Hardcoat anodizing an aluminum alloy is often a lightweight, cost-effective alternative to using another material like chrome, stainless steel, or steel hardened in a salt bath. The reason: hardcoat and polytetrafluoroethylene (PTFE)-penetrated hardcoat anodizing create a hard, corrosion- and abrasion-resistant oxide coating on aluminum alloys which, when combined with high dielectric strength, provide optimum performance in many applications.

In hardcoat anodizing, the current density is higher, the operating temperature of the electrolytic bath is lower, and the resulting coating is more dense than in traditional anodizing. Of course, as with any process a number of variables influence the final product. Planning for the different variables provides greater control, which can reduce manufacturing costs.

What variables affect coating quality and dimensional tolerances?

Each processing stage includes a number of variables that can be controlled to maintain close tolerances and coating integrity.

Design Stage

  • Aluminum alloy
  • Fabrication method
  • Accounting for dimensional changes that occur during anodizing

Processing Stage

  • Current density
  • Electrolyte concentration
  • Temperature
  • Coating thickness

Alloy Composition Affects Density & Hardness

In anodizing, the coating is generated from the aluminum substrate, which means that the alloy’s quality and composition have a direct impact on the quality of the anodic coating. In general, high-purity or commercially pure aluminum produces denser and harder coatings than heat-treated alloys. The latter contains alloying elements that may not dissolve during the anodic process, which may cause microscopic voids that reduce density and ultimately resistance to corrosion and abrasion. When choosing an alloy, keep in mind that alloys with low copper and low silicon content as well as a few cast alloys yield the best coating.

Holding Dimensional Tolerances

That aluminum oxide coating is less dense than the aluminum substrate means that both growth and penetration occur. Coating adds to the original surface, so machining dimensions should be adjusted before coating to account for the change in size. Standard coating on most alloys is 0.002 inches, half of which forms below the surface and half of which is added.

Die castings are an exception. Their surface includes a high silica content that inhibits the growth of anodic film and as a result yields a thinner coating.

What variables affect coating color?

  • Chemical composition and temper of the alloy
  • Coating thickness
  • Aluminum fabrication prior to processing (e.g. Areas that have been machined or welded may have different tempers than the rest of the surface.)

What are the best surfaces for anodizing?

Hardcoat anodizing is well-suited for machining methods like milling, turning, and drilling because of its ability to withstand the abrasive techniques used in these processes. It’s critical to follow guidelines for coating specific parts like holes, corners, and edges and to calculate tolerances correctly.

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