An Overview Of Rapid Prototyping Technologies InManufacturing

Dr. A. DolencInstitute of Industrial AutomationHelsinki University of Technology

July 24, 1994

Abstract

This document overviews a new class of manufacturing processes generally known asRapid Prototyping Techniques or Technologies (RPT) that build parts by adding material ona layer-by-layer basis, in contrast to conventional methods that remove material. We discusstheir basic principles, data transfer, applications, and compare them with their conventionalcounterparts.

Otakaari 1, FIN-02150 Espoo, Finland. Tel:+358-0-4513239. Fax:+358-0-4513293. Email (Internet):ado@mail.cs.hut.fi.

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Contents1 About This Document 5

2 What Is Rapid Prototyping? 5

3 Overview Of Some Processes 63.1 Stereolithography : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 63.2 Solid ground curing : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 83.3 Selective laser sintering : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 93.4 Laminated object manufacturing : : : : : : : : : : : : : : : : : : : : : : : : : 103.5 A short comparison : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 11

4 Data transfer to RPT 124.1 Constraints on the model : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 14

5 RPT In Manufacturing 165.1 Toolings : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 18

6 RPT In Industrial Design 18

7 RPT In Medical Applications 18

8 RPT vs conventional technologies 20

9 Conclusions 20

10 Acknowledgements 21

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List of Figures1 A schematic drawing of an SLA. : : : : : : : : : : : : : : : : : : : : : : : : : 72 A schematic drawing of a SOLIDER process. : : : : : : : : : : : : : : : : : : 83 A schematic drawing of an SLS process. : : : : : : : : : : : : : : : : : : : : : 94 A schematic drawing of a LOM process. : : : : : : : : : : : : : : : : : : : : : 105 A scenario between designer and manufacturer. : : : : : : : : : : : : : : : : : 126 The state transitions of a parametric surface model. : : : : : : : : : : : : : : : 137 The typical scenario of data preparation. : : : : : : : : : : : : : : : : : : : : : 138 A correct triangulation : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 149 Incorrect triangulations. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 1510 Correct vs. incorrect slices. : : : : : : : : : : : : : : : : : : : : : : : : : : : 1511 Changes in the requirements for the manufacturing industry. : : : : : : : : : : 1612 Development time vs. development costs. : : : : : : : : : : : : : : : : : : : : 1713 Obtaining medical models from scanned images. : : : : : : : : : : : : : : : : 1914 RPT vs conventional technologies. : : : : : : : : : : : : : : : : : : : : : : : : 20

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List of Tables1 Other Stereolithography-based processes. : : : : : : : : : : : : : : : : : : : : 72 A short comparison of some RP processes. : : : : : : : : : : : : : : : : : : : 11

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1 About This DocumentThis document has two versions, a printed version and an electronic version. The printed versionis obtained using LATEX, and converted to PostScript using dvips. The compressed PostScriptfile can be obtained via anonymous ftp at sauna.cs.hut.fi (130.233.192.1) in/pub/rp-ml/rp.ps.Z. The electronic version is obtained using latex2html1. It mayhappen that both versions are not updated simultaneously.

Observe that some differences are bound to exist between both versions. Take, for instance,a link to another document. Whereas in the electronic version it suffices to have only the link, inthe printed version, it is replaced by a description of the contents of the other document.

2 What Is Rapid Prototyping?The past decade has witnessed the emergence of new manufacturing technologies that build partson a layer-by-layer basis. Using these technologies, manufacturing time for parts of virtually anycomplexity is measured in hours instead of days, weeks, or months; in other words, it is rapid.

The first commercial process was presented at the AUTOFACT show in Detroit (US) inNovember 1987, by a company called 3D Systems, Inc. At that time, the process was veryinaccurate and the choice of materials was limited. Therefore, the parts obtained where consideredprototypes. Like in software engineering, a prototype is something to look at, serves as a basisfor discussion but cannot be used for anything serious, i.e. in a production environment.

Since then, Rapid Prototyping Technologies (RPT) have taken enormous strides. Nowadays,there are over 30 processes some of which are commercial, while others are under developmentin research laboratories2. The accuracy has improved significantly, and the choice of materialsis relatively large, to the extent that the term prototype is becoming misleading; the parts aremore and more frequently being used for functional testing or to derive tools for pre-productiontesting. It is very likely that a new term, or one of the numerous other expressions that are floatingaround, will replace it in the future3.

