ТЕХНИЧЕСКИЙ ЦЕНТР 7 Сен · 2:45 дп
A Comprehensive Technical Overview
Aluminum Foundry · Engineering & Procurement Reference
1. What Is a Casting Mold / Tooling
A casting mold (also called a pattern or die) is the collective term for the tooling used to form the mold cavity, guide the molten metal during filling, and control the temperature field and shrinkage as the casting solidifies.
Core Functions
- Forming: replicates the part geometry — the cavity is the negative of the product shape.
- Filling: the gating system guides the metal into the cavity smoothly.
- Solidification control: risers and chills govern directional solidification and feeding.
- Dimensional assurance: determines casting tolerance, surface finish, and machining allowance.
2. Classification by Casting Process (Selection Core)
| Тип | Mold Material | Tool Life | Cost | Finish / Tolerance | Batch | Typical Parts |
| Sand Casting | Wood/resin/Al + sand | Single-use | Very low | Ra 12.5 / CT8-10 | One-off~small | Blocks, beds, large parts |
| Permanent Mold | HT250, ductile iron, H13 | 50-100k | USD 4-21k | Ra 3.2 / CT6 | Medium~large | Wheels, motorcycle parts |
| Die Casting | H13 / SKD61 | 200-500k | USD 14-140k | Ra 1.6 / CT4 | High volume | Housings, trans. cases |
| Low Pressure | Metal mold (H13/iron) | Tens of thousands | Medium | CT6 | Medium~large | Wheels, cylinder heads |
| Investment | Wax + ceramic shell | Single-use shell | Medium | Ra 1.6 / CT5 | Small~medium | Turbine blades, precision |
| Lost Foam | EPS foam + dry sand | Single-use | Умеренный | Good | Small~medium | Complex structures |
| Centrifugal | Metal mold (pipe mold) | Long | High | Rough bore | High volume | Pipes, liners, bushings |
Selection Logic (One-Line Rule)
- Batch < 50 pcs / highly complex cavities → sand casting (or 3D-printed sand mold).
- Batch 50-5,000 pcs / non-ferrous alloys → permanent mold / low pressure.
- Batch > 5,000 pcs / thin-wall precision → die casting.
3. Core Elements of Mold Structure Design
3.1 Gating System
The gating system is the channel that guides molten metal into the cavity. It consists of four parts:
- Pouring cup (sprue base): receives the metal, traps slag, acts as a buffer.
- Down-sprue (vertical): directs the flow downward.
- Runner (horizontal): distributes the flow and traps slag (ΣF_runner > ΣF_gate).
- In-gate: connects directly to the cavity; controls filling speed and direction.
Key principles: slag trapping, smooth filling, prevention of mold-wall erosion, and simple material-efficient geometry.
3.2 Riser / Feeder
A riser stores molten metal and compensates for solidification shrinkage — it also handles venting and slag collection.
- Placement: above or beside the last-to-solidify hot spot (directional solidification).
- Neck size: 1.2-1.5 × the hot-spot diameter.
- Types: open (good feeding, more scrap), blind (slower cooling, higher efficiency), insulating / exothermic (smaller volume, higher yield).
3.3 Chill
A chill is a chilling mass that locally accelerates cooling — it is the steering wheel for controlling solidification sequence.
- Function: eliminate hot spots, drive the thermal gradient toward the riser, prevent shrinkage porosity.
- Material: cast iron, copper alloy.
- Thickness: typically 0.8-1.2 × the wall thickness of the casting.
3.4 Other Design Essentials
- Parting line: prefer a flat plane at the maximum cross-section; avoid critical machined surfaces.
- Shrinkage compensation: mold dimension = casting dimension × (1 + linear shrinkage); gray iron ≈ 0.8-1.2%.
- Draft, machining allowance, fillet: ensure easy withdrawal and reduce stress concentration.
- Venting & coating: metal molds need vent slots; cavities are coated with graphite / zirconia-based coatings.
4. Mold Material Selection
4.1 Patterns / Pattern Plates (Sand Molding)
- Wood pattern: low cost, prototyping / small batches.
