investment-casting

name	investment-casting
description	Investment casting process design — wax pattern, ceramic shell build, dewaxing, burnout, casting, shell thickness, Chvorinov's rule, riser design, gating, solidification, dimensional tolerances, surface finish, ASTM B557/A571, aerospace/turbine blade applications, defect analysis.
metadata	{"priority":7,"promptSignals":{"phrases":["investment casting","lost wax casting","ceramic shell casting","precision casting","wax pattern casting","turbine blade casting"],"minScore":3}}

Investment Casting — Complete Skill

Process Overview

Investment casting (lost-wax process):

1. Wax injection → wax pattern
2. Wax pattern assembly onto gating/runner system (wax tree)
3. Ceramic shell build (alternating slurry dip + stucco)
4. Wax removal (dewax/autoclave)
5. Shell burnout (fire shell; remove residual wax)
6. Metal pour (gravity or vacuum)
7. Shell removal (vibration/shotblast)
8. Cutoff, grinding, inspection

Key advantage over sand casting: Tolerances: ±0.1–0.3 mm vs. ±1–3 mm (sand) Surface finish: Ra 1.5–3.2 μm (as-cast) vs. Ra 12–25 μm (sand) Thin walls: 0.5–2 mm feasible vs. 3–5 mm minimum (sand) Complexity: undercuts, internal passages (cores), near-net shape

Wax Pattern

Wax Properties and Injection

Pattern wax types: Filled wax (25–40% filler — talc, PS): lower shrinkage (0.6–1.0%); better dimensional stability; most common Unfilled wax: simpler; higher shrinkage (1.5–2.0%) Water-soluble wax: for removable cores (complex internal passages)

Shrinkage allowance: Overall die compensation = wax shrinkage + metal shrinkage Wax shrinkage: 0.6–1.5% (depends on wax type, section size, injection conditions) Metal shrinkage: 1.5–2.5% (depends on alloy; see below) Die design: oversized by (1 + wax_shrink) × (1 + metal_shrink) factor

Injection parameters: Pressure: 30–200 bar (filled wax at 60–100 bar typical) Temperature: 55–75°C (wax injection temperature; wax must be flowable) Die temperature: 20–30°C (controlled by water cooling) Cycle time: 10–60 s depending on section size

Pattern distortion: thin sections warp during cooling; die inserts needed for critical features; pattern gauging required

Wax Assembly

Runner and gating design: wax runners welded/bonded to central sprue Pattern orientation: heavy sections up (feed metal flows down from top; risers at top) Tree density: balance packing vs. shell permeability; typically 2–4 kg metal per cluster

Ceramic Shell Build

Slurry Formulation

Primary coat (face coat): fine slurry; Ra control; compatibility with alloy Binder: colloidal silica (SiO₂, 20–40% SiO₂) Refractory: zircon flour (ZrSiO₄, mesh 100–200); alumina for high-temperature alloys Viscosity: 20–30 cP (measured by #5 Zahn cup); critical — too thick → drips, too thin → thin coat

Backup coats: coarser slurry + coarser stucco for shell thickness Binder: colloidal silica (same); some proprietary alumina silicate binders Refractory: mullite (3Al₂O₃·2SiO₂), fused silica, or alumina

Stucco (after each dip): Primary: zircon or alumina (100–200 mesh) for good detail reproduction Backup: mullite, fused silica (16–30 mesh) for thermal and mechanical properties

Shell Thickness and Layers

Number of dip layers: Small parts (<1 kg): 5–7 layers (4–6 mm total shell) Medium (1–10 kg): 7–9 layers (6–10 mm) Large (>10 kg): 9–12+ layers (10–15 mm)

Shell drying: Each coat dried 2–4 hours at 20–25°C, 40–60% RH (humidity control critical — premature drying → cracking; too slow → slurry sags) Drying racks with controlled airflow; flash dryers for production

Shell strength requirements: Green strength: > 2 MPa (hold wax during dewax; handle clusters) Fired strength (after burnout): > 5 MPa (withstand metal pour static head)

Thermal Stresses in Shell

Dewax thermal shock: Wax expands before melting → shell must resist burst pressure Flash dewax (autoclave): steam 150–175°C, 5–7 bar → rapid wax melt → pressure <15 bar (wax melts before significant expansion) Furnace dewax: slow heating → wax softens before pressure builds; worse for complex geometries

Burnout temperature: 850–1000°C; 30–60 min hold; removes residual wax and organic binders; sinters shell

Shell permeability: fired fused silica shells: permeability 0.01–0.05 μm² (allow gas escape during pour); essential for preventing porosity from back-pressure

Metal Solidification

Chvorinov's Rule

Solidification time: t_s = C × (V/A)² [s; V = volume [cm³]; A = surface area [cm²]; C = mold constant [s/cm²]] C depends on alloy and shell properties; C ≈ 1.0–3.0 s/cm² for steel in silica shell; higher for thicker shell

Modulus method: M = V/A [cm; "modulus" of a casting section] M_riser > M_casting × f_feed [f_feed ≈ 1.2 for steel; riser must solidify LAST]

Feeding distance: Liquid steel can feed: F ≤ 4.5 × T [T = section thickness in mm; max feeding distance without shrinkage porosity] Place risers within feeding distance of heavy sections

Gating System Design

Gating ratio (for steel): Pressurized: A_sprue : A_runner : A_gate = 1 : 0.8 : 0.6 (choke at gate) Unpressurized: A_sprue : A_runner : A_gate = 1 : 2 : 4 (choke at sprue)

