Weld lines form where two or more flow fronts come together and can create a cosmetic or structural problem..
Figure 1 below shows the development of a weld line. In this case the two flow fronts will meet and there will be very little molecular orientation across the weld. As a result this weld line will be a weak point in this part.
Air can also be trapped at the weld line as the flow fronts meet due to the shape of the flow front. Weld line strength can be improved by creating a transient flow condition through the weld. One method of doing this is to locate the gate in a position such that once a weld is created, flow will continue to flush through the weld location to fill other regions of the cavity. Figure 2 below shows two different gating locations. The top figure results in a butt weld without transient flow. The lower figure creates a transient flow condition through the weld. The resultant molecular orientation occurring under the initial frozen layer can significantly increase its strength.
Another method to create flow through an initial weld is to create a dump outside of the cavity. Again, this can create a transient flow conditions. A third method is to use melt rotation technology to strategically place the weld, create transient flow conditions and position the hottest flow laminates at the weld.
Figure 1
Figure 2
