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How to eliminate the internal stress of injection molded products?

TIME:2021-03-25 Share

A common disadvantage of injection molded products is internal stress. The existence of internal stress is not only an important reason for the warpage and cracking of the parts during storage and use, but also an important factor that affects the optical properties, electrical properties, physical and mechanical properties and apparent quality of the parts. Therefore, find out the regularity of the influence of various molding factors on the internal stress of injection molded products, in order to take effective measures to reduce the internal stress of the parts, and make it evenly distributed on the cross-section of the parts, which is helpful for improving the quality of injection molded products. important meaning. Especially when the parts are subjected to heat, organic solvents and other corrosive media that can accelerate the cracking of the parts under the conditions of use, reducing the internal stress of the parts is more important to ensure their normal operation. In addition, it is also necessary to master the methods of eliminating internal stress and testing methods of injection molded products.

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1.5 Injection molding process conditions Injection molding products inevitably have internal stress due to the characteristics of the molding process, but proper control of the process conditions will minimize the internal stress of the plastic parts and ensure the normal use of the parts. On the contrary, if the process is not properly controlled, the parts will have great internal stress, which will not only reduce the strength of the parts, but also warp, deform and even crack during storage and use. Process conditions that need to be controlled, such as insert preheating, mold temperature, processing temperature, injection speed, injection pressure, holding pressure, injection time, holding time, cooling time, etc. Temperature, pressure, and time are the main factors in the plastic molding process.

1.5.1 When the metal insert is preheated for injection molding, the metal insert should be preheated to a temperature close to the material temperature. The purpose of preheating the insert is to reduce the difference between the shrinkage value of the metal and the plastic during cooling, thereby reducing the thermal expansion coefficient of the two. Different from the shrinkage stress generated around the insert. Shrinkage stress is a kind of internal stress that is easily formed in injection molded products. The existence of such internal stress is an important reason for cracks and strength reduction of injection molded products with metal inserts.

1.5.2 Mold temperature Increasing the mold temperature can reduce the volumetric temperature stress and polymer orientation stress caused by uneven internal and external shrinkage, and can also reduce the crystallization stress of crystalline plastic products. However, the mold temperature should not be too high. The increase of the mold temperature will extend the cooling time and reduce the production efficiency.

1.5.3 Processing temperature Increasing the processing temperature can reduce the orientation stress, but at the same time it will increase the volumetric temperature stress caused by uneven shrinkage, and at the same time increase the sealing pressure and extend the cooling time for smooth demoulding.

1.5.4 Increasing injection pressure, injection speed and injection time injection pressure increases orientation stress and crystalline stress of crystalline plastics, and at the same time increases sealing pressure. The cooling time must be prolonged for smooth demolding, otherwise it will cause demolding stress; injection The increase in speed will also increase the orientation stress and crystallization stress, but it is more advantageous to fill the mold with a high injection speed for plastics with fast condensation, because slow injection of plastics with fast condensation requires higher injection pressure to maintain the flow of the melt; The injection time should not be too long. After the cavity is filled, it is equivalent to maintaining the pressure under the injection pressure, which will also increase the orientation stress of the part.

1.5.5 Holding pressure and holding time In the total deformation of the melt during cooling under external pressure, a considerable part of it is elastic, so the condensation of the melt under high pressure will produce a greater amount of Internal stress and polymer orientation. Pressure reduction immediately after compaction or stepwise pressure reduction during the refilling process is conducive to polymer de-orientation, so reducing the holding pressure and shortening the holding time are conducive to the reduction of the orientation stress; extending the holding time is only within a certain range of orientation Increase, and extend the holding time after the gate is closed to no longer affect the change of orientation.

