In the process of using plastic parts, we will find that plastic parts will crack, there is definitely a reason. Because almost all plastic products will have different degrees of internal stress, especially the internal stress of plastic injection products is more obvious. The existence of internal stress not only causes stress cracking and warping deformation of plastic products during storage and use, but also affects the mechanical properties, optical properties, electrical properties and appearance quality of plastic products. So where does plastic stress come from? Here we take you to analyze the analysis!
According to the causes of internal stress in plastic parts, the internal stress can be divided into the following categories:
1. Orientation internal stress
Orientational internal stress is a kind of internal stress caused by the freezing of macromolecular chains along the flow direction in the process of flow filling and press-holding feeding. The orientation of the macromolecular chain frozen in the plastic product means that there is no relaxation of the reversible high elastic deformation, so the orientation stress is the internal force of the macromolecular chain from the oriented conformation force to the non-oriented conformation. The orientationally internal stress distribution of plastic products is smaller and smaller from the surface to the inner layer of the product, and presents a parabolic change.
2. Cooling internal stress
Cooling internal stress is a kind of internal stress caused by the uneven shrinkage of plastic products during the melting process. Especially for thick-walled plastic products, the outer layer of plastic products is cooled and solidified and contracted first, and the inner layer may still be a hot melt. This core layer will limit the contraction of the surface layer, resulting in the core layer in the compressive stress state, and the surface layer in the tensile stress state. The distribution of cooling stress in plastic products is increasing from the surface layer to the inner layer of the product, and also presents a parabolic change. In addition, plastic products with metal inserts, due to the large difference in thermal expansion coefficient between metal and plastic, easy to form uneven shrinkage of the internal stress.
3. Environmental stress
Environmental stress cracking is a unique phenomenon of polyolefin plastics, which means that when the product is stressed, it will appear brittle cracks in contact with some active media, which may eventually lead to product destruction. "These active substances can be detergents, soaps, water, oils, acids, bases, salts, and organic solvents that have no significant swelling effect on the material." When the raw material is mixed with other impurities or doped with inappropriate or excessive solvents or other additives, cracks can occur at certain locations of stress sets.
The above is the cause of stress cracking of plastic parts. I hope it will be useful for you!
ZIBO HUIWEN Plastics offers a full range of plastic molding capabilities to efficiently mass-produce parts specializing in injection molding and have served customers in variety of industries such as medical, automotive, packaging, construction, home appliances , and agriculture. The company has two factories which occupy a total area of more than 18,000 square meters, and a total of 102 sets of injection molding machines .
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