It is true that rapid prototyping (notice the lowercase) can be achieved using conventionalmethods such as NC milling and hand carving. However, the term RP is normally reserved forthe new technologies that build parts by adding material instead of removing it. In order toregard RP in the right perspective, one would need to compare it with the conventional methods.Unfortunately, this is beyond the scope of the present work. I will explain, though, to the best ofmy ability, the strategic importance of RP and describe in some detail some of the processes thatwill be referenced later.

1http://cbl.leeds.ac.uk/nikos/tex2html/latex2html.tar2A survey dated May 1993 by Wohlers Associates includes 34 processes out of which 11 are commercial with

approximately 525 machines sold worldwide.3Personally, I favour the expression Rapid Prototyping&Manufacturing (RPM).

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3 Overview Of Some ProcessesAll the processes described in this Section take as input a 3D model and a set of parameters thatare process-dependent. The model to be manufactured is sliced by a set of parallel planes. Thespace between two adjacent slices is called a layer. The component of the process where the partis built is called the workspace.

Although the processes described here can differ significantly, e.g. by the use of materialsother than photopolymers, the underlying theme is the same; they all build parts on a layer-by-layer basis. Such processes are generally known as Layered Manufacturing Techniquesor Technologies (LMT). These technologies are changing at a quick pace, and the informationcontained herein may become quickly outdated. For our purposes, it is not important that they bedescribed in great detail. More information can be found in other sources [13, 14, 16, 21, 24, ?].

3.1 StereolithographyOur first example of RPT is the Stereolithography apparatus (SLA) (Figure 1), developed andcommercialized by 3D Systems, Inc. (US). A short explanation on how the process operates isas follows. Initially, the elevator is located at a distance from the surface of the liquid equal tothe thickness of the first, bottom-most layer. The laser beam will scan the surface following thecontours of the slice. The interior of the contour is then hatched using a hatch pattern. Theliquid is a photopolymer that when exposed to the ultra-violet (uv) laser beam solidifies or iscured. The elevator is moved downwards, and the subsequent layers are produced analogously.Fortunately, the layers bind to each other. Finally, the part is removed from the vat, and the liquidthat is still trapped in the interior is usually cured in a special oven.

The laser beam that solidifies the liquid is the HeCd-laser shown in the upper-left corner ofFigure 1. A second, HeNe-laser is used to ensure that the surface of the liquid is in the correctlocation. The sweeper4 breaks the surface tension, ensures that a flat surface is obtained, andminimizes the processing time of each layer.

Because the part is built in a liquid environment, and because the interior of the part stillcontains liquid polymer, it may be necessary to add support structures to increase the rigidityof the part, and to avoid overhangs from sinking to the bottom of the platform or from floatingfreely in the vat. The support structures are usually removed manually after the part is takenaway from the platform.

Scanning time depends on the geometry of the contours, hatch patterns, the speed of the laser,and the recoating time (i.e. the time taken to place a layer of photopolymer over the last solidifiedlayer).

The SLA is not the only process based on Stereolithography. Table 1 lists other organizationsthat commercialize processes based on the same principles.

4Not present in the low-end model of the SLA-family.

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Liquid polymer

HeCd-laser

Lenses

Mirror

Platform

Elevator

HeNe-laser

Sweeper

FIGURE 1. A schematic drawing of an SLA.

Organization Country ProductCMET (Mitsubishi) Japan SOUP 600, 850D-MEC (JSR/Sony) Japan SCS 1000HDLaser 3D France SPL 1000, 5000EOS GmbH Germany STEREOS 400, 600Teijin Seiki Co. Japan Soliform 300, 500TABLE 1. Other Stereolithography-based processes.

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3.2 Solid ground curingThe SOLIDER system was developed and commercialized by Cubital Ltd. (Israel). It also uses aphotopolymer, sensitive to uv-light. It is, however, a significantly different process (see Figure 2).

Mask plate

Electricalcharging

UV-lamp +shutter

Polymerspreader

Residual polymercleaner

Waxspreader

Wax coolingplate

Milling head

Liquid polymer(current layer)

Mask developmentMask erasure

PlatformWax

FIGURE 2. A schematic drawing of a SOLIDER process.

The first difference concerns the vat: it moves horizontally as well as vertically. The horizontalmovements take the workspace to different stations in the machine.

