The Aluminium Remelt Plant produces aluminium billets that are used later in the extrusion process to produce extrusion profiles. Billets are short cylindrical solid pieces of aluminium with a diameter of 171mm and at Almin they are cut to lengths of 420mm, 525mm and 630mm respectively. The key processes are essentially: Charging of aluminium scrap and ingot into the furnace, Casting aluminium logs from the furnace and Cutting the logs into shorter billets using a billet saw.
The major raw materials for the Remelt plant are; high purity aluminium (HP) imported from South Africa as ingot, aluminium scrap either generated from internal processes or bought from scrap dealers and companies outside disposing disused aluminium products. Alloying elements like silicon, magnesium and the grain refiner titanium-boron, which are added to improve the properties of the metal, are also imported from South Africa. The main aluminium alloys produced are 6063 alloys for architectural, engineering and irrigation purposes and 6261 alloys for the transport and engineering products. 99S alloy and 6082 alloys are also produced for selected products.
Relting of the scrap and the ingot is done in a reverbatory furnace, which is a liquid fuel fired furnace that has output capacity of about 5000kg per charge. The reverbatory furnace consists of a steel lining shell with an inside refractory work of brick and castable refractory. The furnace can use the following fuels interchangeably: used-oil, diesel, paraffin, or coal tar. Currently used-oil is the predominant fuel.
The cast logs undergo a heat treatment process called homogenizing to make them easier to extrude. This is carried out by heating the logs in an electric furnace at temperatures of 575 +/- 10 oC depending on the alloy. After homogenizing, the logs are transferred to a cooling chamber where they are cooled under controlled conditions using forced air so as to preserve the state attained during homogenizing.
The cooled logs are then transferred to the billet saw where they are cut into shorter lengths called billets more suitable for the extrusion process. The billets are stamped with an identity number, length and respective alloy. The cut billets are then transferred to the ultimate customer, the extrusion Department.
Extrusion is a process whereby billets from Remelt are squeezed under high pressure, from a closed container through a shaper formed (called a die) to produce long straight semi-finished metal products such as bars, solid and hollow sections, tubes, wire and strips. Both aluminum billets and dies are pre-heated to required temperatures before extrusion.
The extruded products are subjected to further processes of straightening, cutting to required lengths and heat treatment (called ageing) to obtain required strength.
The forming tools called extrusion dies are made in the DIESHOP. This is one of the most important supporting functions of the Extrusion process.
Anodizing is a process by which the surface of aluminium profiles is coated with an artificial oxide layer. The anodizing layer is used to increase corrosion resistance, increase wear resistance or prepare the surface for further electrolytic colouring into bronze and black colours.
Anodizing is an electrolytic process where the aluminium workload acts as the anode immersed in sulphuric acid as the electrolyte (the most common electrolyte used for architectural purposes is sulphuric acid). When an electric current is passed through the acid oxygen is given off at the anode side. This oxygen reacts with the aluminium (extrusions) to form an artificial (forced) oxide layer.
Prior to anodizing, the aluminium profiles go through a series of pre-treatment processes with the following as key stages:
Degreasing is carried out by immersing the extrusions in a tank of detergent to remove oil, grease, and other surface contamination from the profile surface. Another critical step is etching whereby the naturally occurring aluminium oxide layer on the extrusions is removed by dipping in a tank of caustic soda to make way for the artificial oxide layer to be applied more uniformly. Any smut that may result from insolubles such as magnesium oxide, intermetallics, silicon, etc. in the etching tank is removed by a process called de-smutting achieved by dipping in a solution of spent-sulphuric acid.
After anodizing, the extrusions may be coloured by dipping into a tank of cobalt sulphate electrolyte to give a colour that ranges from very light to dark bronze or black depending on how long the extrusions stay in the electrolyte and also the electric voltage used. The patented process is called anolok colouring. The anodizing layer is porous at a microscopic level and a process called sealing carried out in water heated to a minimum of 94 degrees Celsius is necessary to close the pores and protect the surface.
Oxide film thickness tests are carried out on the extrusions as well as inspection for visual surface defects before packing the extrusions.
