Stress in Materials
Stress refers to the force experienced by an object per unit area, emphasizing the internal force distribution within the object. Typically, when an object is subjected to an external force, internal stresses are generated to resist this force. When there is no external force, the inherent internal stress, known as residual stress, occurs due to uneven plastic deformation within the object.
Classification of Residual Stress
Residual stress can be classified into three categories based on the range of its action:
- First Category Residual Stress (Macroscopic Residual Stress): This type of stress is caused by uneven deformation in different parts of the material, creating internal stress within the macroscopic range of the material.
- Second Category Residual Stress (Microscopic Residual Stress): This stress arises from uneven deformation between the individual crystals or sub-crystals within the material (most solid substances in nature are crystalline).
- Third Category Residual Stress (Lattice Distortion Stress): This is caused by lattice distortions, where part of the atoms within the crystal move away from their equilibrium positions, generating internal stress. This is the most significant internal stress in deformed (damaged) materials.
Plastic Residual Stress refers to the internal stress generated during the plastic melt processing due to factors such as molecular chain orientation and cooling shrinkage.
The essence of residual stress is the unbalanced conformation of the polymer chains formed during the melting process. When the material cools and solidifies, it cannot immediately return to a balanced conformation that matches the environmental conditions. This unbalanced conformation is a reversible elastic deformation stored in the plastic product as potential energy. Under suitable conditions, this unstable conformation will transition into a more stable state, releasing energy in the form of kinetic energy.
When the forces between the molecular chains and their entanglements can no longer withstand this kinetic energy, the internal stress balance is destroyed, leading to phenomena like stress cracking and warping deformation in plastic products.
Causes of Plastic Residual Stress
Residual stress arises from several factors, such as intense shear forces during the plastic melt processing, orientation and crystallization effects during processing, difficulty in achieving uniform cooling rates in different parts of the melt, uneven melt plastication, and difficulty in part ejection. Residual stress can be classified into the following types based on its cause:
- Orientation Residual Stress
This stress occurs when polymer chains orient along the flow direction during the molding and holding stages, resulting in internal stress as the oriented conformation of the chains is frozen. - Cooling Residual Stress
Cooling residual stress is generated when plastic parts shrink unevenly during cooling. For thick-walled plastic products, the outer layers cool and solidify first, while the inner layers may still be molten. This restriction on the shrinkage of the inner layers induces compressive stress in the core and tensile stress on the surface. - Crystallization Residual Stress
For crystalline plastic products, differences in crystallization structure and crystallinity between different parts of the product can also create internal stress.
In addition to the above types of residual stress, there are also mold release stress, configuration stress, and other forms of internal stress, but these typically contribute a smaller share to the total residual stress.
Factors Affecting the Generation of Residual Stress in Plastics
- Molecular Chain Rigidity
The greater the rigidity of the polymer chains, the higher the melt viscosity, leading to reduced chain mobility, making it harder for reversible elastic deformation to recover. This can result in larger residual stresses. - Molecular Chain Polarity
The higher the polarity of the molecular chains, the stronger the intermolecular forces, which hinder chain mobility, reducing the extent of reversible deformation and thus increasing the residual stress. - Steric Effects of Substituent Groups
The larger the steric hindrance of the side chains or substituent groups, the more they hinder the free movement of polymer chains, leading to greater residual stress.
The relative order of residual stress in some common polymers is as follows: PPO > PSF > PC > ABS > PA6 > PP > HDPE
Reducing and Dispersing Plastic Residual Stress
- Raw Material Formulation Design
- Choosing high molecular weight resins: Larger molecular weights enhance the interaction and entanglement between polymer chains, which increases resistance to stress cracking.
- Low impurity content: Impurities act as stress concentrators and decrease the overall strength of the plastic. Reducing the impurity content can lower residual stress.
- Blending modification: Mixing resins that are prone to stress cracking with others can reduce the degree of internal stress.
- Reinforcement modification: Using reinforcing fibers can lower internal stress, as the fibers help in entangling the polymer chains, thus improving resistance to stress cracking.
- Nucleating modification: Adding nucleating agents to crystalline plastics can promote the formation of smaller crystals, which lowers and disperses internal stress.
- Control of Molding and Processing Conditions
- Barrel temperature: Higher barrel temperatures help reduce orientation stress by promoting more uniform melt plastication, whereas lower barrel temperatures increase melt viscosity, leading to greater molecular alignment and higher residual stress.
- Mold temperature: Mold temperature affects both orientation and cooling residual stress. Too low a mold temperature accelerates cooling and leads to uneven shrinkage, increasing cooling residual stress.
- Injection pressure: High injection pressure increases shear forces during molding, increasing the likelihood of orientation stress.
- Holding pressure: High holding pressure during the cooling phase can result in forced molecular orientation, increasing orientation stress.
- Injection speed: Faster injection speeds lead to more molecular alignment and greater orientation stress, while too slow an injection speed may cause layering of the melt and result in stress concentration.
- Holding time: Longer holding times increase shear forces on the melt, leading to higher residual stress due to additional molecular orientation.
- Post-Molding Heat TreatmentPost-molding heat treatment involves heating the molded part to a certain temperature to allow the polymer chains to relax, thus reducing internal stresses. This is particularly effective for polymers with higher molecular rigidity or those with thick walls or metal inserts.
- Design of Plastic Products
- Shape and thickness distribution: To disperse residual stress, plastic parts should have continuous shapes without sharp corners or abrupt transitions in thickness.
- Design of metal inserts: For products with metal inserts, the difference in thermal expansion between plastic and metal can cause uneven shrinkage during cooling, creating residual stress. Proper selection of materials with similar coefficients of thermal expansion and using a cushioning layer can reduce this stress.
- Mold Design
- Gate size and position: The size and position of the gate affect the flow of the plastic melt in the mold. A too-large gate leads to prolonged holding time, increasing residual stress, while a too-small gate can result in insufficient filling of the mold.
- Runner design: Shorter, thicker runners can reduce pressure loss and temperature drop, thereby minimizing residual stress.
- Cooling system design: The cooling system should ensure uniform and slow cooling of the mold, particularly at thick-walled and thin-walled areas, to avoid the formation of internal stresses.
- Stress Relief MethodsTo alleviate stress, methods like heat treatment, using acetone or water-based solutions, and improving the design to avoid stress concentration points are commonly used. Heat treatment at temperatures 10-20°C above the service temperature or 5-10°C below the heat distortion temperature helps release residual stresses.