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International Journal of Machine Tools & Manufacture 38 (1998) 12571287

A comparison of rapid prototyping technologies

D.T. Pham*, R.S. Gault

Cardiff Rapid Prototyping Centre, Systems Division, School of Engineering, University of Wales Cardiff, PO Box

688, Cardiff CF2 3TE, UK

Received 16 October 1997

Abstract

Until recently, prototypes had to be constructed by skilled model makers from 2D engineering drawings.This is a time-consuming and expensive process. With the advent of new layer manufacturing andCAD/CAM technologies, prototypes may now be rapidly produced from 3D computer models. There aremany different rapid prototyping (RP) technologies available. This paper presents an overview of the currenttechnologies and comments on their strengths and weaknesses. Data are given for common process para-meters such as layer thickness, system accuracy and speed of operation. A taxonomy is also suggested,along with a preliminary guide to process selection based on the end use of the prototype. 1998 ElsevierScience Ltd. All rights reserved.

Keywords: Rapid prototyping; Stereolithography; Selective laser sintering; LOM; 3D printing; Fused deposition model-ling

1. Introduction

Prototyping is an essential part of the product development and manufacturing cycle requiredfor assessing the form, fit and functionality of a design before a significant investment in toolingis made. Until recently, prototypes were still largely handmade by skilled craftsmen, adding weeksor months to the product development time. Because of this, only a few design iterations couldbe made before tooling went into production, resulting in parts which at best were seldomoptimised and at worst did not function properly.

Rapid prototyping (RP) is a term which embraces a range of new technologies for producing

* Corresponding author.

0890-6955/98/$19.00 1998 Elsevier Science Ltd. All rights reserved.PII: S 0 8 9 0 - 6 9 5 5 ( 9 7 ) 0 0 1 3 7 - 5

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1258 D.T. Pham, R.S. Gault/International Journal of Machine Tools & Manufacture 38 (1998) 12571287

accurate parts directly from CAD models in a few hours, with little need for human intervention.This means that designers have the freedom to produce physical models of their drawings more

frequently, allowing them to check the assembly and function of the design as well as discussingdownstream manufacturing issues with an easy-to-interpret, unambiguous prototype. Conse-quently, errors are minimised and product development costs and lead times substantially reduced.It has been claimed that RP can cut new product costs by up to 70% and the time to market by90% [1].

RP technologies may be divided broadly into those involving the addition of material and thoseinvolving its removal. According to Kruth [2], the material accretion technologies may be dividedby the state of the prototype material before part formation. The liquid-based technologies mayentail the solidification of a resin on contact with a laser, the solidification of an electrosettingfluid, or the melting and subsequent solidification of the prototype material. The processes usingpowders compound them either with a laser or by the selective application of binding agents.

Those processes which use solid sheets may be classified according to whether the sheets arebonded with a laser or with an adhesive. Figure 1 shows Kruths classification, which has beenadapted to include new technologies. In the following, RP technologies are presented accordingto the arrangement shown in this figure.

Fig. 1. Classification of rapid prototyping methods (adapted from [2]).

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1259D.T. Pham, R.S. Gault/ International Journal of Machine Tools & Manufacture 38 (1998) 12571287

2. Material addition technologies

All of the processes reviewed require input from a 3D solid CAD model, usually as slices. Thedesigner therefore first uses a CAD package to design the product which he wishes to manufacture.This model is then tessellated and exported as an STL file, which is the current industry standardfor facetted models, although it may be possible, in future, to slice models directly from the CADsystem without first facetting them [3]. If supports are necessary to brace any overhangs, pro-prietary software may now add these to the model. It is then sliced and the slices sent to the RPmachine for the production of the final physical part. By convention, the data slices are said tobe in the XY plane and the part is built in the Z direction.

An important problem is automatic support generation and part orientation. This is becausepart orientation will influence the final prototype build time and the surface finish of critical areas.The number and position of the supports depend to some extent upon the build direction chosen

and may also adversely affect the build time and surface finish of the prototype [35].

2.1. Processes involving a liquid

2.1.1. Solidification of a liquid polymer

Of the five processes in this category, which all involve the solidification of a resin via electro-magnetic radiation, three construct the part using points to build up the layers whilst the othertwo solidify entire layers or surfaces at once.

2.1.1.1. Stereolithography (SL) The most popular among currently available RP technologiesis perhaps stereolithography. This relies on a photosensitive monomer resin which forms a poly-mer and solidifies when exposed to ultraviolet (UV) light. Due to the absorption and scatteringof the beam this reaction only takes place near the surface. This produces parabolically cylindricalvoxels (three-dimensional pixels) as shown in Fig. 2 which are characterised by their horizontalline width and vertical cure depth [6].

An SL machine consists of a build platform (substrate) which is mounted in a vat of resin anda UV heliumcadmium or argon ion laser (Fig. 3). The first layer of the part is imaged on theresin surface by the laser using information obtained from the 3D solid CAD model. Once thecontour of the layer has been scanned and the interior either hatched or solidly filled, the platformis next lowered to the base of the vat in order to coat the part thoroughly. It is then raised suchthat the top of the solidified part is level with the surface and a blade wipes the resin leaving

exactly one layer of resin above the part. The part is then lowered to one layer below the surfaceand left until the liquid has settled [7]. This is done to ensure a flat, even surface and to inhibitbubble formation. The next layer may then be scanned.

All new SL machines now employ a method to apply the resin that is superior to the deep-dipprocess described above. Because of the high resin viscosity, after the deep dip and recoating,either too little or too much resin is left by the recoating blade, which affects part accuracy. Thenew method involves spreading resin on the part as the blade traverses the vat. Because the bladeapplies only the required amount of resin, good accuracy is achieved. This method also providesa smoother surface finish and reduces non-productive recoat time. Another important advantageis the elimination of trapped volume problems. A trapped volume is a volume of resin that

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Fig. 2. Single cured line of photopolymer (adapted from [6]).

cannot drain through the base of the part (Fig. 4). The presence of a trapped volume in the deep-dip process affects part accuracy and may lead to delamination or collision of the blade and partbecause of a build up of unwanted polymerised resin at the surface.

