mined during the process optimisation exercise. Furthermore, for each run (maximum usually nine runs) the effect of process consistency can be measured using the Price 32-shot technique. This discussion will be continued.
About the author: John Goff is a chartered engineer (CEng), a Fellow of the Institute of Materials, Mining and Metallurgy (FIMMM), and managing director of injection moulding process consultancy and moulding process optimisation software developer G&A Moulding Technology ❙
www.gandamoulding.co.uk
This is the 26th instalment in his Moulding Masterclass series of injection moulding process optimisation articles and is the second part in a discussion on the understanding and effective use of measured process variables. You can read the most recent instalments in this series here, here and here.
www.injectionworld.com
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