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from bits to nuts and bolts
building techniques (continued)
a different approach is to use a block of styrofoam and let a robot cut the mould out of it. this is very fast and precise and when moulds are similar it is possible to use one block several times. styrofoam has its shortcomings in that it is not very heat resistant and only has poor mechanical qualities. it also has a very grainy surface and traces of it may be left on your workpiece. so one may not be able to cast a shiny fiberglass surface unless the mould is prepared accordingly. it is of course not possible to cast glass (or metal) because the styrofoam would evaporate in the very moment it comes into contact with the hot glass. and deep drawing of sheet metal is also not available. but when only minor forces and low temperatures occur, styrofoam is very useful - and that is where thermoplastic materials come in. for the kunsthaus project it was polymethylmethacrylate (PMMA), household names perspex, acrylics, plexiglas. how does the production process work?

a large rectangular block of styrofoam (about the size of a kingsize bed) is being cut by a 5-axis milling cutter - according to the precise curvature of each individual panel. the panel itself is being heated up to a temperature when it becomes flexible and then put on top of the mould. it has to be cooled down slowly in order to avoid tensions and deformation due to different temperature levels inside the panel. despite the rough surface of the mould the panel does not lose its surface tension and retains its glossy appearance.
footnote: why did we not use fiberglass?
fiberglass has very good properties in terms of fire rating, resistance against heat and ultraviolet radiation and is of course mechanically far superior to plexiglas. for a 1:1-scale model we asked two different contractors to manufacture a sample item. one of them, done by the german company hahlbrock even had a degree of transparency that came close to plexiglas. it had less than half of the weight and thickness of a comparable plexiglas sample. therefore we had high expectations but it turned out that the manufacturing process was of course much more elaborate (and thus more expensive) than plexiglas, perhaps due to the glossy surface finish we required. (and this could not be achieved with that cheap mould) i must therefore say that the decision for plexiglas as outer shell is more influenced by the mould than the plexiglas itself.
by the way fiberglass has another advantage over plexiglas: it is potentially endless - it is not constrained to a standard panel format but it can be manufactured in any size. furthermore you do not have to pay for excess material that will be cut off. as opposed, plexiglas can only be cast in rectangular panels that come in predefined sizes you can not influence and you will have to pay for any material that might be wasted. since we knew we would use plexiglas we had to take this into account and find a way to arrange the rectangular panels on the surface and reduce the amount of wasted material to a minimum. this constraint greatly influenced the geometry of the shell as i will explain in a separate chapter.