Melt rupture causes burning

When the melt is injected into the cavity with a large volume under the condition of high speed and high pressure, it is easy to produce melt rupture phenomenon, at this time, the melt surface appears transverse fracture, and the fracture area is rough inclusion in the surface of the plastic parts to form paste spots. Especially when a small amount of molten material is directly injected into the cavity that is easy to be too large, the melt rupture is more serious, and the paste spot presented is larger.

The essence of melt rupture is due to the elastic behavior of polymer melt, when the molten material flows in the barrel, the molten material near the cylinder wall is friction, the stress is large, the flow speed of molten material is small, once the molten material is poured out from the nozzle, the stress of the pipe wall disappears, and the molten material in the middle of the cylinder is extremely high. The molten material at the wall of the cylinder is carried and accelerated by the molten material at the center. Because the flow of molten material is relatively continuous, the flow speed of molten material inside and outside will be rearranged and tend to the average speed.

In this process, the molten material will have a sharp change in stress will produce strain, because the injection speed is very fast, the stress is particularly large, far greater than the strain capacity of the molten material, resulting in melt rupture.

If the molten material meets sudden shape change in the flow channel, such as diameter shrinkage, expansion and dead Angle, etc., the molten material stays and circulates at the dead Angle, it is different from the normal molten material in force, and the shear deformation is large. When it is mixed into the normal flow material, the deformation recovery of the two is inconsistent and cannot be closed. Its manifestation is also melt rupture.

Improper control of molding conditions lead to burning

This is also an important reason for the burning and paste spots on the surface of plastic parts, especially the size of the injection speed has a great impact on it, when the flow material is injected slowly into the cavity, the flow state of the molten material is laminar flow; When the injection speed rises to a certain value, the flow state gradually becomes turbulent.

Under normal circumstances, the surface of the plastic parts formed by laminar flow is relatively bright and flat, and the plastic parts formed under turbulent conditions are not only prone to paste spots on the surface, but also easy to produce pores inside the plastic parts. Therefore, the injection speed can not be too high, and the flow material should be controlled in the laminar flow state.

If the temperature of the molten material is too high, it is easy to cause the decomposition and coking of the molten material, resulting in paste spots on the surface of the plastic part. The screw revolution of the general injection molding machine should be less than 90r/min, and the back pressure is less than 2mpa, which can avoid excessive friction heat from the cylinder.

If the molding process due to the screw back when the rotation time is too long and excessive friction heat, can be properly increased by increasing the screw speed, extend the molding cycle, reduce the screw back pressure, improve the cylinder feed section temperature and the use of poor lubricity of raw materials to overcome.

During the injection process, too much reflux along the screw groove and resin retention at the check ring will cause the melt to fall and decompose. In this regard, the resin with higher viscosity should be selected, the injection pressure should be reduced appropriately, and the injection molding machine with a relatively large length diameter should be used. Injection molding machine commonly used check ring are more likely to cause retention, so that it decomposing discoloration, when the decomposition discoloration of the melt material injected into the cavity, that is, the formation of brown or black focus. In this regard, the nozzle as the center of the screw system should be cleaned regularly.

Mold failure leads to burning

If the mold vent hole is blocked by the release agent and the solidification of the raw material, the mold exhaust setting is not enough or the position is incorrect, and the mold filling speed is too fast, the adiabatic compression of the air that is not discharged in the mold will cause the resin to decompose and coking. In this regard, the blockage should be removed, the clamping force should be reduced, and the poor exhaust of the mold should be improved.

Mold gate form and location determination is also very important, in the design should fully consider the flow state of the molten material and the exhaust performance of the mold.

In addition, the amount of release agent can not be too much, and the surface of the mold cavity should be maintained with a high finish.

Raw materials do not meet the requirements leading to burning

If the content of water and volatile matter in raw materials is too high, the melting index is too large, the excessive use of lubricant will cause burning and paste spot failure.

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

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