How to Avoid a Weld Line in your Plastic Production

After analyzing plastic manufacturing defects including Short ShotsSink MarksFlashing, Bubbles, Bulges and Vacuum Vesicles, we are continuing our discussion on some of the problems you can encounter during your injection molding plastic production. Today, we'll talk about a weld line.


When two or more plastic streams meet at different temperatures as the plastic is filling the mold cavity, the two streams may not correctly mix together. When this happens, it forms a weld line. These are indicated in the following pictures:

 weld line on black plasticWeld line on plastic productionWeld Line example on black plastic

The Reasons and Solutions

There are many reasons that make the plastic with weld line, so we'll list some major reasons and solutions in the sheet below:



The plastic material isn’t fully melted or is under-drying

(1) increase the temperature of feed cylinder;
(2) Make the back-pressure higher;
(3) Increase the screw rotation speed;
(4) Make sure the material is fully dried

The material isn’t pure or contaminated with impurities

Check the material quality to make sure it hasn’t been exposed to external pollutants or change to a different material

There is too much mold release

Decrease the mold release agent or try not to use it if not needed

The mold temperature is too low

Increase the heat applied to the mold

The gate is too small or the location of gate isn’t suitable

Increase the size of gate or change the gate location

There is excess air inside the mold

(1) make the mold vent larger at location with weld line;
(2) Check if the mold vent is blocked or not or use vacuumized forming

The main runner and sub-runner are too thin or too long

Make the size of runner channel wider and/or shorter

Injection speed is too low

Increase the injection speed

The injection pressure is too low

Increase pressure at the nozzle.

The cold-slug well is too small

Increase the cold-slug well size or set flash gate at location with the weld line


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Posted by Bo Li

Bo Li

Bo breaks down complex projects into turnkey-able parts. Educated in Optical Engineering, he is actually a veteran in plastic manufacturing.