Introduction
If you work with die casting, you know that not all defects are visible. Some hide inside the metal, waiting to cause problems. Die casting coils—also called gas porosity or internal voids—are exactly that kind of hidden defect. They form when air or gas gets trapped inside the casting during solidification. You cannot see them by looking, but X-rays reveal the truth: irregular shadows where solid metal should be. For critical parts like engine blocks or hydraulic valves, these hidden voids can cause leaks, reduce strength, and lead to catastrophic failures. This article explains what die casting coils are, what causes them, and how to eliminate them for good.
What Are Die Casting Coils?
Die casting coils are internal voids or porous areas inside a casting. They form when gas becomes trapped in the molten metal and cannot escape before solidification.
How Do They Form?
The process happens in milliseconds. Molten metal shoots into the mold cavity at 3–8 meters per second. At that speed, the metal does not flow smoothly—it splashes and churns. This turbulence creates low-pressure zones that suck in air from the cavity. When the metal solidifies, the air gets locked inside as bubbles.
Sometimes the gas comes from the metal itself. Molten aluminum absorbs hydrogen like a sponge. If you do not remove it before casting, that hydrogen comes out of solution during solidification and forms tiny pores.
What Do They Look Like?
Die casting coils have telltale signs:
| Inspection Method | What You See |
|---|---|
| Visual inspection | Sometimes nothing—defects are internal. Occasionally, small pinhole-like bulges on thick sections. |
| Cut section | Honeycomb-like structure instead of dense metal. Spongy areas visible to the naked eye. |
| X-ray inspection | Irregular, cloud-like shadows inside the casting. Dark areas show where gas is trapped. |
Why Should You Care?
These hidden defects matter because they affect performance in serious ways:
Strength drops: Porosity reduces the load-bearing area. Tensile strength can fall by 10–30%. Fatigue life—how many stress cycles a part survives—can drop by 50% .
Parts leak: For hydraulic components, valves, or anything that holds pressure, pores create leak paths. A part that looks perfect may fail immediately in service.
Costs add up: Even a 5% defect rate in mass production increases overall costs by 15–20% . Rework, scrap, and warranty claims eat into profits.
Real-world example: An automotive supplier produced 200,000 transmission housings per year. Internal porosity caused a 12% scrap rate. At $25 per housing, that was $600,000 in lost product annually.
What Causes Die Casting Coils?
Four main factors work together to create die casting coils. Understanding each one helps you target the right fix.
Hydrodynamic Factors: How Metal Flows
The way molten metal moves through the mold determines whether it traps air.
Injection speed too high: When metal shoots into the cavity at over 6 m/s, it hits the opposite wall and splashes back. This creates vortex rings that suck in air like a tornado.
Poor gating design: Sudden changes in cross-section, sharp turns, or narrow runners disrupt smooth flow. Instead of a steady front, the metal breaks into turbulent streams that trap air.
The numbers: At 4 m/s injection speed, air entrainment is minimal. At 8 m/s, it increases by 300–400% .
Mold Exhaust Limitations: Where Air Goes
The air in the cavity has to go somewhere. If it cannot escape, it gets trapped.
Inadequate exhaust: Traditional exhaust grooves work for slow filling but cannot handle the instantaneous pressure spike of high-speed injection. Air gets compressed and forced into the metal.
Blocked vents: In thin-walled parts, metal solidifies quickly and can block exhaust paths before all air escapes. The remaining air has nowhere to go.
Deep cavities: Air trapped in deep pockets has a long path to freedom. Often, it never makes it out.
Melt Quality: Gas in the Metal
Sometimes the gas comes from the metal itself, not the cavity.
Hydrogen absorption: Molten aluminum loves hydrogen. At 700°C, it can absorb 0.7–1.2ml of hydrogen per 100g of metal. When it solidifies, solubility drops to 0.1ml, so the excess hydrogen forms bubbles.
Contaminated materials: Damp scrap metal, wet charge materials, or moist refining agents decompose when heated, releasing gas directly into the melt.
