How to prevent stress cracking from the design of plastic products? The following points.
Plastic products shape and size
In the specific design of plastic products, in order to effectively disperse the internal stress, should follow the principle that the shape of the product should be as continuous as possible to avoid acute angles, right angles, gaps and sudden expansion or contraction.
For the edge of plastic products should be designed to fillet, fillet radius should be greater than the thickness of the thinner than the adjacent two walls of more than 70%; fillet radius according to the shape of the product to determine.
For parts with large wall thickness difference, it is easy to produce cooling internal stress and orientation internal stress due to different cooling rate.Therefore, it is necessary to design parts with the same wall thickness as possible.If the wall thickness is not uniform, The gradual transition of difference.
Reasonable design of metal inserts
Plastic and metal thermal expansion coefficient difference of 5 to 10 times, so with plastic metal inserts in the cooling, the contraction of the two different degrees, due to plastic shrinkage relatively large and tightly hug the metal inserts, insert Around the plastic inner compression stress, while the outer layer by the tensile stress, resulting in stress concentration.
In the specific set of juice inserts, should pay attention to the following points to help reduce or eliminate the stress:
a. Select plastic parts as inserts if possible.
b. As far as possible choose the material with a small difference in thermal expansion coefficient of plastic materials, such as inserts, aluminum, aluminum and copper.
c. The metal insert coated with a layer of rubber or polyurethane elastic buffer layer, and ensure that the coating does not melt forming, can reduce the difference between the two shrinkage.
d. The surface of the metal insert degreasing, can prevent the grease to accelerate the product stress cracking.
e. Metal inserts are preheated appropriately.
For example, the outer diameter of the insert is D, the plastic thickness around the insert is h, the thickness of the plastic for the aluminum insert is h ≧ 0.8D; for the copper insert, the thickness of the plastic is h ≥ 0.9D.
g. Metal insert should be designed into a smooth shape, preferably with a delicate knurl.
Plastic products on the hole design
Plastic products on the hole shape, hole number and location of the hole will have a great impact on the degree of stress concentration.
In order to avoid stress cracking, it is forbidden to open the prismatic, rectangular, square or polygonal holes on the plastic products.It should be provided with circular holes as much as possible, among which the oval hole has the best effect and the long axis of the oval hole should be parallel to the external force The role of direction.
Such as the opening of the round hole, you can increase the diameter of the process such as the round hole, and the two adjacent holes connecting the center parallel to the direction of the role of external forces, so you can get the effect similar to the oval hole; there is a method that circle Symmetrical slots are provided around the holes to disperse the internal stress.