The common defects of injection molding parts include burn marks, and the causes of burn marks of injection molding parts are mainly four factors, namely melt rupture, improper control of injection molding conditions, mold failure, and raw materials do not meet the requirements.
Below are the 4 Causes of Burn Marks in Injection Molding Parts.
Causes of Burn Marks in Injection Molding Parts 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 surface of the melt appears lateral fracture, 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 easy to be too large, the melt rupture is more serious and the paste spot presented is bigger.
The nature of melt breakage is due to the elastic behavior of the polymer melt, when the melt flows in the barrel, the melt near the barrel is subject to the friction of the barrel wall, the stress is larger, and the flow speed of the melt is smaller.
Once the melt is injected from the nozzle, the stress of the wall disappears, and the melt in the middle of the barrel flows at a very high speed, the melt at the barrel wall is accelerated by the melt at the center, because the flow of the melt is relatively continuous, the flow speed of the melt inside and outside 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, the stress suffered is particularly large, far greater than the strain capacity of the melt, resulting in melt rupture.
If the melt encounters sudden shape changes in the flow channel, such as diameter shrinkage, expansion, and dead ends, the melt stays and circulates at the dead ends.
It is different from the normal melt force, shear deformation is larger when it is mixed into the normal flow of material injected, due to the recovery of the deformation of the two inconsistent, cannot be bridged, if the disparity is large, the fracture rupture occurs, its manifestation is also melted rupture.
From the above, to overcome the trapped melt rupture, to avoid the production of paste spot:
- Pay attention to the elimination of dead ends in the runner so that the runner as much as possible streamline;
- Appropriate to increase the material temperature, reduce the melt relaxation time, so that the deformation is easy to recover and bridge;
- Add low molecular material in the raw material, because the lower the molecular weight of the melt, the wider the distribution, the more conducive to reducing the elastic effect;
- Proper control of injection molding speed and screw speed;
- The reasonable setting of gate location and selection of correct gate form is quite important. Practice shows that it is more ideal to use an enlarged type point gate and latent gate (tunnel gate). It is better to choose the location of the gate after the melt is injected into the transition cavity first and then into the larger cavity, do not make the flow of material directly into the larger cavity.
Causes of Burn Marks in Injection Molding Parts 2: Improper control of molding conditions
This is also one of the important causes of burn marks in injection molding parts, especially the size of the injection speed has a great impact on it.
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, so as to avoid excessive frictional heat generated by the barrel.
If the injection 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 injection 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 molding 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.
Causes of Burn Marks in Injection Molding Parts 3: Mold failure
If the mold exhaust hole is blocked by the release agent and raw material precipitation of curing material, mold exhaust setting is not enough or incorrect location, and the mold filling speed is too fast, the injection mold cannot be discharged from the air adiabatic compression of high-temperature gas will make the resin decomposition coking. 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 mold release agent should not be too much, and the surface of the cavity should be kept with a high degree of finish.
Causes of Burn Marks in Injection Molding Parts 4: Raw materials do not meet the requirements
If the moisture and volatile content in the raw material is too high, the melting index is too large, and the lubricant is used in excess, it will cause scorching and paste spot failure.
In this regard, the hopper dryer or other pre-drying methods should be used to deal with the raw materials, and replace the resin with a smaller melt index as well as reduce the number of lubricants.
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