Aluminium processing

Aluminium is one of the world's most abundant metals however it is not available in its pure elemental form. Aluminium exists in the earths crust as an ore called BAUXITE (named after the French village of Les Baux, where it was first discovered in 1821). The ore is an orangey-brown colour

Sources of Aluminium

Main sources of Bauxite are Australia, Jamaica, Guyana, France, Africa, and India.

The aluminium in Bauxite ore occurs as the compound aluminium oxide - usually called ALUMINA

To extract the metal from the Bauxite two processes are involved.

a) Refining - to get the alumina from the ore.
b) Smelting - to obtain the aluminium from alumina

Refining

This is a chemical process in which the bauxite is first mechanically ground up, mixed with caustic soda and finally heated under pressure to dissolve out the alumina. Then the resulting "mixture" is treated to allow the" pure" alumina to separate out from any impurities, settle out in large tanks and dry out in large drying kilns to give a whitish powder.

Approximately 4 tonnes of bauxite is required to give 1 tonne of alumina.

Smelting

This is the process by which the alumina powder is converted to aluminium.

It is an electrolytic process - whereby chemical compounds can e split down into their elements by passing an electric current through a solution in which they are dissolved.

The extraction of aluminium at this state is still not very easy and to help two catalysts - Cryolite and Felspar are added to the mixture which is contained in huge carbon lined steel pots. A carbon rod is also suspended into each pot from above.

The mixture has to be heated to around 900^oC to keep it liquid.

An electric current is then passed through the carbon lining of the pot (known as the Cathode) and to the carbon rod (the anode) - Aluminium is then attracted out of the alumina to the bottom of the pot; while oxygen is given off at the rod.

Vast quantities of electricity are needed to smelt aluminium - some 17,000 KW (equivalent to 17,000 one bar electric fires) to produce 1 tonne of metal.

The aluminium, thus obtained is run off, still in its white molten state, into moulds to harden off to form what are called ingots.

Formation of Alloys for Aluminium Extrusion

The alloys used most commonly at Simmal are 6063, 6060, 6063A 6082 and 6005. Many other application specific alloys to produce aluminium extrusions are available; these should be discussed for suitability with our experienced personnel.

The alloys are formed by mixing aluminium ingots in a furnace at high temperature i.e. above 620^oC - the melting point of aluminium -with the appropriate quantities of other metals such as magnesium, silicon which helps in achieving the strength required in the resulting alloys.

The molton mix is then poured into cylindrical moulds of the appropriate diameter and after cooling long solid rods are obtained. These are then sawn into logs or billets as we know them.

This gives the starting material for the extrusion process.

Coatings

Anodising - Complies to BS EN 12337 (formerly 3987)

This is an electrolytic process which produces a hard, relatively thick film of Aluminium oxide on the surface of Aluminium extrusions when the metal is made of ANODE in the production tank. The work pieces are immersed normally in sulphuric acid and it is through this acid that the electric current is passed. Aluminium extrusions are anodised in a variety of colours, shades and thickness' both for aesthetic purposes and functionality.

The thickness of film is dependent on acid concentration, current density, immersion time, among other things, The 5 film thicknesses most commonly used on aluminium extrusions:-

5 micron (0.005mm)
10 micron (0.01 mm
15 micron (0.015mm)
20 micron (0.02mm)
25 micron (0.025 mm)

The most popular by end use;

5 micron for internal applications e.g. partitioning secondary windows. 15 micron - 25 micron for external applications e.g. window etc.

An anodised film is porous when it emerges from the anodising tank and will easily finger mark. Therefore the process must be completed by sealing the surface. This is done by immersion in boiling water or a chemical solution.

6063/6060 Alloy is the most suitable for anodising - it gives a natural silvery grey appearance.

6082 Alloy can also be anodised, but is less suitable for decorative anodising. Because of the higher silicon content the anodised film can vary from silver grey to darker grey.

Painting

Aluminium extrusions can be painted to provide a coloured finish or to protect high strength alloys in aggressive conditions.

Types of paint finish

(a) Electrophoretic
(b) Polyester powder
(c) Acrylic wet spray

Electrophoretic paint

In principle the process is very similar to anodising. Material is loaded into "flight bars" to carry the extrusions through a series of pre treatments, paint and rinse tanks.

