Discoloration means the surface color of a molded part is not consistent with the required color. Discoloration may be caused by a variety of reasons, which may cover the several aspects of injection molding machine, mold, process and raw materials, etc.
(1) Injection Molding Machine
① Equipment is not clean and needs to be cleansed. Dusts or powders are deposited in the barrel, which contaminates and discolors the materials;
② Temperature control failure caused by malfunction of thermal couple, temperature controller or the heating system;
③ Obstacles exist in the barrel, causing the plastic material to decompose; foreign metal objects are stuck in the barrel or the screw channel, which keep grinding, causing plastic material discoloration;
① Poor mold venting, causing plastic to be compressed in a heat-insulated environment, so the material tends to be burnt when reacting with oxygen under high-temperature and high pressure conditions;
② Mold gate is too small;
③ Too much lubricants/mold releasing agents exist in the barrel. Clean the material barrel, to remove the additives which possess poorer heat resistant properties than the plastic, such as antistatic agents;
④ Nozzle, sprue and runner sizes are too small;
(3)Injection Molding Process
① Screw speed/pre-molding back pressure is too high;
② Barrel/nozzle temperature is too high;
③ Injection pressure is too high/injection time is too long/injection speed is too fast, causing product discoloration.
(4) Raw Materials
① Material is contaminated;
② Too much moisture/VOC content contained in the material;
③ Coloring agents/additives are decomposed.