Causes and prevention measures of warping, bending and twisting of injection molded parts

1. Insufficient or uneven cooling
When the ejector is not fully cooled, the jacking force of the ejector rod often deforms the formed part, so the forced demoulding without sufficient cooling will cause deformation. The countermeasure is to fully cool the mold cavity and eject it after it is completely hardened. It can also reduce the mold temperature and extend the cooling time. However, the local cooling of some molds is not sufficient, and sometimes deformation can not be prevented under normal molding conditions. In this case, it is necessary to consider changing the path of cooling water, the position of cooling water channel or adding cooling pin holes, especially the use of air cooling instead of water cooling.
2. Caused by ejector pin
Some parts have poor demoulding performance, and deformation is caused by forced demoulding with ejector pin. For plastic parts that are not easy to deform, cracks will occur instead of deformation. For ABS and polystyrene parts, this deformation is shown by the whitening of the pushed part (refer to cracking, cracking, microcracking and whitening). The elimination method is to improve the polishing of the mold and make it easy to demould. Sometimes the use of demoulding agent can also improve demoulding. The most fundamental improvement method is to grind the core, reduce the stripping resistance, or increase the draft angle, and add a ejector rod at the position where it is difficult to eject, while changing the ejection method is more important.
3. Caused by forming strain
The deformation caused by forming strain is mainly caused by the difference in the direction of forming shrinkage and the change of wall thickness. Therefore, increasing the mold temperature, increasing the melt temperature, reducing the injection pressure, and improving the flow conditions of the gating system can reduce the difference of shrinkage in the direction. However, it is difficult to correct only by changing the forming conditions. At this time, it is necessary to change the position and number of gates, such as to inject from one end when forming a long rod.
Sometimes it is necessary to change the configuration of cooling water channel; Longer thin pieces are more prone to deformation. Sometimes it is necessary to change the local design of the parts and set stiffeners on the back of the upwarped side. It is mostly effective to use auxiliary tools to correct this deformation. If it cannot be corrected, the design of the mold must be corrected. The most important thing is to keep the wall thickness of the product consistent. As a last resort, we have to correct the mold in the opposite direction by measuring the deformation of the product.
4. Crystalline plastic
Resins with large shrinkage are generally crystalline resins (such as polyformaldehyde, nylon, polypropylene, polyethylene and PET resins) with greater deformation than non-crystalline resins (such as PMMA resins, polyethylene, polystyrene, ABS resins and AS resins). In addition, glass fiber reinforced resins have greater deformation due to their fiber orientation.
Because of the narrow range of melting point temperature, most of them are deformed and often difficult to correct. The crystallinity of crystalline plastic varies with the cooling rate, that is, the crystallinity of sharp cooling decreases and the molding shrinkage decreases, while the crystallinity of slow cooling increases and the molding shrinkage increases. The special correction method of crystalline plastic deformation is to use this property.
In fact, the correction method used is to make the dynamic and static models have a certain temperature difference. The deformation can be corrected by taking the temperature that causes the strain on the other side of the warp. Sometimes the temperature difference can be as high as 20 ℃, but it must be very evenly distributed. It must be pointed out that when designing crystalline plastic molding parts and molds, if special measures to prevent deformation are not taken in advance, the parts will not be able to use due to deformation. Only if the molding conditions meet the above requirements, the deformation cannot be corrected in most cases.
5. Methods for warping of Jiaozheng products
If the products taken from the mold are to be corrected, the simple way is to put the products to be corrected on the correction tool, and add the weight at the warped place, but the weight of the weight and the position of the weight must be clearly determined.
Or put the warped products on the straightener and put them into the hot water near the thermal deformation temperature of the products, and simply straighten them by hand. However, it should be noted that the temperature of hot water should not be too high, otherwise the deformation of products will be more severe.
After warping correction, there shall be no marks on the product.