What is Black Specks or Streaks
Black or brown specks are spots of degraded plastic or dark spots that show on the injection molded part. This Black or brown specks defects will be more visible on white color or clear plastic molded parts, we also call as carbon buildup, sometime we normally think it is material mix issue.
Black Specks Troubleshooting Molding Process Issues
If Black or brown specks defects caused by the plastic has been degraded to a charred state. This is most often a case of material that has been in the barrel at temperature for too long. There are several factors in the injection molding process that can cause black or brown specks including:
Improper Molding Machine Shutdown
For most plastics materials are critical to properly purge the injection molding machine at shutdown. If the plastics material is held for an extended period of time at melt temperature in the barrel, material degradation will occur. Not that some materials are much more sensitive to this problem than others; materials such as PP is very stable but
PVC will easily get degraded and cause charred materials. When shutting down the molding machine it is important to purge the molding machine clean of the PVC material and instead of PC material in the barrel
High Melt Temperature
Although high melt temperature is not the root reason of black specks it can be a contributor to the black specks defects. If plastic is exposed to higher than recommended melt temperatures it is more easy to degrade. Brown specks are materials that has been severely degraded through an extending exposure to increased material melt temperature. If the material is exposed to higher melt temperature that
it will degrade to produce black specks. Again when dealing with black specks issue usually goes back to length of time at melt temperature, but the higher the temperature the less time before the material get degraded.
Long Residence Time
Excess time at melt temperature is what will create black specks issues. Normally, to reach the point where material turns to black specks defects, it takes more time than the residence time that the plastic through the barrel. Keep in mind that black specks issues are the time and melt temperature problem,
Hot Runner Temperatures
It is similar to high temperature this will usually not be the mean reason to cause the black specks issue but can be a factor. If the hot runner temperatures are set higher for any reason that the plastics resin will be degraded in the hot runner systems, once this reason caused the black specks it will take lots of time clean up hot runner systems.
Verify that the hot runner temperatures are set correctly in the hot runner controllers to determine that the zone is under control. If the current to a given zone is not cycling it may be an indication that the zone is not wired correctly. If a zone is drawing current continuously it may be because the thermocouples for two zones are swapped, resulting in the controller providing constant power to a zone without seeing the temperature rise.
Wrong Material mixed together
If the two or three plastics materials are loaded into a injection molding machine many potential problems can occur including black specks defects, in those materials that some of plastics is overheated so that will cause degrade, which could have black specks. It is important to make sure that the correct pure material is being used in all cases. Normally there will be a lots of molding issues that show up if the wrong material has been loaded, but it is always a good thing to verify when troubleshooting.
When add some regrind material in the raw plastic material there can be inconsistent grind and a level of fines or dust. These regrind fines will have a tendency to melt early in the barrel and as a result will tend to degrade more easily, normally if make clean or white molding parts we do not suggest to use regrind materials.