Aluminium is coated with a “self protective” oxide layer which appears soon after extrusion, although attractive in this plain finish, the coating will eventually fade or blotch due to weathering and atmospheric pollution.
The answer is to anodise! This induces growth of the oxide layer under controlled conditions to a specified depth to ensure a long life, maintenance free surface which is always consistent in appearance. Where aluminium is selected for its decorative advantages, or where regular cleaning and maintenance are difficult, anodising is strongly recommended.
A further step is the Alminok colour anodising process, where shades of bronze to black are induced. Colour fastness is absolute; corrosion and abrasion resistance is enhanced.
The range of surface finshes which can be specified are:
- mill finish
- natural anodised
- colour anodised
- chemically brightened
Please seek our advice on the correct surface treatment and finish for your application
Shaping a brighter future: We’ve been making some changes at the almin Foundry.Big changes. Changes that you, our valid customers will appreciate. To make sure that you get the service and quality of products you deserve, we have implemented a plan that will take us into the next century.
Foundry division specializes in all aspects of gravity castings from product design advice, die making, casting through to fettling and assembling to customer requirements.
We service a broad sector of the Global economy including agriculture, transport, electrical, domestic and general engineering. We supply a wide variety of customers often-competing customers with its own products and products exclusive to specific customers. Customers include virtually all market leaders in their respective sectors. Exclusivity of customer dies is guaranteed and Almin’s reputation for integrity is high.
The Foundry division has design staff (product development staff) that has been specially trained to relate our customers’ needs and manufacturing process.
We have a well-equipped tool-room for manufacturing new dies, repair and die modification. We employ Class 1 artisans, most of them, have been trained by an international consultant in the specialties pertaining to efficient production of gravity casting.
We have a modern, well-equipped and environmentally friendly foundry utilizing electric resistance furnaces and inert gas degassing techniques.
High quality metal alloy is utilized with excellent metal control to close compositional band within recognized international specifications such as BS1490. Compositional control is maintained by frequent analysis, utilizing modern spectrographic techniques.
We also have mechanical testing facilities for use when necessary.
We are certified to ISO 9000 since 1994 to date.
We are the best. We aim to stay the best and will be pleased to discuss any potential application of aluminiumn gravity die-casting with you. Our advice is free.
Powder coating is a process of applying a lacquer coating to protect and decorate a metal surface such as aluminium extrusions. Dry thermo setting powder pigment is sprayed and attaches itself to the metal by means of electro-static force. The powder is then cured in a baking oven where it melts and spreads on the surface to form an even coat before hardening to a final coating, which is tough and resilient. The powders come in a wide range of colours. The powder manufacturer provides a comprehensive 15-year guarantee against deterioration provided that the controls, consistency and quality of the treatment are satisfactory.
The actual spraying of the powder is done in a powder spraying booth where the extrusions to be sprayed are hung on a conveyor system to facilitate easy transfer to the curing oven after powder is applied. The powder is forced out of an electrostatic spraying gun directly onto the surface to be coated. The powder is imparted a negative electrostatic charge by the gun. Meanwhile the extrusions have a positive charge that makes the powder attach itself to the metal. The electrostatic force causes the powder to adhere to the metal surface until it is later melted in the oven.
Key in the coating process is that the aluminium extrusions go through a pre-treatment process before spraying on powder. The main stages are:
The degreasing step using a detergent to remove oil, grease and other surface contaminations from the extrusion surface to obtain a clean surface. Another stage is etching done using caustic soda to remove the thin natural oxide coating on the aluminium surface. The smut which occurs after etching is removed by dipping material into a solution of nitric acid. To enhance the adhesion between the metal surface and the lacquer coating and also improve the corrosion resistance of the lacquered product a stage called chromate conversion is passed by dipping the extrusions into a solution of chromic acid. After chromate conversion the material is dipped in de-ironised water to avoid adhesion and corrosion problems. In this case the conductivity of the water should not exceed 100mS/cm. The extrusions are rinsed in water to remove chemicals from the surface. Prior to powder coating the extrusions have to be completely dry since any moisture on the surface will interfere with the later coating process. Drying is done by forced air from a fan.