The second difference concerns the light source: instead of using a laser beam, a uv-lamp(mercury) is used to flood the chamber and expose and solidify the entire layer at once. Thisavoids the need for post-curing the parts. To select the areas that should be cured, a mask is builton a glass plate, and subsequently, erased after begin used. The mask is built using a processsimilar to the one used in laser printers. The glass plate with the mask is placed between the lampand the surface of the workspace.

The third difference is that the parts are built surrounded by wax, eliminating the need forsupport structures5. Once a layer has been exposed to the uv-lamp, the un-cured areasthoseareas filled with residual, liquid polymerare replaced by wax. This is done by wiping away theresidual polymer and applying a layer of wax. The wax is hardened by a cold metal plate, and

5On the other hand, one must de-wax the part. This can be done even with a dish washer, if the geometry of thepart permits.

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subsequently, the layer is milled to the correct height. The milling station also allows for layersto be removed, i.e. an undo operation is possible. The new layer of polymer is applied when theworkspace moves from the milling station back to the exposure chamber.

The latest improvements announced by Cubital are the ability to change the size of theworkspace and an additional uv-lamp.

3.3 Selective laser sinteringThe University of Texas at Austin developed a method for sintering powder materials. Theprocess is depicted in Figure 3. Instead of a liquid polymer, powders of different materials are

Workpiece

CO2 Laser

Powder Leveling Roller

OpticsScanningMirrors

UnsinteredPowder

Part Cylinderand Powder Bed

Powder Cartridge Feedding/Collecting System

FIGURE 3. A schematic drawing of an SLS process.

spread over a platform by a roller. A laser sinters selected areas causing the particles to melt andthen solidify. Unlike the processes mentioned above where there is only one phase transition, insintering there are two: from solid to fluid, back to solid again. Processes that behave in this way

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are generally known as selective laser sintering (SLS) processes. The materials being used orinvestigated include plastics, wax, metals, and coated ceramics.

It is hoped that parts made of materials other than plastics with the required mechanicalproperties can be made using such processes. Like the SOLIDER system, there is no need forsupport structures, because the surrounding powder supports the parts being built.

The process developed at Austin is being commercialized by DTM Corp. Recently, EOS GmbHhas introduced to the market a process that operates under the same principles.

3.4 Laminated object manufacturingHelysis developed and commercialized a system that cuts and binds foils as illustrated in Figure 4.The undersurface of the foil has a binder that when pressed and heated by the roller causes it to

RaLaser

Optic head

Mirror

FeederCollector

Platform

Heated Roller

FIGURE 4. A schematic drawing of a LOM process.

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glue to the previous foil. The foil is cut by a laser following the contour of the slice. To help theremoval of the excess material once the parts have been built, the exterior of the slice is hatched,as opposed to fluid-based processes (e.g. the SLA process), where the interior is hatched.

The thickness of the foil is not constant. Therefore, a sensor (not shown in the Figure)measures the current foil thickness, and the model is sliced accordingly.

3.5 A short comparisonFrom the users point-of-view, the major aspects taken into consideration in chosing when andhow to obtain a part are: time, cost, and functionality. Regarding RPT, none of the processes excelin all respects; each process has restrictions imposed by costs, accuracy, materials, geometry,and size. Table 26 is a short summary of the differences between the processes discussed in theprevious Sections. The comparison is not complete, in that various other important aspects arenot included, e.g. the price of the equipment, maintenance costs, and material costs.

Process SLA 250 SOLIDER 5600 SLS 2000 LOM 1015Company 3D Systems, Inc. Cubital Ltd. DTM Corp. HelisysMax. part size(mm) 254 254 254 508 508 355

305 381( height) 330 2540 381

Layer thickness(min/max; mm) 0:10:9 0:050:15 0:13 0:0050:05Speed (verti-cal)

Part geometrydependent 60100 layers/hour

Part geometrydependent 10 mm/hour

Accuracy 0:2mm :1% (all directions) 0:050:25mm 0:127mm

Materials Photocurableresins

Photocurableresins, wax

Thermoplastics(PVC, nylon,ABS/SAN),wax

Paper, nylon,polyester

TABLE 2. A short comparison of some RP processes.

When the part does not fit in the workspace of the machine, acceptable results have beenobtained by splitting the model in parts, building the parts separately, and then binding themtogether.

