A review of rapid prototyping technologies and systems

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Vol. 26, No. 4, PP. 307-316. 1996 &pyrisht Q 1996 Ekevier Science Ltd Printed in Great EfitAn. All riahts resewed 0010-4466/96 315.00+ 0.00

Survey A review of rapid prototyping technologies and systems Xue Yan and P Gu* What RP&M can do Rapid Prototyping and Manufacturing @P&M)

technologies have emerged for quickly creating 3D products directly from

To substantially shorten the time for developing patterns, moulds, and prototypes, some manufacturing enterprises have started to use rapid prototyping methods for complex patterns making and component prototyping. Over the past few years, a variety of new rapid manufacturing technologies, general? called Rapid Prototypingand Manufacturing RP&M , have emerged; the technologies develo ed include Stereolithography, Selective Laser Sintering PSLS), Fused DepositionManufacturing (FDM), Laminated Object Manufacturing (LOM), BallisticParticleManufacturing(BPM), and Three Dimensional Printing (30 Printing). These technologies are capable of directly generating physical objects from CAD databases. They have a common important feature: the prototype part is produced by adding materials rather than removing materials. This simplifies the 3D part producing processes to 2D layer adding processes such that a part can be produced directly from its computer model.

computer-aided design systems. These technologies significantly improve the present prototyping practices in industry. This paper reviews the main technologies and applications of RP&M. The principles and the features of those RP&M technologies are presented. Some existing problems and research issues on these new technologies are introduced. We also include two current research and application examples in using rapid prototyping for further illustration. Keywords: rapid prototyping, layered manufacturing

INTRODUCTION Product manufacturing industry is facing two important challenging tasks: (1) substantial reduction of product development time; and (2) improvement on flexibility for manufacturing small batch size products and a variety of types of products. Computer-aided design and manufacturing (CAD and CAM) have significantly improved the traditional production design and manufacturing. However, there are a number of obstacles in true integration of computer-aided design with computer-aided manufacturing for rapid development of new products. Although substantial research has been done in the past for computer-aided design and manufacturing integration, such as feature recognition, CNC programming and process planning, the gap between CADand CAMremains unfilled in the following aspects’: (1) (2)

The basic process of RP&M As shown in Figure I a part is first modelled by a geometric modeller such as a solid modeller. The part is then mathematically sectioned (sliced) into a series of parallel cross-section pieces. For each piece, the curing or binding paths are generated, shown in Figure 2. These curing or binding paths are directly used to instruct the machine for producing the part by solidifying or binding a line of material. After a layer is built, a new layer is built on the previous one is the same way. Thus, the model is built layer by layer from the bottom to top. In summary, the rapid prototyping activities consist of two parts: data preparation and model production.

rapid creation of 3D models and prototypes. cost-effective production of patterns and moulds with complex surfaces.

This is a tutorial paper of rapid prototyping manufacturing (RP & M).

and

Current application areas of RP&M Division of Manufacturing Engineering, Department of Mechanidal Engineering, The University of Calgary, Calgary, Alberta, Canada T2N lN4 * Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9 Paper receiwd: 18 May 1994. Retied: 3 April 1995

Although RP&M technologies are still stage, a number of industrial companies Instruments, Inc., Chrysler Corporation, Ford Motor Co. have benefited from 307

at their early such as Texas Amp Inc. and applying the

Review of rapid prototyping

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becomes simple; the product quality can be improved within the limited time frame and with affordable cost. Iteration. Just like the automotive industry, manufacturers often put new product models into market. Time to market is one of the key features to separate winners from the losers in today’s competitive market. With RP&M technology, it is possible to go through multiple design iterations within a short time and substantially reduce the model development time. Manufacturing We can use the RP&M prototype for producibility studies. By providing a physical product at an earlier design stage, we can speed up process planning and tooling design. In addition, by accurately describing complex geometry, the prototype can help reduce problems in interpreting the blue prints on the shop floor. Another application is tooling development for moulds. The prototypes can also be used as master patterns for castings.

Figure

1

Marketing To assist product sales, a prototype can be used to demonstrate the concept, design ideas, as well as the company’s ability to produce it. The reality of the physical model illustrates the feasibility of the design. Also, the prototype can be used to gain customers’ feedback for design modifications so that the final product will meet customers’ requirements. Meeting customers’ demands in a timely manner is the key to penetrating the market in the 1990s. RP&M technologies have the potential to ensure that quality products are developed quickly for two major reasons: there are almost no restrictions on geometrical shapes; and the layered manufacturing allows a direct and very simple interface from CAD to CAMwhich almost completely eliminates the need for computer-aided process planning.

