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Causes of deformation and warpage of plastic products-injection molding machine manufacturers

TIME:2021-01-27 Share

The cause of deformation and warpage of plastic products refers to the deviation of the shape of the injection product from the shape of the mold cavity. It is one of the common defects in the production of plastic products by the injection molding machine.

There are many reasons for warpage and deformation, and it is often impossible to solve it by process parameters alone. Combining relevant information and actual work experience, the following is a brief analysis of the factors affecting the warpage of injection molded products:

1. Gating system:
The position, form and quantity of the gate of the injection mold will affect the filling state of the plastic in the mold cavity, which will cause the plastic part to deform.

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The longer the flow distance, the greater the internal stress caused by the flow and feeding between the frozen layer and the central flow layer; conversely, the shorter the flow distance, the shorter the flow time from the gate to the end of the flow of the part, and it freezes during mold filling The thickness of the layer is reduced, the internal stress is reduced, and the warpage is greatly reduced. For some flat plastic parts, if only one center gate is used, because the shrinkage in the diameter direction is greater than the shrinkage in the circumferential direction, the molded plastic parts will be deformed; if you switch to multiple point gates or film types The gate can effectively prevent warpage and deformation.

When using point gates for molding, also due to the anisotropy of plastic shrinkage, the position and number of gates have a great influence on the degree of deformation of the plastic part.

In addition, the use of multiple gates can also shorten the plastic flow ratio (L/t), thereby making the melt density in the cavity more uniform and shrinking more uniform. At the same time, the entire plastic part can be filled under a smaller injection pressure. The smaller injection pressure can reduce the molecular orientation tendency of the plastic, reduce its internal stress, and thus reduce the deformation of the plastic part.

2. Cooling system:
During the injection process, the uneven cooling rate of the plastic part will also cause uneven shrinkage of the plastic part. This difference in shrinkage leads to the generation of bending moments and warps the plastic part.

If the temperature difference between the mold cavity and core used in injection molding flat plastic parts (such as mobile phone battery shells) is too large, the melt that is close to the cold mold cavity will quickly cool down, and the mold close to the hot mold cavity The material layer will continue to shrink, and the uneven shrinkage will warp the plastic part. Therefore, the cooling of the injection mold should pay attention to the balance of the cavity and core temperature, and the temperature difference between the two should not be too large (in this case, two mold temperature machines can be considered).

In addition to considering that the temperature of the inner and outer surfaces of the plastic part tends to be balanced, the temperature on each side of the plastic part should also be considered, that is, the temperature of the cavity and core should be kept as uniform as possible when the mold is cooled, so that the cooling rate of the plastic part Balance, so that the shrinkage of all parts becomes more uniform, effectively preventing the occurrence of deformation. Therefore, the arrangement of cooling water holes on the mold is very important. After the distance between the pipe wall and the surface of the cavity is determined, the distance between the cooling water holes should be as small as possible to ensure that the temperature of the cavity wall is uniform.

At the same time, since the temperature of the cooling medium increases with the increase of the length of the cooling water channel, the temperature difference between the cavity and core of the mold is generated along the water channel. Therefore, the length of the water channel of each cooling circuit is required to be less than 2 meters. Several cooling circuits should be installed in a large mold, and the inlet of one circuit is located near the outlet of the other circuit. For long plastic parts, straight-through water channels should be used. (And most of our molds use S-type loops-it is not conducive to circulation, but also prolongs the cycle.)

3. Ejection system:
The design of the ejector system also directly affects the deformation of the plastic parts. If the ejection system is unbalanced, it will cause an imbalance in the ejection force and deform the plastic part. Therefore, when designing the ejection system, it is important to balance the ejection resistance. In addition, the cross-sectional area of ​​the ejector rod should not be too small to prevent the plastic parts from being deformed due to excessive force per unit area (especially when the demoulding temperature is too high).

The arrangement of the ejector rod should be as close as possible to the part with high demolding resistance. Under the premise of not affecting the quality of the plastic parts (including use requirements, dimensional accuracy and appearance, etc.), as many ejectors as possible should be set to reduce the overall deformation of the plastic parts (this is the reason for changing the ejector rod to the ejector block).

When soft plastics (such as TPU) are used to produce deep-cavity thin-walled plastic parts, due to the large demolding resistance and the softer materials, if a single mechanical ejection method is used completely, the plastic parts will be deformed or even The plastic parts are scrapped due to top-through or folding. For example, it will be better to use multi-element combination or the combination of pneumatic (hydraulic) pressure and mechanical ejection (will be used later).

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