What is the dark spot on injection molded products?

You just received a shipment of brand-new plastic parts, but they are ruined by ugly black specks¹. These spots spoil the look, can signal a weak part, and cause costly rejections.

Dark spots on injection molded products are typically specks of carbonized (burned) plastic or foreign contamination². These defects originate from material degradation³ in the machine's barrel or from impurities mixed in with the raw material.

Seeing these specks is incredibly frustrating, especially on a white or light-colored product. I once dealt with a project for a high-end consumer electronic with a beautiful white ABS housing. The first production run was plagued with these tiny black spots. We spent days troubleshooting, tracing every step of the process. In the end, we discovered a "dead spot" in an old machine nozzle where plastic was sitting too long and burning. This experience taught me that solving this problem requires a systematic approach, starting with understanding the two main culprits: carbonization and contamination.

What are the black spots on injection molding?

Black spots appear randomly, making troubleshooting a nightmare. You're left wondering if they come from the machine, the material, or something else. This uncertainty is costly.

Black spots are typically specks of burned plastic (carbon) that have flaked off the screw, barrel, or nozzle. They can also be foreign contaminants like dust or a different type of plastic.

The source of black spots almost always falls into two categories: material degradation³ or contamination. Carbonized material is plastic that has been exposed to too much heat for too long, essentially burning it into small black specks¹. This often happens in worn-out barrels or screws, or if the residence time (the time the plastic spends in the hot barrel) is too long. Contamination, on the other hand, is when foreign impurities get mixed into the clean plastic resin. This could be anything from dust in the air to residue from a previous material that wasn't properly cleaned out.

What causes discoloration⁴ in injection molding?

Your parts aren't just spotted; the entire color is wrong. It might be streaky, faded, or a completely different shade. This makes matching parts impossible and ruins brand consistency.

Discoloration is often caused by inconsistent masterbatch mixing⁵, material degradation³ from excessive heat or shear, or contamination from a previous production run that wasn't properly purged from the machine.

4-in-injection-molding-yo.jpg" alt="Several plastic parts showing streaks and inconsistent coloration" title="Discoloration in injection molding" />

Unlike black spots, general discoloration⁴ is often a broader issue with the material or the process. If your color concentrate (masterbatch) isn't mixed evenly with the base resin, you'll get streaks and swirls. I saw this once when a factory tried to save money by manually mixing masterbatch at the machine instead of using a proper blender. Another common cause is excessive shear heat⁶, which is generated when the screw rotates too fast or with too much back pressure. This heat can actually burn the color pigment itself, changing its shade. Finally, residue from a previous color, especially a dark one, can "bleed" into the next run if the machine isn't cleaned out thoroughly.

Common Causes of Discoloration

• Poor Masterbatch Dispersion: Uneven mixing leads to color streaks.
• Excessive Shear Heat: High screw speed burns the color pigments.
• High Melt Temperature: The base resin or colorant degrades.
• Contamination: Leftover material from a previous run bleeds into the new color.
• Moisture: Wet material can cause splay marks, which look like silver streaks or discoloration⁴.

What are the black spots on plastic?

You know what causes black spots, but how do you stop them? Every time you think you've fixed it, they come back, wasting material, time, and money.

To prevent black spots, implement strict material handling protocols to avoid contamination, use proper purging procedures⁷ between runs, and perform regular maintenance on the screw⁸, barrel, and nozzle to clean out carbon buildup.

Fixing this problem for good requires a preventative mindset⁹. The first step is always to isolate the source. Check the raw material in the bag and in the hopper. Is it clean? If so, the problem is in the machine. Next, purge some material directly from the nozzle into the air (an "air shot") and examine it. If you see black specks¹, the issue is likely carbon buildup. For minor issues, a good purging compound can clean the system between runs. But for persistent problems, there is no substitute for shutting down the machine and pulling the screw for a full, manual cleaning. This is often the only way to remove baked-on carbon from the screw and barrel surfaces.

Troubleshooting Flowchart

1. Inspect Raw Material: Are there specks in the bag/hopper? -> If yes, change material supplier.
2. Take Air Shot: Are there specks in the purged material? -> If yes, proceed to step 3.
3. Use Purging Compound: Does the problem disappear? -> If yes, your process is clean.
4. Problem Persists: Schedule downtime to pull and manually clean the screw and barrel.

What are the marks on injection Mould?

Your parts have marks, but they aren't black spots. You see faint circles, sunken areas, or lines that shouldn't be there. These subtle defects can still lead to part rejection.

Common marks on molded parts include ejector pin marks¹⁰ (from ejection), sink marks¹¹ (from uneven cooling), weld lines¹² (where plastic flows meet), and flash¹³ (excess plastic leaking from the mold).

11 and [flash](https://advancedplastiform.com/how-to-prevent-flash-in-injection-molding/)¹³" title="Common marks on injection molded parts" />

Beyond black spots, a molded part can have many different types of cosmetic or structural marks. These are almost always related to the mold design or the process parameters, not contamination. For example, sink marks¹¹ are small depressions that appear in thick sections of a part because the plastic shrinks as it cools. Weld lines are weak, visible lines that form where two or more fronts of molten plastic meet inside the mold cavity. Each mark tells a story about what is happening inside the mold during that brief, high-pressure injection cycle. A good mold designer anticipates these issues and designs the part and mold to avoid them from the start.

Conclusion

Black spots are frustrating defects caused by burned material or contamination. A clean process, proper material handling, and regular machine maintenance are the keys to producing flawless parts every time.

---