Internal stress (interior stress), referring to the explanation of Baidu Encyclopedia (for more detailed explanation of the concept of internal stress, please refer to this entry), refers to the stress that remains inside the object when the external load is removed. It is generated by uneven volume changes in the internal macroscopic or microscopic organization of the material. Internal stress is a common mechanical phenomenon in polymer materials during molding and processing.

　　As a composite polymer material, thermoplastic elastomer TPE|TPR (mainly based on polystyrene SBC system elastomer), when molding, also often appear the problem of internal stress.TPE material molding of internal stress, there are three main manifestations.

　　Expression I: Sudden cooling stress.

　　This kind of stress sometimes when the product produces bubbles or dents that are self-receding, can be eliminated by annealing methods. Common abnormalities: super-soft TPE, TPR products (such as soft toys, adult products) molding when encountering sudden cold, the surface has obvious shrinkage patterns or even cracking (cracks).

　　Manifestation 2: Volume strain of the configuration

　　This is caused by the different directions of shrinkage formed by the geometry of the part. The thicker and thinner the part, the more significant. It can only be eliminated by annealing.

　　Common abnormalities: severe surface shrinkage and dimensional shrinkage of the product.

　　Manifestation 3: Freezing of molecular orientation.

　　Freezing of molecular orientation. This is the most important type of residual stress. The residual stress that remains in the part due to molecular orientation and freezing is called frozen molecular orientation. This force will cause cracking of the part, dimensional instability when heated, deformation and other defects.

　　The emergence of residual stress is closely related to the injection molding conditions. Mainly from five aspects to consider: mold temperature, part thickness, injection pressure, injection time, material with the temperature. The main thing is to slow down the cooling rate of the molten material in the molding, so that the stress has a more adequate relaxation, so as to minimize the freezing molecular orientation, that is, to minimize the residual stress.

　　Common abnormalities: deformation and warpage of TPE products after molding.

　　Expression four: metal inserts and plastic linear expansion coefficient difference is too large, as well as chemical solvents caused by the chemical reaction fracture (common cases such as improper brush treatment of water resulting in corrosion cracks in the sole).