Once the part is completed, it is removed from the vat and the excess resin drained. Due tothe resin viscosity, this stage may take several hours. The supports are removed and the greenpart is then placed in a UV oven to be postcured. This ensures that no liquid or partially curedresin remains.

Solid or partially solid parts are made with either acrylic or epoxy resins in one of severalbuild styles, the three most common being ACES, STARWEAVE and QuickCast [8]. Com-pletely hollow parts are not normally constructed as these are very fragile in the green state anddeform on handling.

When adopting ACES, the interior of the part is almost wholly cured by the laser (Fig. 5).This is achieved by using a hatch-spacing which is equivalent to half the line width. This spacing

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Fig. 3. Stereolithography.

Fig. 4. Trapped volume in stereolithography.

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Fig. 5. ACES build style: repeated, even laser exposure produces a flat base.

is chosen such that all the solidified resin receives the same cumulative UV exposure and hencethe downward facing surfaces are flat. This style may only be used with epoxy resins that do notshrink much when polymerised otherwise the connected lines would cause warping in the proto-type. It is the most accurate of the three build styles for low-distortion resins and is employedwhen making high precision parts although the drawing time is the longest of the three styles [9].

STARWEAVE provides stability to a solid part by hatching the interior with a series of gridswhich are offset by half of the hatch spacing every other layer (Fig. 6). The grids are drawn suchthat the ends are not attached to the part border to reduce the overall distortion. Also, to keepthe distortion low, the gridlines do not touch one another. However, they are located as closelytogether as possible to improve the green strength of the part [8,9]. This build style should beemployed with acrylic resins which have a higher shrinkage when polymerised. It is sometimesused with epoxy resins in preference to ACES because the draw time is lower.

QuickCast is usually adopted when the prototype is to be employed as a pattern for investment

casting as it produces almost hollow parts. The outline of the layer is drawn before the interioris hatched. Either squares (QuickCast version 1.1) or equilateral triangles (QuickCast version1.0) are used to fill the part and these are offset after a specified vertical build distance to facilitateresin drainage. The triangles are offset such that the vertices of one section are above the centroidsof the triangles in the previous section (Fig. 7). The squares are offset by half of the hatch spacing.Since squares have larger interior angles than triangles, the meniscus of resin will be smaller sobetter drainage is achieved [9]. Horizontal sections that form the outer surface of the part arecompletely solidified and are referred to as skinfill areas. Three layers are drawn with skinfillareas corresponding to the part surface to avoid the formation of pinholes when the supportsare removed and to prevent the upwards-facing horizontal surfaces from sagging [9,10]. These

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Fig. 6. STARWEAVE build style. (a) One layer of STARWEAVE. This is composed of a cross-hatched gridwhich is detached from the part border; (b) alternate layers of STARWEAVE are offset by half the hatch-spacing.

skinfills support the part surface, which means that the hatch spacing may be larger and a smallerpercentage of the prototype is solid [9]. Vents and drains must be designed into these areas toallow the excess resin to bleed from the part. These parts will collapse quickly upon firing sothat little stress is developed on the ceramic investment shell, preventing it from being damaged.Because QuickCast parts have a large surface area and the resin is hygroscopic, they shouldbe used as quickly as possible and stored in an area with controlled humidity to prevent laterdistortion due to water absorption.

Hatch spacing must be determined so that the voxels are situated sufficiently near to each otherto allow the layers to be connected, but not so closely that the laser scan time is unacceptable orresidual stresses are developed through overcure. The layer thickness will obviously affect thecloseness of the voxels in the vertical direction if the layers are too thick, surfaces will notconnect [7]. Voxels on sloped surfaces must be nearer to avoid gaps through which resin maydrain or through which slurry may invade in later processes such as investment casting.

The advantages of stereolithography are that it produces a surface finish that is comparable tothat of NC milling, it is a well proven system with over 500 machines in use worldwide and itis reasonably fast and accurate [11,12]. To utilize the resin vat fully, several parts may be builtat once.

The disadvantages are that the material is expensive, smelly and toxic and must be shieldedfrom light to avoid premature polymerization; there is also a limited choice of resins. The partsmay be brittle and translucent and they need supports which may adversely affect the surfacefinish when removed.

The system has an accuracy of 100 m and can achieve layers 50 m thick [13]. A machinewith a build chamber of 250 250 250 mm, the most common size, will cost approximately

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Fig. 7. QuickCast build style: parts are hatched with offset triangles.

150 000. The largest build chamber commercially available measures 500 500 584 mm [14].The recoat time is 35 s for the new method and more than 50 s for the deep dip method. Thedraw time is proportional to the cross-sectional area of the part; a layer with a cross-section of50 50 mm2 takes about 78 s to solidify, according to the laser power and curing parameters.

Further research is being actively conducted into materials and into the accuracy, warping andshrinkage of the parts.

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2.1.1.2. Liquid thermal polymerization (LTP) This process is similar to SL except that the resinis thermosetting and an infrared laser is used to create the voxels. This difference means that the

size of the voxels may be affected through heat dissipation, which may also cause unwanteddistortion and shrinkage in the part. However, the problems are apparently no worse than thosecaused by SL and are controllable [2]. This system is still being researched.

2.1.1.3. Beam interference solidification (BIS) This process uses two laser beams mounted atright angles which emit light at different frequencies to polymerise resin in a transparent vat (Fig.8). The first laser excites the liquid to a reversible metastable state and then the incidence of thesecond beam polymerises the excited resin.