Regarding the software tools and data exchange formats, the lowest common denominatoris triangulated models represented in STL format. All vendors supply software tools to verify,correct, and slice the models. However, the software architecture and the quality of the toolsvaries considerably. Data transfer is now covered in the next Section.

6This Table was compiled from an internal report of the INSTANTCAM project [3].

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4 Data transfer to RPTAs mentioned earlier, speed is one of the most distinguishing features of RPT when comparedto conventional methods. In fact, in many cases, the use of RPT can only be justified if thepart can be obtained quickly. Quite often, though, the limiting factor is the time spent preparingthe data. Once the data is correct, manufacturing time is known and relatively fast. Figure 5sketches a typical scenario. The designer delivers the model to the manufacturer using surface

or the PartError Description

3D Model

ManufacturerDesigner

FIGURE 5. A scenario between designer and manufacturer.

mail or by electronic means. The model will usually be represented in some neutral format, e.g.VDAFS [27], IGES [25], or STL [1], or in some native format when both have access to thesame CAD system. The model is then verified for correctness and converted to a suitable formif possible. The manufacturer is faced with the following problems:

Is the model correct?

If not, what is the nature of the mistakes and can they be corrected locally?

If the mistakes cannot be corrected locally, how can one describe them to the designer?

RP machines are not yet commonplace and the physical distance between the designer and themanufacturer plays a role in delivery time due to difficulties and delays in communication.Besides, the manufacturing costs are directly related to the amount of work spent preparing thedata and the actual building of the part. The former can represent as much as 2=3 of the totalcosts. Therefore, any software tool that can minimize the number of times the designer and themanufacturer need to communicate or make their communication more efficient is beneficial.

In a nutshell, these are problems related to data transfer. A closer look at how parametricsurface models7 are transferred will give us a better understanding of the problems.

Figure 6 depicts the state transitions of interest undertaken by a model from the moment it issent by the designer until it is manufactured. All the state transitions in the diagram are possible.For instance, one can take slices from medical imaging systems and interpolate intermediaryslices that are subsequently used in a LMT process. In this case, a 3D model is never evaluated

7Henceforth referred to as models.

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ca

Facetted Model in Neutral Format

Surface Model in Neutral Format

of Application Programs (e.g. STL)

(e.g. IGES or VDAFS)

Model in Internal,Native Format

+b-

+-

+- Sliced Model

LMT Process

FIGURE 6. The state transitions of a parametric surface model.

(path b+b). On the other hand, some processes cannot effectively handle sliced modelsor213 D modelstherefore, a faceted model is created and then sliced again (path b+aa+b).The reason is that in some cases it is important to be able to position the model arbitrarily in theworkspace of the machine, and this cannot be done with sliced models.

The typical scenario is shown in Figure 7. A 3D CAD systema surface or solid modeller

a -

a +a -

a +a -

a +

b -

CADSystem

Verifier/Corrector

Processdependent

Slicer LMTProcess

FIGURE 7. The typical scenario of data preparation.

is used for creating the model. The most common step that follows is facetting the model.The current de facto data exchange standard for representing faceted models is called STL [1].This format requires significant redundancy and is restricted to triangles. Normally, each vendorsupplies the software tools for verifying the correctness of the model, generate process-dependentdata, and to perform the slicing of faceted models. The format for representing the slices isproprietary. There is a trend for LMT vendors to develop interfaces based on slicing surface

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models together with major CAD vendors. One of the objectives is to eliminate the need foralways generating intermediary, faceted models. Therefore, the interest in correctly slicingmodels for LMT is growing.

4.1 Constraints on the modelAs mentioned above, data transfer between CAD systems and RP processes is mainly based ondata exchange formats capable of representing faceted models. The current de facto standard isthe STL format [1] which allows one to represent triangulated models, i.e. each facet is a triangle.

FIGURE 8. A correct triangulation

In order for models to be correctly manufactured they must represent a collection of one ormore non-intersecting solids. The manufacturer hopes to receive well-behaved STL-files suchas the one outlined in Figure 8. In a correct STL-file, each triangle has exactly one neighbouralong each edge, and triangles are only allowed to intersect at common edges and vertices. Underthese conditions, it is possible to distinguish precisely the inside from the outside of the model.

