Metal casting design guide
Casting pours liquid metal into a cavity and lets it freeze — so every rule is about how the metal flows, how it shrinks, and how the pattern or die comes out. The same part is designed differently for a sand mold, a wax-and-ceramic shell, a permanent steel mold or a high-pressure die. This guide gives the rules per process, the tolerances each holds, and where to leave stock for machining.
Five casting routes — pick the process before the rules
Quantity, alloy, wall thickness and finish decide the route. Each has its own minimum wall, draft, tolerance and tooling life; the rest of this guide is organised around them.
| Process | Quantity band | Alloys | Min wall | Tolerance | Surface | Typical parts |
|---|---|---|---|---|---|---|
| Sand casting | 1 – 5,000 | Iron, steel, aluminum, bronze, brass | 4–6 mm | ± 1.0–2.5 mm | Ra 12–25 µm | Housings, frames, pump bodies, machine bases; anything large |
| Investment (lost wax) | 10 – 20,000 | Steel, stainless, superalloy, aluminum, bronze | 1.0–1.5 mm | ± 0.1–0.25 mm | Ra 1.6–3.2 µm | Turbine blades, impellers, medical, firearms, detailed steel parts |
| Permanent mold (gravity) | 500 – 50,000 | Aluminum, magnesium, zinc, some copper | 3–4 mm | ± 0.4–0.8 mm | Ra 6–12 µm | Wheels, manifolds, structural aluminum with T6 |
| Low-pressure | 1,000 – 100,000 | Aluminum, magnesium | 2.5–3 mm | ± 0.3–0.6 mm | Ra 3–8 µm | Wheels, pressure-tight housings, heat-treatable structurals |
| High-pressure die (HPDC) | 10,000 – 1,000,000+ | Aluminum, zinc, magnesium | 0.5–1.5 mm (zinc thinnest) | ± 0.1–0.3 mm | Ra 1–3 µm | Electronics housings, brackets, gearbox cases, hardware |
Forging (open-die, closed-die, upset) is a separate family: solid metal deformed hot or warm for the strongest possible part; rules differ (grain flow, flash, parting, 1–3° draft) and are summarised at the end. Cost ranges per route are in the cost guide.
Wall thickness and section change
The metal freezes from the mold wall inward. Thick sections freeze last and pull metal from thin ones — the source of shrinkage porosity, hot spots and tears. Keep sections uniform and blend the changes.
| Rule | Sand | Investment | Perm. mold / low-pressure | Die casting |
|---|---|---|---|---|
| Minimum wall | 4–6 mm (Al 4, iron 5, steel 6) | 1.0–1.5 mm | 2.5–4 mm | Al 1.0–1.5 · Mg 1.0–1.5 · Zn 0.5–0.8 mm |
| Maximum practical wall | 50 mm+ with chills | 15 mm | 25 mm | 6–8 mm (porosity beyond) |
| Section ratio | thick : thin ≤ 2 : 1 | ≤ 2 : 1 | ≤ 2 : 1 | ≤ 1.5 : 1 |
| Transition | taper 1 : 4, blend R ≥ thicker wall | taper 1 : 3 | taper 1 : 4 | taper 1 : 4, R ≥ T |
| Isolated heavy section | core out, or add a chill / riser | core out | core out; the mold acts as a chill | core out — porosity is certain otherwise |
| Cast-in cavities (cores) | sand cores, any shape | ceramic cores, soluble wax | metal cores, must pull straight | straight steel cores, slides for side cores |
Uniform walls are the one rule shared by every casting process. Where a heavy boss or lug must exist, hollow it, rib it or accept a riser/chill on the pattern that the foundry will design.
Draft, radii, fillets and parting line
The pattern or the die has to come away from the casting. Sharp internal corners are stress raisers in the mold and hot spots in the metal — every edge gets a radius.
