The shape, number of holes, and location of the holes on plastic products can have a great impact on the degree of internal stress concentration.
To avoid stress cracking, do not open prismatic, rectangular, square, or polygonal holes in plastic products. As far as possible, circular holes should be opened, of which oval holes have the best effect, and the long axis of oval holes should be parallel to the direction of the external force. If you open a round hole, you can open additional round holes of equal diameter, and make the connection line of the center of two adjacent round holes parallel to the direction of the external force, so that you can achieve similar results as elliptical holes; there is another method, that is, to open symmetrical slot holes around the round hole to disperse the internal stress.
The specific design of plastic products, in order to effectively disperse the internal stress, should follow the principles of
1. For parts with large differences in wall thickness, cooling internal stress and orientation internal stress are easily generated due to different cooling rates. Therefore, it should be designed to be as uniform as the possible wall thickness of the manufactured parts, such as must be uneven wall thickness, the gradual transition of the wall thickness difference.
The shape of the product should be as far as possible to maintain continuity, to avoid sharp angles, right angles, notches, and sudden expansion or contraction.
2. For the edge of the plastic products should be designed into rounded corners, where the inner corner radius should be greater than the adjacent two walls in the thin thickness of more than 70%; the outer corner radius is determined according to the shape of the product.
Plastic and metal thermal expansion coefficient difference of 5 ~ 10 times, and thus with metal insert plastic products in the cooling, the formation of the two shrinkage degree is different because the plastic shrinkage is relatively large and clinging to the metal insert, in the insert around the inner layer of plastic compressive stress, while the outer layer of tensile stress, resulting in stress concentration phenomenon.
In the specific set juice insert, the following points should be noted to help reduce or eliminate internal stress.
1. The metal insert should be designed into a round and smooth shape, preferably with a delicate knurling pattern.
2. The thickness of the plastic around the metal insert should be sufficient. For example, the outer diameter of the insert is D, the plastic thickness around the insert is h, then the aluminum inserts plastic thickness h ≥ 0.8 D; for copper inserts, plastic thickness is h ≥ 0.9 D.
3. In the metal insert coated with a layer of rubber or polyurethane elastic buffer layer, and to ensure that the coating layer does not melt when forming, can reduce the shrinkage difference between the two.
4. As far as possible, choose plastic parts as inserts.
5. Degreasing the surface of the metal insert can prevent grease from accelerating the stress cracking of the product.
6. As far as possible, choose metal materials with plastic thermal expansion coefficient difference is small to do insert materials, such as aluminum, aluminum alloy, and copper, etc.
7. Pre-heat the metal insert properly.
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