Influence of Mold Cooling Systems & Ejection Systems on Warpage of Plastic Parts
In the injection molding process, the uneven cooling rate of the plastic part will also cause the uneven shrinkage of the plastic part, which will lead to the generation of bending moment and cause the plastic part to warp. If the temperature difference between the fixed and movable molds is too great, the residual stress caused by uneven cooling cannot be symmetrical based on the midplane of the plastic part, resulting in a great bending moment, changing the shape of the plastic part, and causing great warping. Therefore, when designing the injection mold, attention should be paid to the reasonable design of the cooling system of the temperature control system to reduce the temperature difference between the fixed mold and the movable mold to reduce deformation.
The formation of the temperature gradient in the thickness direction of the plastic part is usually caused by the different cooling time between the inner and outer surfaces of the plastic part. Where the surface of the mold cavity is closer to the cooling circuit of the mold is greatly influenced by the cooling medium, so that the plastic melt here cools quickly. On the one hand, the time of temperature change is shortened, and the gap between the actual specific volume value of the plastic and that in the equilibrium state is increased. On the other hand, when entering the in-mold shrinkage stage, the temperature of the surface layer of the injection molded product is already very low, so the degree of shrinkage is small.
In addition to considering that the temperature of the inner and outer surfaces of the plastic part tends to be balanced, the consistent temperature on each side of the plastic part should also be considered, that is, when the mold is cooled, the temperature of the cavity and the core should be kept as uniform as possible, so that the cooling rate of the plastic part can be even and the shrinkage of every part can be more uniform, effectively preventing deformation. Therefore, the arrangement of cooling water holes on the mold is very important. Not only the distribution of the distance from the water hole to the cavity should be uniform, but also after the distance from the pipe wall and the cavity surface is determined, the distance between the cooling water holes should be as small as possible to ensure the uniform temperature of the cavity wall. At the same time, since the temperature of the cooling medium increases with the increase of the length of the cooling pipe, a temperature difference is generated between the mold cavity and mold core along the pipe.
Several cooling circuits should be set up in large molds, with the inlet of one circuit near the outlet of the other. It is a feasible method to use the change of mold temperature to compensate for warpage caused by other reasons, which requires the reasonable arrangement of cooling pipes to achieve the purpose of reducing warpage. For example, the use of a center gate often results in saddle-shaped warping or twisting due to orientation effects and shrinkage in the small area. This warpage can be reduced by increasing the cooling near the gate and attenuating the cooling at the end of the cavity through piping arrangements.
How does the mold's ejection system affect the warpage of plastic parts?
The design of the ejection system also directly affects the deformation of the plastic part. In the ejecting and demoulding processes, the temperature of the plastic product is high. The ejection is not straight, uneven, and inconsistent, which will cause mechanical stress, and the product is easy to warp and deform. To avoid mechanical stress, check the ejector system during the application, make necessary adjustments and properly lubricate all moving parts.
The ejector plate of the large mold must use a guide sleeve to prevent the center of the template from changing the ejection direction and position due to its own weight. Therefore, when the ejector system is designed, a balance should be kept between the system and demolding resistance. In addition, the cross-sectional area of the ejector rod should not be too small to prevent the plastic parts from being deformed due to excessive force per unit area, especially when the demolding temperature is too high. The arrangement of the ejector pins should be as close as possible to the parts with great demoulding resistance. Under the premise of not affecting the quality of the plastic parts, including the use requirements, dimensional accuracy and appearance, ejectors should be provided as many as possible to reduce the overall deformation of the plastic parts. In addition, a reasonable ejection method should be selected according to the actual situation. When soft plastic is used to produce large-scale deep-cavity thin-walled plastic parts, the demoulding resistance is great and the material is soft. If a single mechanical ejection method is used, the plastic parts will be deformed, or even scrapped due to piercing or folding. The effect will be better if the combination of multiple components or the combination of pneumatic and hydraulic and mechanical ejection is used.
