What Are the Causes and Solutions of Die Casting Product Sink?

Introduction

If you work with die casting, you have probably seen sink marks on your parts. These smooth, sunken areas usually appear in thick sections. They ruin the look of your products and can hide bigger problems inside. In mass production, sink defects scrap 5–8% of parts—a huge cost over time. For pressure-tight components like hydraulic valves, sinks can cause leaks or complete failure. So what causes these dents? And how do you stop them? This article breaks down the root causes of die casting sink marks and gives you practical, proven solutions to eliminate them.

What Are Die Casting Sink Marks?

Sink marks (also called dents or shrinkage depressions) are smooth, sunken areas on casting surfaces. They typically range from 0.1mm to 2mm deep. Unlike rough defects, sinks have smooth edges and look like someone pressed a thumb into soft metal.

Where Do Sinks Appear?

Sinks always form in thick sections or at wall thickness transitions. Common locations include:

• Ribs and bosses (thick cores)
• Where thin walls meet thick walls (9mm to 3mm transitions)
• Intersections of multiple ribs (hot joints)
• Large flat surfaces with thick areas underneath

Why Should You Care About Sinks?

What Are the Different Types of Sink Marks?

Not all sinks are the same. Their location and appearance tell you what caused them.

What Causes Die Casting Sink Marks?

Sinks follow a simple chain of events: uneven solidification → metal shrinks → no metal to fill the gap → surface collapses. Four main factors trigger this chain.

Design Problems (30–40% of Sinks)

Your part and mold design set the stage for sinks before the first shot.

Wall thickness differences are the biggest culprit. When one area is much thicker than another (ratio over 3:1), the thick section stays hot 2–3 times longer. As it cools and shrinks, it pulls metal from the surface, creating a dent.

Example: A part with a 9mm rib next to a 3mm wall. The rib takes 12 seconds to solidify; the wall solidifies in 4 seconds. Without metal feeding the shrinking rib, the surface sinks.

Hot joints trap heat. Where three ribs meet at 90°, the intersection stays 50–80°C hotter than surrounding metal. Solidification delays of 10–15 seconds create perfect sink conditions.

Poor gate placement means metal cannot reach shrinking areas. If your inner gate sits more than 50mm from a hot joint, or its cross-section is less than 2× the wall thickness, you will get sinks.

Real-world data: A manufacturer of EV battery frames had 18% sink defects at rib intersections. Analysis showed the ribs were 12mm thick with no cooling. Redesigning to 8mm ribs with hollow sections cut sinks to 3%.

Process Parameter Problems (25–35% of Sinks)

Your machine settings either prevent sinks or guarantee them.

Low injection pressure fails to push metal into shrinking gaps. For aluminum, pressure below 50MPa gives a 70% sink rate in thick sections. Zinc needs at least 30MPa.

Short holding time opens the mold before thick sections fully solidify. The rule: holding time should be 1.2× solidification time. A 10mm aluminum section needs 12 seconds. At 5 seconds, you get 60% post-cooling sinks.

High pouring temperature makes everything worse. Above 720°C for aluminum, shrinkage volume jumps 12–18%. More gas also dissolves in the melt, creating internal voids that collapse into sinks.

Case study: An automotive supplier cut sink defects from 22% to 4% by:

• Reducing pouring temperature from 730°C to 690°C
• Increasing holding time from 6 seconds to 11 seconds
• Raising injection pressure from 45MPa to 65MPa

Cooling System Problems (20–25% of Sinks)

Your mold’s cooling channels determine how evenly heat leaves the metal.

Poor channel layout means thick sections cool too slowly. Cooling channels should be within 20mm of thick areas. If they are farther away, temperature differences exceed 40°C between thick and thin sections.

Blocked channels kill cooling efficiency. Scale buildup just 1mm thick reduces heat transfer by 30–40%. Cooling rates drop from 15°C/s to below 8°C/s, and sinks appear.

Inconsistent water flow creates hot spots. Critical channels need at least 2L/min flow. Pressure fluctuations over ±0.2MPa increase cooling unevenness by 25%.

Operational Problems (5–10% of Sinks)

Human factors matter too.

Opening the mold too early by just 2–3 seconds lets surface layers collapse. Operators sometimes rush cycles to hit targets, creating hidden scrap.

Too much release agent acts as insulation. Layers over 10μm thick slow heat transfer in local areas.

Wrong alloy chemistry increases sink risk. Copper over 4% in aluminum raises shrinkage rate by 10–15%. Magnesium under 0.3% reduces fluidity, so metal cannot feed shrinking areas.

How Can You Prevent Sink Marks?

Fixing sinks requires a systematic approach. Address each cause area with specific solutions.

Fix Design Problems First

Balance wall thickness everywhere. Keep thickness ratios under 2:1. Where thickness must change, use gradual slopes (at least 1:5 taper). This alone cuts hot spots by 70%.

