1. Machine side
- (1) The nozzle hole is too large to cause the melted material to flow back and shrink, and the resistance is too small and the material is insufficient to shrink.
- (2) If the clamping force is insufficient, the flash will also shrink. Check the clamping system for any problems.
- (3) If the amount of plasticization is insufficient, a machine with a large amount of plasticization should be used to check whether the screw and barrel are worn.
2. Mold
- (1) The design of the part should make the wall thickness uniform and ensure uniform shrinkage.
- (2) The cooling and heating system of the mold must ensure that the temperature of each part is consistent. (3) The gating system must be unobstructed and the resistance must not be too large. For example, the size of the main runner, runner, and gate must be appropriate, the smoothness must be sufficient, and the transition zone must be arc transitioned.
- (4) The temperature of thin parts should be increased to ensure the smoothness of the material, and the mold temperature should be lowered for thick-walled parts.
- (5) The gate should be opened symmetrically, and should be opened in the thick wall part of the product as much as possible, and the volume of the cold slug well should be increased.
3. Plastic aspect
Crystalline plastics are more harmful than non-crystalline plastics, so the amount of material should be appropriately increased during processing, or a replacement agent should be added to the plastics to accelerate crystallization and reduce shrinkage.
4. Processing aspect
- (1) The temperature of the barrel is too high and the volume changes greatly, especially the forehearth temperature. For plastics with poor fluidity, the temperature should be appropriately increased to ensure smoothness.
- (2) The injection pressure, speed, back pressure is too low, and the injection time is too short, so that the material volume or density is insufficient and the contraction pressure, the speed, the back pressure is too large, and the time is too long to cause flashing and contraction.
- (3) The feeding amount means that the injection pressure will be consumed when the cushion is too large, and if it is too small, the amount of material will be insufficient.
- (4) For parts that do not require precision, after the injection and holding pressure, the outer layer is basically condensed and hardened, but the sandwich part is still soft and can be ejected, and the parts are ejected early and allowed to cool slowly in air or hot water. , Can make the contraction and depression smooth and not so conspicuous without affecting the use.
- (5) “Dent marks” are caused by local internal shrinkage after gate sealing or lack of material injection. The dents or pits on the surface of injection molded products are an old problem in the injection molding process. Dents are generally caused by the increase in the wall thickness of the plastic products and the local increase in the shrinkage of the product. It may appear near the external sharp corners or at the sudden changes in wall thickness, such as protrusions, ribs or behind the supports, and sometimes they may also appear. In some uncommon parts.
The root cause of dents is the thermal expansion and contraction of the material, because the thermal expansion coefficient of thermoplastics is quite high. The degree of expansion and contraction depends on many factors, among which the properties of the plastic, the maximum and minimum temperature range and the cavity holding pressure are the most important factors. The size and shape of injection molded parts, as well as cooling rate and uniformity are also influencing factors.
The amount of expansion and contraction during the molding process of plastic materials is related to the thermal expansion coefficient of the processed plastic. The thermal expansion coefficient during the molding process is called “molding shrinkage”. As the molded part cools and shrinks, the molded part loses close contact with the cooling surface of the cavity. At this time, the cooling efficiency decreases. After the molded part continues to cool, the molded part continues to shrink. The amount of shrinkage depends on the combined effect of various factors.
The sharp corners on the molded part cool the fastest and harden earlier than other parts. The thick part near the center of the molded part separates the cavity cooling surface the farthest, and becomes the last part of the molded part that releases heat, at the corners. After the material is solidified, as the melt near the center of the part cools, the molded part will continue to shrink, and the plane between the sharp corners can only be cooled on one side, 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 pulls the relatively weak surface between the partly cooled and the more cooled sharp corner inward. In this way, dents are produced on the surface of the injection molded part. The presence of dents indicates that the mold shrinkage rate here is higher than the shrinkage of its peripheral parts. If the shrinkage of a molded part is higher than another, it is the cause of the warpage of the molded part.
The residual stress in the mold will reduce the impact strength and temperature resistance of the molded part. In some cases, adjusting the process conditions can avoid the occurrence of dents. For example, in the process of holding the pressure of the molded part, additional plastic material is injected into the mold cavity to compensate for the mold shrinkage. In most cases, the gate is much thinner than other parts of the part. When the molded part is still hot and shrinks continuously, the small gate has been cured. After curing, the pressure holding does not affect the molded part in the cavity. kick in.
Molded parts of semi-crystalline plastic materials have a high shrinkage rate, which makes the dent problem more serious; the molding shrinkage of non-crystalline materials is lower, which will minimize the dent to the greatest extent; the shrinkage rate of filled and maintained reinforced materials Lower, the possibility of dents is less.
Thick injection molded parts take a long time to cool and will produce greater shrinkage. Therefore, large thickness is the root cause of dents. Care should be taken when designing. Thick-walled parts should be avoided as much as possible. If thick-walled parts cannot be avoided, they should be designed as hollow. Yes, thick parts will smoothly transition to the nominal wall thickness. Using large arcs instead of sharp corners can eliminate or minimize the dents generated near the sharp corners.
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Link to this article：The Analysis Of Shrinkage Depression Cause
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