| Feature | Rule | Why |
|---|---|---|
| Draft — sand | 1–2° external, 2–3° internal (cores) | The pattern lifts out of the sand without tearing the mold face |
| Draft — investment | 0–0.5°; often none | Wax is injected into a metal die, then the die opens; the ceramic shell is broken away — no draft needed on the part |
| Draft — permanent / low-pressure | 2–3° external, 3–5° internal | The casting shrinks onto metal cores; more draft inside than outside |
| Draft — die casting | Al 1–1.5° external, 2° internal · Zn 0.5–1° · Mg 1–2° | Steel die, ejected hot; internal faces grip cores as the metal shrinks |
| Internal fillets | R ≥ 0.5 × wall; ≥ 3 mm in sand | Removes hot spots and mold-corner erosion; sharp corners are where castings crack |
| External radii | R ≥ 1.5 mm sand · 0.5 mm investment · 0.5–1 mm die | Sharp external corners on the casting are sharp internal corners in the mold that chip and wash |
| Parting line | flat, in one plane, at the largest cross-section | A stepped parting line costs die machining and shows as a witness line; put it on a non-cosmetic edge |
| Parting-line witness | allow 0.5–1 mm flash removal, 0.1–0.3 mm mismatch | Two mold halves never align perfectly; a small step or bead hides it |
| Ejector pin marks | accept on the non-cosmetic side; state where they may not be | Die casting needs 3–8 pins depending on size; each leaves a 6–12 mm mark |
Holes, bosses, ribs, undercuts and text
Cast what the process casts well; drill, tap and machine the rest. A casting is a near-net shape — machining stock is part of the design.
| Feature | Rule | Why |
|---|---|---|
| Cast holes | sand ≥ 12 mm · investment ≥ 2 mm · die ≥ 2.5 mm (Al) / 1.5 mm (Zn); depth ≤ 4 × Ø | Smaller cores break or bend; drill below these sizes |
| Threads | never cast in aluminum or iron; machine after. Zinc die cast: external threads ≥ M6 can be cast | Cast threads are porous and out of tolerance; a cast pilot hole plus tapping is standard |
| Bosses | height ≤ 2 × wall; hollow if Ø > 3 × wall; fillet to the wall | Solid bosses are hot spots — porosity right where the fastener loads |
| Ribs | thickness 0.6–0.8 × wall; height ≤ 5 × wall; R at the root | Stiffness without heavy sections; a rib also feeds metal to a thin wall |
| Undercuts (die casting) | avoid; each needs a slide or a machining operation | Slides add 15–30 % to die cost and cycle time; investment and sand can cast undercuts with cores |
| Lettering | raised ≥ 0.4 mm, stroke ≥ 0.8 mm, on a flat face parallel to the parting | Raised letters are recesses in the die (cheap); recessed letters are raised die features that break |
| Machining stock | sand 3–5 mm · investment 0.8–1.5 mm · perm. mold 1.5–2.5 mm · die 0.5–1.0 mm | Skin porosity and scale live in the first millimetre; the stock lets the machinist reach sound metal |
| Datum and clamping | three cast pads, machined first, on one face | A casting has no square edges; machined pads give the CMM and the fixture something to hold |
| Pressure tightness | state it; ≥ 3 mm wall; low-pressure or vacuum die cast; impregnation allowed? | HPDC entrains gas; a leak-tight part is designed and quoted differently |
Shrinkage, porosity and heat treatment
Metal shrinks three times: as liquid cooling, as it freezes, and as a solid cooling to room temperature. The pattern is made oversize for the last one; the first two are what riser, gate and chill design fight.
1.0–2.5 %
Pattern-maker’s shrink
Aluminum 1.3 %, gray iron 1.0 %, steel 2.0 %, brass 1.5 %, zinc 0.7 %. The foundry scales the pattern or die — you never scale the CAD. Long thin parts can also bow; state flatness on the drawing.
≤ 2 : 1
Section ratio
The heavier a section is relative to its neighbour, the later it freezes and the more it pulls from them. Above 2 : 1 the foundry will add risers and chills; above 3 : 1 it will ask for a redesign.
T6 / T5
Heat-treatable routes
Sand, investment, permanent-mold and low-pressure castings in A356/A357 take a full T6 (solution + age). Conventional HPDC does not — entrained gas blisters at solution temperature; use T5 or a vacuum die-cast process.
HIP / impregnation
Closing porosity
Hot isostatic pressing closes internal shrinkage porosity in investment and sand castings for fatigue-critical parts. Resin impregnation seals surface-connected porosity in die castings that must hold pressure. Both are specified, never assumed.
