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processing | Moulding masterclass


Effective injection moulding requires an understanding of which variables


control the process and which are


consequential to it. Moulding expert


John Goff explains this vital distinction and its role in


maintaining part quality


Systematic process control should not stop at start-up


There is a very well known – and very truthful – saying in the injection moulding industry that it is easy to produce scrap quickly. Over the past few instalments in this series we have discussed how careful process setting procedures can avoid production of scrap parts. Achieving the most effective and economical component production requires meeting two particular criteria – quality and productivity. As we all know, our goal is to produce injection moulded parts at the right level of quality in the most economical way. Naturally all stages of the moulding process must satisfy both criteria, but the emphasis for each stage may be different. For example, once good melt homogeneity is achieved the level of quality of the moulding is wholly attributable to the manner in which the mould cavity is fi lled with molten material, then how it is compacted and fi nally cooled. As a consequence, greater attention to part quality is given when selecting the process parameters for each of these stages. Component removal and collection of mouldings can


32 INJECTION WORLD | January/February 2013


also affect part quality. However, emphasis is typically placed on how fast this stage of the process can be carried out. It is, therefore, vitally important that a systematic


procedure is undertaken that applies the correct focus for each stage. Without such a systematic approach, the controllable variables mentioned can be arbitrarily selected by the moulding technician, resulting in product quality changes within a production run or from run-to-run. Furthermore, due to the interaction between particu-


lar process variables, changes made in a non-organised or indiscriminate manner can give rise to confusion by providing contradictory evidence as to which change in process parameter settings solved the issue. This is particularly so when several different process param- eters are changed at once. Process variables can be defi ned as controllable or


consequential. Controllable variables are described as those that dictate the base line of component manufac-


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