The solid model of an object

technologies to improve their product development the following three aspects.

in

Design engineering Visualization. Conceptual models are very important in product design. Designers use CAD to generate computer representations of their design concepts. However, no matter how well engineers interpret blue prints and how excellent CAD images of complex objects are, it is still very difficult to visualize exactly what the actual complex products will look like. Some errors may still escape from the review of engineers and designers. The touch of the physical objects can reveal unanticipated problems and sometimes spark a better design. With RP&M, the prototype of a complex part can be built in short time, therefore engineers can evaluate a design very quickly. Verification and optimization. Improving product quality is always an important issue of manufacturing. With the traditional method, developing of prototypes to validate or optimize a design is often time consuming and costly. In contrast, an RP&M prototype can be produced quickly without substantial tooling and labour cost. Consequently, the verification of design concepts

RAPID PROTOTYPING AND MANUFACTURING TECHNOLOGIES As mentioned earlier, there are several technologies available for model production based on the principle of ‘growing’ or ‘additive machining’. The major differences among these technologies are in two aspects: (1) materials used; and (2) part building techniques. The following sections will explain in detail these rapid prototyping technologies with respect to the above two aspects.

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Figure2

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Slicing and scanning

Review of rapid prototyping technologies and systems: X Yan and P Gu

Stereolithography

Model accuracy andperjormance.

Stereolithography apparatus (SLAI SLA was invented by Charle Hull of 3D Systems Inc.*. It is the first commercially available rapid prototyper and is considered as the most widely used prototyping machine. The material used is liquid photo-curable resin, acrylate. Under the initiation of photons, small molecules (monomers) are polymerized into large molecules. Based on this principle, the part is built in a vat of liquid resin as shown in F&-e 3. The SLA machine creates the prototype by tracing layer cross-sections on the surface of the liquid photopolymer pool with a laser beam. Unlike the contouring or zig-zag cutter movement used in CNC machining, the beam traces in parallel lines, or vectorizing first in one direction and then in the orthogonal direction. An elevator table in the resin vat rests just below the liquid surface whose depth is the light absorption limit. The laser beam is deflected horizontally in X and Y axes by galvanometer-driven mirrors so that it moves across the surface of the resin to produce a solid pattern. After a layer is built, the elevator drops a user-specified distance and a new coating of liquid resin covers the solidified layer. A wiper helps spread the viscous polymer over for building the next layer. The laser draws a new layer on the top of the previous one. In this way, the model is built layer by layer from bottom to top. When all layers are completed, the prototype is about 95% cured. Post-curing is needed to completely solidify the prototype. This is done in a fluorescent oven where ultraviolet light floods the object (prototype). There are several features worthy of mention of SLA. Material. There are five commercially available photopolymers. All of them are a kind of acrylate. Support. Because a model is created in liquid, the overhanging regions of the part (unsupported below) sag or float away during the building process. The prototype thus needs some predesigned support until it is cured or solidfied. The support can be pillars, bridges and trusses. Sometimes posts or internal honeycomb sections are needed to add rigidity to tall thin-walled shapes during the process. These additional features are built on the model parts and have to be trimmed after the model building is completed.

The accuracy achieved is about 0.1% of the overall dimension and deteriorates with larger sizes but no more than 0.5%. The layer thickness is between 0.004 and 0.03”. Presently, the SLA machines made by 3D Systems Inc. are the most accurate machines among the RP&M systems. The photopolymer-made prototype is brittle and may not be strong enough to withstand high stress testing. Also, the shrinkage of the material may make the prototype deform. Capacity. The size of the vat that holds the liquid polymer determines the size limit of the prototype that can be built. The machines with larger vat size are usually more expensive. There are three vat sizes available on the market: SLA190 (7.9 X 7.9 X 9.8”) with 7.5 milliwatt Helium-Cadmium laser, SLA250 (10 x 10 x 10”) with 16 milliwatt Helium-Cadmium laser and !&A500 (20 X 20 X 24”) with 200 milliwatt Argon-ion laser. The scan speed is 503 mms-’ for SLA250 and 2540 mms-’ for SLASOO. Recycling. Photopolymers are thermoset material and cannot be melted again for reuse.

Photo-masking Cubital Ltd. developed the Solider systems which use a photo-masking technique to solidi a whole layer of liquid photopolymer at one time9 . In the building process of the Solider systems as shown in Figure 4, a mask is generated by electrostatically charging a glass plate with a negative image of the cross-section of the part. In the meantime, a thin layer of liquid photopolymer is spread across the surface of the work place. Then, the mask plate with the negative image of the cross-section slice is positioned over the thin polymer layer and exposed under the ultraviolet laser lamp for 2 s. All the exposed areas of a photopolymer layer are solidified with one exposure. The area shaded by the mask is left in liquid form. The liquid polymer is wiped off with vacuum pressure and replaced by hot wax which acts as supports for overhangs and isolated parts of the model. After the wax has cooled down to solid, the surface of the entire polymer/wax is milled with a cutter to a specific thickness. The work piece is then ready for being applied with next layer of liquid polymer. The mask plate is discharged and it can be used for the next cross-section’s negative image. This cycle is repeated until the model is completed. When the model

Laser

The vat Polymerized model

’

Negative image on Laser exposure the mask plate

Liquid phoed

I

Wax filling Figure 3

The working principle of SLA

Vacuum off uncured polymer

Figure 4

Millingtoed thickness

”

