Sink marks in injection molding are shallow depressions or indentations that form on the surface of a molded plastic part. They occur when the interior of a thick section cools and shrinks faster than the outer skin, pulling the surface inward. Unlike voids (which are internal air pockets), sink marks are visible surface defects.

Sink marks typically appear in areas of the part where:

• Wall thickness transitions from thick to thin
• Ribs or bosses are attached to a thinner wall
• Bosses or standoffs are located on the exterior surface
• Gates are positioned in thick sections
• Corner radii create localized thickness increases

Sink marks vs. voids

Sink marks are surface-level depressions caused by differential shrinkage. Voids are internal air pockets caused by gas entrapment or insufficient packing. Both can occur in the same area, but they require different diagnostic approaches. If you press on a sink mark, the surface will deform further; a void underneath will feel hollow.

Sink marks vs. warpage

Warpage is a global distortion of the part shape caused by uneven cooling across the entire part. Sink marks are localized, surface-level depressions. A part can have both defects simultaneously, but they originate from different mechanisms.

Root Cause Analysis of Sink Marks in Injection Molding

Sink marks result from one or more of the following root causes. Understanding which factor is dominant in your specific case is critical to selecting the right fix.

1. Excessive or Non-Uniform Wall Thickness

This is the single most common cause of sink marks in injection molding. When a section of the part is thicker than the surrounding walls, the interior material cools more slowly than the outer skin. As the core solidifies and shrinks, it pulls the already-frozen outer surface inward, creating a depression. The recommended wall thickness ratio between a rib and its parent wall is 50-60%. If a rib is 3 mm thick and the parent wall is 2 mm, the rib-to-wall ratio is 150% — far above the acceptable range and almost certain to produce a visible sink mark on the opposite surface.

Common design features that cause sink marks include:

• Thick ribs: Rib thickness exceeding 60% of the adjacent wall thickness
• Oversized bosses: Boss diameter more than twice the nominal wall thickness
• Uneven walls: Abrupt transitions between thick and thin sections without gradual blending
• Hidden thick zones: Areas where multiple ribs or features intersect, creating local mass concentrations

2. Insufficient Packing Pressure or Packing Time

During the packing phase of injection molding, additional material is pushed into the cavity to compensate for volumetric shrinkage as the plastic cools. If the packing pressure is too low or the packing time is too short, the cavity cannot be adequately topped off, and the part surface sinks as the core shrinks. This is particularly common when processing semi-crystalline resins like PP, PA (Nylon), and POM, which have higher shrinkage rates than amorphous resins like ABS, PC, or PS.

The packing phase typically accounts for 70-80% of the total cycle time in thin-wall molding. Insufficient packing is a process-related sink mark cause — the part design may be acceptable, but the machine settings fail to deliver enough material to compensate for shrinkage.

3. High Shrinkage Rate of the Selected Material

Every thermoplastic resin has a specific shrinkage rate — the percentage by which the material contracts as it cools from melt to solid. Materials with high shrinkage rates are inherently more prone to sink marks:

Material	Shrinkage Rate (%)	Sink Mark Risk
PP (Polypropylene)	1.0 - 2.5	High
PA / Nylon	0.7 - 2.0	High
POM (Acetal)	1.5 - 2.2	High
PE (Polyethylene)	1.5 - 4.0	Very High
ABS	0.4 - 0.7	Low-Medium
PC (Polycarbonate)	0.5 - 0.7	Low
PS (Polystyrene)	0.4 - 0.6	Low

Selecting a resin with a lower shrinkage rate — or switching from a semi-crystalline to an amorphous grade — can significantly reduce sink mark risk. Material selection should be part of the DFM review process, ideally conducted before the mold is fabricated.

4. Inadequate or Unbalanced Mold Cooling

The mold cooling system directly affects how uniformly the part solidifies. If the cooling channels are too far from the part surface, poorly positioned relative to thick sections, or have uneven flow rates, certain areas of the part will cool faster than others. The slower-cooling zones shrink more and create sink marks. Effective cooling system design ensures that all areas of the cavity cool at a similar rate, minimizing differential shrinkage.

Common cooling-related causes of sink marks include:

• Distance to surface: Cooling channels located more than 2× the channel diameter from the cavity surface
• Uneven cooling: One side of the mold significantly hotter than the other (typically the core side)
• Insufficient flow rate: Coolant velocity below 1 m/s, creating laminar flow and poor heat transfer
• Blocked channels: Scale buildup or debris restricting coolant flow in aging molds

5. Poor Gate Location or Gate Size

The gate is the entry point where molten plastic enters the cavity. If the gate is located in a thin section of the part, the packing pressure cannot effectively reach thick sections that need additional material to prevent shrinkage. Similarly, a gate that is too small restricts the flow of packing material into the cavity. Gate location and size are critical process design parameters that should be optimized during the mold design phase. Poor gate placement is one of the hardest sink mark causes to fix after the mold is built — making early DFM review essential.

How to Diagnose Sink Marks Systematically

Before applying fixes, identify which root cause is dominant. Use this systematic approach:

1. Map the sink mark location: Identify exactly where on the part surface the sink marks appear. Are they opposite ribs? Behind bosses? At thick-to-thin transitions?
2. Measure local wall thickness: Use calipers, ultrasonic thickness gauge, or cross-section the part. Compare the wall thickness at the sink mark location with surrounding areas.
3. Check material and process data: Record resin grade, MFI, melt temperature, mold temperature, injection pressure, packing pressure, packing time, and cooling time.
4. Run a short-shot study: Gradually reduce injection volume to visualize the fill pattern. This reveals whether thick sections fill last (indicating packing pressure cannot reach them).
5. Perform a packing pressure study: Incrementally increase packing pressure while holding all other parameters constant. If sink marks improve, the root cause is insufficient packing.

