Executive Summary: The Silent Cost of Poor Cooling

Sink marks (sink marks) are one of the most frustrating defects in thick-wall injection molding, affecting both the cosmetic appeal and structural integrity of your products.

At Shenzhen Ulite Precision Technology Limited, our team has successfully eliminated over 500+ instances of sink marks through advanced process control, mold flow simulation, and design optimization.

This injection molding troubleshooting guide breaks down the root causes and provides practical eliminate sink marks solutions to ensure your molded parts leave the factory free from defects.

Key Takeaways:

• Sink marks occur due to uneven shrinking of material during cooling.
• Proper conformal cooling channels can reduce sink risk by up to 70%.
• Strategic rib design saves cost without sacrificing stiffness.
• Ulite’s Moldflow Analysis predicts sink marks before cutting steel.

Understanding Sink Marks: The Science of Plastic Shrinkage

When plastic cools inside the mold cavity, it shrinks. If the outer skin solidifies faster than the interior core, the shrinking material pulls away from the walls, creating a depression known as a sink mark (A type of injection molding defect).

The Physics Behind It:

1. Material Density Change: As polymer melts cool, density increases.
2. Core Contraction: Inner material contracts more than the skin.
3. Surface Pull: Skin gets pulled inward towards the center.

Common Scenarios Where Sink Marks Happen:

7 Proven Methods to Eliminate Sink Marks

Method 1: Optimize Mold Temperature

Lower mold temperatures speed up cycle time but increase sink risk because the skin sets too fast.

Solution: Increase mold temperature slightly (e.g., 60°C -> 80°C) to allow slower cooling and better packing pressure transmission.

Method 2: Adjust Holding Pressure Profile

Constant holding pressure often isn’t enough. A multi-stage profile compensates for viscosity changes.

Solution: Start with high pressure immediately after filling, then gradually ramp down over time. Use Scientific Molding techniques to pinpoint exact settings.

Method 3: Add Conformal Cooling Channels

Standard drilled holes create hot spots. Uniform cooling reduces differential shrinkage.

Solution: Use moldflow simulation to identify hot spots near thick sections. Install conformal cooling channels (e.g., via

DMLS 3D printing inserts) to maintain uniform temperature.

Method 4: Design Ribs Instead of Thick Walls

Thick walls shrink more than ribs. Increasing rib height is stronger than increasing base wall thickness.

Rule: Keep rib thickness at 50–60% of nominal wall thickness to prevent sink marks at the root.

Method 5: Gate Location Optimization

If the gate is far from the thick section, material may freeze off before compacting fully.

Solution: Move gates closer to thickest features. Consider using multiple small gates instead of one large gate for balanced filling.

Method 6: Material Selection for Low Shrinkage

Some resins naturally shrink less.

Recommendation: Use glass-filled materials (PA66 GF30) which have lower shrinkage compared to virgin ABS or PP.

Method 7: Vacuum Venting System

Trapped air prevents full packing, leading to internal voids that collapse into external sinks.

Solution: Implement vacuum venting to remove air before material enters thick zones.

Case Study 1: Large Electrical Enclosure (Structural Part)

Client Challenge: European electrical equipment manufacturer producing IP65-rated enclosure. Required smooth front surface, but significant sink marks appeared on thick mounting bosses.

Material Used: PC/ABS Blend.

Ulite Engineering Intervention:

🔧 Design Fix: Reduced boss thickness from 6mm to 3.5mm; added support ribs instead.

🔧 Process Fix: Increased holding pressure from 90bar to 110bar using stage-profile technique.

🔧 Tooling Fix: Installed conformal cooling insert directly behind the boss.

Results Achieved:

✅ Zero visible sink marks on front face.

✅ Cycle time improved by 8% due to better cooling efficiency.

✅ Weight reduced by 12% (material savings).

Product Manager Testimonial:
“We were failing every shot initially due to sinking in the boss area. Ulite’s combination of rib redesign and conformal cooling completely solved it.”
— Product Manager, European Electrical Co.

Case Study 2: Smart Pet Feeder Base (Aesthetic & Functional)

Client Background: US-based pet tech startup launching an automatic feeder requiring stability (weight) and a clean look for home interiors.

Challenge: To prevent the feeder from being tipped over by pets, the base required a thick, heavy footprint. However, this thick geometry caused severe sink marks on the bottom exterior, making it unretail-ready.

Material Selected: High-Impact PP (Polypropylene).

Ulite Eliminate Sink Marks Solution :

🔧 Internal Structural Design: Replaced solid thick walls with a honeycomb ribbing pattern internally. This maintained structural rigidity and weight while reducing solid mass that causes shrinkage.

🔧 Precision Cooling: Used conformal cooling technology on the base plate to equalize heat dissipation across the thick section.

🔧 Vacuum Packing Control: Added vacuum-assisted injection packing to pull the melt tightly against the mold surface in critical corners.

Results Achieved:

✅ Bottom exterior achieved Class C finish without any visible sinking.

✅ Stability unchanged (same overall weight as solid design).

✅ Production cost reduced by 15% due to lighter resin usage.

Founder Testimonial:
“Our prototypes looked like cheap toys due to ugly sink marks on the bottom. Ulite kept the weight we needed for stability but removed the visual defects entirely. Our crowdfunding campaign succeeded because the product looked premium inside and out.”
— Founder, US Pet Tech Company

Prevention First: DFM Review Saves You Money

Eliminate sink marks is possible, but preventing them during Design for Manufacturing (DFM) is cheaper.

Ulite’s DFM Checklist for Thicken Sections:

• Are ribs connected properly? (No dead ends).
• Is there enough space for cooling lines around thick zones?
• Is gate position optimal? (Avoiding freezing off).
• Is material shrinkage coefficient accounted for?
• Do we have sufficient ejection force?

Why Partner with Ulite for Thick-Wall Injection Molding Components?

While many shops claim to solve sink marks, only those with data-driven capabilities truly master it.

Get Your Free Consultation Today

Don’t let uncertainty slow you down. With Ulite Precision, you get direct access to technical experts ready to answer your toughest questions.

Next Steps:

1. Send Your Files: Upload STEP/IGES to our secure portal.

2. Ask Your Question: Email us any concerns via Email: inquiry@ulitemech.com

3. Get a Response: Receive a detailed answer within 24 hours.

Special Offer for New Clients:

✨ Free Initial Feasibility Review ($500 Value)

✨ Free DFM Analysis for New Projects

✨ Free Shipping Estimate (Air/Sea Options)

✨ Free Sample Quote Package

📞 Contact us for FREE DFM Analysis and Custom Injection Molding Advice and Injection Molding Quote within 48 hours.

📧 Email: inquiry@ulitemech.com

🌐 Visit: https://ulitemech.com

Trusted by over 3,000 companies worldwide and numerous renowned brands.

Conclusion: Achieving Flawless Surfaces Starts Now

Eliminating sink marks in thick-wall injection molding requires a holistic approach combining precise design, advanced tooling, and expert processing.

By choosing Shenzhen Ulite Precision Technology Limited, you gain access to world-class engineering resources dedicated to solving complex Injection Molding Defects. Don’t let defects compromise your brand reputation—partner with experts who understand the science of plastic.

Final Call to Action:

Send us your drawings today! Let our DFM team perform a free analysis and suggest the best way to eliminate sinks in your specific application.