In the plastic injection molding, internal stress is a big problem for engineers, so how should we do to avoid this problem?
The plastic part design is a very import process.
1.Shape and size of plastic products
In the design of plastic products, in order to effectively disperse the internal stress. There are some principles: the shape of the product should be as continuous as possible, avoiding acute angles, right angles, and sudden expansion or reduction.
The edges of plastic products shall be designed with rounded corners. The radius of the inner corners shall be greater than 70% of the thickness of the thinner of the two adjacent walls. We should determine the radius of the outer corners according to the shape of the product.
For parts with large differences in wall thickness, due to different cooling speeds, cooling internal stress and orientation internal stress are liable to occur. Therefore, it should be designed as a part with as uniform wall thickness as possible.
If the wall thickness must be uneven, a gradual transition in wall thickness difference should be performed.
2.Reasonable design of metal inserts
The thermal expansion coefficients of plastic and metal differ by 5 to 10 times.
Therefore, when plastic products with cold metal inserts, the degree of shrinkage between the two is different.
In the plastic injection molding,due to the relatively large shrinkage of plastic, we need fix the metal inserts tightly.
The inner plastic layer is subjected to compressive stress. While the outer layer is subjected to tensile stress, resulting in stress concentration.
When designing the juice insert, the following points will help reduce or eliminate internal stress:
a. Whenever possible, choose plastic parts as inserts.
b. Whenever possible, choose a metal material with a small thermal expansion coefficient that is different from the plastic as the insert material, such as aluminum, aluminum alloy, and copper.
c. Apply a rubber or polyurethane elastic buffer layer on the metal insert, and ensure that the coating layer does not melt during injection molding, which can reduce the shrinkage difference between the two.
d. Degreasing the surface of the metal insert can prevent the oil from accelerating the stress cracking of the product.
e. The metal insert is appropriately after pre-heat treatment.
f. The thickness of the plastic surrounding the metal insert should be sufficient. For example, if the outer diameter of the insert is D and the thickness of the plastic 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≥0.9 D.
g. Metal inserts need in a smooth shape, preferably with a delicate rolling pattern.
3.Design of holes in plastic products
The shape, number of holes, and location of holes in plastic products all have a large effect on the degree of internal stress concentration.
To avoid stress cracking, do not open prismatic, rectangular, square or polygonal holes in plastic products. Circular holes need open as much as possible. The best effect is the oval hole .And the long axis of the oval hole should be parallel to the direction of external force.
If a circular hole is open, we can add a technological circular hole of equal diameter and the center connecting line of two adjacent circular holes to the direction of the external force.
To achieve an effect similar to that of an elliptical hole; there is also a method of opening symmetrical grooves around the circular hole to disperse the internal stress.