Anodize racking is a popular way to cost-effectively electroplate parts in large numbers with minimal gaps in coverage. If you didn’t know, anodizing is another name for electroplating. It involves using one metal to coat or design another. Racking is also called jigging and it’s one of the most important steps in the anodizing process. The basic functions of racking are 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 create a uniform layer on the aluminum part.

Racking serves two functions–It provides a way to physically hold the part during the anodizing process, and it is the means by which electrical current enters the part.

Multiple racking methods are used depending on material type and requirements. There are many potential process steps and many differences in an aluminum anodizing process, but the main objective is for the parts to move up and down as they enter and exit tanks, side to side in chemicals and water. For example, to complete a generic color anodizing process, a part goes through four major process stages (pre-clean, process, dye, and seal). Each of these processes consists of one or more chemical tanks and usually multiple rinse tanks. 

This means that components need to be secured during this process. If left unracked, parts get lost in the tank or damaged by accidentally falling off the rack. And with heavier parts, sturdier racks are needed, and the larger the part, the bigger the rack marks. In order to keep larger components stable, the part needs to be securely fixtured to restrict movement, and this usually means multiple contact places—causing rack marks.

It is the latter of the two racking functions that frequently gets overlooked by designers and engineers, yet it’s an essential one because supplying the correct current is crucial to developing an anodic coating. Anodizing is an electro-chemical process where coatings are created by converting the aluminum on the surface of the part to aluminum oxide. The amount of current required to drive this process is in direct relation to the surface area of the part. The most common racking techniques used in the industry are clamping, bolting, or welding. The electrical contact where the rack touches the part must be able to accept 10 – 50 amps per square foot (ASF) of surface area, depending on whether the anodic coating being applied is Type II (decorative) and or Type III (hard coat).  There are many important features of this basic requirement:

1) Type II anodizing leaves smaller rack marks than Type III, as less current is required.
2) The size of the rack mark is a function of the surface area of the part. Larger surface area parts have larger rack marks as more total current is applied. Larger surface area parts also may need to have more than one rack location to create a uniform coating – as the coating grows from the racking location until there is a consistent coating thickness across the part.
3) The rack mark depends also on the conductivity of the rack. Racks can be made out of aluminum or titanium. Titanium, which is a more electrically resistant metal, generally requires more surface area to conduct the same amount of electrical current than aluminum racks. The size of the rack mark cannot always be an indicator. In addition, the contact between the rack and the part needs to be very tight, as loose racking creates higher resistance and heat, which can lead to burned parts.

Regardless of the racking method, it is critical that the aluminum is held tightly and has significant electrical contact throughout the process.

A Step-by-Step Walkthrough of the Racking Process

When the raw parts arrive in our anodizing facilities, they are unpacked and racked. Before anything happens, we make a point of securing a clear agreement with our clients on how the parts are to be racked after explaining the areas on the components where the rack and the part touch. These areas will not be anodized. 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 to both materials.

A few advantages of using aluminum racks: High electrical conductivity, low price, and easy to machine.

Disadvantages of using aluminum racks:  Gets attacked by chemicals and the electrochemical process and it’s inherently a soft material that can be damaged during the process.

Advantages of using titanium racks: High strength and durability, and it’s chemical inert, except from fluoride. Disadvantages of using titanium racks are it’s expensive, has specialized needs for repairing, and titanium is less conductive 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 provide the best anodizing results in the aluminum part. That’s because titanium only forms a very thin oxide layer which is easily removed while racking new parts. The only exception is that titanium racks should not be used when the parts are electrolytically colored.

Aluminum will anodize along with the part and if not carefully selected, it can take all the current from the parts that are supposed to be anodized. So following the anodizing process, aluminum racks have to be stripped clean to remove the oxide layer that forms on its surface. Not cleaning/stripping the aluminum after anodizing considerably reduces the life of aluminum racks. 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.

Rather than using racks, many anodizers use plastic clamps for fast racking of extrusions. Using plastic clamps will produce less aluminum in the waste treatment because plastic clamps do not need to be stripped after the anodizing process. Plastic clamps can also be used together with bolts and nuts, as the photo below shows. 

Racking Without Visible Rack Marks

Sometimes it is possible to anodize parts without a visible rack mark on the exposed surface. You can accomplish that by simply attaching the part to the rack in areas that are not visible on the finished product. Parts that must be racked on exposed surfaces will almost always have some sort of visible rack mark. It’s a matter of how visible the rack mark is, or looking at it from another viewpoint, how difficult it is to find the rack mark. The size and weight of the part will also be a factor in how easy or difficult it is to keep it on the rack.

The type of anodizing process also will determine the size of the rack mark. If a very small part is to receive a relatively thin anodic coating, it may be possible to process it with only a very small rack mark, one that you have to search for to see. If that same part were to require a Type III (hardcoat) anodize, for example, it would most likely have to be held very firmly in order to maintain a good electrical contact during the entire anodizing cycle. This might result in a more visible rack mark. If the parts are to be dyed, especially a dark color, it might be even more difficult to process the parts with no visible mark.

Sometimes a part can be redesigned to make it easier to rack for a very small visible mark. If your part is going to be a long running one, it might be worth a try to see if the part design could be changed enough to make it easier to rack. There are times when the parts in question simply cannot be racked and successfully anodized without a visible mark. 

Here are a few common techniques to reduce rack marks:

  • Squeezing the part between two or more contacts that wish to remain closed.
  • Squeezing the contacts that wish to remain open and inserting in a hole or an opening in the piece.
  • Threading a bolt or rod into an existing threaded hole or using a nut and bolt combination in a hole and tightening to make contact.
  • Clamping with a C-clamp type device against an aluminum or titanium bar.

When engineering a part that will be anodized, it is best to identify where the part will be racked and not leave it up to the anodizer to guess what location will be acceptable. Typically, the racking location is chosen to minimize the visibility of the mark after the part is assembled.