warping reason and solution
1. Insufficient or uneven cooling
When pushed out before complete cooling, the pushing force of the push rod often causes deformation of the formed part, so forced demolding without sufficient cooling will result in deformation. The solution is to fully cool the mold cavity and wait for complete hardening before ejecting. It can also lower the mold temperature and extend the cooling time.
However, some molds have insufficient local cooling and sometimes cannot prevent deformation under normal forming conditions. In this situation, it is necessary to consider changing the path of the cooling water, the position of the cooling water channel, or adding cooling pin holes, especially considering the use of air cooling instead of water cooling.
2. Top rod caused
Some parts have poor demolding performance and are deformed due to forced demolding with a top rod. For plastic parts that are not easily deformed, cracks may occur instead of deformation. For ABS and polystyrene parts, this deformation is manifested by the whitening of the pushed part (referring to cracking, cracking, micro cracking, and whitening). The elimination method is to improve the polishing of the mold and make it easy to demold, and sometimes the use of demolding agents can also improve demolding.
The most fundamental improvement methods are grinding the core, reducing demolding resistance, or increasing the draft angle, adding top rods in areas that are not easily ejected, and changing the ejection method is more important.
3. Caused by forming strain
The deformation caused by forming strain is mainly caused by differences in the direction of forming shrinkage and changes in wall thickness. Therefore, increasing the mold temperature, increasing the melt temperature, reducing injection pressure, and improving the flow conditions of the pouring system can all reduce the difference in shrinkage rate in direction. However, changing the forming conditions alone is often difficult to correct. In this case, it is necessary to change the position and number of gates, such as injecting from one end when forming long rods.
Sometimes it is necessary to change the configuration of the cooling water channel; Longer thin sheet parts are more prone to deformation, and sometimes it is necessary to change the local design of the parts by setting reinforcement ribs on the back of the upturned side. Using auxiliary tools for cooling to correct this deformation is mostly effective. When it cannot be corrected, it is necessary to modify the design of the mold.
Among them, the most important thing is to pay attention to making the wall thickness of the product consistent. In the last resort, the mold had to be corrected in the opposite direction by measuring the deformation of the product.
4. Crystalline plastics
Resins with higher shrinkage rates are generally crystalline resins (such as polyoxymethylene, nylon, polypropylene, polyethylene, and PET resins) with greater deformation than amorphous resins (such as PMMA resin, polyethylene, polystyrene, ABS resin, and AS resin). In addition, due to the fiber orientation of glass fiber reinforced resins, deformation is also greater.
Due to the narrow temperature range of the melting point, deformation often occurs and is difficult to correct. The crystallinity of crystalline plastics varies with different cooling rates, namely a sharp decrease in crystallinity and a decrease in molding shrinkage rate, while a slow increase in crystallinity and an increase in molding shrinkage rate. The special correction method for the deformation of crystalline plastics is to utilize this property.
The actual correction method used is to create a certain temperature difference between the dynamic and static modes. It is to adopt a temperature that causes strain on the other side of the warp to correct deformation. Sometimes the temperature difference can reach over 20 ℃, but it must be evenly distributed. It must be pointed out that when designing crystalline plastic molded parts and molds, if special measures are not taken in advance to prevent deformation, the parts will become unusable due to deformation, and only the molding conditions meet the above requirements. In most cases, deformation cannot be corrected.
5. The method of warping in Jiaozheng products
If the product taken out of the mold needs to be corrected, a simple method is to place the product to be corrected on the correction tool and add a heavy object to the warped area, but the weight of the heavy object and its position must be clearly determined.
Alternatively, place the warped product on a straightener and place it in hot water near the product's deformation temperature, simply straightening it by hand. But it is important to note that the temperature of hot water should not be too high, otherwise it will cause more severe deformation of the product. After warping correction, no spots should be left on the product.