Fill time: t_fill = (2 × M_metal) / (C_d × A_gate × ρ × √(2 × g × h)) [s; h = effective head height; C_d ≈ 0.8] Target fill time: 3–10 s for most investment castings (balance turbulence vs. cold shut risk)

Melt superheat: Typically 50–100°C above liquidus; ensures complete fill in thin sections before freeze Excess superheat: increases grain size; longer solidification → more shrinkage porosity

Alloy Shrinkage

Alloy	Linear Shrinkage [%]	Notes
Stainless 316	2.1	Austenitic
17-4 PH	2.1	Martensitic PH
Inconel 718	1.9	Nickel superalloy
Ti-6Al-4V	1.8	Vacuum cast
Aluminum 356	1.3	Semi-solid possible
Carbon steel 1025	2.0	Low carbon
Cobalt alloy (Stellite)	2.2	High-carbon cobalt

Hot tear susceptibility: High hot-tear risk: alloys with wide freezing range (Ni alloys, some stainless) Prevention: reduce restraint (flexible shell); hot-top feeders; modify gating to reduce temperature gradient

Vacuum Casting

Required for: reactive alloys (Ti, Ni superalloys); prevent oxidation; improve cleanliness Vacuum level: < 0.01 torr (1.3 Pa) during melt and pour Centrifugal vacuum casting: for thin-wall titanium medical implants; fills thin sections by centrifugal force

Tolerances and Surface Finish

Dimensional Tolerances

ASTM B557 / Investment Casting Institute (ICI) Standard: Linear dimensions: ±0.13 mm for first 25 mm, ±0.05 mm per additional 25 mm Flatness/straightness: 0.25 mm per 100 mm

As-cast tolerances (typical): Small parts (< 50 mm): ±0.1–0.2 mm Medium (50–200 mm): ±0.2–0.5 mm Large (200–500 mm): ±0.5–1.0 mm

Factors affecting tolerance: Wax shrinkage variation: ±0.05% (dominant for small dimensions) Shell expansion: +0.1–0.3% (fused silica shell expands on heating → compensate in die) Metal shrinkage variation: ±0.1% (composition and section size dependent)

Surface Finish

As-cast Ra: 1.5–3.2 μm (fused silica shell with fine zircon face coat) After electropolish: Ra 0.4–0.8 μm (stainless steel parts) Machined features: added by CNC post-casting for critical tolerances (< ±0.05 mm)

Ceramic Core Technology (Internal Passages)

Application: turbine blade cooling channels; complex internal geometry impossible to pull Material: fused silica (90%+ SiO₂); alumina/silica ceramic Removal after casting: leach in hot NaOH solution (silica cores); HF acid (alumina cores)

Core design: Minimum wall thickness (metal between core and surface): 0.8–1.5 mm Core-to-shell fixture: held by chaplets (metal pins) or shell engagement Core offset during pour: < 0.2 mm (critical for cooling channel dimensions in turbine blades)

Aerospace and Turbine Applications

Equiaxed casting: conventional; random grain orientation; for moderately stressed parts Directional solidification (DS): columnar grains aligned with thermal gradient → better creep resistance in one direction (turbine blades); 10–15°C/min withdrawal rate Single crystal (SX): no grain boundaries → best creep and oxidation resistance; 1000+ hours in turbine Selector/pig-tail geometries initiate single grain; rejected if more than one grain nucleates (X-ray inspection)

Turbine blade alloys: Equiaxed: IN713, B1914 DS: MAR-M247, CM247LC SX: CMSX-4, René N6, PWA1484

Defect Analysis

Defect	Cause	Prevention
Shrinkage porosity	Insufficient feeding; thick section without riser	Add/enlarge riser; adjust gating
Gas porosity	Dissolved gas (H₂, N₂) in melt; damp shell	Degas melt; dry shell thoroughly; vacuum cast
Cold shut	Premature freezing; insufficient superheat	Increase melt temperature; reduce fill time
Misrun	Metal froze before filling thin section	Increase superheat; preheat shell
Hot tear	Restraint during solidification	Reduce restraint; widen risers
Shell crack	Thermal shock during dewax; inadequate green strength	Flash dewax; more backup coats
Inclusions	Dross, refractory spall, ceramic contamination	Filter use; careful pouring; shell inspection

Standards

Standard	Scope
ASTM B557	Mechanical testing of wrought/cast aluminum and Mg
ASTM A571	Austenitized/quench/tempered stainless investment castings
AMS 5354	Investment cast Inconel 718
AMS 4928	Ti-6Al-4V investment castings
ICI Standards	Investment Casting Institute dimensional standards
NADCA (Die Cast Assoc.)	Design guide (for reference/contrast)
ISO 11971	Steel and Fe alloy castings — visual surface inspection

Output

Provide: alloy (grade, liquidus T [°C], linear shrinkage [%]), part mass [kg] and largest section thickness [mm], shell build plan (primary coat slurry, N backup layers, total thickness [mm]), gating type (pressurized/unpressurized) and gating ratio, fill time [s] and effective head height [mm], solidification time t_s [s] from Chvorinov (V [cm³], A [cm²], C constant), riser modulus M_riser [cm] vs. M_casting × 1.2, melt superheat [°C], wax shrinkage allowance [%] + metal shrinkage [%] → die oversize, dimensional tolerance achievable [±mm], surface finish as-cast [μm Ra], core required (yes/no, material), vacuum required (yes/no), defect risk assessment (porosity/hot tear/cold shut), and applicable standard (ASTM B557, AMS 4928/5354, ICI).