How to optimize mold gate design to avoid plastic part sink marks

Optimizing Gate Design to Avoid Sink Marks (Dents) in Injection Molding

The core principle behind optimizing gate design to prevent sink marks is to ensure that the molten material can effectively pack out the thick-walled areas during the holding pressure phase. Key measures include placing the gate at the thickest section of the part, using large or fan gates (with a thickness of approximately 50% to 80% of the part wall thickness), employing multi-point feeding to shorten flow lengths, setting up auxiliary runners far from the gate, and combining these design features with higher holding pressures and longer holding times.

Specific optimization details are as follows:

1. Gate Location & Setup

• Placement at Thickest Sections: Gates should be positioned directly in the thick areas of the part, particularly near ribs, bosses, and other features prone to sink marks. This ensures these regions receive ample melt replenishment as they begin to cool.
• Symmetry & Balance: For parts with complex geometries, use symmetric or multi-point gate layouts. This prevents localized slow cooling and uneven shrinkage.
• Avoid Thin-to-Thick Flow Paths: Do not place gates such that the melt must flow through a thin wall section to reach a thick wall. This can cause the gate to freeze off prematurely, “starving” the thick area of packing pressure and leading to sink marks.

2. Gate Type & Size Optimization

• Enlarge Gate Dimensions: Small gates tend to freeze off too early during the packing phase. The gate cross-section should be appropriately enlarged. As a general guideline, gate height should be between 50% and 80% of the part wall thickness to extend the effective packing time.
• Selecting Appropriate Gate Types:
  • Fan Gate: Ideal for large, flat, or planar parts. It promotes uniform filling and minimizes pressure loss.
  • Submarine / Pin Gate (Sub/Pin Gate): Suitable for small parts and can be positioned close to thick sections.
  • Tab Gate: Particularly useful for thick-walled parts.
• Reduce Resistance Upstream of the Gate: Ensure smooth transitions in the runner system to minimize flow resistance, allowing pressure to be transmitted effectively to the farthest or thickest areas of the cavity.

3. Supporting Mold Features

• Auxiliary Cooling: Implement robust cooling systems near thick areas to ensure their cooling rate is as close as possible to that of the thin sections.
• Design Auxiliary Runners: Runner systems can be designed to channel melt directly towards thick sections prone to shrinkage.
• Venting Design: Properly position venting grooves or ejector pins to ensure air in the cavity can escape, preventing trapped air from hindering melt packing.

4. Supporting Process Optimization

• Extended Hold Time & Delay: Prolonging the holding pressure time ensures the cavity can be effectively filled to compensate for volumetric shrinkage before the gate seals off completely.
• Increase Injection Pressure & Speed: A combination of high pressure and lower speed often provides better packing results and reduces the occurrence of sink marks.

In summary, by adhering to the design principles of “large gates, feeding at thick walls, and efficient packing,” sink mark defects in injection molded plastic parts can be significantly minimized or eliminated.