A virtual prototyping system for rapid product development

S.H. Choi*, A.M.M. Chan

Department of Industrial and Manufacturing Systems Engineering, The University of Hong Kong,

Pokfulam Road, Hong Kong, China

Abstract

This paper describes a virtual prototyping (VP) system that integrates virtual reality with rapid prototyping (RP) to create virtual or digital

prototypes to facilitate product development. The proposed VP system incorporates two new simulation methodologies, namely the

dexel-based and the layer-based fabrication approaches, to simulate the powder-based and the laminated sheet-based RP processes,

respectively. The dexel-based approach deposits arrays of solid strips to form a layer, while the layer-based approach directly forms a

complete layer by extruding the slice contours. The layer is subsequently stacked up to fabricate a virtual prototype. The simulation

approaches resemble the physical fabrication processes of most RP systems, and are therefore capable of accurately representing the

geometrical characteristics of prototypes. In addition to numerical quantification of the simulation results, the system also provides

stereoscopic visualisation of the product design and its prototype for detailed analyses. Indeed, the original product design may be

superimposed on its virtual prototype, so that areas with dimensional errors beyond design limits may be clearly highlighted to facilitate

point-to-point analysis of the surface texture and the dimensional accuracy of the prototype. Hence, the key control parameters of an RP

process, such as part orientation, layer thickness and hatch space, may be effectively tuned up for optimal fabrication of physical prototypes

in subsequent product development. Furthermore, the virtual prototypes can be transmitted via the Internet to customers to facilitate global

manufacturing. As a result, both the lead-time and the product development costs can be significantly reduced.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Virtual prototyping; Visualisation; Dexel-based and layer-based simulation

1. Introduction

1.1. Rapid prototyping

Rapid prototyping is an innovative technology developed

in the past two decades. It aims to produce prototypes

relatively quickly for visual inspection, ergonomic

evaluation, form-fit analysis, and as master patterns for

production tools, etc. to help speed up an entire product

development process. Various RP systems [1,2] are now

commercially available. According to the materials used,

common RP processes can be classified into three main

types, namely powder-based, resin-based and laminated

sheet-based. Powder-based RP processes include the

Selective Laser Sintering (SLS) and the 3D Printing (3DP)

processes that all use powder material to make prototypes.

Resin-based RP processes, such as the StereoLithography

Apparatus, use a liquid resin, which is solidified by exposure

to a ultra-violet laser beam. Laminated sheet-based

processes include the Laminated Object Manufacturing

(LOM) process, in which a prototype is fabricated from

laminated materials.

Despite the advantages, current RP technology is far

from ideal. Indeed, it is plagued by some major problems,

which undermine the accuracy and quality of prototypes.

The performance of an RP process is affected by a multitude

of process parameters. It is not an easy task to choose an

appropriate combination of these parameters for optimal

fabrication of a prototype, which depends on the quality

requirements, such as accuracy, build-time, strength and

fabrication efficiency. However, the quality requirements

vary from visual aids to master patterns for secondary

processes. Hence, a significant degree of expertise is

required to produce prototypes of consistent quality.

The process is of a trial-and-error basis and is therefore

both time-consuming and very costly.

1.2. Virtual prototyping

Virtual prototyping (VP) may alleviate the shortcomings

of RP. It makes use of a digital model called a virtual

0010-4485/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0010-4485(03)00110-6

Computer-Aided Design 36 (2004) 401–412

www.elsevier.com/locate/cad

* Corresponding author. Tel.: þ852-2859-7054; fax: þ852-2858-6535.

E-mail address: shchoi@hkucc.hku.hk (S.H. Choi).

prototype, in lieu of a physical prototype, for testing and

evaluation of specific characteristics of a product or a

manufacturing process in a computational environment.

Therefore, in virtual prototyping, faults concerning

fabrications, product design and production planning can

be detected in a compressed time frame before great

expenditures are committed. This significantly reduces the

number of physical iterations and thereby the associated

manufacturing overheads that leads to faster and cost-

effective product development. Once the virtual prototyping

is finished, the model may be sent directly to physical

fabrication or via the Internet to customers to solicit

comments. Since digital models are mostly used in VP,

the cost incurred in repeating the process to optimise

prototype quality is minimal.

Dedicated VP systems have been successfully developed

and used in automobile and aerospace industries.

