What Is Shot Blasting? Media, Machines and When Castings Need It

Shot blasting is a surface treatment in which small steel or stainless steel particles are thrown at a metal part at high speed to strip scale, sand, rust, and residue and leave a clean, uniformly textured surface. In a foundry, it is the step that turns a rough, gray casting with ceramic still clinging to it into the part you would recognize from the drawing.

Every steel casting that leaves Precimetal passes through a shot blasting machine at least once, and most pass through twice. This article explains how the process works, which media and machines are used, what finish you should expect, and how to decide whether your casting needs it, needs something gentler, or needs nothing at all.

How the shot blasting process works

A shot blasting machine has three working parts: a media supply, a way of accelerating that media, and a chamber that holds the parts while they are blasted. The media is usually cast steel shot (round) or grit (angular), sized and graded to a standard. The most common one in North America is SAE J444, which sets the sieve distribution for each size; European buyers will more often see ISO 11124 on the media certificate, but the sizes correspond closely.

In a wheel blast machine, the shot is fed to the center of a spinning bladed wheel and thrown by centrifugal force at somewhere around 60 to 80 meters per second. In an air blast machine, compressed air pushes it through a nozzle instead. Wheel blast moves far more media per minute at lower operating cost, which is why foundries use it for volume work. Air blast gives the operator a nozzle to point, which is better for small batches, awkward shapes, and spot cleaning.

Whatever the machine, the shot hits the surface, knocks off whatever is loose or bonded, and falls to a hopper. A separator pulls out dust, broken shot, and sand before the media is recycled. Steel shot lasts for thousands of cycles, which is the main reason it replaced sand.

Shot blasting vs sand blasting: what is the difference?

The two terms get used interchangeably, but they describe different equipment and different media. Sandblasting uses compressed air to propel an abrasive (rarely actual sand any more) through a hand-held or fixed nozzle. Shot blasting uses a mechanical wheel to throw steel shot. Shot blasting is more aggressive, faster, and cheaper per part; sandblasting is gentler and more controllable, which suits thin, delicate, or non-ferrous parts.

There is also a health reason the word “sand” has fallen out of use. Crystalline silica dust causes silicosis, and OSHA’s respirable silica rule and the EU’s carcinogens directive both restrict its use. A modern foundry using “sandblasting” is almost always using aluminum oxide, glass bead, or garnet, not silica sand.

Shot blasting media: shot, grit, and stainless

Steel shot is spherical. It peens the surface as it cleans, closing pores slightly and leaving a matte, dimpled texture. Steel grit is angular, cuts rather than peens, and leaves a rougher, sharper profile that paint and coatings grip better. Foundries cleaning castings for final use mostly run shot; fabricators preparing steel for painting mostly run grit.

Sizes are designated by number: S110, S170, S230, S330, S460 for shot (the number is the nominal sieve opening in ten-thousandths of an inch, so S230 is about 0.6 mm), and G80, G50, G40, G25 for grit. Smaller shot reaches into tight features and leaves a finer finish; larger shot cleans faster and hits harder. For investment castings in the 20 g to 50 kg range, S110 to S230 covers most work.

One detail that matters if you buy stainless steel castings: carbon steel shot leaves microscopic iron particles embedded in the surface. On a 304 or 316 casting, those particles rust within days in a humid warehouse and show up as orange freckling that customers often mistake for a defective alloy. The fix is to blast stainless castings with stainless steel shot, or with a non-metallic media, and follow with passivation to ASTM A967 or A380. Ask your supplier which shot they use on stainless before you approve a first article. It is a five-second question that prevents a lot of rejected shipments.

Types of shot blasting machine

Foundries pick the machine by part size, shape, and how much surface must be reached:

• Tumble blast machines carry small, robust parts in a rotating rubber belt or drum while wheels blast from above. They are cheap to run and give complete coverage, but parts touch each other, so they suit brackets, fittings, and small valve bodies rather than parts with a cosmetic face.
• Hanger or spinner hanger machines hang larger or fragile parts from hooks that rotate in front of the wheels. Nothing touches the part except the shot. Pump housings, impellers, and manifolds are typically blasted this way.
• Table machines rotate parts on a turntable inside the chamber, blasting one side at a time. They are useful for flat or one-sided parts and for medium-sized single pieces.
• Continuous mesh belt and pass-through machines feed parts on a conveyor and are used for high-volume automotive components where every part is the same.

Air blast cabinets sit alongside these for touch-up, for internal passages the wheel cannot reach, and for small trial batches before production tooling exists.

When does a casting need shot blasting?

