Every die casting ships trimmed, deburred and shot blasted to Ra 1.6 to 3.2 µm at no added cost, and for internal parts that is the correct specification. Beyond it, powder coating at 60 to 120 µm is the default for industrial parts and reaches 500 to 1,000 hours neutral salt spray over a chromate conversion coat, while conversion coating alone at 0.5 to 4 µm is the only finish that protects while staying electrically conductive below 5 mΩ. Anodizing is the one finish to avoid on castings: die casting alloys carry 7.5 to 12% silicon that does not convert to oxide, so the film grows around the particles and produces a mottled dark grey at 5 to 15 µm instead of the uniform 15 to 25 µm film wrought 6061 gives, and Type III hardcoat is not viable at all.
Finish specifications on die castings go wrong in two directions. Parts get a coating they do not need, because a callout was inherited from a previous drawing revision, and parts get a finish the substrate cannot deliver, because the specification was written for wrought aluminum and applied to a casting.
Both are avoidable with an understanding of what each process physically does to a cast Al-Si surface. This article covers every finish available on die cast aluminum with the numbers that decide between them, and it is direct about the one process that does not work well on castings and never will.
The baseline every casting already has
Before any specified finish, every casting goes through the same three operations: gates, runners and overflows removed on a trim press, flash along the parting line and around slides removed by hand deburring or belt sanding, then shot blasting to a uniform matte texture.
The result is bare matte grey aluminum at Ra 1.6 to 3.2 µm, and it is included in the piece price rather than quoted as a finish. For internal components, machine parts and anything neither seen nor exposed to weather, this is the right specification and adding to it is money spent on nothing.
Blasting is not purely cosmetic. It removes residual release agent and oxide, evens out the visual difference between the smooth as-cast skin and the rougher trimmed areas, and produces a mechanically keyed surface that every subsequent coating adheres to better.
Every finish compared
| Finish | Appearance | Film thickness | Corrosion protection | Cost tier | Best application |
|---|---|---|---|---|---|
| As-cast, trimmed and deburred | Matte grey, faithful to die texture, visible witness marks | None beyond native oxide | None | Included | Internal parts, anything hidden |
| Shot or sand blasting | Uniform matte grey, Ra 2-4 µm | No added thickness | None | Included as standard | Baseline appearance, and the key for every coating |
| Vibratory deburring | Softened edges, light satin | Removes 0.05-0.3 mm at edges | None | Very low | High-volume small parts, edge break, handling safety |
| Mechanical polishing | Bright to near-mirror, Ra 0.2-0.4 µm | Removes 0.05-0.2 mm | None on its own | High, labour intensive | Small bright features, pre-plating preparation |
| Chromate conversion, trivalent | Clear to light iridescent | 0.5-4 µm | 150-250 h alone; roughly doubles the coating over it | Low | Grounding paths, EMI contact faces, paint and powder primer |
| Chromate conversion, hexavalent | Yellow-gold | 1-4 µm | 336 h alone | Low | Only where a defence or aerospace specification demands it |
| Powder coating | Any RAL or Pantone, 10-90 GU gloss | 60-120 µm | 500-1,000 h over conversion coat | Moderate | Industrial housings, outdoor equipment, any coloured part |
| Wet paint, 2K polyurethane | Any colour, thin film, sharp detail retained | 20-60 µm | 300-500 h | Moderate to high | Exact colour match, thin film, heat-sensitive assemblies |
| Anodizing, Type II | Dark mottled grey, visible silicon specks | 5-15 µm on castings | 200-400 h | Moderate | Rarely correct on a casting; see the section below |
| Anodizing, Type III hardcoat | Not achievable | Not achievable | Not applicable | Not offered | Use a hardened insert or electroless nickel instead |
| Electroless nickel plating | Uniform satin metallic, 500-600 HV | 5-25 µm | 500-1,000 h at 25 µm | High | Wear surfaces, controlled contact resistance, complex geometry |
| Decorative copper-nickel-chrome | Bright metallic | 20-40 µm total stack | 300-600 h | High | Bright trim, and only on impregnated castings |
Salt spray figures are neutral ASTM B117 on correctly pre-treated A380 and vary with alloy, geometry and edge coverage. Where a drawing states a specific hour requirement, it is qualified on panels before production release rather than assumed. The in-house process list and inspection records are on the surface finishing page.
The mechanical finishes
Blasting media, and why to specify it
Media choice sets the resulting texture, and the difference is obvious side by side and impossible to correct later without re-blasting. It is worth a line on the drawing.
