A dexel‐based virtual prototyping system for product development

A dexel-based virtualprototyping system forproduct development

S.H. Choi and

A.M.M. Chan

The authors

S.H. Choi and A.M.M. Chan are based at the Department

of Industrial and Manufacturing Systems Engineering,

The University of Hong Kong, Pokfulam Road, Hong Kong.

Keywords

Prototyping, Data visualisation, Simulation

Abstract

This paper proposes a dexel-based virtual prototyping

system, which builds a virtual prototype with dexels or

rectangular strips of solid. The approach resembles the

physical fabrication process of most powder-based rapid

prototyping (RP) systems. It simulates an RP process to

create a virtual prototype. Colour virtual prototypes may

also be fabricated relatively easily. Thus, the designer can

perform design validation and accuracy analysis easily in a

virtual environment as if using a physical prototype. In

addition to numeric quantification of the RP process, the

system provides vivid visualisations of the prototype for

studying its characteristics. Furthermore, the prototype

may be superimposed on the product model, and the areas

with dimensional errors beyond design limits may be

clearly highlighted for subsequent improvement. The

designer may thus analyse and compare the surface

texture point-by-point of the prototype with the product

design.

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1. Introduction

1.1 Rapid prototyping

Rapid prototyping (RP) is a relatively new

technology for fabrication of prototypes much

more quickly than conventional methods. It

plays an essential role in product

development, starting from conceptual design

to final product verifications, such as aesthetic

analysis, ergonomic evaluation, functional

testing, process planning, etc. This helps

explore and solve potential problems of a

product prior to actual production. Indeed,

early discovery of design problems fruitfully

leads to substantial saving in both time and

costs.

Various RP systems (Pham and Gault,

1998; Yan and Gu, 1996) are now

commercially available. According to the

materials used, RP systems can be classified

into three types, namely powder-based, resin-

based and laminated sheet-based. Powder-

based RP systems include the selective laser

sintering (SLS) and the 3D printing (3DP)

processes that all use powder material to make

prototypes. Resin-based RP systems, such as

the stereolithography apparatus (SLA), use a

liquid resin, which is solidified by exposure to

a ultra-violet laser beam. The laminated sheet-

based systems include the laminated object

manufacturing (LOM) process, in which a

prototype is fabricated from sheet materials.

Despite the advantages, current RP

technology is far from ideal. In fact, it is

plagued by some major problems, which

undermine the accuracy and quality of

prototypes. Indeed, 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

Rapid Prototyping Journal

Volume 8 · Number 5 · 2002 · pp. 300–314

q MCB UP Limited · ISSN 1355-2546

DOI 10.1108/13552540210451778

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.

Received: November 2001

Revised: August 2002

Accepted: August 2002

300

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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 and enhance its

functions. It makes use of a computer-

generated digital prototype, in lieu of a

physical one, for the testing and evaluation of

specific characteristics of a product or a

manufacturing process. It is often carried out

in a virtual reality (VR) environment, which

provides a stereoscopic and compelling

illusion of a model. The stereoscopic viewing

ability allows the designer to gain a “being

there” feeling, as if a real object is being

manipulated. This provides the designer with

a tool to perform a number of what-if studies,

and broadens the designer’s understanding of

the product. Once the VP process is finished,

the model can be physically fabricated, or sent

via the Internet to customers to solicit

comments. This reduces the number of

physical iterations and thereby the associated

manufacturing overheads that leads to faster

and cost-effective product development.

Since digital models are mostly used in VP,

there is no need to worry about the costs and

quality of physical prototypes.

Dedicated VP systems have been developed

and used by automobile and aerospace

companies. Resseler (1995) presented a

summary of applications of VR in

manufacturing projects. The task performed

at the Boeing’s Huntsivlle laboratory is to

model lunar rover radiation effects, especially

during a solar proton event. The VR world

developed allows the designer to insert models

to evaluate the dosage levels of radiation based

on their position. The work at Naval Research

Institute (Rosemblem, 1996) focused on

experiments in shipboard fire fighting to verify

the effectiveness of VR as a mission-planning

tool, and VR was used as a visualisation tool in

the preliminary design of new naval ships. The

engineers at Volkswagen employed VR to

accelerate their car design processes (Purchke

et al., 1998). Research efforts have also been

focused on studying the change of the

products under specific operating conditions.

