Causes of plastic internal stress

There are many reasons for internal stress. For example, plastic melt is subjected to strong shear during processing, the orientation and crystallization in processing, the cooling rate of each part of the melt is extremely difficult to achieve uniformity, and the melt is plasticized Unevenness, difficulty in demoulding products, etc., can cause internal stress. Depending on the cause of the internal stress, the internal stress can be divided into the following categories.

(1) Oriented internal stress Oriented internal stress is a kind of internal stress that occurs when the plastic melt is frozen in the process of flow filling and pressure maintaining and the macromolecular chains are aligned along the flow direction and the conformation is frozen. The specific process of orientation stress generation is as follows: * The melt near the wall of the flow channel causes the outer melt viscosity to increase due to the fast cooling speed, so that the flow velocity of the melt in the center layer of the cavity is much higher than the surface flow velocity, resulting in the melt Shear stress is applied between the inner layers, resulting in an orientation along the flow direction. The freezing of oriented macromolecular chains in plastic products means that there is unrelaxed reversible hyperelastic deformation, so the orientation stress is the internal force of the macromolecular chain transition from the orientation conformation to the non-orientation conformation. The heat treatment method can reduce or eliminate the orientation stress in the plastic products. The orientation stress distribution of the plastic product is getting smaller and smaller from the surface layer to the inner layer of the product, and changes in a parabola.
(2) Cooling internal stress Cooling internal stress is a kind of internal stress generated by the uneven shrinkage of plastic products during cooling and shaping during melt processing. Especially for thick-walled plastic products, the outer layer of the plastic product is cooled and solidified first, and the inner layer may still be hot melt. This will limit the shrinkage of the surface layer, resulting in the core layer being in a state of compressive stress and the surface layer being in tension Stress state. The distribution of the internal stress of plastic products during cooling is increasing from the surface layer to the inner layer of the product, and it also changes parabolically. In addition, for plastic products with metal inserts, due to the large difference between the thermal expansion coefficients of metal and plastic, it is easy to form internal stress with uneven shrinkage. In addition to the above two main internal stresses, there are the following internal stresses: For crystalline plastic products, the internal stress will also be generated when the crystal structure and crystallinity of various parts inside the product are different. In addition, there are internal stress, stress and internal stress of demoulding, but its internal stress is very small.