Die casting surface finishing covers the mechanical and chemical processes applied after trimming and machining: shot blasting, vibratory deburring, powder coating, wet painting, polishing, chromate conversion coating and anodizing. CharMax Precision applies these finishes in-house on A380, ADC12 and A360 castings, colour matched to any RAL or Pantone reference.
Die casting surface finishing covers the mechanical and chemical processes applied after trimming and machining: shot blasting, vibratory deburring, powder coating, wet painting, polishing, chromate conversion coating and anodizing. CharMax Precision applies these finishes in-house on A380, ADC12 and A360 castings, colour matched to any RAL or Pantone reference.
Every casting leaves the plant trimmed, deburred and shot blasted as a baseline, giving a uniform matte surface at Ra 1.6 to 3.2 µm. Beyond that, powder coating is the most common specification for industrial parts because it covers the minor surface variation inherent to casting, resists handling damage and delivers 500 to 1,000 hours of salt spray protection over a conversion coat.
One honest caveat runs through this page: die cast aluminum does not finish like wrought aluminum. The 8 to 12% silicon that makes the alloy castable is the same silicon that makes anodizing dark and uneven and makes a mirror polish risky. Where that matters, we say so and propose the finish that will actually meet the requirement.
| As-cast roughness | Ra 1.6 to 3.2 µm |
|---|---|
| Baseline finish | Trim, deburr and shot blast on every part |
| Blast media | Steel shot, stainless shot, ceramic bead, aluminum oxide |
| Powder coat thickness | 60 to 120 µm |
| Powder cure | 180 to 200 °C for 10 to 20 minutes |
| Wet paint thickness | 20 to 60 µm, 2K polyurethane |
| Chromate conversion thickness | 0.5 to 4 µm, trivalent RoHS compliant |
| Anodize thickness on castings | 5 to 15 µm Type II, dark grey only |
| Polished finish | Ra 0.2 to 0.4 µm |
| Salt spray, powder over conversion | 500 to 1,000 h to ASTM B117 |
| Adhesion | ASTM D3359 cross-hatch, 4B to 5B |
| Colour matching | RAL, Pantone or customer sample, gloss 10 to 90 GU |
Before any specified finish is applied, every casting goes through the same three steps. Gates, runners and overflows are removed on a trim press or by sawing, flash along the parting line and around slides is removed by hand deburring or belt sanding, and the part is shot blasted to a uniform matte texture.
Blasting is not only 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. A part blasted and then coated the same week performs measurably better in salt spray than one that sat in storage picking up an oxide layer first.
If the drawing specifies no further finish, the part ships in this condition: uniform matte grey aluminum at Ra 1.6 to 3.2 µm. It is entirely acceptable for internal components, machine parts and anything that will not be seen or exposed to weather.
Blasting propels abrasive media at the casting under compressed air or from a spinner wheel. The media choice sets the resulting texture, and it is worth specifying rather than leaving open, because the visual difference between steel shot and ceramic bead is obvious side by side and impossible to correct later without re-blasting.
Machined faces, threads, sealing lands and bores are masked before blasting. Media will round a sharp machined edge and will embed in a soft surface, and neither is recoverable, so the masking plan is agreed at the same time as the finish specification.
Most finish decisions come down to four questions: does the part need to be a specific colour, does it need corrosion protection, does it need to conduct electricity, and does anyone look at it. The table below answers all four across the finishes we run in-house.
| Finish | Appearance | Corrosion protection | Typical thickness | Relative cost | Best for |
|---|---|---|---|---|---|
| Shot blasting | Uniform matte grey, Ra 2-4 µm | None, bare aluminum only | No added thickness | Included as standard | Internal parts, machine components, base for other coatings |
| Vibratory deburring | Softened edges, light satin | None | Removes 0.05-0.3 mm at edges | Low | High volume small parts, edge break, pre-coating preparation |
| Powder coating | Any RAL or Pantone, 10-90 GU gloss | 500-1,000 h salt spray over conversion coat | 60-120 µm | Moderate | Industrial housings, outdoor equipment, most coloured parts |
| Wet painting | Any colour, thin film, sharp detail | 300-500 h salt spray | 20-60 µm | Moderate to high | Exact colour match, thin film needs, large or heat-sensitive assemblies |
| Mechanical polishing | Bright to mirror, Ra 0.2-0.4 µm | None on its own | Removes 0.05-0.2 mm | High, labour intensive | Cosmetic parts, pre-plating preparation, small polished features |
| Chromate conversion | Clear or light iridescent | 150-336 h salt spray on its own | 0.5-4 µm | Low | Electrical grounding paths, EMI contact surfaces, paint primer |
| Anodizing | Dark, mottled grey, not uniform | 200-400 h salt spray | 5-15 µm Type II | Moderate | Rarely the right choice on castings, see the section below |
Salt spray figures are to ASTM B117 on properly pretreated A380 and vary with alloy, geometry and edge coverage. Where a specific hour requirement is on your drawing, we run a qualification panel before production release.
