analysis of dents in injection molded parts

The dents of injection molded parts are shrinkage dents caused by the cooling and hardening of plastics, which mainly appear in the thick wall position, ribs, casing, back of nut inserts, etc.

1、 Equipment:

(1) Insufficient supply.

The screw or plunger is seriously worn, and the molten material leaks during injection and pressure holding, which reduces the mold filling pressure and material quantity, resulting in insufficient molten material.

(2) The nozzle hole is too large or too small.

If it is too small, it is easy to block the feed channel. If it is too large, it will make the injection force small and it is difficult to fill the mold.

2, mold aspects:

(1) The gate is too small or the runner is too narrow or shallow, the runner efficiency is low, the resistance is large, and the melt cools prematurely

The gate should not be too large, otherwise the shear rate will be lost, the viscosity of the material will be high, and the product will not be full. The gate shall be set at the thick wall of the work in process. The necessary cold material well with sufficient capacity is set in the flow channel to eliminate the cold material from entering the mold cavity and make the mold filling continue.

The gate length of point gate and needle gate must be controlled below 1mm, otherwise the plastic will solidify quickly at the gate and affect the pressure transmission; If necessary, the number of point gates or gate positions can be increased to meet the actual needs; When the flow channel is long and thick, an exhaust groove shall be set at the edge of the flow channel to reduce the blocking effect of air on the material flow.

(2) For multi gate mold, the filling speed of each gate should be adjusted, and it is best to set the gate symmetrically.

(3) The key parts of the mold shall be effectively provided with cooling water channels to ensure that the cooling of the mold has a good effect on eliminating or reducing shrinkage.

(4) The whole mold shall be free of burrs and have reliable mold sealing performance, and can withstand the mold filling of high-pressure, high-speed and low viscosity melt.

3、 Process:

(1) Increase the injection pressure, pressure holding pressure and prolong the injection time.

For plastics with high fluidity, high pressure will produce flash and cause pit collapse. Properly reduce the material temperature, reduce the temperature of the front section of the barrel and the nozzle, so as to reduce the change of molten material volume entering the cavity and easy to cold solidify; For high viscosity plastics, the barrel temperature should be increased to make mold filling easy. When shrinkage occurs in the gate area, the holding time shall be extended.

(2) Increasing the injection speed can easily fill the workpiece and eliminate most of the shrinkage.

(3) For thin-walled parts, the die temperature should be increased to ensure smooth material flow; For thick wall parts, the mold temperature shall be reduced to accelerate the curing and shaping of the skin.

(4) Prolonging the cooling residence time of the parts in the mold, maintaining a uniform production cycle, increasing the back pressure, and retaining a certain cushion in the front section of the screw are conducive to reducing the shrinkage phenomenon.

(5) Low precision products should be molded early so that they can be cooled slowly in air or hot water, which can make the shrinkage depression smooth without affecting the use.

4、 Raw materials:

If the raw material is too soft, it is easy to sag. The effective way is to add nucleating agent to the plastic to speed up the crystallization.

5、 Product design:

The product design shall make the wall thickness uniform and try to avoid the change of wall thickness. For plastics with large shrinkage such as polypropylene, when the thickness change exceeds 50%, it is best to replace the thickened part with reinforcement.