Causes and Handling Techniques for Excessive Surface Bubbles in Medical Injection-Molded Parts

In the production process of medical injection-molded parts, the issue of excessive surface bubbles is a common and challenging defect. It directly impacts the appearance quality and performance stability of the products and may even lead to product rejection due to non-compliance with strict medical standards. This article will delve into the causes of bubble formation from four dimensions: raw materials, process parameters, mold design, and equipment status, and propose targeted handling techniques.

I. Analysis of the Causes of Bubble Formation

(A) Raw Material Factors

1. Excessive Moisture Content: Medical-grade plastic raw materials are highly sensitive to moisture. If the raw materials are not fully dried or stored in a high-humidity environment, the moisture in them will rapidly vaporize during the high-temperature injection molding process, forming water vapor bubbles. For example, the moisture content of polycarbonate (PC) raw materials needs to be controlled below 0.02%; otherwise, bubbles are highly likely to form.
2. Raw Material Degradation: Excessively high barrel temperatures or prolonged residence time of raw materials in the barrel can lead to thermal degradation of the raw materials, generating volatile gases. For instance, polyamide (PA) is prone to decomposition at temperatures above 280℃, and the decomposition products can be trapped in the melt to form bubbles.
3. Raw Material Contamination: The presence of foreign plastics, powders, or impurities in the raw materials can alter the fluidity of the melt, causing air to be entrapped during the mold-filling process and resulting in bubble formation.

(B) Process Parameter Factors

1. Improper Injection Speed: When the injection speed is too fast, the melt impacts the mold cavity at high speed, compressing the air, which cannot be discharged in time, thus forming bubbles. When the injection speed is too slow, the melt cools prematurely in the runner, leading to insufficient mold filling and the formation of vacuum bubbles on the surface.
2. Insufficient Packing Pressure: Insufficient packing pressure prevents the melt from being adequately replenished during the cooling and shrinkage process, creating voids in the mold cavity, which manifest as bubbles.
3. Uncontrolled Barrel Temperature: Excessively high barrel temperatures can cause raw material degradation and gas generation, while excessively low temperatures increase the viscosity of the melt, reducing its fluidity and making mold filling difficult, which can also lead to bubble formation.
4. Mismatched Back Pressure and Screw Speed: Low back pressure or excessively fast screw speed can cause the melt to entrain excessive air in the barrel, which then enters the mold cavity with the melt, forming bubbles.

(C) Mold Design Factors

1. Defective Venting System: The lack of vent holes on the mold parting surface or blocked or improperly located vent holes prevent the air in the mold cavity from being discharged, allowing it to be encapsulated by the melt to form bubbles. For example, when the depth of the mold venting slot exceeds 0.03mm, the melt is prone to leakage, forming flash; when the depth is insufficient, the venting effect is poor.
2. Unreasonable Gate Design: Poor gate location, undersized gates, or asymmetrical gate arrangements in multi-gate parts can disrupt the continuous flow of the melt, blocking the air passage and leading to bubble formation. Direct gates are prone to vacuum hole phenomena because after packing is completed, the pressure in the mold cavity is higher than that in front of the gate. If the melt has not solidified, backflow can occur, creating holes.
3. Defective Runner Design: Narrow, long, and narrow runners or the presence of air-trapping dead spots in the runners increase the flow resistance of the melt, causing air to be trapped and form bubbles.

(D) Equipment Status Factors

1. Nozzle Problems: A nozzle with a too-small orifice can impede the smooth flow of the melt, entraining air. Nozzle drooling, stringing, or the presence of obstacles or burrs in the barrel or nozzle can generate frictional heat as the high-speed melt passes through, causing the material to decompose and produce gas.
2. Worn Screw and Barrel: Wear between the screw and the barrel increases the clearance, allowing air to be easily entrained into the melt during transportation, resulting in bubble formation.

II. Bubble Handling Techniques

(A) Raw Material Handling

1. Strictly Dry the Raw Materials: Set the drying temperature and time according to the characteristics of the raw materials. For example, PC raw materials need to be dried at 120℃ for 4 - 6 hours to ensure that the moisture content meets the requirements. Use a desiccant dryer to ensure that the drying air circulates in a closed loop and is not affected by atmospheric humidity.
2. Prevent Raw Material Contamination: Strengthen the storage management of raw materials to avoid the mixing of foreign plastics or impurities. Screen the raw materials before use to remove powders and foreign objects.
3. Control Raw Material Degradation: Optimize the barrel temperature settings to avoid local overheating. Reduce the residence time of raw materials in the barrel and reasonably adjust the screw speed and back pressure.