To date, there are no commercial applications of this technology because there are still technicaldifficulties to be solved:

Shadows are cast from previously solidified sections. There is a problem with light absorption because the intensity of the lasers drops with depth. It is hard to intersect the laser beams due to diffraction variations in the resin caused by

temperature gradients or solid sections [2].

Fig. 8. Beam interference solidification (adapted from [2]).

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2.1.1.4. Solid ground curing (SGC) This system again utilizes photopolymerising resins andlight. Data from the CAD model is used to produce a mask which is placed above the resin

surface. The entire layer can then be illuminated with a UV lamp (Fig. 9). Once the layer hasbeen cured, the excess resin is wiped away and any spaces are filled with wax. The wax is cooledwith a chill plate, milled flat and any chips removed. A new layer of resin is applied and theprocess is repeated.

The mask itself is a sheet of glass which is prepared whilst the current layer is being waxed,cooled and milled. The negative image of each subsequent layer is produced electrostatically onthe glass and developed using a toner in a similar manner to laser printing.

Because wax is used to fill the gaps in the cured resin, no further supports need to be addedby the interface software. The wax supports any overhangs in the design and anchors any discreteprotrusions which may be drawn on a layer. It also theoretically reduces distortion due to warpingand curl since the part is surrounded and means that the machines do not need to be vibration

proofed as the part cannot move in the vat [2,15]. Builds may also be paused to allow other,more urgent parts to be made [16]. An advantage of this system is that the entire layer is solidifiedat once, reducing the part creation time, especially for multi-part builds. Parts may also be nestedto utilise the build volume fully. All the resin within a layer is completely cured by this method,and so no postcuring is required, parts may be more durable than the hatched prototypes created

Fig. 9. Solid ground curing.

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using other processes and operators need not handle partially cured, toxic resin [16]. The waxmay be removed automatically in a special machine.

The disadvantages of this system are that it is noisy, large and heavy and needs to be constantlymanned. It wastes a large amount of wax which cannot be recycled and is also prone to break-downs [1,9]. The mask is produced by raster scanning the image [16] which may cause steps inthe XY plane, affecting accuracy. The resin models produced using SGC are solid and so cannotbe used for later investment casting since the coefficient of thermal expansion of the resin is anorder of magnitude greater than that of the ceramic system so the ceramic moulds will crackwhen the sacrificial part is burnt out [1].

The resolution is 100 m in the horizontal XY plane and 100 m in the z direction. The leastexpensive SGC machine costs around 180 000 and weighs about 5000 kg. The largest buildchamber available is 500 350 500 mm. Typically, a layer can be built in 65120 s, dependingon the machine used. Of this building time, 3 s are for exposing the layer to a 2000 W UV

lamp, the remaining time being needed to clear the part of resin and to add, chill and mill thewax [16,17].

2.1.1.5. Holographic interference solidification (HIS) A holographic image is projected intothe resin causing an entire surface to solidify. Data is still obtained from the CAD model, althoughnot as slices. The build space is 300 300 300 mm [2]. There are no commercial systemsavailable yet.

2.1.2. Solidification of an electroset fluid: electrosetting (ES)

Electrodes are printed onto a conductive material such as aluminium. Once all the layers havebeen printed, they are stacked, immersed in a bath of electrosetting fluid and energised. The fluidwhich is between the electrodes then solidifies to form the part. Once the composite has beenremoved and drained, the unwanted aluminium may be trimmed from the part.

Advantages of this technology are that the part density, compressibility, hardness and adhesionmay be controlled by controlling the voltage and current applied to the aluminium. Parts may bemade from silicon rubber, polyester, polyurethane or epoxy. The hardware for such a systemmay be bought off the shelf and costs about 5000. The software for the system is still beingdeveloped [18].

2.1.3. Solidification of molten material

There are four technologies which involve the melting and subsequent solidification of the part

material. Of these, the first three deposit the material at discrete points whilst the fourth manufac-tures the whole layer at once.

2.1.3.1. Ballistic particle manufacture (BPM) A stream of molten material is ejected from anozzle. It separates into droplets which hit the substrate and immediately cold weld to form thepart (Fig. 10). If the substrate is rough, thermal contact between it and the part is increased whichwill reduce stresses within the part [19].

The stream may be a drop-on-demand system or a continuous jet. When a continuous jet isadopted, it is ejected through the nozzle which is being excited by a piezoelectric transducer ata frequency of about 60 Hz [20]. To avoid melting the transducer, it is located at a distance from

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Fig. 10. Ballistic particle manufacture.

the nozzle. Although a capillary stream will naturally decompose into droplets [21], the disturb-ance at the nozzle forces the production of a stream of small, regular droplets with uniformspacing and distance. Using a low-frequency carrier wave modulated by a higher frequency dis-turbance, tailor made streams have been produced where the user is able to specify larger dropletseparations than would otherwise be obtainable with just a single frequency. Regular streams havealso been produced consisting of a few small, close droplets followed by larger, more distantdroplets [22]. This should allow more time for the nozzle to move to a new position or for thedroplets to solidify if necessary.

Parameters that will affect the eventual part characteristics are the temperature and velocity ofthe droplets and the charge that they carry. The charge is acquired electrostatically when thestream is ejected and can be used for the accurate placement of the material. Since the maximum

charge which may be held by a drop is limited, the maximum deflection of such a drop is alsolimited and the substrate or the jet must therefore be movable in order to produce a large enoughbuild area. The temperature will control the speed at which the molten material solidifies. If thedroplets are too cold they will solidify midflight and will therefore not weld to the part. If theyare too hot, the part will lose shape. The deformation and placement accuracy of the dropletdepend on its velocity. If it is moving too slowly, placement accuracy will be poor; if it movestoo quickly the droplet will be highly deformed on impact [19].