These systems can be classified into two areas, namely

product design and process simulation. For product design,

various analyses regarding design validation, such as

functional testing, form-and-fit testing, ergonomic testing,

assembly testing and disassembly testing, are performed on

a model in the virtual environment. Bennett presented how

VP could assist the different stages of product development

of complex aerospace products [3]. Rooks described the use

of digital mock-ups for finding out possible errors

concerning product assembly in the early design stage [4].

Jayaram et al. carried out virtual assembly in VR [5].

The parts were firstly designed in a CAD system and

subsequently assembled in a virtual environment. Siddique

and Rosen suggested using VP to generate complete

disassembly processes of a product design [6].

For process simulation, a manufacturing process is

simulated in a computer to determine possible

manufacturing problems or bottlenecks in production such

that expensive physical mock-ups can be saved.

Schulz applied VR technique to simulate the forming

process of stamping products in order to study the residual

stress and material distribution [7]. Bowyer et al. developed

a virtual milling machine that could cut a virtual block to

produce a part with the desired shape by a virtual tool [8].

Bickel developed a virtual welding cell for precise weld

path generation for die re-forging [9].

Apart from manufacturing, VP has also had a profound

impact on the medical field. It was used in training, surgical

planning, and creation of digital human organs [10]. Virtual

organs were created according to the patient’s data for

subsequent simulations. During surgical education,

students could ‘fly’ inside and around the organs, which

responded like real ones to operations. They could thus

practise surgeries and other medical procedures.

1.3. A new approach to virtual prototyping

Visualisation has been recognised as an effective way

to present real scenarios that facilitate effective

communication of designs and ideas. An IT manager of a

car company applied this concept to development of new

cars [11]. A car model is built virtually and projected on a

large screen, and people from different departments may

conveniently share a true 3D image of the car by wearing

stereo glasses to evaluate the design and to identify

problems before getting too far down the production.

Tseng et al. combined VP with design to explore the

customer perception on the target products [12]. The VR

technology allows the customer to be immersed in the

virtual environment for detailed design visualisation and

modification. The virtual prototype is then put in

simulations to find out an optimal assembly process.

Chuang and O’Grady worked on visualisation of assembly

process to provide the designer with the parts’ interaction in

assembly operations and at the same time, to track the paths

for subsequent assembly [13]. The design for assembly

process may thus be improved by expressing the results

fully and naturally in a visual manner, rather than in

abstractive numerical figures.

The strength of visualisation has been explored and

applied successfully in many VP systems. However, little

research work has been done to date on using the technique

to study and enhance the quality of prototypes before

physical fabrication. Indeed, VP provides a test-bed and

much valuable information that may otherwise have

required time-consuming and expensive physical exper-

imentation. Furthermore, it provides results in a natural way

that allows the designer to make corrective actions.

This paper therefore proposes a new virtual prototyping

system that exploits visualisation to facilitate product

development. It is based on the mathematical model

developed by the authors for modelling and optimisation

of rapid prototyping [14]. The model incorporates various

process parameters like layer thickness, hatch space,

bed temperature, laser power and sinter factor, etc. to

quantify the measures of prototype quality, which include

accuracy and build-time. The proposed system simulates an

RP process to create a virtual prototype with quantified

measures of prototype quality for assessment of the RP

process. To exploit visualisation to facilitate product

development, the virtual prototype may be superimposed

on the original model to provide a clear visualisation for

direct comparison of the product design and the resultant

prototype that the RP machine will subsequently deliver.

The superimposition allows a designer to perform validation

of the product design and analyses of the dimensional

accuracy conveniently. This is particularly useful in that

the designer can conveniently analyse and compare the

surface texture and the dimensional accuracy point-by-point

of the prototype with the product design. Specific areas of

the prototype where the dimensional deviations are beyond

the design limits can be easily identified and highlighted

for subsequent improvement. With such a virtual prototype

in the computer, the product design can be scrutinised

easily, and its aesthetic and functional characteristics

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412402

simulated and analysed accordingly. Subsequently, the

virtual prototype may be transmitted via the Internet to

customers and/or designers in other parts of the world to

solicit improvements of the product design. This facilitates

global manufacturing and hence helps reduce both the

lead-time and the product development cost significantly.

The proposed VP system incorporates two new simulation

methodologies, namely the dexel-based and the layer-based

fabrication approaches, to simulate the powder-based and

the laminated sheet-based RP processes, respectively.

