Weld Line Defects in Plastic Injection Molding

Every manufacturing process can introduce different defects into the final product. The goal of every manufacturer is to have the machines, process, and quality assurance in place to reduce and even remove flaws from final parts and products. In this blog, we’re looking at one of the more common part defects that can come from injection molding: weld lines. Let’s go over what weld lines defects are, how they are caused, and what injection molding companies do to avoid them.

What are Weld Line Defects?

Weld line defects – also known as knit lines – is a defect that occurs when the injected plastic meets within a mold. As the polymer resin is injected into a mold, it flows through all parts of the mold. In places where the plastic flows around a solid piece of the mold, from a simple circle to forming complex shapes like rings or squares, the two or more fronts of molten plastic rejoin into a single piece – or at least they are supposed to. This defect forms when they don’t “knit” completely back together.

How Do Weld Lines Affect the Part?

When the two melt flows collide in a mold cavity, they are supposed to fuse back together. Depending on a host of factors, they can fuse together perfectly, or imperfectly and cause the weld line. If this is minor, they have completely fused together, and the weld line is purely cosmetic. However, it could be the case that the weld between the two sides is very poor, undermining the strength of the part. Particularly to loadbearing or moving parts, this reduction in strength can be a major defect in the final project.

What are the Causes of Weld Lines?

Just like with many part defects, there are multiple causes for weld lines. The two major factors here are the temperature of the plastic and the pressure pushing it together. There are many sources, but here are some of the primary factors:

• Temperature: There are multiple places the temperature needed to keep plastic flowing properly can fail – the injection molding machine, the runners to the mold, and the mold itself.
• Pressure: Without the proper pressure, the flow fronts may not be pushed together enough for them to meld back together. This could be the machine (back pressure) or issues with the mold design that cause bottlenecks in pressure – such as gates, runners, or parts within the mold.
• Speed: A subset of the two above factors, when the pressure is low, the rate the plastic moves through the mold is lessened, which means the fronts may cool before they touch.
• Mold Design: A poorly designed mold (including gates and runners) can cause all the above problems as well, from improperly placed gates to wall thickness.

A lot of these can also cause a more extreme version of weld lines, where the plastic doesn’t completely fill the mold, called short shots. Learn more about them in this blog: Troubleshooting Short Shot in Plastic Injection Molding.

Troubleshooting Weld Line Defects

Many of the solutions to this defect directly solve the causes. For temperatures, the temperature of the injection molding machine’s barrel may be increased, the runners to the mold replaced by hot runners, or heating the mold directly. Pressure too can be increased at the machine, replacing the location and number of gates, or designing the mold to avoid bottlenecks. The speed can be adjusted alongside pressure. Changing the type of resin used can also improve performance, as some types of resin have longer flow times than others.

Avoiding these and other injection molding defects is all about using the right mold manufacturer and having a part manufacturer that has the right quality assurance to catch these defects and troubleshoot them. If you’re interested in working with someone who does it both, it’s time to contact Midstate Mold & Engineering. Interested in reading up on more part defects? Check out part one and part two of our Uncovering Molding Defects and How To Troubleshoot series.