Causes and Solutions for Surface Defects of Medical Injection Molding Products

Surface defects in medical injection molding products can significantly impact both the aesthetic quality and functional integrity of the final product. Common defects include cracks, silver streaks, grooves, ripples, wave marks, and embrittlement. These defects often arise from stress induced within the product, which can exceed its material strength. Understanding the causes of these defects is essential for producing high-quality medical devices.

Common Surface Defects and Their Causes

1. Grooves and Ripples:

– These defects typically appear at the leading edge of the fluid flow. They result from a pause in the flow front, causing pressure to build up before the material flows forward again. This issue is often linked to insufficient injection pressure or low injection speed.

2. Embrittlement:

– This defect can occur due to overfilling or underfilling during the injection process. Additionally, contamination, polymer degradation, or exposure to environmental stress cracking agents can contribute to embrittlement.

3. Cracking:

– Cracking can manifest locally or throughout the entire part. It is often caused by rapid injection speeds, leading to excessive molecular orientation, particularly in thin-walled products.

4. Silver Streaks:

– These are fine lines or small cracks that appear as whitening in localized areas, often indicating issues with the process or the polymer used.

Key Processing Issues

Surface ripples can arise from several processing problems:

– Pressure or Volume Issues: Inadequate filling pressure or poor speed control can cause ripples. The maximum pressure during the first filling stage should be maintained at 200-400 psi (14-28 kg/cm²) below the limit pressure.
– Position or Transfer Problems: Incorrect transitions between the first and second injection stages can lead to defects. For instance, reducing the second stage holding pressure too much can result in underfilling, particularly in thin-walled products.

To mitigate these issues, it is crucial to improve hydraulic responsiveness during the transition between injection stages, ensuring that pressure is adequately increased and then quickly adjusted to the set value for the second stage.

Material and Temperature Considerations

– Melt Temperature: Low melt temperatures or mold temperatures can contribute to defects. Using thermal probes or infrared sensors can help verify that the melt temperature aligns with the material supplier’s recommendations.
– Material Properties: Stress-related defects, such as cracking or silver streaks, can often be traced back to processing conditions. Injection speeds should be carefully controlled to prevent excessive molecular orientation.

Design Considerations

Design flaws can exacerbate surface defects. For example, improper gate location can lead to cracking at weld lines. Gates should be strategically placed to minimize stress concentrations. Additionally, sharp corners in the mold design can create stress points, leading to defects.

Degradation and Contamination Issues

Degradation can occur if processing temperatures are too high or if hydrolysis affects the polymer chains. This can significantly reduce the material’s properties. Regularly checking the condition of the injection equipment and monitoring the residence time of the resin in the barrel is essential to prevent degradation.

Recycling and contamination of materials can also lead to cracking or brittleness in medical products. It is important to assess the quality of recycled materials and ensure they match the specifications of virgin materials.

Conclusion

To achieve high-quality medical injection molding products, it is essential to understand the various surface defects and their underlying causes. By meticulously monitoring processing conditions, material properties, and design elements, manufacturers can minimize defects and ensure the production of reliable medical devices. Regular inspections and adjustments based on these insights will contribute to the overall success of the injection molding process.