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ALEX DEN OUDEN |
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Oude techniek en werktuigbouw, industriële geschiedenis en archeologie |
Historical engineering and technology, industrial archaeology and history |
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In mask exposure stereo lithography, models are built from horizontal layers of ultraviolet-cured photopolymer. The CAD representation of the 3D shape is used to calculate exact raster images of a sequence of horizontal sections. The vertical stepping distance between these sections equals the layer thickness that will be deposited in the process. The created raster images are used as masks during exposure of the photopolymer. One by one, the images are sent to the mask plotting unit of the machine, where a high resolution optical image is projected on a flat glass platen. The image is then treated with black toner powder to form the actual mask.
At the same time the mask is being generated, a thin layer of liquid photopolymer is spread on the working platform of the machine. The platform and the toner mask are then positioned under the exposure station and carefully brought in register. A shutter opens, to expose the workpiece for about 3 seconds through the mask, to a high power (2,000 W) ultraviolet lamp. The UV light passing through the clear sections of the mask fully cures a horizontal section of the pattern of that slice. Unexposed areas of the layer surface remain in a liquid state.
The workpiece passes under a wiper, that removes all residual liquid photopolymer, leaving behind only the cured layer. It then travels to the next station, where a thin layer of melted wax is spread over the surface, filling all the voids and cavities left after removal of the untreated photopolymer. A cold plate is then lowered onto the wax surface, cooling and solidifying it. The results is a fully solid layer that is composed partly of cured polymer and partly of wax.
Next station. The workpiece is brought under a milling cutter that trims the layer's thickness down to the predetermined thickness, creating a flat smooth surface ready for the next layer. Also, the glass mask plate is returned to the mask plotter, where it is discharged. The previous mask is cleaned off. The platen is then ready to receive the next mask.
The process can now be repeated, building the next layer, until finally all layers are finished. When the last layer has been completed, we are left with a block of wax, within which the model (or models!) are embedded. The wax can be melted away in a microwave oven, by the use of a hot air gun or even by using warm water (60° C), after which the finished models are ready for use.
A special advantage of this method is the layer thickness milling operation, which ensures high accuracy of the finished model(s). No final curing of the finished model is required, as with laser scanning stereo lithography. These machines can generate layers from 0.05 mm to 0.15 mm thickness. The finest resolution is only used where sharply radiused curves in the vertical direction occur. Elsewhere a larger thickness is chosen, to speed up the building process.
Shrinkage and distortion are relatively small with mask exposure stereo lithography. Accuracy is within 0.1%. Undercuts (overhanging shapes) and concave surfaces cause few problems. A special feature of the mask process is, that multiple models can be built at the same time in the same process. Very good for improving productivity. Also, the process can easily be modified to produce wax models for use in the lost wax casting (investment casting) process. The latter method is particularly useful when castings small products in difficult metals and/or complex shapes.
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