Powder coating is the default finish for die cast industrial parts, and the reason is practical rather than aesthetic. At 60 to 120 µm it is thick enough to cover the minor surface variation, flow lines and light texture differences that are normal on a casting, whereas a 25 µm wet paint film reveals every one of them.
The pretreatment chain does most of the work. Parts are degreased in an alkaline bath, rinsed, etched, rinsed again, given a trivalent chromate or zirconium conversion coat, then dried before powder is applied. Skipping or shortening pretreatment is the single most common cause of coating failure in the field, and it is invisible at incoming inspection because the part looks identical either way. Adhesion is verified by cross-hatch test to ASTM D3359 on production samples.
One casting-specific problem is worth designing around. Gas trapped inside a casting expands during the 180 to 200 °C cure and escapes through the wet powder film, leaving pinholes and craters. It appears most often on thick sections and on parts that were machined deeply into porosity. The fix is a pre-bake at 200 to 220 °C for 20 to 30 minutes to degas the part before coating, which we apply as standard on any part with sections over 6 mm or with heavily machined faces.
Wet paint gives an exact colour match against a physical sample, applies at 20 to 60 µm where a thicker film would fill a detail or interfere with a fit, and cures at low temperature so it can go on an assembly containing seals, electronics or plastic parts that would not survive a 200 °C powder oven.
Two-component polyurethane over an epoxy primer for durability, or 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.
Belt and buff polishing takes a casting from Ra 1.6-3.2 µm down to Ra 0.2-0.4 µm through progressive grits. It removes 0.05 to 0.2 mm of material, which means it removes most of the dense chill layer, so any subsurface porosity in that area appears as pits in the bright surface.
Small features, edges and localised bright areas polish reliably. A large Class A mirror surface on a die casting is genuinely difficult and should be planned with A360 alloy or vacuum assist, gated so the polished face is not over a last-to-freeze region, and quoted with a realistic scrap allowance.
Chromate conversion coating is a chemical treatment that grows a thin protective film, typically 0.5 to 4 µm, by reacting with the aluminum surface rather than being deposited on it. It has two roles: standalone corrosion protection on parts that stay bare, and the primer layer under powder or paint that is largely responsible for how long the coating survives once it is scratched.
The property that makes it irreplaceable is electrical conductivity. Anodizing insulates, powder insulates, paint insulates. Conversion coating protects while remaining conductive, with contact resistance below 5 mΩ per MIL-DTL-5541 Class 3. Any casting that has to act as a ground path, an EMI shield with metal-to-metal contact at the joint, or a bonding surface in an electrical enclosure gets conversion coating on those specific faces.
We use trivalent chromium chemistry as standard, which is RoHS and REACH compliant and produces a clear to light iridescent film. Hexavalent chromium to MIL-DTL-5541 Type I, with its characteristic yellow-gold colour and higher standalone salt spray resistance, is available where a defence or aerospace specification explicitly calls for it and the customer confirms the compliance position.
Anodizing converts the aluminum surface into a controlled layer of aluminum oxide by making the part the anode in an acid electrolyte. The process only works on aluminum. Anything in the alloy that is not aluminum does not convert, and on a die casting that is a large fraction of the surface.
Die casting alloys carry 7.5 to 12% silicon by design, because silicon is what gives the fluidity and low shrinkage the process depends on. Those silicon particles sit in the surface and simply do not anodize. They remain as unconverted dark grey specks interrupting the oxide film, which is why an anodized die casting looks mottled charcoal grey rather than the uniform light finish you get from 6061. A380 adds a second problem: at 3 to 4% copper, the copper dissolves preferentially in the acid bath, leaving pits and further darkening.
The practical consequences are consistent across the industry, not specific to any one supplier. Achievable Type II coating is 5 to 15 µm rather than 15 to 25 µm on wrought alloy. Dyeing is unreliable because the base colour is already dark and uneven, so black is the only colour that looks deliberate. Batch-to-batch appearance varies with alloy chemistry within its allowed range. Type III hardcoat is not viable on high-silicon die castings at all.
So when a drawing arrives specifying clear anodize on an A380 casting, we raise it before quoting rather than after delivery. There is almost always a better route to the requirement behind the specification.
| Attribute | Die cast A380 | Wrought 6061 |
|---|---|---|
| Silicon content | 7.5-9.5% | 0.4-0.8% |
| Copper content | 3.0-4.0% | 0.15-0.40% |
| Achievable Type II thickness | 5-15 µm | 15-25 µm |
| Appearance | Dark mottled grey, visible silicon specks | Uniform, light, semi-translucent |
| Colour dyeing | Unreliable, black only | Full colour range |
| Batch-to-batch consistency | Poor, varies with alloy chemistry | Good |
| Type III hardcoat | Not viable | 25-50 µm achievable |
| Corrosion protection | 200-400 h salt spray | 500-1,000 h salt spray sealed |
The same limitations apply to ADC12 and A413, which carry even more silicon than A380. A360 anodizes marginally better because its copper is below 0.6%, but the silicon problem remains.
