Coil Anodizing

Saves Time, Material Handling & Finishing Costs

Aluminum coil anodizing was created as an alternative to piece part anodizing. Customers demanded improved products and cost containment by anodizers in the mid 1930’s. The response was a new concept: The concept of pre-anodizing aluminum coil and strip prior to fabrication. Eventually, manufacturers were able to produce and assemble products without having an intermittent finishing step. The aluminum received into their plants and fed into their production lines was already anodized. Savings in time, material handling, and finishing costs were immediate and significant.

Reprinted with permission of Continuous Coil Anodizers Association.

Anodizing Basics

Aluminum is anodized for corrosion resistance, abrasion resistance, insulation from electricity, adhesion, or aesthetic enhancement.

Definition
Anodizing is the successful development and control of a natural oxidation process that occurs when aluminum is exposed to the atmosphere. Electricity and chemicals are used jointly to produce a hard, transparent surface that is integral with base aluminum.

Characteristics
Hard, comparable to a sapphire
Transparent, similar to glassInsulative and static-resistantWide variety of colors and finishes
Integral with aluminum surfaces, non-flaking

The Anodizing Process

Batch (or piece), sheet, and coil anodizing all consist of three processing stages:

Pretreatment
The aluminum surface is first cleaned then chemically treated. Etching results in a satin matte appearance. Various degrees of etching can be specified (i.e. light, medium, heavy). Bright -dipping will enhance an already bright aluminum surface, and result in varying degrees of reflective finishes.

Anodizing
Once the surface is prepared, the anodic film is built. Electrical current is passed through an electrolyte bath in which the aluminum has been immersed. The anodize film is built from the aluminum itself, not applied. It is a hard and porous film. The coating thickness may be tightly controlled, based on the end use product.

Post Treatment
The porous anodic film can be colored in this stage. Organic dyes can be used to fill pores with color, or metal salts can be electrochemically deposited at the base of the pores to create a broad spectrum of colors. Many of the colors are fade-resistant. Sealing the anodic film normally consists of a hot water bath that basically swells the pores shut.

Methods of Anodizing

Regardless of the method, the anodizing process is basically the same.

Continuous Coil Anodizing

Uses
High volume
Coiled sheet
Foil
Products with less severe forming

Advantages
Wide range of metal and film thicknesses
Less material handling
Precise color control and uniformity
Cost effective

Disadvantages
Bare edges on stamped parts
Crazing when severely formed
Limited to sheet and foil

*Coil anodizing involves continuous unwinding of coils through a series of anodizing tanks and then rewinding.

Continuous Coil Capabilities

Sheet Anodizing

Uses
Wide widths
Plate
Large fabricated products

Advantages
Small runs
Thicker films
Anodized edges

Disadvantages
Color variance
High costs
Film thickness variance
Crazing when severely formed

*Sheet anodizing involves racking or framing of sheets and immersing them in large tanks.

Batch Or Pierce Anodizing

Uses
Extrusions
Castings
Parts with severe forming

Advantages
Small runs
Thicker films
Anodized edges

Disadvantages
Color variance
Excessive handling
High costs

*Piece anodizing involves racking parts and immersing them in a series of treatment tanks.