Plastic and metal thermal expansion coefficient difference of 5 ~ 10 times, and therefore 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 by compressive stress, while the outer layer by 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.
a.As far as possible, choose plastic parts as inserts.
b. As far as possible to choose and plastic coefficient of thermal expansion of the small difference between the metal material as an insert material, such as aluminum, aluminum alloy and copper, etc..
c. In the metal insert coated with a layer of rubber or polyurethane elastic buffer layer, and ensure that the coating layer does not melt when forming, can reduce the difference between the two shrinkage.
d.Degreasing the surface of the metal insert can prevent grease from accelerating the stress cracking of the product.
e. Appropriate preheating treatment for metal insert.
f. 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 insert plastic thickness h ≥ 0.8 D; for copper inserts, plastic thickness h ≥ 0.9 D.
g.Metal inserts should be designed into a rounded shape, preferably with a delicate knurling pattern.
preferably with a delicate knurling pattern.
Demoulding slope of plastic parts.
The size of the release angle is no certain guidelines, most of them are based on experience and in accordance with the depth of the product to decide. In addition, the molding method, wall thickness and choice of plastic are also taken into account.
Generally speaking, a certain amount of release slope is required for any side of the molded product in order for the product to come out of the mold smoothly. The size of the release slope is generally 0.5° to 1°.
Specific selection of the release slope pay attention to the following points.
a.If the surface of the plastic part is glossy, the dimensional accuracy is high, and the shrinkage rate is small, a smaller release slope should be selected, such as 0.5°.
b. For higher and larger sizes, a smaller release slope should be taken according to the actual calculation.
c. If the shrinkage rate of plastic parts is large, a larger slope value should be selected.
d. When the wall thickness of plastic parts is thicker, it will increase the shrinkage of molding, and the slope of demolding should adopt a larger value.
e.When the structure is designed to be inserted in pairs, the slope of the inserting surface is generally 1°～3°.
f.Take the direction of the slope, the general bore to the small end shall prevail, in line with the drawings, the slope from the direction of expansion, the shape of the large end shall prevail, in line with the drawings, the slope from the direction of reduction.
g.Shell surface demoulding slope greater than or equal to 3 °. In addition to the shell surface, the remaining features of the shell release slope to 1 ° as the standard release slope. Special can also be taken in accordance with the following principles: below 3mm high reinforcement of the release slope to take 0.5 °, 3 to 5mm to take 1 °, the rest to take 1.5 °; below 3mm high cavity of the release slope to take 0.5 °, 3 to 5mm to take 1 °, the rest to take 1.5 °.
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