The Internal Stress of Injection Molded Products
There are many reasons for the generation of internal stress, such as:
- Strong shear forces acting on the plastic melt during plasticizing.
- The presence of orientation and crystallization effects during processing.
- Extreme difficulties in achieving uniform cooling rates for different parts of the melt.
- Uneven plasticization of the melt.
- Difficulties in demolding of the product.
All of these factors can lead to the generation of internal stress. Depending on the reasons for the internal stress, it can be categorized into the following types.
1. Orientation internal stress
Orientation internal stress is an internal stress that occurs when the polymer melt freezes with the macromolecular chains aligned in a directional conformation during the plastic injection and back-pressure holding.
The specific process that generates orientation stress is as follows:
The melt near the mold cavity walls experiences rapid cooling, resulting in an increase in viscosity in the outermost layer of the melt.
As a result, the melt in the central layer of the cavity flows much faster than the surface layer, causing shear stress to act between the inner layers of the melt, resulting in orientation along the flow direction.
The frozen orientation of large polymer chains within plastic products implies the existence of unreleased reversible high elastic deformation. Therefore, orientation stress is the internal force when polymer chains transition from an oriented configuration to an unoriented one.
Through heat treatment methods, it is possible to reduce or eliminate orientation stress within plastic products.
The distribution of orientation stress within plastic products decreases from the surface to the interior and follows a parabolic pattern.
2. Cooling-Induced Internal Stress
Cooling-induced internal stress is a type of internal stress generated in plastic products during the cooling and solidification phase due to uneven shrinkage.
This is especially noticeable in thick-walled plastic products. In such cases, the outer layer of the plastic product cools and solidifies first, while the inner layer may still be in a molten state.
As a result, the core layer restricts the shrinkage of the surface layer, leading to compressive stress in the core layer and tensile stress in the surface layer.
The distribution of cooling-induced internal stress in plastic products increases from the surface to the interior and follows a parabolic pattern.
Additionally, plastic products with metal inserts can also develop internal stresses due to the significant difference in the thermal expansion coefficients between metal and plastic, leading to uneven shrinkage.
Apart from the two main types of internal stresses mentioned above, there are several other types of internal stresses.
- For crystalline plastic products, differences in the crystalline structure and crystallinity among various parts of the product can also generate internal stresses.
- There are also configuration-induced stresses and demolding stresses, although these internal stresses typically account for a smaller proportion.