The resolution of the prototypes is related to the droplet diameter which is typically 50100 m.Droplets may be released in nitrogen or in vacuo to avoid their oxidation and dispersion. Thedeposition rate is up to 15 000 droplets per second using a single nozzle and a continuous jet

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[19]. In this process, the supports are usually made from a different material which facilitatestheir subsequent removal from the part.

Advantages of BPM are that it is cheap and environmentally safe and that metal parts madeusing this technology have a finer grain structure than the equivalent cast parts. This is becausethe splat cooling of the droplets means that they retain an amorphous structure instead of crystallis-ing, giving the prototype good mechanical properties. Materials which may currently be employedfor part construction are tin, zinc, lead, other low ( 420C) melting point alloys and thermoplas-tics. Systems are being developed to deposit copper which melts at 1100C [19].

A disadvantage is the small range of commercial materials available to construct the prototypes.Of the systems available, either speed or accuracy is possible, but not both attributes.

There are several commercial dual material systems available which can deposit either thermo-plastic or wax. One of the most accurate, BPM1, uses a drop-on-demand jet to eject the moltenmaterial. The droplets are spheres, 76 m in diameter, which flatten on impact to give discs which

have a diameter of 101 m and are 63 m thick. After each layer is deposited, the part is milledto achieve accurate dimensions in the z direction. In order to maintain the tolerances in the hori-zontal plane, the layer contours are drawn using linear interpolation (not raster scanning) beforethe interior of the part is filled. The system is able to vary the layer thickness in order to providespeed in areas where the geometry remains unchanged from layer to layer without losing accuracyin critical areas. A future improvement is the use of a larger nozzle to deposit material withinthe boundary of the part. This should significantly reduce the build time.

The system is claimed to have an exceptionally good accuracy of 25 m, layer thicknessesof 13130 m and resolution of 101 m in the XY plane. It operates at 1824C and can buildat a linear speed of 310 mm s1 [23]. The cost of a machine with a build chamber of 300 150 220 mm is about 60 000. It is intended to produce parts for downstream manufacturing andso offers a very high accuracy and low layer thickness.

A similar system, BPM2, employs a head with 5 d.f. to deposit the material. This ensures thatthe direction of the jet is perpendicular to the normal of the surface and should eliminate stepsin the build direction. The system uses a proprietary thermopolymer material to build modelswith a maximum size of 250 203 150 mm. A BPM2 machine will cost approximately 25 000.It has a resolution of 558 m and an accuracy of 17 m [24].

Another implementation of this technology, known as Multi Jet Modelling (MJM), employs96 jets which scan each layer in a raster fashion. Parts are constructed from a thermopolymermaterial within a 250 200 200 mm build envelope. The parts have a layer thickness of 33 m,an XY resolution of 85 m and a droplet placement accuracy of 100 m [25]. The cost of an

MJM machine is around 50 000. The machine offers a high part creation speed and is intendedprimarily for model visualisation.

2.1.3.2. Fused deposition modelling (FDM) The FDM machine consists of a movable headwhich deposits a thread of molten material onto a substrate. The build material is heated to 0.5Cabove its melting point so that it solidifies about 0.1 s after extrusion and cold welds to theprevious layers (Fig. 11). Factors to be taken into consideration are the necessity for a steadynozzle speed and material extrusion rate, the addition of a support structure for overhanging parts,and the speed of the head which affects the overall layer thickness [15,26].

The latest FDM system includes two nozzles, one for the part material and one for the support

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Fig. 11. Fused deposition modelling.

material. The latter is cheaper and breaks away from the prototype without impairing its surface.It is also possible to create horizontal supports to minimise material usage and build time [26,27].

An advantage of this system is that it may be viewed as a desktop prototyping facility in adesign office since the materials it uses are cheap, non-toxic, non-smelly and environmentallysafe. There is also a large range of colours and materials available, such as investment castingwax, ABS plastic, medical grade ABS (MABS) and elastomers. Parts made by this method havea high stability since they are not hygroscopic [26].

A disadvantage is that the surface finish of the parts is inferior to that produced using SL dueto the resolution of the process which is dictated by the filament thickness [28]. It has not yetbeen demonstrated whether the material extrusion may be stopped quickly enough to producesmall holes in vertical sections [9].

A typical commercially available machine is a stand alone system measuring 660 914 1067 mm which weighs 160 kg and operates at about 80C. The build chamber in such a systemmeasures 254 254 254 mm. The system costs around 100 000, deposits approximately

380 mm of material a second, produces layer thicknesses of 50762 m and has an accuracy of 127 [27].

2.1.3.3. Three dimensional welding (3DW) This experimental system uses an arc-welding robotto deposit weld material on a platform as simple shapes which may then be built into morecomplex structures. Unlike most RP technologies, therefore, the prototypes are not built usingsliced CAD files. Parts with a resolution of a few millimeters have been made which may beused for sandcasting or directly as tooling.

Several problems still remain to be solved. Since there is no feedback, heat buildup duringmanufacture can cause the prototypes to melt and because the layers do not form a smooth surface

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the torch may hit the part [11,29]. It is also not known whether complex structures can be built.Some method needs to be found to generate the robot program directly from the CAD file. The

orientation of each section to be built should be generated as well as the order in which thesections are to be assembled.Another system which is being researched deposits the weld material in layers. Feedback control

is established by the use of thermocouples which monitor the temperature and operate an on-linewater cooling system. There is a grit blasting nozzle to minimise the oxidisation of the part anda suction pump and vacuum nozzle to remove excess water vapours and grit [18].

