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Causes and solutions of dents in plastic moulds

Source:   Pubdate:2020-09-28 11:41;   Click:

Causes of dents in plastic products: shrinkage depression caused by cooling and hardening of plastics mainly occurs in thick wall position, ribs, casing, back of nut insert, etc.
First, equipment
(1) Insufficient supply. The screw or plunger wear seriously, and the melt leakage occurs during injection and pressure maintaining, which reduces the filling pressure and material quantity, resulting in insufficient melt.
(2) The nozzle hole is too large or too small. If it is too small, it is easy to block the feeding channel; if it is too large, the injection force will be small and the mold filling will be difficult.
Second, plastic mold
(1) If the gate is too small or the runner is too narrow or shallow, the efficiency of the runner is low, the resistance is high, and the molten material is cooled 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 should be set in the thick wall part of the product in process. A necessary cold material well with enough capacity is set up in the flow passage, which can eliminate the cold material from entering the cavity and make the mold filling continue. The gate length of point gate and needle gate must be controlled below 1 mm, otherwise the plastic will solidify quickly in the gate, which will affect the pressure transmission; if necessary, the number or position of the gate can be increased 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) In order to adjust the filling speed of each gate, it is better to set the gate symmetrically.
(3) The key parts of the mold should be effectively set with cooling channels to ensure the cooling of the mold, which has a good effect on eliminating or reducing shrinkage.
(4) The whole mold should have no burr and reliable sealing performance, and can bear the filling of high pressure, high speed and low viscosity melting material.
Three aspects of Technology
(1) The injection pressure and holding pressure were increased and the injection time was prolonged. For the plastic with high fluidity, flash will be produced under high pressure, which will cause collapse pit. The temperature of material in front of barrel and nozzle should be lowered properly to reduce the change of melt volume into the cavity, which is easy to cold set; for high viscosity plastics, the barrel temperature should be increased to make mold filling easier. When shrinkage occurs in the gate area, the packing time should be prolonged.
(2) With the increase of injection speed, the parts can be easily filled and most of the shrinkage can be eliminated.
(3) The mold temperature should be increased for thin-walled parts to ensure smooth material flow; for thick wall parts, the mold temperature should be reduced to accelerate the curing and shaping of skin.
(4) Prolonging the cooling residence time of the parts in the mould, keeping the production cycle uniform, increasing the back pressure, and retaining a certain cushion in the front of the screw can reduce the shrinkage.
(5) Low precision products should be early out of the mold, so that it can be slowly cooled in air or hot water, which can make the shrinkage depression smooth and does not affect the use.
4. Raw materials:
The effective way is to add nucleating agent into plastic to accelerate crystallization.
5. Product design:
The product design should make the wall thickness uniform and avoid the change of wall thickness as far as possible. For plastics with large shrinkage such as polypropylene, when the thickness change is more than 50%, it is better to replace the thickened part with ribs
The above is Hubei Huangshi Yike for you to bring about the design principle of blister mold, welcome you for blister mold customers with ideas to our company to learn more about consulting!

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