What You Need to Know About Anodising Individual Aluminium Parts Post-Machining

When machining an anodised aluminium profile, every cut, hole, and machined feature removes part of the anodised layer and exposes raw aluminium.

For applications where the entire component must remain protected or maintain a consistent appearance, anodising after machining is the solution, but this comes with additional cost and manufacturing considerations.

Here's what you need to know before choosing post-machining anodisation.

How aluminium anodisation works

Before understanding why anodising after machining may be necessary, it helps to understand how the anodisation process works.

Anodising is an electrolytic process that converts the aluminium surface into a controlled oxide layer. Depending on the method used, such as chromic acid anodising (Type I), sulphuric acid anodising (Type II), or hard anodising (Type III), the resulting layer can range from a thin protective film to a dense, wear-resistant coating.

The resulting aluminium oxide layer is:

• Harder than the base metal

• Highly corrosion- and wear-resistant

• Electrically insulating

• Naturally porous, allowing optional colouring before sealing

Layer thickness typically ranges from 5 - 10 µm for decorative indoor applications, 15 - 25 µm for architectural and outdoor applications, and 25 - 70 µm for hard anodising where increased wear resistance is required.

Why anodise after machining your aluminium profile?

Machining operations remove any anodised surface they come into contact with. If a profile is anodised before machining, all machined areas will expose raw aluminium.

Post-machining anodisation is therefore chosen when the entire component must remain protected by the anodised layer and maintain a consistent appearance after all manufacturing operations have been completed.

This is often the case for:

• Visible or decorative aluminium components

• Outdoor applications exposed to corrosion

• Medical equipment

• Architectural components

• High-end consumer products

Taking the medical equipment example, aluminium in the medical industry is often anodised after machining to ensure the entire part is protected by the anodised layer. Because this layer provides increased surface hardness and wear resistance, improved corrosion resistance and biocompatibility when exposed to bodily fluids, sterilisation processes, and harsh chemical cleaners.

What post-machining anodisation involves

In post-machining anodisation, aluminium components are anodised individually after all machining operations have been completed.

Each part is mounted on custom jigs or racks before being immersed in the electrolytic bath. Unlike anodising long extruded bars, where many parts may share electrical contact points and experience minor variations, individual fixturing allows significantly better control of layer thickness, colour consistency, and surface finish.

However, it is important to note that the contact points used to hold the component during anodising will remain uncoated. These locations, often referred to as rack marks, should be considered during the design stage, particularly for visible components.

How anodising affects dimensions and tolerances

One of the most important considerations when anodising machined aluminium components is dimensional change.

The anodised layer grows both into and out of the aluminium surface. As a general rule, approximately 50% of the coating thickness penetrates the aluminium surface while 50% builds outward.

For example, a 20 µm anodised layer will typically increase a dimension by approximately 10 µm per surface.

While this may seem insignificant, it can become important on tight-tolerance features such as bearing fits, sliding components, and threaded holes.

When designing aluminium profiles or machined aluminium components that will be anodised after machining, the coating thickness should be considered during the design phase.

Surfaces and machined features you don't want to anodise

Not every surface should be anodised. For example, threaded holes usually should not be anodised.

A common solution, and one we regularly see and use at ALUCAD, is to complete all other machining operations, anodise the individual parts, and then machine or tap the threads afterwards.

There are also applications where electrical grounding surfaces are required, and anodisation can interfere with conductivity.

These are important considerations before deciding whether your bespoke aluminium profile should be anodised after or before machining.

When you don’t need post-machining anodisation

Piece-by-piece anodising is not always necessary.

Anodising before machining reduces handling and fixturing requirements, making production faster and more cost-effective. The trade-off is that any subsequent machining operation will expose raw aluminium and may result in less control over colour consistency, coating uniformity, and cosmetic appearance.

If these factors are not critical to the application, anodising the extruded bars before machining is usually the better option. It's simpler and more economical.

The decision to anodise before or after machining should be made during the design stage, as it affects cost, appearance, corrosion resistance, dimensional tolerances, and manufacturing lead times.

Understanding the differences between pre- and post-machining anodisation will help you make the right decision for your application.

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If your bespoke aluminium profile or machined aluminium component requires anodising after machining, ALUCAD can provide a cost-effective solution while ensuring the quality and consistency your application demands.