PC is the general name of polycarbonate, due to its excellent mechanical properties, commonly known as a bulletproof adhesive.PC has the characteristics of high mechanical strength, wide temperature range, good electrical insulation performance (but the arc resistance performance remains unchanged), good dimensional stability, transparency and so on.It is widely used in electrical products, electrical instrument shell, electronic products structural parts. PC has many modified products, usually adding glass fiber, mineral filler, chemical flame retardant, other plastics. The fluidity of PC is poor and the processing temperature is high, so the processing of many grades of modified PC materials needs special plasticizing injection structure.
The phenomenon of the bubble in the injection molding of PC plastic will have serious consequences. In order to prevent poor backflow or uneven cooling resulting in bad plastic molding, surface defects and deterioration, the following points should be noted in the mold design:
- The wall thickness should be as uniform as possible and the demoulding slope should be large enough.
- In the transition part, sharp corners or edges should be eliminated, especially for PC products.
- Gate. The runner should be as wide and short as possible, the gate position should be set according to the shrinkage condensation process, and the cooling material well should be used when necessary.
- The surface of the die should be smooth and clean with low roughness (preferably less than 0.8) exhaust holes. Holes and grooves must be sufficient to ensure timely discharge of gas from melting while avoiding too thin wall thickness, generally not less than l.0mm.
PC has a large water absorption, processing must be preheated drying. Pure PC 120 ℃ drying, modified PC generally with 110 ℃ drying temperature more than 4 hours, time can’t more than 10 hours. Any impurities in PC plastics may affect the transparency of the product, so it must be kept dry during storage and transportation. In the process of drying, the air input should be filtered, dehumidified and the barrel, screw and its accessories should be cleaned to prevent the pollution of raw materials and old materials or impurities in the screw and accessories depression. The recycled material cannot be mixed with different color masterbatch at the same time, otherwise, it will seriously damage the property of the finished product. Must be sealed when feeding, to ensure that the raw materials clean. Because the water will cause the deterioration of raw materials after heating. So in injection molding, must use the dry hopper. Therefore, before and after the use of screw cleaning agent should be used to clean all parts so that they can not stick impurities, or use PE/ Ps instead.
To prevent raw material at high temperature for long time cause degradation, temporary outage, drier and barrel temperature should be reduced, such as PC, PMMA, barrel temperature will drop to below 160 ℃.(hopper temperature for PC should have fallen to below 100 ℃). Transparent plastic products due to high light transmittance, its surface quality requirements are strict, can not have any markings, blowhole, white, fog halo, black spots, discoloration, poor luster and other defects, so in the whole process of injection of raw materials, equipment. Mold and even product design should be very careful and put forward strict and even special requirements.
Because transparent plastic has high melting point and poor fluidity. In order to ensure the surface quality of the product, the need for fine adjustment of the machine high temperature, injection pressure, injection speed and other process parameters, so that the injection can be filled with mold and will not produce internal stress caused by product deformation and cracking. Therefore, it is necessary to strictly control the raw materials, equipment and mold requirements, injection molding process.