What Is Permanent Mold Casting? (And Why Names Get Confusing)
Permanent mold casting is a metal casting process where molten aluminum is poured into a reusable mold made from steel or cast iron, relying on gravity to fill the mold cavity. The mold (also called a "die" in some regions) is not consumed in the process鈥攊t is used for thousands of cycles.
Here is where the naming confusion starts:
| Term | Region/Context | Same Process? |
|---|---|---|
| Permanent mold casting | North America (ASTM, AFS) | �?Yes |
| Gravity die casting | Europe, UK, Asia (EN standard) | �?Yes鈥攕ame process |
| Gravity casting | Common shorthand worldwide | �?Yes鈥攕ame process |
| Die casting | General usage | �?No鈥攗sually refers to **high-pressure die casting (HPDC)** |
Permanent mold casting and gravity die casting are the same process. The American Foundry Society (AFS) and ASTM standards use "permanent mold casting." European and Asian standards (e.g., EN 1706) more commonly use "gravity die casting." When a Chinese or European supplier says "gravity casting," they almost always mean this process.
"Die casting" without a qualifier almost always means high-pressure die casting (HPDC)鈥攁 fundamentally different process that uses high-speed injection (up to 100 m/s gate velocity) to force molten aluminum into a steel die. Do not confuse the two. They produce castings with very different properties.
How the Process Works
The permanent mold casting process follows these steps:
- •Mold preparation. The steel mold halves are preheated to 200�?50掳C and coated with a ceramic-based release agent (mold wash). This controls solidification rate and protects the mold surface.
- •Pouring. Molten aluminum (typically 680�?40掳C depending on alloy) is poured into the mold under gravity. No external pressure is applied鈥攖he metal fills the cavity by its own weight and metallostatic head.
- •Solidification. The metal solidifies from the mold walls inward. Because the steel mold conducts heat much faster than sand, solidification is rapid, producing a fine-grained, dense microstructure.
- •Mold opening and part ejection. Ejector pins or mold tilting release the casting. Cycle times typically range from 2 to 5 minutes depending on part size and wall thickness.
- •Optional sand cores. When the part requires internal cavities or undercuts that cannot be formed by the mold halves alone, sand cores are placed inside the mold before pouring. This variant is called semi-permanent mold casting.
Permanent Mold Casting vs. Other Processes: A Direct Comparison
The real question for most procurement professionals is not "what is permanent mold casting?" but "when should I choose it over the alternatives?" Let us compare it head-to-head against the three processes you are most likely evaluating.
Permanent Mold vs. Sand Casting
Sand casting uses expendable molds made from bonded sand. Each mold is destroyed after a single pour.
| Factor | Permanent Mold Casting | Sand Casting |
|---|---|---|
| Mold material | Reusable steel/iron mold | Expendable sand mold |
| Tooling cost | $8,000�?40,000 | $1,500�?10,000 (pattern + core boxes) |
| Surface finish (as-cast) | 3.2�?.3 渭m Ra | 6.3�?5 渭m Ra |
| Dimensional tolerance | �?.3�?.5 mm (per ASTM B108) | �?.8�?.5 mm |
| Minimum wall thickness | 4�? mm | 5�? mm |
| Grain structure | Fine (fast solidification) | Coarse (slow solidification) |
| Mechanical properties | 15�?5% higher than sand (same alloy, same heat treatment) | Baseline |
| Porosity level | Low | Moderate to high |
| Ideal volume range | 500�?0,000+/year | 1�?,000/year |
| Lead time (first article) | 6�?0 weeks (mold build) | 3�? weeks (pattern build) |
When permanent mold beats sand casting:
- •You need better surface finish without excessive post-machining. Permanent mold castings come out of the mold at 3.2�?.3 渭m Ra, cutting grinding and polishing time.
- •You need tighter tolerances to reduce CNC machining stock (saving cycle time and material).
- •You need higher mechanical properties. The faster solidification in a metal mold produces finer dendrite arm spacing (DAS), per the Hall-Petch relationship, directly improving strength and ductility. According to ASM Handbook Volume 15 (Casting), A356-T6 permanent mold castings typically achieve 15�?5% higher tensile strength than identical alloy sand castings.
- •Your annual volume justifies the mold investment. Above roughly 500 pieces, the per-part cost drops below sand casting because the reusable mold eliminates per-cycle mold-making labor and material.
