On a rack line, throughput is limited by rectifier capacity and cycle time. On a basket line it is limited by something less obvious: the proportion of parts that come out with a witness mark where a neighbour touched them. Get packing density right and a basket line is the cheapest anodising plant you can build. Get it wrong and you will run it at half its saleable output without ever seeing a fault on the control panel.

Why contact marks are unavoidable — and why that is fine

Current has to reach every part. In a basket, it does so by passing through the mass of parts themselves: from the basket contact, through part to part, to the piece furthest from the connection. Every point where two parts touch is a point where no electrolyte reaches, so no oxide grows.

Unlike a rack line, you cannot choose where that happens. What you can control is how many contacts each part makes, how large each contact is, and whether the contacts move during the cycle.

The rule that surprises people: parts must not move

"Barrel anodising" is a misleading name and the industry is drifting away from it. Parts are not tumbled while the current is on. They are packed tightly enough that they cannot shift at all.

The reason is electrical. An anodic coating is an insulator. The instant a part moves and breaks its contact path, the oxide it has already grown prevents it from re-establishing one. It stops anodising and comes out of the tank partially coated — and no extension of cycle time recovers it, because the problem is not time.

This inverts the intuition from plating, where movement is desirable and helps uniformity. In anodising, movement is the failure.

The density band

  • Too loose: parts shift under agitation and solution flow. You get partially anodised pieces, scattered randomly through the load, with no pattern that points at a cause.
  • Too tight: more contact points per part, larger shielded areas, and restricted solution flow through the middle of the mass — which shows up as thin or patchy coating at the centre of the basket while the outside is correct.

The band between those two failures is narrower than most operators expect, and it moves with part geometry. A load of hex-head screws and a load of round knobs do not pack the same way at the same weight.

What actually suits bulk processing

Good candidates

Small parts without long flat faces — rivets, screws, spacers, standoffs, rings, small knobs. Curved and irregular shapes touch each other at points rather than along faces, so each mark is small.

Poor candidates

Flat plates, discs, anything with a large visible face. Two flat faces in contact shield a wide area, and on a catalogue product that patch is exactly where the customer looks.

Borderline

Handles and long brackets. Often better racked than bulked, even at higher cost per piece, because the visible face justifies it.

Never

Parts that nest or interlock. Two components that seat into each other create a large sealed contact area and will come out joined by an unanodised patch on both.

The four controls that matter

1. Load weight consistency

Weigh every basket. If load weight varies between batches, packing density varies, current per part varies, and coating thickness varies with it. This is the single cheapest control on a basket line and the one most often skipped.

2. Basket and contact material

Titanium for anything carrying current. Aluminium baskets anodise along with the load, so contact resistance climbs cycle by cycle and the whole load gets progressively thinner — which the operator usually mistakes for a rectifier fault. Non-current-carrying structure can be plastic-coated.

3. Solution flow through the mass

The centre of a packed basket is the hardest place for fresh electrolyte to reach and for heat to leave. Agitation has to move solution through the load, not just around it. Thin or burnt coating in the middle with correct coating at the edges is a flow problem, not a chemistry problem.

4. Current density calculated on real surface area

The total surface area in a basket of small parts is far larger than operators estimate — a load of small components has an enormous area-to-volume ratio. Setting current from load weight rather than calculated area is a common cause of under-coating on a basket line.

Reading the defects

  • Small scattered bare points, otherwise good coating: normal contact shadow. Reduce contacts per part by adjusting packing, or accept them if the part's visible faces are clean.
  • Partial coating on random pieces: movement during the cycle. Pack tighter, do not run longer.
  • Thin or patchy in the centre of the load, fine at the edges: solution flow, or packing too dense.
  • Whole load progressively thinner over successive runs: anodised aluminium baskets or contacts. Strip or replace before touching the rectifier.
  • Burn marks at basket contact points: too much current through too few contact paths.

What it costs to get wrong

A contact-mark reject costs twice: the revenue is gone and the power, chemicals and labour were already spent on the piece. Our plant cost and payback calculator puts an annual figure on each percentage point of reject rate — set the archetype to Bulk / basket so the tanks are sized by pack volume rather than part length, which is how these plants are actually built.

For the wider picture, see bulk anodising for small parts. For how the same problem behaves when parts are racked rather than packed, see rack marks on anodised cookware.

Frequently asked questions

Why do some parts come out only partly anodised in a basket load?

They moved during the cycle and lost electrical contact. An anodic coating is an insulator, so once a part shifts and breaks its current path, the oxide already grown prevents it from re-establishing one and it simply stops anodising. Extending the cycle does not help. The fix is packing the load tightly enough that parts cannot move at all.

Is barrel anodising the same as barrel plating?

No, and the name causes real confusion. In plating, tumbling helps uniformity. In anodising, movement is the failure mode: parts must be packed so they cannot shift, because a part that loses contact cannot regain it. Many practitioners prefer the terms bulk or basket anodising for exactly this reason.

Which small parts are suitable for bulk anodising?

Small parts without long flat faces — rivets, screws, spacers, standoffs, rings and small knobs. Curved and irregular shapes touch at points, so each contact mark is small. Flat plates, discs and anything with a large visible face are poor candidates because two flat faces in contact shield a wide area. Parts that nest or interlock should never be bulk processed.

Why is my whole basket load getting thinner run after run?

Almost always aluminium baskets or contacts. They anodise along with the parts, so contact resistance rises with every cycle and current falls across the entire load. It is commonly mistaken for a rectifier fault. Use titanium for anything carrying current, and inspect it before adjusting the rectifier.