Unfortunately, quite often incorrect faceted models are used. The mistakes can be numerous(Figure 9). The models can contain gaps due to missing facets, facets may intersect at incorrectlocations, the same edge may be shared by more than two facets, etc. Special cases of theseerrors may occur that require separate treatment, e.g. overlapping facets (coplanar facets whoseintersection results in another facet). The reasons for such errors are related to the applicationthat generated the faceted model, the application that generated the original 3D CAD model, andthe user. Many STL interfaces in CAD systems fail to inform the user that the result is not correctand problems remain undetected until the manufacturer attempts to process the model.

Errors in the model can interfere with the building process. For instance, if a slice containsa gap when the internal structure of the slice is built, stray vectors might be created (Figure 10).The possibility of this happening is great, due to the fact, that the tool in this case is a laser

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Gap

FIGURE 9. Incorrect triangulations.

beam of small diameter (approximately 0:2mm in an SLA), and the distance between the hatchlines may be likewise apart8. These stray vectors damage the resulting part and possibly otherparts being built in the same platform. The internal structure is process-dependent and is usually

Stray vectorsContour of the slice

Hatch pattern

FIGURE 10. Correct vs. incorrect slices.

proprietary information. Therefore, the internal structures used in practice will probably differfrom the ones shown in Figure 10, but they all require simple, non-intersecting contours to besuccessfully created. These, in turn, can only guaranteed to be obtained if the original model iscorrect, i.e. a solid.

8Not all processes use a laser beam but similar problems may occur with other processes.

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5 RPT In ManufacturingRPT can be useful to anyone who manufactures a product or needs a physical object. To illustratethe strategic importance of RPT, we will use, as an example, the manufacturing industries.Figure 11 [26] illustrates how the requirements for the manufacturing industries have changedover the past three decades. One partner in the INSTANTCAM project markets appliances in

1970 1980 1990

1970 1980 1990

Number of Variants

1970 1980 1990

Product Complexity

Product Lifetime

Required Delivery Time

1970 1980 1990

FIGURE 11. Changes in the requirements for the manufacturing industry.

10 countries. The same product family may have 6 different motors and 4 different technicalfeatures. The different technical features can be simple such as different materials, plugs, orcolors, or complex such as differences in the internal housing. These differences are needed inorder to attend to specific needs of users or to differenciate oneself from the competition. Inaddition, product lifetimes are becoming shorter, forcing a design group to develop new productswhithin a shorter time.

During the development process, one is frequently faced with the choice of either extendingthe development time or increasing the resources in order to meet the deadlines. Under thesecircumstances, time to market has been identified as a key factor in profitability; it is thedevelopment time and not the cost that is critical for the results (Figure 12 [18]).

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DEVIATION

DECREASEDPROFITS

Product Lifetime: 5 Years

by about 6 monthsExtending development time Increasing development costs

by about 50%

30%

5%

FIGURE 12. Development time vs. development costs.

This scenario requires changes on how a product is developed. Different groupsdesign,engineering, marketing, productionmust cooperate more closely towards a common goal andwork concurrently. The goal must be clear to everyone involved, and if cooperation is to beeffective, it is essential to avoid communication problems. RPT allows a physical model to beavailable as soon as a 3D CAD model is ready. The physical model is a perfect communicationtool; if a picture is worth a thousand words, then a physical model is worth a thousand pictures.

In addition, parts produced via RPT are more and more frequently being used for functionaltests and for obtaining tools that can be used for pre-series production tests. In this way, errorscan be found at an earlier stage when changes are not so costly. Requirements can be refined andbetter understood leading to better products that meet the market demands. It has been estimatedthat using RPT effectively, the development time for toolings can be reduced by half.

Another important aspect is the cost of introducing changes in the design of a product. In thisrespect, development of a physical product does not differ from software development: the costof introducing changes increases significantly as one reaches the final stages of development.RPT can be an effective means for evaluating a design before costly committments are made,commitments that affect manufacturing costs and, ultimately, the final cost of the product. Again,the analogy holds: prototype software is developed for the same reason!

However, Rapid Prototyping cannot be used effectively by product developers that do not usea 3D CAD system to create a model of the product.

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5.1 ToolingsThe ideal situation is the ability to build any part with any material. Clearly, this is not yet possible,but there is relief in sight. Soligens DSPC process is capable of producing ceramic shells, andis now in beta testing [21, 29]. Two Institutes of the Fraunhofer Gesellschaft in Germany,the Institute for Manufacturing Engineering and Automation (IPA, Stuttgart, Germany) and theInstitute for Applied Material Research (IFAM, Bremen, Germany), are developing a processthey call Multiphase Jet Solidification which can build plastic, metallic, and ceramic parts [12].Another exciting project is being carried out at the Carnegie-Mellon University (USA) where aprocess capable of building composite parts is being developed [11].