Before processing a calculation is made to ensure that the total area of extrusion on each flight bar equals 20 square meters. This is essential to ensure that the eventual paint coverage equals 25 microns.

Prior to painting, the extrusions are lightly etched, cleaned, desmutted and rinsed in ultra clean de-mineralised water.

The paint is applied electrophoretically in a tank of water based polyurethane paint using the extrusion as an anode.

The paint film thickness is uniform and there is no build up of paint in grooves and an even coverage on all faces is achieved.

After rinsing the painted surface in de-m neralised water and allowing to drain the extrusions are stoved at 160^oC for 15 minutes.

Powder Coating - Complies with Qualicoat standard

This process differs from all other finishes in as much as it is the only dry product. It is fusion bonded and is a much thicker coating. It is now probably the most widely used finish for aluminium extrusions offering extremely good adhesion, outstanding weathering and an infinite colour range.

The correct coating of the product is very reliant upon the accurate marking of they die drawings as the powder is applied by spraying not immersion. Drawings must clearly indicate primary (always visible) and secondary (sometimes visible) areas.

The powder is available in a vast array of colours and in three different gloss levels. Matt or 30^o gloss level, which is used for aluminium extrusions in architectural environments; Semi or 70 gloss level. Which is slowly being phased out, and High or full at 90^ogloss level aesthetically similar to our traditional gloss decorating paint? As a general rule the lower the gloss level the fewer problems are experienced with surface finish after coating.

Powder is electrostatically applied by spraying onto the extrusion and subsequent stoving at 200'C for ten minutes.

Polyester is extremely durable and recommended for exterior use whereby Epoxy or Epoxy/Polyester mix is not as light fast and should only be considered for an internal application.

Wet Spray Paints - Complies with BS 4842

This process has shown a steady decline over the years as powder coating has grown in popularity, but is still used on aluminium extrusions in certain applications

Pre treatment varies dramatically; from the minimal consisting of a spray tricoethylene degreasing and spray zinc chromate primer to an 11 stage immersion powder coat pre treatment.

Application of the paint is nearly always by manual method. The surface finish is therefore very much reliant upon the expertise of the individual sprayer and accurately marked die drawings showing critical faces.

Film thickness on primary areas is approximately 25 microns making it the ideal finish for coloured work with tight tolerances or fine detail.

The paint is available in virtually any colour and is reasonably quick to match.

These products are rapidly becoming the forgotten finish, which is unfortunate as they have many attributes.

Fabrication

Aluminium extrusions can be fabricated in much the same way as other metals an in general the same equipment and tooling can be used, although good quality tools with well polished surfaces are essential.

Care is necessary when handling aluminium when surface finish is important. Simmal offer a wide range of in house finished aluminium extrusions, machined, formed, drilled, tapped, thread formed, with inserts if required, painted, anodised, poly sleeved or packed to suit. Expert advice is always available.

Many other specialist finished are available from hard anodising to chrome plating we are happy to assist you in choosing the most appropriate for your end product

Forming & bending of aluminium extrusions

Pure aluminium and non-heat treatable alloys in groups 1000, 2000, 3000 series, can be easily formed drawn or spun, although the more highly alloyed materials in the 5000 series may need inter-stage annealing to allow further bending/forming to take place.

In the heat treatable 6000 series, into which our alloys fall, forming is effected after annealing or immediately after solution treatment (T4 condition). Forming or bending in the fully heat treated condition (T6) may not be possible, depending on the section thickness and shape.

For punching and shearing, drilling, tapping, reaming, the harder the temper (ie T6 condition) the better for the resulting edges of the machined parts are cleaner and the cutting tools will not clog and will work more efficiently.

New aluminium extrusion enquiries.

Information that helps us to help you:
(a) What is application of the aluminium extrusion
(b) Weight/Metre
(c) Finish required - Mill or Anodising or painting quality
(d) Visible face of sections
(e) Lengths required from us (final used/component length)
(f) Can random lengths be supplied
(g) Can short off cuts be supplied
(h) Packaging requirements
(i) Maximum weight/carton or bundle
(j) Mating/sliding sections (if any) get details
(k) Details of any plastic inserts/snap-ins/wool pile, etc
(l) Thread forms, Pem inserts- details if applicable
(m) T slots - details if applicable
(n) Important features
(o) Fabrication requirements (if any)

Find out more about : Designing Extruded Aluminium Components

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