When sand casting still wins:
- •Very low volumes (under 200�?00 parts total)
- •Extremely large parts (sand casting handles parts over 500 kg more readily)
- •Early-stage prototyping where design changes are expected (modifying a sand pattern costs less than modifying a steel mold)
Permanent Mold vs. High-Pressure Die Casting (HPDC)
HPDC injects molten aluminum into a steel die at pressures of 70�?00 MPa and gate velocities up to 30�?00 m/s.
| Factor | Permanent Mold Casting | High-Pressure Die Casting |
|---|---|---|
| Fill mechanism | Gravity (no pressure) | High-pressure injection (70�?00 MPa) |
| Cycle time | 2�? minutes | 30�?0 seconds |
| Tooling cost | $8,000�?40,000 | $25,000�?150,000+ |
| Tooling lead time | 4�? weeks | 8�?6 weeks |
| Wall thickness range | 4�?5 mm | 1.0�? mm |
| Porosity | Low (gravity fill, controlled) | Higher (turbulent fill, trapped gas) |
| Heat treatable? | Yes (T6 standard for A356) | Generally no (porosity causes blistering) |
| Mechanical properties (A356-T6 vs ADC12-F) | UTS 260�?10 MPa, elongation 3�?% | UTS 228�?60 MPa, elongation 1�?% |
| Weldability | Good | Poor (porosity issues) |
| Ideal volume range | 500�?0,000/year | 10,000�?,000,000+/year |
| Post-machining needs | Moderate | Low (tight as-cast tolerances) |
When permanent mold beats HPDC:
- •Structural integrity is critical. Gravity-filled permanent mold castings have dramatically lower porosity than HPDC parts. This means they can be heat-treated (T6), welded, and used in pressure-containing applications. HPDC parts typically cannot鈥攖he trapped gas porosity expands during solution heat treatment, causing blisters.
- •Production volumes are moderate. Below 10,000�?0,000 units/year, permanent mold casting's lower tooling cost results in lower total cost. HPDC only becomes economical at higher volumes where the fast cycle times amortize the tooling investment.
- •Wall thickness exceeds 5�? mm. HPDC is optimized for thin walls. Thick-section HPDC parts suffer from excessive internal porosity (shrinkage and gas) because the outer skin solidifies rapidly while the interior remains molten. Permanent mold casting, with its slower, directional solidification, handles thick sections far better.
- •The part must be pressure-tight without impregnation (e.g., hydraulic manifolds, compressed air fittings). Per ASTM E505 radiographic standards, permanent mold castings routinely meet Level 2 or better, while HPDC castings frequently require Level 3�? acceptance criteria.
When HPDC wins:
- •Volumes above 20,000 units/year where cycle time savings dominate
- •Thin-walled parts (1.0�?.5 mm) that gravity cannot reliably fill
- •Parts where structural demands are low (covers, housings, brackets that do not bear significant load)
- •Complex geometries with many features that would require extensive machining if gravity-cast
Permanent Mold vs. Low-Pressure Die Casting (LPDC)
Low-pressure die casting uses a sealed furnace below the mold. Compressed air (0.3�?.0 bar) pushes molten aluminum up through a riser tube into the mold cavity.
| Factor | Permanent Mold (Gravity) | Low-Pressure Die Casting |
|---|---|---|
| Fill mechanism | Gravity pour | Low pressure from below (0.3�?.0 bar) |
| Fill control | Good (tilt-pour improves it) | Excellent (controlled, non-turbulent) |
| Cycle time | 2�? minutes | 4�? minutes |
| Tooling cost | $8,000�?40,000 | $15,000�?60,000 |
| Porosity | Low | Very low |
| Yield (metal utilization) | 60�?5% | 80�?0% |
| Ideal applications | General structural castings | Wheels, symmetrical parts, premium castings |
| Equipment cost | Lower | Higher (sealed furnace, pressure system) |
When permanent mold beats LPDC:
- •Asymmetric or complex geometries. LPDC works best with parts that have a central gate and relatively symmetrical geometry (classic example: automotive wheels). Gravity casting accommodates off-center gating and complex runner systems more flexibly.
- •Lower equipment investment. Gravity casting cells are simpler and cheaper to set up than LPDC systems.
- •Faster cycle times. Gravity casting is typically 30�?0% faster than LPDC, beneficial for cost-sensitive medium-volume production.
When LPDC wins:
- •Extremely low porosity requirements (safety-critical components like wheel rims per SAE J452)
- •Very high metal yield is critical to cost control (LPDC returns unused metal to the furnace)
- •Parts with symmetrical geometry that suit bottom-gating
The Sweet Spot: Where Permanent Mold Casting Excels
Based on the comparisons above, permanent mold aluminum casting occupies a clear niche. It is the optimal choice when your application meets most of these criteria:
Volume: Medium Production Runs
The process is most cost-effective for 500 to 50,000 parts per year. Below 500, sand casting's lower tooling cost may win. Above 50,000, die casting's cycle time advantage starts to dominate (though many programs run permanent mold at higher volumes when properties demand it).