The following sections describe the simulation approaches

and the implementation of the proposed VP system.

2. Dexel-based and layer-based prototyping processes

To perform an accurate analysis of physical prototypes in

VR is not a simple task. An essential and prerequisite

criterion is that a virtual prototype should accurately

represent the physical one. Since there are many RP

processes commercially available in the market, it is not

practical to develop individual modelling approaches for

each of them. Therefore, the proposed VP system focuses on

two major types of RP processes, namely the powder-based

and the laminated sheet-based. Section 2.1 discusses the

characteristics of these two types of RP processes.

2.1. Dexel-based virtual fabrication

Physical prototypes made by powder-based RP systems,

such as SLS and 3DP, may be regarded as being made up of

strips of material that are sintered/solidified by a laser or

binder beam.

During fabrication of a prototype on a powder-based

RP system, the beam is positioned to a point on the

surface to solidify a small portion (typically of the beam

width) of the material. It continues to solidify the

neighbouring points by travelling along a hatch vector,

as shown in Fig. 1. A hatch vector represents the path that

the beam has to follow within a contour to build a portion

of the layer. The scanning motion is so fast that it appears

as if the beam is solidifying a complete strip of material

along the hatch vector at a time.

Hatch vectors are obtained by passing rays onto the layer

contour at grids of resolution of the hatch distance. The beam

moves along successive hatch vectors to build the

layer. Each hatch vector can be considered as a dexel [15,

16], which represents the centre of the beam trajectory.

Hence, a strip of material of a physical prototype may be

represented by building a volume of a specific height and

width around a dexel. Such a volume is regarded as a voxel,

and its width, height and length are the beam diameter, the

layer thickness and the length of the dexel, respectively.

Therefore, building a voxel per dexel simulates the sintering

or the solidification process, and a layer is formed by

building voxels around the dexels in the layer contour.

This is a new approach to virtual fabrication in that

rectangular finite solid strips are laid to form a layer, which

is subsequently stacked up to form a virtual prototype. Fig. 2

shows the process of dexel-based virtual prototyping of a

gearbox housing. It should be pointed out that in order to

represent different RP systems better, the shape of the

voxels may be varied. For example, the end of the voxels

may be changed from square to semi-circular in shape,

which may simulate the material sintering by the laser beam

of SLS process more accurately. However, this would be

graphically complicated for the simulation. Hence, square-

ended voxels are currently adopted in the system.

2.2. Layer-based virtual fabrication

On the other hand, physical prototypes made by

laminated sheet-based RP processes, such as LOM, may

be regarded as being made up of complete layers, instead of

strips, of material. During fabrication, an entire layer is

deposited at a time and subsequent layers are stacked up to

form a physical prototype. Based on this fabrication

approach, the Solid Ground Curing process may also be

classified as laminated sheet-based.

Since a prototype fabricated by this type of RP processes

is composed of entire layers, smoothness of layer edges, and

therefore the absence of the horizontal staircase effect,

characterises the prototypes. However, the vertical

staircase effect still exists. Hence, simulation of laminated

sheet-based RP processes should fabricate a virtual proto-

type layer-by-layer. To achieve this, a layer-based fabrica-

tion approach is developed such that all contours of a layer

are extruded at a time to form a complete layer.

Fig. 1. Rectangular finite strip of solid built around a dexel.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412 403

2.3. Virtual prototypes for design validation

Virtual prototypes may represent the physical ones

accurately. They facilitate design validation in the early

stage of product development as the designer can have a

clear representation of the product to examine its aesthetic

and structural features. If any problems are identified,

the design can be promptly improved before it goes too far

down the development cycle. This is particularly important

to help enhance the competitiveness of the manufacturing

industry, which is faced with increasing pressure to satisfy

demands for small-batch production of different varieties of

customised products. In such situations, it would not be

economical to make a mould for small-batch production.

On the other hand, rapid prototyping may be a convenient

tool for direct production of customised products, provided

it can fabricate prototypes of the required accuracy and of

appropriate materials. Indeed, some researchers [17,18]

recognised the significance and they have worked on the

techniques to produce metallic or functional prototypes. It is

envisaged that when RP becomes economical for direct

manufacture of customised products, it will be of profound

importance to validate the accuracy and quality of the

prototypes before committing to physical fabrication.

