Why does PC and metal crack when molded together?
Plastic and metal coefficient of thermal expansion difference of 5 to 10 times, so with metal inserts of plastic parts in the cooling, the two constitute the shrinkage of the size of the Division, because the plastic shrinkage ratio is large and firmly embrace the metal inserts, in the inserts around the inner layer of the plastic by the compressive stress, and the outer layer by the tensile stress, resulting in a stress concentration phenomenon.
In the design of inserts, should pay attention to the following points to assist in reducing or eliminating internal stress.
① As far as possible to choose plastic parts as inserts
② As far as possible to choose and plastic coefficient of thermal expansion difference between small metal materials as insert materials, such as aluminum, aluminum alloy and copper, etc.
③ In the metal inserts coated with a layer of rubber or polyurethane cool elastic cushioning layer, and to ensure that the coating layer does not melt when molding, can reduce the two shrinkage difference.
④ Nominal degreasing of metal inserts can avoid the acceleration of stress cracking of plastic parts by grease.
⑤ Appropriate preheating of metal inserts.
⑥ The thickness of the plastic around the metal insert should be adequate. For example, the insert outer diameter of D, the thickness of plastic around the insert for the aluminum insert plastic thickness h20.8D; for copper inserts, plastic thickness h20.9D.
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