The properties offered by RPT part are sufficiently good nowadays to enable the productionof prototype toolings by using a process chain. A well-established method is vaccum casting.From the RPT model, one obtains a silicon mould from which approximately 20 parts can bemade in Epoxy or investment casting wax. Various process chains have been reported in theliterature [19, 15]. Each one has limitations concerning the geometry of the part, precision,number of parts that can be manufactured, and materials that can be obtained in the final part. Inaddition, some process chains, such as QuickCast, are restricted to a certain RP process.

6 RPT In Industrial DesignWhen comparing industrial design applications to the manufacturing of toolings, the role ofdimensional accuracy is not as significant as the quality of the surface. It should be clear to thereader by now that parts made via LMT exhibit a staircase effect. This effect can be minimizedby choosing a suitable building direction, but rather often this is done at the expense of buildingtime and costs.

The staircase effect can be addressed in several ways. Firstly, good software tools can helpminimize the problem [5]. Secondly, post-treatment can be applied, and in this case, the part isusually polished. Finally, the processes can be improved to virtually eliminate the problem. Forinstance, the technology developed by Laser 3D [2] can use a layer thickness as low as 0:015mmresulting in parts with no noticable staircase effect to the naked eye. Soligen claims that theirprocess can also eliminate the staircases using different principles.

7 RPT In Medical ApplicationsApplying RPT in the medicine is a new and exciting field. Many applications have become pos-sible due to the convergence of three distinct technologies, namely Medical Imaging, ComputerGraphics and CAD, and RPT.

Computer-Assisted Tomography (CT) and Magnetic Resonance Imaging (MRI) provide highresolution images of internal structures of the human body, e.g. bone structures and organs. Once

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these images have been processed by suitable software tools, it is possible to transfer the result toa RP process and obtain a physical part, called a medical model. Figure 13 depicts this process.

ImageProcessingToolbox

EdgeDetection

Contours

RP-specificDataGeneration

Contours,Internal vectors,Support structures

RPProcess

PhysicalPart

Images (pixmaps)

FIGURE 13. Obtaining medical models from scanned images.

Together, these technologies provide doctors and surgeons with a new toolphysical modelsof human internal structuresto better plan and prepare complex surgeries. If the surgeries canbe carried out more successfully, less costs associated to post-operative treatment are expected,in addition to reduced risks, reduced patient suffering, and improvements in the quality of theresults.

Another recent application has been the manufacturing of a human chromosome9. In thiscase, a chromosome was depleted of DNA by enzymatic digestion, leaving just the scaffold.Markers where introduced to serve as reference points for the 3D reconstruction. Next, EMphotographs of tilted series from 0o to 60o, with steps of 3o, were taken. A 3D CAD modelcontaining 700000 triangles was obtained from the pictures, and a physical model was thenbuilt. Chromosomes are extremely complex. Several visualization techniques are used tounderstand their structure, including electron microscopic tomography, hidden line removal,stereo projections, and animations. Physical models, though, may become an indispensible toolfor researches in the field.

9A description of this episode can be found through the World Wide Web athttp://www.cs.hut.fi/ado/chromosome in the document chromosome.html. It contains colorphotographs and videos of the model, and the original pictures of the chromosome.

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8 RPT vs conventional technologiesRPT does notand will notreplace completely conventional technologies such NC and high-speed milling, or even hand-made parts. Rather, one should regard RPT as one more option in thetoolkit for manufacturing parts. Figure 14 depicts a rough comparison between RPT and millingregarding the costs and time of manufacturing one part as a function of part complexity10. It is

Costs

Complexity Complexity

TimeLMT

NC milling

High-speed milling

FIGURE 14. RPT vs conventional technologies.

assumed, evidently, that the part can be manufactured by either technology such that the materialand tolerance requirements are met. The axis have no values; these are company dependent.RPT offers clear advantages when more than one copy of a complex part must be made.

