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Plastic shrinkage of common problems and solutions on

Plastic shrinkage is the problem of plastic shrinkage

1. Four kinds of plastic shrinkage

Thermal shrinkage, phase change shrinkage, orientation shrinkage, compression shrinkage and elastic recovery.
Shrinkage process consists of three parts: shrinkage before the gate solidification, cooling shrinkage and shrinkage after demoulding.

2. Main causes of shrinkage

①Insufficient injection volume;
②High melt temperature;
③Injection pressure and holding pressure are too small;
④Injection time and holding time is too little;
⑤Injection speed is too large;
⑥Inadequate mold temperature.

3. The main causes of shrinkage

①Insufficient injection volume;
②Injection pressure is too low;
③Inadequate injection speed;
④Mold temperature is too low.

4. Injection molding parts lack of glue, not full mold

Analysis of the causes: plastic melt is not completely filled cavity. Plastic material fluidity is not good.

Countermeasures: Improper matching of products and injection molding machine, plasticizing ability of injection molding machine or insufficient injection volume. Material temperature, mold temperature is too low, plastic in the current pressure flow difficulties, injection speed is too slow, holding pressure or holding pressure is too low. Insufficient melting of plastic and poor flowability, resulting in large injection pressure loss. Increase the number of gates, gate location arrangement should be reasonable, multi-cavity unbalanced arrangement filling. Insufficient or improperly reserved cold material well in the runner, cold material head enters the cavity and hinders the normal flow of plastic, increasing the cold material cavity. The nozzle, runner and gate are too small, the flow is too long, the plastic filling resistance is too large. Mold exhaust poor, air can not be excluded.

5. Phi peak (hairy edge)

Analysis of the causes: plastic melt into the mold surface or insert mating surface will occur. Clamping force is sufficient, but in the main sprue and the diversion of the convergence of the film-like excess rubber.

Countermeasure: Insufficient clamping force, high pressure plastic injected into the cavity so that the parting surface or insert mating surface to produce a gap, plastic melt overflow into this gap. Mold (fixed side) is not fully in contact with the machine nozzle, male and female mold gap. (Not installed tightly) The effect of mold temperature on crankshaft type clamping system. Improve the strength and parallelism of the mold plate. Mold guide sleeve friction damage / mold mounting plate damage / bending of tie bar (goring post) with insufficient strength, resulting in parting surface offset. Foreign matter adheres to the parting surface. The air venting slot is too deep. The cavity projection surface is too large/the plastic temperature is too high/over-pressure.

6. Surface shrinkage, shrinkage hole (vacuum bubble)

Analysis of the causes: the phenomenon of product surface depression. By the plastic volume shrinkage generated, commonly in the local meat thick area, such as reinforcement or column position and face intersection area. Product local flesh thickness in the cooling process due to volume contraction of the vacuum bubble, called shrinkage (Void). Plastic melt contains air, moisture and volatile gas, in the injection molding process air, moisture and volatile gas into the product inside the residual cavity called bubble (Bubble).

Countermeasures: Increase the size of the gate and runner, so that the pressure is effectively applied to the thick part of the molded product. If necessary, adjust the position of the glue mouth. Increase the holding pressure, extend the holding time. Increase the filling speed to achieve full compression before the plastic cools and cures. The plastic injection and pressure-holding is too fast. Make the thickness of the flesh change more smoothly and improve the cooling efficiency of this part. Pre-drying the rubber pellet to remove water. Barrel temperature setting should not be high, can effectively prevent plastic decomposition gas generation. Change to a smaller screw or machine to prevent the screw from over-shearing. Increase the back pressure so that the gas can be discharged from the barrel. Reduce the filling speed appropriately, so that the gas can be discharged in sufficient time.

7. Silver pattern (material flowers, water flowers)

Analysis of the causes:The surface of the product or near the surface, along the direction of plastic flow presented silver-white stripes. Silver streaks are generally caused by the vaporization of water or volatiles in the plastic or attached to the surface of the mold, etc. The injection molding machine screw involved in the air sometimes also produces silver streaks. The material has impurities.

Countermeasures:Plastic containing moisture, volatiles, insufficient drying. Plastic melt overheating or stagnant barrel for too long and decomposition, producing a large amount of gas, incomplete discharge, in the curing will produce silver wire. Mold temperature is too low, the plastic melt quickly cured, resulting in incomplete exhaust. Oil or water or mold release agent is attached to the surface of the mold and evaporates to become gas-like, which liquefies as the plastic melt cools and cures. The screw is involved in the air, the lower part of the hopper is cooled sufficiently, then the temperature of the hopper side is low and there is a temperature difference with the barrel, and the rubber particles often rub the screw and easily bring in air. Poor exhaust at the beginning of injection. The initial injection of the plastic melt quickly solidified, so the gas discharge is incomplete, and the silver line occurs. Injection pressure is too high and injection speed is too fast. When the flesh thickness changes drastically, the compressed plastic melt in the flow rapidly decompresses and expands, and the volatile decomposition gas liquefies after contact with the mold cavity.
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