Plastic products should be designed to prevent stress cracking. There are three points:
1. 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 be as continuous as possible, avoiding acute angles, right angles, gaps and sudden expansion or contraction.
For the edge of the plastic product, it should be designed as rounded corners, wherein the radius of the fillet should be greater than 70% of the thickness of the thinner adjacent walls; the radius of the fillet is determined according to the shape of the product.
For parts with large wall thickness differences, cooling internal stress and orientation internal stress are likely to occur due to different cooling rates. Therefore, it should be designed as a part with the same wall thickness as possible. If the wall thickness is not uniform, a gradual transition of the wall thickness difference is required.
2. Reasonable design of metal inserts
The thermal expansion coefficient of plastic and metal differs by 5-10 times. Therefore, when the plastic products with metal inserts are cooled, the degree of shrinkage formed by the two is different. Because the shrinkage of the plastic is relatively large, the metal inserts are tightly held, and the inserts are embedded. The surrounding plastic inner layer is under compressive stress, while the outer layer is subjected to tensile stress, resulting in stress concentration.
When specifying the juice insert, the following points should be noted to help reduce or eliminate internal stress:
a. Select plastic parts as inserts as much as possible.
b. Select a metal material that is different from the thermal expansion coefficient of the plastic as an insert material, such as aluminum, aluminum alloy and copper.
c. Apply a layer of rubber or polyurethane elastic buffer layer on the metal insert and ensure that the coating layer does not melt during molding, which can reduce the difference in shrinkage between the two.
d. The surface degreasing treatment of the metal insert can prevent the grease from accelerating the stress cracking of the product.
e. The metal insert is subjected to a suitable pre-heat treatment.
f. The thickness of the plastic around the metal insert should be sufficient. For example, the outer diameter of the insert is D, the thickness of the plastic around the insert is h, the thickness of the aluminum insert plastic is h ≥ 0.8D; for the copper insert, the thickness of the plastic h ≥ 0.9D.
g. Metal inserts should be designed in a rounded shape, preferably with a fine knurling.
3. Design of the upper hole of plastic products
The shape of the holes in the plastic product, the number of holes and the position of the holes all have a great influence on the degree of internal stress concentration.
In order to avoid stress cracking, it is forbidden to open prismatic, rectangular, square or polygonal holes in plastic products. A circular hole should be opened as much as possible, wherein the elliptical hole has the best effect, and the long axis of the elliptical hole should be parallel to the direction of the external force. For example, if a circular hole is opened, the round hole of the same diameter can be opened, and the center connecting line of the adjacent two circular holes can be parallel to the direction of the external force, so that the effect similar to the elliptical hole can be obtained; there is also a method, that is, in the circle Symmetrical slots are formed around the holes to disperse internal stresses.
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