Apply new polymer layer

The working process of photo-masking

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is constructed, the supporting wax is melted with either hot water, hot blown air, microwave energy, or solvent. Some features of the Solider system are: Materials. The Solider system also uses liquid photopo lymer-acrylate. The model does not need any post-cure. Support. The system uses wax to fill the cavities left by the uncured polymer. The wax acts as supports for those overhanging regions of a part. Therefore, no pre-designed support structure is needed. Model accuracy and peflomance. The machine produces models with 0.1% dimensional accuracy. The layer thickness is about 0.004-0.006”. The built-in wax support can also help reduce the distortion due to the shrinkage or gravitational effects. Capacity. For the Cubital’s product, Solider 5600, the model size is as large as 20 X 20 X 14”. It uses 2 kw ultraviolet laser lamps. A cycle takes about 90 s.

designed support structures. The unfused powder on every layer acts as a support during the building process. Model accuracy and petiormance. The average accuracy achieved ranges from +0.005 to +O.OlS’ for a part with 12” diameter and 15” height. The layer thickness is between 0.003 and 0.02”. The product may suffer shrinkage and warpage due to sintering and cooling. These two factors can be partly eliminated by choosing powder particles which have a small size, and a high aspect ratio and air flow temperature above the softening point of the powder, but below the sintering point. Capacity. The maximum part dimensions are 12” in diameter and 15” deep for the commercialized SLS Model 125 with 20 watt CO, laser power. Recycling. The prototype can be ‘crushed’ into powder for reuse.

Selective laser sintering (SLS)

Fused deposition modelling (FDM)

DTM Corp. (Austin TX) offers an alternative to liquid-curing systems with its SLS systems which were developed by Carl Deckard and Joseph Beaman at the Mechanical Engineering Department of University of Texas at Austin4T5. SLS uses a carbon dioxide laser to sinter successive layers of powder instead of liquid. In SLS processes, a thin layer of powder is applied by a counter-rotating roller mechanism onto the work place. The powder material is preheated to a temperature slightly below its melting point. The laser beam traces the cross-section on the powder surface to heat up the powder to the sintering temperature so that the powder scanned by the laser is bonded. The powder that is not scanned by the laser will remain in place to serve as the support to the next layer of powder, which aids in reducing distortion. When a layer of the cross-section is completed, the roller levels another layer of powder over the sintered one for the next pass. Figzue 5 shows the working principle of SLS. SLS has several features. Material. SLS uses a wide range of materials for model production including polycarbonate, PVC (polyvinyl chloride), ABS (acrylonirile butadine styrene), nylon, resin, polyster, polypropane, polyurethane and investment casting wax. The machine that is capable of using metal and ceramic powder is in the process of develop ment. Support. The SLS systems usually do not need pre-

Rapid prototyping system-3D modeller developed by Stratasys Inc.-constructs parts based on deposition of extruded thermoplastic materials called FDM proces$. In an FDM process, a spool of thermoplastic filament feeds into a heated FDM extrusion head. The movement of the FDM head is controlled by computer. Inside the flying extrusion head, the filament is melted into liquid (P above the melting temperature) by a resistant heater. The head traces an exact outline of each cross-section layer of the part. As the head moves horizontally in x and y axes the thermoplastic material is extruded out a nozzle by a precision pump. The material solidifies in l/10 s as it is directed on to the workplace. After one layer is finished, the extrusion head moves up a programmed distance in z direction for building the next layer. Each layer is bonded to the previous layer through thermal heating. Figure 6 shows the working principle of FDM. The FDM has the following main features: Material. The FDM technology allows a variety of modelling materials and colours for model building. Available materials are wax-filled plastic adhesive material,

Thermoplastic filament

Laser

Mirror ’ %

1

--4X

I

Fixed workplace Figure 5

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The working principle of SLS

Figure 6

The working principle of FDM

Review of rapid prototypingtechnologies and systems: X Yan and P Gu

proprietary nylon, and investment casting wax. All the materials are non-toxic and can be in different colours. There is minimum material wastage in the method. No post-curing is required. Suppolz. In many cases, the FDM process does not need support to produce part. The FDM extrusion head forms a precision horizontal support in mid-air as it solidifies. For overhanging parts, a support may still be required to reduce part distortion. Model accuracy and performance. The overall tolerance is * 0.005”. Successive laminations are within the range of O.OOl-0.05”, and wall thickness ranges from 0.01 to 0.25”. The prototype built in the FDM process has 1.2% of the maximum shrinkage. Capacity. The working envelope of the 3D modeller is up to 12 X 12 X 12”. The 3D modeller operation speed is up to 900” min-’ (15” s-i).