Strength: High Mechanical Performance Required
When parts must meet demanding mechanical specifications鈥攑articularly after T6 heat treatment鈥攑ermanent mold casting with alloys like A356 delivers:
- •Tensile strength: 260�?10 MPa
- •Yield strength: 185�?30 MPa
- •Elongation: 3�?%
These numbers satisfy automotive structural standards like IATF 16949 requirements for safety-relevant components.
Surface Quality: Better Than Sand, Close to Die Casting
As-cast surface finish of 3.2�?.3 渭m Ra means less post-processing. For parts with cosmetic or sealing surfaces, this is a significant cost and quality advantage over sand casting.
Integrity: Pressure-Tight and Weldable
Permanent mold castings can be:
- •Pressure-tested to customer specifications without vacuum impregnation
- •Welded using TIG or MIG (common for repair or joining)
- •X-ray inspected to meet ASTM E505 Level 2 or better radiographic quality
Cost Structure: What You Are Actually Paying For
Understanding the cost structure helps you evaluate supplier quotes accurately and avoid comparing apples to oranges.
Tooling Costs
| Component | Typical Cost Range | Notes |
|---|---|---|
| Steel mold (2-piece, simple) | $8,000�?15,000 | Small-to-medium parts, no slides |
| Steel mold (complex, with slides) | $15,000�?30,000 | Multiple slides, complex parting |
| Steel mold (large, multi-cavity) | $25,000�?40,000 | Large parts or multi-impression |
| Sand core tooling (if needed) | $2,000�?8,000 | Per core box |
| Mold life | 30,000�?0,000 shots | Depends on alloy, part geometry, maintenance |
Mold cost is a one-time investment amortized over the production life. For a $20,000 mold producing 20,000 parts, the tooling amortization is $1.00/part. Over 50,000 parts, it drops to $0.40/part.
Unit Cost Components
| Component | % of Unit Cost (Typical) | Notes |
|---|---|---|
| Raw material (aluminum ingot) | 30�?5% | Fluctuates with LME aluminum price |
| Casting labor + overhead | 20�?0% | Includes mold handling, pouring, inspection |
| Heat treatment (T6) | 8�?5% | Solution treatment + aging furnace cycles |
| CNC machining | 15�?0% | Depends on number of machined features |
| Surface treatment | 5�?0% | Powder coating, anodizing, or painting |
| Quality inspection | 3�?% | CMM, X-ray, pressure testing |
Total Cost Example
For a 2 kg A356-T6 gravity casting with moderate machining complexity at 5,000 units/year:
| Item | Estimated Cost |
|---|---|
| Casting (including material, labor, overhead) | $6.50�?9.00 |
| T6 heat treatment | $1.00�?1.50 |
| CNC machining (3 setups) | $3.00�?5.00 |
| Surface treatment (powder coat) | $0.50�?1.00 |
| Inspection and packaging | $0.50�?0.80 |
| **Total unit cost** | **$11.50�?17.30** |
| Tooling (amortized over 5,000 pcs) | $3.00�?5.00/pc |
These figures are illustrative. Actual pricing depends on part geometry, tolerance requirements, alloy, and supplier location. Chinese manufacturers with in-house capabilities across all these operations typically offer 20�?0% lower total costs than equivalent European or North American suppliers, with competitive quality when properly qualified.
Real-World Application Examples
Example 1: Automotive Suspension Crossmember
- •Material: A356-T6
- •Part weight: 4.5 kg
- •Annual volume: 15,000 units
- •Why permanent mold: The part is safety-critical (structural suspension component), requiring high fatigue strength, T6 heat treatment, and IATF 16949 quality certification. HPDC cannot deliver the required ductility (minimum 5% elongation spec). Sand casting's surface finish would require excessive machining.
- •Result: Permanent mold casting delivered all mechanical requirements with as-cast surfaces meeting sealing specs, reducing total machining time by 35% compared to the previous sand-cast version.
Example 2: Industrial Hydraulic Manifold Block
- •Material: A356-T6
- •Part weight: 3.2 kg
- •Annual volume: 3,000 units
- •Why permanent mold: Internal pressure rating of 25 bar required zero porosity in the flow passages. The part needed pressure testing to 37.5 bar (1.5�?service pressure) per customer specification. Semi-permanent mold casting with sand cores formed the internal channels.
- •Result: 99.4% first-pass pressure test rate, eliminating the vacuum impregnation step that was previously required with the sand-cast version.
Example 3: EV Motor Housing
- •Material: A356-T6
- •Part weight: 6.8 kg
- •Annual volume: 8,000 units
- •Why permanent mold: The motor housing requires high thermal conductivity (151 W/m路K for A356 vs. 96 W/m路K for ADC12) to dissipate heat from the electric motor. It also integrates cooling jacket passages formed by sand cores, and the mounting interfaces require structural integrity for NVH (noise, vibration, harshness) performance.
