Standard anodic films are generally limited to a maximum of 30 microns in thickness. But if the temperature of the acid bath is lowered to near freezing point and the voltage is increased, anodic thicknesses of up to 100 microns can be achieved. These thicker anodic coatings have exceptional hardness characteristics and are very suitable for tougher engineering applications. This variation to the anodising process is commonly know as ‘hard’ anodising or ‘hard coat’ anodising.
The performance of the coating is dependant on the type of aluminium alloy used. The alloying ingredients will also influence the final colour of the product. Generally, hard anodic films have a dark grey or yellowish-brown tint.
The process is unlike other finishing methods, where the surface is coated with an organic or inorganic deposit. In contrast, anodising is a conversion of the base material to form a protective oxide coating.
Almost all anodising processing requires the components to be racked. Although we have many standard racks in stock, it may be necessary to construct special racks for some jobs, which may incur extra charges.
Before the anodising process commences, the part must be chemically cleaned. This most often involves a mixture of alkaline cleaners to remove oils and surface contamination. This is generally followed by an acid or caustic etching process.
Due to the density of the coating formed in the hard anodising process, it is always difficult to apply dyes to the resulting finish. The standard anodising process accepts dyes much more readily than hard anodising.
You can read more about standard and coloured anodising here.