Print Page   |   Contact Us   |   Sign In   |   Join the AAC
Coil Anodizing
Share |


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.


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   Sheet Anodizing   Batch or Piece Anodizing
         
Uses
High volume
Coiled sheet
Foil
Products with less severe forming.
  Uses
Wide widths
Plate
Large fabricated products.
  Uses
Extrusions
Castings
Parts with severe forming
         
Advantages
Wide range of metal and film thicknesses
Less material handling
Precise color control and uniformity
Cost effective
  Advantages
Small runs
Thicker films
Anodized edges
  Advantages
Small runs
Thicker films
Anodized edges
         
 Disadvantage
Bare edges on stamped parts
Crazing when severely formed
Limited to sheet and foil
  Disadvantages
Color variance
High costs
Film thickness variance
Crazing when severely formed 
  Disadvantages
Color variance
Excessive handling
High costs
         
         
         
*Coil anodizing involves continuous unwinding of coils through a series of anodizing tanks and then rewinding.   *Sheet anodizing involves racking or framing of sheets and immersing them in large tanks.   *Piece anodizing involves racking parts and immersing them in a series of treatment tanks





































Reprinted with permission of Continuous Coil Anodizers Association.

Back