(B) Process Parameter Optimization

1. Adjust the Injection Speed: Use multi-stage injection speed control, such as medium-speed filling of the runner, slow filling of the gate, fast injection, and low-pressure slow filling of the mold to ensure that the air in the mold is discharged in a timely manner at each stage. For thin-walled parts, increase the injection speed to prevent insufficient mold filling; for thick-walled parts, reduce the injection speed to avoid air entrainment.
2. Increase the Packing Pressure and Time: Set the packing pressure and time reasonably according to the wall thickness and material characteristics of the part to ensure that the melt is adequately replenished during the cooling and shrinkage process and eliminate bubbles.
3. Control the Barrel Temperature: Gradually reduce the barrel temperature to avoid local overheating that can cause raw material degradation. The temperature of the feeding section should not be too high to prevent backflow and bubble formation.
4. Optimize the Back Pressure and Screw Speed: Appropriately increase the back pressure and reduce the screw speed to reduce the likelihood of air entrainment in the melt in the barrel.

(C) Mold Improvement

1. Improve the Venting System: Add vent holes at the parting surface, inserts, ejector pins, and other positions of the mold to ensure smooth venting. Regularly clean the vent holes to prevent blockage. For closed areas or cold runners, vacuum extraction points can be opened and connected to a vacuum pump to extract the air in the mold cavity during injection.
2. Optimize the Gate Design: Select the appropriate gate location and type according to the part structure to avoid direct gates. The gate size should be proportional to the part weight to ensure uniform melt flow. For multi-gate parts, arrange the gates reasonably to avoid discontinuous melt flow.
3. Improve the Runner Design: Shorten and widen narrow, long, and narrow runners and eliminate air-trapping dead spots. Optimize the runner layout to reduce the flow resistance of the melt.

(D) Equipment Maintenance

1. Check the Nozzle Status: Regularly check the orifice size of the nozzle and clean nozzle drooling or stringing. Check the barrel and nozzle for obstacles or burrs and repair or replace them in a timely manner.
2. Maintain the Screw and Barrel: Regularly check the wear condition of the screw and barrel and replace worn parts in a timely manner. Keep the screw and barrel clean to prevent residual materials from decomposing and producing gas.

III. Case Study

A medical injection-molded part manufacturer produced polypropylene (PP) infusion connector parts, which frequently had surface bubble defects. After analysis, the following reasons were identified:

1. Raw Material Problems: The raw materials were stored in a high-humidity environment, resulting in excessive moisture content. A small amount of recycled materials was mixed in the raw materials, reducing fluidity.
2. Process Parameter Problems: The injection speed was too fast, the packing pressure was insufficient, and the barrel temperature was too high.
3. Mold Problems: The mold vent holes were blocked, and the gate size was too small.

In response to these problems, the manufacturer took the following measures:

1. Raw Material Handling: Improved the raw material storage environment and fully dried the raw materials using a desiccant dryer. Stopped using recycled materials and switched to brand-new medical-grade PP raw materials.
2. Process Parameter Optimization: Reduced the injection speed, increased the packing pressure and time, and adjusted the barrel temperature to a reasonable range.
3. Mold Improvement: Cleaned the mold vent holes and enlarged the gate size.

After these improvements, the surface bubble defects of the infusion connector parts were significantly reduced, and the product pass rate increased from 85% to 98%, effectively reducing production costs and improving production efficiency.

IV. Conclusion

The issue of excessive surface bubbles in medical injection-molded parts involves multiple links such as raw materials, processes, molds, and equipment, and requires comprehensive analysis and improvement from a systematic perspective. By strictly managing raw materials, optimizing process parameters, improving mold design, and strengthening equipment maintenance, bubble defects can be effectively reduced, and product quality and production efficiency can be improved. At the same time, enterprises should establish a sound quality management system and strengthen process monitoring and recording to ensure that products comply with the strict standards of the medical industry.