Black Specks
Dark specks caused by wear, thermal damage or contamination. Prevented by systematic purging and regular mechanical cleaning.
Black specks are sharply defined dark particles inside or on the surface of the part. Their source is one of three places: old material that has built up and degraded in the plasticizing unit, contamination entering the line from outside, or worn metal particles. The first two are solved by cleaning; the third shows that there is a problem with the equipment itself.
How to Recognize It
- Scattered black/brown specks on the surface.
- If it started after a material change, it is residue of the old material.
- When PC is processed, material stuck to the unit surface can flake off as "black spots".
Physical Causes
- Process: excessive melt temperature/residence; excessive temperature in the hot runner.
- Mold: a dirty gate system, dead spots in the hot runner.
- Machine: a dirty plasticizing system, screw-barrel wear.
- Material: impurities in the granulate, high reclaim, unsuitable colorant.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- At the start of production, purge the system THOROUGHLY with the material to be processed; minimize interruptions.
- During a long interruption lower the barrel temperature; at the end of production purge with a cleaning material such as PP.
- Bring the melt temperature and residence time below the critical range.
Mold, Design and Material Solutions
- In long production runs, clean the barrel, screw and non-return valve mechanically on a REGULAR basis (low-pressure blasting — sand and steel brushes are FORBIDDEN, the oxide layer increases sticking).
- Check the hot runner and gate system for dead spots/wear.
- Reduce the reclaim ratio; use a material deduster.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Burn Marks | A burn mark is a diffuse discoloration. A black speck is a single, sharply defined particle. |
| Glass Fiber Marks | Glass fiber marks make light-colored, feather-like lines on the surface. A black speck is dark and point-like. |
Frequently asked questions
I cleaned the screw but the black specks continue — where should I look?
Look at the hot runner and the line. If there is a dead zone in the hot runner, the material waiting there degrades and comes to the surface from time to time. On the raw material side, foreign matter getting into the line and worn pipe particles are the most frequent sources.
How do I know if it is a metal particle?
Examine the particle with a magnifying glass: if it has a metallic shine, its source is not degraded plastic. In that case, check whether there is a magnetic separator at the machine inlet; a metal particle leads to much more expensive damage than a black speck — screw and barrel wear.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice