What causes the bending deformation of plastic mold injection molded parts?

Created on 01.13

Usually, we encounter some products made by plastic mold injection that are warped and deformed. What exactly is the reason? Are you still scratching your head in confusion? Is it a product structure issue, a mold issue, or a material issue? Next, let's analyze together the common root causes and solutions for deformation of plastic mold injection products.

Caused by molding strain, the deformation resulting from molding strain is primarily due to differences in shrinkage in different directions and variations in wall thickness. Therefore, increasing the mold temperature, raising the melt temperature, reducing injection pressure, and improving the flow conditions of the gating system can all reduce the difference in shrinkage rates in different directions. However, simply changing the molding conditions is often difficult to correct. In such cases, it is necessary to alter the position and number of gates, such as injecting from one end when molding long rod-shaped parts. Sometimes, it is necessary to change the configuration of the cooling water channels. Long, thin sheet-type parts are more prone to deformation, and sometimes it is necessary to alter the local design of the part, such as adding reinforcement ribs on the back of the upturned side. Using auxiliary tools for cooling to correct this deformation is mostly effective. If correction is not possible, it is necessary to modify the design of the plastic mold. Among these, it is most important to ensure that the wall thickness of the product is consistent. In situations where this is not possible, it is necessary to measure the deformation of the product, correct the mold in the opposite direction, and calibrate it.
Crystalline plastics, resins with a high shrinkage rate. Generally, crystalline resins such as nylon, POM, PP, and PET exhibit greater deformation than non-crystalline resins such as PMMA, PS, ABS, and PC. Additionally, due to the fiber orientation of glass fiber reinforced resins, deformation is also significant, often resulting in deformation that is difficult to correct due to the narrow melting temperature range. The crystallinity of crystalline plastics varies with different cooling rates, i.e., rapid cooling reduces crystallinity and decreases molding shrinkage, while slow cooling increases crystallinity and increases molding shrinkage. The special correction method for deformation of crystalline plastics utilizes this property, where the actual correction method involves creating a certain temperature difference between the moving and stationary molds, i.e., applying a temperature that causes strain on the other side of the warpage, thereby correcting the deformation. Sometimes this temperature difference can be as high as 20°C or more, but it must be evenly distributed. It must be noted that when designing molded parts and molds for crystalline plastics, if special measures to prevent deformation are not taken in advance, the molded parts may become unusable due to deformation. In most cases, deformation cannot be corrected simply by meeting the aforementioned molding conditions.
Insufficient or uneven cooling, and ejection before complete cooling often cause deformation of the molded parts due to the pushing force of the ejector pin. Therefore, forcibly demolding before sufficient cooling can lead to deformation. The countermeasure is to allow sufficient cooling within the mold cavity and perform ejection only after complete hardening. Alternatively, the mold temperature can be lowered and the cooling time can be extended. However, in some cases where the local cooling of the mold is insufficient and deformation cannot be prevented under normal molding conditions, consideration should be given to changing the path of the cooling water, the position of the cooling water channel, or adding cooling holes. In particular, it is advisable to consider using air cooling instead of water cooling.
Due to the use of ejector pins, some parts may exhibit poor demolding properties, leading to deformation when forcibly demoulded using ejector pins. For plastic parts that are not prone to deformation, this may result not in deformation but in cracking. For ABS and polystyrene parts, this deformation manifests as whitening at the pushed area (refer to cracking, cracks, microcracks, and whitening). The solution is to improve the polishing of the mold to make it easier to demold. Sometimes, using a mold release agent can also improve demolding. The most fundamental improvement method is to grind the core to reduce demolding resistance, or increase the draft angle, add ejector pins at areas where ejection is difficult, etc. Changing the ejection method is even more important.
For some small-batch plastic molds or products with special structural performance requirements, it is too cumbersome to solve the problem from the mold itself or the plastic itself. We can also make fixtures to fix the product shape after injection molding, and then use the fixtures to hold the product until it is completely cooled. Another simple method is to place the corrected product on a correction tool, add weights to the warped area, but it is necessary to clearly determine the weight of the weights and the placement. Or place the warped product on a straightener and immerse it in hot water near the heat distortion temperature of the product. Simply straighten it by hand, but be careful not to use hot water that is too hot, otherwise it will cause more severe deformation of the product. However, this method is not recommended, as manual processing is more time-consuming and resource-consuming than mechanical automation, and the efficiency is not so high, and quality control is not easy.

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