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Analysis of internal stress in injection molding products Dec 12, 2017

In the process of injection molding, there is an inherent quality problem in the injection molding, that is, internal stress. The sources of internal stress are related to the types of plastic materials used, the types of injection machines, the structure of plasticizing system, the structure and accuracy of molds, the structure of plastic products, the setting and control of injection molding process parameters, the production environment and the state of operators. Any of these problems will affect the quality of the products.

Moreover, due to the surface quality of the products is a reflection of the intrinsic quality, so that can cause the internal quality of the product factors, can also cause the surface quality of the products and other quality problems, such as reducing caused by products cracking and crazing, warpage, deformation, mechanical strength, and even loss of use value.

The types and causes of internal stress

There are four main internal stresses in the injection products.

1. Temperature stress:

It is the stress produced by the uneven temperature of the product when the product is cooled. When the melt enters a lower temperature mold, the melt close to the cavity wall is rapidly cooled and solidified. Due to the poor thermal conductivity of the solidified polymer layer, a larger temperature gradient is produced in the direction of the thickness of the product. The first solidification of the outer melt prevents the shrinkage of the inner melt after solidification, resulting in compressive stress (shrinkage stress) in the outer layer, and tensile stress (oriented stress) in the inner layer. On the other hand, the cooling rate is not uniform because the wall thickness is uneven and the cooling temperature is uneven.

2. Orientation stress:

It is the stress produced by the large molecular orientation in the interior of the product. For linear resin and fiber reinforced plastics, it is the most easy to produce orientation stress in processing. As a result, the degree of molecular orientation along the flow direction is the largest. Under fast cooling condition, if the stretched chain is not enough to relax, the orientation stress is generated in this direction.

3. Contractile stress:

In the process of injection molding, the equilibrium state of the plastic molecule itself is destroyed and the stress in the unbalanced volume is produced. For example, the internal stress produced by the uneven contraction of the crystalline and amorphous regions of the crystalline plastic is caused by the uneven contraction.

4. Demoulding stress:

The stress produced by the deformation of a product when demoulding. This is mainly due to the poor machining precision and unreasonable design of the mold, such as the inaccuracy of stripping, the inadequacy of the number of thimble or the unbalance of the top.

The above 2 to 4 types of stress can be summed up as forming stress, and it is the stress produced by the forming process. In general, there are several stresses in the injection products at the same time, and the damage of the products is the result of the joint action of several stresses.