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Weld lines | DFM

Weld lines have a negative impact on part quality but their presence can be predicted using fl ow analysis tools as part of the DFM process. André Eichhorn explains how

Plan to manage weld lines

Aside from part defl ection and distortion, there is another critical factor that has a huge impact on the quality of an injection moulded component – weld lines. Weld lines do not only affect the visual quality of a part, they are also a major cause of mechanical weakness in the product and this can give rise to costly failures during assembly or in service. All of those that have spent time working in the

plastics industry will have experienced parts that always tend to break in the same position. One of the major reasons for such failures is poor weld line quality. When such problems emerge in parts in production, it is sometimes possible to improve the situation by applying a structured injection moulding process optimisation routine and adjusting process parameters such as injection speeds, melt and mould temperatures to achieve the best mechanical performance. While modifi cations to the moulding process can

resolve weld line issues, or at least minimise their impact, processing around design issues is rarely a good use of process engineering time. Far better that we avoid weld line issues in the fi rst place by giving them due consideration in our DFM procedures. With the tools available to design engineers today, it is quite easy to predict where weld lines will appear. But let us fi rst consider the basics. Weld lines are formed where two melt fl ow fronts meet and can be caused by a number of factors, including the use of multiple gate points or by mould features that interrupt and divides the path of the fl ow front.

A typical example is often seen where the melt is divided in the mould cavity to fl ow around a ‘hole’. The main image above shows a weld line which has formed where the divided melt fl ow fronts meet again after encountering such a mould feature. This specifi c example resulted in visual issues, as well as consider- ably reducing the strength of the part. The result was that the moulding broke very easily and failed the required drop tests on almost every occasion. In this case, optimising the injection moulding

process did not resolve the problems so the component design was analysed in a further DFM loop. Signifi cant changes were made to the tooling design, involving costly relocation of the gate points and other tooling

Main image: The visible

weld line on this part shows the typical

characteristics of inconsistent melt fl ow front temperatures

Figure 1: Weld line location can easily be predicted in the part design process using fl ow analysis software

March 2014 | INJECTION WORLD 49

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