- •Result: Gravity casting with sand cores achieved all thermal and structural requirements at a lower total cost than the LPDC alternative that was also evaluated (20% lower tooling cost, 15% faster cycles).
Example 4: LED Streetlight Heat Sink Housing
- •Material: A356-T61 (modified aging for higher conductivity)
- •Part weight: 2.1 kg
- •Annual volume: 12,000 units
- •Why permanent mold: The heat dissipation fins require good thermal performance and corrosion resistance (outdoor installation, 15-year life expectancy). The moderate volume does not justify HPDC tooling. Surface finish must be cosmetically acceptable for powder coating.
- •Result: Permanent mold casting provided the balance of thermal performance, corrosion resistance, surface quality, and cost that neither sand casting nor die casting could match at this volume.
Frequently Asked Questions
1. Is "permanent mold casting" the same as "gravity die casting"?
Yes. These are two names for the same process. "Permanent mold casting" is the standard term in North America (per AFS and ASTM), while "gravity die casting" is more common in Europe, the UK, and Asia. When your supplier says "gravity casting," they almost certainly mean this process. If they say "die casting" without specifying gravity, ask for clarification鈥攖hey likely mean high-pressure die casting, which is a different process with different properties.
2. What alloys work best with permanent mold casting?
The most widely used alloy is A356 (AlSi7Mg0.3), which offers the best combination of castability, strength (after T6), and ductility. Other common choices include:
- •A357 (AlSi7Mg0.5): Higher magnesium for higher strength; used in aerospace and demanding structural applications.
- •ZL114 (Chinese standard, similar to A357): Common in Chinese foundries for high-performance castings.
- •535 (AlMg3.5): Excellent corrosion resistance without heat treatment; used in marine applications.
Alloys with high silicon content (>10% Si, like ADC12) are generally not used in permanent mold casting because they are formulated for high-pressure die casting.
3. What is the minimum order quantity (MOQ) for permanent mold castings?
There is no universal MOQ, but practical economics set a floor. Most foundries prefer a minimum first order of 300�?00 pieces to justify mold setup, preheating, and process qualification. For repeat orders, quantities can be lower (100�?00 pieces per batch). However, below about 200 total lifetime pieces, sand casting may be more economical because the mold investment cannot be adequately amortized.
4. How long does it take from order to first delivery?
A typical timeline for a new permanent mold casting project:
- •Mold design and review: 1�? weeks
- •Mold manufacturing: 4�? weeks
- •Trial casting and adjustment (T1 samples): 1�? weeks
- •Sample approval and PPAP: 1�? weeks
- •First production delivery: 2�? weeks after approval
Total: approximately 8�?3 weeks from order to first production shipment. Repeat orders typically ship in 3�? weeks depending on quantity.
5. Can permanent mold castings pass automotive quality standards?
Absolutely. Permanent mold castings are used extensively in automotive applications including safety-critical components. Qualified manufacturers operate under:
- •IATF 16949 quality management for automotive
- •ASTM B108 casting standard for permanent mold aluminum alloy castings
- •ASTM E505 radiographic inspection standards
- •Customer-specific PPAP (Production Part Approval Process) requirements
The process's inherent low porosity and consistent repeatability make it well-suited for the statistical process control (SPC) requirements of automotive OEMs and Tier 1 suppliers.
6. How does permanent mold casting compare on sustainability?
Permanent mold casting has favorable sustainability characteristics:
- •Mold reuse: One steel mold replaces 30,000�?0,000 sand molds, eliminating sand waste and binder chemicals.
- •Recyclability: Aluminum is 100% recyclable. Production scrap (runners, risers) is remelted in-house.
- •Energy efficiency: Shorter cycle times than sand casting reduce energy consumption per part.
- •Reduced machining: Better as-cast tolerances mean less material removed (and less energy consumed) in secondary machining.
Choose the Right Casting Process for Your Application
Permanent mold aluminum casting is not the right process for every part. But when your application demands the combination of structural integrity, medium production volumes, good surface finish, and heat-treatable performance, it consistently outperforms the alternatives.
The key is working with a manufacturer that understands the process boundaries and will recommend the right approach鈥攅ven if it is not permanent mold casting.
Bohua Machinery operates permanent mold (gravity) casting, high-pressure die casting, and CNC machining under one roof. We cast A356, A357, ZL114, and ADC12 alloys daily and deliver fully finished components鈥攈eat-treated, machined, surface-treated, and inspected鈥攖o automotive and industrial customers worldwide.
#0f1e3d]">Need help choosing the right casting process? [Contact our engineering team for a free process evaluation and quote.