Hence, the significance of the proposed VP system will be

further highlighted.

2.4. Visualisation of RP process

The proposed VP system facilitates design validation

through visualisation of the RP process and the resultant

prototypes. Visualisation also helps the designer understand

what will possibly happen to a particular part of the

prototype. It is not common that all features of the model are

required for a specific analysis [19]. Indeed, numerical

values indicate only the overall average quality of a

prototype. On the other hand, a clear visualisation facilitates

detailed assessment of specific parts of the prototype.

2.4.1. Superimposition of product model

on virtual prototype

For this purpose, the proposed VP system can display a

virtual prototype and the product model simultaneously.

These two images are superimposed for direct comparison

of the resultant prototype with the original design.

This allows point-by-point investigation of any discrepancy

in the characteristics of the prototype and the product

design. For example, the surface texture of the prototype can

be easily studied, and specific areas with dimensional errors

beyond tolerance limits may be clearly identified and

highlighted for subsequent improvement.

Fig. 2. Dexel-based virtual prototyping of a gearbox housing.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412404

2.4.2. Staircase effects and optimisation

of process parameters

RP machines fabricate prototypes layer by layer. Hence, a

prototype may be regarded as a staircase approximation of

the intended product model. The staircase between two layers

along the build-direction affects adversely the surface texture

and the dimensional accuracy of the prototype. This effect is

directly related to the layer thickness. Furthermore, in

powder-based RP processes, there exists horizontal staircase

effect within a layer. Similarly, it also affects the surface

texture and dimensional accuracy of the prototype.

A horizontal staircase occurs when the laser beam or binder

head deposits one strip of material next to another to form a

layer. Hence, it is related to the hatch space, which is the

distance between two hatch lines.

The surface texture and the dimensional accuracy of a

prototype may be improved by reducing both the layer

thickness and the hatch space. In fact, a curved surface can be

accurately produced only if the layer thickness and the hatch

space are infinitesimally small. However, this will make the

build-time impractically too long. Indeed, the quality and

the build-time of a prototype are significantly affected by

some major process parameters, particularly the orientation,

the layer thickness and the hatch space, etc. Therefore, an

optimal combination of process parameters must be carefully

chosen for efficient production of prototypes of the required

quality.

Visualisation of RP will therefore be very useful to help

choose a proper set of process parameters for optimal

production of prototypes. The designer can see the effects of

changing the process parameters on the prototype quality

clearly. Subsequently, an optimal set of process parameters

may be chosen quickly for efficient production of prototypes.

3. The proposed virtual prototyping system

The main objective of the proposed VP system is to

facilitate visualisation and optimisation of RP processes, and

thus faster product realisation. Based on a product model

designed in a CAD package, the system simulates the

characteristics of an RP process to perform virtual fabrication

of product prototypes. The virtual prototypes may then be

used in various analyses. As shown in Fig. 3, the proposed VP

system consists of three main steps, namely (1) creation of a

product model; (2) virtual fabrication; and (3) visualisation

and tuning of process parameters.

3.1. Product model

Creating a product model is the first step to provide the

necessary information of the design, which includes the

geometry and the attributes of material and colour, etc.

In general, the product model is designed using a CAD

package and then converted to a STL model.

3.2. Virtual fabrication

Before performing virtual fabrication, preparation work is

carried out in several modules, including the Model Viewer,

the Slicer, the Hatcher, the Contour Sorter and the Part

Fabricator. The Model Viewer reads a model in STL format

and displays it in the virtual world (VW) to allow the designer

to have an idea of the original model. The Slicer and Hatcher

modules are integral part of the VP system developed in the

Department, and they can handle relatively large and

complex STL models [20]. The Slicer slices the STL model

to produce the contour information of each layer. The Hatcher

performs hatching of all the layers to generate the laser/

binder path, while the Contour Sorter constructs the

hierarchical relationship of contours for the layer-based

simulation. Subsequently, the Part Fabricator reads in the

hatch information and simulates the fabrication process to

form a virtual prototype. It allows the designer to visualise

the process by displaying the modelled results, such as the

surface quality, with respect to the process parameters.

Indeed, it is vital to show the effect of different process

parameters on the prototype in real-time.