In investment casting, shot blasting appears at two points in the 15-step process. The first is right after knockout, when the ceramic shell has been broken off but a film of refractory still clings to the surface, especially inside pockets and around gates. The second is after heat treatment, which leaves a dark oxide scale that would otherwise carry through to the customer.

Beyond those two, blasting is worth specifying when:

• The casting will be painted, powder coated, or plated. Coatings adhere to a blasted profile far better than to a smooth or scaled surface. ISO 8501-1 grade Sa 2½ (“very thorough blast cleaning”) is the standard most coating specifications call for.
• The part needs a uniform cosmetic appearance. Blasting removes color variation from heat treatment and hides minor surface marks.
• You want a slight fatigue improvement. Shot leaves the surface in mild compression. This is not true shot peening (which is a controlled process with Almen strip verification), but it does no harm and often helps.
• Internal passages must be clean. Loose ceramic inside a valve body or an EGR passage will end up in the customer’s pump or engine. Air blast with a lance, sometimes combined with water-jet cleaning, is used for these.

When shot blasting is the wrong choice

Shot blasting is not free of side effects, and there are parts where we recommend something else.

Thin-walled castings under about 2 mm can distort under heavy shot; a smaller size at lower wheel speed, or glass bead in an air cabinet, avoids it. Parts with a machined face already finished to 0.8 Ra or better should be blasted before machining, not after, or masked. Parts that will be electropolished or bright-finished do not want the matte dimpled texture that shot leaves; a chemical descale is the right route. And if your drawing specifies a maximum surface roughness, tell the foundry which faces it applies to. Shot blasting on our floor leaves an as-cast surface at 3.2 Ra or better; a tighter value on one sealing face should be called out on that face only, otherwise it gets quoted across the whole part.

What to specify on your drawing or RFQ

Three lines on the drawing or in the purchase order avoid most misunderstandings:

1. Media type and size (for example, “stainless steel shot S170” for stainless, “cast steel shot S230” for carbon steel).
2. Cleanliness or finish requirement (ISO 8501-1 Sa 2½ if the part is to be coated; a maximum Ra value if roughness matters; “free of loose refractory” for internal passages).
3. Post-blast treatment (passivation standard for stainless; rust preventive oil for carbon steel going by sea).

If you are unsure what to write, send the end-use of the part and we will recommend the callout. This is a routine conversation during the design-for-manufacturing review that precedes every new tool at Precimetal.

Frequently asked questions

Is shot blasting the same as shot peening?

No. Shot blasting is mainly used to clean and prepare casting surfaces, while shot peening is a controlled process used to improve fatigue resistance by introducing compressive stress. At Precimetal Cast, shot blasting is used as part of the finishing process for investment castings and should not be treated as a substitute for specified shot peening.

Does shot blasting change dimensions?

Very slightly. A cleaning pass with S110 to S230 shot removes scale, not base metal, and has no measurable effect on dimensions held to IS 11166 or VDG P690 tolerances. Heavy grit or prolonged blasting can remove material and round edges, which is why we blast before finish machining.

What surface finish does shot blasting leave on a casting?

A uniform matte texture. On our investment castings the as-cast surface after blasting is 3.2 Ra (about 125 RMS) or better; finer finishes come from machining, polishing, or electropolishing rather than from blasting.

Can stainless steel castings be shot blasted?

Yes. Stainless steel investment castings can be shot blasted using suitable media selected according to the material and surface requirements. Precimetal Cast can select the appropriate finishing process for stainless steel castings, with additional treatments such as passivation available where required for corrosion resistance.

How much does shot blasting add to the cost of a casting?

The cost depends on the casting size, batch volume, media, surface requirements, and finishing specifications. For standard production components, shot blasting is generally an efficient finishing operation because multiple castings can be processed in a cycle and blasting media can be reused. At Precimetal Cast, requirements such as special media, masking, or additional surface treatment can affect the final casting cost and should be considered during quotation.

Conclusion

Shot blasting is more than a cleaning step. The right media, machine, and blasting specification can determine the surface quality, coating performance, and condition of your casting.

Before approving a casting, confirm three things: the media type and size, the required surface finish or cleanliness, and any post-blast treatment. For stainless steel, also confirm that stainless or non-metallic media will be used to prevent iron contamination.

If you are unsure what finish or blasting specification your casting requires, send your drawing, alloy, and end-use requirements to Precimetal before finalizing the RFQ. Our team can recommend the appropriate blast and finish callout for your application.

Talk to us about your casting finish

If you are sourcing stainless steel castings, pump and valve components, or automotive castings and want the finish specified correctly from the first sample, send us your drawing. Tell us the alloy, the end use, and whether the part will be coated, and we will come back within 24 hours with a recommended blast and finish callout as part of the quotation.