The internal stress of plastics modification of control
almost all plastic products will exist in different degrees internal stress, especially the internal stress of plastic injection products is more apparent. The existence of internal stress not only make the plastic products in the process of storage and use in warping deformation and cracks, also affects the mechanical properties of plastic products, optical properties, electrical properties and the appearance quality. So we must find out the causes of internal stress and the stress relief method, the greatest degree of reduce internal stress of the plastic products, and make the residual internal stress as uniform distribution on the plastic products, avoid to produce stress concentration phenomenon, thus improving plastic products of mechanical and thermal properties, etc.
plastic internal stress is in the process of plastic melt processing due to the orientation of macromolecule chain and cooling contraction of factors such as an inner stress. The essence of the internal stress of macromolecular chains in the process of the molten form conformation, the imbalance of the imbalance conformation during cooling and solidification can't immediately return to adapted to the environmental conditions of balance conformation. The imbalance of conformation in a reversible high elastic deformation, high elastic deformation and freeze at ordinary times in the form of potential energy stored in the plastic products, under appropriate conditions, this forced the unstable conformation to the freedom of the stability of the conformation transformation, releasing potential energy into kinetic energy. When forces and mutual entanglement between the macromolecular chain force bear the kinetic energy, internal stress balance is destroyed, the plastic products can produce stress cracking and buckling deformation phenomenon.
the causes of stress in plastic
there are many reasons for the internal stress, such as the plastic melt in the machining process by the strong shearing action, existing in the processing of orientation and crystallization, melt each place extremely difficult to achieve uniform cooling speed, melt uneven plasticizing, products demoulding difficulty etc, will cause the generation of internal stress.
cooling internal stress is plastic products in the process of melting due to uneven contraction when cooling to finalize the design of a kind of internal stress. Especially for thick wall plastic products, plastic products outer first cooling solidification shrinkage, the inner layer is likely to be hot melt, such core layer can limit surface shrinkage, in core layer in the compressive stress state, the surface layer is in a state of tension stress. Plastic cooling of the internal stress distribution of products from the surface to the inner layer is more and more big, and also a parabolic variation. The other is the plastic with metal insert, because the expansion coefficient of metal and plastic is large, easy to form the uneven shrinkage internal stress.
orientation internal stress of the plastic melt flow in the mold filling and pressure maintaining in the process of filling material, macromolecule chain along the flow direction is arranged directional conformation was frozen and produce a kind of internal stress. Orientation stress produced by the specific process is: melt near the port wall outer melt viscosity caused by cooling speed increased, which makes the melt layer flow rate is much higher than in the center of the cavity surface velocity, lead to internal melt under shear stress between layer and layer, produce orientation along the flow direction. Orientation of macromolecule chain freeze in plastic products, that is, there is no slack of reversible high elastic deformation, so stress orientation is macromolecular chain conformation to the transition from the orientation to no orientation conformation of internal force. Use the method of heat treatment, can reduce or eliminate the orientation of the stress in the plastic products. Plastic products orientation of internal stress distribution of products from the surface to the inner layer is more and more small, and a parabolic variation.
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