Fig. 3. Flow of the proposed VP system.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412 405

3.3. Visualisation and tuning of process parameters

Once a virtual fabrication simulation process is

completed, the designer can manipulate the resultant

virtual prototype using the utilities provided to visualise

the quality of the product prototype that the RP machine

will subsequently deliver. The designer can navigate

around the internal and opaque structures of the prototype

to investigate the product design. Furthermore,

superimposing the STL model on its virtual prototype

may highlight dimensional deviations. The system also

calculates the maximum and the average cusp heights that

indicate the overall accuracy of the prototype. To study the

dimensional errors, a tolerance limit may be set and

any locations with deviations beyond the limit will be

clearly highlighted. The designer may thus identify and

focus on the parts that need modifications. To improve the

accuracy and the surface quality of specific features of the

prototype, the process parameters, such as the orientation

of the model, the layer thickness or the hatch space, may be

changed accordingly.

4. Implementation

The proposed virtual prototyping system has been

implemented in Cþþ language, with the WorldToolKit

(WTK) virtual reality support libraries.

4.1. The Model Viewer module

The Model Viewer displays the product model is in a VR

environment created by WTK. It provides two modes for

viewing, namely the normal mode and the stereo mode.

The stereo mode provides 3D stereoscopic viewing of the

virtual prototype. The system adopts a semi-immersive VP

interface [21], which requires only an emitter and a pair of

CrystalEyes shutter glasses. The designer wears a pair of

shutter glasses that generates a stereoscopic feeling

by synchronising with the display device to switch on and

off the images to the left eye and the right eye alternatively.

This creates a depth perception and therefore a ‘being there’

illusion. However, if stereoscopic display of the prototype is

not required, the normal mode can be chosen, and no emitter

and glasses are needed.

4.2. The Slicer module

The Slicer consists of a slicing algorithm [20] that slices

a STL model into a number of layers of a predefined

thickness. It generates layer contours by determining the

intersection points of the slicing plane and the facets.

The Slicer module offers two slicing approaches, as shown

in Fig. 4. The model in Fig. 4a was sliced with the normal

un-offset approach, which is generally adopted in most

commercial RP systems. It can be noticed that when

the surface converges in the upward direction, there are

excessive materials at the edges of each layer.

Consequently, this leads to deformation and the original

round shape of the prototype becomes a little oval, as shown

in Fig. 4c. To solve this problem, the offset slicing shown in

Fig. 4b is proposed. It performs slicing at the middle of each

layer. Hence, by forming a layer by extruding the slice both

upward and downward, the prototype will not be deformed

in a particular direction and its shape can be maintained, as

shown in Fig. 4d.

The layer contours are stored in a data file in Common

Layer Interface (CLI) format, which will be further

processed to represent the virtual part. The designer can

perform slicing with a different layer thickness to suit the

fabrication requirements. The layer contours are

subsequently hatched for virtual fabrication.

4.3. The Contour Sorter module

The Contour Sorter is used to identify the topological

hierarchy relationship of contours for sorting out different

levels of internal contours to prepare for layer extrusion in

the layer-based virtual fabrication [20]. It first establishes

the parent-and-son relationships of the contours in the same

layer, and then groups them into different families. Fig. 5

shows an example of the contour hierarchy relationship of a

layer. According to the result of grouping, hierarchy

information is generated to represent the relationship of

contours. During layer extrusion, only the contours in the

same family are extruded at the same time for creation of

internal details in a solid layer.

Fig. 4. Normal (un-offset) and offset slicing approaches.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412406

4.4. The Hatcher module

The Hatcher processes the layer contours in CLI format

to determine the coordinates of the endpoints of each hatch

line for virtual fabrication. The hatch information is stored

in another CLI file. As virtual fabrication normally involves

complex layer contours, hatching errors due to ambiguity

may sometimes occur when hatch lines are very close to

small and intrinsic contours such that it is difficult

to determine whether there is an intersection. Therefore,

to enhance the stability of the Hatcher module, a

small tolerance zone is implemented. It will be regarded

as an intersection if any part of a contour is within the

tolerance zone around the hatch line.

