How to prevent stress cracking from the design of plastic products
How to prevent stress cracking from the design of plastic products
In the design of plastic products, in order to effectively disperse the internal stress, the following principle should be followed: the shape of the products should be as continuous as possible to avoid acute angle, right angle, gap and sudden expansion or reduction.For plastic products, the edge should be designed as a fillet, in which the radius of the inner fillet should be greater than 70% of the thickness of the thinner one of the two adjacent walls; the radius of the outer fillet should be determined according to the shape of the product.
For the parts with large wall thickness difference, cooling internal stress and orientation internal stress are easy to occur due to different cooling speed. Therefore, we should design the parts with uniform wall thickness as much as possible. If the wall thickness must be uneven, the gradual transition of wall thickness difference should be carried out.
2.Reasonable design of metal insert
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.
Design of holes in plastic products
The shape, number and location of holes in plastic products have great influence on the internal stress concentration.
In order to avoid stress cracking, do not open prismatic, rectangular, square or polygonal holes on plastic products. Circular holes should be opened as far as possible, and the effect of elliptical holes is the best, and the long axis of the elliptical holes should be parallel to the direction of external force.
If a round hole is opened, a process round hole of equal diameter can be added, and the central connecting line of two adjacent circular holes is parallel to the direction of external force, so that the effect similar to that of an elliptical hole can be obtained; another method is to set up symmetrical slot holes around the circular hole to disperse the internal stress.
