Sand casting is the process of forming a cavity in bonded sand around a pattern, pouring molten metal into that cavity, and breaking the sand away once the metal has solidified. The mold is used once; the pattern that shaped it goes back on the rack for the next one. That is why the process carries most medium-to-large ferrous work: the tooling is a pattern rather than a steel die, and sand’s refractory limit of 1650–1820 °C covers the pouring temperature of every iron and steel grade.
How Does Sand Casting Work
Sand casting runs in seven steps: tooling, molding, coring and closing, melting, pouring, cooling and shakeout, and finally cleaning, heat treatment and inspection.
Step 1: Pattern and core boxes
The pattern is a replica of the part, built oversize and reshaped so sand can be formed around it and it can be pulled back out. It carries three allowances the part drawing does not show: shrinkage for the metal’s contraction as it cools, draft of 1° to 2° on faces perpendicular to the parting line so the pattern releases, and machining stock on every surface that will be cut. Core boxes are built alongside it to form the sand cores for internal passages.
Shrink allowance is the foundry’s number, not a handbook one: what a pattern actually needs depends on the molding process and how much the mold restrains each feature. Send the part drawing and let the foundry apply the allowances for the process it will run.
Step 2: Molding the cope and drag
Sand is packed around each half of the pattern in its own flask and the pattern is withdrawn, leaving the cavity split between the drag (bottom half) and the cope (top half). Runners and gates are formed in the parting face at the same time.
Step 3: Cores and mold closing
Cores are set into the drag to form holes, passages and undercuts, the cope is lowered onto it, and the assembled mold is clamped or weighted against the lifting force of the metal. Each core seats in a core print, the recess the pattern left for it. A feature formed between a core and the mold carries the placement error of both pieces, which is why wall thickness around a cored bore is the dimension a foundry most wants room on.
Step 4: Melting
The charge — steel scrap, returns, pig iron and ferroalloys — is melted in an induction furnace, and the melt’s chemistry is checked by spectrometer before the ladle is tapped. Ductile iron is treated with magnesium in the ladle at this point. Chemistry outside the grade’s range is corrected in the furnace, not accepted in the casting, and the numbers from this check are what appear on the material test report.
Step 5: Pouring
Metal is poured from the ladle into the sprue at a temperature set by the alloy. Too cold and thin sections misrun; too hot and the metal erodes the sand, picks up gas, and shrinks more. The gating system’s job during the pour is to fill the cavity quickly but without turbulence, so no air or loose sand is folded into the metal.
Step 6: Cooling and shakeout
The casting stays in the mold until its heaviest section has solidified and the whole part has cooled enough to handle, then the mold goes onto a vibrating grid that shakes the sand off. Cooling time scales with section thickness, so a heavy counterweight sits in the sand far longer than a flange. Steel and heavy ductile iron sections are not shaken out early: pulling them hot invites cracking and distortion.
Step 7: Cleaning, heat treatment and inspection
Gates and risers are knocked or cut off, the contact areas ground flush, and the casting shot blasted; then it is heat treated if the grade requires it, machined, and inspected against the drawing and the specification. Inspection means chemistry and mechanicals on the MTR, dimensions on the CMM, and NDT where the drawing calls for it.
Main Components of a Sand Mold
A sand mold is two halves — cope and drag — meeting at the parting line, cores seated in core prints for internal features, a gating system that carries the metal in, risers that feed the shrinkage, and chills and vents that steer how the metal cools.
The drag is the bottom half of the mold, the cope the top, and the plane where they meet is the parting line. Slag, gas and loose sand float up to the cope surface, so a cope surface that will be machined needs at least 6 mm (0.25 in) of stock, while drag surfaces and side walls get by with less. Put critical machined faces in the drag when the geometry allows, and expect any dimension that crosses the parting line to carry the mismatch between the halves.
A core is a bonded-sand shape that forms an internal passage or an undercut the pattern could not release; the core print is the extension that seats it in the mold. Cores are placed by hand or fixture, so a wall between a core and the cavity is thicker or thinner by whatever the core shifted. One large core holds position better than several small ones meeting inside the casting.
The gating system — pouring basin, sprue, runners and gates — is the metal’s path into the cavity. Gates leave witness marks where they are removed, so tell the foundry which faces must stay untouched.
A riser is a reservoir of metal that solidifies after the casting and feeds the contraction as the section it serves freezes. Where risers go, how large the neck is, and whether a heavy boss can be reached at all are worked out in gating and risering design, checked by solidification simulation before the pattern is cut. An isolated heavy section with no path to a riser is the most common cause of shrinkage porosity in housings.
