Racking, a sensitive part of the anodizing process

When the parts arrive in the anodizing shop, even before starting to process the aluminum parts, they need to be unpacked and racked. It is important to have a clear agreement with the anodizer on how the parts are to be be racked. The areas where the rack and the part will touch will not be anodized.

Racking, also called jigging, is one of the most important steps in the anodizing process. The basic functions of the racking is to hold the part tight during transfer between the different process tanks and to provide sufficient contact to the part, so that enough current can run, to form a uniform layer on the aluminum part.

In addition to this it is important that the racks are designed so that no solution from the process tanks can be entrapped.

Aluminum and titanium are the most common materials for racking before anodizing. There are advantages and disadvantages of both materials.

Advantages in using aluminum rack:
  • Low price
  • Easy to machine
  • High electrical conductivity

Disadvantages in using aluminum rack:

  • Attacked chemical and electrochemical
  • Soft material

Advantage in using titanium rack:

  • High strength and durability
  • Chemical inert, except from fluoride

Disadvantage in using titanium rack:

  • Expensive when purchased
  • Specialized needs for repairing
  • Less electrical conductivity than aluminum

The electrical conductivity is a very important property when anodizing. The most efficient utilization of the energy is when all the current is sent to the parts.

Titanium has an electrical conductivity of around 10% of aluminum. The conductivity of aluminum is highly dependent on the alloy used. The rule of thumb for design of aluminum racks is 1,000 amperes per square inch. The comparable value for solid titanium is 350 amperes per square inch.

Titanium racks can be designed and used to give the most optimal anodizing result of the aluminum part. The reason for this is that titanium only forms a very thin oxide layer which is readily removed in the process of racking new parts. The only "but", is that titanium racks should not be used when the parts are electrolytical colored.

Aluminum will anodize along with the work and if not carefully selected it will actually steal all the current from the parts to be anodized. So after anodizing, aluminum racks have to be stripped to remove the formed oxide layer. This issue reduces the life of the aluminum racks tremendeously.

Two aluminum alloys which are considered as good racking materials are 6061 and 2024. The reason for this is that they anodize very slowly compared to a lot of other aluminum alloys.

Instead of using racks, a lot of anodizers use plastic clamps for fast racking of extrusions. Using plastic clamps will give less aluminum in the waste treatment due to the fact that the plastic clamps should not be stripped after the anodizing process as the aluminum bolts and nuts.

Courtesy of Burlingham International

The plastic clamps can also be used together with bolts and nuts, as photo below show.

If you find this article useful and you would like to know more please contact me blog@aluconsult.com