Out of context, part complexity cannot be defined precisely, but it certainly contains thefollowing ingredients: model size, wall height and thickness, and the ratio between these two,total number of surfaces in the CAD model, tolerance requirements, type of CAD system usedto generate tool paths, and so on. Again, what is, and what is not, a complex part varies, to someextent, from one company to another.

Concerning material requirements, it is clear that when using milling one can always obtaindirectly a part with the desired mechanical properties. This is usually the choice when manufac-turing production toolings. But, as mentioned earlier, using a chain of processes that includes aRPT part, it is many times possible to obtain, indirectly, the same results in a shorter period oftime.

9 ConclusionsIt is impossible to cover all aspects of these relatively new manufacturing processes without beingbrief at times. The reader should browse the literature to overcome the obvious limitations of thiswork. More important, though, is their effective introduction in the current working practices ofcompanies. It is clear that these technologies, when applied correctly, can bring benefits in theform of better products in shorter lead times, and at reduced costs.

10The original figure is part of an unpublished report of the INSTANTCAM project.

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10 AcknowledgementsI was introduced to RPT while participating in the INSTANTCAM project, an European Con-sortium of partners from both industry and research centers. A lot of material presented herewas derived from reports of that project. I am very gratefull to these partners, particularly HansMuller (BIBA, Germany), Ulrich Reetz (Black&Decker GmbH, Germany), Bent Mieritz andKarsten L. Jensen (the Danish Technological Institute, Denmark), Reidar Hovtun (NTH-SINTEFProduction Engineering, Norway), and my collegue Ismo Makela. The schematic drawings of theprocesses were made available by Joakim Simons and Benjamin Sederholm from the MechanicalEngineering Department of our Institute. At HUT, we received the financial support of TEKES.

References[1] 3D Systems, Inc. Stereolithography Interface Specification, July 1988.[2] A.-L. Allanic, C. Medard, and P. Schaeffer. Stereophotolithography: A Brand New Ma-

chinery. In Solid Freeform Fabrication Symposium, pages 260271. University of Texas atAustin, August 1992. Austin, Texas, USA.

[3] V. Burguete. LMT Processes Comparison. Technical report, Instituto Superior Tecnico,1991. Available from the author. Address: Av. Rovisco Pais 1, P-1096 Lisboa Codex,Portugal.

[4] Cubital Ltd., 13 HaSadna St., Industrial Zone North, Raanana 43650, Israel. Cubital FacetList Syntax Guide.

[5] A. Dolenc. Software Tools for Rapid Prototyping Technologies in Manufacturing. PhDthesis, Helsinki University of Technology, October 1993. Published in ACTA POLY-TECHNICA SCANDINAVICA, Mathematics and Computer Science Series No. 62.

[6] A. Dolenc and I. Makela. LEAF: A Data Exchange Format for LMT Processes. In ThirdInternational Conference on Rapid Prototyping, pages 155160, Dayton, Ohio USA, June710 1992.

[7] A. Dolenc and I. Makela. Optimized Triangulation of Parametric Surfaces. In AdrianBowyer, editor, Computer-aided Surface Geometry and Design (Mathematics of SurfacesIV), number 48 in The Institute of Mathematics and its Applications Conference Series,pages 169183. Clarendon Press (Oxford), 1994. The Conference took place at BathUniversity (UK) in September 1990. An improved version of this work can be found in [5].

[8] A. Dolenc and I. Makela. Slicing Procedures for Layered Manufacturing Techniques.Computer-Aided Design, 26(2):119126, February 1994. This article is a subset of [5].

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[9] A. Dolenc, I. Makela, and R. Hovtun. Better Software for Rapid Prototyping with IN-STANTCAM. In G. L. Olling and F. Kimura, editors, Human Aspects in Computer Inte-grated Manufacturing, pages 449455. North-Holland, 1992. Also available as TechnicalReport TKO-B66 from the Helsinki University of Technology.

[10] R. J. Donahue and R. S. Turner. CAD Modeling and Alternative Methods of InformationTransfer for Rapid Prototyping Systems. In National Conference on Rapid Prototyping,pages 221235. University of Dayton and EMTEC, June 1991. Dayton, OH, USA.

[11] D. Dutta, N. Kikuchi, P. Papalmbros, F. Prinz, and L. Weiss. Project MAXWELL: TowardsRapid Realization of Superior Products. In H. L. Marcus, J. J. Beaman, J. W. Barlow, D. L.Bourell, and R. H. Crawford, editors, Solid Freeform Fabrication Symposium, pages 5462.University of Texas at Austin, August 1992. Austin, Texas, USA.

