Bead Blasting
Also: glass bead blasting, media blasting, abrasive blasting, sandblasting, sand blasting, grit blasting, bead blast finish, aluminum oxide blasting, vapor blasting, wet blasting, blast finish
Bead blasting throws glass bead or grit at a part to clean it, hide tool marks and leave a uniform matte finish: the usual prep before anodize and paint.
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Abrasive blasting drives media at the surface with compressed air through a nozzle, inside a cabinet for small parts or a blast room for weldments; production lines use wheel blast machines that throw the media centrifugally. What the media does depends on its shape. Round glass bead peens and smooths, removes almost nothing, and leaves a soft, even satin matte that hides machining marks. Angular grit (aluminum oxide, garnet, silicon carbide, steel grit) cuts: it strips scale, rust and old paint and leaves an anchor profile for coatings. In a job shop "bead blasting" means the cosmetic glass-bead finish, and "grit blasting" or "sandblasting" means the cleaning and profiling job. Silica sand itself is little used in the U.S. because of the silicosis hazard.
The finish depends on media type and size, air pressure (commonly 2-6 bar, 30-90 psi), nozzle distance and angle, dwell time, and the condition of the media, since glass bead breaks down into finer particles with use. On machined aluminum, bead blasting before anodize is the classic cosmetic finish for enclosures and panels: a uniform, low-glare matte that the anodize preserves. Anodize also shows every variation in the blast, so cosmetic lots are blasted together, by one operator or in an automated cabinet, against an approved sample.
Blasting has limits. It rounds sharp edges slightly, it can bow thin sheet by peening one side, and media lodges in blind holes, threads and internal passages unless they are plugged. Media must be dedicated: steel grit or shot used on aluminum or stainless embeds iron that later rusts. And a blast cabinet is not a shot-peening machine: uncontrolled bead blasting leaves some compressive stress but earns no fatigue credit, which takes controlled shot peening to a specified Almen intensity.
At a glance
| Typical tolerances | Glass bead removes very little, typically under about 5 µm (0.0002") per surface, and does not change dimensions measurably on most parts. Aluminum oxide and other grits remove more, roughly 5-25 µm (0.0002-0.001"), and leave a surface profile of about 25-75 µm (1-3 mils) peak to valley for coating adhesion. Both round sharp edges slightly. Sheet thinner than about 1.5 mm (0.06") can bow when blasted on one side. Bearing bores, seal faces, threads and datum faces are masked. |
|---|---|
| Size limits | Blast cabinets commonly take parts up to about 0.6-1.2 m (24-48") across; blast rooms take weldments, frames and trailers. Automated cabinets with turntables or conveyors handle production cosmetic parts, and tumble and wheel blast machines handle bulk small castings and stampings. Very small parts go in baskets or on fixtures so they are not lost in the media. |
| Surface finish | Glass bead on machined aluminum: a uniform satin matte, typically about 1-3 µm Ra (40-125 µin) depending on bead size and pressure, finer with fine bead; clear anodize over it gives the familiar soft silver-grey. Aluminum oxide grit: a duller, rougher etched look, about 2-6 µm Ra (80-250 µin), with a 25-75 µm anchor profile for paint and powder. Stainless comes out satin grey. Blasting adds no coating and no controlled change in hardness; it leaves a clean, active surface that should be coated, anodized or passivated soon after. |
| Lead time | Often in-house at machine shops, same day or next day; 1-3 business days at a finishing shop. Blast plus anodize or powder coat usually runs 1-2 weeks through the finisher. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Aluminum is the most common, as cosmetic prep before anodize or chem film. Stainless steel with glass bead or aluminum oxide on dedicated media, followed by passivation. Carbon steel with grit before paint, powder or thermal spray. Titanium, nickel alloys, brass and copper, and castings in every alloy to remove sand, scale and investment shell. Tool steel molds for texture. Plastics and 3D-printed parts with plastic media, walnut shell or fine glass bead at low pressure for deflashing and an even matte.
What drives the cost
- Masking of threads, bores, seal faces and datums, which is often more labour than the blasting
- Cosmetic requirements: A-surface grading, matching an approved sample, lot-to-lot consistency
- Part size: cabinet work is cheap; blast-room work on large weldments is billed by the hour
- Media: glass bead is inexpensive; aluminum oxide, ceramic and dedicated contamination-free media cost more
- Cleaning after blasting: blowing out holes, washing, and checking for lodged media
- Quantity: automated or tumble blasting drops the per-part cost at volume
When to use it
- A uniform matte cosmetic finish on machined aluminum before anodizing
- Hiding tool marks, handling scratches and minor die lines on visible parts
- Removing mill scale, rust, heat-treat scale and old paint before coating
- Creating an anchor profile for powder coat, paint or thermal spray
- Cleaning castings, forgings and 3D-printed metal parts of sand, scale and loose powder
- A low-glare surface on optical, instrument and camera hardware
When not to
- Precision bores, seal faces and bearing fits that cannot be masked
- Thin sheet parts that will bow when one side is blasted
- Parts with internal passages, blind holes or crevices where media can lodge and later come loose (hydraulic manifolds, fuel system parts), unless they are plugged and cleaned
- Fatigue-critical parts that need a compressive layer: specify controlled shot peening instead
- Bright or mirror finishes: polish or electropolish instead
Design tips
- Call out the media and size, and the pressure if it matters: "Glass bead blast, fine, to match approved sample" or "Aluminum oxide grit blast, 25-50 µm profile".
- For cosmetic anodized parts, approve a sample and ask for the lot to be blasted and anodized together.
- Mark every surface to be masked: threads, bores, seal faces, electrical contacts, datum faces.
- Deburr before blasting; blasting softens a burr but does not remove it.
- Require dedicated media for aluminum and stainless, and passivate stainless after blasting.
- Avoid one-sided blasting on sheet thinner than about 1.5 mm, or have both sides blasted to balance the stress.
- Coat or anodize soon after blasting; freshly blasted steel flash-rusts within hours in humid air.
Frequently asked questions
- What is the difference between bead blasting and sandblasting?
- Bead blasting uses round glass beads that peen the surface to a smooth satin matte and remove almost nothing. Sandblasting, more accurately grit blasting, uses angular media that cuts: it strips rust, scale and paint and roughens the surface for coating. Silica sand itself is rarely used now because of the health hazard.
- Does bead blasting remove material?
- Very little. Glass bead typically takes under about 5 µm (0.0002") per surface and does not change most dimensions. Angular grit removes more, about 5-25 µm, and rounds edges, so mask any fit that matters.
- Should aluminum be bead blasted before anodizing?
- For cosmetic parts, usually yes. Blasting hides tool marks and gives an even matte that anodize preserves; without it, anodize makes every machining mark more visible. Run the whole lot through blast and anodize together for a consistent look.
- What media is used to blast stainless steel?
- Glass bead for a cosmetic satin finish, or aluminum oxide for a rougher profile, on media never used on carbon steel. Steel shot or grit embeds iron that rusts. Passivate the part afterwards to restore the chromium oxide layer.
- Is bead blasting the same as shot peening?
- No. Both throw round media, but shot peening is a controlled process, with a specified Almen intensity and measured coverage, that puts a known compressive stress in the surface for fatigue life. Cosmetic bead blasting has no such control and earns no fatigue credit.