The problem: If your melt has high gas content, you are fighting a losing battle. No amount of process adjustment can fix gas that is already dissolved.
Improper Process Parameters: Timing Matters
Even with good flow and good metal, wrong settings create defects.
Injection timing off: Switching from slow to fast injection at the wrong moment causes problems. Too early—metal splashes. Too late—filling incomplete.
Short holding time: After filling, pressure must hold while metal solidifies. If you release pressure too soon, shrinkage pulls bubbles larger instead of compressing them.
Cold mold: Low mold temperature chills the surface immediately. Solid metal forms a skin that traps gas inside before it can escape.
How Can You Eliminate Die Casting Coils?
Fixing die casting coils requires a systematic approach. Address each cause area with specific solutions.
Optimize Your Injection Parameters
The injection profile is your first and best tool.
Use three-stage injection:
- Slow start (1–2 m/s): Metal enters the cavity smoothly, no splashing
- Fast fill (4–6 m/s): Main cavity fills quickly while metal is hot
- Slow finish (1–3 m/s): Smooth transition to pressurization, no turbulence
Match speed to geometry:
- Thin walls: Lower speed (3–4 m/s) prevents premature solidification
- Thick sections: Moderate speed (5–6 m/s) with buffers to reduce impact
Extend holding time: After fill, apply 1.5× working pressure for 2–5 seconds. This compresses existing bubbles and pushes metal into shrinking areas.
Real result: A manufacturer of pump housings reduced porosity defects from 18% to 4% by:
- Lowering injection speed from 7 m/s to 4.5 m/s
- Adding 3 seconds to holding time
- Implementing three-stage injection control
Improve Mold Exhaust
Your mold design determines whether air can escape.
Add serpentine exhaust grooves: Place them at the last places to fill. Make depth at least 0.1mm. The winding path gives air time to exit while letting metal slow down.
Install vacuum assist: For complex parts, pull vacuum in the cavity before injection. Targets: -90kPa or better. This removes over 90% of the air before metal enters.
Fix your gating: Use inclined sprues or tangential inlets that let gas separate from metal naturally. Add buffer zones where flow can smooth out.
Polish out dead zones: Sharp corners and sudden transitions create vortices. Radius all corners to at least 1mm to keep flow attached.
Case study: An electronics manufacturer struggled with pinholes in magnesium phone frames. Adding vacuum assist and optimizing exhaust grooves cut defects from 15% to 2%. Yield hit 98% for the first time.
Control Melt Quality Strictly
Clean metal is non-negotiable for defect-free castings.
Degas aggressively: Use rotary degassing with argon or nitrogen. Target gas content below 0.15ml/100g Al. Test every shift with a gas analyzer.
Dry your materials:
- Bake scrap returns at 120–200°C for 4–6 hours
- Store ingots indoors, away from moisture
- Cover furnaces when not in use
Clean regularly: Remove dross and oxides from the furnace surface every 8–12 hours. These trap moisture and reintroduce gas.
The standard: Aerospace foundries maintain melt quality below 0.10ml/100g. Automotive should aim for 0.15ml max. Above 0.20ml, porosity is guaranteed.
Consider Advanced Technologies
For critical parts, basic improvements may not be enough.
Vacuum die casting pulls air from the cavity before injection. Reduces internal porosity by 90%+ . Essential for:
- Automotive powertrain parts
- Pressure-tight components
- Parts that will be heat treated or welded
Semi-solid die casting uses partially solidified slurry instead of fully liquid metal. The thick, toothpaste-like flow traps almost no air. Combines forging density with casting complexity.
Local pressurization: Hydraulic pins in the mold apply pressure to thick sections during solidification. Squeezes bubbles before they form.
Cost-benefit: Vacuum adds $20,000–50,000 to equipment cost but can cut scrap by 80% on high-value parts. Payback is often under 6 months.
Real-World Success Stories
Case 1: Automotive Gearbox Housing
The problem: A major supplier had 12% scrap from internal porosity in aluminum gearbox housings. Leaks caused failures in the field.
