Weekly topic: 8 wealth points for those who answer correctly, 2 wealth points for participation； Question 124

When the injection-molded part is demolded, the deformation of the macromolecules has not yet ceased; further deformation of these macromolecules during storage and use can cause the part to deform. The main reason for product shrinkage is that rapid cooling during melt molding causes the macromolecules to pack more loosely (i.e., there is \"free volume\"). During storage and use, the development of rearrangement movements of macromolecules causes the packing to become increasingly dense, resulting in an increase in density and a decrease in volume. Polymers that can crystallize cause the volume of the molded product to shrink as a crystalline structure gradually forms.

What are the main reasons for internal stress in injection-molded products? When the injection-molded part is demolded, the deformation of the macromolecules has not yet ceased; further deformation of these macromolecules during storage and use can cause the part to deform. The main reason for product shrinkage is that rapid cooling during melt molding causes the macromolecules to pack more loosely (i.e., there is \"free volume\"). During storage and use, the development of rearrangement movements of macromolecules causes the packing to become increasingly dense, resulting in an increase in density and a decrease in volume. Polymers that can crystallize cause the volume of the molded product to shrink as a crystalline structure gradually forms.

Macromolecular deformation, product volume shrinkage

1. Rigidity of the molecular chain 2. Polarity of the molecular chain 3. Steric effect of substituents

Answer: Strong shear forces acting on the plastic melt during processing, orientation and crystallization processes that occur during processing, the difficulty in achieving uniform cooling rates across different parts of the melt, uneven plasticization of the melt, and difficulties in demolding the finished product – all these factors can lead to the generation of internal stresses.

When the injection-molded part is demolded, the deformation of the macromolecules has not yet ceased; further deformation of these macromolecules within the part during storage and use can cause the part to deform. The main reason for product shrinkage is that rapid cooling during melt molding causes the macromolecules to pack more loosely, resulting in the presence of \"free volume\". During storage and use, the development of rearrangement movements of macromolecules causes the packing to become increasingly dense, resulting in an increase in density and a decrease in volume. Polymers that can crystallize experience volume shrinkage in the molded products as crystal structures gradually form

Factors such as strong shear forces acting on the plastic melt during processing, orientation and crystallization processes that occur during processing, the difficulty in achieving uniform cooling rates across different parts of the melt, uneven plasticization of the melt, and difficulties in demolding the finished product can all lead to the generation of internal stresses. Depending on the causes of these internal stresses, they can be classified into the following categories: (1) Orientation stress: Orientation stress is a type of internal stress that arises when, during the flow of the plastic melt into the mold and during the holding pressure phase, the polymer chains arrange themselves in a oriented configuration along the direction of flow, with this configuration being fixed in place. The process by which orientation stress is generated is as follows: The melt near the walls of the mold cools more rapidly, resulting in an increase in the viscosity of the outer layer of the melt. This causes the flow velocity in the central layer of the melt to be much higher than that in the outer layer, leading to shear stress between the different layers within the melt and thus an orientation along the direction of flow. When the oriented polymer chains are fixed in place within the plastic product, it means that there is reversible high-elastic deformation that has not yet relaxed. Therefore, orientation stress is essentially the internal force exerted by the polymer chains as they attempt to transition from an oriented configuration to an unoriented one. Heat treatment can be used to reduce or eliminate orientation stress in plastic products. The distribution of orientation stress in plastic products decreases from the surface layer to the inner layers, following a parabolic pattern. (2) Cooling stress: Cooling stress is a type of internal stress that occurs when plastic products contract unevenly during the cooling and setting process after melting. Especially in thick-walled plastic products, the outer layer cools and solidifies first, while the inner layer may still be in a molten state. This results in the inner layer restricting the contraction of the outer layer, causing the inner layer to be under compressive stress while the outer layer is under tensile stress. The distribution of cooling stress in plastic products increases from the surface layer to the inner layers, also following a parabolic pattern. Additionally, in plastic products with metal inserts, the significant difference in thermal expansion coefficients between metal and plastic can lead to uneven contraction and thus internal stresses

1. Polymer orientation results in unreleased elastic deformation within the fabricated parts； 2. Volumetric thermal stress is caused by uneven contraction of the part during cooling; 3. It is practically impossible for the part to reach its equilibrium volume immediately after demolding when polymers solidify in the mold cavity, even under extremely slow conditions; 4. Internal stresses related to the deformation that occurs when the part is ejected ； 4. Unreasonable design of the injection-molded product’s shape, inadequate mold design, incorrect molding process conditions, and inappropriate selection of the injection machine can all result in significant internal stresses within the product

When the injection-molded part is demolded, the deformation of the macromolecules has not yet ceased; further deformation of these macromolecules during storage and use can cause the part to deform. The main reason for product shrinkage is that rapid cooling during melt molding causes the macromolecules to pack more loosely (i.e., there is \"free volume\"). During storage and use, the development of rearrangement movements of macromolecules leads to increasingly tighter packing, resulting in an increase in density and volume contraction. Polymers that can crystallize experience volume shrinkage in the molded products as crystal structures gradually form

Answer: When the injection-molded part is demolded, the deformation of the macromolecules has not yet ceased; further deformation of these macromolecules during storage and use can cause the part to deform. The main reason for product shrinkage is that rapid cooling during melt molding causes the macromolecules to pack more loosely (i.e., there is \"free volume\"). During storage and use, the development of rearrangement movements of macromolecules causes the packing to become increasingly dense, resulting in an increase in density and a decrease in volume. Polymers that can crystallize experience volume shrinkage in the molded products as crystal structures gradually form