Cause analysis of shrinkage depression

- Sep 11, 2019-

1. Machine

(1) too large nozzle hole causes melting material backflow and shrinkage; too small resistance and insufficient material quantity cause shrinkage.

(2) shrinkage of flash edges caused by insufficient clamping force should be checked to see if there is any problem with the clamping system.

(3) if the amount of plasticizing is insufficient, the machine with large amount of plasticizing should be selected to check whether the screw and the barrel are worn.

2.mold

(1) the parts shall be designed to ensure uniform wall thickness and uniform shrinkage.

(2) the cooling and heating system of the mold should ensure that the temperature of each part is consistent.(3) the pouring system should be unobstructed and the resistance should not be too large. For example, the size of the main flow channel, shunt channel and gate should be appropriate, the smoothness should be adequate, and the transition area should be circular arc.

(4) for thin parts, the temperature should be increased to ensure smooth material, for thick wall parts, the mold temperature should be reduced.

(5) the gate should be opened symmetrically, as far as possible in the parts of thick wall, should increase the volume of cold material well.

3.In plastic

Crystalline plastic than non-crystalline plastic shrinkage experience harm, processing should be appropriate to increase the amount of material, or in the plastic adding agent, in order to accelerate crystallization, reduce shrinkage depression.

Cause analysis of shrinkage depression

Cause analysis of shrinkage depression

4.processing

(1) the temperature of the charging drum is too high, and the volume changes greatly, especially the temperature of the front furnace. For plastics with poor fluidity, the temperature should be appropriately raised to ensure smooth flow.

(2) injection pressure, speed, back pressure is too low, injection time is too short, so that the material quantity or density is insufficient and shrinkage pressure, speed, back pressure is too large, time is too long, resulting in flash and shrinkage.

(3) feeding amount, that is, if the buffer pad is too large, the injection pressure will be consumed; if it is too small, the feeding amount will be insufficient.

(4) for the parts that do not require precision, after the injection pressure is maintained, the outer layer is basically condensed and hardened, and the sandwich part is soft and can be pushed out. The mold is released early, allowing it to slowly cool in air or hot water, which can make the contraction depression gentle and not so conspicuous and does not affect the use.

(5) the "dent" is caused by local internal contraction caused by the sealing of the gate or the injection of missing materials.Dimples or micro-depressions in the surface of injection molding products are an old problem in the process of injection molding.Indentation is usually caused by the increase of the wall thickness of plastic products caused by the local shrinkage of products and produced, it may appear in the outer corner or wall thickness mutation, such as the bulge, reinforcement or the back of the support, sometimes also appear in some unusual parts.

The root cause of indentation is the thermal expansion and cold contraction of materials, because the thermal expansion coefficient of thermoplastic is quite high.The degree of expansion and contraction depends on many factors, among which the performance of plastics, *, * small temperature range and cavity pressure are important factors.The size and shape of the injection parts, as well as the cooling speed and uniformity are also factors.

The amount of expansion and contraction of plastic materials in the molding process is related to the thermal expansion coefficient of the processed plastic, which is called "molding shrinkage".With the cooling and shrinkage of the molding parts, the molding parts lose close contact with the cooling surface of the mold cavity, and then the cooling efficiency decreases. After the molding parts continue to cool, the molding parts keep shrinking, and the amount of shrinkage depends on the comprehensive effect of various factors.

Molding the wedge Angle of the fast cooling * and earlier than other parts hardening, close to the thick part of the molding parts center far from cavity cooling surface *, become a part of the heat released after molding pieces of the *, the corners of the material after curing, as close to the center of the melt cooling parts, mold parts will continue to shrink, the Angle between the plane gets only one cooling, the strength of material of high strength no sharp corners.

The cooling shrinkage of the plastic material at the center of the piece pulls in the relatively weak surface between the partially cooled and the sharply cooled corners.This creates a dent in the surface of the molded part.The presence of indentation indicates that the molding shrinkage here is higher than that of the surrounding area.If the shrinkage of a molded part is higher in one place than in another, the cause of warping occurs in the molded part.

The residual stress in mold will reduce the impact strength and temperature resistance of molded parts.In some cases, the process conditions can be adjusted to avoid indentation.For example, additional plastic material is injected into the mold cavity to compensate for mold shrinkage during the compression of the molded part.In most cases, the gate is much thinner than the rest of the piece, and the small gate has been solidified while the molded piece is still hot and continues to shrink.

The shrinkage rate of semi-crystalline plastic is high, which makes the indentation problem more serious.The molding shrinkage of amorphous material is lower, which will reduce the indentation to * degree.Materials that fill and maintain reinforcement have lower shrinkage and are less likely to produce dents.

Thick plastic cooling time is long, will produce larger shrinkage, so big thickness is the fundamental cause of indentation, the design should pay attention to avoid thick wall parts, if not be able to avoid a thick wall, should be designed as a hollow, thick parts is smooth transition to the nominal wall thickness, with a large circular arc instead of Angle, can eliminate or * reduce the dent near Angle to produce as much as possible