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In order to anodize aluminum, the part being anodized must be held (racked) by either a titanium or aluminum rack and then immersed in a sulfuric acid bath. While being immersed, an electrical charge is applied to the part creating the anodic coating on the surface of the part. Rack marks are the small contact points leaving exposed areas on the anodized parts as a result of the process. Electrical contact must be made to each part to be anodized. The more electrical current required, the greater the electrical contact. The size of the contact therefore depends on the anodizing process and the size of the part being anodized.

At least with hardcoat anodizing (Type III) the surface area needed for sufficient current requirements can be pretty substantial depending on the size of your part. Most rack marks are going to be approximately the size of 1-2 pencil lead points in width, if the racks are titanium.

Mil-A-8625F, the Mil spec for anodize allows for contact marks. See Mil-A-8642F sections; 3.13.1 and 6.14 for more information.

Sometimes it is possible to anodize parts without a visible rack mark on the exposed surface. One way is to attache 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 then of how visible the rack mark is, or looking at it from another viewpoint, how difficult it is to find the rack mark. It sounds like the part you are talking about may not be able to be anodized without a visible rack mark. The larger the part, the more difficult it will be to hold it on a rack with fingers that grip the part and hold it firmly. Even if the rack mark is small, you might not be able to hold the part firmly enough to keep from losing it during processing. 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.

As you can see, there are many variables that come into play in determining the size and visibility of the rack mark. Part size, weight, shape and even the alloy play an important role, as does the type of anodizing process. Obviously, having a variety of rack styles to choose from will also be an important part of the equation for racking parts with as small a visible rack mark as possible. 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 while hiding the marks. In the meantime, the best way to handle this is to denote on your drawings where on your parts the rack marks are acceptable while keeping in mind that there are times when the parts in question simply cannot be racked and successfully anodized without leaving a visible mark.

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Aluminum anodizing is an electrochemical process in which an oxide (anodic) layer is chemically built on the surface of the metal. This oxide layer acts as an insulator and can be dyed in a wide variety of colors. Anodizing provides surface corrosion protection along with an excellent substrate for decorative finishes. Anodize is typically specified by the military standard MIL-A-8625F.

The MIL-A-8625F spec. characterizes anodize by Type and Class. Type II refers to regular anodize while type III refers to hardcoat anodize. Class 1 is non-dyed or “clear” while class 2 is dyed either black or other colors.

Below is a comparison of some of the primary characteristics of the 2 anodize types.

Regular Type II Anodize

Regular Type II anodize should not require masking since the part is etched first removing .0004 material and the anodize buildup is minimal, usually .0007 with half impregnated into the alloy the other half as surface buildup. As a result, total buildup is .0001 to .0002 per surface face.

Hardcoat Type III Anodize

Hardcoat is a highly abrasion resistant, non-conductive aluminum oxide (Al₂O₃-xH₂O) coating that makes an aluminum surface harder than tool steel due to greater thickness and weight than conventional anodic coatings. Anodic coatings form an excellent base for dry film lubricants, Teflon, paint, and adhesives.

Note: You cannot hardcoat over hardcoat, anodize over anodize, build up hardcoat over anodize, chromic over hardcoat, or just add another 0.0005″ to the surface.

Design Considerations

Avoid the following when designing for an anodized finish:

• blind holes
• hollow weldments
• steel inserts
• sharp corners
• heavy to thin cross sections

Weldments and Welded aluminum parts

When two or more parts are welded together, acid is entrapped in the weld and the area around the weld.  Color variations exist when a welding rod alloy is vastly different from the alloy used to make the part.  Halos appear around welds because of the high temperature used in the welding process.  The area around the weldment will be slightly lighter in color, causing the welded area to appear larger than it is.

Hardcoat and Thread Coating

Hardcoat thickness is typically 0.002″ (0.0508 mm).  Half the coating thickness is build-up and half is penetration into the base metal.  For the threaded rod on the right, the diameter increased by 0.002″ since half of the coating thickness (0.001″) built up the diameter on each side of the rod.

Hardcoat Blind Holes or Through Holes

Through holes (TH on drawing at right) will hardcoat evenly up to twice the length of the diameter (X).
Blind holes (BH on drawing at right) will only hardcoat to a depth equal to the diameter of the hole (X=Y).

Coating Thickness, Color and Alloys

See chart at right.  Example: Alloy 2024 may have a max. coating thickness of 0.005″ and will be gray in color.

++Note: Coatings over 0.0035″ tend to chip and become milky in color and should only be used in the salvage of parts.

Properties of Hardcoat

Coating Thickness vs. Corner Radius

Masking

Masking is required where no build up is desired or when a part needs both anodizing & chemical conversion. All threaded holes, 1/4″ or smaller, are typically masked when hardcoating unless otherwise specified. Holes with heli-coils must be masked. Any dissimilar metal (steel, brass, bronze) or any form of plating will burn off in the anodizing tank unless masked.

