Structural Design of Injection-molded Plastic Products for Stress Reduction

The shape and size of plastic products

In the specific design of plastic products, in order to effectively disperse internal stress, the principle should be followed: the shape of the product should maintain continuity as much as possible, avoiding sharp angles, right angles, notches, and sudden expansion or contraction.

The edges of plastic products should be designed with rounded corners, where the inner radius of the rounded corner should be greater than 70% of the thickness of the thinner of the adjacent walls; The outer corner radius is determined based on the shape of the product.

For areas with significant differences in wall thickness, different cooling rates can easily generate cooling and orientation stresses. Therefore, it should be designed as a component with as uniform a wall thickness as possible. If uneven wall thickness is necessary, a gradual transition of wall thickness differences should be carried out.

Reasonably design metal inserts

The thermal expansion coefficients of plastic and metal differ by 5-10 times, so plastic products with metal inserts have different degrees of shrinkage when cooled. Due to the large shrinkage of plastic, they tightly embrace the metal insert, and the inner layer of plastic around the insert is subjected to compressive stress, while the outer layer is subjected to tensile stress, resulting in stress concentration.

When designing specific embedded components, the following points should be noted to help reduce or eliminate internal stress. a. Choose plastic parts as inserts as much as possible. b. Try to choose metal materials with a small difference in thermal expansion coefficient from plastic as the insert material, such as aluminum, aluminum alloy, and copper. c. Coating a layer of rubber or polyurethane elastic buffer layer on the metal insert and ensuring that the coating layer does not melt during molding can reduce the shrinkage difference between the two. d. Surface degreasing treatment of metal inserts can prevent grease from accelerating stress cracking of the product. e. Metal inserts should undergo appropriate preheating treatment. f. The thickness of the 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 of the aluminum insert is ≥ 0.8D; For copper inserts, the plastic thickness h ≥ 0.9 D. g. Metal inserts should be designed in a smooth shape, preferably with delicate rolling patterns.

Design of holes on plastic products

The shape, number, and position of holes on plastic products can have a significant impact on the concentration of internal stress.

To avoid stress cracking, it is important to avoid opening angular, rectangular, square, or polygonal holes in plastic products. Circular holes should be opened as much as possible, with elliptical holes having the best effect, and the long axis of the elliptical hole should be parallel to the direction of external force. If a circular hole is opened, equal diameter process circular holes can be added, and the center connecting lines of adjacent circular holes should be parallel to the direction of external force. This can achieve a similar effect as an elliptical hole; Another method is to create symmetrical slots around the circular hole to disperse internal stress.Please contact us for industrial plastic parts injection molding.