（29） Causes of Brittleness of Plastic Products Injection Molding Process Defect Handling Manual

Friability
The brittleness of products is largely caused by internal stress. There are many reasons for product brittleness, mainly including:
Equipment
(1) There are * corners or obstacles in the barrel, which can easily promote the degradation of molten material.
(2) The plasticizing capacity of the machine is too small, and the plastic is not fully plasticized in the machine barrel; The plasticizing capacity of the machine is too large, and the plastic is subjected to heat and shear action in the cylinder for too long. The plastic is prone to aging, making the product brittle.
(3) The ejection device is inclined or unbalanced, and the cross-sectional area of the ejector shaft is small or improperly distributed.
II. Mould
(1) If the gate is too small, consider adjusting the gate size or adding an auxiliary gate.
(2) If the shunt channel is too small or improperly configured, try to arrange it in a balanced and reasonable manner or increase the size of the shunt channel.
(3) Poor mold structure causes abnormal injection molding cycles.
III. Process
(1) The temperature of the barrel and nozzle is too low, adjust it higher. If the material is prone to degradation, the temperature of the barrel and nozzle should be increased.
(2) Reduce the back pressure and rotation speed of the screw pre molding to slightly loosen the material and reduce the degradation of the plastic due to shear overheating.
(3) The mold temperature is too high, making it difficult to demould; The mold temperature is too low, the plastic cools prematurely, the fusion joints are poor, and it is easy to crack, especially for high melting point plastics such as polycarbonate.
(4) The cavity core should have an appropriate release angle. When the core is difficult to demould, it is necessary to increase the cavity temperature and shorten the cooling time; When the mold cavity is difficult to detach, lower the mold cavity temperature and extend the cooling time.
(5) Minimize the use of metal inserts, and brittle plastics such as polystyrene with a high specific heat and cold capacity should not be used for injection molding.
Four aspects of raw materials
(1) When the raw material is mixed with other impurities or doped with inappropriate or excessive solvents or other additives.
(2) Some plastics, such as ABS, can undergo catalytic cracking reactions with water vapor when heated under humid conditions, causing large strain to the parts.
(3) Too many times of plastic recycling, too high recycled material content, or too long heating time in the barrel can promote brittle cracking of the workpiece.
(4) The quality of plastics itself is poor, such as a large molecular weight distribution and excessive occupancy of components with uneven structures such as rigid molecular chains; Or contaminated by other plastics, harmful additives, dust, impurities, etc. are also causes of embrittlement.
V. Product Design
(1) The product has sharp corners, notches, or parts with large thickness differences that are prone to stress cracking.
(2) The product design is too thin or has too many hollows.