casting-forging

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.