2.1.3.4. Shape deposition manufacturing (SDM) This still experimental layer-by-layer processinvolves spraying molten metal in near net shape onto a substrate, then removing unwantedmaterial via NC operations. Support material is added in the same way either before or after theprototype material depending on whether the layer contains undercut features (Fig. 12). The added

material bolsters subsequent layers. If the layer is complex, support material may need to be

Fig. 12. Shaped deposition manufacturing (adapted from [30]).

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added both before and after the prototype material. Each layer is then shot-peened to removeresidual stresses. The prototype is transferred from station to station using a robotised pallet

system which can position the workpiece to within an accuracy of

5

m. Droplets of 13 mmdiameter are deposited at a rate of 15 droplets per second.To date, stainless steel parts supported with copper have been produced. The copper may then

be removed by immersion in nitric acid. These prototypes have the same structure as cast orwelded parts with the accuracy of NC milling. Multiple materials may be employed and compo-nents can be embedded in the structure. As yet, no temperature control system for the substratehas been implemented, and the temperature, size and trajectory of the droplets are also not con-trolled [30].

2.2. Processes involving discrete particles

These processes build the part by joining powder grains together using either a laser or aseparate binding material.

2.2.1. Fusing of particles by laser

Selective Laser Sintering (SLS) is the main process in this category. With Gas Phase Deposition(GPD), the discrete grains are the result of the interaction between a reactive gas and a laser.However, the laser is also used to fix the grains with respect to the part.

2.2.1.1. Selective laser sintering (SLS) SLS uses a fine powder which is heated with a CO2laser of power in the range of 2550 W such that the surface tensions of the grains are overcomeand they fuse together. Before the powder is sintered, the entire bed is heated to just below themelting point of the material in order to minimize thermal distortion and facilitate fusion to theprevious layer [31]. Each layer is drawn on the powder bed using the laser to sinter the material.Then the bed is lowered and a powder-feed chamber raised. A new covering of powder is nextspread by a counter-rotating roller. The sintered material forms the part whilst the unsinteredpowder remains in place to support the structure and may be cleaned away and recycled oncethe build is complete (Fig. 13).

There is a large range of materials available for this process basically any material whichcan be pulverised may be employed. At present, nylon, nylon composites, sand, wax, metals andpolycarbonates are in use, and it is claimed that these materials have engineering grade properties

[32]. They are cheaper than the resins used for SL, are non-toxic and safe and may be sinteredwith relatively low-powered lasers. However, parts need a long cooling cycle on the machinebefore they can be removed. For example, wax parts require 12 h to cool down. The materialsemployed by the system are sensitive to the different heating and laser parameters and eachmaterial requires distinct settings. These can be difficult and time-consuming to obtain.

Parts may be finished by infiltration with molten metal to achieve 100% density. A drawbackis that the recycled powders require sieving to ensure that no globules are present that wouldinterfere with the smooth application of the next powder layer. The system also requires an inertnitrogen atmosphere in which to sinter the materials [32].

The least expensive machine which sinters thermoplastics costs around 250 000. The

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Fig. 13. Selective laser sintering (adapted from [2]).

maximum build chamber size is 330 380 425 mm. The layer thickness is 76 m with anaccuracy in the horizontal plane of 51 m. The build speed is 1225 mm h1 [32].

A similar system, still under development, involves feeding powder through a nozzle onto thepart bed whilst simultaneously fusing it with a laser. The powder nozzle may be on one side ofthe bed, or coaxial with the laser beam. If it is to the side, a constant orientation to the partcreation direction must be maintained to prevent solidified sections from shadowing areas to bebuilt. If the powder feeder is coaxial, there may be inaccuracies in the geometry of the part andthe layer thickness if the beam and the powder feeder move out of alignment.

The heating of the powder can lead to thermal distortion of the prototype. It is necessary to

cool the part when it becomes too hot in order to prevent distortions in the final piece. Analternative would be to add a temperature control system. The minimum wall thickness dependson the feed rate and the width of the particle stream and the laser spot size, speed and power.Walls of 0.50.7 mm have been achieved [31].

2.2.1.2. Gas phase deposition (GPD) In this process, the molecules of a reactive gas are decom-posed using either light or heat to leave a solid. The solid result of the decomposition then adheresto the substrate to form the part (Fig. 14). Three slightly different methods of constructing thepart are currently being researched.

In the first, called SALD (Selective Area Laser Deposition), the solid component of the decom-

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Fig. 14. Gas phase deposition.

posed gas is all that is used to form the part. It is possible to construct parts made from carbon,silicon, carbides and silicon nitrides in this way. The second method, SALDVI (Selective AreaLaser Deposition Vapour Infiltration), spreads a thin covering of powder for each layer. Then thedecomposed solids fill in the spaces between the grains. In the third method, SLRS (SelectiveLaser Reactive Sintering), the laser initiates a reaction between the gas and the layer of powderto form a solid part of silicon carbide or silicon nitride. A resolution of 1 m is hoped for [11,33].

2.2.2. Joining of particles with a binder

2.2.2.1. Three dimensional printing (3DP) Layers of powder are applied to a substrate thenselectively joined using a binder sprayed through a nozzle (Fig. 15). In order to avoid excessivedisturbance of the powder when it is hit by the binder, it is necessary to stabilise it first by mistingwith water droplets [34]. Once the part is completed, it is heated to set the binder then the excesspowder, which was supporting the part, is removed by immersion in a water bath [35]. The partis next subjected to a final firing at 900C for 2 h in order to sinter it [15]. It is possible to pressthe green part isostatically before this final firing to increase its density to over 99% of that of asolid part [36]. After firing, the part may be dipped in binder and refired so that its strength is

improved. Since there is no state change involved in this process, distortion is reduced [28].The resolution is dependent on the size of the binder droplets and the powder grains, the

placement accuracy of the nozzle and the way that the binder diffuses through the powder dueto capillary action. Neighbouring grains which have been wetted by a binder droplet are pulledtogether into a voxel of approximately spherical shape due to the surface tension. The entire voxelthen shrinks as it dries [34]. The layer thickness is affected by the compression of the powderdue to the weight of subsequent layers. This compression is most noticeable in the center of thepart. At the base, there is no room to compact the powder. At the top of the part, there are fewerlayers to cause the compaction. However this effect is mitigated when using more densely packedpowders [34].

