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Hard Coat Anodizing Services Type III

PFI Inc. / June 5, 2024

What is Hard Coat Anodizing?

Hard coat anodizing is a process in which an abrasion resistant outside coating is applied to aluminum. This coating is extremely hard and offers a much higher level of durability compared to other types of anodizing all while maintaining a smoother finish. Because of these properties, Type III hard coat anodizing is extremely popular in a wide range of applications.

Aluminum is already a premier material choice in terms of versatility and weight, but hard coat anodizing improves its functionality even further. This coating increases corrosion resistance and strength all while providing a smooth finish. The finish most commonly comes in a clear or black dye finish for uniformity, and the aluminum material is easier to clean and maintain, which is essential for preventing panel edge staining. With its improved lubrication, it is ideal for applications that involve moving parts. The coat is non-conductive, can act as a way to repair an aluminum surface, and it is non-contaminating, which makes it ideal for medical instruments.

Hard Coat Anodizing vs Plating

Hard coat anodizing is distinct from plating in that the process and end result is different. Plating involves adding coating to the surface of the material. The entire thickness of the coating will be added to the outside. Meanwhile, hard coat anodizing is formed through penetration in the material as well as build up on the outside. Half of the coating penetrates while the other half builds up on the exterior surface. That means hard coat anodizing does not add as much dimensional changes to the material as plating does even when using the same coating thickness.

Attributes of Hard Coat Anodizing

  • Increased abrasion resistance (optimized when unsealed)
  • Increased corrosion resistance (optimized when sealed)
  • Increased wear resistance
  • Acts as an electrical insulator
  • Increased lubrication
  • Sterilized for medical use
  • Smooth outward appearance.

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What Makes Type III Special?

There are three total types of hard coat anodizing, but Type III is the ideal process for pinpoint results in the coating. Type I anodizing is a chromic acid anodize or other types of chromic alternatives. This kind of coating is exceptionally thick with a thickness of 0.0001”. Type II anodizing is sulfuric acid anodizing, which is typically used for coatings that need to be dyed any color. The thickness of Type II coating ranges from 0.0002” to 0.0006 inches. With Type III anodizing, the process conditions are more exacting, which results in a harder, thicker, denser, and more abrasion resistant coating. Thickness of Type III coating ranges between 0.0005” and 0.0030”, though this varies based on which alloy is being anodized.

The Type III Anodizing Process

The comprehensive Type III anodizing process is what creates the durable thick coating on aluminum. It works by submerging the material in a sulfuric bath, which permeates half into the aluminum itself with the other half built up on the surface. This is similar to the Type II process, but there is a significant temperature difference. Type II anodizing uses an acid bath temperature of about 70 degrees fahrenheit while Type II is much cooler around freezing temperatures.

With the temperature difference, the end result of Type III is significantly thicker and tougher compared to Type II. The exact thickness can vary based on which aluminum alloy is being anodized, but Type III is always going to be thicker than Type II. This is particularly useful for end applications that will receive a lot of abrasion in its use.

The Science Behind the Process

The anodizing process is a high voltage process in conventional cases up to 100V that creates the oxide layer via the sulfuric acid bath, which is typically concentrated at around 200g/liter. The aluminum material is the positive electrode in the process, which is also known as the anode. That, of course, is the source of the term “anodizing” in the first place. When submerged in the acid, Al3+ ions move away from the aluminum towards the electrolyte interface while O2- ions do the opposite, moving from the electrolyte to the metal. If the part needs to be sealed after anodizing, that can be accomplished via precipitation or hydrothermal methodology, but not all applications require a sealant.

Applications of Type III Hard Coat Anodizing

Type III hard coat anodizing has multiple applications most often in industries that require strong abrasion resistance and easy lubrication. These applications include the following:

  • Sliding parts like pistons and some valves
  • Hinge mechanisms
  • Gears
  • Wear and mold plates
  • Anything subject to wear and tear where aluminum is preferred over steel
  • Swivel joints
  • Insulation plates
  • Blast shields

Sealing Hard Coat Anodizing

In most cases, sealing after hard coat anodizing is unnecessary. In fact, attributes like corrosion resistance are more effective when the coating is unsealed. If the project requires the hardcoat to be dyed, however, sealing is an effective way to help limit the dye from fading. The natural appearance of Type III hard coat anodizing is black since it is the easiest way to create a uniform appearance.

Hard Coat Anodizing and Dimensional Specifications

Adding a hard coat to material will change its thickness, but that change is typically minute enough to not affect the part in its planned applications. We have experience working with tight tolerances that can maintain very little thickness changes despite the fact that half of the coating builds up on the surface. The final product dimensions can be maintained with help from our experienced team.

Hard Coat Teflon for Type III Anodizing

Type III anodizing can be performed using polytetrafluoroethylene (PTFE) based formulas. The most well-known brand name for this is Teflon. This anodizing solution is notably durable, and the hardcoat can lower the coefficient of friction up to 50% compared to hard anodizing. Since it is effectively more slippery, Teflon anodizing is particularly useful for parts that require high levels of lubricity. This also comes with superior release characteristics and ability to withstand relatively high temperatures. With these properties, PTFE based formulas are often used for hard coat anodizing in the aerospace, firearms, electronics, military, and marine industries.

Progress for Industry Inc., A Leader in Aluminum Anodizing

Progress for Industry, Inc has been creating excellent metal finishing services for the molding industry for more than 30 years. With our expert craftsmanship and extensive knowledge of precision mold and component construction, your next hard coat anodizing process can be completed with tight tolerances. Many of our workers have been here since the beginning back in 1982, and their expertise has only grown with time. Learn more about all the services we have to offer.

Learn more about all the services we have to offer.

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