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Causes and prevention of warpage, bending and distortion of injection molded parts
- Oct 08, 2018 -

1. Inadequate or uneven cooling.

When the ejection is not completely cooled, the ejection force of the ejector rod often causes deformation of the molding parts, so the deformation will occur if the ejection is not fully cooled. The countermeasure is to cool fully in the mold cavity, and then fully harden before ejecting. It can also reduce the mold temperature and extend the cooling time. However, some molds are not fully cooled locally, and sometimes deformation can not be prevented under normal molding conditions. In this case, it should be considered to change the path of cooling water, the location of cooling water channel or to add cooling pinholes, especially not to use water cooling, air cooling and so on.

2. The ejector pin causes.

Some of the parts are demoulding bad, and the ejection of the ejector pin is used to cause deformation. For plastic parts which are not easy to deform, they do not produce deformation but produce cracks. For ABS and polystyrene parts, this deformation is represented by whitening of the pushed part (referring to cracking, cracking, microcracking and whitening). The elimination method is to improve the polishing of the mold and make it easy to demould. Sometimes demoulding agent can also improve demoulding. The most fundamental improvement method is to grind the core, reduce the demoulding resistance, or increase the inclination of the drawing die, add the ejector in the difficult ejection position, and so on, and change the ejection method is more important.

3. Caused by molding strain.

The deformation caused by forming strain is mainly caused by the difference of forming shrinkage direction 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 casting system can reduce the difference of shrinkage in the direction. However, it is difficult to rectify only by changing the molding conditions, so it is necessary to change the position and number of gate, such as injecting long rods from one end.

Sometimes it is necessary to change the configuration of the cooling water channel; longer sheet parts are more likely to deform; sometimes it is necessary to change the local design of the parts to set stiffeners on the back of the upside side of the upside. If it can not be corrected, the design of the mold must be corrected. Among them, the most important thing is to pay attention to uniform wall thickness. In case of necessity, the mold must be corrected in the opposite direction by measuring the deformation of the product.

4, crystalline plastics

Resins with larger shrinkage generally have larger deformation of 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 resins). In addition, glass fiber reinforced resins have greater deformation due to fiber orientation.

Due to the narrow temperature range, most of them are deformed and often difficult to correct. The crystallinity of crystalline plastics varies with the cooling rate, that is, the crystallinity decreases sharply and the molding shrinkage decreases, while the crystallinity increases slowly and the molding shrinkage increases. The special correction method for the deformation of crystalline plastics is to use this property.

In fact, the correction method used is to make the dynamic and static modes have a certain temperature difference. It is to take the temperature on which the other side of the warping produces strain to correct the deformation. Sometimes the temperature difference is above 20 C, but it must be evenly distributed. It must be pointed out that in the design of crystalline plastic molding parts and molds, if special measures are not taken in advance to prevent deformation, the parts will be unable to use because of deformation, only to make the molding conditions meet the above requirements, most cases still can not correct deformation.

5. Correct the warpage of products.

If the part removed from the mold is to be corrected, the simple method is to place the product to be corrected on the correcting tool and add a weight to the warping area, but the weight of the weight must be clearly determined with the position placed.

Or put the warped products on the straightener, together with the products into hot water near the thermal deformation temperature, simply straighten by hand. But we should pay attention to the temperature of hot water can not be too high, otherwise it will make the deformation of products more harmful.

No warpage can be found after warpage correction.