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

- Aug 27, 2019-

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


 


1. Insufficient cooling or unevenness


When it is not fully cooled, the top thrust of the ram tends to deform the molded part, so if it is not sufficiently cooled, the mold release will be deformed. The countermeasure is to fully cool in the cavity, and then it can be ejected after being completely hardened. It is also possible to reduce the mold temperature and extend the cooling time. However, some of the molds have insufficient local cooling, and it is not possible to prevent deformation under normal molding conditions. In this case, consider changing the path of the cooling water, the location of the cooling water channel or the additional cooling tip hole. In particular, consider using no water cooling and air cooling.


 


2, caused by the ejector


Some of the parts have poor mold release properties, and the ejector rods are forcibly demolded to cause deformation. For plastic parts that are not easily deformed, no deformation occurs at this time, but cracks are generated. For ABS and polystyrene parts, this deformation is manifested by the blush of the pushed part (see cracking, cracking, microcracking, and blushing). The elimination method is to improve the polishing of the mold, making it easy to release the mold, and sometimes using the release agent to improve the mold release. The most fundamental improvement method is to grind the core, reduce the mold release resistance, or increase the draft angle, and add a ejector at the point where it is difficult to ejector, and it is more important to change the ejector mode.


 


3, caused by molding strain


The deformation caused by the forming strain is mainly caused by the difference in the direction of the forming shrinkage and the change in the wall thickness. Therefore, increasing the mold temperature, increasing the melt temperature, lowering the injection pressure, improving the flow conditions of the casting system, etc., can reduce the difference in the direction of shrinkage. However, it is difficult to correct only the molding conditions, and it is necessary to change the position and the number of the gates, such as injection from one end when forming a long rod.


Sometimes it is necessary to change the configuration of the cooling water channel; longer sheet-like parts are more likely to be deformed, and sometimes the partial design of the part needs to be changed to provide reinforcing ribs on the back side of the upper side. It is mostly effective to use auxiliary tool cooling to correct this deformation. If it cannot be corrected, the mold design must be corrected. Among them, the most important thing is to make the wall thickness of the product consistent. In the case of last resort, the mold must be corrected in the opposite direction by measuring the deformation of the product.


 


4, crystalline plastic


The resin with a large shrinkage ratio is generally a crystalline resin (such as polyacetal, nylon, polypropylene, polyethylene, and PET resin) than a non-crystalline resin (such as PMMA resin, polyethylene, polystyrene, ABS resin, and AS). The deformation of the resin or the like is large, and since the glass fiber reinforced resin has fiber orientation, the deformation is also large.


Most of the melting point temperature range is deformed and is often difficult to correct. The crystallinity of the crystalline plastic varies depending on the cooling rate, that is, the sharp cooling crystallinity decreases, the molding shrinkage rate decreases, and the slow cooling crystallinity increases and the molding shrinkage rate increases. A special correction method for the deformation of crystalline plastics is to use this property.


The correction method actually used is to make the dynamic and static modes have a certain temperature difference. The deformation is corrected by taking the temperature at which the other side of the warpage is strained. 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 molded parts and molds, if special deformation prevention measures are not taken in advance, the parts may not be used due to deformation, and only the molding conditions meet the above requirements, and most of the cases still cannot Correct the deformation.


 


5, the method of warping the product


If the product removed from the mold is to be corrected, the product to be corrected is placed on the correcting tool in a simple manner, and the weight is added to the warped portion, but the weight of the weight must be clearly determined. Or put the warped product on the straightener, put it into the hot water near the heat distortion temperature of the product, and simply straighten it by hand. However, 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 harmful. Do not leave any marks on the product after warping correction.