Analysis of the root cause of scorching of injection molded products

Melt rupture leads to scorching

When the melt is injected into a large volume cavity at high speed and high pressure, it is easy to produce a melt rupture phenomenon, at this time, the melt surface appears lateral fracture, the fracture area is roughly interspersed in the surface layer of the plastic parts to form a paste spot. Especially when a small amount of melt is injected directly into the cavity which is easy to be too large, the melt rupture is more serious and the paste spot presented is bigger.

When the melt flows in the barrel, the melt near the barrel is rubbed by the barrel wall, the stress is higher, the flow speed of the melt is smaller, once the melt is injected from the nozzle, the stress of the wall disappears, and the flow speed of the melt in the middle of the barrel is extremely high, the melt at the barrel wall is accelerated by the melt at the center, and the flow speed of the melt inside and outside will be rearranged to the average speed because the flow of the melt is relatively continuous.



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

If the melt in the runner encounters sudden shape changes, such as diameter shrinkage, expansion and the emergence of dead ends, etc., the melt in the dead-end stay and circulation, it is different from the normal melt stress, shear deformation is larger, when it is mixed into the normal flow of material injection, due to the recovery of the two deformation is not consistent, can not be bridged, if the disparity is very large, the fracture rupture, its manifestation is also melted rupture.

Improper control of molding conditions leads to scorching

This is also an important cause of scorching and pastes spots on the surface of the plastic parts, especially the size of the injection speed has a great impact on it when the flow of material into the cavity at a slow speed, the flow state of the melt for the laminar flow; when the injection speed rises to a certain value, the flow state gradually becomes turbulent.

In general, the surface of the plastic parts formed by laminar flow is brighter and flatter, while the plastic parts formed under turbulent flow conditions are not only easy to have paste spots on the surface, but also easy to produce air holes inside the plastic parts. Therefore, the injection speed should not be too high, and the flow of material should be controlled to fill the mold in the laminar flow state.

If the temperature of the melt is too high, it will easily cause the decomposition and coking of the melt, which will lead to paste spots on the surface of the plastic parts. General injection molding machine screw rotation should be less than 90r/min, the backpressure is less than 2mpa, to avoid excessive frictional heat generated by the barrel.

If the molding process is too long due to the rotation time of the screw back and excessive friction heat, can be overcome by increasing the screw speed, extending the molding cycle, reducing the screw back pressure, increasing the temperature of the barrel feed section, and the use of poor lubrication of raw materials.

During the injection process, too much melt backflow along the screw groove and resin retention at the non-return ring can lead to melt degradation and decomposition. In this regard, the use of higher viscosity resin should be selected, appropriate to reduce the injection pressure, and switch to a larger diameter of the injection molding machine. Injection molding machines commonly used non-reverse rings are more likely to cause stagnation, so that the decomposition of discoloration, when the decomposition of discoloration of the melt material into the cavity, that is, the formation of teal or black focus. In this regard, the screw system centered on the nozzle should be cleaned regularly.

Mold failure causes scorching

If the mold exhaust hole is blocked by the release agent and raw material precipitation of curing material, the mold exhaust setting is not enough or incorrect position, and the mold filling speed is too fast, the adiabatic compression of the air in the mold can not be discharged to produce high-temperature gas will make the resin decomposition scorching. In this regard, the blockage should be cleared, the closing force should be reduced, and the bad exhaust of the mold should be improved.

Mold gate form and location are also very important, in the design should be fully considered the flow state of the melt and mold exhaust performance.

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

Raw material does not meet the requirements leading to scorching

If the moisture and volatile content in the raw material is too high, the melt index is too large, and the lubricant is used too much, it will cause scorching and paste spot failure.

In this regard, you should use a hopper dryer or other pre-drying methods to deal with raw materials, and replace the resin with a smaller melt index and reduce the number of lubricants.