Ask ten buyers what “good surface finish” means on a casting, and you will get ten different answers: “smooth,” “like machined,” “no pits,” “better than sand casting,” and “Ra 3.2 everywhere.” Most of those answers cost money—usually more than they need to. Surface finish is one of the most over-specified requirements in casting procurement and one of the most misunderstood. This guide explains what investment casting actually delivers, how Ra values work on cast surfaces, and how to specify finish so you pay for exactly what your part needs—nothing more.
What “Surface Finish” Means on a Casting
On a machined part, surface finish is a matter of tool geometry, feed rate, and spindle speed — a controlled process producing a measurable, repeatable result. On a casting, the as-cast surface is a copy of the inside of the ceramic shell: it inherits the shell’s roughness, modified by how the metal fills and solidifies. That copy is remarkably good — which is precisely why investment casting is chosen for cosmetic and functional parts alike — but it is a different kind of smoothness than machining, and it should be specified differently.
The standard measure is Ra (average roughness), in micrometers (µm). As a rough mental model for investment castings:
- Ra 0.8–1.6 µm — fine as-cast surfaces, achievable on shell-facing surfaces of small parts in silica sol process; often the best as-cast you can rely on
- Ra 1.6–3.2 µm — the typical commercial as-cast range for most steel and ductile iron investment castings
- Ra 3.2–6.3 µm — normal for water glass process parts and larger castings; still far better than sand casting
- Ra 6.3–12.5 µm — sand casting territory; also where heavy as-cast surfaces on large investment castings can land
- Ra 0.4–1.6 µm — the machined world; only achievable by removing material
The gap between “best as-cast” and “any machined surface” is the boundary where most specification mistakes happen.
Why Investment Casting Surfaces Are Different From Sand Castings
Two reasons, both worth understanding because they explain the price difference.
Shell quality. The silica sol ceramic shell used in quality investment casting has a fine, dense inner surface — essentially a ceramic layer that copies the wax pattern with low roughness. Water glass shells are coarser and cheaper. Sand casting’s mold surface is, well, sand: individual grains leave their imprint on the casting, producing the characteristic rough, gritty surface finish (and the classic “sanding” of the surface where the pattern raps against the mold). This is why “it’s a casting, of course it’s rough” is a sand-casting mindset — it does not automatically apply to investment castings.
No parting line. Sand casting and die casting parts carry a parting line where the two mold halves meet, plus ejector pin marks and draft angles. Investment casting has none of these: the wax pattern is an integral body, so the casting has no parting flash and no draft requirement. That single difference eliminates the most visible “casting look” cues and is one of the main reasons investment cast parts are specified where appearance matters — pump components, valve trim, automotive brackets, decorative hardware.
Cosmetic vs Functional Surface Requirements
The most expensive specification mistake is treating a cosmetic surface as functional. The question to ask for every surface is: what does this surface actually do?
Functional surfaces — sealing faces, bearing seats, gasket faces, running surfaces, grooves that guide O-rings — have real performance requirements. But note what they rarely have: an as-cast finish requirement. Functional surfaces are almost always machined anyway, because the tolerance and finish they need (Ra 0.8, flatness 0.05) cannot come from any casting process. So the functional-finish question on an investment casting is almost always answered by “machine it,” and the as-cast Ra is irrelevant there.
Cosmetic surfaces — the visible outside of the part, the body of a valve, a bracket face seen in an engine bay — are where as-cast finish actually matters, and where over-specification is rampant. An as-cast Ra 3.2 requirement on a water glass part forces a more expensive silica sol process or additional blasting/polishing operations for zero functional benefit. An as-cast Ra 6.3 requirement would have passed just as well.
The rule of thumb we use at Ningbo Ruican: specify as-cast finish only where it matters visually or for subsequent coating adhesion, and keep it realistic for the process — Ra 3.2 for silica sol, Ra 6.3 for water glass, unless you genuinely need better and are ready to pay for it.
