Analysis of scorching reason of injection molding products

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The rupture of the melt results in scorching


When the melt is injected into the cavity with a large volume under high speed and high pressure, it is easy to produce melt rupture. At this time, the melt surface appears transverse fracture, and the fracture area is roughly mixed 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 which is easy to be too large, the melt rupture is more serious and the paste spot is larger.


The essence of the melt fracture is due to the elastic behavior of polymer melt material, when the liquid flow in the cylinder, near the cylinder of liquid by the wall friction, the stress is larger, the flow of the molten material velocity is small, once the molten material from the nozzle outlet, stress in the wall effect of the disappeared, and the central cylinder of liquid flow rate is extremely high, are compared. In the molten material is the center of molten material carrying and acceleration, Since the flow of molten material is relatively continuous, the flow velocity of inner and outer molten material will rearrange to the average velocity.


In this process, the molten material will undergo a sharp stress change 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.

 Molten Material

If molten material in the flow channel in case of sudden change of shape, such as diameter shrinkage, expand and dead Angle, etc., molten material stay in the corner and circulation, it is different from the normal force of melting, shear deformation is bigger, when its mixed in the normal flow of material out, because of the inconsistent deformation recovery, can not close, if the disparity is very big, the fracture rupture occurred, It also takes the form of melt rupture.


Improper control of forming conditions leads to scorching


This is also an important cause of scorching and pasting on the surface of plastic parts, especially the size of the injection speed has a great influence on it. When the flow material is slowly injected into the cavity, the flow state of the molten material is laminar flow. When the injection velocity rises to a certain value, the flow state gradually becomes turbulent.


In general, the surface of the plastic parts formed by laminar flow is relatively bright and smooth, 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 should not be too high, the flow material should be controlled in laminar flow state of mold filling.

If the temperature of molten material is too high, it is easy to cause molten material decomposition and coking, resulting in paste spots on the surface of plastic parts.

General injection molding machine screw rotation should be less than 90r/min, the back pressure is less than 2MPa, which can avoid excessive friction heat produced by 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 screw speed, extend the molding cycle, reduce the back pressure of the screw, improve the cylinder feeding temperature and the use of poor lubrication of raw materials and other methods to overcome.

 Molten Material

In the process of injection, too much backflow of molten material along the screw groove and resin retention at the stop ring will lead to polymer degradation of molten material. In this regard, the resin with higher viscosity should be selected, the injection pressure should be appropriately reduced, and the injection molding machine with larger diameter should be replaced. Injection molding machine commonly used to stop the ring are easy to cause retention, so that the decomposition of discoloration, when the discoloration of melted material injected into the cavity, that is, the formation of brown or black focus. In this regard, the nozzle centered screw system should be cleaned regularly.


Burning caused by mold failure


If the exhaust hole of the mold is blocked by the release agent and the solidified material precipitated out of the raw material, the exhaust hole of the mold is not set enough or the location is not correct, and the filling speed is too fast, the air in the mold too late to discharge is adiabatic and compressed to produce high temperature gas, and the resin will decompose and coke. In this regard, the blocking material should be removed, the clamping force should be reduced, and the poor exhaust of the mold should be improved.


It is also very important to determine the form and position of the die gate. The flow state of the molten material and the exhaust performance of the die 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 finish.


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