4.5. The Part Fabricator module

The Part Fabricator incorporates both the dexel-based

and the layer-based fabrication approaches. For the

simulation of powder-based RP processes, the dexel-

based approach is chosen, with a hatched contour file as

input. The hatch information is read in and the

rectangular solid strips will be displayed one by one

accordingly at an appropriate z-height to simulate the solid

material solidified by the laser/binder head. For the

simulation of laminated sheet-based RP processes, the

layer-based approach is used, for which the input contour

file does not include hatch information. The contours

together with the topological hierarchy information are

read to form an entire layer directly by extrusion. Based

on the choice of the fabrication approach, the Part

Fabricator simulates the RP process to build a virtual

prototype in VW. During fabrication, the designer can

observe how the prototype is built. When the virtual

fabrication is completed, the virtual prototype is dis-

played and/or superimposed on the product model for the

visual inspection. Similar to using the Model Viewer, the

designer may choose the stereo mode to view the virtual

prototype in a virtual environment.

5. Case studies

Examples are now presented to demonstrate how the VP

system facilitates quality analyses and parameter

optimisation for RP process planning. For the sake of

clarification, parameters that produced relatively rough

texture were adopted for the fabrications.

Fig. 5. Example of topological relationship of contours.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412 407

5.1. Prototypes fabricated by the dexel-based simulation

5.1.1. A turbine fan

To demonstrate the use of the system for optimisation of

RP process parameters, fabrication of a turbine fan on a

Sinterstation 2000 SLS machine with nylon was simulated,

and the result is shown in Fig. 6. The laser diameter was

0.04 mm, while the hatch space and the layer thickness were

both set as 0.1 mm. The turbine fan has different features

like a hollow cylinder and several freeform surfaces around

it. Depending on the distribution of the facets, the surface

accuracy and build-time could be different for different

orientations.

Fig. 6 also indicates the surface accuracy, build-time, and

the number of layers when the part was rotated about the

x-axis. The surface accuracy was mainly dependent on the

cosine of the angle between the facet normal and the build-

direction. Thus, it was only necessary to rotate the part

between 0 and 908. The part at orientation of 08 gave the

minimum build-time of 0.71 h and the surface accuracy of

0.033 mm. The possibilities of reducing the build-time while

achieving the best surface accuracy possible at this

orientation were thus further considered. Simulations were

also performed to reduce the build-time by increasing

the layer thickness to 0.15 mm and the hatch space to

0.2 mm while keeping all the other values constant. It was

observed that the increase in the layer thickness changed the

surface accuracy from 0.033 to 0.049 mm, while the build-

time was reduced from 0.71 to 0.51 h. This was expected

since the number of layers was reduced from 127 to 84.

Despite the fact that the scan-distance per layer was not

changed, the total laser scan-time was reduced because

the number of layers scanned was reduced. It was also found

that increasing the hatch space to 0.2 mm did not affect the

surface accuracy. However, the build-time was reduced from

0.71 to 0.60 h, when the hatch space was increased from 0.1

to 0.2 mm.

5.1.2. A spider

A toy spider was chosen to demonstrate the study of the

dimensional deviations of a prototype from its STL model,

and the normal (un-offset) slicing approach was used to slice

the model. Fig. 7a shows two spiders, one of which was a

STL model and the other a virtual prototype. Without the

VP system, it would be difficult to study the dimensional

deviations even if a real prototype was available. However,

when they were superimposed in a virtual environment, as

shown in Fig. 7b, the surface texture and the dimensional

deviations were clearly illustrated. The prisms indicated the

excessive materials in fabrication, which were located

Fig. 6. Simulation result of a turbine fan.

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mainly at the upper part of the model. This was because the

layers were normally formed on the sliced planes during

physical fabrication. When the surface converged in the

upward direction, such as the body of the spider, there were

excessive materials at the edges of each layer. This shows

the phenomena that most RP machines cannot produce a

regular sphere, which generally becomes a little oval.

Through this simple case study, the strength of visualisation

is explored.

The system also calculated the cusp heights to

evaluate the overall accuracy of the prototype. In this

case, the average and the maximum cusp heights were

0.601 and 1.278 mm, respectively. Suppose that any

deviations more than 1.270 mm were not acceptable, the

designer might choose to highlight the areas which are

out of the design limit for subsequent investigation of

these important features. Fig. 8 shows the same spiders

with some pins on them. The pins indicated the facets

Fig. 7. STL model and virtual prototype of a spider.

Fig. 8. Areas of spider with dimensional deviations beyond design limit.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412 409

of the STL model with cusp heights more than

1.270 mm. The colour of the pins may be red or green.

