Even before surface treatment, the appearance and surface quality of extruded aluminium profiles is perfectly satisfactory for many applications. Thanks to good corrosion resistance, surface treatment is rarely necessary simply to provide corrosion protection. However, there are many other reasons for treating the surfaces of profiles. Examples of attributes that can be changed by surface treatment include:
– surface structure
– corrosion resistance
– wear resistance
– electrical insulation
Surfaces do not always need treatment after extrusion. Load-bearing structures and machine parts are examples of products where the surface quality is satisfactory without any treatment.
- Profile design
- Mechanical surface treatment
- Powder coating
- Screen printing
- Function specific surfaces
Lines and extrusion stripes that would be noticeable on visible surfaces can easily be hidden using decoration. Such patterns or optical effects are an integral part of the profile solution created at the design stage. Refer also to Extrusion design guide – decorate.
Mechanical surface treatment
Grinding is one of the methods used for improving surface quality. The process leaves a fine striation in the direction of grinding. The resultant surface can be “very fine”, “medium” or “coarse”. Grinding is most commonly used for furnishing and interior design products. Ground surfaces are often anodised. Grinding before painting can further improve the surface finish.
Polishing smoothes the surface. Quality and gloss are determined by customer specifications. Polished surfaces normally go on to be anodised. To achieve a high-gloss finish, polishing is followed by bright anodising.
Tumbling (barrel polishing)
Tumbling is mainly used for deburring. Determined by the polishing medium used in the drum, surfaces range all the way from matt to gloss.
Anodising, one of the most common surface treatments, is used to (amongst other things):
– maintain a product’s “as-new” appearance.
– enhance corrosion resistance.
– create a dirt repellent surface that satisfies stringent hygiene requirements.
– create a decorative surface with durable colour and gloss.
– create a “touch-friendly” surface.
– create function-specific surfaces, for example, slip surfaces, abrasion-resistant surfaces for use in machine parts, etc.
– give surfaces an electrically insulating coating.
– provide a base for the application of adhesives or printing inks.
Recommended layer thickness when anodising
The anodising process
There are normally four stages in the process: pre-treatment, anodising, colouring (where required) and sealing. The most frequent type of anodising is natural anodising. The electrolytic process takes place once the metal surface has received the appropriate mechanical or chemical pre-treatment and has been thoroughly cleaned.
The profile is connected to a direct current source and becomes the anode (hence anodising). An electrolytic cell is formed. Dilute sulphuric acid at room temperature is normally used as the electrolyte. During electrolysis, the surface of the metal is oxidised. The process continues until the desired layer thickness (usually 5 – 25 μm) is reached.
The oxide layer contains a large number of pores, approx. 1011/cm2 (i.e. around a hundred billion). The diameter of the pores is between 120 and 330 Å. To obtain an impermeable surface, the pores have to be sealed. Sealing is achieved by treating the surface in de-ionised water at 95 – 98°C. This changes the aluminium oxide into bohemite, the attendant increase in volume closing the pores.
The oxide layer formed in natural anodising is transparent. Coloured oxide layers are also possible (see pages 108 and 109). Natural anodised profiles are delivered with matt or semi-matt surfaces.
Maintenance – cleaning
The anodic oxide layer has good corrosion resistance in most environments. With the proviso that the surface is cleaned, anodised profiles are virtually maintenance-free. The surface cleans easily in both water with a little neutral detergent and in white spirits. Although solvents do not affect aluminium, strong alkaline solutions should be avoided.
Resistance to corrosion, discoloration and abrasion increases with layer thickness. Recommendations for suitable thicknesses are given in the table above.
As the anodic oxide layer has poor cold formability, forming should take place before anodising. Cutting and drilling can be carried out after anodising but the exposed surfaces will, of course, be untreated. Welding is to be carried out before anodising.
Properties of anodised aluminium
Corrosion resistance is very good, especially where pH is between 4 and 9. In contact with strongly alkaline substances, surfaces can stain and be damaged. Thus, it has to be borne in mind that aluminium should be protected against lime, cement and gypsum (e.g. on building sites). Visible surfaces can be protected using tape.