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Fig. 15. Three dimensional printing.

Parts made using this process do not require supports to brace overhanging features. They dohowever need to include a hole so that excess powder can be removed [20]. Disadvantages ofthis technology are that the final parts may be fragile and porous, and it can be hard to removethe excess powder from any cavities. A further drawback is that the layers are raster-scanned bythe printhead which leads to a stair-stepping effect in the XY plane as well as in the builddirection [9].

The materials employed by 3DP are metal or ceramic powders, or metalceramic compositeswith colloidal silica or polymeric binders [20]. At present, this technology is available through a

service bureau only and is used to create cast metal parts. A 3DP machine has a build chambermeasuring 355 457 355 mm, a layer thickness of 177 m, a resolution of 508 m and anaccuracy of 127 m. The build speed is 1825 mm h1 [37].

A similar technology, known as Topographic Shape Formation (TSF) is used primarily forrapid production of moulds, which may then be used to create the prototype. The system printsparaffin wax about a centimeter below the surface of a silica powder. Once each layer has beencompleted, more powder is applied and the process is repeated. The wax binds the powder toform the part and also partially melts the previous layer to ensure good adhesion. Once the partis completed, it is sanded, coated in wax and then employed as a mould for the customers part.Materials in use include concrete, fibreglass and expanding foam.

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An advantage of this technology is that it can build very large parts quickly and cheaply, whichmay be expensive and time-consuming if constructed by other RP methods. A disadvantage is

that the moulds have a gritty surface finish and may need to be finished by an operator.At present, TSF is in use in a service bureau only. The machine has a maximum build envelopeof 3353 1829 1219 mm, a layer thickness of 12703810 m, resolution of 12 700 m andan accuracy of 1270 m [38].

2.2.2.2. Spatial forming (SF) This technology is being developed for prototyping specialisedmedical equipment with metal. It is designed to produce high precision parts within a small buildenvelope of 2 2 300 mm. A negative of each layer is printed onto a ceramic substrate witha ceramic pigmented organic ink. The layer is then cured with UV light and the process repeated.After approximately 30 layers, the positive space left by the printing, which corresponds to thepart cross section, is filled using another ink which contains metal particles. This is then cured

and milled flat. The process continues until the whole part is finished. Once the prototype iscomplete, it is heated in a nitrogen atmosphere to remove the binders in both the positive andnegative inks and to sinter the metal particles. The ceramic negative can then be removed inan ultrasonic bath to reveal the final piece, which is infiltrated with liquid metal to produce themetal prototype.

The sintering process causes shrinkage of up to 20% in all directions which needs to be takeninto account when designing the part. Further research includes optimizing the binder removalprocess and automating the addition of the positive material and the later milling [39].

A prototype of this system is currently being employed to construct preassembled microstruc-tures for medical purposes. To date, no commercial system is available and only extruded partswith a constant cross-section can be produced. In theory, however, completely arbitrary geometriesshould be feasible.

2.3. Technologies which use a solid

There are two different technologies which use solid foils to form the part. Laminated ObjectManufacture (LOM) bonds the different sheets with an adhesive and then cuts the part contourusing a laser. The second, Solid Foil Polymerisation (SFP), bonds sheets of foil by curing themwith UV light.

2.3.1. Sheets bonded with adhesive: laminated object manufacture (LOM)

The build material is applied to the part from a roll, then bonded to the previous layers usinga hot roller which activates a heat-sensitive adhesive. The contour of each layer is cut with alaser that is carefully modulated to penetrate to a depth of exactly one layer thickness. Unwantedmaterial is trimmed into rectangles to facilitate its later removal, but remains in place during thebuild to act as supports (Fig. 16). The sheet of material used is wider than the build area so that,once the part cross-section has been cut, the edges of the sheet remain intact. This means that,after the layer has been completed and the build platform lowered, the roll of material can beadvanced by winding this excess onto a second roller until a fresh area of the sheet lies over thepart. The whole process can then be repeated.

The system employs a 25 or 50 W CO2 laser to cut the material. Smaller hatches must be used

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Fig. 16. Laminated object manufacture (LOM1).

on up- and down-facing surfaces to facilitate the removal of waste material which has bonded tothe part. It may also be necessary to stop the build to excavate paper from otherwise hard-to-access places. Once the parts have been completed, they should be sealed with a urethane, silicon

or epoxy spray if made of paper to prevent later distortion of the prototype due to water absorption.The height is measured and the cross-sections are calculated in real time to correct for any errorsin the build direction [9].

Advantages of LOM include the wide range of relatively cheap materials available partsmay be made using paper for example, or from more expensive materials such as plastic or fiberreinforced glass ceramic. The parts may be quite large compared to those produced by other RPmethods. Since they have the appearance of wooden pieces when finished, they are popular withmodel makers. Speed is another strong point of LOM. As only the outlines of the parts need tobe traced, this method is about 510 times faster than other processes [40].

A drawback is the need to prise the finished parts off the table which adversely affects their

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surface finish. It is also hard to make hollow parts due to the difficulty in removing the core andthere are serious problems with undercuts and re-entrant features. Other disadvantages of this

technology are that there is a large amount of scrap, the machine must be constantly manned,parts need to be hand finished and the shear strength of the part is adversely affected by thelayering of adhesive and foil [1,41]. Because the laser cuts through the material, there is a firehazard which means that the machines need to be fitted with inert gas extinguishers. The dropsof molten material (dross) which form during the cutting process also need to be removed [2].

