Casting is one of the primary ways manufacturers produce complex three-dimensional metal geometry without machining the entire part from solid stock. Internal cavities, curved surfaces, thick sections, bosses, ribs, passages, and large structural shapes can often be created close to final form.
The casting process determines how the mold is made, whether the mold is reusable, how cores create internal geometry, which alloys can be poured, what surface finish is practical, how much machining stock is required, and what production quantity makes economic sense.
What Is Metal Casting?
Metal casting is a manufacturing process in which molten metal is introduced into a shaped mold cavity, allowed to cool and solidify, and then removed as a component that may undergo additional cleaning, machining, heat treatment, finishing, and inspection.
The cavity represents the intended component geometry while allowances may be included for shrinkage, machining, draft, gating, risers, parting lines, and core placement.
Major Metal Casting Processes
Investment Casting
Wax patterns are coated with ceramic material to create detailed molds capable of producing complex parts with relatively good surface finish and fine features.
Sand Casting
Sand is compacted around patterns to form mold cavities. The process is flexible for a wide range of component sizes, alloys, quantities, and geometries.
Permanent Mold Casting
Molten metal is introduced into reusable metal molds, providing repeatability and surface quality between sand casting and high-pressure die casting for suitable products.
Low-Pressure Casting
Controlled pressure moves molten metal into the mold and can support predictable filling and reduced turbulence for suitable parts.
Centrifugal Casting
A rotating mold uses centrifugal force to distribute molten metal and is commonly associated with cylindrical, ring, sleeve, pipe, and tubular geometry.
Die Casting
Molten nonferrous metal is forced into reusable steel tooling under pressure for repeatable medium- and high-volume production.
For high-pressure reusable tooling, see the separate die casting guide.
Investment Casting
Investment casting is often called the lost-wax process because expendable wax patterns are used to create ceramic molds. The wax pattern is removed before molten metal is poured into the resulting cavity.
Why Manufacturers Use Investment Casting
Investment casting is widely considered when the desired geometry would be expensive to machine, difficult to create in a sand mold, or impractical to produce with conventional two-part tooling.
How Investment Casting Works
Tooling is created for producing wax patterns that represent the final casting geometry.
Individual wax parts are produced and inspected before assembly.
Wax patterns may be attached to a central runner system to create a casting tree.
The wax assembly is repeatedly coated with ceramic slurry and refractory material until a strong shell develops.
Wax is removed from the hardened ceramic shell, leaving the casting cavities and runner system.
Molten alloy is poured into the prepared ceramic shell.
Metal cools and solidifies inside the shell.
Ceramic material is broken away after the casting has cooled.
Individual castings are separated from the tree, cleaned, ground, heat treated, machined, and inspected as required.
Sand Casting
Sand casting uses specially prepared sand to create expendable molds. Patterns form the external cavity while cores can create internal passages, holes, recesses, and hollow geometry.
Sand casting generally requires more draft and greater machining allowance than highly precise molding processes. Surface finish also reflects the mold material and casting conditions.
Casting Molds and Cores
A mold creates external casting geometry while cores create internal features that molten metal flows around. Core design can therefore make complex internal cavities possible without machining all material from a solid casting.
Other Metal Casting Methods
| Process | General Characteristics | Typical Fit |
|---|---|---|
| Permanent Mold | Reusable metal molds with gravity or controlled filling. | Repeat production of suitable nonferrous components. |
| Low-Pressure Casting | Controlled pressure moves metal into a mold from below or through a managed feed system. | Components requiring controlled filling and suitable production volume. |
| Centrifugal Casting | Rotating molds distribute molten metal using centrifugal force. | Rings, sleeves, cylinders, tubes, and related rotational geometry. |
| Continuous Casting | Molten metal solidifies continuously into long intermediate forms. | Billets, slabs, bars, and feedstock for later manufacturing. |
| Die Casting | High-pressure filling of reusable steel dies. | Repeatable medium- and high-volume nonferrous parts. |
Common Metal Casting Alloys
| Material Family | General Casting Considerations |
|---|---|
| Carbon & Alloy Steel | Used for structural, wear, machinery, transportation, and high-strength cast components where ferrous properties are required. |
| Stainless Steel | Used for corrosion-resistant castings in process equipment, food systems, valves, pumps, machinery, and specialty components. |
| Cast Iron | Common for machine bases, housings, pump bodies, engine components, heavy equipment, and vibration-damping structures. |
| Aluminum | Lightweight and corrosion resistant, with broad use in housings, machinery, automotive parts, structures, and general OEM components. |
| Bronze | Used for bearings, bushings, marine components, wear parts, valves, pumps, and corrosion-resistant hardware. |
| Brass | Common for valves, fittings, hardware, plumbing components, decorative products, and electrical parts. |
| Nickel Alloys | Used where high temperature, corrosion, oxidation, or demanding mechanical requirements justify specialized casting. |
| Titanium | Investment casting can be used for specialized lightweight, corrosion-resistant, aerospace, medical, and industrial parts. |
Design for Metal Casting
Abrupt transitions between thin and thick regions can create uneven cooling and shrinkage.
