Design of extrusion regarding surface treatment (Topic: 14002)

When designing extrusions which are going to be surface treated, it is important to know what kind of surface treatment the customer wants, or to inform the customer which surface treatment that is possible. The surface treatment methods mentioned in this manual have the following properties:

Anodising

An anodic oxide coating will give a slight increase in thickness of the profile depending on the anodising method and of course the thickness of the layer. It is usual to say that thickness of the oxide layer will give an increase in the material thickness (total thickness) like 1/3 of the oxide thickness, and 2/3 of the total oxide thickness has been produced of the aluminium (origin) metal. (But etching of the aluminium surface will remove material, and the final thickness of the profile may be unchanged). See figure below.

Formation of the anodic oxide coating.

Requirements for anodic film thickness depend upon service condition of the product and also whether there is a need for further processing (e.g. for colouring). Normal thicknesses is mentioned in table below.

Normal thicknesses of the anodic oxide coating.

If an anodic oxide layer is required inside hollow sections auxiliary cathodes have to be used. Due to this arrangement there is a limitation in the diameter of the tube regarding availability for anodising. Dependant upon anodising bath condition, there will be a limited "throwing power" of the process to provide an anodic oxide layer inside hollows without auxiliary cathodes.

Lacquering

A lacquer coating will give an increase of the profile thickness. The thickness of a lacquer coating is usually in the range between approximately 20 µm and 120 µm, dependent of the application method, type of lacquer, and the service conditions. One has to adapt the lacquering method to the design of the extrusion, but it is also important not to design a profile which is very difficult to lacquer with the coating thickness that is demanded.

Plating

The plating thickness, or the uniformity of the thickness will be very dependent of the plating method and the geometry of the profile. With electrolytic plating higher coating thicknesses are possible at sharp edges and corners. Thinner coatings are possible in shielded areas and recesses. With electroless plating the thickness will be uniform independent of the profile geometry.