However, the choice of ejection method and ejection position is determined by the structure of the injection molded product to a greater extent. Usually, the warpage of the product mainly comes from the shrinkage in the mold. Therefore, the design of the injection mold structure should be improved in order to improve the warpage. The improvement of gating systems and cooling systems should be given priority to.
The formation of the temperature gradient in the thickness direction of the plastic part is usually caused by the different cooling time between the inner and outer surfaces of the plastic part. Where the surface of the mold cavity is closer to the cooling circuit of the mold is greatly influenced by the cooling medium, so that the plastic melt here cools quickly. On the one hand, the time of temperature change is shortened, and the gap between the actual specific volume value of the plastic and that in the equilibrium state is increased. On the other hand, when entering the in-mold shrinkage stage, the temperature of the surface layer of the injection molded product is already very low, so the degree of shrinkage is small.
In addition to considering that the temperature of the inner and outer surfaces of the plastic part tends to be balanced, the consistent temperature on each side of the plastic part should also be considered, that is, when the mold is cooled, the temperature of the cavity and the core should be kept as uniform as possible, so that the cooling rate of the plastic part can be even and the shrinkage of every part can be more uniform, effectively preventing deformation. Therefore, the arrangement of cooling water holes on the mold is very important. Not only the distribution of the distance from the water hole to the cavity should be uniform, but also after the distance from the pipe wall and the cavity surface is determined, the distance between the cooling water holes should be as small as possible to ensure the uniform temperature of the cavity wall. At the same time, since the temperature of the cooling medium increases with the increase of the length of the cooling pipe, a temperature difference is generated between the mold cavity and mold core along the pipe.
Several cooling circuits should be set up in large molds, with the inlet of one circuit near the outlet of the other. It is a feasible method to use the change of mold temperature to compensate for warpage caused by other reasons, which requires the reasonable arrangement of cooling pipes to achieve the purpose of reducing warpage. For example, the use of a center gate often results in saddle-shaped warping or twisting due to orientation effects and shrinkage in the small area. This warpage can be reduced by increasing the cooling near the gate and attenuating the cooling at the end of the cavity through piping arrangements.
How does the mold's ejection system affect the warpage of plastic parts?
The design of the ejection system also directly affects the deformation of the plastic part. In the ejecting and demoulding processes, the temperature of the plastic product is high. The ejection is not straight, uneven, and inconsistent, which will cause mechanical stress, and the product is easy to warp and deform. To avoid mechanical stress, check the ejector system during the application, make necessary adjustments and properly lubricate all moving parts.
The ejector plate of the large mold must use a guide sleeve to prevent the center of the template from changing the ejection direction and position due to its own weight. Therefore, when the ejector system is designed, a balance should be kept between the system and demolding resistance. In addition, the cross-sectional area of the ejector rod should not be too small to prevent the plastic parts from being deformed due to excessive force per unit area, especially when the demolding temperature is too high. The arrangement of the ejector pins should be as close as possible to the parts with great demoulding resistance. Under the premise of not affecting the quality of the plastic parts, including the use requirements, dimensional accuracy and appearance, ejectors should be provided as many as possible to reduce the overall deformation of the plastic parts. In addition, a reasonable ejection method should be selected according to the actual situation. When soft plastic is used to produce large-scale deep-cavity thin-walled plastic parts, the demoulding resistance is great and the material is soft. If a single mechanical ejection method is used, the plastic parts will be deformed, or even scrapped due to piercing or folding. The effect will be better if the combination of multiple components or the combination of pneumatic and hydraulic and mechanical ejection is used.
However, the choice of ejection method and ejection position is determined by the structure of the injection molded product to a greater extent. Usually, the warpage of the product mainly comes from the shrinkage in the mold. Therefore, the design of the injection mold structure should be improved in order to improve the warpage. The improvement of gating systems and cooling systems should be given priority to.
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