Internal stress in plastics refers to an inherent stress generated during the molten plastic processing due to factors such as the orientation of macromolecular chains and cooling contraction.
The essence of internal stress is the unbalanced conformation formed by macromolecular chains during molten processing. This unbalanced conformation cannot immediately return to an equilibrium conformation adapted to environmental conditions upon cooling and solidification. This unbalanced conformation is essentially a reversible, highly elastic deformation. This frozen elastic deformation is normally stored in the plastic product as potential energy. Under suitable conditions, this forced, unstable conformation will transform into a free, stable conformation, and the potential energy will be released as kinetic energy.
When the forces and entanglement forces between macromolecular chains cannot withstand this kinetic energy, the internal stress balance is disrupted, and the plastic product will exhibit stress cracking and warping deformation.
I. Causes of Internal Stress in Plastics
1. Orientation Internal Stress
Orientation internal stress is an internal stress generated when the orientation of macromolecular chains along the flow direction is frozen during the flow, filling, and pressure-holding processes of the molten plastic.
The detailed process of orientation stress generation is as follows: The melt near the runner wall cools rapidly, causing an increase in the viscosity of the outer melt layer. This results in a much higher flow velocity in the core layer of the mold cavity compared to the surface layer, leading to shear stress between the internal layers and resulting in orientation along the flow direction.
The unfreezing of oriented macromolecular chains within plastic products signifies the presence of unrelaxed, reversible, highly elastic deformation. Therefore, orientation stress is the internal force driving macromolecular chains to transition from an oriented conformation to a non-oriented conformation. Heat treatment can reduce or eliminate orientation stress within plastic products.
The distribution of orientation stress in plastic products decreases from the surface to the inner layers, exhibiting a parabolic change.
2. Cooling Stress
Cooling stress is an internal stress generated in plastic products during the melt processing due to uneven shrinkage during cooling and setting. Especially for thick-walled plastic products, the outer layer cools and solidifies first, shrinking while the inner layer may still be a hot melt. This core layer limits the shrinkage of the surface layer, resulting in the core layer being under compressive stress and the surface layer under tensile stress.
The distribution of cooling internal stress in plastic products increases from the surface to the inner layer, exhibiting a parabolic change.
Furthermore, plastic products with metal inserts are prone to uneven shrinkage internal stress due to the significant difference in thermal expansion coefficients between metal and plastic.
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 of different parts of the product also generate internal stress. There are also structural stresses and demolding stresses, but their proportion is generally small.
II. Factors Affecting the Generation of Internal Stress in Plastics
1. Molecular Chain Rigidity
The greater the rigidity of the molecular chain, the higher the melt viscosity, and the poorer the mobility of the polymer molecular chain. Therefore, the recovery from reversible elastic deformation is poor, easily leading to residual internal stress. For example, some polymers containing benzene rings in their molecular chains, such as PC, PPO, and PPS, have corresponding products with higher internal stress.
2. Molecular Chain Polarity
The greater the polarity of a molecular chain, the stronger the intermolecular attraction, making intermolecular movement more difficult and reducing the degree of recovery from reversible elastic deformation, resulting in higher residual internal stress. For example, some plastics containing polar groups such as carbonyl, ester, and nitrile groups in their molecular chains have corresponding products with higher internal stress.
3. Steric Hinderment Effect of Substituent Groups
The larger the volume of the substituent groups on the side groups of the macromolecule, the more it hinders the free movement of the macromolecular chain, leading to increased residual internal stress. For example, the phenyl group in polystyrene has a larger volume, resulting in higher internal stress in polystyrene products.
III. Three Methods for Testing Internal Stress in Injection Molded Parts
1. Solvent Method
▶ Acetic Acid Immersion
The acetic acid (CH3COOH) used must be 95% or higher, and the number of repeated tests should not exceed 10.
① Surface Stress Test: Pour acetic acid (glacial acetic acid) into a glass container and completely immerse the product in the acetic acid for 30 seconds. After 30 seconds, remove the sample with clamps and immediately rinse it with clean water (tap water is acceptable). Observe the sample surface for whitening and cracks.
Judgment: There should be no cracking. Slight whitening of the surface is permissible.
② Internal Stress Test: After drying the sample that has passed the surface stress test, completely immerse it in acetic acid for 2 minutes. After 2 minutes, remove the sample and immediately rinse it with clean water (tap water is acceptable). Observe the sample for whitening and cracks.
Judgment: There should be no breakage. Slight cracks at the insert and surface whitening are permissible.
▶Methyl ethyl ketone (MEK) + Acetone Immersion Method
Immerse the entire machine completely in a 1:1 mixture of MEK and acetone at 21°C. Remove and immediately spin dry. Inspect as described above.
Principle: Based on the phenomenon of medium stress cracking, solvent molecules penetrate between the resin macromolecules, reducing the intermolecular forces. Areas with high internal stress already have weakened intermolecular forces before immersion; these weakened forces are further reduced after immersion, leading to cracking. Areas with low internal stress will not crack in a short time.
Therefore, the magnitude and location of internal stress in the part to be plated can be determined by the time and extent of cracking on the surface. This determines whether the plastic part should be electroplated.
2. Instrumental Method
Illuminate the plastic part with polarized light. Analyze the strength of internal stress by observing the amount of colored light bands. This method is only applicable to transparent parts. The polarized light method requires expensive equipment, is complex to operate, and has low accuracy because the changes in the part before and after treatment are not significant. The light bands appearing on the spectrum are not necessarily due to internal stress; for example, surface ripples can also affect the test results.
This method has no impact on the function of the part, making it a non-destructive test. Parts that have passed the test can continue electroplating and use.
3. Temperature Sudden Change Method
This method involves repeatedly subjecting the plastic part to heating and cooling, and assessing the magnitude of internal stress based on the time it takes for cracks to appear. It is suitable for various plastic molded parts. The temperature sudden change method requires simple equipment, but the testing time is relatively long.
The plastic part after inspection is damaged and cannot be used again.
IV. Elimination of Internal Stress
Similar to metals, plastic products can also have some stress eliminated through a post-molding "annealing" process, just like metals. However, this is only a stopgap measure when design and manufacturing processes cannot meet the requirements, and it is not recommended as a regular method.
This method has several limitations:
1. It cannot effectively eliminate defects in glass fiber fillers;
2. Tests show that during post-molding heating, the material's strength and chemical resistance decrease, necessitating controlled annealing time to prevent failure;
3. Prolonged heating and annealing significantly increases the cost of the final product;
4. Stable heating and cooling during annealing are crucial to avoid thermal shock caused by rapid temperature changes.