- Steel shot gives a uniform matte grey at Ra 2 to 4 µm and lightly peens the surface. The default for industrial castings.
- Stainless shot gives the same texture without leaving ferrous particles embedded, specified where iron contamination would cause corrosion staining.
- Ceramic bead gives a fine, even satin, and is the usual choice where the blasted surface is the final visible finish.
- Glass bead gives a brighter satin, used on cosmetic parts and before polishing.
- Aluminum oxide cuts rather than peens, used to key a surface heavily or remove scale.
Machined faces, threads, sealing lands and bores are masked before blasting, because media rounds a sharp machined edge and embeds in a soft surface, and neither is recoverable. Which features get masked follows directly from the machining after casting plan, so the two specifications are agreed together.
Vibratory deburring
Parts tumble with abrasive media and a water-based compound in a vibrating bowl. For high-volume small castings it is far cheaper than hand deburring, and every part comes out the same, which hand work never quite achieves. Typical edge break is 0.1 to 0.3 mm over a 30-minute to 4-hour cycle.
The limits are real. Media cannot reach deep pockets, small holes or narrow slots, it will round a machined edge that was meant to stay sharp, and parts with thin fins or fragile bosses are excluded because part-on-part impact in the bowl damages them.
Mechanical polishing
Belt and buff polishing takes a casting from Ra 1.6-3.2 µm to Ra 0.2-0.4 µm through progressive grits, removing 0.05 to 0.2 mm of material in the process.
That removal figure is the whole story. The dense chill layer on a die casting is only 0.3 to 0.5 mm thick, so polishing removes most of it, and any subsurface porosity in that area appears as pits in an otherwise bright surface. Small features, edges and localised bright areas polish reliably. A large Class A mirror face on a die casting is genuinely difficult, needs A360 or vacuum assist and gating that keeps the polished face away from a last-to-freeze region, and should be quoted with a realistic scrap allowance rather than as a standard operation.
Chromate conversion coating
Conversion coating grows a 0.5 to 4 µm protective film by reacting with the aluminum surface rather than depositing onto it. It does two jobs, and the second is the one people underrate.
As a standalone finish it gives 150 to 250 hours salt spray in trivalent chemistry. As a pre-treatment under powder or paint, it roughly doubles the life of the coating above it and is largely responsible for how long that coating survives once it is scratched. Skipping pre-treatment is the most common cause of coating failure in the field, and it is invisible at incoming inspection because the part looks identical either way.
The property that makes conversion coating irreplaceable is electrical conductivity. Anodizing insulates, powder insulates, paint insulates. Conversion coating protects while keeping contact resistance below 5 mΩ per MIL-DTL-5541 Class 3. Any casting that has to act as a ground path, provide metal-to-metal EMI contact at a joint, or serve as a bonding surface gets conversion coating on those specific faces and something else everywhere else.
Trivalent chromium is standard and is RoHS and REACH compliant, producing a clear to light iridescent film. Hexavalent chemistry reaches 336 hours and is yellow-gold, and is used only where a specification explicitly demands it.
Powder coating
Powder coating is the default finish for die cast industrial parts, and the reason is practical rather than aesthetic. At 60 to 120 µm the film is thick enough to cover the minor surface variation, flow lines and texture differences that are normal on a casting. A 25 µm wet paint film reveals every one of them.
Powder chemistry matters more than colour:
- Polyester TGIC for outdoor and UV exposure, holding gloss and colour for years rather than months.
- Epoxy for indoor parts needing chemical and abrasion resistance, though it chalks under UV.
- Epoxy-polyester hybrid for general indoor use at lower cost.
- Texture, wrinkle and matte finishes where a smooth gloss would show casting variation.
One casting-specific failure mode is worth designing around. Gas trapped inside the casting expands during the 180 to 200 °C cure and escapes through the wet film, leaving pinholes and craters. It appears most often on sections over 6 mm and on faces machined deeply into porosity. The remedy is a pre-bake at 200 to 220 °C for 20 to 30 minutes to degas the part before coating, applied as standard on parts with heavy sections or extensive machining. It is a good example of a finishing defect whose root cause is in the casting, which is why the defect catalogue and the finish specification belong in the same conversation.
Film thickness is checked with a magnetic-induction free gauge on every batch, adhesion by cross-hatch test to ASTM D3359, and colour and gloss against a signed master. Threads, sealing faces, bearing bores, grounding pads and machined datums are masked with silicone plugs and high-temperature tape.
