How to solve PC/ABS or PC internal stress cracking?

Before analyzing the various causes of stress cracking, we first pave the way for basic knowledge:

1) Introduction to the basic structure of PC:

Polycarbonate PC is a thermoplastic resin containing [OROC=O] links in the main chain of the molecule. According to the different ester groups in the molecular structure, it can be divided into aliphatic, alicyclic, and aliphatic aromatic types. Among them, the most valuable is aromatic. Type polycarbonate PC, and bisphenol A type polycarbonate PC is the most important.

2) Structure determines nature, and nature determines external phenomena

A. PC microstructure leads to PC internal stress cracking: PC material is prone to internal stress cracking due to its own molecular structure, that is, polycarbonate has a benzene ring in the molecular structure, so orientation is more difficult. After molding, the oriented chain links There is a tendency to restore the natural state, but because the molecular chain links have been frozen and the force between the molecular chains may cause stress in the product, this is the phenomenon of stress cracking, especially the recycled PC. The relative molecular mass decreases, the relative molecular mass distribution becomes wider, and the presence of a small amount of moisture, pigments, impurities, solvents, etc. is easy to cause cracking.

B. Stress classification

Shear stress: refers to the stress caused by shear flow during plastic processing, which is affected by the flow rate and viscosity of the plastic in its molten state.

Orientation stress: Under the high pressure and high-speed shearing of the plastic material molecules during the molding process, the molecular chain links drastically change and freeze before the molecules completely return to their natural state, which forms the orientation stress.

Shrinkage stress: In the process of plastic molecules from melting to cooling, the cooling temperature is uneven due to the difference in product thickness and the difference in the cooling water path, which leads to different shrinkage rates of different parts. In the parts with different shrinkage rates, the stretching and shearing Generate stress. Based on the above basic knowledge, it can be said that there are many forces in the microcosm of PC stress cracking polycarbonate, and the more important one is the "cracking resistance". The magnitude of this force depends on the length of the PC molecular chain. The number of entanglements with the chain, the force between the molecular chain and the molecular chain, simply speaking, the PC material is better, and the anti-cracking ability is better.

When the anti-cracking force is balanced with the internal stress, the product will not crack;

When the anti-cracking force is less than the internal stress, cracking will occur.

Through simple analysis, we understand the causes of stress cracking, that is, we have found a solution to PC internal stress cracking.