In the world of injection molding, surface defects are not just cosmetic issues—they can be indicators of deeper process instability or material degradation. Today, I’d like to share a real-world case involving a 1.2 mm thick plastic part molded with RTP material at a processing temperature of 200–210°C. The defects observed were bubbles, white spots, and black specks. Below is a breakdown of the root cause analysis and the corresponding solutions.
Problem Description
The molded parts exhibited three types of surface/internal defects:
- Bubbles (voids or gas pockets)
- White spots (often associated with material degradation or moisture)
- Black specks (typically burnt particles or decomposed resin)
The part thickness was 1.2 mm, and the material was RTP, a well-known compounder of engineered thermoplastics. The production temperature was set between 200°C and 210°C.
Root Cause Analysis
After initial investigation, the primary cause was identified as material degradation due to excessive processing temperatures. While the specified temperature range of 200–210°C is typical for many engineering resins, actual thermal exposure during processing—including residence time in the barrel and shear heating—can push the material beyond its thermal stability limit.
When the material overheats:
- Bubbles form due to the release of volatile gases from thermal breakdown.
- White spots may appear as a result of localized degradation or filler separation.
- Black specks are direct evidence of carbonized (burnt) resin particles contaminating the melt stream.
In this case, the combination of prolonged residence time and slightly elevated temperatures likely pushed the material into the degradation zone, leading to these defects.
Solutions Implemented
Two main corrective actions were taken to resolve the issue:
1. Optimize Molding Parameters
The first step was to refine the injection molding process to reduce thermal stress on the material:
- Lower the melt temperature to the lower end of the recommended range, ensuring it stays within the material’s thermal stability window.
- Reduce residence time by matching shot size to barrel capacity, avoiding unnecessary dwell time at high temperatures.
- Optimize back pressure and screw speed to minimize shear heating during plastication.
- Adjust cooling and cycle times to ensure consistent solidification without gas entrapment.
2. Conduct Material Trials
To verify whether the issue was process-related or material-specific, alternative lots of the same RTP material were trialed. In some cases, even slight variations in material formulation or moisture content can influence thermal stability. Running controlled trials helped confirm that with optimized parameters, the material could perform without defects.
Key Takeaways
- Temperature control is critical. Even within a manufacturer’s recommended range, localized overheating or extended residence time can cause degradation.
- Defects tell a story. Bubbles, white spots, and black specks often share a common root cause—thermal degradation—but may appear differently depending on the material and part geometry.
- Process + Material = Solution. Addressing such issues typically requires both parameter optimization and material verification. Relying solely on one may not fully resolve the problem.
Conclusion
In this case, reducing the processing temperature and optimizing the cycle parameters eliminated the defects without the need for a material change. However, having the flexibility to trial alternative material lots provided valuable confirmation that the process was the primary contributor.
For molders working with engineered compounds like RTP, maintaining strict control over thermal exposure is essential—especially for thin-wall parts where melt flow and residence time dynamics are more critical.
Have you encountered similar defect issues in your injection molding production? Feel free to contact us—I’d be happy to discuss troubleshooting ideas with you.