Improve hot joints by:

• Adding 3–5mm cooling holes at rib intersections
• Using hollow ribs (2–3mm walls) instead of solid thick ribs
• Placing cooling channels directly under hot joints

Redesign gate systems:

• Put gates within 30mm of hot joints
• Make gate area 2.5× the wall thickness
• Add auxiliary feeders (volume 5–10% of hot joint size) for large parts

Design impact: A pump housing redesigned with balanced walls and hollow ribs saw sink defects drop from 25% to 2%. Tooling cost increased 8% but scrap savings paid back in 3 months.

Optimize Process Parameters

Key insight: Small adjustments matter. Dropping aluminum pouring temperature by just 20°C (from 720°C to 700°C) reduces shrinkage volume by 8–10% with no negative effects.

Upgrade Your Cooling System

Install targeted cooling: Use profiled channels that match part geometry. Spiral channels around thick ribs boost heat transfer by 35%.

Apply high-pressure cooling: Push 0.8–1.2MPa water through channels at hot joints. This thickens the quenched layer by 0.5–1mm, speeding solidification.

Maintain regularly: Clean channels every 500 cycles with descaling agents. Replace corroded pipes. Restore flow to at least 2L/min in critical channels.

Use Advanced Technology for Tough Parts

Local extrusion eliminates sinks completely. Install hydraulic pins (5–10mm diameter) in the mold. When metal reaches the semi-solid stage (60–70% solid), apply 80–120MPa pressure through the pins. This pushes metal into shrinking gaps—95% sink elimination.

Solidification simulation predicts problems before cutting steel. Software like MAGMA or Flow-3D shows exactly where sinks will form. One EV battery maker used simulation to identify sink risks, tweaked the design, and cut defects by 70% before building the mold.

Hollow out thick sections: Replace solid thick areas with thin walls and ribs. This reduces heat accumulation and shrinkage volume by 25% while maintaining strength.

How Do You Fix Existing Sinks?

For parts that already have sinks, these repair methods work:

Warning: Repairs cost money and time. Preventing sinks is always cheaper—by a factor of 5–10×.

FAQ About Die Casting Sink Marks

Can you detect sinks before the part comes out of the mold?

Yes, with real-time monitoring:

• Temperature sensors alert if thick sections stay over 300°C after set holding time
• Pressure sensors detect pressure drops in hot joints (means insufficient metal feed)
• Ultrasonic testing during solidification finds internal shrinkage that will become surface sinks

These tools cut wasted parts by 60% .

Do all die casting materials get sink marks?

All are at risk, but severity varies:

• Magnesium shrinks 4.5%—most prone to sinks
• Zinc shrinks 2.5%—less risk
• Aluminum in the middle—3.5–4% shrinkage
• Copper alloys need strict cooling control—sinks form if mold exceeds 250°C

Solutions work for all materials, but parameters must be tailored to each alloy.

Is it cheaper to fix sinks in design or after production?

Design fixes are 5–10× cheaper. A design change costing $500–1,000 prevents $5,000–10,000 in rework and scrap for a 10,000-part run. Invest in solidification simulation upfront—it identifies 90% of sink risks before mold manufacturing.

Can sinks appear days after casting?

Yes—post-cooling sinks happen in very thick parts. The center stays hot for hours after ejection. As it finally solidifies and shrinks, it pulls the already-solid surface inward. This is why thick parts need longer holding time and better cooling.

What is the fastest way to reduce sinks on an existing job?

Check three things first:

1. Increase holding time by 20–30%
2. Lower pouring temperature by 10–20°C
3. Verify cooling channel flow in thick sections

These three adjustments solve 60–70% of sink problems without mold changes.

Conclusion

Die casting sink marks are preventable defects. They form when thick sections solidify slowly, shrink, and lack metal to fill the gap. The causes fall into four areas: design problems (uneven walls, poor gates), process parameters (low pressure, short hold time), cooling issues (bad channel layout), and operational errors (early demolding).

The solutions follow directly from the causes:

• Balance wall thickness and improve hot joints
• Set correct parameters: 60–80MPa pressure, 1.2× solidification time hold, 680–700°C aluminum temperature
• Upgrade cooling with targeted channels and regular maintenance
• Use advanced methods like local extrusion and simulation for tough parts

Fixing sinks in design is 5–10× cheaper than repairing them after production. With the right approach, you can reduce sink defects from 5–8% to under 1%—saving money and improving quality.

Discuss Your Die Casting Projects with Yigu Rapid Prototyping

At Yigu Rapid Prototyping, we have helped dozens of manufacturers eliminate sink marks and other die casting defects. Our team combines decades of hands-on experience with advanced simulation tools to identify problems before they cost you money.

Whether you need:

• Sink analysis for a problematic part
• Design review to prevent defects
• Process optimization to improve quality
• Simulation services to predict issues upfront
• Quality parts from our own production lines

We are ready to help.

Contact Yigu Rapid Prototyping today to discuss your challenges. Send us photos of defective parts, your process data, or your part drawings. We will give you honest, practical advice based on real-world experience. Let’s work together to eliminate sink marks and make your die casting better.