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Question about PET extrusion line: Why is some PET sheets brittle?

2022-10-26 Page view : 13 views

A large part of the brittleness of PET sheets is due to internal stress. There are many causes of brittleness in products, the main ones being.

Equipment problems
(1) The presence of dead space or obstructions in the barrel that tend to promote degradation of the molten material.
(2) The plasticising capacity of the machine is too small and the plastic in the barrel is not adequately plasticised; the plasticising capacity of the machine is too large and the plastic is heated and sheared in the barrel for too long, making the plastic prone to ageing and the product brittle.
(3) The injector is tilted or unbalanced, and the top torso section is small or unreasonably distributed.

Mould problems
(1) If the gate is too small, adjust the size of the gate or increase the auxiliary gate.
(2) If the manifold channel is too small or improperly configured, try to balance or increase the size of the manifold channel in a reasonable way.
(3) Bad mould structure leads to abnormal injection cycle.

Technical aspects
(1) The barrel and nozzle temperature is too low, adjust. If the material degrades easily, the barrel and nozzle temperature should be increased.
(2) Reduce screw pre-moulding back pressure and speed to loosen the material slightly and reduce degradation of the plastic due to shear overheating.
(3)The mould temperature is too high, making demoulding difficult; the mould temperature is too low and the plastic cools prematurely, resulting in poor fusion joints and easy cracking, especially for plastics with high melting points such as PET flakes.
(4) The cavity core should have a proper demoulding slope. When the core is difficult to take off, the cavity temperature should be increased and the cooling time shortened. When the cavity is difficult to take off, the cavity temperature should be lowered and the cooling time extended.
(5) Use metal inserts as little as possible, such as polystyrene brittle cold and heat ratio of the larger plastic, more can not add inserts injection moulding.

Raw material level
(1) Raw materials mixed with other impurities or mixed with inappropriate or excessive solvents or other additives.
(2) Some plastics, such as PET flakes, are subjected to heat in humid conditions and will react with water vapour in a catalytic cracking reaction, resulting in high strain on the part.
(3) Too many times the plastic is regenerated or the recycled material content is too high, or the barrel is heated for too long, all of which can promote brittle cracking of the parts.
(4) The quality of the plastic itself is not good, such as a large molecular weight distribution, containing too many components with uneven structure such as rigid molecular chains; or contaminated by other plastic adulteration, bad additive contamination, dust impurity contamination, etc. is also a cause of brittleness.

Product design level
(1) Products with sharp corners, notches or parts with large thickness differences are prone to stress cracking.
(2) Products designed with too thin or too much hollowing.

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