Wet paint
Wet paint beats powder in three specific situations: when an exact match to a physical sample is required, when the film has to stay between 20 and 60 µm because a thicker one would fill a detail or interfere with a fit, and when the part is an assembly containing seals, electronics or plastics that would not survive a 200 °C oven.
Two-component polyurethane over an epoxy primer is the durable choice; single-component acrylic where cost matters more than chemical resistance. Both go over the same conversion-coated substrate as powder, and both can be spot repaired on site, which powder cannot. The trade-offs are lower transfer efficiency, more masking labour and 300 to 500 hours salt spray against powder’s 500 to 1,000.
Plating
Plating on castings is run through a qualified partner rather than in-house, and it comes with one dominant caveat.
Aluminum cannot be plated directly, because its native oxide re-forms faster than a deposit can nucleate. The process therefore begins with a zincate immersion or an electroless nickel strike, which is routine. What is not routine is porosity: plating solutions wick into any surface-connected pore, sit there through the rinse stages, and bleed out over the following weeks, producing blisters and corrosion staining that appear after shipment rather than at inspection. Vacuum resin impregnation before plating is often mandatory rather than optional, and it should be priced in from the start.
Where plating earns its cost:
- Electroless nickel at 15 to 25 µm gives a 500 to 600 HV surface with genuinely uniform thickness on complex geometry, including inside bores and around fins. It is the practical answer to a wear requirement on a casting, and the correct substitute for the Type III hardcoat that is not available.
- Decorative copper-nickel-chrome at a 20 to 40 µm stack gives bright metallic trim, and it is the finish least tolerant of casting porosity of anything in this article.
Anodizing die cast aluminum, and why it comes out patchy
This is the section worth reading before writing a finish callout, because the specification arrives on drawings regularly and the substrate cannot deliver it.
What anodizing actually does
Anodizing makes the part the anode in an acid electrolyte and converts the aluminum surface into a controlled layer of aluminum oxide. The film grows by consuming aluminum. On wrought 6061, with 0.4 to 0.8% silicon mostly in solid solution or as fine Mg2Si precipitates, there is almost nothing in the surface that is not aluminum, so the film grows uniformly, is optically near-transparent, and reaches 15 to 25 µm in Type II. That is why 6061 anodizes to a clean, even tone and takes dye predictably.
What the silicon does
Die casting alloys carry 7.5 to 12% silicon by design, because silicon is what gives the melt enough fluidity to fill a 1.5 mm wall in milliseconds and what suppresses hot tearing as the casting shrinks against rigid steel. Since silicon is less dense than aluminum, that is roughly 11 to 14% by volume, present as coarse eutectic plates and primary particles rather than in solution.
Elemental silicon is essentially inert in the sulphuric acid bath at anodizing voltages. It does not dissolve and it does not convert to oxide. Three consequences follow directly:
- The film cannot grow through the particles, so it grows around them. Each silicon particle ends up embedded in or protruding from the oxide, making the film discontinuous at the micron scale.
- Those embedded particles scatter light. A film that would be transparent on 6061 becomes translucent grey on a casting, so the surface reads as dark charcoal rather than clear, and dyed colours come out muddy and shifted.
- Film growth is limited. Achievable Type II thickness on a casting is 5 to 15 µm against 15 to 25 µm on wrought alloy, which is why salt spray performance lands at 200 to 400 hours rather than 500 to 1,000.
What copper adds
A380 carries 3.0 to 4.0% copper and ADC12 carries 1.5 to 3.5%. Copper-rich phases dissolve preferentially in the acid bath, leaving voids at the surface, causing local burning at the current densities used, and darkening the film further with pitting and streaks. Copper above roughly 0.5% is generally treated as incompatible with a decorative anodize, which rules out both of the most commonly specified die casting alloys.
Why it looks blotchy rather than merely dark
Uniform darkness would be acceptable to most people. Patchiness is the real complaint, and it comes from microstructure varying across the part.
Silicon and intermetallic distribution depends on local cooling rate. The chilled skin against the die is fine-grained with finely divided silicon; slower-cooling thick sections and the last regions to freeze have coarser eutectic and more segregation. Anodizing is a conversion process, so it reports whatever microstructure it finds. A face that was blasted or machined through the skin exposes a coarser structure than an untouched as-cast face beside it, and after anodizing the two read as visibly different areas of the same part. Add lot-to-lot chemistry variation inside the allowed alloy range and batch-to-batch appearance will not hold to a tight standard.
