Discoloration refers to the inconsistency between the surface color of the formed parts and the required color. Discoloration may be caused by a variety of reasons, including injection molding machine, plastic mold, injection process and raw materials.
(1) Injection molding machine
① The equipment is not clean and needs cleaning. Dust or powder deposited in the barrel contaminates and discolors the material;
② Temperature control failure caused by temperature controller or heating system failure;
③ There are obstacles in the barrel, which lead to the decomposition of the plastic; foreign metal objects are stuck in the barrel or spiral channel, which are grinded continuously, resulting in the discoloration of the plastic;
(2) Plastic mold
① The poor exhaust of the plastic mold results in the compression of the plastic in the heat insulation environment, so the material is easy to burn when it reacts with oxygen under high temperature and pressure;
② Too much lubricant / release agent in the barrel. When cleaning the barrel, remove additives with worse heat resistance than plastics, such as antistatic agents;
③ The size of nozzle, gate and sprue is too small;
(3) Injection molding process
① Screw speed / back pressure of preform is too high;
② Too high barrel / nozzle temperature;
③ Injection pressure is too high / injection time is too long / injection speed is too fast, resulting in product discoloration.
(4) Raw materials
① There are impurities in the material;
② Excessive moisture content in the material;
③ Colorant / additive decomposition.
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Link to this article：Discoloration of common problems in plastic injection molding
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