Where Finish Really Matters: Coating and Corrosion
There is one place as-cast roughness deserves real attention: where the part will be painted, powder-coated or electroplated. Rough cast surfaces have sharp peaks that show through thin coatings (“orange peel”), and coatings can bridge over surface pits and blister later. For painted parts, buyers often specify “shot blasted, then painted” — the blasting does two jobs: cleaning scale and creating the anchor profile for the coating.
For powder coating and e-coat, a reasonably smooth as-cast surface (Ra 3.2–6.3) plus a light blasting pass gives a professional, even finish. Chasing Ra 1.6 on the whole casting before painting is wasted money — the coating will level the microscopic peaks anyway, and the cost of polishing cast surfaces to that level is significant.
How to Specify Surface Finish Correctly
Practical specification guidance for your next RFQ:
- Separate machined and as-cast surfaces on the drawing. Mark functional surfaces as machined with IT-grade tolerance and Ra. Mark the rest as-cast with one global callout.
- State Ra only where it is verifiable. On as-cast surfaces, Ra measurement is tricky (profilometer traces across a curved, blasted surface are noisy). A realistic callout like “as-cast Ra ≤ 6.3 µm, shot blasted” is testable and honest; “Ra 1.6 as-cast” is not and will be priced as if it were machined.
- Use visual acceptance where possible. For cosmetic surfaces, agree a visual/analog sample (“matches approved sample A”) instead of a number. This is standard practice in the industry and far more enforceable than a Ra call on a blasted surface.
- Match the callout to the process. Don’t specify silica-sol-level finish on a water glass part, and don’t pay for silica sol just for finish if blasting after the fact meets your need.
- Ask the foundry what they actually hold. A foundry that quotes its real as-cast capability, with samples, is a partner; one that promises Ra 1.6 as-cast on everything is pricing in risk.
Blasting, Tumbling and Polishing: The Finish Toolkit
Beyond as-cast, there is a spectrum of cheap-to-expensive surface treatments worth knowing:
- Shot blasting (most common, included): removes shell residue and scale, uniform matte finish, opens the surface for coating. Standard on nearly all our castings.
- Sand/bead blasting (finer): lighter etch for cosmetic parts or thin sections that shot blasting would peen.
- Vibratory tumbling (cheap smoothing): deburrs edges and rounds sharp corners; good for small parts with edges that touch assembly operators.
- Mechanical polishing / buffing: only where the surface must look machined or mirror-finished; cost rises quickly with area.
- Glass bead peening: controlled, uniform finish plus mild compressive stress benefit; used on cosmetic and fatigue-sensitive surfaces.
For most industrial castings, shot blasting is the default finish and it meets the requirement. Every step beyond that should have a reason.
The Cost Reality of Over-Specifying Finish
Concrete numbers make the point. On a typical 2 kg steel investment casting:
- As-cast Ra 6.3, shot blasted: baseline cost — included in the quote
- As-cast Ra 3.2 (silica sol surface): +5–15% process cost, sometimes zero if the shell is fine anyway
- Machined critical surfaces only: +20–40% total part cost (machining time on a casting that is otherwise near-net)
- Polished cosmetic surfaces: +30–60% or more on those surfaces
The uncomfortable truth: surface finish is where foundries recover margin from over-specified drawings. The buyer who says “smooth it all” gets a quote that pays for polishing they never see. The buyer who says “machine these six surfaces, blast the rest, approve by sample” gets a competitive quote and a part that does its job.
Why Ningbo Ruican Cares About This
We manufacture silica sol and water glass investment castings in carbon steel, alloy steel, stainless steel and ductile iron, with an in-house CNC shop and full surface treatment capability from blasting through polishing. When we review a drawing, surface specification is one of the first things we discuss — not because we want to avoid work, but because our quoting engineers are trained to separate what the part needs from what the drawing says, and to tell you the difference honestly. The result is a quote for the finish you actually require, delivered at the price the market demands.
Send us your drawing and a clear statement of which surfaces are functional: we will confirm what we can hold as-cast, what should be machined, and what your realistic finish specification should be.
Get a surface-finish review with your next quote at http://www.cnsnadcasting.com.