Laminated object manufacturing (LOM) The LOM processes produce parts from bonded paper plastic, metal or composite sheet stock3. LOM machines bond a layer of sheet material to a stack of previously formed laminations, and then a laser beam follows the contour of part of a cross-section generated by CADto cut it to the required shape. The layers can be glued or welded together. The excess material of every sheet is either removed by vacuum suction or remains as next layer’s support. Figure 7 shows the working principle of LOM. The features of LOM are as follows: Material. Virtually any foil (sheet material) can be applied: paper, metals, plastics, fibres, synthetic materials, glass or composites. Helisys Inc. uses cellulose foils now. Support. The LOM process uses solid-state materials and therefore usually does not need predesigned support structure. Model accuracy and performance. The models can be constructed ivith the accuracy of f0.005” and do not shrink or distort because of the use of sheet material. The thickness of a layer is between 0.002 and 0.02”. The materials used make the prototypes less fragile than those made from photopolymers. Capacity. The LOM machine LOM-1015 uses a 40 watt carbon dioxide laser beam. The size of the prototypes that can be constructed by LOM-1015 is 15 x 10 x 15”. Since only profile cutting is needed instead of curing a solid area, LOM is comparatively faster.

Ballistic particle manufacturing (BPM) The ballistic particle manufacturing technique, developed by Perception Systems uses a piezo-driven inkjet mechanism to shoot droplets of melted materials, which cold-weld together, onto a previously deposited layer’. A layer is created by moving the droplet nozzle in x and y directions. After a layer is formed, the base plate lowers a specified distance and a new layer is created on the top of the previous one. Automated Dynamics Co. also developed a similar machine independently. Figure 8 shows the working principle of BPM. The features of BPM are: Material. The materials should be easily melted and solidified, such as thermoplastics, aluminium, and wax. Perception Systems Inc. now uses wax, Automated Dynamics Co. aluminium. Support. During model building processes, support structures for overhangs and voids are needed. The material for the support is a water-soluble synthetic wax. When the model is completed, it is washed off by warm water. Model accuracy and performance. The overall accuracy is + 0.004”. The layer thickness is about 0.0035”. No performance is reported. Capacity. The BPM printer can spray 50pm droplets of wax at a rate of 10000 droplets s-l with an array of 32 inkjet nozzles. The maximum work piece size is 12 X 2 x 12”.

Three-dimensional printing (3D Printing) Three-dimensional printing was developed at Massachusetts Institute of Technology’. In the 3D Printing process, a 3D model is sliced into 2D cross-section layers in computer. A layer of powder is spread on the top of the piston, the powder bed, in a cylinder, and then an inkjet printing head projects droplets of binder material onto the powder at the place where the solidification is required according to the information from the computer model. After one layer is completed, the piston drops a predefined distance and a new layer of powder is spread out and selectively glued. When the whole part is completed, heat treatment is required to enhance the bonding of the glued powder, and then the unbonded powder is removed. Figure 9 shows the working process of 3D Printing. Features of 3D Printing are summarized below: Materials. The 3D Printing process can use aluminumoxide and alumina-silica ceramic powders. The binder material is amorphous or colloidal silicon carbide. Shooting guns

Workpiece I

Figure 7 The working principle of LOM

g 0-

Molten particles

Figure 8 The working principle of BPM

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Drop piston

Figure 9 The working process of 3D Printing

Support. With the 3D Printing technique, the design of

support structure for the part is not needed, since the unbonded powder of each layer remains to form a natural support during the layering process. Model accuracy and pe$ormance. Little quantitative data are available since 3D Printing is still in the testing stage. For the testing sample, the layer thickness is 178pm and the minimum feature size is 0.017”. Capacity. The 3D Printing process can be used to produce both functional parts and tooling for prototypes. The current maximum part size is 12 X 12 X 24’. The technique has the potential of building parts over 20 X 20” layers that are 100 pm thick at a rate of approximantely 2 s per layer-O.18 m h-i.

P Gu

scribe a model-geometric data and topological data. Geometric data consist of the basic shape-defining parameters. Topological data include the connectivity relationships among the geometric components. There are various types of representation schemes. The most commonly used representation schemes are: CSG (construction solid geometry), Brep (boundary representation) and Polyhedra model. In surface modellers, individual surfaces are assembled to form the desired design. The surfaces generated by 3D surface modellers are of zero thickness. One of the prominent advantages of surface modelling is that it is easy to control the surface shapes during the modelling process, especially when systems utilize non-uniformed rational B-spline (NURBS) to define entities. A major functional difference between solid modelling and surface modelling is that the surface model does not have topological data connecting the surfaces and lacks the capability to describe the interior of the part. Surface modellers are often used to describe aerodynamic and aesthetic shapes. The surfaces themselves may be mathematically precise. However, when they are used as descriptions of the boundaries of an object, unclosed surfaces and Mobius strips may exist since the unambiguity cannot be guaranteed by surface modellers.

