Systematic approach for Rapid Prototyping processes development

This paper investigates major issues in developing a new Rapid Prototyping (RP) process and presents a systematic approach to the required R&D. A procedure for simultaneous study of the many inter-related issues, including hardware and software design and development, material selection, and optimization of various variables will be shown using realistic examples from a recently developed RP process (Selective Inhibition Sintering). Also, further study on another recently developed RP process (Contour Crafting) shows that the systematic approach is applicable for this process as well. 1-Introduction 1-1Major commercial RP processes Rapid prototyping (RP) is a computer based manufacturing technique that produces products in a short period of time. Several rapid prototyping techniques have been proposed since 1970. Some of these techniques are being used by the manufacturing industries as well as in other fields such as medical engineering and art. Some of the RP processes have been commercialized. SLA (Stereolithography), SLS (Selective Laser Sintering), FDM (Fused Deposition Modeling), LOM (Laminated Object Manufacturing), and 3DP (3D Printing) are the best known commercial RP systems. 1-2Two newly developed RP processes There are two newly patented additive fabrication technologies that have been developed at the University of Southern California: Selective Inhibition Sintering and Contour Crafting. After a short description of each of these processes, they will be used as examples to explain the paper’s objective. 1-2-1Selective Inhibition Sintering (SIS) The Selective Inhibition Sintering process is a new RP method that, like many other RP processes, builds parts in a layer-by-layer fabrication basis. The SIS process works by joining powder particles through sintering in the part’s body, and by sintering inhibition at the part boundary. As shown in Figure 1, the SIS process starts by laying a thin layer of powder slightly above the previous layer, by sweeping a roller over both a powder supply tank and the build tank (A). Then, the areas of the powder bed selected for sintering inhibition are wetted by a printer (B). A radiationminimizing frame is positioned to prevent areas of the powder layer which lie outside the part envelope from sintering (C). The entire layer is then sintered with a blast of thermal radiation from an infrared heater (D). As implemented on the Alpha machine, the heater is a coiled wire that is mounted on a carriage. This allows the heating element to be passed over the surface of the powder bed. Steps A-D are repeated until the part is completed. In the end, a solid polymeric block remains that is totally sintered except for those areas wetted by inhibitor (E). The final part can be easily extracted from surrounding material (F)[1]-[6]. Institute of Industrial Engineering Research Conference, Houston, TX. 2004 Figure 1. SIS process steps (A-D) and fabricated part extraction (E and F) 1-2-2Contour Crafting (CC) Contour Crafting uses computer control to exploit the superior surface-forming capability of troweling. To produce exceptionally smooth and accurate planar and free-form surfaces, the process utilizes trowels that function as solid planar surfaces [7]. Figure 2 shows major elements of the Contour Crafting process. The extrusion nozzle is equipped with a top and a side trowel. As the material is extruded, the traversal of the trowels creates smooth outer and top surfaces on the layer. The side trowel can be deflected to create non-orthogonal surfaces. The extrusion process builds only the outside edges (rims) of each layer of the object. After complete extrusion of each closed section of a given layer, a filler material such as concrete can be poured into the volume defined by the extruded rims [8]. Figure 2. Contour Crafting process In this process, large-scale parts can be fabricated quickly, a key advantage over other prototyping methods. Consequently, Contour Crafting has a great potential in automated construction of whole structures, as well as subcomponents. 2-Major issues in RP processes development Because they use many different materials and also many different material processing mechanisms, RP systems seem to be very different from each other. However, an investigation of the process development histories for these systems reveals a set of common steps required to reach an acceptable, functional system. For example, it’s hard to find similarities in the SIS and CC processes. But, as will be shown in the following, there are many similarities in Institute of Industrial Engineering Research Conference, Houston, TX. 2004 the development of these two processes. In this study, we use experiences from research on these two processes as well as similar commercial processes to organize the required developmental activities in the form of general steps, to help researchers save time and cost in future research. Required activities for developing a new RP process are not sets of sequential steps. Because of the interplay between activities and simultaneous studies, it is difficult to separate out developments into well-defined categories. Therefore, several activities should be considered and conducted simultaneously. 3Developmental activities for new RP processes Developmental activities for a new RP process may be categorized in three groups: experimental research, developmental research, and analytical research (Figure 3). These categories of activities are not independent of each other. Simultaneous theoretical study, experimental research, and software and machine development and modification need to be conducted. After analysis of each experiment, new set of experiments, new machine and software modifications, or a new theoretical subject study might be proposed and pursued. However, experimental research is the key to evaluate and validate related theories and developments. Figure 3. Research activity interplay Developmental Research Analytical Research Experimental Research 3.1. Experimental research A RP development research project usually begins with feasibility study experiments, to validate the developers’ initial concept. In addition, experiments will be conducted to evaluate the machine, software, material, and any modification in them, as well as part fabrication and process improvement/optimization. In the SIS process feasibility study experiments, many experiments were conducted to study the sintering phenomenon of different polymer powders (polystyrene, polycarbonate, polyamide, and their mixtures), powder wetability, liquid-powder reaction, the compatibility of the liquids with available desktop printer technologies for future machine development, and also to see the effects of very small droplets applied to the powder bed. In addition, after any change in the process hardware or software, new sets of experiments were designed to evaluate and analyze the effectiveness of the change. Many new ideas for hardware, software, and material modification were generated through experimental observations. Also, many experiments were designed and analyzed by statistical tools to optimize the fabricated part quality. 3.2. Developmental research In the developmental research, several activities are being done. Machine development (i.e., mechanical structure and control systems) and CAD/CAM software development (NC path generator) are the major activities in the developmental research. To study the SIS concept, an Alpha machine was designed and constructed for the process. This machine has been used for several experiments. The SIS machine contains several subsystems: a mechanical structure, printer system, electronics control system, heating system, and the graphical user interface. Some of the machine elements such as the heater and printer systems were revised, redesigned, and rebuilt several times as part of the evolutionary process of conducting experiments and evaluating results. While other machine elements such as the build tank heater were added only after the requirements of the evolutionary process were observed to exceed the initial machine capabilities. Institute of Industrial Engineering Research Conference, Houston, TX. 2004 Figure 2.2. Selective Inhibition of Sintering machine Figure 2.3. SIS Alpha machine Build Tank Feeder Tank Printer Heater Linear Guide