Sand casting is an old process, and most of the design world uses it the same way: for small, complex forms that are too expensive to machine or stamp, usually finished with a uniform polish or patina and photographed once. That use is fine. This entry is about what it doesn’t have to be.
The honest constraints
Sand casting produces a rough surface that requires significant post-cast finishing — grinding, filing, and polishing at minimum. Dimensional tolerance is typically ±0.5 mm on smaller features; tight tolerances require machining after casting, which is why mating surfaces, threads and seats get a machining allowance on the drawing rather than a hope. Porosity (small internal voids from gas in the melt) is a real risk in thick sections and affects both structural integrity and how the surface responds to finishing — a porous face shows as pinholes the moment you polish it. Lead times are longer than for sheet metal or turned parts: a new pattern, a test pour, finishing, and quality check.
The split line is a design decision
The mould split line — where the two halves of the sand mould meet — leaves a seam on the casting. Its location is a design decision, not a manufacturing default. If you design for it deliberately, it becomes a feature; if you don’t, it appears somewhere inconvenient and costs time to remove. Talk about the split line at the drawing stage.
What’s possible that most quotes miss
The surface of a sand casting carries the texture of the sand mould — coarse, directional, alive in a way that no polished or machined surface is. Leaving that texture in place (or designing for it) gives you something that reads as made, not manufactured. The same design can be cast with different final finishes on different faces: polished at the front, rough at the back, patinated at edges. Chemical patination — using ammonia, salt, acid, or heat — can produce colour ranges from green and blue-green to brown and near-black. These are not stable without sealing, but a well-sealed patina is durable for interior use.
Cost drivers
- The pattern — a one-time cost, but it defines everything downstream; cheap patterns make expensive castings
- Finishing labour — every face you want smooth is hand-hours; every face you leave as-cast is free texture
- Porosity rejects — thick, uneven sections raise the loss rate; even wall thickness is the cheapest insurance
- Machined features — each post-cast machining setup adds cost; group them on one face where possible
- Patination — chemical patinas are craft labour, priced per piece, and need a sealed sample standard
What to put on the drawing
- The split line location, agreed, drawn, and signed off
- Draft angles on every face perpendicular to the mould opening — 1.5–3°
- Wall thickness as even as the design allows; call out any section over ~20 mm for porosity review
- Which faces are as-cast, which are finished, and to what grit — with a boundary sample
- Machining allowance on every face that mates, seats or threads
- Patina against a sealed physical sample, never a photograph — patinas don’t photograph honestly