Article - Issue 17, October/November 2003
Professor Duc Pham OBE FREng and Dr Stefan Dimov
A time compression tool
Rapid prototyping (RP) has emerged as a key enabling technology, with its ability to shorten product design and development time. This article discusses the role of RP in ‘time compression’ engineering and provides a brief description of three RP processes with the highest commercial impact on the market. The article also outlines different applications and future developments of this technology.
Global competition, product customisation, accelerated product obsolescence and continued demands for cost savings are forcing companies to look for new technologies to improve their business processes and speed up the product development cycle. Rapid prototyping (RP) has emerged as a key enabling technology, with its ability to shorten product design and development time.
Rapid prototyping is a technology for quickly fabricating physical models, functional prototypes and small batches of parts directly from computer-aided design (CAD) data. This technology has also been referred to as layer manufacturing, solid free-form fabrication, material addition manufacturing and three-dimensional printing. Rapid prototyping is a means of compressing the time-to-market of products and, as such, is a competitiveness enhancing technology.
This article focuses on RP technology for building physical models and discusses the role of this technology in ‘time compression’ engineering (TCE). A brief description of three existing RP processes with the highest commercial impact is provided. Examples of RP applications in five different areas are then presented. Finally, future developments to achieve long-term growth in this field and realise the full potential of RP technology are outlined.
Rapid prototyping – an enabling technology for time compression engineering
The main enabling technology behind time compression engineering is 3-D computer-aided design modelling. If different design and manufacturing activities are carried out concurrently it is possible to compress the overall product development time. This can also allow engineers to be creative by providing more time for design iterations (Figure 1).
Concurrent engineering environments have evolved considerably during the last few years to integrate 3-D modelling with computer-aided manufacturing (CAM), computer-aided engineering (CAE), rapid prototyping and manufacturing, and a number of other applications. The 3-D model becomes a central component of the whole product or project information base, so that in all design, analysis and manufacturing activities the same data are utilised. There is no duplication or possibility for misunderstanding. Product information captured in this way can be copied and re-used; it is readily available for different downstream applications.
Three-dimensional models and virtual prototypes allow most problems with fitting to become obvious early in the product development process. Assemblies can be verified for interference. Structural and thermal analysis can be performed on the same models, employing CAE applications as well as simulating downstream manufacturing processes. Ultimately, these very accurate and data-rich models can be taken directly to RP and CAM applications, speeding up process planning and in some cases eliminating the need for drawings.
Currently, there are a number of RP machines available on the market but only three technologies have a significant commercial impact. These RP processes are described below.2
Stereolithography (SL): This process relies on a photosensitive liquid resin which forms a solid polymer when exposed to ultraviolet (UV) light (Figure 2). Stereolithographic systems consist of a build platform (substrate) which is mounted in a vat of resin and a UV helium–cadmium or argon ion laser. The first layer of the part is imaged on the resin surface by the laser using information obtained from the 3-D solid CAD model. Once the contour of the layer has been scanned and the interior hatched, the platform is lowered and a new layer of resin is applied. The next layer may then be scanned. Once the part is completed, it is removed from the vat and the excess resin drained. The ‘green’ part is then placed in a UV oven to be postcured.
Selective laser sintering (SLS): SLS uses a fine powder which is heated with a carbon dioxide laser so that the surface tension of the particles is overcome and they fuse together. Before the powder is sintered, the entire bed is heated to just below the melting point of the material in order to minimise thermal distortion and facilitate fusion to the previous layer. The laser is modulated such that only those grains that are in direct contact with the beam are affected. A layer is drawn on the powder bed using the laser to sinter the material. The bed is then lowered and the powder-feed cartridge raised so that a covering of powder can be spread evenly over the build area by a counter-rotating roller. The sintered material forms the part whilst the unsintered powder remains in place to support the structure and may be cleaned away and recycled once the build is complete (Figure 3).
There is another process, laser sintering technology (LST), that employs the same physical principles. Figure 4 shows an LST system equipped with two laser beams working in parallel. Currently such dual-laser systems are available for processing thermoplastics and sand.
Fused deposition modelling (FDM): FDM systems consist of two movable heads (one for building the part and one for the supports) which deposit threads of molten material onto a substrate (Figure 5). The material is heated to just above its melting point so that it solidifies immediately after extrusion and cold welds to the previous layers.
Applications of rapid prototyping technology
Rapid prototyping models are becoming widely used in many industrial sectors. Initially conceived for design approval and part verification, rapid prototyping now meets the needs of a wide range of applications, from building test prototypes with material properties close to those of production parts, to fabricating models for art and medical or surgical uses. In order to satisfy the specific requirements of a growing number of new applications, various software tools, build techniques and materials have been developed. Five examples in different application areas are described in this section.2
Functional models: Selective laser sintering, one of the three rapid prototyping processes detailed above, is widely used for producing polyamide-based models for functional tests. The SLS production of polyamide parts is generally costeffective when a small number of parts (one to five) is required. The housing in Figure 6 is a test part built in glass-filled polyamide (a blend of 50% by weight polyamide powder with a mean particle size of 50 Ìm and 50% by weight spherical glass beads with an average diameter of 35 Ìm) because it has to withstand harsh test conditions, including temperatures of about 100°C. As a base part for mounting precision components, it has to keep its dimensions within close limits. To produce this functional component to the specified requirements, the thickness of its walls was reduced to 2 mm and made uniform. Furthermore, 2 mm non-functional ribs were added across the housing to stiffen it. The errors in 90% of all functional dimensions of the built component were between + 0.35 and – 0.31 mm.
