Weld line strength is primarily influenced by the amount of interaction that can be caused at the weld line location. Gas traps and low pressure at the weld line location are some of the primary causes for low weld line strength.
Concern: I am having some issues with one of our newer molds. We are using a PPE 20% glass. We are getting weld lines in around screw wholes. We have added another gate to move the weld line over, which it did not, and still having some issues with the part cracking. So part of the problem we believe is the mold, part processing, and part the material.
My Response: Although there are many potential problems, I will address the strength of the weld line here.
If you have gas trapped at the weld line location, it displaces the polymer, reducing the interaction. This typically causes a notch in the surface of the part making it a stress concentration and potential fracture point. If you can feel the weld line with your fingertip or a knife edge, you likely have a gas trap issue.
If you have too little pressure at the weld line location, the polymer chains will not interact, resulting in a weaker weld line. There are many ways to lose pressure but the most common reasons are 1) a slow injection speed will result in a high pressure loss during injection 2) Injecting too much material during 1st stage will cause a spike in pressure causing stress in the part, but the screw will bounce back resulting in a less effective packing phase. This is a common problem and may be contributing to the cracking issues you are encountering.