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The three main reasons for the injection mold burning?
2019-01-09 Clicks: 111
Injection molds are important process equipment for the production of various industrial products. With the rapid development of the plastics industry and the promotion and application of plastic products in the aviation, aerospace, electronics, machinery, shipbuilding and automotive industries, the more demanding the products are. The higher it is. I believe that everyone often encounters injection mold scorching, which is very headache. Now Ward Mould gives you an analysis of the three major reasons for the injection mold burning.

Injection mold burning

1. Melt fracture leads to charring

When the melt is injected into a cavity with a large volume under high-speed and high-pressure conditions, the melt fracture phenomenon is easily generated. At this time, the surface of the melt is laterally fractured, and the fracture area is coarsely mixed with the surface layer of the plastic part to form a paste. In particular, when a small amount of molten material is directly injected into an easily oversized cavity, the melt fracture is more serious, and the appearance of the paste is larger.

The nature of the melt fracture is due to the elastic behavior of the polymer melt. When the melt flows in the barrel, the melt near the barrel is rubbed by the wall of the barrel, the stress is larger, and the flow rate of the melt is higher. Small, once the melt is injected from the nozzle, the stress acting on the pipe wall disappears, and the melt flow rate in the middle of the cylinder is extremely high, and the melt at the wall is accelerated by the melt at the center, because the flow of the melt is Relatively continuous, the flow velocity of the inner and outer melt will be rearranged, tending to an average speed.

During this process, a sharp stress change in the melt will cause strain, and the injection is extremely fast, and the stress is particularly large, far greater than the strain capacity of the melt, resulting in melt fracture.

If the melt encounters a sudden shape change in the flow path, such as diameter shrinkage, enlargement, and dead angle, the melt stays and circulates at the dead corner, which is different from the normal melt, and the shear deformation becomes larger when it is When it is mixed into the normal flow material, the deformation recovery of the two is inconsistent and cannot be bridged. If the disparity is large, the fracture fracture occurs, and the form of the fracture is also melt fracture.

2. Improper control of molding conditions leads to charring

This is also an important cause of scorching and smudging on the surface of plastic parts. In particular, the size of the injection speed has a great influence on it. When the flow is slowly injected into the cavity, the flow state of the melt is laminar; when the injection speed When it rises to a certain value, the flow state gradually becomes turbulent.

Under normal circumstances, the surface of the plastic part formed by the laminar flow is relatively bright and flat, and the plastic parts formed under the turbulent flow condition are not only prone to surface streaks, but also the pores are easily generated inside the plastic part. Therefore, the injection speed should not be too high, and the flow should be controlled in a laminar flow state.

If the temperature of the melt is too high, it is easy to cause the melt to decompose and coke, resulting in a surface on the surface of the plastic part. Generally, the screw rotation speed of the injection molding machine should be less than 90r/min, and the back pressure should be less than 2mpa, so as to avoid excessive friction heat generated by the barrel.

If excessive rotation heat is generated due to too long rotation time when the screw is retracted during the molding process, the screw rotation speed can be appropriately increased, the molding cycle can be extended, the screw back pressure can be reduced, the temperature of the barrel supply section can be increased, and the raw materials with poor lubricity can be used. The method is overcome.

During the injection process, the melt reflows too much along the groove and the resin stays at the anti-reverse ring, which will cause the melt to depolymerize and decompose. In this regard, the resin with higher viscosity should be selected, the injection pressure should be appropriately reduced, and the injection molding machine with a relatively long diameter should be replaced. The anti-reverse ring commonly used in injection molding machines is relatively easy to cause retention, causing it to decompose and discolor. When the disintegrating molten material is injected into the cavity, a brown or black focus is formed. In this regard, the nozzle-centric screw system should be cleaned regularly.

3, mold failure leads to burnt

If the mold vent hole is blocked by the release agent and the solidified material deposited by the raw material, the mold exhaust is not set enough or the position is incorrect, and the filling speed is too fast, the mold can not be decomposed and coked by the adiabatic compression of the discharged air. . In this regard, the obstruction should be removed, the clamping force should be reduced, and the exhaust of the mold can be improved.

The determination of the form and location of the mold gate is also very important. The flow state of the melt and the exhaust performance of the mold should be fully considered in the design.

In addition, the amount of release agent should not be too much, and the surface of the cavity should maintain a high degree of finish.

Raw materials do not meet the requirements, causing charring

If the moisture and volatile content of the raw material is too high, the melt index is too large, and excessive use of the lubricant may cause burnt and mottling failure.

In this regard, the hopper dryer or other pre-drying method should be used to treat the raw materials, replacing the resin with a smaller melt index and reducing the amount of lubricant.

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