This diagnostic method eliminates guesswork and directs your corrective action to the right cause. For production environments running multiple materials or mold configurations, systematic diagnosis prevents unnecessary adjustments that could introduce other defects.

Sink Mark Solutions: From Design to Production Floor

The following solutions address each root cause category, ordered from most effective (design-stage prevention) to corrective (in-production fixes).

Design Optimization: The Most Effective Prevention

• Follow the 60% rib-to-wall rule: Rib thickness should not exceed 60% of the adjacent nominal wall thickness. For a 2.5 mm wall, ribs should be no thicker than 1.5 mm. If structural stiffness requires thicker ribs, use multiple thinner ribs instead of one thick rib.
• Core out thick sections: Where wall thickness must be greater for structural reasons, core out the back side to create a uniform wall. This eliminates the mass concentration that causes sink marks.
• Add fillets at transitions: Gradual transitions between thick and thin sections reduce differential shrinkage. Use a minimum 3:1 slope ratio for thickness changes.
• Relocate bosses: If possible, move bosses away from cosmetic surfaces. Bosses on hidden (non-cosmetic) sides will not produce visible sink marks.
• Specify texture or stipple: A light surface texture can mask minor sink marks that are unavoidable in the design. This is a common technique for consumer electronics enclosures and automotive interior parts.

Process Parameter Optimization for Sink Mark Reduction

When design changes are not feasible, process adjustments can reduce or eliminate sink marks:

• Increase packing pressure: Raise packing pressure in 5% increments until sink marks disappear. Monitor for over-packing symptoms (flash, sticking, part distortion).
• Extend packing time: Ensure the gate freezes before packing pressure is released. Extend packing time until gate seal study confirms proper timing.
• Optimize melt temperature: Higher melt temperature improves flow and packing, but increases cycle time and shrinkage. Find the optimal balance point.
• Increase mold temperature: A warmer mold surface allows the skin to remain flexible longer, enabling packing pressure to push more material into the cavity. This is particularly effective for semi-crystalline materials.
• Reduce cooling rate asymmetry: Adjust coolant temperature on the core and cavity sides to minimize differential cooling.

Material Selection Strategies to Minimize Sink Marks

If the design and process cannot fully resolve sink marks, material changes offer a powerful alternative:

• Switch to a lower-shrinkage resin: e.g., from PP to ABS, or from PA6 to PC-ABS blend
• Use mineral-filled or glass-filled grades: Fillers reduce overall shrinkage and improve dimensional stability
• Select nucleated grades: Nucleated semi-crystalline resins solidify more uniformly, reducing sink mark depth
• Consider impact-modified grades: These often have lower shrinkage than unfilled versions of the same resin

Mold Modifications for Persistent Sink Mark Problems

When design and process changes are insufficient, mold-level modifications may be necessary:

• Improve cooling near thick sections: Add conformal cooling channels or baffles/bubblers to increase cooling rate at problem areas
• Relocate or resize gates: Move the gate closer to thick sections to improve packing pressure transmission
• Add overflow wells: Strategic overflow wells can increase effective packing pressure in localized areas
• Modify venting: Improved venting allows better material flow into thick sections during packing

Mold modifications are the most expensive fix but may be unavoidable for complex parts. For tooling projects, investing in Moldflow simulation before mold fabrication can predict sink mark locations and allow design corrections before steel is cut.

Sink Mark Prevention Checklist for Engineers

Use this checklist during design review and production setup to minimize sink mark risk:

• ☐ All wall thickness transitions follow the 3:1 slope rule
• ☐ Rib thickness ≤ 60% of adjacent wall thickness
• ☐ Boss diameter ≤ 2× the nominal wall thickness
• ☐ No hidden thick sections at feature intersections
• ☐ Material shrinkage rate reviewed and documented
• ☐ Gate location optimized for thick-section packing
• ☐ Cooling channels within 2× diameter of cavity surface
• ☐ Mold temperature balanced between core and cavity
• ☐ Packing pressure and time optimized via DOE or scientific molding
• ☐ Gate seal study completed to confirm packing time
• ☐ Moldflow analysis performed for complex or cosmetic parts
• ☐ Surface texture specified for cosmetic areas where minor sinks are acceptable

Why Partner with Eastmaster for Sink-Mark-Free Injection Molding

At Eastmaster Manufacturing Limited, sink mark prevention starts at the design review stage — not on the production floor. Our engineering team conducts a comprehensive DFM analysis for every injection mold project, evaluating wall thickness ratios, rib design, gate placement, and cooling system layout before any steel is cut.

With 25+ years of production experience, ISO 9001 and ISO 13485 certification, and DNB verification (D-U-N-S Number: 66-849-8033), we deliver consistent, defect-free parts across a wide range of materials and industries. Our capabilities span rapid prototyping through full production, with Moldflow simulation, scientific molding, and full dimensional inspection included as standard.

Whether you need a single prototype or thousands of parts, our team is ready to help you design and manufacture sink-mark-free plastic components that meet your specifications.

Contact Eastmaster today to discuss your injection molding project and learn how our DFM-first approach eliminates sink marks before they start.

Email:  info@eastmaster.com 

Phone: +86 755 22676100