Data requirement for RP&M DATA PREPARATION

IN RP&M

An RP&M

Techniques in geometric modelling Geometric modelling techniques are used in CAD to construct digital geometric models for a wide range of objects. There are many modelling techniques available, each of which possesses strengths and weaknesses in constructing different types of objects. The criteria used to evaluate a particular modelling technique can be: The suitability of this technique for a certain application area: a technique that is very useful to construct the wing surface of an aeroplane may not be suitable to construct a simple screw. The intuitiveness of the design process: geometric modelling usually involves human interactions with a computer system. Hence, the technique must be comprehensible to a human operator. The accuracy of digital models: the technique should produce digital models with certain required tolerance specifications. The ease of model modification: geometric modelling involves trial-and-error processes. Therefore, the technique should allow the user to effectively modify the resulting models. Based on the application areas, geometric modelling techniques can be classified into two categories: those that are suitable for modelling complex surfaces and those that are suitable for modelling solids. We call them surface modelling and solid modelling, respectively. A solid model is the unambiguous and informationally complete mathematical representation of the shape of a physical object’. A solid modelling system usually maintains two principle types of data to de312

system is highly dependent on its data input, since it converts the data descriptions directly into solid objects. Therefore, an RP&M system requires unambiguous data description of the object geometry. Also, the model data must facilitate the generation of closed paths and differentiate between the inside and outside of the object. As a result, solid models are more suitable for 3D model descriptions for RP&M systems. For products with free-form surfaces, additional topological information is required from the users, so as to accommodate this kind of product.

Converting geometric modelling into model production instructions Currently, there is no fundamental difference for the data preparation among the existing RP&M technologies. A product is first designed with a 3D modeller. Surfaces of the product are then approximated with multiple facets (usually triangles) whose vertices are ordered to indicate which side of a triangle contains the mass. In the approximation, the precise representations of surfaces such as spline surfaces or boundaries of CSG primitive solids are tessellated into the facet format. This has originated from an RP & M system-SLA of 3D Systems Inc. called STL format. Now, the STL format has become the de facto standard for the data input of all types of RP&M systems. Figure 20 shows the tessellation of a sphere. The accuracy of a non-planar surface depends on the number of facets used to approximate the surface. The STL file is a faceted representation of the exterior surfaces of the object. In order to build the object, the facet representation is then sliced into cross-section data. That means that the object (part) is

Review of rapid prototyping technologies and systems: X Yan and P Gu

Multiple facets STL file

(2) @

D

Single facet

Figure 10 The tessellation of a sphere

(3)

mathematically sectioned (sliced) by a slicing algorithm into a series of parallel horizontal planes. For each of these planes, the scan line algorithm is used to generate the curing paths (the ST1 file)-the trace of ‘machining’. Figure 2 shows the slicing and scan line algorithms. The ST1 file is directly used as the instructions of model production. One of the main reasons for using facet representation is that software systems for generating data for rapid prototyping are based on the existing CADsystems which usually use the facet (polygon) representation to generate shaded images or to do finite element analysis. A majority of the alternative modelling techniques in computer graphics can ultimately be defined in terms of a number of polygons. Polygons are a general and flexible modelling primitive in computer graphics. However, there are several disadvantages of using facet (polygon) representations’. Some of them are given below. (1)

PROBLEMS AND RESEARCH RP&M is still in its infancy. The physical models made by most of these systems cannot be used as working parts, due mostly to material and economic constraints. The major problems in the current RP&M systems include: part accuracy, limited material variety and mechanical performance.

The problem about part accuracy A large number of factors limit the ability of rapid prototyping systems to create parts as accurate as the CAD designs on which they are based. The most common sources of error among the RP&M systems could be categorized as mathematical, process-related or material-related. Mathematical errors include facet approximation of the part surfaces in the standard input to RP&M systems, limited layer resolution along the 2 axis, such

It is a time consuming process to create any but the most simplistic models. A polygon is not a sufficiently ‘high level represenation’ to allow for easy specification of complex models. For example, in order to create even a simple cylinder, each of 20 or so polygons must be specified accurately. The higher the part accuracy is, the higher the resolution for the tessellation of the CADmodel is

LA i

Z

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X

required. The higher tessellating resolution means longer process lead time, much smaller triangles, and a bigger STL model size. It may cause storage and computing problems. Because of the limited precision of computer systems, a triangle may collapse into a line whenever one of its sides become too small. The result is the lack of connectivity in the ‘3D’ triangle matrix, which leaves ambiguous gaps in the representation of geometric objects. A gap can mislead the scan line algorithm to think that a solid mass extends to the edge of the RP&M universe. Figure II shows the gap that results in an incorrect curing path. Incorrect normals can be generated if two adjacent triangles suggest the mass of the object is on the opposite side. As the incorrect normal propagate, Mobius-strip conditions appear in which the surfaces transform one another.

