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Causes and solutions of cracks in injection molding processing
Cracking is a relatively common defect in injection molding processing, which is mainly manifested as cracking in the position where stress is easy to concentrate or welding lines, or paint cracking occurs after painting for a period of time. The causes of cracking mainly include external stress, stress deformation caused by external environment and residual stress.
1. Injection molding processing - cracking caused by external stress:
The external stress here is mainly the stress concentration caused by unreasonable design, especially in sharp corners.
2. Injection molding processing - cracks caused by external environment:
Water degradation caused by chemicals, moisture absorption, and excessive use of recycled materials can deteriorate physical properties and lead to cracks.
3. Injection molding processing - cracking caused by residual stress:
Residual stress is mainly caused by metal inserts, overfilling, and mold ejection.
(1) When metal parts are embedded in the process of injection molding, stress is easy to occur, and it is easy to crack after a period of time. This is due to the fact that the metal and resin will be stressed due to the large difference in the coefficient of thermal expansion, and over time, the stress will gradually exceed the strength of the resin material and cracks will occur. To avoid this situation, it is necessary to choose a material suitable for inlay metal, for example, general-purpose polystyrene is not suitable for inlay, and glass fiber reinforced resin material is more suitable for inserts because of its small coefficient of thermal expansion. In addition, preheating the metal insert before molding can also largely avoid the appearance of cracks.
(2) Cracks caused by overfilling can be solved by the following methods:
(1) Under the premise of ensuring that the resin does not decompose and does not deteriorate, the temperature of the resin can be reduced appropriately. Low melt viscosity improves fluidity and can also reduce . Low injection pressure to reduce stress.
(2) Excessive injection and holding time will also produce stress, which will be appropriately shortened or Th pressure holding switching effect. The result is better.
(3) Due to the small pressure loss of the straight gate, if the crack is mainly generated near the straight gate, the method of multi-point distribution point gate, side gate and shank gate can be considered instead.
(4) Compared with amorphous resins such as AS resin, ABS resin, and PMMA resin, crystalline resins such as polyethylene and polyoxymethylene are more likely to produce residual stress.
(5) Under normal circumstances, stress is more likely to occur when the mold temperature is low, which can be solved by appropriately increasing the temperature. However, when the injection speed is high, even if the mold temperature is low, the stress can be alleviated to a certain extent.
(3) When the demolding is launched, due to the small demolding slope, mold-type glue and rough convex die, the ejection force is too large, resulting in stress, and sometimes even whitening or cracking around the ejection rod. Just by looking closely at where the cracks occur, the cause can be determined.
1. Injection molding processing - cracking caused by external stress:
The external stress here is mainly the stress concentration caused by unreasonable design, especially in sharp corners.
2. Injection molding processing - cracks caused by external environment:
Water degradation caused by chemicals, moisture absorption, and excessive use of recycled materials can deteriorate physical properties and lead to cracks.
3. Injection molding processing - cracking caused by residual stress:
Residual stress is mainly caused by metal inserts, overfilling, and mold ejection.
(1) When metal parts are embedded in the process of injection molding, stress is easy to occur, and it is easy to crack after a period of time. This is due to the fact that the metal and resin will be stressed due to the large difference in the coefficient of thermal expansion, and over time, the stress will gradually exceed the strength of the resin material and cracks will occur. To avoid this situation, it is necessary to choose a material suitable for inlay metal, for example, general-purpose polystyrene is not suitable for inlay, and glass fiber reinforced resin material is more suitable for inserts because of its small coefficient of thermal expansion. In addition, preheating the metal insert before molding can also largely avoid the appearance of cracks.
(2) Cracks caused by overfilling can be solved by the following methods:
(1) Under the premise of ensuring that the resin does not decompose and does not deteriorate, the temperature of the resin can be reduced appropriately. Low melt viscosity improves fluidity and can also reduce . Low injection pressure to reduce stress.
(2) Excessive injection and holding time will also produce stress, which will be appropriately shortened or Th pressure holding switching effect. The result is better.
(3) Due to the small pressure loss of the straight gate, if the crack is mainly generated near the straight gate, the method of multi-point distribution point gate, side gate and shank gate can be considered instead.
(4) Compared with amorphous resins such as AS resin, ABS resin, and PMMA resin, crystalline resins such as polyethylene and polyoxymethylene are more likely to produce residual stress.
(5) Under normal circumstances, stress is more likely to occur when the mold temperature is low, which can be solved by appropriately increasing the temperature. However, when the injection speed is high, even if the mold temperature is low, the stress can be alleviated to a certain extent.
(3) When the demolding is launched, due to the small demolding slope, mold-type glue and rough convex die, the ejection force is too large, resulting in stress, and sometimes even whitening or cracking around the ejection rod. Just by looking closely at where the cracks occur, the cause can be determined.
