Moulding masterclass | processing
Once a stable moulding process is achieved, continuous process monitoring techniques will ensure optimum performance is maintained throughout the production run. John Goff explains how
Real time process monitoring
In the previous edition, the simple 32-shot analytical tool developed by statistician Frank Price was ex- plained. It was said that a stable moulding process, where the percentage variation measured by the Price principle is 0.4% or less, would be classed as “free and easy” – meaning that the process has a certain level of inherent robustness to counteract external infl uences that could cause the moulded product to become non-compliant. The degree of resilience is dependent upon the level or severity of variance encountered. For this reason it is common for data generated from the moulding process to be continually monitored so that trends - minor and/or step changes - can be readily identifi ed, giving sound technical evidence for the processor to make effective process changes. Real time monitoring of selected process variables provides the individual moulder with objective and factual informa- tion regarding the level of performance of the moulding process throughout a production run. Moulding companies that do not enjoy the luxury of
having a real time monitoring facility can use the moulding machine instead. The on-board process monitoring facility will undertake cyclic duties by comparing the actual values derived to those set and subsequently record the offset. This offset is then compared to the upper and lower limits selected for the set value and, if found to be in excess, is then recorded as a deviation.
The number of deviations encountered for a particular number of moulding cycles, for example 1000, 2000 or 5000, is often governed by the overall AQL
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for both the component and quality system employed. A typical process monitoring page is shown in Figure 1, where the tolerance band specifi ed for each of the process parameters would be selected as a result of the information achieved from either a Process Window Study or Design of Experimentation (DoE) exercise. The main difference between the machine-based
system and the external monitoring package is the manner in which the data is displayed. However, whether it is the off-line or machine-based version, the selection of such process parameters and extent of permitted variability is critical to ensure the overall stability of the moulding process is effectively assessed and correlates with the quality requirements of moulding. The tolerance band provides invaluable information in addition to process stability assessment, as it offers good technical guidance if altering the process conditions. Upon such a condition change, if the resultant monitored value falls between the upper and lower limits then the moulded component will be compliant to the quality standard. Alternatively, the moulder has the knowledge that when such boundaries are breached, particular quality attributes of the moulding are affected causing non-conformal mould- ings to be produced. Unfortunately, within many moulding companies such monitoring capabilities are not used for a number of reasons: Too great a variability of the moulding process prevents sensible boundary values being selected or evaluated; Too wide a boundary limit (tolerance) is applied, so
April 2013 | INJECTION WORLD 33
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