The cost of a LOM machine is between 120 000 and 235 000 depending on the size of thebuild chamber. Available machines have a maximum build chamber of 813 559 508 mm.The minimum layer thickness that they can handle is 76203 m and their maximum accuracyis 127 m. The maximum cutting speed achievable is 508 mm s1 [40].

A similar process, LOM2, includes the ability to bond the sheets selectively to the part cross-section. Here, the cross-section of the part is printed onto a sheet of paper which is applied to

the work-in-progress and bonded using a hot roller. A knife is then used to cut the outline of thepart and cross-hatch the waste material. This process is repeated until the part is finished, whenthe excess material may be peeled away from the model. This can then be sealed with epoxy.

Since a knife is used to cut the paper, this system should be less hazardous and cheaper thanLOM1. The waste material is also easier to remove and so finer features may be built. A LOM2machine costing approximately 130 000 has a build chamber of 400 280 300 mm. Thesystem has a throughput of 1 sheet per minute. The parts have a layer thickness of 100 m, XY resolution of 25 m and an accuracy of 200 m [42].

Another development which yields a low-cost machine involves using layers cut from adhesivematerial on backing paper or from foam laminating material. These layers are then assembled byhand using special positioning marks and the backing is removed. Once the prototype is com-pleted, it may be coated to protect and strengthen it. This RP technology (LOM3) is perhaps oneof the most inexpensive available, with machines costing approximately 8500 [43], although thefinished parts are somewhat tacky and the assembly process has to be performed manually.

2.3.2. Sheets bonded with UV light: solid foil polymerisation (SFP)

In SFP, the part is built up using semi-polymerised foils. On exposure to UV light, the foilsolidifies and bonds to the previous layer. It also becomes insoluble. Once the cross-section hasbeen illuminated, a new foil can be applied. The areas of foil which do not constitute the eventualpart are used to support it during the build process, but remain soluble and so are easy to remove.Once the part is complete, the non-bonded pieces can be dissolved to leave the finished part

[2,44]. No commercial systems are available yet.

2.4. Material removal technology: desktop milling (DM)

This is a process which removes material from the workpiece as in traditional machining pro-cesses instead of creating the part by gradual material buildup. The prototypes can be made witha high degree of accuracy because they do not deform after they have been completed. If NCmachining is to be employed to manufacture the finished design, features which are difficult tocreate will also be detected at this stage.

Any CNC machine may be employed to make prototypes from an inexpensive material such

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as wax. There is a commercial entry-level desktop milling system available which is capable ofdealing with STL files, even those which contain gaps and self-intersecting surfaces. This means

that the designer does not need to spend time verifying and correcting the files, as is the casewith the material accretion technologies. The machine is inexpensive and can handle a widevariety of materials. It generates the NC tool path automatically and may be operated with noNC training. The cost of the basic model is approximately 4500. Cutting speeds of 0.063.6 m s1

can be obtained depending on the model purchased. The resolution can be as high as 10 m foran inexpensive entry level machine, with an accuracy of 10 m [45,46].

3. Applications

There are many uses for RP. Unlike conventional prototypes which may take a skilled artisanweeks or months to produce, RP parts may be made cheaply by a machine in a few days or less,with little human intervention. Therefore the designer may prototype the part as often as necessaryto check for appearance and function. Changes may then be easily incorporated into the modeland another prototype generated. This facilitates the optimisation of the design and saves time-consuming and expensive alterations at a later production date. There are many other applicationsfor the prototyped parts which would have been impractical with conventional models. Some ofthese applications are listed below.

3.1. Visualisation

Parts may be employed to facilitate communication of ideas in a concurrent engineeringenvironment.

Some companies now routinely include a prototype made from the CAD file with their salesproposal to allow the customer to see and assess the part [27].

Complex models may be produced for teaching purposes [47].

3.2. Working models/functional parts

Small batches of plastic parts can be commercially manufactured. Because patterns for injectionmoulding are expensive to produce, the break-even point for a production run is a few thousandparts [2]. RP technologies can be used on their own or in conjunction with other more conven-tional technologies to manufacture parts in quantities as low as one.

Parts may be produced with intricate internal shapes that could not be manufactured usingtraditional technologies. Examples include medical equipment such as the interlocking tipassembly for a catheter system to investigate arteries [39] and monolithic ceramic filters [48].

One-of-a-kind parts such as bone replacements may be made accurately from a scanned modelof the original. The bone may be imaged using X-ray tomography and the data translated toa CAD file which is then used to drive the RP process [47].

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Parts can be produced with well-defined microstructures by using technologies which candeposit different materials [47].

3.3. Tooling/manufacturing pattern

It is possible to employ RP parts directly as tooling. SLS or ceramic 3DP parts may be infil-trated with liquid metal to produce a dense tool with a well-defined distribution of ceramic ormetal particles [19,37]. RP models may be sprayed with metal to produce EDM electrodeswhich may be used to manufacture up to 1000 parts [9,11]. TSF parts may be used as mouldsfor concrete, fibreglass or expanding foams [38].

Parts made by RP may be used to produce tools indirectly. Tooling lead-times may be reduced

from 1226 weeks to 16 weeks. Parts made of wax or other low melting point materials maybe sprayed with metal and the wax subsequently removed by melting. The metal shells maythen be employed for plastic injection moulding [20].

