Nenad Grujović2, Dejan Divac

1. 
   INTRODUCTION
   

Rapid Prototyping (RP) refers to a broad set of additive manufacturing technologies used in various stages of product development, with significant application in arts, education, and medicine [1]. By using any of these technologies in traditional processes, time and cost of modeling and planning is greatly reduced. The fastest RP systems today are based on powder binding processes, such as 3D printing by ZCorporation and SLS by 3D Systems. Still, considerable time is required for parts post-processing, thus overall efficiency of RP system may be reduced. One of required steps in powder-based systems is removing produced parts from loose powder, which is usually performed by trained person using vacuuming system. Nevertheless, vacuuming and cleaning must be done with great caution, because the parts are buried in loose powder and gradually appear during the extraction. Usually, the whole of build chamber is used for parts, in order to increase efficiency. Not only this is time consuming process, but parts may also be damaged while extracting, which requires RP process to be repeated. It would be of great benefit if some visual aid would be provided for operating technician, so that part extraction can be accelerated and risks of damaging parts diminished.

Recent advances in augmented reality applications provide cheap and effective solutions for marker-based systems with wearable displays [2]. Such system is presented in this paper as an aid for part extraction procedure in 3D printing process. The main idea is to help operating technician by visualizing the position of parts within build chamber, while they are still buried in loose powder. The system relies on printing setup generated by proprietary software, and the use of fiducial markers applied on top surface of 3D printer.

2. 
   3D PRINTING
   

ZCorporation was founded in 1994, in Massachusetts, USA. Based on MIT-patented technology [3], this company developed and commercialized their first 3D printer, Z402 System, in 1997. 3D printing is RP powder-based technology that uses liquid binder for layer generation.

The binder is applied on powder via printing head, using the same process as in thermal inkjet printing. This also allows for printing to be performed using multiple binder liquids in various colors. Figure 1 illustrates fabrication of a single layer[4]. The following parts are of main interest: on the left-hand side is reservoir chamber with feed piston at the bottom, the gantry with roller are depicted as circle and square, respectively, in the middle is build chamber with build piston at the bottom, while on the right-hand side is overflow chute. The gantry traverses from left to right side, and vice versa, whiles the print head it carries travels along the gantry, allowing the binder to be printed on whole top surface of the build chamber.

Fig.1. The process of 3D printing
At first, the feed piston is raised, so that the powder is slightly over the top of reservoir chamber. The gantry then moves from left to right over reservoir chamber, while the roller picks up free powder. The second step is spreading this powder over build chamber. The build piston is already lowered for height of one layer, so that the powder could be spread evenly. At the rightmost position, the excess powder is dropped through the chute to overflow bin, so it can be recycled later. The fourth step is actual printing of layer, where the print head applies binding liquid on new layer of powder in build chamber, and thus binds this layer to previously printed layers. The final step is at leftmost position of gantry, where the feed piston is raised and the build piston is lowered, and the whole process is repeated.

Fig.2. Fabrication of single layer
This way, the model is built, layer by layer, and the whole process results in solid part buried in unbound powder. The unbound powder also provides support for printed parts, so that complexity of geometry is not an issue. The loose powder is removed usually by vacuuming and may be used for printing (Figure 3a). After initial cleaning, the parts usually undergo post processing, such as depowdering (removing remains of unbound powder by blowing), curing (drying in an oven at around 100°C), and infiltrating (e.g., strengthening the surface of the part with epoxy)[5].

a)
b)
Fig.3. Part removal: a) vacuuming loose powder; b) damaged part due to improper extraction
One of the main problems is how to extract printed parts from unbound powder. The reason for this is that usually multiple parts are printed in one batch, in order to utilize the whole build volume and reduce print times. Even if a single part is printed, but one with complex geometry or thin features (Figure 3b), the operating technician may damage the part if he is not aware of its actual position within build chamber.

The presented augmented reality system is designed to provide technicians visual aid by superimposing parts layout within build chamber.

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end -> George huntington, M. D. Charles s. Stevenson bulletin of the institute of the history of medicine
end -> Endi 2011 / Daniel/Jason/Kevin/Marc/MiHe/Parth/Simrun
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end -> Unit IV: End of the Cold War and Contemporary America The Americans
end -> Index 1 shell 2
end -> A. background
end -> End Times Colloquium 23-25 July 2014 Wits Club, West Campus, University of the Witwatersrand