The standard methods of weld line prediction fail when the gas-assist technique is used and weld lines develop in thick-walled areas. Low flow resistance in the gas channels leads to increased gas bubble propogation. At the end of the melt filling phase when one flow front lags slightly behind the other, the cushion of melt in front of the gas bubble on the lagging side will be smaller than the cushion of melt in front of the other bubble. This means that the flow resistance of the melt will become smaller too and the flow front will advance at a correspondingly fast pace. This leads to a shift in the position of the weld line. The expected weld line position is shown in the diagram below. Inevitably, this will lead to an accumulation of material at the end of flow.

The designer should avoid a thick-walled gas-assist injection molding ring (exterior car mirrors are a typical example). The best alternative is a balanced separation of the two gas channels by means of a thin-walled area without a gas bubble. The melt flow can be balanced with the aid of rheological computation programs suited for gas-assist injection molding.

 

Click in the graphic area below to view an actual mold filling which demonstrates the unpredictable location of the weld line. The video shows four different mold fillings, one of which is in slow motion.