Injection molded parts Dents are caused by shrinkage depressions due to cooling and hardening of plastic, mainly in thick-walled positions, bars, housings, backs of nut inserts, etc.
I. Equipment.
(1) Insufficient material supply.
Screw or plunger wear serious, injection and pressure-holding molten material leakage, reducing the mold filling pressure and material volume, resulting in insufficient molten material.
(2) Nozzle hole is too big or too small.
Too small is easy to block the feed channel, too large will make the injection force is small, mold filling difficulties occur.
Second, mold aspects.
(1) The gate is too small or the runner is too narrow or too shallow, the runner efficiency is low, the resistance is high, the melt is cooled too early
The gate should not be too large, otherwise, the shear rate will be lost and the viscosity of the material will be high, which also cannot make the product full. The gate should be opened in the thick-walled part of the product. Cold material wells with sufficient capacity in the runner are necessary to exclude cold material from entering the cavity so that mold filling can continue.
The gate length of the point gate and needle gate must be controlled below 1mm, otherwise, the plastic will solidify fast at the gate and affect the pressure transfer; if necessary, increase the number of point gate or gate positions to meet the actual needs; when the runner is long and thick, exhaust grooves should be set at the edge of the runner to reduce the blocking effect of air on the material flow.
(2) Multi-gate molds should adjust the filling speed of each gate, and it is better to open the gate symmetrically.
(3) The key parts of the mold should be effectively set up with cooling water channels to ensure that the cooling of the mold plays a good role in eliminating or reducing shrinkage.
(4) The whole mold should be burr-free and have reliable mold sealing, and can withstand high pressure, high speed, and low viscosity melt filling.
Third, the process.
(1) Increase the injection pressure, holding pressure, and extend the injection time.
For plastics with high fluidity, high pressure will produce flying edges and cause a collapse of the pit. The material temperature should be lowered appropriately and the temperature of the front section of the barrel and the nozzle should be lowered so that the volume change of the melt entering the cavity will be reduced and it will be easy to cold solidify. Shrinkage occurs in the gate area, the holding time should be extended.
(2) Increase the injection speed to make the part filled more easily and eliminate most of the shrinkage.
(3) Thin-walled parts should increase the mold temperature to ensure smooth material flow; thick-walled parts should reduce the mold temperature to speed up the curing of the skin and setting.
(4) extend the cooling residence time of the parts in the mold, maintain a uniform production cycle, increase the backpressure, the front section of the screw to retain a certain buffer pad, etc. are conducive to reducing shrinkage phenomenon.
(5) low precision products should be early out of the mold to let it cool slowly in the air or hot water, which can make the shrinkage depression gentle and does not affect the use.
Fourth, the raw materials.
Raw materials are too soft and easy to dent, an effective method is to add nucleating agents in the plastic to speed up the crystallization.
Fifth, the product design.
Product design should make the wall thickness uniform, try to avoid changes in wall thickness, such as polypropylene shrinkage of such plastics, when the thickness change of more than 50%, it is best to use the rib instead of thickening the part.