Injection Molding Sink Marks: Top Causes and How to Eliminate Them

Sink marks are common yet frustrating defects in injection molding, appearing as small craters or depressions on the surface of molded parts. These flaws often occur in thicker sections of a product, where cooling and solidification happen unevenly. Beyond compromising aesthetics, sink marks can signal underlying issues with structural integrity. Understanding their causes and implementing proven solutions is critical for manufacturers aiming to deliver high-quality, reliable products.

What Are Sink Marks?

Sink marks typically form when the inner layers of a plastic part cool and shrink more slowly than the outer surfaces. This uneven shrinkage creates a vacuum effect, pulling the exterior inward and causing visible depressions. They are most prevalent in areas with thick walls, ribs, bosses, or other geometric features that create inconsistent cooling rates. While often cosmetic, severe sink marks can weaken a part’s mechanical performance.

Top Causes of Sink Marks

1. Insufficient Packing or Holding Pressure: If the injection pressure is too low during the packing phase, there won’t be enough material forced into the mold to compensate for natural shrinkage as the plastic cools .
2. Inadequate Cooling Time or Temperature: Rapid cooling of the outer layers while the core remains molten exacerbates shrinkage. Low mold temperatures can accelerate this imbalance .
3. Material Issues: Some plastics, like semi-crystalline polymers (e.g., polypropylene), are more prone to shrinkage due to their molecular structure. High material viscosity can also hinder flow into critical areas .
4. Part Design Flaws: Thick sections or abrupt transitions in wall thickness create natural hotspots for sink marks. For instance, ribs that are too thick relative to the main wall can cause depressions on the opposite surface .
5. Incorrect Gate Size or Location: Small gates may freeze off too early, preventing additional material from reaching and packing out the cavity effectively .

How to Eliminate Sink Marks

1. Optimize Process Parameters

· Increase Holding Pressure and Time: Higher pressure applied for a longer duration helps push more material into the mold to counteract shrinkage .
· Adjust Cooling Time and Temperature: Ensure uniform cooling by elevating mold temperature and optimizing cooling channel layout. Slower, more controlled cooling reduces internal stresses .
· Raise Melt Temperature: A hotter melt flows more easily into thicker sections, but avoid excessive heat that degrades the material .

2. Improve Part and Mold Design

· Reduce Wall Thickness: Design parts with uniform, thinner walls (ideally ≤4mm). Use coring or ribs to maintain strength without adding mass .
· Modify Gate Design: Enlarge gates or relocate them to ensure material reaches thick sections before solidifying .
· Incorporate Gas-Assisted Injection Molding: This technology uses pressurized gas to hollow out thick areas while maintaining surface pressure, effectively minimizing sinks .

3. Select the Right Material

· Choose low-shrinkage resins (e.g., filled compounds like glass-filled nylon) or materials with higher flow rates to improve packing .
· Pre-dry hygroscopic polymers to prevent moisture-related inconsistencies .

4. Utilize Advanced Molding Techniques

· Sequential Valve Gating: Controls melt flow through multiple gates to pack specific zones progressively.
· Mold Flow Analysis: Simulate filling and cooling phases to identify potential sink areas before manufacturing .

Key Takeaways

Sink marks stem from a combination of material behavior, process settings, and design flaws. A systematic approach—balancing pressure, temperature, and cooling with intelligent design—is essential for elimination. Manufacturers should prioritize:

· Process Fine-Tuning: Validate parameters through DOE (Design of Experiments).
· Design for Manufacturability (DFM): Collaborate with mold engineers early in product development.
· Technology Adoption: Leverage simulation software and advanced molding methods.

By addressing these factors, companies can significantly reduce defects, enhance product quality, and boost customer satisfaction.

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