[12] M. Geiger, W. Steger, M. Greul, and M. Sindel. Multiphase Jet Solidification. EARPNewsletter, (3):8, January 1994. This Newsletter is published by the EARP Project andprinted at the Danish Technological Institute (Arhus, Denmark).

[13] P. F. Jacobs, editor. RAPID PROTOTYPING & MANUFACTURING: Fundamentals ofStereolithography. SME, 1992.

[14] K. L. Jensen. Desktop manufacturing, the next Industrial Revolution. Technical report,Danish Technological Institute, Teknologiparken, DK-8000 Aarhus, Denmark, 1991. Thisdocument contains a rather complete technical description of all RP processes, includingsome efforts that never reached the market. It was, though, no longer updated after 1993.

[15] K. L. Jensen and R. Hovtun. Making Electrodes for EDM with Rapid Prototyping. In ThirdInternational Conference on Rapid Prototyping, pages 295301, Dayton, Ohio USA, June710 1992.

[16] J. P. Kruth. Material Incress Manufacturing by Rapid Prototyping Techniques. In Annalsof CIRP, volume 40/2, pages 603614, 1991.

[17] I. Makela and A. Dolenc. Some efficient procedures for correcting triangulated models. InH. L. Marcus, J. J. Beaman, J. W. Barlow, D. L. Bourell, and R. H. Crawford, editors, SolidFreeform Fabrication Symposium, pages 126134. University of Texas at Austin, August1993. Austin, Texas, USA.

[18] McKinsey&Co. Various sources are cited. Apparently, this is part of a study conductedby McKinsey&Co. The Figure was kindly supplied by Mr. Ulrich Reetz (Black&Decker),1990.

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[19] T. H. Pang and P. F. Jacobs. StereoLithography 1993: QuickCastTM . In Solid FreeformFabrication Symposium, pages 158167. University of Texas at Austin, August 1993.Austin, Texas, USA.

[20] S. J. Rock and M. J. Wozny. A Flexible File Format for Solid Freeform Fabrication. InH. L. Marcus, J. J. Beaman, J. W. Barlow, D. L. Bourell, and R. H. Crawford, editors,Solid Freeform Fabrication Symposium Proceedings, pages 110. The University of Texasat Austin, September 1991.

[21] E. Sachs, M. Cima, P. Williams, D. Brancazio, and J. Cornie. Three Dimensional Printing:Rapid Tooling and Prototypes Directly from a CAD Model. Transactions of the ASME,114:481488, 1992.

[22] X. Sheng and B. E. Hirsch. Triangulation of trimmed surfaces in parametric space.Computer-Aided Design, 24(8):437444, August 1992.

[23] X. Sheng and U. Tucholke. On triangulation surface model for SLA. In Second InternationalConference on Rapid Prototyping, pages 236245, Dayton, Ohio USA, June 2326 1991.A better version of this paper was published in [22].

[24] Technical Insights, Inc., PO Box 1304, Fort Lee, NJ 07024-9967, USA. Rapid Prototyping:Strategic Technology for Product Development Success, Winter 19911992.

[25] U.S. Department of Commerce, National Bureau of Standards, Gaithersburg, MD 20899,USA. Initial Graphics Exchange Specification (IGES) Version 4.0, June 1988.

[26] VDMA. The Figures where found by Mr. Ulrich Reetz (Black&Decker) in a brochure,1991.

[27] Verband der Automobilindustrie e.V. (VDA), Westendstrasse 61, D-6000 Frankurt am Main,Germany. VDA Surface Interface, version 2.0, January 1987.

[28] L. E. Weiss, E. L. Gursoz, F. B. Prinz, P. S. Fussel, S. Mahalingam, and E. P. Patrick.A Rapid Tool Manufacturing System Based on Stereolithography and Thermal Spraying.Manufacturing Review, 3(1):4048, March 1990.

[29] J. Yoo, M. J. Cima, S. Khanuja, and E. M. Sachs. Structural Ceramic Components by 3DPrinting. In H. L. Marcus, J. J. Beaman, J. W. Barlow, D. L. Bourell, and R. H. Crawford,editors, Solid Freeform Fabrication Symposium, pages 4050. University of Texas at Austin,August 1993. Austin, Texas, USA.

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