The causes and prevention of warping, bending, and twisting of injection molded parts

1. Insufficient or uneven cooling

In not completely cooled ejected, the ejector rod thrust often deforms the molded parts, so not fully cooled on the forced release will produce deformation. The countermeasure is to fully cool in the mold cavity, and so completely hardened before ejecting. You can also lower the mold temperature and extend the cooling time.

However, some molds are not sufficiently cooled locally, and deformation cannot be prevented under normal molding conditions. This situation should consider changing the path of cooling water, the position of cooling water channel or additional cooling tip hole, especially should consider not using water cooling, using air cooling and other ways.
 

2. The top bar causes

Some parts have bad demolding properties, and the deformation is caused by forcible demoulding with the top bar. For the plastic parts that are not easily deformed, this is not a deformation but a crack. For ABS and polystyrene parts, this deformation is manifested by whitening of the part to be pushed (refer to cracking, crazing, microcracking, and whitening). It is eliminated by improving the polish of the mold, making it easy to release, and sometimes by using a release agent to improve release.

The most fundamental improvement method is to grind the core, reduce the mold release resistance, or increase the mold slope, and add the ejector rod in the part that is not easy to eject, etc., while changing the ejector method is more important.

3. Caused by molding strain

The deformation caused by molding strain is mainly caused by the difference of molding shrinkage in the direction and the change of wall thickness. Therefore, raising the mold temperature, increasing the melt temperature, lowering the injection pressure, and improving the flow conditions of the pouring system can reduce the difference in shrinkage in the direction. However, most of the time, it is difficult to correct the problem by changing the molding conditions, so it is necessary to change the location and number of gates, for example, to inject from one end when molding long rod parts.

Sometimes it is necessary to change the configuration of the cooling water channel; longer sheet-type parts are more likely to be deformed, sometimes it is necessary to change the local design of the part to set reinforcement bars on the back of the upturned side, etc. The use of auxiliary tool cooling to correct this deformation is mostly effective. When it cannot be corrected, the design of the mold must be revised.

Among them, the most important thing is to pay attention to making the product wall thickness consistent. In the case of last resort, we have to correct the deformation of the product by measuring it and correcting the mold in the opposite direction.
 

4. Crystalline plastic

Resins with large shrinkage, generally crystalline resins (such as polyacetal, nylon, polypropylene, polyethylene, and PET resin, etc.) than non-crystalline resins (such as PMMA resin, polyethylene, polystyrene, ABS resin and AS resin, etc.) deformation is large, in addition, due to the glass fiber reinforced resin with fiber matching, deformation is also large.

Most of the deformation occurs due to the narrow range of melting point temperatures and is often difficult to correct. The crystallinity of crystalline plastics varies with the cooling rate, i.e., the crystallinity decreases with rapid cooling and the molding shrinkage decreases, while the crystallinity increases with slow cooling and the molding shrinkage increases. The special correction method for the deformation of crystalline plastics is to take advantage of this property.

The actual correction method used is to make a certain temperature difference between the moving and static mold. This is the temperature at which the other side of the warp is strained to correct the deformation. Sometimes this temperature difference is as high as 20°C or more, but it must be distributed very evenly. It must be pointed out that in the design of crystalline plastic molding parts and molds, if special means are not taken in advance to prevent deformation, the parts will be deformed and unusable, only to make the molding conditions meet the above requirements, most cases still can not correct the deformation.
 

5. Way to correct warpage of products

To correct the products taken out from the mold, the simple way is to put the products to be corrected on the correcting tool and add weights to the warped area, but the weight of the weights and the position of the weights must be decided.

Or put the warped product on the straightener and put it into hot water near the thermal deformation temperature of the product, and simply straighten it by hand. But be careful that the temperature of the hot water is not too high, otherwise, it will make the deformation of the product more severe. Do not leave a mark on the product after warpage correction.