Casting alloys — which metal casts how
Cast alloys are not the wrought grades. A356 is not 6061; ADC12 / A380 is not 6082. Choose from the casting alloy families, then match to the process that casts them.
| Family | Key grades | Processes | Castability | Use it for |
|---|---|---|---|---|
| Aluminum — sand / permanent | A356, A357, 319, 355 | Sand, investment, permanent, low-pressure | Good | Structural castings heat-treated to T6: wheels, suspension, pump and gearbox housings |
| Aluminum — die casting | A380 / ADC12, A383, A360, 413 | HPDC | Excellent | Thin-wall housings, brackets, covers in volume; as-cast, not heat-treated |
| Zinc | Zamak 3, 5, 7; ZA-8, ZA-27 | Hot-chamber HPDC | Excellent | Small intricate parts, 0.5 mm walls, plating-grade finish; hardware, connectors, decorative |
| Magnesium | AZ91D, AM60B, AZ31 | HPDC, low-pressure, sand | Good | Lightest structural castings: housings, seat frames, laptop and camera bodies |
| Gray & ductile iron | GG-25 / Class 30, GGG-40/50 (65-45-12) | Sand | Excellent | Machine bases, brackets, housings, brake parts; damping, cheap, machines beautifully |
| Carbon & low-alloy steel | WCB, 1030/1040, 4130, 8620 | Sand, investment | Fair | Load-bearing frames, valve bodies, mining and rail parts |
| Stainless steel | CF8 / CF8M (304 / 316 cast), 17-4PH, 410 | Investment, sand | Fair | Pump and valve internals, food and marine, medical |
| Copper alloys | C83600 (85-5-5-5), C95400 Al-bronze, C86300 Mn-bronze | Sand, investment, permanent | Good | Bearings, marine, valves, decorative; wear and corrosion |
| Superalloy / cobalt | Inconel 713, 718; Stellite 6 | Investment | Specialist | Turbine and hot-section parts; hardfacing |
Wrought-to-cast equivalents: 6061 → A356-T6 (structural) or A380 (housings in volume); 304 → CF8; 316 → CF8M; 1045 → WCB / 1040 cast. Each grade’s data is in the material library’s casting collection.
Tolerances by process
ISO 8062-3 grades castings by process and size (CT grades). These are the working numbers; machine any feature that needs better.
| Process | ISO 8062 grade | Linear, 25 mm | Linear, 100 mm | Linear, 400 mm | Across parting |
|---|---|---|---|---|---|
| Sand, hand-molded | CT 11–13 | ± 1.4–2.8 mm | ± 2.0–4.0 mm | ± 3.0–6.0 mm | add 0.5–1.5 mm |
| Sand, machine-molded / shell | CT 8–10 | ± 0.5–1.0 mm | ± 0.8–1.6 mm | ± 1.2–2.6 mm | add 0.3–0.8 mm |
| Investment | CT 4–6 | ± 0.13–0.3 mm | ± 0.2–0.5 mm | ± 0.4–0.9 mm | n/a (no parting on the part) |
| Permanent mold / low-pressure | CT 7–9 | ± 0.35–0.7 mm | ± 0.55–1.1 mm | ± 0.9–1.8 mm | add 0.3–0.5 mm |
| Die casting, aluminum | CT 5–7 | ± 0.15–0.35 mm | ± 0.25–0.55 mm | ± 0.5–0.9 mm | add 0.15–0.3 mm |
| Die casting, zinc | CT 4–6 | ± 0.1–0.25 mm | ± 0.2–0.4 mm | ± 0.35–0.7 mm | add 0.1–0.2 mm |
| Machined feature (any casting) | — | ± 0.05 mm | ± 0.05 mm | ± 0.1 mm | the machining datum, not the casting |
Dimensions that cross the parting line or depend on a moving core get the extra allowance in the last column. Flatness: roughly 0.3 % of length as-cast; machine the mating faces.
Forging — the rules that differ
Forging shapes solid metal between dies. It gives the highest strength and fatigue life of any route, at the price of die cost, generous stock and simpler shapes.
- Draft 3–7° (steel), 1–3° (aluminum). The forging must release from a die under tons of force with no ejectors; more draft than casting.
- No undercuts, no through-holes. Holes are pierced or drilled after; internal features are machined. Design for a two-die parting.
- Radii large: fillets R ≥ 3 mm, corners R ≥ 1.5 mm. Metal must flow around them; sharp corners cause laps and die wear.
- Stock 1.5–3 mm on all machined faces. Scale and decarb on steel; the forging skin is removed wherever tolerance or finish matters.