Vibratory finishing tumbles parts with abrasive media and a water-based compound in a vibrating bowl or tub. For high volume small castings it is far cheaper than hand deburring: one load processes hundreds of parts in the time an operator would take on a dozen, and every part comes out the same, which hand work never quite achieves.
It has clear limits. Media cannot reach into deep pockets, small holes or narrow slots, and it will round a sharp machined edge that was meant to stay sharp. It also does not replace machining deburr on precision features. We use it for general edge break and for pre-coating preparation, and hand deburr the features that need judgement.
The equipment, controls and finished-part evidence used to deliver this operation in production.
Real parts we have manufactured that involved this capability.
A European industrial pump manufacturer converted a billet-machined housing to A360 die casting to reach 8,000 units a year, holding 6 bar pressure tightness, an H7 bearing bore and a 0.05 mm sealing face.
A collaborative robot manufacturer moved a joint housing from machined prototypes to ADC12 die casting at 12,000 units a year, cutting 180 g per joint while holding ±0.02 mm concentricity between the harmonic drive seat and the encoder bore.
An LED lighting brand replaced a bonded two-piece design, an extruded heat sink adhered to a die cast housing, with a single A413 casting, removing the thermal interface and taking 11 °C off the LED junction temperature at 60,000 units a year.
Yes, but the result is a dark, mottled grey rather than the uniform finish anodizing produces on wrought alloys. Die casting alloys contain 7.5 to 12% silicon, and silicon does not convert to oxide, so it remains as dark unconverted specks in the surface. A380 also carries 3 to 4% copper, which dissolves in the acid bath and darkens the film further. Achievable Type II thickness is 5 to 15 µm, dyeing is unreliable beyond black, and Type III hardcoat is not viable at all.
Powder coating, matched to any RAL or Pantone reference at 60 to 120 µm with gloss from 10 to 90 GU. It is the right answer for casting because the film is thick enough to cover the minor surface variation that is normal on a cast part, whereas anodizing exaggerates it and thin wet paint reveals it. Where you need an exact match to a physical sample, or a film thin enough not to affect a fit, wet paint at 20 to 60 µm is the alternative.
60 to 120 µm for a standard single-coat system, measured with a magnetic-induction free gauge and recorded per batch. Below about 50 µm coverage becomes unreliable on edges and casting texture starts to show through. Above about 150 µm the film is prone to sagging on vertical faces and to chipping on impact. Allow for the thickness on any feature with a fit requirement, or mask that feature instead.
Gas trapped inside the casting expands during the 180 to 200 °C powder cure and escapes through the wet film, leaving pinholes and craters. It is most common on sections thicker than 6 mm and on faces machined deeply into subsurface porosity. The remedy is a pre-bake at 200 to 220 °C for 20 to 30 minutes to degas the part before coating, which we apply as standard on parts with heavy sections or extensive machining.
Powder coating over a chromate conversion coat reaches 500 to 1,000 hours neutral salt spray to ASTM B117, which covers most outdoor industrial requirements. Wet paint over the same pretreatment reaches 300 to 500 hours, conversion coating alone 150 to 336 hours, and anodizing on castings 200 to 400 hours. Alloy matters too: A360 with its low copper content outperforms A380 in every system. Where your drawing states a specific hour requirement we qualify it on panels before production release.
Yes, using silicone plugs, caps and high-temperature tape, with the masking plan agreed alongside the finish specification. Sealing faces, bearing bores, tapped holes, dowel holes, grounding pads and any machined datum are masked as a matter of course, since 60 to 120 µm of powder would take a threaded hole or an H7 bore out of tolerance. Mark masked features clearly on the drawing so they are not open to interpretation.
Production tooling takes 25 to 40 days depending on part complexity and cavity count, followed by 5 to 7 days for T1 sample production. After you approve samples, production lead time is 15 to 25 days including machining, finishing and inspection. For repeat orders on existing tooling, expect 15 to 20 days.
There is no rigid minimum once tooling exists, and we regularly run batches of 500 pieces. For new programs, the economics rather than a policy set the floor: below roughly 1,000 annual pieces, we will tell you honestly whether CNC machining from billet would serve you better.
Send your drawing with the finish callout, or tell us the requirement behind it: colour, salt spray hours, conductivity or appearance. We will confirm what is achievable on your alloy and supply a sample before production release.