How to Avoid Sink Marks in Injection Molding

I. The Essence of Sink Marks: A "Runaway Scene" of Cooling Shrinkage

(1) The "Shrinkage Gene" of Raw Materials

(2) The "Butterfly Effect" of Process Parameters

• Temperature Traps: Too low barrel temperature can cause a sharp increase in melt viscosity, making it as difficult to flow and compensate for shrinkage as "solidified syrup." Too high mold temperature prolongs cooling time, allowing crystalline plastics to "over-shrink." For instance, when the mold temperature for polyethylene increases from 40°C to 60°C, the shrinkage rate may rise by 20%.

• Pressure Disruption: The holding pressure stage is critical for shrinkage compensation. Insufficient holding pressure or too short a holding time mean the melt in the mold cavity is like a "faucet with cut-off water," unable to fill the voids formed by shrinkage. In one case, extending the holding time of a product from 10 seconds to 15 seconds reduced the sink mark depth from 0.2mm to 0.08mm.

• Uneven Cooling: When the thin-walled areas of a product (e.g., the edges of a phone case) solidify rapidly, while the thick-walled areas (e.g., internal support pillars) continue to shrink, the thin walls act like "taut strings," unable to supply material to the thick walls, eventually causing surface sink marks.

(3) "Compensation Obstacles" in Mold Design

• Too "Stubborn" Runner: A runner with too small a diameter (e.g., less than 4mm) or too many bends causes significant energy loss during melt flow, making it difficult to supply material to the far end of the mold cavity during the holding pressure stage. This is like a narrow water pipe failing to deliver enough water—small runners also cannot supply material to thick-walled areas.

• Gate in the "Wrong Position": If the gate is far from thick-walled areas, the melt must "travel a long distance" to reach the shrinkage compensation zone, often losing fluidity due to cooling midway. For example, relocating the gate from the product edge to the vicinity of a thick-walled boss can increase compensation efficiency by over 30%.

• Inadequate Venting: Insufficient mold venting slots allow gas to accumulate in the cavity, forming high-pressure air pockets that not only hinder melt filling but also cause local uneven cooling. A deep-cavity product with no venting slots once had a 50% sink mark defect rate, which dropped to 5% after adding venting slots.

(4) "Inherent Shortcomings" in Product Structure

II. Full-Process Prevention and Control: Making Sink Marks "Nowhere to Hide"

(1) Raw Material Selection: Choosing the "Right Personality" for Plastics

• Priority for Low-Shrinkage Formulations: Amorphous plastics or glass fiber-reinforced materials are "main forces" against sink marks. For example, 30% glass fiber-reinforced polypropylene reduces the shrinkage rate from 2.0% to 0.7%, making it nearly free of sink marks when used to produce car grilles.

• Thorough Drying Treatment: Hygroscopic raw materials (e.g., nylon, PC) must be thoroughly dried; otherwise, moisture vaporizes at high temperatures, exacerbating shrinkage. Nylon 66 needs to be dried at 80°C in a vacuum for 8 hours to reduce the moisture content below 0.1%, or it may form "honeycomb-like" internal voids after molding.

• Additives to Regulate Fluidity: Adding 0.5%-1% lubricants (e.g., stearic acid) can reduce melt viscosity and facilitate shrinkage compensation. Adding silicone oil to polyoxymethylene (POM) can increase its flow length by 15%, reducing sink marks caused by insufficient flow.

(2) Process Tuning: Precise Control of the "Shrinkage Rhythm"

• Temperature Strategy:

  • Barrel Temperature: Appropriately increase the temperature within the raw material’s heat resistance range. For example, increasing the barrel temperature of ABS from 220°C to 240°C can boost the melt flow rate by 20%, significantly enhancing compensation capability.

  • Mold Temperature: Use medium-temperature molds (50-70°C) for crystalline plastics (e.g., PE) to delay cooling and extend compensation time; use low-temperature molds (30-50°C) for amorphous plastics (e.g., PS) to accelerate surface solidification and prevent shrinkage from transferring to the surface.

  
  

• Pressure Management:

  • Injection Pressure: Adjust based on product wall thickness. Use 80-100MPa for thin-walled parts (<2mm) and 120-150MPa for thick-walled parts (>4mm) to ensure the melt "fills the mold cavity completely."

  • Holding Pressure and Time: Holding pressure is typically 60%-80% of the injection pressure, and the holding time should last until the gate freezes. The "weight method" can be used for testing: gradually shorten the holding time until the product weight stabilizes (usually 10-20 seconds).

  
  

• Cooling Optimization: Adopt "conformal cooling" technology, arranging dense cooling channels in thick-walled areas and reducing cooling in thin-walled areas. For example, 3D-printed conformal cooling molds can reduce product temperature differences from 20°C to within 5°C, decreasing sink mark depth by over 60%.

(3) Mold Improvement: Opening the "Lifeline for Shrinkage Compensation"

• Runner Design: Increase the runner diameter (e.g., from 6mm to 8mm), shorten the runner length, and reduce bends. For large products, use fan-shaped or trapezoidal runners to allow the melt to spread evenly across the mold cavity like a "uniform stream."

• Gate Optimization:

  • Position: Place the gate in thick-walled areas or at sink mark risk points, such as using a submarine gate at the bottom of ribs to directly "precisely compensate" thick walls.

  • Size: The gate thickness should be no less than 1.5 times the product wall thickness to avoid premature freezing during the holding pressure stage. For example, a 2mm-thick product should have a 3-4mm-thick gate, extending the compensation time by 3-5 seconds.

  
  

• Upgraded Venting: Machine 0.02-0.05mm-deep venting slots at the highest points of the mold cavity or insert breathable steel (e.g., Corrax breathable steel from ASSAB, Sweden) to leave no room for gas. For transparent products, set up gas-collecting chambers at the ends of venting slots to prevent scorch marks from gas combustion.

(4) Product Design: Eliminating Hazards at the Source

• Uniform Wall Thickness: Avoid abrupt wall thickness changes; use gradual transitions (e.g., chamfer radii ≥1.5 times the wall thickness) and control the difference between the maximum and minimum wall thicknesses within 30%. For example, designing a R0.8mm fillet at the junction between a home appliance casing’s boss and main body can reduce sink mark risk by 40%.

• Structural Thinning: Replace solid thick walls with hollow or honeycomb designs. For example, changing the support ribs of a motor end cover to hollow tubes not only reduces weight but also cuts shrinkage volume by over 50%.

• Reinforcement Rib Design: The thickness of reinforcement ribs should not exceed 60% of the main wall thickness (e.g., ≤1.8mm for a 3mm-thick main wall), with a height no more than three times the wall thickness and a R0.3mm fillet at the root to avoid stress concentration causing sink marks.

III. Practical Case: From "30% Defect Rate" to "Zero Sink Marks"

1. Raw Material Replacement: Switched from ordinary ABS to 20% mineral-filled ABS, reducing the shrinkage rate from 0.6% to 0.3%.

2. Process Adjustment: Increased holding pressure from 70MPa to 100MPa, extended holding time from 12 seconds to 18 seconds, and reduced mold temperature from 55°C to 40°C.

3. Mold Modification: Added an 8mm-diameter circular runner in the thick-walled area of the handle and created 0.03mm-deep annular venting slots.

4. Structural Optimization: Reduced the internal wall thickness of the handle by 1mm and added three 0.8mm-thick reinforcement ribs to disperse shrinkage stress.
   After these improvements, sink marks disappeared completely, and the product’s first-pass qualification rate rose to 98%, meeting medical-grade aesthetic requirements.

IV. Conclusion: Achieving Perfection in Details