Sand Inclusions: How to Prevent Erosion and Clean Metal

You’ve just finished machining a casting. The surface looks good, but then you see it: small, hard, light‑colored spots. Your cutting tools feel dull. Some spots flake out, leaving tiny pits.

You’re looking at sand inclusions.

Sand inclusions are one of the most frustrating defects in sand casting. They come from the mold itself—eroded sand grains carried by the molten metal into the casting. Unlike slag or dross (which come from the melt), sand inclusions are hard, abrasive, and often difficult to remove.

This guide explains what causes sand inclusions, how to tell them apart from other defects, and—most importantly—how to prevent them.

What Are Sand Inclusions?

Definition

A sand inclusion is a trapped particle of molding sand or core sand inside a casting. These particles are typically:

How They Form

1. Molten metal flows through the gating system or mold cavity

2. High velocity or turbulence erodes sand from the mold wall

3. Loose sand grains are carried by the metal stream

4. Grains become trapped in the solidifying casting

Key point: Sand inclusions originate from the mold, not from the melt.

Sand Inclusions vs. Other Defects

Simple test: If you can scratch it with a steel tool, it’s probably slag. If it’s hard and gritty, it’s likely sand.

Root Causes of Sand Inclusions

1. Poor Mold or Core Strength

2. High Metal Velocity or Turbulence

3. Poor Gating Design

4. Inadequate Coating or Washes

5. Mold or Core Damage

How to Prevent Sand Inclusions

1. Optimize Sand and Core Properties

For cores:

• Verify core hardness (use a core hardness tester)

• Inspect cores for cracks or friable edges

• Use core coatings on all surfaces that contact metal

2. Improve Gating System Design

Recommended velocities:

3. Apply Mold and Core Coatings

Application tips:

• Apply evenly with spray or brush

• Ensure coating thickness is consistent (0.3‑0.5 mm typical)

• Dry thoroughly before closing the mold

• Avoid puddling or runs that can flake off

4. Use Paper Casting Runners (for Iron/Steel)

Paper runners eliminate sand erosion in the runner system entirely. The metal flows through a smooth paper tube, not a sand channel.

Advantages for sand inclusion prevention:

• No sand to erode in the runner

• Smooth surface reduces turbulence

• Consistent geometry

📖 Related article: Paper Casting Runners: Complete Guide

5. Use Ceramic Pouring Cups

A ceramic pouring cup at the sprue top prevents sand erosion at the entry point.

Benefits:

• No sand exposed at the pour basin

• Smooth transition into sprue

• Reduces initial turbulence

6. Optimize Pouring Practice

The Role of Filtration

Do Filters Stop Sand Inclusions?

Yes, ceramic foam filters are very effective at capturing sand particles that have already been eroded and are traveling in the metal stream.

Where to Place Filters for Sand Inclusion Control

• After the sprue — captures sand eroded at the sprue bottom

• Before the casting — catches any sand that enters the runner

• As close to the casting as possible — prevents re‑erosion after filter

Important Note

Filters treat the symptom, not the cause. If you have severe sand erosion, a filter will capture some of it, but it may clog quickly. Always address the root cause first.

Inspection and Quality Control

Visual Inspection of Molds

Before closing the mold:

Blow or Vacuum

Before closing the mold, blow or vacuum out any loose sand from:

• Runner system

• Core prints

• Cavity corners

Testing Methods for Sand Inclusion Defects

Common Mistakes to Avoid

Frequently Asked Questions

Q1: How can I tell if a defect is sand or slag?

A: Sand inclusions are hard and gritty. Slag/dross is softer and can often be scratched with a steel tool. Sand also appears lighter in color (tan, brown) while slag is usually dark gray or black.

Q2: Can a filter completely eliminate sand inclusions?

A: A properly sized and placed ceramic foam filter can capture most sand particles. However, if erosion is severe, the filter may clog or bypass may occur. Always fix the erosion source first.

Q3: What’s the best way to prevent sand erosion in the runner?

A: Use paper casting runners (for iron/steel). They eliminate sand from the runner entirely. For aluminum, use ceramic fiber launders or ensure runner design keeps velocity low.

Q4: Does higher sand strength always help?

A: Generally yes, but excessively high strength can cause other problems (poor collapsibility, higher binder costs). Target the optimal strength for your casting size and metal.

Q5: How do I know if my gating velocity is too high?

A: Use gating design software or manual calculations. As a rule of thumb, runner velocities above 50 cm/s for iron and 60 cm/s for aluminum increase erosion risk significantly.

Q6: Can sand inclusions cause machining tool breakage?

A: Yes. Sand particles are very hard and abrasive. They can quickly dull or break cutting tools, especially carbide inserts.

Conclusion

Sand inclusions are preventable. The key is addressing the source—the mold and gating system—before relying on filters to catch the particles.

When you combine these strategies, sand inclusions become rare. Your castings will be cleaner, your tools will last longer, and your scrap rate will drop.

At SF-Foundry, we supply products that help you prevent sand inclusions:

Contact us for technical support or product recommendations:

• Email: info@sf-foundry.com

• Technical Support: 8618636913699

• Website: www.sf-foundry.com