Y

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/ Gap

Incorrect solid line due to gap

Figure 11 The gaps resulting from an incorrect curing path

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Review of rapid prototyping technologies and systems: X Yan and P Gu as stairsteps and accuracy of vertical dimensions. Alternative data preparation methods based on CSG solid modelling which can input precise part surface to RP& M systems are under development’0-12. Parts are represented with CSG and a ray-tracing algorithm is used to generate the curing path. Carnegie-Mellon University (CMU>‘3 uses a 5axis CNC mill to remove the excess material to shape the geometry of the layer in a newly developed RP&M system (shape deposition manufacturing) which can eliminate the stairsteps. This S-axis mill could also be used to maintain the accuracy of vertical dimensions by milling away the excess thickness. Process-related errors affect the shape of the layer in the X-Y plane and along the Z axis, the registration between different layers and the overall 3D shape. These errors are mainly dependent on the accuracy of the RP&M machines and operators’ experiences’4,15. Major material-related errors are shrinkage and distortion. The shrinkage is a by-product of solidification-the cooling down of material with rapid prototyping processes. Predictable dimensional shrinkage can be compensated for by scaling the CAD model. Sometimes, the shrinkage is not identical along X, Y and Z axes. During the processes of building parts, stresses due to shrinkage may be locked into parts. Eventually, these stresses may cause the part to creep and distort. There may be several ways to minimize the effect of shrinkage: selection of appropriate manufacturing control parameters, development or exploration of materials with relatively small shrinkages or stressfree properties, and stress relief methods. All these approaches require in-depth research on the materials and understanding of the processes. For the SLA system, 3D System’s Star-Weave method2 can limit some of the distortion by curing more resin in the vat, and hatching the inside cross-section area on each layer. Many people study the physical and chemical characteristics of the materials used in stereolithography systems that affect the produced parts’ accuracy 6 19. In Stratasys Inc. experimental research for manufacturing control parameters and material selection is carried out for FDM systems 2o. Data are presented to help the user choose the appropriate material for specific applications. The in-depth research on FDM processes results in system refinements in accuracy, speed and surface finish2i. The physical mechanisms that drive the SLS process are being studied in University of Texas to enhance the technology22,23. The physical and chemical characteristic of materials in 3D Printing are being studied for better product dimensionsz4. Japanese Synthetic Rubber Co. Ltd. and DMEC Ltd.” developed a new resin for stereolithography, which was effective in reducing the distortion. DuPont Imaging Systems also developed a low-shrinkage resin for its stereolithography system-SOMOS solid image’. CMU uses a method of striking metallic balls or shots under pressure on each solidified layer to relieve large stresses generated during the processes of depositing metal material for its rapid prototyping systems25.

The problem about materials The current RP&M systems use very limited types of materials. Parts built by RP&M systems tend to be 314

weak and fragile compared to those made conventionally from metals and engineering plastics. Some materials for the RP& M machines are expensive or toxic. Most of the research and development efforts have been focused on improving part materials, which are carried out in two different directions. One is that plastic companies with products already on the market, particularly those based on SLA, SLS, and FDM, have developed plastics with better physical properties-less brittle, lower shrink, lower viscous resin and similar to the end-user applications. Chemists developed a new less-brittle polymer that is used in 3D System’s SLA-500’. Another method for improving the mechanical properties of the material uses fibre-resin composites composed of glass, carbon, or graphite fibre bonded together by polymeric resin and cured into a solid part . Some researchers are trying to mix ceramics powder in the photopolymer resin to create ceramic green bodies such as structural ceramic parts and refractory ceramic parts for investment casting using stereolithography systems2’. The other direction is to focus on metals. The users of RP&M technologies tend to build models with materials whose properties are similar to the materials they might use in their end-use applications. Metal is most-commonly used in current industry. All rapid prototyping machines on the market can be used to produce metal parts indirectly through various casting processes such as investment casting. The direct production of metal parts is still in development. CUM is developing a metal based RP&M system-Shape Deposition in which a part is built by successively depositing molten metal materials such as zinc, steel and copper in thin layers =. With MIT’s 3D Printing technique, direct shell production casting generates a ceramic shell or mould complete with core8. Then an alloy is poured into the shell to case a functional metal part. The University of Texas is pursuing the direct sintering of metal powder (SLS). Several metal mixtures are used in the SLS process to produce metallic parts, such as low melting-point binary metal powders: Cu-70Pb-30Sn solder, Cu-Sn28,29 and Ni-Sn ‘car; and high temperature materials: pre-alloyed 90Cu-1OSn bronze mixed with Ni powder32,33. With BPM technique, Incre Inc. in Oregon has been shooting droplets of molten tin and aluminum from a moving nozzle to build 3D objects.

EXAMPLES OF RAPID PROTOTYPING RESEARCH AND APPLICATIONS In this section, we present two examples of our research and application of rapid prototyping technologies.

Rapid prototyping without tessellation The model production technologies of RP&M have received much more attention than the data preparation, However, the quality and the accuracy of input data are equally important in implementing this technology. We have analysed the current approximation approaches in data preparation, and pointed out the problems earlier. Based on this analysis, a new tech-

Review of rapid prototyping technologies and systems: X Yan and P Gu

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(2)

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(i) (ii) (iii) mm----

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If R intersects with A but not with B. Then:

(3)

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(4)

If R intersects with both A and B. Then:

(A U BjR = (a1,u2) U (b,,b,). (A n BIR = (a,, a,> n (b,,b,). (iii> (A - BjR = (u1,u2) - (b,,b,). (8

