What are the reasons for the brittleness of PET plastic sheet?

Date:2020-10-26 17:19:37 星期一
Summary:

A large part of the brittleness of the product is cause […]

A large part of the brittleness of the product is caused by internal stress. There are many reasons for the brittleness of products, mainly:
1. Equipment
(1) There are dead corners or obstacles in the barrel, which easily promote the degradation of the melt.
(2) The plasticizing capacity of the machine is too small, and the plastic is not fully plasticized in the barrel; the plasticizing capacity of the machine is too large, and the time for the plastic to be heated and sheared in the barrel is too long, the plastic is easy to age, and the product is changed. brittle.
(3) The ejector device is inclined or unbalanced, and the cross-sectional area of ​​the ejector stem is small or improperly distributed.
2. Two molds
(1) The gate is too small, you should consider adjusting the gate size or adding auxiliary gates.
(2) If the runner is too small or the configuration is improper, it should be arranged as balanced as possible or increase the size of the runner.
(3) Poor mold structure causes abnormal injection cycle.
3. Process
(1) The temperature of the barrel and nozzle is too low, increase it. If the material is easy to degrade, the temperature of the barrel and nozzle should be increased.
(2) Reduce the screw pre-plastic back pressure and speed to make the material looser and reduce the degradation of the plastic due to shearing overheating.
(3) Mold temperature is too high, demoulding is difficult; mold temperature is too low, the plastic is cooled prematurely, the weld seam is poorly fused, and it is easy to crack, especially for high melting point plastics such as polycarbonate.
(4) The cavity and core must have a proper demoulding angle. When the core is difficult to demold, increase the cavity temperature and shorten the cooling time; when the cavity is difficult to remove, reduce the cavity temperature and extend the cooling time.
(5) Use metal inserts as little as possible. Brittle plastics with a large heat and cold specific capacity such as polystyrene should not be used for insert injection molding.
4. Raw materials
(1) When the raw materials are mixed with other impurities or doped with inappropriate or excessive solvents or other additives.
(2) Some plastics, such as ABS, will undergo catalytic cracking reaction with moisture when heated under damp conditions, causing large strains on the parts.
(3) Too many times of plastic recycling or too high content of recycled materials, or too long heating time in the barrel, will cause the parts to be brittle.
(4) The quality of the plastic itself is not good, for example, the molecular weight distribution is large, and the composition containing the uneven structure of the rigid molecular chain is too large; or it is contaminated by other plastics adulteration, bad additives, dust and impurities, etc., which can also cause brittleness. the reason.
5. Product design
(1) The product has sharp corners, gaps or parts with large differences in thickness that are prone to stress cracking.
(2) The product design is too thin or too much hollow.

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