Surface porosity contributes a final mechanism: pores trap acid, incomplete rinsing bleeds out later, and the result is staining that shows up after the part has shipped.
Where anodizing on a casting is defensible
Not never, but narrowly. A360 with copper below 0.6% takes a functional sealed clear or black Type II film that is dimensionally consistent and corrosion-protective. If the part is not a visible cosmetic surface and the requirement is protection rather than appearance, it is a legitimate specification, and we will supply a sample before committing a batch. Expect medium-to-dark grey with visible mottling, and do not expect two parts from the same shot to match closely.
What to do instead
| If the requirement is | Do this on a casting | Not this |
|---|---|---|
| A specific, repeatable colour | Powder coat to RAL or Pantone, ΔE ≤ 1.0 against a signed master | Dyed anodize |
| A Class A anodized cosmetic appearance | Machine the part from 6061 billet at charmaxprecision.com | Anodized die casting |
| Corrosion protection | Chromate conversion plus powder coat, 500-1,000 h | Anodize at 200-400 h |
| A hard wear surface | Electroless nickel 15-25 µm, or a pressed-in hardened bushing | Type III hardcoat, which is not viable |
| Surface electrical conductivity | Chromate conversion, Class 3, under 5 mΩ | Anodize, which is an insulator |
| A metallic appearance | Blast plus clear powder, or plate | Anodize |
| A thin film that will not affect a fit | Chromate conversion at 0.5-4 µm | Anodize, which grows into the part as well as out |
The last row deserves expanding, because it catches people. An anodic film grows roughly half outward and half into the substrate, so a 20 µm film adds about 10 µm per surface and closes a bore by about 20 µm on diameter. On a machined H7 bore that is most of the tolerance band. Mask bores, threads and any fit surface, or specify conversion coating there instead.
If a genuinely anodized cosmetic finish is a hard requirement for your part, the honest recommendation is to change process rather than chase the finish. That is the conversation in die casting versus CNC machining, and it is one we would rather have during DFM review than after first articles.
Choosing a finish by application
| Application | Specify | Why |
|---|---|---|
| Internal bracket, never seen | As-cast, trimmed and blasted | No functional need, and it is already included |
| Indoor equipment housing, coloured | Blast, trivalent chromate, polyester powder 60-100 µm | Colour control, hides normal casting variation |
| Outdoor or coastal enclosure | A360 alloy, chromate, epoxy primer plus super-durable polyester topcoat 100-140 µm | UV and salt resistance; qualify against the stated hour requirement |
| EMI-shielded electronics enclosure | Chromate Class 3 on gasket lands and grounding pads, powder elsewhere with masking | Contact resistance must stay low and stable |
| LED or power heat sink | Blast plus matte black powder at 40-70 µm | Emissivity rises from about 0.05 to 0.85 or more; conduction penalty is small below 70 µm |
| Sealing or gasket face | Machined to Ra 0.8-1.6 µm, masked from all coating | Coating on a gasket land changes clamp load and creeps under it |
| Sliding or wear surface | Electroless nickel 15-25 µm | 500-600 HV, uniform on complex shapes |
| Bright decorative trim | Machine from billet, or polish and plate an impregnated casting | A cast substrate will not hold a Class A bright finish |
| Wipe-down medical or food equipment | Smooth powder coat, gloss 60 GU or above, no texture | Cleanability, and resistance to alcohol and hypochlorite wipes |
| Part with a tight machined fit | Chromate conversion only on that feature | 0.5-4 µm does not meaningfully affect dimensions |
Specifying a finish so it can be inspected
A finish callout that cannot be measured will be argued about. Six items make it enforceable:
- Process and system, including primer if there is one, not just “powder coat”.
- Colour reference: RAL, Pantone or a signed physical master, with a ΔE limit.
- Gloss window in GU, typically ±5 GU at 60°.
- Film thickness range, which is a dimension anywhere a fit is involved.
- Corrosion requirement in salt spray hours to a named standard, and whether panel qualification is required before production release.
- Masked features, listed explicitly: threads, bores, sealing faces, grounding pads, machined datums.
Include those on the drawing and the finish becomes a specification rather than an expectation. The submission checklist for the rest of the drawing is in how to prepare CAD files for die casting.
If you have a finish callout you are unsure about, send the drawing and the requirement behind it through contact. Where the substrate will not deliver what the specification asks for, that is what the DFM report will say, along with the finish that will.
