In medical device injection molding production, sink marks are one of the most common and challenging quality defects. Medical products have extremely high requirements for dimensional accuracy, surface quality, and structural integrity. Sink marks not only affect appearance but can also lead to uneven wall thickness, seal failure, and even endanger patient safety. This article systematically explains how to effectively address sink mark issues in medical injection molded products, from cause analysis to solutions.

1. What Is Injection Molding Sink Mark?

An injection molding sink mark refers to depressions, voids, or sink marks on the surface or interior of a product caused by volume shrinkage when the plastic melt cools and solidifies in the mold cavity. In the medical injection molding field, common manifestations include:

• Surface Sink Marks: Localized depressions on the product surface
• Internal Voids: Hollow cavities formed inside the product, affecting structural strength
• Dimensional Deviations: Product dimensions exceeding tolerance ranges due to uneven shrinkage

2. Core Causes of Sink Marks

1. Improper Process Parameters

2. Mold Design Issues

• Gate size too small: Insufficient compensation channel, unable to effectively transmit pressure during the holding phase
• Unreasonable gate location: Far from thick-walled areas, with excessively long compensation paths
• Uneven cooling system design: Local cooling is too fast or too slow, causing differential shrinkage
• Poor venting: Trapped air causes local incomplete filling, forming voids after cooling

3. Product Structural Design

• Uneven wall thickness: Thick-walled areas shrink more while thin-walled areas shrink less, making sink marks prone to occur at the junctions
• Excessive thick-walled areas: Common features in medical products such as ribs and snap-fit positions easily accumulate sink marks
• Sudden local thickness changes: Abrupt transitions from thin to thick walls cause obvious shrinkage differences

4. Material Factors

• High material shrinkage rate: Crystalline materials such as PP and PE can have shrinkage rates of 1.5%-3.0%
• Excessive moisture content in material: Moisture vaporizes at high temperatures, forming internal voids (especially sensitive for medical-grade materials like PC and PA)
• Excessive regrind ratio: Recycled material has reduced flowability and unstable shrinkage behavior

3. Systematic Solutions

✅ Solution 1: Optimize Holding Process (Most Direct and Effective)

Holding pressure is the first line of defense against sink marks. Follow these steps for adjustment:

1. Gradually increase holding pressure: Increase by 5-10% each time and observe the improvement in sink marks
2. Extend holding time: Extend until the gate is completely frozen (usually determined by the weight method)
3. Adopt multi-stage holding pressure: First use high pressure for rapid compensation, then low pressure for holding, reducing internal stress
4. Adjust the V/P switch point: Move the switch point from the end of injection to 95%-98% fill

✅ Solution 2: Improve Mold Design

✅ Solution 3: Optimize Product Structural Design

During the product design stage (DFM), follow these principles:

• Uniform wall thickness: Keep wall thickness as consistent as possible; for medical products, wall thickness tolerance should be controlled within ±10%
• Avoid thick-wall concentration: Control rib thickness at 50%-70% of the main wall thickness
• Use progressive wall thickness transitions: Wall thickness change ratio should not exceed 1:3
• Use ribs instead of thickening: Replace overall thickening with rib strips where reinforcement is needed

✅ Solution 4: Material and Drying Management

Common medical injection molding materials (PC, PP, POM, PA, etc.) are extremely sensitive to moisture:

• Use a dew point dryer to ensure controllable drying results
• Dried material should not stay in the hopper for more than 2 hours (hygroscopic materials)
• Regrind ratio should be controlled within 20%; for medical products, ≤10% is recommended

✅ Solution 5: Leverage Mold Flow Analysis (Moldflow)

Conducting mold flow analysis before mold opening can:

• Predict sink mark locations and severity
• Optimize gate location and quantity
• Simulate shrinkage behavior under different holding conditions
• Adjust wall thickness and rib design in advance

4. Quick Troubleshooting Table for Different Scenarios

5. Special Considerations for the Medical Industry

1. Cleanliness requirements: Any process adjustments must not introduce contamination; mold venting grooves need regular cleaning
2. Regulatory compliance: Process changes must be verified and documented according to ISO 13485 requirements
3. Biocompatibility: When adjusting temperature parameters, ensure no harmful degradation products are generated (e.g., BPA from PC decomposition at high temperatures)
4. Traceability: The handling process for sink mark issues should be fully documented and incorporated into CAPA management

Frequently Asked Questions (FAQ)

Q1: How to distinguish between sink marks and burn marks in medical injection molded products?
A: Sink marks have a smooth surface with uniform depressions, mostly appearing in thick-walled areas. Burn marks (gas marks) show silver streaks or scorch marks on the surface, mostly appearing at the end of filling. They have different causes—sink marks are mainly due to shrinkage, while burn marks are mainly due to trapped air and excessive shear heating.

Q2: Will increasing holding pressure cause flash?
A: It is possible. It is recommended to first increase holding pressure; if flash occurs, then appropriately reduce clamping force or optimize the mold clamping surface. You can also adopt a "high pressure short time + low pressure long time" staged holding pressure strategy to achieve both compensation and flash prevention.

Q3: If sink marks appear during the trial molding stage, can they still be solved through subsequent process adjustments?
A: If the sink marks during trial molding are structural (e.g., excessive wall thickness, unreasonable gate location), process adjustments alone can only improve 50%-70%. Mold modifications are essential. It is recommended to conduct mold flow analysis during the trial stage to optimize the design in advance, avoiding costly and time-consuming repeated mold modifications later.

Q4: Medical-grade PC material has particularly severe sink marks during injection molding. Any special recommendations?
A: PC has a high shrinkage rate (0.5%-0.7%) and is temperature-sensitive. Recommendations: ① Control barrel temperature at 280-310°C, not too high; ② Mold temperature at 80-120°C; ③ Holding pressure at 70-100MPa; ④ Ensure thorough drying (moisture content <0.02%); ⑤ Consider using PC/ABS alloy, which can reduce the shrinkage rate to 0.4%-0.5%.