Key Words : 3D Printing Augmented Reality, Visualization, Zcorp

Abstract:3D printing generally refers to a set of technologiesthat create 3D physical prototypes by solidifying layers of base material using various binding techniques. By definition 3D printing is an extremely versatile and rapid process accommodating geometry of varying complexity in hundreds of different applications, and supporting many types of materials.Powder-based processes, such as ZCorp 3DP provide great versatility and speed. But there are several bottlenecks related to human factor. When the 3D printing process completes, loose powder surrounds and supports the part in the build chamber. Users remove the part from the build chamber after the materials have had time to set, and return unprinted, loose powder back to the feed platform for reuse.The problem is that entire build area is filled with powder so the user cannot perceive exact position of parts. Usually it is more effective to position several parts within build volume using printing software and print them in one cycle. This increases the risk of damaging the parts while removing them. This paper presents the system for augmented reality assisted part removal, which helps the user to see printed parts through unbound powder. The virtual image of printed parts is generated using data from printing software 3D document that contains both parts geometry and their position within build chamber. This way the risk of damaging the parts during their removal is greatly reduced.

Key words: 3D printing augmented reality, visualization, ZCorp.

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 systemstoday 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.

1. 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.

1. AUGMENTED REALITY

Augmented Reality (AR) is the synthesis of real and virtual imagery[6]. In contrast to Virtual Reality (VR) in which the user is immersed in an entirely artificial world, augmented reality overlays extra information on real scenes: typically computer generated graphics are overlaid into the user’s field-of-view to provide extra information about their surroundings, or to provide visual guidance for the completion of a task. In its simplest form, augmented reality could overlay simple highlights, arrows or text labels into the user’s view - for example, arrows might guide the user around a foreign city. More complex applications might display intricate 3D models, rendered in such a way that they appear indistinguishable from the surrounding natural scene.

Major advantages of AR are ease of collaboration, intuitive interaction, integration of digital information, and mobile computing. AR enables a user to work in a real world environment. At the same time user can receive additional computer-generated information. Until recently, AR has been mainly engaged in scientific visualization and gaming environments, but the latest investigations are aimed at supporting activities like surgery, training and collaborative work[7]. Some early uses of AR in architecture were for sketching.

Fig.4.Pipeline of AR system

Figure 4 illustrates the pipeline of AR system[8]: the user points image capturing device to a marker; the image is captured and converted to black-and-white format; the marker is located in the image and coordinate system is computed based on position and orientation of marker; using computed coordinate system, an image of 3D object is rendered; the resulting image is overlaid on device display. The process is near real-time, depending on hardware capabilities of the system.

1. AR ASSISTED SYSTEM FOR PART REMOVAL

The solution presented here is designed to visually assist technicians in part removal procedure using marker-based AR that supports both handheld and head-mounted devices. The parts layout in build chamber setup by technician using ZPrint software represents input for visualization. The platform for AR is ARToolKit, a reliable GNU library for single-camera applications.

Fig.5.Structure of the system

The structure of the system is illustrated in Figure 5. The central part is the visualization module, which can be hosted both on head-mounted or handheld devices. This module uses data from parser module to generate scene for rendering based on marker orientation and position. The parser module is used to convert build layout to geometry data used in visualization module.

A build batch consists of multiple parts arranged within build volume. It holds information on part geometry and layout. Since native format for saving build in ZPrint software is undocumented binary file, the technician needs to export the build to STL file, using menu command File > Export STL. The resulting file represents input data for the parser module, which converts triangle list into object geometry used by visualization module.

Fig.6.AR assisted part removal

Up to 4 fiducial markers may be used by visualization module, so that the technician can freely move while extracting printed parts. It is only necessary that at least one marker is always in field of view, so marker placement is important when preparing the system. By using special markers, it is possible to calibrate the system through guided procedure. The calibration is performed in order to establish reference coordinate system for visualization. Once reference coordinate system is established, the system will require subsequent calibration only if the marker placement changes.

