Analysis of the causes of shrinkage and depression in injection molding

1、Machine

(1) the injection nozzle hole is too large to cause the melt material reflux and shrinkage, too small resistance to a large amount of material is not enough to appear shrinkage.

(2) Insufficient clamping force causes shrinkage due to flying edges, and the clamping system should be checked for problems.

(3) insufficient plasticizing volume should be used plasticizing volume of the machine, check whether the screw and barrel wear.
 

2、Mold

(1) The design of the parts should make the wall thickness uniform to ensure consistent shrinkage.

(2) The cooling and heating system of the mold should ensure that the temperature of each part is the same. (3) The pouring system should be smooth and the resistance should not be too large, such as the main flow channel, diversion channel, gate size should be appropriate, the finish should be sufficient, and the transition area should be rounded.

(4) The temperature should be increased for thin parts to ensure smooth material flow, and the mold temperature should be reduced for thick-walled parts.

(5) The gate should be opened symmetrically, as much as possible in the thick-walled part of the part, and the volume of the cold material well should be increased.
 

3、Plastics

Crystalline plastic is more harmful than non-crystalline plastic shrinkage, processing should be appropriate to increase the amount of material, or add the plastic into the agent, in order to speed up the crystallization, reduce shrinkage depression.
 

4、Processing

(1) The barrel temperature is too high, the volume changes a lot, especially the temperature of the front furnace, the liquidity of poor plastic should be appropriate to increase the temperature, to ensure smooth.

(2) injection pressure, speed, back pressure is too low, injection time is too short so that the amount of material or density is not enough and shrinkage pressure, speed, back pressure is too large, too long caused by flying edge and shrinkage.

(3) The amount of material added, that is, the buffer pad is too large when the injection pressure is consumed, too small, the amount of material is not enough.

(4) for the precision of the parts, in the injection pressure is completed, the outer layer of basic condensation hardening and the sandwich part is still soft and can be ejected parts, early out of the mold, so that the air or hot water in the slow cooling, you can make the shrinkage depression is gentle and not so obvious and does not affect the use.

(5) "dent" is caused by the local internal shrinkage caused by the sealing of the gate or the lack of material injection. The surface of the injection molded products produced by the dent or micro-sink is an old problem in the injection molding process. Dents are generally caused by a local increase in shrinkage due to an increase in the wall thickness of the plastic product and can occur near external sharp corners or at abrupt changes in wall thickness, such as behind bumps, reinforcements, or supports, or sometimes at uncommon locations.

The root cause of dents is the thermal expansion and contraction of the material, as thermoplastics have a fairly high coefficient of thermal expansion. The degree of expansion and contraction depends on many factors, of which the properties of the plastic, the maximum and minimum temperature range, and the holding pressure of the mold cavity are the most important ones. The size and shape of the molded part, as well as the cooling rate and uniformity, are also influential factors.

The expansion and shrinkage of plastic materials during the molding process is related to the coefficient of thermal expansion of the processed plastic, which is called "molding shrinkage". As the molded part cools and shrinks, the molded part loses close contact with the cooling surface of the mold cavity, at which time the cooling efficiency decreases.

The sharp corners on the molded part are cooled fastest and hardened earlier than other parts. The thick part near the center of the molded part is farthest from the cooling surface of the cavity and becomes the last part of the molded part to release heat. After the material at the corners is cured, the molded part continues to shrink as the melt near the center of the part cools, and the plane between the sharp corners can only get one-sided cooling, and its strength is not as high as the strength of the material at the sharp corners.

The cooling shrinkage of the plastic material at the center of the part will be partially cooled with the cooling degree of the relatively weak surface between the sharp corners pulled inward. As a result, a dent is created on the surface of the injection molded part. The presence of a dent indicates that the shrinkage of the molded part here is higher than the shrinkage of its surrounding parts. If the shrinkage of the molded part is higher in one place than in another, then the molded part is the cause of warpage.

Residual stresses in the mold can reduce the impact strength and temperature resistance of the molded part. In some cases, dents can be avoided by adjusting the process conditions. For example, additional plastic material is injected into the mold cavity during the holding process of the molded part to compensate for molding shrinkage. In most cases, the gate is much thinner than the rest of the part, and while the molded part is still hot and continues to shrink, the small gate has cured, and after it has cured, the holding pressure does not work for the molded part in the cavity.

Semi-crystalline plastic materials have high shrinkage of molded parts, which makes the dent problem more serious; non-crystalline materials have lower molding shrinkage and will minimize dents; filled and maintenance enhanced materials have lower shrinkage and are less likely to produce dents.

Thick injection molded parts with long cooling time will produce larger shrinkage, so the thickness is the root cause of dents, the design should be paid attention to, to avoid thick-walled parts as far as possible, if you can not avoid thick-walled missing, should be designed as hollow, thick parts on the smooth transition to the nominal wall thickness, with a large arc instead of sharp corners, can eliminate or minimize the dents generated near the sharp corners.