How to avoid warping and deformation of injection molded parts?

1. What is the main cause of warping
The shrinkage of crystalline engineering plastics is relatively high and is influenced by many factors. If the demolding is too fast, the mold temperature is too high, the mold temperature is uneven, and the flow channel system is asymmetric.
For crystalline engineering plastic materials, warping is greatly affected by wall thickness and mold temperature. The uneven wall thickness or uneven corners of the workpiece result in uneven cooling and shrinkage during forming, leading to warping and deformation. For the sake of surface aesthetics, some parts have runner gates located at the corners of the product. During molding, the molten adhesive can only be injected into the mold cavity at high speed from one end, and the solidified plastic is straightened and arranged in the same direction (orientation). During demolding, the raw material molecules are pulled back to their original state to form warping. Improper mold temperature can also cause warping and deformation.
Due to the directionality of glass fibers, glass fiber reinforced engineering plastics exhibit completely different shrinkage characteristics during injection molding, with slight shrinkage effects caused by wall thickness differences. Therefore, the main reason for warping deformation is the shrinkage difference between the horizontal and vertical flow of reinforced fibers.
From the analysis of the mold gate, the design of the mold should ensure that the plastic can smoothly enter the mold cavity, avoid using a right angle turning form for the diversion channel, and use an arc transition zone for the turning point. Short and thick diversion channels are ideal for reducing fluid resistance and fluid orientation phenomena,
In order to avoid deformation caused by difficulty in demolding due to different levels of tightness during melt filling, the cross-sectional shape and size of the diversion channel should be changed depending on the amount of glue injected and the shape of the product. After the difficultly formed parts of the product are thickened, the main flow channel also increases accordingly, making the cross-sectional area of the main flow channel equal to the sum of the cross-sectional areas of the main flow channel.
There are two other noteworthy issues: one is the form of the ejection device for the workpiece. If there are too few ejector pins set, it is easy to cause deformation and warping; However, if there are too many ejector pins, it will affect the appearance of the product. At this time, it should be considered to use a push plate method for ejection. The second is the design of the cooling channel in the mold cavity, which should ensure uniform overall shrinkage of the workpiece. If necessary, the workpiece should be equipped with hidden mold cores.
In short, the factors that affect warping deformation are wall thickness distribution, gate position, flow resistance, splitter channel, and the rigidity of the formed product. These different reasons lead to warping and deformation of injection molded products. Depending on whether the raw material contains reinforcing fibers, the same part often experiences opposite warping deformation.
2. How to prevent warping and deformation?
When injection molding crystalline engineering plastics, it is required that the wall thickness of the parts be uniform to avoid the accumulation of molten adhesive as much as possible. The use of multi-point gates to meet high pressure injection molding and minimize shrinkage differences, and the design of the mold temperature control system should consider average heat dissipation as much as possible.
For glass fiber reinforced plastics, the symmetry of the formed product structure is equally important as uniform wall thickness. Asymmetric parts hinder the flow of molten adhesive and cause deviation, ultimately resulting in warping and deformation. Asymmetric parts must be balanced by combining hidden mold cores during the mold planning and design stages. The position of the gate is also important, as each diversion and fusion line is a potential factor causing warping.
In order to ensure the smooth filling of the mold cavity with molten plastic, the design should try to avoid the following points as much as possible:
*The difference in thickness between the same workpiece is too large;
*Having excessive sharp angles;
*The buffer zone is too short, resulting in a significant difference in thickness transition.
3. Process measures to prevent warping deformation during injection molding
Assuming that the molded product, gate, and mold have been designed correctly, the warping deformation of the product can be controlled by adjusting the holding pressure and mold temperature during injection molding. The mold can also use most thermal loop systems to achieve complete heat dispersion.
For reinforced engineering plastics, changing the injection rate or lowering the mold temperature can slightly improve warping deformation. If possible subsequent warping is not foreseen during the mold manufacturing and molded product design stages, subsequent corrections cannot be made by changing the molding process conditions.