The hardness of the oxide layer depends on the anodising process used. Generally, the layer is harder than glass and as hard as corundum. The oxide layer is transparent. Whether natural or coloured, its appearance depends on the viewing angle.
At temperatures above 100°C, fine cracks form in the oxide layer. From an aesthetic point of view, this may be an undesirable effect.
The reflectivity of bright etched aluminium is high. The gloss value is 90 units (ISO 7599, 60° viewing angle). This decreases slightly with anodising. The oxide layer is an electrical insulator. A sealed, 15 μm oxide layer has a breakdown voltage of 500 – 600 V. An anodised profile can be recycled with no pre-treatment. Before remelting, painted profiles must first have the paint removed.
Painting offers a limitless choice of colours and very good colour matching (repeatability). Powder coating is now easily the most widespread method of painting aluminium profiles.
To ensure the right adhesion for the paint, it is important that pre-treatment, paint application and subsequent curing are all carried out correctly. As maximum adhesion and durability are prime goals, pre-treatment is of crucial importance. Pre-treatment normally comprises degreasing and pickling of the surface, followed by a chemical treatment.
The chemical treatment (chrome-free or chrome-based) gives good adhesion and effective corrosion resistance. The chrome-free titanium based process is GSB approved and is now our standard method. It has undergone extensive testing.
Rinse water from the chromating process is treated in efficient cleaning plants. The sludge is drawn off and sent away for appropriate disposal. Pre-treatment is the same for both powder coating and wet painting.
Broadly speaking, there are absolutely no limits to the choice of colour. Powder coatings are applied and cured without solvents. This gives a good work environment and has no negative impact on the external environment.
In a wet coating plant, half the paint is lost through evaporation and the waste involved in over-spraying. In a modern powder coating plant, up to 98% of the powder is used. Powder that does not adhere to the product is recirculated via a reclamation system.
Powder coating qualities
The prime qualities of powder coating and powder coats are:
– No risk of running or blistering.
– High repeatability.
– Powder coatings withstand knocks and abrasion far better than
wet paint coatings.
– Good formability (e.g. can be formed after coating).
– Suitable for outdoor use – good resistance to UV and corrosion.
Coating thickness is normally 60 – 140 μm. In some designs, the thickness of the coating has to be taken into consideration when determining profile dimensions and tolerances.
Screen printing (formerly silk-screen printing) is an ancient printing method. The original design is reproduced on a transparent film that is then placed on a fine-meshed screen (usually nylon nowadays). This is then exposed and developed photographically. The screen is next fitted into a frame. Either manually or automatically, a squeegee is dragged along the screen to transfer the design onto the printing surface.
Tampon printing is a technique that makes it possible to use screen printing on both concave and convex surfaces.
Natural and colour anodising on the same profile
Using screen printing, a profile’s surfaces can combine natural anodising and colouring. Anodising is interrupted when the oxide layer has formed. The profile areas that are not to be printed are then coated with a special masking ink. After printing, the profile is sealed in the normal way.
Unanodised surfaces on anodised profiles
A masking technique is also used when parts of a profile are to emerge unanodised from the anodising process. This preserves the surface’s electrical and thermal conductivity (the anodic oxide layer is insulating).
Function specific surfaces
A function-specific surface is defined as one where certain function-related properties are of critical importance.
Slip, friction and sealing surfaces
Here, the surface roughness (i.e. the Ra values, axially and radially) is of the utmost importance. Cylinder tubes are an example. Direct from the press, tubes can be delivered where the insides have Ra values as low as 0.6 axially and 1.2 radially. The Ra values can, of course, be further improved by machining.
Abrasion resistant surfaces
These surfaces have to be anodised.
Four height adjustable legs made from telescoping aluminium profiles – slip surfaces direct from the press (no machining). The product: Control cabinet lift columns from MPI.