sand-casting

name	sand-casting
description	Sand casting process — gating system design, riser sizing (Chvorinov's rule, modulus method), shrinkage, casting defects, pattern allowances, green sand vs no-bake, tolerances.
metadata	{"priority":7,"promptSignals":{"phrases":["sand casting","casting gating","riser design","Chvorinov","casting defects","pattern allowance","green sand"],"minScore":3}}

Sand Casting — Complete Skill

Process Overview

Sand mold formed around pattern → removed → cavity filled with molten metal → solidifies → shakeout

Types of sand processes:

• Green sand: clay-bonded, wet; most common; fast, low cost
• No-bake (furan, phenolic): chemically bonded; better dimensional accuracy; slower
• Shell mold: pre-coated sand heated on pattern; good surface finish
• Coldbox (PUCB): gas-cured binder; fast; used in automotive (cores)

Pattern Allowances

Shrinkage allowance: pattern larger than final casting by shrinkage amount

Metal	Shrinkage allowance
Gray cast iron	1.0% (8.3 mm/m) — graphite expansion partially compensates
White cast iron	2.0%
Steel (carbon)	2.0%
Aluminum	1.3–1.6%
Copper alloys	1.6%
Magnesium	1.3%

Draft angle: 1–3° on vertical surfaces (allows pattern removal) Machining allowance: 1.5–6 mm (added where surfaces are to be machined) Distortion allowance: for complex shapes that warp during cooling

Gating System Design

Pressurized vs. unpressurized: Pressurized: small gates, large sprue → back pressure; controls flow; less turbulence Unpressurized: gates larger than sprue → stream fills gates; good for Al

Gating ratio: A_sprue : A_runner : A_gate Pressurized (Al): 1:2:1.5 or 1:3:3 Unpressurized (Fe): 1:1.5:2 or 1:2:4

Sprue design: Tapered to prevent aspiration; A_sprue,bottom = A_sprue,top × √(h_top/h_bottom) (Bernoulli-based taper to maintain full flow)

Runner: trapezoidal cross-section preferred; extends beyond last gate (runner extension traps dross)

Ingate velocity: v_gate = √(2g h_metal) [m/s; h_metal = effective metallostatic head] Target: v_gate < 0.5 m/s (Al); < 2 m/s (iron) to reduce turbulence

Riser Design

Risers compensate for volumetric shrinkage as metal solidifies

Chvorinov's Rule

Solidification time: t_s = B × (V/A)² [= B × (modulus)²] B = mold constant [min/cm²]; depends on metal + mold type

Modulus method: M_casting = V/A = casting modulus [cm] M_riser must > M_casting × 1.2 (riser solidifies last → feeds casting)

Riser shapes: Cylindrical riser: M = (π r² h) / (2π r h + π r²) = r h / (2h + r) For slender (h > r): M ≈ r/2 Sphere: M = r/3 (most efficient; rarely practical)

Riser sizing for feeding: V_riser ≥ S_v × V_casting S_v = volumetric shrinkage (steel: 4–7%; Al: 3–5%; cast iron: 1–2%)

Blind riser: more efficient (insulated top); extends feeding range Exothermic sleeves: dramatically extend feeding range by reducing heat loss

Feeding Distance

Region fed by riser: end effect + riser effect Feeding distance (steel plate): L_feed ≈ 4.5 T (T = section thickness) from riser Difficult sections: need additional risers or chills

Chills: Inserted metal blocks in mold → increase local cooling rate → control solidification direction Internal chills: become part of casting External chills: in mold face adjacent to casting

Casting Defects

Shrinkage

Macro-shrinkage (pipe): centerline cavity; improper riser Micro-shrinkage (micro-porosity): dispersed voids; mushy solidification zone Fix: redesign riser (size + location); directional solidification; chills

Porosity

Gas porosity: dissolved gas (H₂ from moisture in steel/Al; N₂); spherical pores Shrinkage porosity: irregular; interdendritic Fix: degas melt (rotary degassing for Al); dry molds and ladles

Sand-Related Defects

Penetration: metal penetrates sand → rough surface (undercounted mold strength) Inclusion: sand particles entrapped → hard spots in casting Blow holes: steam/gas from binder → pockets near surface

Solidification Defects

Hot tear (hot cracking): tensile stress during solidification → crack at grain boundary Cold shut: two metal streams meet but fail to fuse (temperature too low; redesign gate) Misrun: metal solidifies before filling mold (increase temperature or gate size)

Dimensional Tolerances

Green sand: ±1.5–6 mm (general); better with good tooling No-bake: ±0.8–3 mm Shell mold: ±0.5–1.5 mm Investment casting: ±0.1–0.3 mm (higher cost)

Output

Provide: gating ratio recommendation, A_sprue [cm²], M_riser [cm], riser diameter × height [mm], feeding distance check, shrinkage allowance [%], defect risk assessment (shrinkage/porosity/cold shut), draft angles, machining allowance.