How to solve weld lines in injection moulded parts.

Weld lines

What are weld lines?

Weld lines in a plastic part can be a source of weakness causing the part to slit and break on impact. They can also cause visual problems on the consumer-facing surface of the part, which can be a critical defect. Also, weld lines on a seal surface can cause leaks to occur when the part is assembled. So even thought weld lines seem like a small issue, the more people look at them they seem to get found more often and with a greater effect on quality.

Define the weld line issue.

The first thing you need to do is understand the issue and if it really is an issue. A part quality log should be opened, this is a simple document that outlines the quality requirements for the part during manufacturing. Some companies call this a product specification or quality control form. No matter whats its called, it should be at the press where when the product is running so that people can clearly understand what the part quality requirements are. So for weld line issue, a section within the document should show images of acceptable and unacceptable samples with the dimension or some measurable specification for the weld line. If you don’t have this, you will only go around in circles with a quality standard that is not clear to all, and essentially only end up frustrating people. So do yourself a favour and define fully the issue and where possible get your customer to sign the quality control sheet so that there is no ambiguity.

How weld lines are formed.

Weld lines are formed by two or more melt flows coming together. As the outer layers of the melt is the coldest and has a skin formed on it, so when two of more melt front come together, the outter skins of the melt flow into each other producing a line. The melt may slipt as it flows around a pin or may front down a wall quicker then other areas causing the melt to flow around itself and causing melt front to come to gethers. Without getting into to much detail essentially that is what weld lines are. Depending on how hot the melt is and how much pressure is used the strength of the weld will be determined.

How to Solve weld lines.

In any part design the location of the weld lines will depend on the gating locations as well as the part geometry and to the processing conditions. It is always a good idea to do a flow analysis to determine weld line location, the last thing you want is weld lines showing up in areas where mechanical strength of the part is critical or in areas that is consumer-facing.

When a moldflow is done it will predict the weld lines location, pretty actually however it will not really tell you how visible or how strong the weld line will be. So you are going to have to rely on some general guideline.

Weld line at the end of fill will be the weakest, as the melt fronts will be the coldest.

Weld weld furthermost from the gate will generally be more visible, again due the the melt been colder at the end of fill. A rule of thumb would be the melt front drops 10 degrees for every second of injection.

Grey or silver material tends to show weld lines more easily, I think it is something to do with the colorant molecular structure.

Weld lines on high gloss parts, tend to how up a more defined line.

Using filled material will show weld line clearly as the filler fibers a line to the flow direction.

So the best way to solve weld lines is obviously avoid them by optimizing your design so the parts will not form weld lines when moulded. This will mean choosing a gate location that keeps weld lines away from critical areas. Also by understanding how the plastic flows in the mould you can use the wall thickness to promote or restrict flow. Also by incorporating features to strength or hid weld lines within your design, this is the best way to solve weld lines. However as most of us know only too well weld lines are