Black specks/black streaks


What are black specks and black streaks?

Black specks and black streaks are dark spots or dark streaks found on the surface or throughout a molded part. Brown specks or streaks refer to the same type of defect, except the burning or discoloration is not as severe.
 


FIGURE 1. Black specks (left) and black streaks (right)


Causes of black specks/black streaks
Black specks and black streaks are caused by overheated (degraded, burned) material or by contamination of the resin.

Material degradation  
Overheated materials can degrade and lead to black streaks. Material that stays in the nicked rough surfaces of the barrel wall and screw surfaces for a prolonged period of time after heating will char and degrade, resulting in the defect.

Material contamination  
Contaminants in the air or material, such as dirty regrind, foreign material, different color material, or a lower melt-temperature material, are what most often lead to black specks and black streaks. Airborne dirt can also cause dark spots on the surface of a molded part.


Other defects resulting from the same causes

Remedies


Handle the material carefully


Make sure no contaminated materials, such as dirty regrind, are blended into the original material.


Put the cover on the hopper and all bins of material. Airborne dirt can contaminate the original material, leading to black spots.


Alter the mold design


Clean the ejectors and slides. The streaks could by caused by the grease or lubricants on the slides or ejectors.


Improve the venting system. If the black specks are found at the end of flow paths or blind spots, they are likely caused by a poor venting system. Compressed air trapped in the cavity is sometimes ignited, leading to the defect.


Clean or polish any nicked surface on the runner system to keep dirt from lodging in these areas.


Clean the mold before molding.


Select a proper machine


Size a proper injection machine for a specific mold.
The typical shot size should be between 20 and 80 percent of machine injection capacity. For temperature-sensitive materials, the range should be narrowed down more. Plastics simulation software can help you select the right size injection machine for a specific mold. This will help avoid resin remaining in the heated barrel for prolonged periods of time.


Check for scratched or dented barrel/screw surfaces that trap material.
This could lead to the material becoming overheated or burned.


Check for local overheating by a run-away heater band or a malfunctioning temperature controller.


Adjust the molding conditions


Lower the barrel and nozzle temperature.
Material degradation can result from a high melt temperature.


Purge and clean the injection unit.
The black streaks might be caused by contamination from the barrel wall or the screw surface. When molding with two materials, after switching from one material to the other, the old material might not be purged from the barrel completely. This could generate defects during the molding of the second material.


Avoid recycling rejected parts with black specks and black streaks.
Recycling such parts could lead to further contamination, unless they'll be used for parts that are in black or for which such defects are acceptable.