Cast metal: Liquid free form
Few architects know about the possibilities of cast metal. Few designers use the marvelous products created with this technology. Force lines precisely shaped in metal, creating fluid and exciting shapes. The molding industry is primarily focused on the technical engineering side so that the structural designer is missing a lot. To illustrate and inspire we line up a number of applications.
Before at the end of the nineteenth hundreds rolled products became available as linear products, construction steel was mainly forged and cast iron. The benefits of rolled products, and of steel against iron, were so great that cast iron gradually vanished from the architectural landscape. Steel is more ductile, has greater tensile strength, can be welded, is less labor intensive and therefore cheaper to produce than cast iron. Some, like constructor Peter Rice of Ove Arup & Partners, still apply it in modern architecture (Centre Pompidou, Paris) but applications and benefits were not known to enough people to make it popular. Hence the image of cast metal as a material for artisans, labor intensive and expensive.This is not really true. Whereas in construction cast iron is used only for restoration purposes, mechanical engineering developments have progressed over the years. Cast metal products can be produced manually and in an automated process. The construction business can make use of these new technologies.
Structural of not?
There is also the misconception about the application of materials. The subject of cast metal in architecture mainly addresses metals suitable for structural qualities. Steel, brick, wood and concrete known in every detail as building materials are as important to architectonic expression as are non-structural materials. How about cast aluminum and cast magnesium? These materials are used for products like tank aerators, ship’s bells and components of mechanical engineering equipment. The link to architectonic application is readily made. For instance the banisters and door handles in shopping mall “De Barones” in Breda.
Cost price in some cases attractive
The benefit of casting is that the end shape is directly realized, no milling required. Cast pieces are relevant not only for their design appeal: under certain conditions its cost price is more attractive than that for welded constructions. Cast metal is also a very versatile material. Today’s metal types, alloys, casting methods and engineering options convert this old skill into a modern production technology with many interesting facets.
The market and the industry
The Netherlands has some sixty foundries, half of which are ferro based (i.e. metal) foundries. This industry focuses on mechanical engineering projects and not so much on construction. Cast metal is found mainly in cars, bicycles, railroad industry, sanitation, heating, pipes, sewage, non-electrical machines, and ship building.
Three dimensional facade panels
One exception to the purely industrial application of cast metal are the facade elements made of enameled cast iron, by Lovink in Terborg. This product was originally developed for a job on the London underground. The order concerned the production of cast iron wall cladding. The project was cancelled, but Lovink continued developing the element into an external facade element. It is not an obvious step, making a level, two-dimensional object from cast iron. Material and technology are best suited for complex three dimensional products after all. The client in this case was also the manufacturer and wanted to incorporate his own product in the building. Plus, the suspension construction was cast in on the back, ensuring invisible fastenings. The joints are open, primarily for reasons of low maintenance. The facade panels are made of gray cast iron. With its high carbon content, it is highly corrosion resistant and weather resistance is enhanced through enameling. The entire panel is protected by a primer enamel and the front is coated with a color enamel. The flat panels are machine cast; the round corners hand-made. The cast pieces for the banister in shopping mall De Barones in Breda are a modern application of artisan technology. The aluminum profiles are enameled in a bronze color. Bronze proved too expensive. Project architects are London based CZWG and ir. Dirk Jan Postel of Kraaijvanger.
On the noise reduction walls around traffic junction Kleinpolderplein the force lines in the nodes are cast in metal. This sample of efficient materials use is a design by ir. Jouke Post of XX Architects and manufactured by Hardinxveld Foundry. The melting temperatures table to the right for various metals also indicates processing parameters. A low melting temperature signifies easy casting. A high carbon content means a lower melting pint. Steel for instance may be strong, ductile and hard, but is difficult to cast compared to e.g. nodular cast iron.
Mold is biggest cost item
The biggest cost item for cast products is the mold. So that the product is only profitable at multiple units. However it is not realistic to claim that it only makes sense with enormous quantities of the same product to switch to cast metal. Subject to options, metal type, intended performance and alternative production methods, casting is certainly a viable option. Not only for purely structural elements, but also for interior nodes, and locks and hinges.
The metal is selected to fit the intended performance to be delivered by the product. Important aspects such as casting and welding properties are governed by carbon content; the higher the carbon content, the poorer the welding and casting properties.
Also pertinent are tensile strength, compression strength, ductility and plastic stretch properties of a material. Design is an essential element and must be agreed with the foundry.
Gray cast iron / lamellar is not elastic and is used where compression strength is the only factor. It has a lamellar graphite structure. Tensile strength is about one quart of compression strength. Gray cast iron has a high carbon content and cannot be welded. Unalloyed it is proof against the elements. It can be enameled and galvanized and is used in the construction industry for restoration projects only.
Malleable cast iron or temper cast iron consists of de-carbonized fused iron. Temper cast iron has a very high compression strength and can be welded. Welding properties are subject to the specific type of temper cast iron.
Nodular cast iron was developed fifty years ago. It is stronger and more ductile than lamellar cast iron with manganese sulfur added to the smelt creating a nodular graphite structure. Nodular cast iron has a high carbon content ensuring a lower melting point. It is easily cast so it can be used to make thinner walls than when steel is used. Tensile strength is about 75% of compression strength. Nodular cast iron is used much in bollards and bumper poles; it absorbs collision energy well.
Steel has a lower carbon content than iron giving it’s a higher melting point. Strength, ductility, tear resistance and hardness are much better than in nodular cast iron. Steel is harder to cast making casting costs higher. It low carbon content makes steel easier to weld.
Stainless steel has the same density as steel and the same elasticity modus, but a higher thermal expansion coefficient and lower thermal ductility.
Aluminum has a lower melting temperature and hence easier processed. It is suitable for injection molding methods.
Zinc has the lowest temperature, is malleable and suitable for injection molding methods.
Magnesium and its alloys have the lowest density compared to other metallic construction materials. Magnesium is injection molded and used mainly in the automotive and aerospace industries. Current technology enables 1 mm wall thickness. Magnesium cast pieces were first made in 1909. Prior to WWII magnesium casting was widely used in the production of air cooled airplane engines.
Making a cast piece
Before it’s possible to pour a shape there has to be a cavity or mould. This is possible with sand or metal. The liquid metal solidificates in the cavity, after which the unprocessed cast can be taken out. Metal moulds are made with metal alloys: this is responsible for the high expenses for this process and is only cost efficient for mass production.
Handmade sand-form casting is suitable for casting small series of very large pieces. By adding a bindingagent, the shape stiffens.
Machine sand-form casting: machines are used, with man-made steps involved as well.
Automatic sand-form casting: fully automatic process, no man-made steps involved.
Vacuum casting: essentially a sand-form casting method. The sand form is shaped not with the aid of binding agents but by extracting the trapped air. To achieve this, the sand is covered with a film.
Bowl-form casting: resin sand, also known as furan sand, is used. Heat treatment gives the sand its rigidity. The mold consists of two semi-bowls enabling the production of complex shapes like fine ribs.
Lost wax model: is an ancient method. A model of wax is packed in ceramic material. Heating melts the wax and bakes the ceramic material. Once poured full, the mold is broken. The use of robots makes this method very suitable for mass production.
Lost foam model: is a relatively new technology based on the principle of the lost wax model. The foam model is made in a mold, into which polystyrene is injected.
Die-cast method: the liquid material is pressurized into a mold. This method is applied in mass production of cast pieces made of low melting-point metals such as zinc, aluminum and magnesium. Copper is just possible, but iron alloys place greater demands on mold and compression channel and cannot be profitably processed.