Patterns for investment and vacuum casting: Rapid prototyping is widely used for building patterns for investment and vacuum casting. For example, models built using any of the three technologies listed above can be employed as patterns for both casting processes. The heat exchanger assembly of a Pratt & Whitney PW6000 engine shown in Figure 7 was produced using SLS patterns. The assembly includes three cast aluminium components that have to withstand high temperature and pressure. These complex castings are essentially pressure vessels with multiple portings, mountings and sensor pads. The largest component measures 600 mm in height and 325 mm in diameter (Figure 7). As a relatively small number of exchangers was required per year, the SLS process was approved as a production method for the fabrication of the required casting patterns. In general, RP patterns are a cost-effective alternative when a small number of parts, say up to 50, of complex design is required and the cost of a mould tool for wax patterns is prohibitive.
Medical or surgical models: Rapid prototyping technologies are applied in the medical/surgical area for building models that provide visual and tactile information. In particular, RP models can be employed in the following applications: operation planning, surgery rehearsals, training and prosthesis design. For instance, two stereolithographic medical models were built for a patient suffering from a secondary carcinoma of the right superior orbital margin and the adjacent frontal bone. The first model was used to plan the resection of the cancerous bone and also as an operation reference and patient consent tool. The fabricated plastic template was placed over the model to check the match with the surgeon’s resection line (Figure 8). The second model was then employed to construct an acrylic custom implant (Figure 9). The unaffected left superior orbital margin was mirrored across to assist the design of the implant. The operation was reported as a complete success and the surgeon was fully satisfied with the quality and the cost of utilising RP models.
Art models: Another growing application area for RP technologies is art and design. Through building RP models, artists can experiment with complex artwork which supports and enhances their creativity. Initially, the high cost of RP models meant strict limits on the size of the models. However, recently, with the introduction of relatively inexpensive RP machines for quickly producing design models, it has become cost-effective to employ RP techniques in many artistic applications. The following example was part of work conducted within the CALM (creating art with layer manufacture) Project supported by the Higher Education Funding Council for England in an initiative to promote the use of IT within the academic art and design community. The example is an artwork representing a splash spanning the inside of a plexi-glass vitrine (Figure 10). In its final installation, the SLS model (Figure 11) is to be incorporated into a plexi-box exactly the width of the splash itself.
Engineering analysis models: Various software tools exist, mainly based on finite elements analysis (FEA), to speed up the development of new products by enabling design optimisation before physical prototypes are available. However, the creation of accurate FEA models for complex engineering objects sometimes requires significant amounts of time and effort. By employing RP techniques it is possible to begin test programmes on physical models much earlier and complement the FEA data. RP models could be used for visualisation of flow patterns, thermoelastic tension analysis, photoelastic stress analysis and fabrication of models for wind tunnel tests. The opening image shows an example of an RP part produced for photoelastic stress analysis. The part was fabricated by stereolithography because the SL resin material exhibits birefringence when under stress and irradiated with polarised light. The fringe patterns seen in this image, which indicate the stresses and strains in the part, were ‘frozen’ by warming the loaded SL model to a level above the resin–glass transition temperature and then gradually cooling it back to room temperature.
The field of rapid prototyping is just over ten years old. In spite of this, significant progress has been made in widening the use of this technology and in the development of new processes and materials. To achieve long-term growth in this field and realise its full potential, a number of challenges remain. These challenges could be grouped under the following categories5:
Productivity/cost of RP machines: To benefit truly from the ‘direct’ fabrication capabilities of RP processes, especially when the serial production of parts is planned, their productivity should be increased and machine costs reduced significantly. It is expected that long-term growth in the RP industry will come from applications that are impossible or very difficult, costly and time-consuming to implement with conventional manufacturing techniques. Therefore, new RP machines should address the specific requirements of these applications. Furthermore, it is expected that wider use of RP machines for rapid manufacturing will lead to reduction of the cost of RP machines.
Materials: One of the main limitations of RP processes is the limited variety of materials and their properties, and also their relatively high cost. Significant research efforts are focused on the development of a broader range of materials that simulate very closely the properties of the most commonly used engineering plastics. Recently, the fabrication of multi-materials and heterogeneous objects has attracted the attention of the research community. Developments in this area will make possible the fabrication of objects with multiple and conflicting functional requirements.
Process planning: With the increase of part complexity and the range of available RP materials and RP machines, there is a need for more advanced process planning tools, in particular tools that could relate process variables to part quality characteristics and address the process-specific requirements associated with the fabrication of parts from heterogeneous materials.
Rapid prototyping data formats and design tools: The stereolithography format, a de facto standard for interfacing CAD and RP systems, has a number of drawbacks inherent in the representation scheme employed. Work on the development of new formats continues in order to address the growing need of RP applications for more precise methods of data representation. Also in recent years, with the emergence of RP processes for fabrication of heterogeneous objects, there is increasing interest in developing new CAD tools that enable the design of objects with varying material composition and/or microstructure.
The authors wish to thank the Institution of Mechanical Engineers to re-use material from Reference 5 in this article.
Duc Pham OBE FREng and Stefan Dimov
Manufacturing Engineering Centre, Cardiff University
Stefan Dimov is Distinguished Senior Research Fellow and Operations Director of the Manufacturing Engineering Centre at Cardiff University. He is the recipient of the 2000 and 2002 Thomas Stephen Group Prizes from the Institution of Mechanical Engineers.
Duc-Truong Pham is Professor of Computer-Controlled Manufacture and Director of the Manufacturing Engineering Centre at Cardiff University. The MEC has won the Queen’s Anniversary Prize and the Secretary of State for Trade and Industry’s First Prize for its research and programme of practical technology transfer and partnership building.