The system supports the use of either head-mounted or mobile AR devices. When using head-mounted AR device, the technician is free to move while extracting printed parts. The overlaid image of printed parts guides the technician in process of part removal, thus reducing the risk of damaging the parts buried in loose powder. When using the mobile AR device, the technician needs to periodically check part layout because using the AR device interferes with handling the other equipment. Nevertheless, for more complex build layouts even sporadic use of AR helps the technician to plan the part removal and reduce the associated risks.

1. SUMMARY AND CONCLUSION

The presented system for AR assisted part removal in 3D printing provides new way of helping technicians in sensitive operation of printed part extraction. The risk of damaging printed parts in the process of removal from loose powder is greatly reduced through use of such a system, especially with use of head-mounted AR which enables technician to keep visualized build layout constantly in field of view, while the use of mobile AR device provides periodical checks of layout. The best results can be achieved when building layouts that are relatively complex or when printing delicate parts.

Further work on this system will be directed mostly to visualization manipulation, since visualizing of complex builds in some cases proves to be distractive to the operating technician. The tools for manipulation of AR visualization that will be developed are cutting plane and object visibility. The technician will be able to hide objects through selection of visible objects or by means of moveable cutting plane. By hiding already extracted parts, AR visualization may provide better compliance with progress of removal operation, while cutting plane also resembles the process of removing loose powder from top to bottom of the build chamber.

The application of system for augmented reality assisted part removal, in presented state and with further improvements, will improve productivity and lessen the manufacturing costs in powder-based 3D printing services.

REFERENCES

[1]JACOBS, P.F. (1992)Rapid Prototyping & Manufacturing– Fundamentals of StereoLithography, Society of Manufacturing Engineers, (ISBN 0-87263-425-6), Dearborn, MI, USA, s. 4-23

[2]AZUMA, R., BAILLOT, Y., BEHRINGER, R., FEINER, S. MACINTYRE, B. (2001)Recent advances in augmented reality. Computers & Graphics, IEEE, November/December, 21, 6, 34-47.

[3]LAUDER, A., CIMA, M.J., SACHS, E., FAN, T. (1991)Three Dimensional Printing: Surface Finish and Microstructure of Rapid Prototyped Components, Synthesis and Processing of Ceramics: Scientific Issues, Boston, MA, USA, s. 331-336.

[4]ZPrinter 310 User Manual (2003), ZCorporation,.

[5]TRAJANOVIĆ, M., GRUJOVIĆ, N., MILOVANOVIĆ, J., MILIVOJEVIĆ, V. (2008)Računarski podržane brze proizvodne tehnologije, monografija, Mašinski fakultet, Kragujevac

[6]FEINER, S.K. (2002)Augmented reality: a new way of seeing. Scientific American, Vol. 4 No.24, 48-55.

[7]POUPYREV, I., TAN, D.S., BILLINGHURST, M., KATO, H., REGENBRECHT, H. TETSUTARI, N. (2002)Developing a generic augmented-reality interface. IEEE Computer, 35, 3, 44-50.

[8]WAGNER, D, SCHMALSTIEG, D. (2007)ARToolKitPlus for Pose Tracking on Mobile Devices, Computer Vision Winter Workshop 2007, Graz Technical University

ACKNOWLEDGMENT: The part of this research is supported by Ministry of Science in Serbia, Grants III41007.

CORRESPONDENCE

Nikola Milivojevic, Research Associate, PhD

Institute for Development of Water Resources „Jaroslav Černi“, Beglrade, Serbia

Nenad Grujović, Full Professor, PhD

Department for Applied Mechanics and Automatic Control, Faculty of Mechanical Engineering, University of Kragujevac, Sestre Janjić 6, Kragujevac, Serbia

Dejan Divac, Senior Research Associate, PhD
Institute for Development of Water Resources "Jaroslav Černi", Belgrade, Serbia

Vladimir Milivojević, Dipl. Eng.

Institute for Development of Water Resources „Jaroslav Černi“, Beglrade, Serbia

Jelena Borota, Dipl. Eng.

Faculty of Mechanical Engineering, University of Kragujevac, Sestre Janjić 6, Kragujevac, Serbia