What are the differences between oriented internal stress and cooling internal stress?

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There are many causes for the generation of internal stress. For example, strong shear action exerted on the plastic melt during processing, orientation and crystallization occurring in the processing process, the extreme difficulty in achieving uniform cooling rate across different parts of the melt, uneven plasticization of the melt, difficult demolding of products, etc., all can lead to the generation of internal stress. According to the different causes of internal stress, it can be divided into the following categories. ### Oriented Internal Stress Oriented internal stress is a type of internal stress generated by the freezing of the oriented conformation of macromolecular chains arranged along the flow direction during the flow filling and pressure-holding replenishment stages of plastic melt. The specific generation process of oriented stress is as follows: *The melt near the runner wall experiences a rapid cooling rate, which increases the viscosity of the outer-layer melt. Consequently, the flow velocity of the melt in the central layer of the mold cavity is much higher than that in the surface layer, resulting in shear stress between different layers inside the melt and thus inducing orientation along the flow direction.*

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The frozen oriented macromolecular chains in plastic products mean the presence of unreleased reversible high-elastic deformation. Therefore, oriented stress refers to the internal force that drives the macromolecular chains to transition from an oriented conformation to a random one. The oriented stress in plastic products can be reduced or eliminated through heat treatment.

The distribution of oriented internal stress in plastic products decreases progressively from the surface layer to the inner layer, exhibiting a parabolic variation trend.

Cooling Internal Stress

Cooling internal stress is a type of internal stress generated by uneven shrinkage during the cooling and setting stage of plastic products in melt processing. Especially for thick-walled plastic products, the outer layer solidifies and shrinks first upon cooling, while the inner core may still remain in a molten state. In this case, the core layer restricts the shrinkage of the surface layer, resulting in the core layer being in a state of compressive stress and the surface layer in a state of tensile stress.

The distribution of cooling internal stress in plastic products increases gradually from the surface layer to the inner layer, also showing a parabolic variation characteristic.

In addition, for plastic products with metal inserts, significant differences in the thermal expansion coefficients between metal and plastic tend to cause internal stress due to uneven shrinkage.

Besides the two main types of internal stress mentioned above, there are several other types: for crystalline plastic products, differences in the crystalline structure and crystallinity among different parts inside the products can also lead to internal stress. Additionally, there are configuration internal stress and demolding internal stress, but their proportions in the total internal stress are relatively small.

Source: Micro Injection Molding

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What are the differences between oriented internal stress and cooling internal stress?

Cooling Internal Stress