Key points of metal insert design for injection molding products​

Key points of metal insert design for injection molding products

The difference of thermal expansion coefficient between plastic and metal is 5 ~ 10 times. Therefore, when the plastic products with metal inserts are cooled, the shrinkage degree of them is different. Because of the large shrinkage ratio of plastic, they tightly hold the metal insert, and the plastic inner layer around the insert is stressed, while the outer layer is subjected to tensile stress, resulting in stress concentration.

The following points should be paid attention to when designing the insert to help reduce or eliminate the internal stress
a. As far as possible, select plastic parts as inserts.
b. As far as possible, the metal materials with small difference with the thermal expansion coefficient of plastic should be selected as the insert materials, such as aluminum, aluminum alloy and copper.
c. A layer of rubber or polyurethane elastic buffer layer is coated on the metal insert, and the coating layer is not melted during molding, which can reduce the shrinkage difference between them.
d. The degreasing treatment on the surface of the metal insert can prevent the grease from accelerating the stress cracking of the product.
e. The metal insert shall be properly preheated.
f. The thickness of plastic around the metal insert should be sufficient. For example, if the outer diameter of the insert is D and the plastic thickness around the insert is h, then the plastic thickness h ≥ 0.8d for the aluminum insert and H ≥ 0.9d for the copper insert.
g. Metal inserts shall be designed in a smooth shape, preferably with a fine roll pattern.