1.5.6 Cooling time When the injection pressure, holding pressure, melt temperature increase, and the gate size is larger, the sealing pressure will increase. At this time, the cooling time must be extended to reduce the residual pressure in the cavity before the mold is opened. Very low or close to zero, otherwise it will be difficult to eject the part from the mold smoothly. If demolding is forced, the part will produce a lot of stress when it is ejected, and the part may be scratched, or cracked in severe cases. However, the cooling time should not be too long, otherwise the production efficiency is low, and further cooling after the internal pressure of the part drops to zero may form a negative pressure inside the part, that is, tensile stress between the inner and outer layers of the part due to cooling shrinkage.
A common disadvantage of injection molded products is internal stress. The existence of internal stress is not only an important reason for the warpage and cracking of the parts during storage and use, but also an important factor that affects the optical properties, electrical properties, physical and mechanical properties and apparent quality of the parts. Therefore, find out the regularity of the influence of various molding factors on the internal stress of injection molded products, in order to take effective measures to reduce the internal stress of the parts, and make it evenly distributed on the cross-section of the parts, which is helpful for improving the quality of injection molded products. important meaning. Especially when the parts are subjected to heat, organic solvents and other corrosive media that can accelerate the cracking of the parts under the conditions of use, reducing the internal stress of the parts is more important to ensure their normal operation. In addition, it is also necessary to master the methods of eliminating internal stress and testing methods of injection molded products.

1.5 Injection molding process conditions Injection molding products inevitably have internal stress due to the characteristics of the molding process, but proper control of the process conditions will minimize the internal stress of the plastic parts and ensure the normal use of the parts. On the contrary, if the process is not properly controlled, the parts will have great internal stress, which will not only reduce the strength of the parts, but also warp, deform and even crack during storage and use. Process conditions that need to be controlled, such as insert preheating, mold temperature, processing temperature, injection speed, injection pressure, holding pressure, injection time, holding time, cooling time, etc. Temperature, pressure, and time are the main factors in the plastic molding process.

1.5.1 When the metal insert is preheated for injection molding, the metal insert should be preheated to a temperature close to the material temperature. The purpose of preheating the insert is to reduce the difference between the shrinkage value of the metal and the plastic during cooling, thereby reducing the thermal expansion coefficient of the two. Different from the shrinkage stress generated around the insert. Shrinkage stress is a kind of internal stress that is easily formed in injection molded products. The existence of such internal stress is an important reason for cracks and strength reduction of injection molded products with metal inserts.

1.5.2 Mold temperature Increasing the mold temperature can reduce the volumetric temperature stress and polymer orientation stress caused by uneven internal and external shrinkage, and can also reduce the crystallization stress of crystalline plastic products. However, the mold temperature should not be too high. The increase of the mold temperature will extend the cooling time and reduce the production efficiency.

1.5.3 Processing temperature Increasing the processing temperature can reduce the orientation stress, but at the same time it will increase the volumetric temperature stress caused by uneven shrinkage, and at the same time increase the sealing pressure and extend the cooling time for smooth demoulding.

1.5.4 Increasing injection pressure, injection speed and injection time injection pressure increases orientation stress and crystalline stress of crystalline plastics, and at the same time increases sealing pressure. The cooling time must be prolonged for smooth demolding, otherwise it will cause demolding stress; injection The increase in speed will also increase the orientation stress and crystallization stress, but it is more advantageous to fill the mold with a high injection speed for plastics with fast condensation, because slow injection of plastics with fast condensation requires higher injection pressure to maintain the flow of the melt; The injection time should not be too long. After the cavity is filled, it is equivalent to maintaining the pressure under the injection pressure, which will also increase the orientation stress of the part.

1.5.5 Holding pressure and holding time In the total deformation of the melt during cooling under external pressure, a considerable part of it is elastic, so condensing the melt under high pressure will cause a greater amount of deformation in the part. Internal stress and polymer orientation. Pressure reduction immediately after compaction or stepwise pressure reduction during refilling process is conducive to polymer de-orientation, so reducing the holding pressure and shortening the holding time are conducive to the reduction of orientation stress; extending the holding time is only within a certain range of orientation Increase, and extend the holding time after the gate is closed to no longer affect the change of orientation.

1.5.6 Cooling time When the injection pressure, holding pressure, melt temperature increase, and the gate size is larger, the sealing pressure will increase. At this time, the cooling time must be extended to reduce the residual pressure in the cavity before the mold is opened. It is very low or close to zero, otherwise it will be difficult to eject the part from the mold smoothly. If demolding is forced, the part will produce a lot of stress when it is ejected, and the part may be scratched, or cracked in severe cases. However, the cooling time should not be too long, otherwise the production efficiency is low, and further cooling after the internal pressure of the part drops to zero may form a negative pressure inside the part, that is, tensile stress between the inner and outer layers of the part due to cooling shrinkage.

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