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The residual stress is too high. When the residual stress in the plastic part is higher than the elastic limit of the resin, the surface of the plastic part will be cracked and cracked.

(1) When injection molding, the molecular arrangement of the polymer melt, under the action of external forces will produce molecular chain orientation, when the polymer chain from a natural stable state forced transition to another orientation state, and finally by When frozen in the mold, residual stresses will occur in the cooled plastic parts.

At the same time, due to the large temperature difference in the cold mold, the molten material will quickly change from a viscous fluid state to a glass state. The oriented macromolecules will not be frozen until the initial stable state is restored, and a part of internal stress remains on the surface of the plastic part.

Under normal circumstances, cracks and cracks caused by residual stress are most likely to occur near the gate, because the molding pressure at the gate is higher than other parts, especially when the main flow path is a direct gate.

In addition, when the wall thickness of the plastic part is not uniform and the cooling rate of the molten material is not uniform, due to the difference in the amount of shrinkage in the thick and thin parts, the former is subjected to the tensile force of the latter, and residual stress is also generated. Because the residual stress is a major cause of cracks and cracks in plastic parts, it is possible to prevent cracking and cracking of plastic parts by reducing the residual stress. The main method to reduce the residual stress is to improve the structure of the pouring system and adjust the forming conditions of the plastic parts.

In the design and manufacture of molds, direct gates with minimal pressure loss and high injection pressure can be used. The forward gate can be changed to multiple pin point gates or side gates and the gate can be reduced. diameter. When designing the side gate, the tab gate form that can remove the broken part after molding can be used.

For example, raw materials such as polycarbonate, polyvinyl chloride, and polyphenylene ether have poor melt flow properties and need to be injection molded under high pressure conditions. Cracks are easily generated at the gate. If a tab or side gate is used, molding can be performed. After that, the crack portion produced in the tab portion is removed. In addition, rational use of annular stiffeners around the gate can also reduce cracks at the gate.

In the process operation, reducing the residual stress by reducing the injection pressure is the easiest method because the injection pressure is in direct proportion to the residual stress. If the cracks generated on the surface of the plastic part are blackened around, it means that the injection pressure is too high or the amount of material to be added is too small, and the injection pressure or the amount of feed should be reduced appropriately. When the molding temperature is lower and the molding temperature is lower, the injection molding pressure must be higher in order to fill the cavity, resulting in a large amount of residual stress in the plastic part.

In this regard, the temperature of the barrel and the mold should be appropriately increased, the temperature difference between the melt and the mold should be reduced, and the cooling time and speed of the mold inside the mold should be controlled so that the oriented molecular chain has a long recovery time.

In addition, under the premise of ensuring that there is insufficient feeding and not causing shrinkage of the plastic parts, the holding time can be appropriately shortened, because the holding pressure for a long time is also prone to cracks caused by residual stress.

(2) External forces cause residual stress concentration.

Before demolding the plastic parts, if the cross-sectional area of the ejection ejection mechanism is too small or the number of the top rods is not enough, the position of the top rod is unreasonable or the installation is inclined, the balance is poor, and the mold draft angle is insufficient. Too much resistance will lead to stress concentration due to external forces, causing cracks and cracks on the surface of plastic parts.

In general, such failures always occur around the jack. After such failures occur, the ejector should be carefully checked and calibrated. The ejector pin is set at the place where the mold release resistance is the greatest, such as protrusion, reinforcement, etc.

If the number of jacks to be set cannot be increased due to the condition that the pushing-out area is limited by the conditions, a method using a small-area multi-piercing rod may be adopted.

If the demoulding angle of the mold cavity is not enough, the surface of the plastic part will be scratched to form a wrinkle pattern. When selecting the draft angle, the shrinkage of the molding material must be taken into consideration as well as the structure of the ejector system. Under normal circumstances, the draft angle should be greater than 0.85%, and the draft angle of small plastic parts should be 0.1 to 0.5%. Large plastic parts stripping slope of up to 2.5%.

(3) There is a difference in the coefficient of thermal expansion between the molding material and the metal insert. The coefficient of thermal expansion of the thermoplastic is 9 to 11 times larger than steel and 6 times larger than aluminum. Therefore, the metal inserts in the plastic parts will hinder the overall shrinkage of the plastic parts, resulting in a large tensile stress, and a large amount of residual stress will be accumulated around the inserts to cause cracks on the surface of the plastic parts. In this way, metal inserts should be preheated, especially when cracks on the surface of the plastic parts occur just after power-on, mostly due to too low insert temperatures.

In addition, in the selection of the material of the insert, a material having a coefficient of expansion close to that of the resin should be used as much as possible. For example, inserts made of light metal materials such as zinc and aluminum are superior to steel.

In the selection of molding raw materials, high-molecular-weight resins should also be used as much as possible. If low-molecular-weight molding raw materials must be used, the thickness of the plastic around the inserts should be designed to be thicker for polyethylene, polycarbonate, polyamide, and acetic acid. For cellulosic plastics, the thickness of the plastic around the insert should be at least equal to half the diameter of the insert; for polystyrene, metal inserts are generally not suitable.

(4) The raw material was improperly selected or not pure.

The sensitivity of different raw materials to produce residual stress is different. Generally, non-crystalline resin is more likely to produce residual stress-induced cracks than crystalline resin; for water-absorbent resins and resins with more recycled materials, the water-absorbing resin will decompose and decompose when heated. As a result, the smaller residual stress will cause brittle fracture, while the resin with higher content of recycled materials has more impurities, higher volatile content, and lower strength of the material, and is also prone to stress cracking.