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Why does PET sheet become brittle?

October 26, 2022

Why does PET sheet become brittle?

A large part of PET sheet brittleness is caused by internal stress. There are many reasons for brittleness, including:

Equipment problems

(1) There are dead ends or obstructions in the barrel that tend to promote degradation of the molten material.

(2)The plasticizing capacity of the machine is too small, and the plastic in the barrel is not plasticized enough; the plasticizing capacity of the machine is too large, and the plastic is subjected to heat and shear in the barrel for too long, which makes the plastic easy to age and the product brittle.

(3) The injector is tilted or unbalanced, and the top trunk section is small or unreasonably distributed.

Mold problem

(1)If the gate is too small, adjust the gate size or increase the auxiliary gate.

(2)If the diversion channel is too small or improperly configured, try to balance or increase the size of the diversion channel reasonably.

(3) Abnormal injection cycle caused by poor mold structure.
Technical level

(1) Cylinder and nozzle temperature is too low, adjust. If the material is easily degraded, the cylinder and nozzle temperature should be increased.

(2)Reduce the back pressure and speed of screw preform to make the material slightly loose and reduce the degradation of plastic due to shear overheating.

(3) The mold temperature is too high, making it difficult to release the mold; the mold temperature is too low, making the plastic cool prematurely, fusing poorly and cracking easily, especially for high melting point plastics such as PET flakes.

(4) The cavity core should have a proper demoulding slope. When the core is difficult to release, the cavity temperature should be increased and the cooling time should be shortened. When the cavity is difficult to release, the cavity temperature should be reduced and the cooling time extended.

(5) minimize the use of metal inserts, such as brittle cold heat capacity of polystyrene plastic, not to insert injection molding.

Raw material level

(1) raw materials mixed with other impurities or inappropriate or excessive solvents or other additives.

(2) Some plastics, such as PET flakes, are heated under humid conditions and undergo catalytic cracking reactions with water vapor, resulting in higher strain on the part.

(3) Too many times of plastic recycling or too much recycled material content, or too long heating time in the barrel, can promote parts brittle cracking.

(4) Bad quality of the plastic itself, such as large molecular weight distribution, rigid molecular chains and other structural inhomogeneities are too many components; or pollution caused by other plastics, bad additives, dust impurities, etc. It is also the cause of brittleness.

Product design level

(1) Products with sharp corners, notches or parts with large thickness differences are prone to stress cracking.

(2) Product design is too thin or too much hollowing.