Causes of warping, bending and warping of injection parts and preventive measures

1. Insufficient or uneven cooling

When it is ejected before it is completely cooled, the pushing force of the ejector rod tends to deform the molded product, so if it is not sufficiently cooled, it will be deformed if it is barely demolded. The countermeasure is to fully cool in the mold cavity and wait for it to harden completely 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 molding conditions. In this case, consideration should be given to changing the cooling water path, the location of the cooling water channel, or adding cooling tip holes. In particular, consideration should be given to using water cooling instead of air cooling.

2. Caused by the jack

Some parts have poor mold releasability, and they are deformed by forced ejection with a jack. For plastic parts that are not easily deformed, cracks are generated instead of deformation. For ABS and polystyrene parts, this deformation is manifested by blushing of the pushed item (see cracking, cracking, microcracking, and blushing). The elimination method is to improve the polishing of the mold and make it easy to demold, and sometimes the mold release agent can also improve the demolding. The most fundamental improvement method is to grind the core, reduce the demolding resistance, or increase the draft angle, and add the ejector rod to the part that is not easy to eject, and it is more important to change the ejection method.

3. Caused by molding strain

The deformation caused by molding strain is mainly caused by the difference in the direction of molding shrinkage and the change in wall thickness. Therefore, increasing the mold temperature, increasing the melt temperature, reducing the injection pressure, and improving the flow conditions of the casting system can all reduce the difference in shrinkage in the direction. However, it is often difficult to correct them by simply changing the molding conditions. In this case, it is necessary to change the position and number of gates, such as pouring from one end when molding long rods.
Sometimes it is necessary to change the configuration of the cooling water channel; longer sheet-like parts are more easily deformed, and sometimes it is necessary to change the local design of the part to set reinforcement ribs on the back of the upturned side. Use auxiliary tools to cool to correct this deformation. Effective. When it cannot be corrected, the mold design must be revised. The most important thing is to pay attention to make the product wall thickness consistent. In the case of last resort, you have to measure the deformation of the product and correct the mold in the opposite direction to correct it.

4. Crystalline plastic

Resins with larger shrinkage ratios are generally crystalline resins (such as polyoxymethylene, nylon, polypropylene, polyethylene and PET resins) than non-crystalline resins (such as PMMA resins, polyethylene, polystyrene, ABS resins and AS The deformation of the resin, etc.) is large, and the glass fiber-reinforced resin has fiber alignment, and the deformation is also large.
Due to the narrow melting temperature range, most deformations occur and are often difficult to correct. The crystallinity of crystalline plastics changes with the cooling rate, that is, the sharp cooling decreases the crystallinity and the molding shrinkage decreases, while the slow cooling increases the crystallinity and the molding shrinkage increases. The special correction method for the deformation of crystalline plastic is to use this property.
The correction method actually used is to make the dynamic and static modes have a certain temperature difference. That is, the temperature at which the other side of the warpage is strained can be used to correct the deformation. Sometimes this temperature difference is as high as 20 ° C or more, but it must be very evenly distributed. It must be pointed out that when designing crystalline plastic molded parts and molds, if special precautions against deformation are not taken in advance, the parts will be unusable due to deformation. Only the molding conditions meet the above requirements, and most cases still cannot Correct the deformation.

5. Method of correcting warpage of products

If the products taken out of the mold are to be corrected, a simple method is to put the products to be corrected on the correction tool and add heavy objects to the warped places, but the weight of the heavy objects must be clearly determined with the place where they are placed.
Or put the warped product on the straightener and put it in hot water near the heat deformation temperature of the product, and simply straighten it by hand. But it should be noted that the temperature of the hot water should not be too high, otherwise the deformation of the product will be more severe.
Do not leave marks on the product after warpage correction.