Plastic internal stress refers to an intrinsic stress caused by factors such as orientation and cooling shrinkage of macromolecular chains during melt processing of plastics . The essence of internal stress is the unbalanced conformation formed by the macromolecular chain during melt processing. This unbalanced conformation cannot be restored to the equilibrium conformation suitable for environmental conditions immediately upon cooling and solidification. The essence of this unbalanced conformation is one. Reversible high-elastic deformation, while the frozen high-elasticity is flattened in a plastic product in a potential energy situation. Under suitable conditions, this forced unstable conformation will transform into a free and stable conformation. Change to kinetic energy and release. When the force between the macromolecular chains and the intertwining force cannot withstand such kinetic energy, the internal stress balance is destroyed, and the plastic products are subject to stress cracking and warping deformation.
1. Oriented internal stress
The orientation internal stress is an internal stress generated by the macromolecular chain being aligned in the flow direction during the flow filling and pressure-holding process. The detailed process of orientation stress generation is as follows: the melt of the near-flow channel wall increases the viscosity of the outer layer melt due to the fast cooling rate, so that the flow rate of the melt in the core layer of the cavity is much higher than the surface flow velocity, resulting in the inside of the melt. The layers are subjected to shear stresses to produce an orientation along the flow direction. The thawing of oriented macromolecular chains in plastic articles also symbolizes the existence of unrelaxed reversible high elastic deformation, so the orientation stress is the internal force of the macromolecular chain transitioning from the oriented conformation to the non-oriented conformation. By means of heat treatment, the orientation stress in the plastic article can be reduced or eliminated.
The orientation of the internal stress of the plastic article is smaller and smaller from the surface layer to the inner layer of the article, and changes in a parabola.
2, cooling internal stress
The cooling internal stress is an internal stress generated by the uneven shrinkage of the plastic product during the melt processing due to cooling and setting. Especially for thick-walled plastic products, the outer layer of the plastic product first cools and solidifies and shrinks, and the inner layer may still be a hot melt. This core layer will shrink the surface layer, causing the core layer to be under compressive stress, while the surface layer is at Pull stress state.
The distribution of internal stress in the cooling of plastic products is increasing from the surface layer to the inner layer of the product, and also changes in parabola.
In addition, plastic products with metal inserts, because of the large difference in thermal expansion coefficient between metal and plastic, it is easy to form a uniform internal stress without shrinking.
In addition to the above two important internal stresses, there are also the following types of internal stress: for crystalline plastic products, internal stress is also generated when the crystal structure and crystallinity of various parts inside the product are different. In addition, there are also internal forces in the configuration, and the internal stress of the demoulding, but the proportion of the internal stress is very small.
Factors affecting the internal stress of plastics:
1. The rigidity of the molecular chain
The greater the rigidity of the molecular chain, the higher the melt viscosity, the poor mobility of the polymer molecular chain, and thus the reversible high elastic deformation recovery is poor, and the residual internal stress is easily generated. For example, some polymers containing a benzene ring in the molecular chain, such as PC, PPO, PPS, etc., the internal stress of the corresponding products is too large.
2, the polarity of the molecular chain
The greater the polarity of a molecular chain, the greater the attraction between the molecules, so that the interaction between the molecules increases arduously, and the degree of restoration of the reversible elastic deformation decreases, resulting in a large residual internal stress. For example, some types of plastics containing a polar group such as a carbonyl group, an ester group, or an eye group in a molecular chain have a large internal stress of the corresponding article.
3. The steric effect of the substitution group
The larger the volume of the pendant group of the macromolecular group, the greater the residual internal stress caused by the free activity of the macromolecular chain. For example, the phenyl group of the polystyrene substituent group is bulky, and thus the internal stress of the polystyrene article is large. The order of internal stress of several common polymers is as follows: PPO>PSF>PC>ABS>PA6>PP>HDPE.
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