Parts made with a low-melting point material may be used for investment casting purposes.The parts are coated with a ceramic slurry and then burnt out. As mentioned previously, SLparts should be built using a draw-style such as QuickCast to avoid cracking the ceramicmoulds. The FDM, BPM and SLS investment casting waxes burn out, leaving little to no ashcontent ( 0.002%), and therefore are ideal for investment casting. LOM parts made of papermay be burnt out at 760C leaving approximately 3% in ash [40]. When adopting 3DP, theceramic moulds may be made directly, which has the effect of tightening tolerances as there

are fewer shape transfers. It is also possible to produce moulds with integral cores. This meansthat they do not have to be manually located and again tolerances are tightened. Another possi-bility is to print the cores in a different material so that they are easy to remove at a later date[20]. An advantage of these RP technologies is that the expensive conventional tooling usedto produce the mould which makes the sacrificial wax patterns is not needed to create theprototype, allowing multiple trials before the design is finalised [1,49].

SL, SLS and LOM prototypes may be used in the sand casting process for short runs of castparts [1].

4. Selection of RP processes

Tables 1 and 2 contrast the main features of the different RP systems. The technologies aresplit into those which are commercially available and those which are still being researched. Thereare alternative systems listed under each of the categories of BPM and LOM and data for thesealternatives have been included in the table. LOM1 is the fully automatic LOM process, employinga laser, LOM2 is the selective bonding process which uses a knife and LOM3 is the manualassembly process. As described previously, the BPM processes are the dual-jet BPM1, the 5-axis BPM2 machine and the multi-jet MJM. The figures for DM refer to the entry level systemmentioned earlier.

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Table 1

Features of rapid prototyping processes (commercial)

SL SGC BPM1 BPM2 MJM FDM SLS 3DP TSF LOM1 LOM

Postcuring Yes No No No No (firing Yes No No

required may be

required)

Supports Yes No Yes No Yes Yes No No No No required

Material Epoxy or Resin Thermo- Thermo- Thermo- ABS, Nylon, Ceramic Sand and Paper, Pape

used acrylic plastic or polymer polymer MABS, metals, or metal wax plastic or

resin wax wax or wax, or ceramic

elastomers poly

carbonate

Laser Yes No No No Yes No No Yes No

used

Layer 50 100200 13130 Not 33 50762 76 177 1270 76203 100

thickness available 3810

(m)

XY 200250 100 101 558 85 254 Not 508 12 700 203254 25

Resolution available

(m)Accuracy 100 500 25 17 100 127 51 127 Not 127 20

(m) available

Scan N/A N/A 310 12 000 6200 380 0.001 0.007 Not 508 N/A

speed particles 0.008 available (cutting

(mm s1) per speed)

second

Time to 113 (50 65 N/A N/A N/A N/A N/A N/A Not N/A 60

complete 50 mm) available

a layer (s)

Maximum 500 500 300 250 250 254 330 355 3353 813 559 400

part 500 350 150 203 200 254 380 457 1829 508 30

dimensions584 500 220 150 200 254 425 355 1219

(mm3)

Cost 150390 180300 60 25 50 100 250365 Bureau Bureau 120235 130

(1000) service service

only only

1Since prototypes made with the LOM3 system are assembled manually there is no height constraint.2These figures refer to an entry-level system only.

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Table 2

Features of rapid prototyping processes (non-commercial)

LTP BIS HIS 3DW SDM GPD SF SF

Postcuring Yes Yes No No No Yes No

required

Supports Yes No No Yes Yes No No No

required

Material used Resin Resin Resin Weld beads Metal Reactive gas Metal Re

Laser used Yes Yes Yes No No Yes No Ye

Layer 100 N/A 1450 0.5

thickness

(m)

XY 100 300600 10

Resolution

(m

Accuracy 500 25

(m)

Scan speed 8(mm s 1)

Time to

complete a

layer (s)

Maximum 300 300 300 300 2

part 300 300

dimensions

(mm3)

Cost (1000)

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The accuracy data in the tables was obtained from technical publications and from companyliterature. In the main, these represent the best accuracies achievable with finely tuned equipment

when operated by a skilled technician and not the average accuracies and resolutions achievedby the users. The layer thickness shown in the table is taken to be equivalent to the z resolutionof the part. Depending on whether the part is built point by point or layer by layer, either thelinear build rate is quoted, or the time to complete a layer is given.

Of the systems listed, the most accurate is the dual-jet BPM1 machine. However, the buildchamber and therefore the maximum part size is small. The cheapest systems are the LOM3machine and the entry-level DM system. As already mentioned, the drawback of the LOM3 systemis that the parts produced are tacky and need to be assembled manually. Disadvantages of thelow-cost DM machine are that its work envelope is small and it cannot manufacture shapes ascomplex as those created using the material accretion technologies.

There was less information available for the non-commercial processes and for some techno-

logies no accurate figures could be obtained.Figure 17 is a quick guide to selecting RP processes. The selection is based on the end use of

the part, part size, whether or not all features may be freely accessed, whether or not the part ishollow, part accuracy and part strength. For completeness, approximate capital and running costinformation is provided on each process and this is then used to rank the different alternatives.Only commercially available processes are represented.

5. Conclusion

Rapid prototyping is an enabling technology for concurrent engineering. Its goal is to reduceproduct development and manufacturing costs and lead times, thereby increasing competitiveness.Impressive steps towards that goal have been made. However, the field of RP is still new, withmuch effort to be expended on improving the speed, accuracy and reliability of RP systems andwiden the range of materials for prototype construction. Another area of improvement will becosting, as most RP systems are currently too expensive to be affordable by any but the largerfirms. Although RP technology will continue to be available to all companies via bureaux which,often in partnership with traditional model makers, can provide a comprehensive service fromdesign through to short-run production, the future is likely to see more user-owned RP machinesas their costs are reduced. There will also be two different types of RP systems for two distinctmarkets: the design-office 3D-plotter for rapidly generating parts for design verification and theworkshop/model-making shop machine for producing accurate functional parts and tooling.

Acknowledgement

This work was supported by the European Regional Development Fund which is administeredby the Welsh Office for the European Commission.

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Fig. 17. RP process selection guide.

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Fig. 17. Continued

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