After the read, you will learn about the common 4 types of a cause of burn marks in injection moulding products.
Cause of burn marks in injection moulding 1 - Melt rupture
When the melt is injected into the cavity at high speed and high pressure, it is easy to produce melt rupture. At this time, the melt surface appears to have lateral fracture, and the fracture area is roughly interspersed in the surface layer of the plastic parts to form a paste.
Especially when a small amount of melt is injected directly into the cavity, which is easily 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 lower, and once the melt is injected from the nozzle, the stress of the wall disappears. The flow speed of the melt in the middle of the barrel is extremely high. The melt accelerates the melt at the barrel wall carried in the center.
Because the flow of the melt is relative as the flow of melt is relatively continuous, the flow speed of internal and external melt will be rearranged and tend to the average speed.
In this process, the melt will undergo a sharp stress change that will produce strain because the injection speed is very fast, and the stress is particularly large, much larger than the strain capacity of the melt, resulting in melt rupture.
Suppose the melt encounters sudden shape changes in the runner, such as diameter shrinkage, expansion, and dead ends. In that case, the melt stays and circulates at the dead end, which is different from the normal melt force, and the shear deformation is larger.
Due to the inconsistent recovery of the two deformations, it cannot be bridged when mixed into the normal flow of material injected. If the disparity is large, the fracture rupture occurs, and its manifestation is also a melted rupture.
Cause of burn marks in injection moulding 2 - Improper control of molding conditions
This is also an important Cause of burn marks in injection moulding 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 is laminar;
The flow state gradually becomes turbulent when the injection speed rises to a certain value.
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 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 material flow 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, and the backpressure should be less than 2mpa to avoid excessive frictional heat generated by the barrel.
Suppose the molding process is too long due to the rotation time of the screw back and excessive friction heat. In that case, it 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, 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 use non-reverse rings are more likely to cause stagnation, so the decomposition of discoloration when the decomposition of discoloration of the melting 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.
Cause of burn marks in injection moulding 3 - Mold defects
Suppose the mold exhaust hole is blocked by the release agent and raw material precipitation of curing material. In that case, the mold exhaust setting is not enough or is in an incorrect position. The mold filling speed is too fast, and the air in the mold cannot be discharged adiabatically compressed to produce high-temperature gas will make the resin decomposition cause of burn marks in injection moulding.
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 and should fully consider the melt and mold exhaust performance flow state.
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.
Cause of burn marks in injection moulding 4 - Raw material does not meet the requirements
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 of burn marks in injection moulding 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.
These are the four common types of the cause of burn marks in injection moulding products, have you got it?
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