Chills are metal inserts in the mold face that pull heat out of a heavy section so it freezes before the riser does; vents are channels that let steam and binder gas escape ahead of the metal. Neither appears on your drawing, but a chill leaves a locally harder, finer-grained zone on the casting, which matters if that face is going to be drilled.
Types of Sand Casting
Three sand systems cover almost all ferrous sand casting, distinguished by what holds the sand together: green sand (clay and water), no-bake or resin sand (a chemical binder that sets at room temperature), and shell molding (resin-coated sand cured against a heated metal pattern). The choice sets tooling cost, the size of part the mold can hold, and how much dimensional variation to expect.
Green sand is silica sand bonded with bentonite clay, water and a small amount of carbonaceous additive, rammed around the pattern and poured wet. It is the cheapest sand to make and reuse and the fastest to mold, so it wins on volume work and on smaller castings. Its limit is that the mold is only as stiff as the ramming, so large, heavy castings and deep pockets move more.
No-bake sand is mixed with resin and catalyst, packed around the pattern, and left to harden chemically so the mold is rigid before the pattern is drawn. It holds the large and heavy parts green sand cannot and supports deep pockets and cores with less draft. It carries one penalty green sand does not: dimensions that cross the parting line pick up about 1 mm (0.040 in) of additional 6σ variation, so keep critical dimensions on one side of the parting line.
Shell molding forms a thin shell of resin-coated sand on a heated metal pattern and joins two shells into a mold. It gives the tightest tolerances and the best surface of the sand processes, but the shell is thin and the pattern is metal, so it is limited to small castings and run lengths that pay back the tooling.
The choice between them follows size and tolerance. Below about 50 lb the three are dimensionally similar; above 200 lb no-bake holds tighter than green sand; shell is tightest but size-limited.
Take that ordering — shell tightest, then no-bake, then green sand — loosely. In SFSA’s study of more than 140,000 features on production steel castings across 15 foundries, the gap between processes was smaller than the gap between foundries running the same process. Ask a foundry for its own capability data rather than a tolerance grade inferred from the process name.
What Metals Can Be Sand Cast
Any metal that can be melted can be sand cast, but the process does most of its work in ferrous alloys — gray iron, ductile iron, carbon steel and stainless steel — because sand stands up to their pouring temperatures. Aluminum, brass and bronze are sand cast as well, at far lower temperatures; they are outside what we pour.
Which family to specify follows what the part has to survive. Gray iron’s graphite is in flakes, each acting as a crack, so it is weak in tension but damps vibration and machines easily — counterweights, flywheels and housings that see no shock. Ductile iron carries the same graphite as nodules, which gives it steel-like ductility at iron pouring temperatures and iron cost — pump and valve bodies, bearing and gearbox housings.
Steel pours hotter, shrinks more and needs heavier risering, so the same housing costs more in steel. It is chosen where weldability, impact strength or a pressure-temperature rating demand it, and stainless where corrosion does.
| Metal | ASTM specification | Grades named in the spec | Pouring temperature | Typical parts |
|---|---|---|---|---|
| Gray iron | A48 | Classes by minimum tensile strength of a separately cast test bar | 1340–1480 °C | Counterweights, flywheels, gearbox housings |
| Ductile iron | A536 | 60-40-18, 65-45-12, 80-55-06, 100-70-03, 120-90-02 | 1340–1480 °C | Pump and valve bodies, bearing housings, hydraulic manifolds |
| Carbon steel | A27 (general), A216 (pressure parts, high-temperature service) | 60-30, 65-35, 70-36, 70-40; WCA, WCB, WCC | 1565–1700 °C | Flanges, weldable structural parts, valve bodies |
| Stainless steel | A351 (pressure parts), A743 (general corrosion service) | CF3, CF8M | 1480–1600 °C | Corrosion-service pump and valve bodies |
Ductile iron under A536 is called out as three numbers — tensile strength, yield strength and elongation — and two of the five grades, 65-45-12 and 80-55-06, are supplied as-cast, the cheapest condition. Carbon steel splits into A27 for general parts and A216, where WCB is the everyday grade, for pressure parts that will be welded.
Conclusion
Sand casting is a pattern, a two-part sand mold with cores, a controlled pour, and a chain of cleaning, heat treatment and inspection that ends in a part certified to an ASTM grade. What the foundry needs from you is the part drawing with the material called out by specification and grade, the machined faces and the tolerances that matter marked, and the process left open unless size or tolerance forces it. Then ask for that foundry’s own capability data; the process name alone does not promise a tolerance.