The fix:
- Redesigned sprue from straight to spiral (reduced turbulence)
- Added three-stage exhaust grooves plus vacuum assist
- Dropped injection speed from 6 m/s to 4.5 m/s
- Extended holding time by 3 seconds
The results:
- Internal porosity down 82%
- Tensile strength up 15%
- All housings passed 1.2MPa pressure test for 5 minutes
- Scrap rate dropped to 0.8%
Case 2: Mobile Phone Middle Frame
The problem: Magnesium phone frames had surface pinholes from internal gas. Yield was stuck at 85%.
The fix:
- Added local pressurized pins in thin-walled areas
- Switched to argon-protected casting (reduced air contact)
- Optimized cooling to slow surface solidification (gave gas time to escape)
The results:
- Surface pinholes eliminated completely
- Yield hit 98%
- Production increased 15% due to less rework
FAQ About Die Casting Coils
Can you detect die casting coils without cutting parts apart?
Yes. X-ray inspection is the standard method. It reveals the location and size of internal pores with over 95% accuracy. For mass production, automated X-ray lines scan every part at production speed. Ultrasonic testing also works by detecting how sound waves reflect off internal voids.
Will reducing injection speed always fix the problem?
No—it is not that simple. While excessive speed causes turbulence, too slow (under 2 m/s) leads to incomplete filling or cold shuts. The key is matching speed to geometry. Use three-stage profiles and adjust based on where defects appear.
Is vacuum die casting worth the extra cost?
For critical parts, absolutely. Vacuum adds 10–15% to equipment cost but can cut porosity by 90%. For high-value components like engine blocks or transmission housings, the payback comes from lower scrap and fewer warranty claims. For simple decorative parts, optimized exhaust may be enough.
What is the maximum acceptable gas content in aluminum?
For quality castings, keep hydrogen below 0.15ml/100g Al. For aerospace or pressure-tight parts, aim for 0.10ml or less. Above 0.20ml, you will fight porosity no matter what else you do.
Can die casting coils be repaired?
Sometimes, but it is not ideal. Impregnation seals surface-connected pores with resin—works for pressure-tightness but does not restore strength. Welding can fill voids but risks distortion and new defects. Prevention is always better and cheaper.
Do different alloys have different porosity risks?
Yes. Aluminum-magnesium alloys (like A535) are most prone because magnesium increases hydrogen absorption. Aluminum-silicon alloys (like A380) are more forgiving. Zinc alloys have lower melting points and less gas pickup—porosity is usually from trapped air, not dissolved gas.
Conclusion
Die casting coils are serious defects that hide inside your parts, waiting to cause failure. They form when air or gas gets trapped during solidification—either from turbulent flow, poor exhaust, contaminated metal, or wrong process settings.
The solutions are clear:
- Control injection with three-stage profiles and appropriate speeds
- Improve mold exhaust with serpentine grooves, vacuum assist, and smooth flow paths
- Manage melt quality through degassing, drying materials, and regular cleaning
- Use advanced methods like vacuum or semi-solid casting for critical parts
Real-world examples prove these work. Gearbox housings went from 12% scrap to 0.8%. Phone frames hit 98% yield. The investment in prevention pays back quickly through lower scrap, fewer warranty claims, and happier customers.
The key is treating die casting coils as a symptom of systemic issues—not a random defect. Look at your whole process, find the root causes, and fix them systematically. Your parts will be stronger, your customers will be happier, and your bottom line will thank you.
Discuss Your Die Casting Projects with Yigu Rapid Prototyping
At Yigu Rapid Prototyping, we have helped dozens of manufacturers eliminate die casting coils and other internal defects. Our team combines decades of hands-on experience with advanced simulation and testing tools to find and fix problems fast.
Whether you need:
- Porosity analysis for a problematic part
- Process optimization to reduce scrap
- Mold flow simulation to predict defects before cutting steel
- Vacuum die casting for critical components
- 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 internal defects and make your die casting better.