Weld Lines

Causes of injection molded parts weld lines
The leading end of the molten material is cooled but not solidified at the confluence of the strip, and then causing weld lines.
The air and the volatile material is sealed.
The mould cavity has residual release agent.

Associate knowledge about weld lines
Using only one sprue (gate), without confluence the material from two directions, in this way the molded parts generally do not have weld line. If you want to avoid weld lines, you can only change the position of the sprue in the injection mould so that the welding mark will not occur in obvious places. (Figure A)

A. Change the location of the sprue

Pin (hole in the molded plastics) on the injection mould usually generates weld lines. (Figure B)

B. Weld line caused by the hole on the product

There is a lot of weld lines caused by air sealing, it is necessary to complete discharge the air.
Make the weld line on the extra sheets of the injection molded parts, and then cut off the sheet (Figure C)

C. Let the weld line occurs on the extra sheet

Excessive release agent leaving in the molten material feeding float cavity, it will certainly cause weld lines.

D. Burrs in the weld portion

Solutions
Instant: Increase injection speed, mould temperature, barrel temperature and injection pressure.
Short-term: increase the gas escape groove.
Long-term: Change the sprue location of the injection mould.