- Resin-sand / aluminum pattern: small-medium batches, better accuracy.
- 3D-printed sand mold: no pattern required — CAD direct-to-mold, ideal for complex parts and small batches.
4.2 Metal Molds / Die-Casting Dies
| Материал | Characteristics | Заявка |
| HT250 / HT300 (gray iron) | Cheap, good thermal conductivity & damping; poor thermal fatigue | Simple small parts, low volume |
| QT500 / QT600 (ductile iron) | Good strength & toughness, resists cracking; economical | Medium-large permanent molds |
| H13 (4Cr5MoSiV1) | Most common hot-work steel; balanced thermal fatigue & toughness; retains hardness at 600°C | Mainstream Al die casting / permanent molds |
| SKD61 (JIS H13) | Equivalent to H13 | Widely used |
| 3Cr2W8V | Higher red hardness, better erosion resistance; lower toughness | Copper alloys, high-temp molds |
| 1.2344 / 8407 / DAC | ESR-refined H13 variants, high purity | High-end, high-cycle, complex cavities |
| BeCu / CuCrZr | Extremely high thermal conductivity; local chill inserts | Eliminating shrinkage porosity |
Selection basis: production volume, casting material (melting point), part size & complexity, and required service life.
5. Manufacturing Process & Cost Comparison
5.1 Traditional Sand Mold vs. 3D-Printed Sand Mold
| Dimension | Traditional Sand Mold | 3D-Printed Sand Mold |
| Lead time | 6-12 weeks (pattern making) | 2-7 days |
| NRE cost | USD 700-4,200+ | USD 70-700 (digital prep) |
| Design changes | High (new pattern needed) | Very low (edit CAD) |
| Dimensional accuracy | ±1.0 mm | ±0.3 mm |
| Economics per piece | Better for high volume | Optimal for small-medium (1-100 pcs) |
| Break-even point | ≈ 50-1,500+ pcs | High ROI on complex parts (first article pays back) |
Trend: 3D printing solves speed + complexity; traditional wood / metal patterns solve high volume + low cost — the two are complementary, not substitutive.
5.2 Cost Breakdown
- In sand casting, the pattern (mold tooling) accounts for 30-50% of total cost.
- Machining methods: CNC milling, EDM, 3D printing, manual carving.
6. Common Defects & Mold Countermeasures
| Defect | Cause | Mold / Process Countermeasure |
| Shrinkage cavity / porosity | Insufficient feeding, broken directional solidification | Add risers, chills, insulating risers |
| Gas holes / slag inclusion | Poor venting, poor slag trapping | Vent risers, runner traps, filter mesh |
| Cold shut / fold | Improper filling speed | Optimize gating, raise pouring temperature |
| Crack | Hot spot, poor collapsibility | Add chills, optimize fillet, allow collapsibility |
| Sticking / scoring | Mold surface / coating issue | Apply coating, polish, select proper mold steel |
7. Recommended Focus for an Aluminum Foundry (Application Notes)
- Broad process coverage: full process chain — sand casting / permanent mold / low pressure / die casting.
- Tooling capabilities: wood, resin, and 3D-printed sand molds for rapid prototypes (2-7 day delivery); H13 permanent molds / dies (200k+ cycles).
- Material compatibility: aluminum alloys (A356, ZL101A-T6, etc.), magnesium, zinc.
- Dimensional assurance: CT6-CT8, Ra 3.2-12.5 μm, CNC-machined datum surfaces.
- Engineering service: gating / riser / chill design and simulation (e.g., MAGMA / ProCAST).
8. Summary
A casting mold is the decisive piece of tooling that bridges part design and molten metal. The correct process and material choice — driven by batch size, alloy, geometry, and required mold life — directly determines casting quality, lead time, and unit economics.
For aluminum foundries, the practical default is: use sand / 3D-printed sand molds for prototyping and low-volume complex parts; move to H13-based permanent molds and dies as volume ramps up. Mastering riser, chill, and gating design is the key to controlling shrinkage, porosity, and dimensional accuracy across every process.