The red ones pointed to the maximum deviations,

while the green ones pointed to the unacceptable

deviations. The process parameters may be deemed

acceptable, unless unsatisfactory deviations were located

at important parts of the model, in which case the

designer might choose either to change the model

orientation to shift the deviations or to reduce the layer

thickness and the hatch space to improve the cusp

heights. Fig. 9 shows a comparison of the two virtual

prototypes of different layer thickness and hatch space.

Although 0.02 mm thickness and 0.05 mm hatch space

gave a finer prototype as in Fig. 9b, such process

parameters may not be practical for most SLS processes.

5.2. Prototypes fabricated by the layer-based simulation

5.2.1. A hand skeleton

Applications of RP in the medical field for making

prototypes of human skeletons and organs have become

widespread in the past decade. A hand skeleton, as shown in

Fig. 10, was thus chosen to illustrate possible applications of

the VP system in the medical field.

In a surgery, for example, a patient’s hand was

scanned to produce a prototype for doctors to study

injury or deformity more clearly. Suppose a prosthesis

was to be put into the fourth finger of the patient’s hand.

Visualisation of the virtual prototype would therefore be

helpful for doctors to choose a prosthesis that would best

fit to minimise the possibility of mismatch. Fig. 11a

shows two virtual hands superimposed on each other.

The model was sliced with the offset approach and the

excessive materials were thus evenly distributed. Its

average and maximum cusp heights were 0.946 and

1.866 mm, respectively. Fig. 11b highlights the

deviations that exceeded 1.854 mm.

Although there appeared many unacceptable deviations,

the highlighted deviations actually represented a very small

portion of the total 110,000 facets of the hand model.

Moreover, most of the deviations were located at the back of

the hand, whilst the main concern of the surgery was the

fourth finger. Therefore, this prototype may be deemed good

enough and physical fabrication can be carried out using

these process parameters. In general, it is not necessary to

aim at producing a prefect prototype. Instead, an optimal

prototype with good accuracy at selected areas will be more

practical and economical.

6. Limitations and further development

A major limitation of the current system is that it does not

incorporate shrinkage and warpage effects to enhance

accuracy estimation. In fact, for RP processes that employ

heat energy to solidify/sinter the material, the subsequent

Fig. 10. Hand skeleton.

Fig. 9. Comparison of spider prototypes of different layer thickness and hatch spacing.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412410

prototypes tend to shrink after cooling, resulting in

dimensional deviations and geometric distortion of

a physical prototype. Warpage is another kind of inaccuracy

caused by uneven distributions of heat energy and

the resultant binding force. These dimensional errors vary

with the geometry of the prototypes and the characteristics

of the RP processes. Indeed, to predict the effects of

shrinkage and warpage may require complex

thermodynamics and binding force models, which are not

yet available.

Hence, for the time being, the VP system builds a

virtual prototype without taking the shrinkage and the

warpage effects into account. However, the individual

voxels of a dexel-based virtual prototype may provide

a convenient vehicle for analysis of such effects, when

appropriate thermodynamics and binding force models

become available for incorporation into the VP system.

Indeed, voxels may be treated as finite strips of solid for

modelling the energy density and binding force

distribution based on heat dissipation of such strips.

Research effort is now being devoted to developing a

simulation model that incorporate voxels with finite

element analysis technique. When it becomes available,

the VP system may be able to predict the dimensional

changes due to shrinkage and warpage effects. It would

then be possible to modify the model design to

compensate for these effects, and consequently,

fabrication of high precision prototypes would become

possible.

Acknowledgements

The authors would like to acknowledge the Research

Grant Council of the Hong Kong SAR Government and

the CRCG of the University of Hong Kong for their

financial support for this project.

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S.H. Choi is associate professor in the IMSE

Department at the University of Hong Kong.

He obtained both his BSc and PhD degrees at

the University of Birmingham. He worked in

computer industry as CADCAM consultant

before joining the University of Hong Kong.

His current research interests include CAD-

CAM, advanced manufacturing systems and

virtual prototyping technology.

A.M.M. Chan got her BEng degree from the

IMSE Department at the University of Hong

Kong. She continued her postgraduate

research study in the Department, and her

research interest is in virtual prototyping

technology.

S.H. Choi, A.M.M. Chan / Computer-Aided Design 36 (2004) 401–412412