DFM | Weld lines
Figure 2: Metallic pigment fl akes in this ABS component highlight the weldline, as predicted in fl ow analysis studies.
modifi cations. If this analysis had been carried out in the fi rst place, the additional costs and delays on the ramp up of the complete device could have been avoided. So, what can be done to minimise weld line issues during the DFM process? First of all, consideration should be given to the material to be moulded. For reinforced materials such as glass fi bre reinforced PA it is a big disadvantage in any case to have weld lines as the resulting fi bre orientation will show up in both visual and mechanical terms. It is also important to minimise the number of gate
points onto the component. This becomes even more important where there are thin wall thicknesses on the fl ow path as high injection speeds generally result in poor weld line strength (this was the case for the component mentioned earlier). The positioning of the gate points is also an impor-
tant factor. It is very important that all weld lines meet at the same time during the mould fi lling cycle. This is because of the switchover from fi lling to packing pressure, which can only ideally be done once the cavity is fully fi lled. If some weld lines meet early in the mould fi lling cycle then the fl ow fronts may start to freeze off early and, as a result, will not receive the full amount of packing pressure required to obtain optimal mechanical properties. This latter point may sound like a processing issue
rather than design consideration, but this understand- ing of the moulding process is very important if weld lines are to be considered properly during DFM. Modern fl ow analysis software provides the essential tools required to check how weld line behavior is
50 INJECTION WORLD | March 2014
effected by injection speed, fl ow-front temperature, cooling time, fi nal point of fi ll, and freeze off of the gate. Some applications also offer a module allowing fi bre orientation at the weld line to be modelled. By knowing where weld lines will be formed, information can also be passed on to the toolmaker to help in deciding where to place venting features. The following list highlights some of the key points that need to be considered when looking at weld lines as part of a fl ow study of any component and gating system design: Limit the number of gate points and calculate the right gate and runner size; Place all weld lines on the last point of fi ll so that suffi cient packing pressure can be applied to each; Maintain a maximum 10°C temperature difference between fl ow fronts; Avoid fast fi lling speeds (especially on thin wall thicknesses as high shear rates have a negative impact on the mechanical properties of weld line); Consider the presence of fi llers and pigments in the material which may cause highly visible cosmetic issues (Figure 2).
About the author: André Eichhorn is general manager of Germany-based AST Technology. This is the latest instalment in a series of articles in which he discusses how product manufac- turing problems can be overcome at the start of a project by the application of Design for Manufacturing techniques. You can read the most recent articles in this series here, here, and here.
www.injectionworld.com
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