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2026年3月24日
Professional Analysis of Injection Molding Defects in Plastic Products (Part 2)
In our previous discussion, we covered unstable plasticization and gate blush. Today, we delve deeper into three critical defects that impact structural integrity and aesthetics: Voids, Sink Marks, a
In our previous discussion, we covered unstable plasticization and gate blush. Today, we delve deeper into three critical defects that impact structural integrity and aesthetics: Voids, Sink Marks, and Black Spots.
4. Voids (Internal Vacuums)
What are Voids? Voids are hollow pockets that form inside a molded part, typically at the thickest sections. They are generally classified into two types:
- Gas Voids: Caused by trapped air or gas within the melt.
- Vacuum Voids: Caused by localized shrinkage during the cooling process.
Root Causes of Vacuum Voids (Insufficient Packing) Vacuum voids occur when the outer skin of the part solidifies quickly while the molten center continues to shrink, pulling the material outward.
- Process: Early transition to holding pressure (VP switchover), low holding pressure, or insufficient holding time.
- Tooling: Small gate sizes or thin runners that freeze before the part center is fully packed.
- Equipment: Wear on the screw’s "three-piece set" (check ring, thrust ring, and nozzle) leading to backflow.
- Material: High-shrinkage resins are more prone to this defect.
Expert Tip: Mold temperature plays a decisive role. Lower mold temperatures harden the surface quickly, making it resistant to deformation but prone to internal voids. Conversely, higher mold temperatures keep the skin soft, leading to sink marks.
Solutions
- Optimize Feed: Increase holding pressure and extend holding time.
- Tooling Modification: Enlarge gates and runners; position gates as close to thick sections as possible.
- Maintenance: Regularly inspect the screw and check valve for wear to ensure consistent pressure delivery.
5. Sink Marks
What are Sink Marks? A sink mark is a surface depression caused by the contraction of resin during cooling. Since crystalline resins undergo significant volume reduction upon solidification, these marks usually appear on thick walls or above internal ribs.
Root Causes
- Low Effective Packing: If the volume of resin injected during the holding phase is less than the volume lost to shrinkage, a sink mark forms. This is often due to small gates, thin runners, or a malfunctioning check valve.
- Slow Cooling: Thicker sections cool slowly, leading to higher total shrinkage. High mold temperatures can exacerbate this.
- Insufficient Cooling Time: If the part is ejected before the solidified "skin" has enough structural rigidity, the internal vacuum will pull the surface inward.
Solutions
- Pressure Management: Increase packing pressure. If the gate freezes too early, consider enlarging the gate or runner.
- Thermal Control: Gradually lower the mold temperature to reduce total shrinkage.
- Design Optimization: Follow the "1/3 rule"—rib thickness should ideally be around 1/3 of the base wall thickness to minimize sink potential.
- Material Choice: Use high-viscosity grades or add glass beads/fillers to reduce the shrinkage rate.
6. Black Spots and Dark Streaks
What are Black Spots? These are black dots or dark streaks embedded in the part, usually consisting of carbonized (burnt) resin, degraded additives, or external contaminants.
Root Causes
- Resin Degradation: Material "hanging up" in dead spots within the barrel, nozzle, or flange. Excessive melt temperature or long residence times (machine too large for the shot weight) are common culprits.
- Inadequate Purging: Residual resin from a previous run or burnt material stuck to the screw flights.
- Contamination: Dust, foreign materials, or non-compatible regrind entering the hopper.
- Trapped Air (Diesel Effect): If the screw starts dosing too early, air can be trapped in the melt. Under high pressure, this air compresses and heats up, scorching the plastic.
Solutions
- Thorough Cleaning: Use high-viscosity PE/PP or specialized chemical purging agents. For high-optical parts, dedicated barrels for specific materials are recommended.
- Temperature Management: Use a probe to verify the actual melt temperature. Pay close attention to the nozzle and check ring areas where stagnation is likely.
- Equipment Sizing: Ensure the machine's barrel capacity is appropriately matched to the part's shot weight to minimize residence time.
- Check Valve Care: When cleaning the screw, use a copper brush. Never use a blowtorch, as it softens the metal and accelerates wear.
Thank you for your reading, we will keep update of next part continue. We are welcome if you have any new ideas or sharing to us by info@jstmold.com.