Barrel plating vs rack plating: how small fasteners are actually plated
We run both lines in house. This page explains which one your part belongs on, and what changes on the drawing and in the price when the answer is rack.
Barrel plating vs Rack plating
Short answer: bulk fasteners go in the barrel; rack is for parts that cannot tumble
Barrel plating tumbles thousands of loose parts in a perforated rotating barrel, with current arriving through danglers and part-to-part contact. It is by far the cheapest route per kilogram and the normal one for screws, nuts, bolts, washers and pins. The price is a wider thickness spread — heavy on edges, corners and thread crests, thin in recesses, blind holes and thread roots — plus the tumbling itself, which nicks threads, bends thin pins, tangles springs and lets thin washers nest face to face and come out with unplated patches. Rack plating hangs each part on a jig with its own contact point: more uniform, controllable where it matters, capable of heavier deposits, and free of tumbling damage — at a cost per piece several times higher and a fraction of the throughput. A part genuinely has to go on a rack when it is too large or heavy to tumble without battering itself, too delicate or too easily deformed, needs a heavy closely controlled thickness, or carries a visible surface that a tumble mark would reject.
Barrel plating versus rack plating
| Decision factor | Barrel plating | Rack plating |
|---|---|---|
| How the part is held | Loose in a perforated rotating barrel; current through danglers and part-to-part contact | Hung individually on a jig with its own contact point |
| Load size | Thousands of pieces per barrel, loaded by weight | Tens to a few hundred pieces per rack, hung by hand |
| Thickness uniformity | Wider spread — the minimum must be met at the worst point, so the average overshoots | Tighter and controllable; jig and anode position can favour the difficult face |
| Thickness commonly achievable | Around 5–12 µm zinc on fasteners is routine | About 12–25 µm and above is practical |
| Edges versus recesses | Dog-boning: heavy on corners, edges and thread crests, thin in recesses, blind holes and thread roots | Same physics, but the difficult geometry can be presented to the anode |
| Part size and weight | Parts must tumble freely; large or heavy parts batter each other and the barrel | No tumbling limit — bounded by jig and tank size |
| Damage modes | Nicked threads, bent thin pins, tangled springs, nested washers with unplated faces | Contact marks where the jig grips; the part is otherwise untouched |
| Cost basis | Lowest cost per kilogram — the reason bulk fasteners are barrel plated | Cost per piece, typically several times higher |
| Hydrogen relief baking | Long pickle and plating cycles; parts above the ISO 4042 trigger must still be baked, started promptly | Same rule — hardness decides whether baking applies, not the plating method |
| Typical parts | Screws, nuts, bolts, washers, pins, small springs | Long thin parts, delicate or cosmetic parts, parts needing heavy uniform thickness |
How to choose
Barrel plating
Choose barrel plating for bulk small fasteners — screws, nuts, bolts, washers and pins in the thousands, where the requirement is a service condition and a minimum thickness rather than a show surface. This is the normal route for almost everything we plate, and it is the only reason the cost per piece stays low enough for a fastener.
Rack plating
Choose rack plating when the part cannot survive tumbling, or cannot meet the specification in a barrel: long thin parts that bend, fine springs that tangle, thin flat washers that nest, parts with a visible surface, and parts needing a heavy, closely controlled thickness. Budget for a per-piece price and more time per load, and send samples so the jig can be confirmed before you commit a lot.
Limits before putting it on the drawing
- Barrel thickness is a distribution, not a single number — state on the drawing where thickness is measured, or it becomes an argument after the lot is plated.
- Thickness and salt-spray figures here are typical guidance for small fasteners and depend on the part, the bath and the governing specification; confirm with trial pieces.
- Thin flat washers, cup washers and stamped clips nest in the barrel — tell us before the lot arrives so the load size or the fixturing can be changed.
- Rack plating fixes uniformity, not hydrogen: parts at or above roughly 320 HV (class 10.9 and up) need de-embrittlement baking either way, started as soon as practical after plating per ISO 4042.
- We run trivalent Cr3+ zinc, phosphate and black oxide. Hot-dip galvanizing and zinc-flake systems are different processes we do not apply.
What to send the plater
- 1Part drawing with the thickness requirement and the point it is measured at
- 2Part weight, maximum dimension and lot quantity or weight
- 3Whether the part nests, tangles or bends — thin washers, springs, long pins
- 4Service condition or standard, e.g. ASTM B633 SC1–SC4 or ISO 2081
- 5Property class or hardness, so relief baking can be assessed
- 6Any cosmetic requirement, and whether jig contact marks are acceptable
- 7Sample pieces for a trial load before a production lot
Frequently asked questions
Do you offer both barrel and rack plating?
Yes. Barrel is the workhorse for bulk small fasteners and rack is available for parts that cannot tumble. Send the part and its thickness requirement and we will tell you honestly which line it belongs on — including when the honest answer is that a barrel cannot hold the tolerance you need.
Why did some of my washers come back with bare patches?
That is nesting: thin flat parts stick face to face in the barrel, so the mated faces see neither current nor solution. It is fixed with smaller loads, separating media, or by moving the part to a rack — but it has to be identified before the lot runs, not at incoming inspection.
Can barrel plating hold a 15 µm minimum?
Sometimes, with a longer run and a smaller load, and the average thickness then overshoots the minimum considerably. Whether that beats racking depends on the part and the quantity — send the drawing and the lot size and we will cost both routes.