Plastic stress refers to a kind of internal stress in the plastic melting process which is caused by the orientation and the cooling shrinkage factor of macromolecular chain. The essence of internal stress is not balanced conformation of molecular chain formed in the melting process, the imbalance in the conformation of curing not immediately cooling return to the equilibrium conformation and adapt to environmental conditions, the essence of unbalanced conformation for high elastic deformation of a reversible, high elastic deformation and the usual to freeze energy stored in plastic products, under suitable conditions, the forced unstable conformation will transform to free stable conformation, can change into kinetic energy and release. When a large force between molecular chains and mutual entanglement cannot afford this kinetic energy, internal force balance is destroyed, plastic products will cause stress cracking and warping deformation phenomenon.

Causes of plastic stress:

1, the orientation of internal stress

The orientation of the internal stress of plastic melt in the mold filling flow and pressure feeding process, a large internal stress along the flow direction of molecular chain orientation conformation were frozen and produced. With the orientation of the stress generated was melt near wall flow due to fast cooling caused by the melt viscosity increased from the outer layer, the core melt in the cavity is far higher than the surface velocity of laminar flow velocity caused by the melt, between the internal layer and the layer shear stress generated orientation along the flow direction. The macromolecular chain orientation of the thaw in plastic products also symbolizes the high elastic deformation is not reversible relaxation, so that the orientation of stress is the macromolecular chain conformation to the internal force from the orientation transition to non oriented conformation. With heat treatment, can reduce or eliminate the stress orientation of plastic products.

The orientation of the stress distribution of plastic products from the surface to the inner layer is more and more small, and a parabolic curve.

2, the internal stress of cooling

Cooling stress is a kind of internal stress of plastic products in the melt processing process due to cooling shrinkage caused by uneven.Especially for the thick wall plastic products, plastic products, outer cooling and solidification shrinkage, the inner layer may still be hot melt, which will limit the surface of the core layer best shrinkage, lead core layer in a compressive stress state, and surface tension.

The distribution of internal force for cooling plastic products from the surface to the inner product is more and more big, and also a parabola change.

In addition, plastic products with metal insert, because the expansion coefficient of metal and plastic is large, easy to form a uniform of the contraction stress.

In addition to the above two kinds of important internal stress, there are many kinds of stress on crystallization of plastic products, the crystal structure of each part of the internal products with different crystallinity will occur stress. In addition to internal force and configuration. The stress release, just listen to the stress of the proportion is very small.