(ii)

nique called model preparation without tessellation for improving RF&M part modelling and generating control data for curing paths from CADdata was explored. The technique uses the ray tracing algorithm to intersect with solid primitives of CSG representation, so it avoids the tessellation processes. The advantages of this technique over current approximation methods include better dimensional accuracies, less data conversion, smaller data files and simpler control strategies. The ray tracing algorithm developed by Appel was used originally for visible-surface determination in computer graphics 34. Goldstein and Nagel were the first researchers to ray trace combinations of simple objects produced with Boolean set operations35. Calculating the union, difference, or intersection of two 3D solids is a difficult issue when it must be done by direct comparison of one solid with another. However, ray tracing allows a 3D problem to be reduced to a set of 1D calculations. The intersections of each ray and a primitive object yield a set of values, each of which specifies a point at which the ray enters or exits the object. The Boolean operations are performed on one ray at a time by determining the 1D union, difference, or intersection of spans from the two objects along the same ray. Each ray hierarchically traverses the CSG tree by evaluating the left and right intersection lists at each node. Iiigure 12 shows the spans defined by a ray passing through two objects and the combination of spans when the set operations are applied. To explain the algorithm clearly, the following notations are used. If a ray R intersects with a solid primitive P at points pi and pz, then it is expressed as PR= (p,, p&, where (p,, pz) is the span of the ray within the object P. If the ray R intersects with P at only one point p, then it is represented as PR= (p,p). If R does not intersect with P, then the situation can be expressed as PR = NULL. If two solid primitives A and B intersect with a ray R at points a, and u2, and 6, and b,, they are described by A, =(a1,u2) and BR = (b,,b,), respectively. The following is the enumeration of the ray-primitive intersection cases: (1)

If R intersects with B but not with A. Then:

For each solid primitive supported by the Autosolid package, a method called INTERSECTis defined. For example, if the primitive is a sphere, then its INTERSECT method is defined as:

Span asphere: INTERSECTWtuy) BEGIN /*Cc,,, yo, z,)-The centre of the sphere;*/ / *r-The radius of the sphere;*/ a = r x r - (&y.z - zO>x (uRay.t -to); IF(a > = 0) THEN b = u - W?uy.x -no) x 6zRay.x-x0); IF(b>=O)THEN s1 =yo-

sqdb); $2=yo + sqrt(b); RETURN (s,,s,); ELSE RETURNNULL; END-IF; ELSE RETURNNULL, END-IF; END; For each CSG tree, a method called TRACINGis defined as the following:

List _Of -Span aCSG : TRACING&~) BEGIN IF(aCSG is a primitive, say aPrimitive)THEN RETURNaPrimitive: INTERSECT@@‘); ELSE SpanListl = aCSG . leftTree : TRACIN~UR~~); SpanList = aCSG . rightTree : ~~~mduRay); RETv&Merge(SpanListl, SpanList2, aCSG . operator)); END-IF; END;

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and systems:

X Yan and P Gu

An example of applying the algorithm is shown in resolutions of the model. Currently, we are comparing the technique with the tessellation technique.

Figure 13. It shows different

Development of complex pattern with rapid prototyping Polyurethane bicycle tyres have gained popularity in P . recent years. A Calgary tyre manuracrurmg company is manufacturing such tyres using casting methods. In order to produce moulds for casting, a master tyre pattern must be first developed. The master pattern is used to produce a temporary mould that produces a number of casting patterns. These casting patterns are then used to make the final moulds. After the moulds are manufactured, these patterns are also destroyed. However, the master pattern is preserved and it can be used again to produce casting moulds. The entire process including design, pattern development and

mould manufacturing takes 6-8 weeks to produce a set of moulds. In order to reduce lead time and economically produce moulds, we have developed a computer-integrated design and manufacturing sytem and the rapid production technique of complex patterns36”7. The traditional method of making the master patterns uses machines to cut the initial rough shapes of the tyre patterns and complete other features by machining, then manually finishes the tread patterns. Because of the complexity of tread patterns of bicycle tyres, it usually takes days to make such patterns and the quality is also a problem, especially the consistency of the tread patterns. A Solider 4600 of Cubital America Inc. (shown in Figure 24) was used to produce the tyre master pattern. The working principles of Solider systems have been discussed earlier in this paper. A complete Solider 4600 system includes a DEC Vax workstation, slicing and control software, the model-building machine, and a cleaning (dewax) station. The system uses light-curable acrylate photopolymer and a photo-masking technique

(b)

(a>

(4 Figul re 13

316

An example

of using the algorithm

Review of rapid prototyping technologies and systems: X Yan and P Gu

similar to the process used for the manufacture of printed circuit boards. Models as large as 14 X 14 X 14” can be made. According to the description of Cubital Ltd., the machine can produce models with 0.1% (or 0.002”) dimensional accuracy in x, y and z directions. Cubital’s CAD interface accepts both industry-standard STL files and Universal files developed by Structural Dynamics Research Corp. (SDRC); the latter allows precise curve-fitting techniques to be used. The light-curable acrylate photopolymer called Solider G5601/N was used for making the pattern. It has the following properties: Modulus @ 25” C, 50% rh: Tensile strength (at break): Elongation (to break): Creep (mm): Water absorption (afer 7 days):

1000 Mpa 45 Mpa 5% 5% 1.2

To make a 10” tyre master pattern, the STL file was created from its CAD model in Pro-Engineer and then transferred to the Solider 4600 system. To make the pattern, that is about 1.5” thick, the Solider system sliced it into 254 layers with 150pm in thickness. It took about 8 h to make the pattern (and a mould and many other parts in one setup). Figure 15 is a 10” lyre master pattern made by the Solider 4600 systems. In the experiments, the cordheight was set to 0.01 mm for reducing the number of facets and thus a small sized data file. The quality can be further improved by decreasing the cord-height, say to 0.001 mm. The experiments show that the overall efficiency can be improved by using the technology. Currently, we are working on the development of moulds directly using rapid prototyping methods.

