| name | casting-forging |
| description | Casting and forging design rules — draft angles, parting lines, shrinkage allowances, wall thickness, gating, risers, forging flow lines, die design considerations, defects and prevention. |
| metadata | {"priority":5,"promptSignals":{"phrases":["casting","forging","die casting","sand casting","investment casting","draft angle","parting line","riser","shrinkage"],"minScore":4}} |
Casting and Forging Design — Complete Skill
Casting Processes Comparison
| Process | Material | Tolerance | Surface | Min Wall | Production Rate | Cost |
|---|
| Sand casting | Most metals | ±1.5-3mm | Rough | 4-6mm | Low-medium | Low tooling |
| Die casting | Al, Zn, Mg | ±0.1-0.3mm | Good | 0.8-3mm | Very high | High tooling |
| Investment (lost wax) | Most metals | ±0.1-0.3mm | Excellent | 0.8-1.5mm | Medium | High |
| Permanent mold | Al, Cu | ±0.3-1mm | Good | 2-4mm | Medium-high | Medium |
| Centrifugal | Fe, Cu | ±1mm | Good | 6mm | Medium | Medium |
| Squeeze casting | Al | ±0.2mm | Good | 3mm | Medium | High |
Casting Design Rules
Draft Angles (Taper for Pattern Removal)
External surfaces: 1-3° (minimum 0.5° for die casting)
Internal surfaces (cores): 2-5°
Die casting: 1-2° external, 2-3° internal
Parting Line
Where mold separates — keep straight when possible
Design parting line at widest cross-section perpendicular to draw direction
Avoid features crossing parting line (flash location)
Wall Thickness
Uniform wall preferred (avoids hot spots, reduces shrinkage)
Sand casting: 4-6mm min for iron, 3-5mm for aluminum
Die casting: 1-3mm Al, 0.5-2mm Zn
Transition between thick/thin: taper over 3:1 length ratio
Rule: ratio of wall thicknesses ≤ 2:1 to avoid porosity
Shrinkage Allowances (linear, added to pattern)
| Metal | Shrinkage |
|---|
| Gray cast iron | 1.0% |
| White cast iron | 2.0% |
| Ductile iron | 0.8% |
| Steel (carbon) | 2.0% |
| Aluminum alloys | 1.3-1.6% |
| Copper alloys | 1.5-2.1% |
| Zinc | 1.3% |
Bosses and Ribs
Boss height < 2× boss diameter (avoid shrinkage cavity)
Rib thickness = 0.6-0.7× adjacent wall (avoids hot spot at junction)
Rib height: < 3× rib thickness
Rounded fillets: r ≥ 3mm (reduces stress concentration, prevents hot tears)
Avoid isolated thick sections (use coring)
Holes and Coring
Minimum cored hole diameter:
Sand casting: 20-25mm (smaller holes machined after)
Die casting: 2-3mm min, L/D ≤ 3 (blind), L/D ≤ 6 (through)
Draft on core: 0.5-2° for removal
Gating System (Sand Casting)
Pouring cup → sprue → runner → gate → casting → riser
Gating ratio: R_sprue:R_runner:R_gate (pressurized system: 1:2:1.5; unpressurized: 1:4:4)
Gate into thick sections (riser at thin/last to solidify)
Avoid turbulence: Re < 10,000 at gate
Risers (Feeding)
Function: compensate for solidification shrinkage
Modulus of riser M_r ≥ 1.2×M_casting [M = V/A_cooling, Chvorinov's rule]
Riser must solidify AFTER the section it feeds
Top riser vs. side riser: top preferred (gravity assists feeding)
Blind risers with ceramic inserts: more efficient (smaller riser volume)
Forging Design Rules
Forging Processes
Open die (smith) forging: simple shapes, large parts, low quantities
Impression die forging: moderate-complex shapes, medium-high volumes
Closed die: near-net shape, high volume, tight tolerances
Flash forgings: excess material escapes through flash land (required for fill)
Flashless (precision): no flash, near-net
Draft Angles (Die Forging)
External surfaces: 5-7° (hot, open die), 1-3° (cold, precision)
Internal pockets: 7-10° (harder to fill, more friction)
Minimum draft: 3° for aluminum, 5° for steel
Parting Line in Forging
Straight parting line: simplest tooling
Broken parting line: allows deeper features
Locate at maximum cross-section of part (facilitates die opening and flash flow)
Minimum Section Thickness
Hot steel: 3-6mm
Hot aluminum: 2-4mm
Cold steel: 1-3mm
Flash
Flash thickness: ~2-4% of die opening height
Flash gutter width: 3-6mm (holds back metal for complete fill)
Flash removal: trimming die (within 24hr for hot trim, after cooling for cold trim)
Forging Flow Lines
Forging preserves grain flow (wrought structure)
Flow lines should follow part contour (not be cut by machining)
Critical for fatigue: align flow lines with principle stress direction
Compare forged vs. machined-from-bar: forged has superior fatigue life
Common Forging Defects
Laps/cold shuts: folded surface — caused by improper flash flow or sharp corners
Underfill: incomplete die fill — increase temp, reduce speed, redesign gating
Internal cracks: excessive reduction per pass
Misalignment: die shift
Design for Strength Comparison
| Property | Cast | Forged | Wrought (machined) |
|---|
| Tensile strength | 100% | 130-150% | 110-120% |
| Fatigue life | 100% | 150-200% | 120-140% |
| Impact (Charpy) | 100% | 200-300% | 150-200% |
| Consistency | Variable | Good | Excellent |
| Cost (high volume) | Low | Medium | High |
Use forging when: fatigue, impact, or high reliability required
Use casting when: complex internal geometry, low volume, cost-sensitive
Output
Provide: draft angles, parting line recommendation, wall thickness check, minimum riser size, shrinkage allowance, key defect risks for the chosen process.
