Creative casting The Bronze Age process of casting now incorporates computer simulation to ensure that it is right the first time
Castings are still made as ancient axes were made: by pouring a liquid metal into the void of a mould. It originated in the Bronze Age but is continually renewing itself both in its structural applications and its aesthetic qualities.
The first stage of a job today is likely to be a series of computer simulations, starting with the component shape which the architect or engineer requires and ending up with something which should actually do the job.
John Wadsworth of Goodwin Steel Castings, Stoke on Trent, describes the basic process. Once the design of a component has been established, a pattern, usually a wooden representation of the required shape, is made and encased in moulding sand. The modern moulding sand is bonded with resins and catalysts to high-mould definition. The pattern is removed to leave the void, usually in two halves. The halves are then treated with an inert coating. This helps obtain the 'homogeneous as cast' surface that is characteristic of the process. The mould is then reassembled to receive the chosen metal: stainless and carbon steels are the most common for structural applications.
Goodwin has developed the use of higher strength 'duplex' and nitrogen- bearing stainless steel, which combines the strength of carbon steel with the looks and corrosion resistance of stainless steel. Stainless and duplex materials look expensive but when whole life costs and environment impacts are taken into consideration (no painting, coating, repainting, recoating etc), they are remarkably cost-effective.
The casting is manufactured to the specified strength, ensured by quality heat treatment which eliminates any stresses within the casting - giving it an isotropic quality, a homogeneity of performance throughout the component's structure. The advantages of castings include economy, particularly where long runs are involved, durability, quality and good tolerances.
There are several production methods. Sand casting, lost wax casting, lost-foam casting and die casting are most common.
Cast components can range from the 50+ tonne steel node of an oil-production platform to the intricate sub-assemblies in missile guidance system created using the 'lost wax' process. This method was used in the manufacture of Bronze Age axes: a pattern of beeswax was made and encased in clay, which hardened as it dried. It was fired to melt the wax, leaving a mould into which metal was poured. After cooling, the mould was broken off and the casting was hand finished. Today sand-based ceramic is used instead of clay and the wax patterns are made with the use of metal dies. The process produces castings with a high dimensional accuracy and a good surface finish and is appropriate for repeat runs (hundreds) of small components.
Sand casting is appropriate for small and large runs and most sizes of component. For larger runs the pattern may be made of metal or resin. The skill of the founder is to achieve a good flow of molten metal into the mould produced from the pattern; many foundries use solidification simulation software to ensure that the required casting integrity will be obtained.
The manner in which the molten metal is introduced into the mould cavity is critical to the process. Goodwin has pioneered the development of ceramic filters to control the metal flow in to the mould cavity. This technique greatly reduces the amount of oxides which manifest as surface discontinuities.
All metals, once solid, contract and expand at different rates. This has to be allowed for when sizing the pattern and consequently it is not possible to change the metal specified for a job - say from stainless to aluminium - and re-use the pattern.
All castings require finishing. Hand polishing, used for yacht components, is expensive. Grit blasting or shot blasting, often used in combination are cheaper. The cost of the rough casting is modest and labour intensive finishing can easily exceed it, but this may still not achieve the desired finish. Consequently the quality of finish must be agreed before manufacturing begins. The casting process chosen partly determines the quality of finishes available.
Complex castings are used in many structures to form strong junctions between rolled steel elements like tubes, bars and channels homing in from several directions. Castings can manage and direct such vital loads and stresses.
The incoming members, tubes, bars etc can be attached to the castings by the use of either welded or mechanical connections. The complexity of junctions speak of the skill and cunning of their creators. Castings are often well worth exposuring to public view - they are things of strength and subtlety and frequently beauty too.
Goodwin began as an engineering company and iron foundry in 1883 and is expert in the casting and the aod (argon/oxygen/de-carbonising process) refining of carbon and stainless steels and nickel alloys - refining in an aod vessel removes volatile trace elements and produces a cleaner metal by removing non-metallic inclusions.
Goodwin is making the thick section nickel alloy castings for the European Union's Thermie project, which will enable steam power plants to operate at 700degreesC - an advance of 60degreesC which will increase their efficiency and reduced emissions.
Goodwin castings can be seen at Paddington Station (page 24). The node points in the bowstring truss roof over the Lawn Area show a combination of carbon and stainless castings. The arms and bases on the customer information system totems were cast in stainless steel, as were the support arms under the glass walkways. Goodwin is particularly proud of the 44 stainless-steel mullions in the glass screen that separates the Lawn Area from the main station concourse. At 5.3m high they are made up of four slender elements, with the tallest at over 2m. Maintaining surface finish and straightness was paramount to the design.
The company also made the castings for Stratford Station on London's Jubilee Line extension - components ranging from a few kilograms in weight to the three-tonne main base castings in a structural steel that strike off the main ellipse of the station roof - 70 tonnes of components in total.
'There were very difficult engineering problems to overcome,' says John Wadsworth. 'Solidification simulations, models and drawings were utilised prior to manufacture to prove all the cast components. The internals of key components were shape-modified in the simulator. With the ability to transfer drawings and solid model files by e-mail, the exchange of concepts is both accurate and quick.'
Many of the castings are intentionally visible and finished to show off the skill of designers and craftsmen as well as the architect.
Wadsworth says: 'Our main message to architects must be that they should involve their casting specialist as early in the concept design as possible. We have the ability to prove casting concepts and reduce the possibility of costly changes at later stages. In the past we have also been successful in stripping out cost, when involved early enough.'