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How to Avoid a Weld Line in your Plastic Production

Aug 7, 2018 8:00:00 PM

After analyzing the Short Shot, the Sink Marks, the Flashing and Bubbles and the Bulges and Vacuum Vesicles issues, 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 exemple 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:

Reason Solution
The plastic material isn’t fully melted or is underdrying

(1) increase the tempereture of feed cylinder;

(2) Make the back-pressure higher;

(3) Increase the screw speed;

(4) Make sure the material is fully dried;

The mould tempereture is too low Increase the heat applied to the mold
Injection speed is too low Increase the flow speed
The injection pressure is too low Increase injection nozzle pressure
The material isn’t pure or mixed with impurity 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 or try not to use it if not needed
The runner and in-gate is too small or the location of gate isn’t suitable Increase the size of runner and in-gate or change the gate location
There is excess air inside the mold

(1) make the exhaust groove larger at location with weld line;

(2) Check if the exhaust groove is blocked or not or use vaccuumized injection;

The main runner and sub-runner are too thin or too long Make the size of runner channel larger
The cold slug well is too small Increase the cold slug well size or set flash groove at location with weld line