CONCLUSIONS Product features, quality, cost and time to market are important factors for a manufacturer to remain competitive. Rapid prototyping systems offer the opportunities to make products faster, and usually at lower

Figure 14

costs than using conventional methods. Since RP&M can substantially reduce the product development cycle time, more and more businesses are taking advantage of the speed at which product design generated by computers can be converted into accurate models that can be held, viewed, studied, tested, and compared. Several new and promising rapid prototyping manufacturing techniques were discussed. They are all based on material deposition layer by layer. Each of them has particular features in terms of accuracy, material variety and the cost of the machine. Some present problems and research issues were also discussed. This is a rapid development area. Capacities and the potential of rapid prototyping technologies have attracted a wide range of industries to invest in these technologies. It is expected that greater effort is needed for research and development of those technologies so that they will be widely used in product-oriented manufacturing industries.

ACKNOWLEDGEMENT This research is supported by the National Sciences and Engineering Research Council of Canda (NSERC) through CRD Grant #143281 and Polyair Tires Inc. through Grant-in-Aid.

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Figure 15 Tyre master pattern Crump, SS ‘Fast, precise, safe prototypes with

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519-528 Fan, T, Lauder, A, Sachs, EE and Cima, MJ ‘The surface finish in the three-dimensional printing’ Third Int. Conf Rapid Prototyping (June 1992) pp 63-72 Amon, CH, Finger, S, Prinz, FB and Weiss, LE ‘Modeling novel manufacturing processes’ Proc. Manufacturing Science and Engineering Chicago, Illinois, ASME (June 1994) pp 535-546 Barlage, WB, Jara-Almonte, CC, Bagchi, A, Ogale, AA and Dooley, RL ‘Fiber/resin composite manufacturing using solid freeform fabrication’ Third Znt. Conf Rapid Prototyping (June 1992) pp 15-24 Griffith, ML and Halloran, JW ‘Ultraviolet curable ceramic suspensions for stereolithography of ceramics’ Proc. Manufacturing Science and Engineering Chicago, Illinois, ASME, (June 1994) pp 529-534 Manriquez-Frayre, JA and Bourell, DL ‘Selective laser sintering of binary powder’ Solid Freeform Fabrication Symp. Proc. Austin, TX (August 19901pp 99-106 Manriquez-Frayre, JA and Bourell, DL ‘Selective laser sintering of Cu-Pb/Sn solder powder’ Solid Freeform Fabrication Symp. Proc. Austin, TX (August 1991) pp 252-260 Bourell, DL, Marcus, HL and Weiss, WL ‘Selective laser sintering of part by compound formation of precursor powders’ US Patent _5,156,697 issued 20 October (1992) Weiss, WL and Bourell, DL ‘Selective laser sintering to produce Ni-Sn intermetallic parts’ Solid Freeform Fabrication Symp. Proc. Austin, TX (August 1991) pp 267-274 Agawala, MK, Bourell, DL, Manthiram, A, Birmingham, BR and Marcus, HL ‘Synthesis, selective laser sintering and infiltration of high tc dual phase y2bacu3o7-x superconductor composites’ Solid Freeform Fabrication Symp. Proc. Austin, TX (August 1993) pp 339-349 Zong, G, Wu, Y, Tran, N, Lee, I, Beaman, JJ, Bourell, DL and Marcus, HL ‘Direct selective laser sintering of high temperature materials’ Solid Freeform Fabrication Symp. Proc. Austin, TX (August 1992) pp 72-85 Appel, A ‘Some techniques for shading machine renderings of solids’ SICC (1968) pp 37-45 Foley, JD, Van Dam, A, Feiner, SK and Hughes, JF Computer Graphics--principles and Practice (2nd Ed.) Addision-Wesley (1990) Gu, P ‘Computer-integrated design and manufacturing of polyurethane bicycle tires and moulds’ NSERC CRD Grant Report (November 1993) Gu, P ‘Rapid prototype manufacturing of tires patterns and moulds’ NSERC CRD Grant Report (November 1994)

X Yan is currently a research associate in the Department of Mechanical Engineering at the Universi~ of Calgary, Canada. She received her MSc from the University of Calgaty, MEng from the Chinese Academy of Science, and B Eng fromZhjing Uniuersity in China. Her recent research interests include rapid prototyping design automation and automated assembly.

P Gu is Professor and NSERC /

AECL

the Department

has also held a tenured faculty position the Department the Universi~ the Uniuersi~ was a Senior Manufacturing Engineer Ltd. .^ . . current research life cycle design rapid product realization and manufactuting systems.