Internal stress in plastics refers to an internal stress generated during the melting process of plastics due to factors such as the orientation of macromolecular chains and cooling shrinkage.

The essence of internal stress is the unbalanced conformation formed during the melting process of macromolecular chains, which cannot be immediately restored to an equilibrium conformation suitable for environmental conditions upon cooling and solidification. The essence of this unbalanced conformation is a reversible high elastic deformation, while the frozen high elastic deformation is usually stored in plastic products in a state of failure under suitable conditions, This forced unstable conformation will be transformed into a free and stable conformation, and the potential energy will be released by changing into kinetic energy.

When the forces and entanglement forces between macromolecular chains cannot withstand this kinetic energy, the internal stress balance is destroyed, and plastic products will experience stress cracking and warping deformation.

Analysis of the causes of internal stress in plastics:

1. Orientation internal stress

The oriented internal stress is an internal stress generated by the freezing of macromolecular chains aligned in the flow direction during the process of plastic melt filling and pressure maintaining replenishment.

The detailed process of producing orientation stress is as follows: the melt near the flow channel wall increases the viscosity of the outer layer due to the rapid cooling rate, which causes the flow rate of the melt in the core layer of the mold cavity to be much higher than the flow rate of the surface layer, resulting in shear stress acting between the layers within the melt, resulting in an orientation along the flow direction.

The thawing of oriented macromolecular chains in plastic products also indicates the presence of unrelaxed reversible high-elastic deformation. Therefore, the orientation stress is the internal force that the macromolecular chain strives to transition from an oriented conformation to a non oriented conformation. Heat treatment can reduce or eliminate the orientation stress in plastic products.

The orientation internal stress distribution of plastic products is becoming smaller and smaller from the surface layer to the inner layer of the product, with a parabolic change.

2. Cooling internal stress

Cooling internal stress is an internal stress generated during the melting process of plastic products due to uneven shrinkage during cooling and shaping. Especially for thick wall plastic products, the outer layer of the plastic product first cools, solidifies, and contracts. The inner layer may still be a hot melt, which limits the shrinkage of the surface layer, causing the core layer to be in a compressive stress state, while the surface layer is in a tensile stress state.

The distribution of internal cooling stress in plastic products is increasing from the surface layer to the inner layer of the product, and also changes in a parabola.

In addition, plastic products with metal inserts are prone to uneven internal stress due to the large difference in thermal expansion coefficients between metals and plastics.

In addition to the two important internal stresses mentioned above, how many other internal stresses are there? For crystalline plastic products, internal stresses can also occur due to differences in the crystalline structure and crystallinity of various parts within the product. In addition, there are structural internal stresses, stresses, and demolding internal stresses, but the proportion of internal stresses is very small.

Analysis of factors affecting the generation of internal stress in plastics:

1. The rigidity of molecular chains

The greater the rigidity of the molecular chain, the higher the viscosity of the melt, and the poorer the mobility of the polymer molecular chain. Therefore, the poor resilience to the generated reversible high elastic deformation is prone to generate residual internal stress. For example, some polymers containing benzene rings in the molecular chain, such as PC, PPO, and PPS, have relatively high internal stress in their corresponding products.

2. Polarity of molecular chains

The greater the polarity of a molecular chain, the greater the force of attraction between molecules, which increases the mutual movement between molecules, reduces the degree of recovery of reversible elastic deformation, and leads to greater residual internal stress. For example, some plastic types with polar groups such as carbonyl, ester, and eyegroup in their molecular chains have high internal stress in their corresponding products.

3. Steric hindrance effect of substituent groups

The larger the volume of the side substituent group of the macromolecule, the greater the residual internal stress caused by the obstruction of the free movement of the macromolecule chain. For example, the phenyl volume of the polystyrene substituent group is larger, and therefore the internal stress of the polystyrene product is larger.