Cause analysis of warpage of injection molded parts

The deformation, bending and distortion of injection molded parts are mainly due to the higher shrinkage rate in the flow direction than that in the vertical direction, which makes the parts warped due to different shrinkage rates in each direction. Moreover, due to the large residual internal stress in the parts during injection filling, the warpage is caused by high stress orientation. Therefore, fundamentally speaking, the mold design determines the warpage tendency of the parts. It is very difficult to restrain this tendency by changing the molding conditions. The final solution to the problem must start with the mold design and improvement. This phenomenon is mainly caused by the following aspects:

1. Mould:

(1) The thickness and quality of the products should be uniform.

(2) The design of the cooling system of injection molding machines should make the temperature of each part of the mold cavity uniform, the gating system should make the material flow symmetrical, avoid warpage caused by different flow direction and shrinkage rate, appropriately thicken the shunt channel and main channel of the difficult part, and try to eliminate the density difference, pressure difference and temperature difference in the mold cavity.

(3) The transition zone and corner of the workpiece thickness should be smooth enough and have good demoulding performance. For example, increase the stripping redundancy, improve the polishing of the die surface, and keep the ejection system balanced.

(4) Exhaust well.

(5) By increasing the wall thickness or increasing the anti warping direction, the anti warping ability of the part can be enhanced by reinforcing rib.

(6) The material used in the mould is not strong enough.

2. Plastics:

In addition, the crystallized plastics can use the crystallization process that the crystallinity decreases with the increase of cooling rate and the shrinkage rate decreases to correct the warpage deformation.

3. Processing:

(1) If the plastic injection pressure is too high, the holding time is too long, the melting temperature is too low and the speed is too fast, the internal stress will increase and the warpage will appear.

(2) The mold temperature is too high and the cooling time is too short, so that the parts are overheated and the ejection deformation occurs.

(3) The internal stress is limited by reducing the screw speed and back pressure and reducing the density while keeping the minimum charge.

(4) If necessary, soft setting or demoulding can be carried out for the parts easy to warp and deform.