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3D-Scanning and rapid prototyping
This method of scanning makes the production of an accurate 3-D computer model of the original object possible without the need to make contact with the object’s surface. Two types of scanner were tested, the Konica-Minolta Vivid VI-700 and the Model Maker X made by 3-D scanners Ltd now owned by Metris. The most successful scanner for these objects was the Model Maker X, due to the small scale and highly reflective surface of the figures. With the Model Maker X, the object to be scanned is placed on a foam support while the laser head moves around it. Instead of turning the object around a static laser source, the pin-joint arm enables scanning whilst the object is kept still. The pin-joint arm contains sensors which automatically locate the laser head in a coordinate system, as long as neither the tripod nor the object moves. The scanner head emits a thin strip of low-power laser light. Light reflected from the surface of the object is recorded by an off axis CCD-camera. This low power radiation does not harm the object. The data, which represents an image of the 3-D surface, is collected as a set of points, in which each point is measured every 35 microns (= 1/1000 mm). This data set describes a 3-D point cloud, consisting of many millions of points, accurate to ~0.1-0.2 mm. The results can be accessed immediately on a portable computer monitor with a software application called Pre-Scan. Each side of the object is scanned, forming two point-cloud shells, which are then joined. The resulting virtual object can be turned and examined from all angles. At this point, if necessary, it is possible to re-scan certain details if the resolution or accuracy of the original scan is not sufficient. For example, it was necessary to rescan the undercut elements, such as crossed legs or facial features, as the dark and hollow areas of the figures are difficult for the scanner to interpret. This is because the light from the laser beam is scattered, preventing it from being accurately detected by the camera chip. The final file size for a high quality scan of each figure is approximately 10 MB. Scanned figure10 seconds © The Conservation Centre, Liverpool These scanning tests were carried out with Martin Cooper in the Department for Conservation Technologies at the National Conservation Centre in Liverpool and Angela Geary from the Research Unit at the University of the Arts, London. Rapid prototyping is a technique developed in the 1980s to enable designers to create models or prototype parts for testing prior to widespread production. For the purposes of this project, the following prototyping techniques were tested at CRDM Ltd in Buckinghamshire to compare different materials for the replicas of the figures: SLA – Stereolithography apparatus SLA was the first established Rapid Prototyping process. The Stereolithography apparatus consists of a vat of liquid resin, which solidifies under exposure to an ultra violet laser. The technique can create models from a range of epoxy or acrylic resins. The model is built up in layers which map the points contained in the data file. The photosensitive resin cures on exposure to the UV light with each layer being created individually, working from the core of the object to its surface. In the tests carried out for the figures from Letter to the Censors, the thickness of each layer was set to 0.1mm. The range for this setting can be chosen between 0.025 mm up to 0.15 mm. When the model is complete the excess resin is removed.
The result is a solid translucent resin model, accurate to within 0.1mm
of the original. SLS – Selective Laser Sintering The SLS technology makes it possible to use a wider variety of materials for prototyping. In our tests, the models were made of white nylon (Polyamide). These prototypes are highly durable and accurate to ± 0.15 mm. They are also heat resistant up to temperatures of 150°C. SLS uses a computer operated CO2 laser to fuse or melt together the Nylon powder in 0.1 mm layers in cross-section. The system sinters layer after layer until the object is complete. The colour of the samples can vary depending on the material used. Sintering is a method for making objects from powder by heating the material (below its melting point) until its particles adhere to each other. Sintering is traditionally used for manufacturing ceramic objects, and has also found uses in such fields as powder metallurgy.
Schema: SLA prototyping
Schema: SLS prototyping From these tests SLA was found to be a more effective technique than SLS for creating the replica figures. This was largely due to the difference in the surface appearance of the models. The surface produced by the SLS technique tends to be either slightly powdery or granular. A powdery surface is produced if the temperature is slightly too low, this is because during fabrication some of the particles of the base material fuse together without completely melting. If the temperature of the uncured SLS powder is too high during the sintering process, excess fused material can collect on the surface. Although applying a coating to the SLS models helps
to even out the irregularities in the surface this has the undesirable
effect of reducing the surface detail.
Scanning apparatuses at The Conservation Centre, Liverpool
Software application during scanning
Original and replica figures |
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