Rounded internal transitions improve metal flow and reduce sharp geometry that can concentrate stress.
Pattern-based processes commonly require draft to remove the pattern or casting from tooling and molds.
Mold separation affects flash, finishing, dimensional relationships, core placement, and pattern complexity.
Internal cores enable complex passages but add cost, handling, dimensional variation, and cleaning requirements.
Critical sealing faces, bores, threads, bearing seats, and datums may require stock for final machining.
Large concentrated sections can remain hot longer and increase shrinkage or internal defect risk.
Sand, ceramic shell, core material, scale, and blasting media must be removable from the finished casting.
Common Casting Defects
Casting Tolerances and Inspection
Dimensional capability varies considerably by casting process. Investment casting generally supports finer detail and tighter as-cast control than conventional sand casting, while sand casting provides greater flexibility for large parts and lower tooling investment.
Characteristics Commonly Inspected
Depending on the component, inspection may include CMM measurement, gauges, visual examination, X-ray, ultrasonic inspection, magnetic particle testing, dye penetrant testing, pressure testing, chemical analysis, hardness testing, and mechanical testing.
Secondary Operations for Castings
Gate & Riser Cutoff
Feed systems, gates, risers, runners, and casting-tree connections are removed after solidification.
Blasting & Cleaning
Sand, ceramic, scale, oxide, and other casting residue are removed from the surface.
CNC Machining
Bores, threads, datums, mounting faces, sealing surfaces, and precision interfaces are machined to final size.
Heat Treatment
Castings may be normalized, hardened, tempered, solution treated, aged, annealed, or stress relieved.
Plating & Coating
Paint, plating, passivation, powder coating, and other finishes may protect or modify the cast surface.
Assembly
Castings may receive bearings, seals, fasteners, inserts, fittings, electronics, or other components.
What Drives Metal Casting Cost?
Patterns, wax tooling, core boxes, dies, fixtures, and gauges create upfront production cost.
Material price, melting temperature, density, recovery, availability, and required chemistry influence recurring cost.
Large castings require more metal, mold material, furnace capacity, handling equipment, and finishing effort.
Multiple or intricate cores add tooling, assembly, inspection, handling, and removal requirements.
Gates, risers, runners, rejected castings, machining stock, and scrap affect how efficiently poured metal becomes final product.
Grinding, blasting, polishing, coating, and cosmetic requirements can add substantial finishing labor.
Tight final tolerances, deep bores, threads, sealing surfaces, and complex machining increase recurring cost.
X-ray, nondestructive examination, pressure testing, material testing, documentation, and traceability add quality cost.
Related Casting and Manufacturing Resources
Metal castings frequently move into machining, grinding, deburring, heat treatment, plating, inspection, and assembly. Foundry selection should therefore account for the complete finished-component requirement.
Casting & Secondary Process Research
These manufacturing references correspond with casting and common downstream processes used to complete metal components.
How to Select a Metal Casting Supplier
Foundries should be evaluated according to the required casting process, alloy, component size, internal geometry, tooling, tolerance, quantity, inspection, machining, and finish requirements.
Confirm the foundry specializes in the appropriate investment, sand, shell, permanent mold, centrifugal, or other casting process.
Verify routine experience melting and controlling the required steel, iron, aluminum, bronze, nickel, titanium, or other alloy.
Furnace size, mold equipment, handling systems, cranes, and finishing capacity should match the component.
Review patternmaking, wax tooling, core boxes, rapid tooling, and engineering-change support.
Complex internal passages require reliable core making, assembly, location, venting, and removal.
Integrated or qualified machining capability can simplify control of datums, bores, threads, faces, and final tolerances.
Determine whether dimensional, material, pressure, X-ray, ultrasonic, penetrant, magnetic, and mechanical testing are available.
Cleaning, blasting, grinding, heat treatment, coating, plating, and assembly should align with final product requirements.
Different Casting Processes Solve Different Geometry and Volume Problems
Investment casting favors detailed near-net-shape components, sand casting provides broad flexibility in size and alloy, and reusable-mold methods improve repeatability when volume supports additional tooling. The best casting process depends on component geometry, material, wall thickness, cores, production quantity, surface finish, tolerances, machining, defect requirements, inspection, and total finished-part cost.