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1. Hardcoat IS different; Hardcoat is NOT plating.
2. Hardcoat PENETRATES the base metal as much as it builds up on the surface and the term THICKNESS includes both the buildup and the penetration.
3. Hardcoating a shaft .002 thick will increase the diameter by only .002.  Plating the same shaft .002 thick would increase the diameter .004, since plating is 100% surface build up.
4. Be positive before you machine your parts that you are allowing for the hardcoat buildup and not a plating buildup.
5. When you call for hardcoat, the use of the term “Build up per surface” will make it impossible to misunderstand what you are requesting.
6. Exacting dimensions can be maintained with the Hardcoat Process. Standard commercial tolerance is +.0005 on a coating thickness of .002.  For closer tolerance requirements, consult the finisher in advance.
7. Allowing a tolerance on coating buildup means that you must machine closer than blueprint dimensions.  For example: A shaft diameter which is to finish at 1.500 + .001 and is to be hardcoated .002 thick (.001 +  .0001 buildup per surface), your planning should call out Machine to 1.498 +.0008”, your part will then be to finish dimensions after hardcoating.
8. When a “V” thread is to be cut to allow for hardcoating, the formula is “Build up per surface”, multiplied by “Four”; this will equal the pitch diameter change.  A typical example is: Desired P.D. = .405/.4091 (7/16 N.F. Internal Thread) Coating Thickness .002 + .0002 (.001 + .0001 buildup per surface).  Minimum buildup per surface is .0009 x 4 = .004 P.D. change.  Machine P.D. to .4094/.4127.

HOW TO ORDER THE HARDCOAT PROCESS

To save time, trouble, and possible errors, information on the following four items must be known:

1. Alloy
2. Coat Thickness
3. Masking Requirements (if any)
4. Racking Instructions (if possible)

1. ALLOY – Hardcoat can be applied to virtually any aluminum alloy.  However, since the coating builds up at different rates on each alloy in order to control coatings accurately, it is important to specify the alloy.  Also, some alloys require different procedures from others.  If the alloy is not properly designated, there is a possibility of damage.

2. COATING THICKNESS – Hardcoat may be provided in thicknesses ranging from a few .0001’s to .008” or .009”, depending on the alloy and the application.  Like other coatings, Hardcoat changes the dimensions of the basic part.  One half of the Hardcoat build-up and one half is penetration, i.e., .002” hardcoat consists of .001” penetration and .001” added to the original dimension.  Therefore, in machining the part, it is essential to allow for the change and to request a specific coating thickness on blueprints and/or purchase orders.

3. MASKING – It may be necessary to exclude (or mask) the coating from certain areas of a part.  If so, areas to be masked (threaded hole, bored holes, ground points, etc.) should be clearly specified. In designing for hardcoat, remember that masking is a hand operation which often, but not always means added total cost. For instance, even if Hardcoat is only required on one area of a part, it is usually much less expensive to permit the part to be coated all over if at all possible. It is also important to remember that masking on small parts may not be an effective solution since electrical anodic contact points of the rack leave small voids in the coating.

4. RACKING – Firm electrical and mechanical “contact” must be made with every part to be Hardcoated. That is, each part must be “racked”.  Proper racking is a key to economical and effective processing of parts.  Since each rack contact point leaves a small void in the coating, it is essential that such contacts be made in non-critical area. Any guidance which can be provided as to where best to rack the part will aid in proper processing. Before designing a part for Hardcoat, if you have any questions as to how to handle your specific part, please give us a call. We will be glad to advise by telephone or make a personal visit.

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By definition, anodizing is “a process to coat a metallic surface electrolytically with a protective or decorative oxide.” A coating of aluminum oxide is grown from the aluminum by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications. Unlike most other finishes, anodizing preserves the natural luster, texture, and beauty of the metal itself. The anodized coating is hard, durable, will never peel, and, under normal conditions, will never wear through.

What is anodizing aluminum used for?

Because anodizing is such a versatile process there are literally thousands of different applications. These applications include but are not limited to:

• Recreational Vehicles
• Architectural products like windows and doors
• Appliances
• Automotive
• Lighting
• Food preparation equipment
• Furniture
• Sporting goods
• Medical Equipment
• Marine
• Construction
• Military

Does anodizing alter the bolt/nut thread pattern and diameter or overall size of a part?

Yes. Which means the dimension of the anodized part changes. The amount of change anodizing is a process that converts aluminum to its oxide. The oxide is thicker than the aluminum that is consumed will depend on the anodizing process conditions (temperature, current density, etc.) and alloy. Keep in mind that when calculating the shrinkage of a hole, you must double the amounts given because a hole has two sides. Different anodizes use slightly different process parameters. With all these variables, it is a good idea for the design engineer to contact the anodizing plant under consideration and ask for their input.

Can steel and/or stainless steel be anodized?

Steel and stainless steel can’t be anodized; the process baths used to anodize aluminum would attack and dissolve steel parts. Your parts need to be completely stripped of any steel otherwise the steel will not make it through the process.

What is the purpose of anodizing?

The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has much higher corrosion and abrasion resistance than aluminum. There are some types of anodizing that produce a porous oxide layer that can be colored with organic dyes or metallic pigments giving the aluminum a decorative and protective finish. In short, the main purposes for anodizing are corrosion resistance, abrasive / wear resistance and cosmetics.

Given that anodizing is an electrochemical process, can areas be masked or holes plugged to prevent coverage of an area? (E.g. To create letters or symbols on a surface or to prevent buildup in machined bores or threads).

Yes, sections of a part can be masked. Flat areas can sometimes be more difficult to mask; holes and bores can usually be masked without too much difficulty. Lettering may be accomplished more satisfactorily either by casting them into the part or using laser engraving after anodizing.

From a fabricator’s viewpoint, is it best to do as much machining as possible before anodizing because of the increased surface hardness?

Machining before hardcoat anodizing is much easier and saves considerable wear and tear on the tools. A good rule of thumb is that the hardcoated surface has about the same hardness as nitrided steel (about 50 + Rockwell “C”). Is there a simple process for stripping the anodizing from a part? There is a common procedure performed by anodisers to remove anodise, and such parts can be re-anodised, but there are hazards associated with the removal of anodic oxide from a part. Consult a professional anodiser to explore the details of removing anodise. Do not attempt to remove anodize before sending a part in.

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