Black spots on the surface of injection molded products are a very common yet frustrating quality problem. When technicians encounter this issue, their first instinct is often to blame the raw material. However, the causes of black spots are far more complex than one might think. Screw problems can be just as much of a culprit, and in some cases, both factors may coexist. This article provides an in-depth analysis from three dimensions: raw material, screw, and process parameters, to help you quickly identify the root cause.

1. Characteristics of Black Spots Caused by Raw Material Contamination

Impurities in the raw material are one of the most direct sources of black spots. These impurities may come from other colored plastics mixed into regrind material, dust, carbonized particles, or contamination of virgin material during storage and transportation.

Black spots caused by raw material contamination typically have the following characteristics: the size of the black spots varies and their distribution is relatively random with no obvious pattern; when trial molding with different batches of material, the frequency of black spots changes noticeably; if the black spots disappear or are significantly reduced after filtering the material, the raw material is almost certainly the cause.

The solution is relatively straightforward: switch to a clean batch of material, strengthen material drying and filtration, install magnetic separators or filter screens at the hopper, and reduce the proportion of regrind material used.

2. Characteristics of Black Spots Caused by Screw Problems

Many people tend to overlook the impact of the screw on black spots. In reality, after prolonged use, the screw can develop several conditions that directly lead to black spots on the product surface.

The first is screw wear. The clearance between the screw and the barrel increases due to long-term friction, causing the material to stay in the clearance for too long. This leads to excessive shear and thermal decomposition, forming black carbonized particles. These particles enter the mold with the melt and ultimately appear as black spots on the product surface.

The second is damage to the mixing section of the screw. If the mixing elements are worn or broken, the mixing effect deteriorates, causing localized overheating, which in turn leads to thermal decomposition and black spots.

The third is carbon buildup on the screw surface. When processing materials that are prone to decomposition over a long period, a layer of carbonized residue gradually accumulates on the screw surface. This carbon buildup periodically flakes off into the melt, causing intermittent black spot problems.

Black spots caused by screw problems have their own unique characteristics: the appearance of black spots has a certain periodicity, possibly occurring once every few shots; when trial molding with clean virgin material, the black spots still appear; after replacing the screw or performing cleaning and maintenance on the screw, the black spots are significantly reduced or even disappear.

3. How to Quickly Determine Whether the Problem Is Raw Material or Screw

The most effective method is to conduct a controlled experiment. First, trial mold using brand new, fully dried material. If the black spots disappear, the problem lies with the raw material. If the black spots persist even with new material, the focus should shift to inspecting the screw and barrel condition.

You can also observe the distribution pattern of the black spots. If the black spots are concentrated in a fixed position on the product, it is very likely related to carbon buildup flaking off from a specific location on the screw. If the black spots are completely randomly distributed, raw material contamination is more likely the cause.

In addition to raw material and screw issues, carbon buildup in dead zones of the mold runner, excessively high barrel temperatures, and excessive back pressure can all cause black spots. Therefore, when troubleshooting, do not focus on just one direction; consider all factors comprehensively.

4. Summary

Black spots on the surface of injection molded products can be frequently caused by either raw material contamination or screw problems, and the characteristics of each are distinctly different. By switching materials for trial molding and observing the distribution pattern of the black spots, the root cause can generally be identified in a short time. In daily production, proper raw material management and regular screw maintenance are the most effective ways to prevent black spot problems.