- Grain flow follows the shape. Orient the die so the flow lines follow the load path — the whole point of forging a bracket or a crank instead of machining it.
- Open-die for low volume. Below a few hundred pieces, open-die or ring-rolled blanks plus machining beat a closed-die tool.
What to send for a quote
Casting is engineer-quoted at FabDigit. These items let the engineer choose the route, plan the pattern or die and return one price.
- The 3D model of the FINISHED part (STEP). The foundry adds draft, stock, radii and the pattern shrink; a drawing shows which faces are machined.
- Annual quantity and first-order quantity. Sand for 50, permanent mold for 5,000, die casting for 50,000 — the number picks the process and the tooling.
- Alloy and heat treatment, or the requirement. “A356-T6” — or “aluminum, 250 MPa yield, pressure-tight to 5 bar” and let the engineer choose.
- Machined faces, datums and finish. Mark the machined surfaces and their tolerances; state as-cast finish (Ra), shot blast, paint, powder, anodize or plate.
- Inspection and certification. Material certs, X-ray / dye penetrant / pressure test, first-article dimensional report, ISO 8062 grade.
- Cosmetic faces and where marks may not be. Parting line, ejector pins, gates and risers all leave witness — say where they are unacceptable.
Cost ranges per route, tooling cost bands and the crossover with machining are in the metal casting cost guide. For under a hundred aluminum or steel parts, compare with machining from stock before tooling anything.
Metal casting design — FAQ
What is the minimum wall thickness for a casting?
Sand casting 4–6 mm (aluminum 4, iron 5, steel 6); investment casting 1.0–1.5 mm; permanent-mold and low-pressure 2.5–4 mm; aluminum die casting 1.0–1.5 mm; zinc die casting 0.5–0.8 mm. Keep sections within a 2 : 1 ratio and blend the transitions.
How much draft does a casting need?
Sand 1–2° external and 2–3° on cores; permanent-mold 2–3° external and 3–5° internal; aluminum die casting 1–1.5° external and 2° internal, zinc 0.5–1°. Investment castings need little or none because the ceramic shell is broken away.
What tolerances can casting hold?
Per ISO 8062-3: sand CT 8–13 (± 0.5–4 mm at 100 mm depending on molding method), permanent-mold CT 7–9 (± 0.55–1.1 mm), aluminum die casting CT 5–7 (± 0.25–0.55 mm), zinc die casting and investment CT 4–6 (± 0.2–0.5 mm). Anything tighter is machined; machined features on castings hold ± 0.05 mm.
Sand casting, investment casting or die casting?
Sand for large parts, iron and steel, and low quantities — cheap tooling, rough finish, thick walls. Investment for detailed steel, stainless and superalloy parts with thin walls and a fine finish, from tens to tens of thousands. Die casting for aluminum, zinc or magnesium parts above roughly 10,000 per year, where a steel die pays for itself with thin walls, tight tolerances and no machining.
How much machining stock should I leave on a casting?
Sand 3–5 mm, permanent-mold 1.5–2.5 mm, investment 0.8–1.5 mm, die casting 0.5–1.0 mm. The first millimetre of a casting holds skin porosity and scale; the stock lets the tool reach sound metal. Cast three pads to machine first as datums.
Can die castings be heat treated?
Conventional high-pressure die castings cannot take a T6 — the entrained gas blisters at solution temperature — so they are used as-cast or T5. For a heat-treated aluminum casting choose permanent-mold, low-pressure, sand or investment in A356 / A357, or specify a vacuum die-cast process.
Which aluminum alloy for casting?
A356-T6 (or A357) for structural sand, permanent-mold, low-pressure and investment castings; A380 / ADC12 for high-pressure die castings used as-cast; 413 for thin pressure-tight die castings; 319 for engine components. Wrought 6061 is not a casting alloy — its casting equivalent is A356.
What should I send for a casting quote?
The STEP model of the finished part, annual and first-order quantity, alloy and heat treatment (or the performance requirement), marked machined faces with tolerances, surface finish, inspection and certification needs, and where witness marks are not allowed. A FabDigit engineer chooses the route and returns a quote with tooling and per-part lines.
Design for the pour, machine the rest.
Upload the finished-part model with quantity and alloy — a FabDigit engineer picks the casting route, checks walls, draft and stock, and returns the tooling and per-part quote.