Schulz et al. (1998) simulated the sheet metal

forming process to predict the distribution of

residual stress and thickness distribution of a

stamping product. Bowyer et al. (1996)

developed a desktop VP system for milling

operations, while Bickel (1998) developed a

virtual welding cell for die re-forging. VP has

had a profound impact on the medical field

too. It is mainly used in training, surgical

planning, and creation of digital human organs

according to the patient’s data for subsequent

simulations (Zajtchuk and Satava, 1997).

However, RP still plays a very vital role,

despite the advent of VP. One of its

applications that cannot be replaced by VP is

tooling. For example, RP makes master

patterns for moulds and dies used in injection

moulding and investment casting. Fabricating

these tools rapidly facilitates speeding up the

whole production process. Therefore, it is of

vital importance to continue to improve RP.

Indeed, the characteristics of VR may help

alleviate some major shortcomings of RP.

However, few researchers have exploited the

strengths of VR to facilitate RP. Gibson et al.

(1993) investigated the contributions of VR

and RP towards efficient product

development, with regard to ergonomic,

aesthetic and functional aspects of design.

They suggested the use of VR as a

complementary technology to RP with an

interface accommodated through a CAD

system. Morvan and Fadel (1996) linked VR

with RP to visualize the support structures of a

part and to aid the designer to identify

improper support structures. They discussed

the translation issues related to head mounted

display (HMD) and computer screen, and

further coupled RP with VR by developing the

Interactive Virtual Environment for

Correction of STL files (IVECS) system. The

system detects errors in STL files and allows

triangular facets to be added, removed,

reversed or offset. It allows the designer to have

their hands virtually on the STL models to fix

the errors. It also consists of tools that help the

designer to visualize the part in different

modes. Indeed, correcting a faulty STL file

interactively is tedious. Jee and Sachs (1998)

developed a visual simulation system for 3DP.

However, their system is aimed at developing a

visual tool to examine surface texture only.

To conclude, VP is well developed and used

in automobile and aerospace industries for

replacement of large and expensive physical

mock-ups. However, applications of VP to RP

have been overlooked, especially to studying

the quality of the resultant prototypes. Most

research work generally employed numerical

values to represent the quality of the

prototypes. Thomson and Crawford (1995)

estimated the volumetric error between the

A dexel-based virtual prototyping system

S.H. Choi and A.M.M. Chan


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intended part and the layered approximation.

Zhao and Laperriere (1998) quantified the

surface accuracy by the relative surface area

deviation between the successive slices. Hur

and Lee (1998) evaluated the prototype

accuracy in numerical values by the projected

staircase area and the cross-sectional cusp

area. Although this process may be

interactively repeated to optimise the quality,

there is a lack of visualisation of the prototype,

which may impair the decision making of the

designer. Since the numerical values indicate

only the overall average quality, detailed

assessment of specific parts of the prototype is

very difficult.

1.3 A new approach to VP

Visualisation has been recognized 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 (Rippinner, 1999). 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. (1998)

combined VP with design to explore the

customer perception on the target products.

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 (1999) 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. The

design for assembly (DFA) 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. On one hand, VP

provides a test-bed and much valuable

information that may otherwise have required

time-consuming and expensive physical

experimentation. On the other hand, it

provides results in a natural way that allows

the designer to make corrective actions.

This paper proposes a new VP system that

exploits visualisation to facilitate product

design and development. It builds a virtual

prototype with dexels or rectangular strips of

solid. This approach resembles the physical

fabrication process of most powder-based RP

systems. It simulates an RP process to create a

virtual prototype, which allows the designer to

perform validation of the product design and

analyses of the dimensional accuracy

conveniently. 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.

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 scrutinized easily,

and its aesthetic and functional characteristics

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

improvement of the product design. This

facilitates global manufacturing and hence

helps reduce the cost and lead-time of product

development significantly. The following

sections describe the proposed approach and

the implementation of the VP system.

2. Dexel-based virtual prototypes

2.1 Dexel-based virtual fabrication

Physical prototypes made by powder-based

RP processes, such as SLS and 3DP, may be

regarded as being made up of strips of material

that are solidified/sintered by the laser or

binder beam. The beam is positioned at a

point on the surface to solidify a small portion

(typically of laser diameter width) of the

material. It continues to solidify the neigh-

bouring points by travelling along a hatch

vector, as shown in Figure 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.




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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

(Hook, 1986; Stifter, 1995), which represents

the centre of the beam trajectory. Hence,

building a volume of a specific height and

width around a dexel may represent a

rectangular finite solid strip. Such a volume is

called 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 solidification process, and

subsequently building voxels around the

dexels in the slice contours forms a layer.

This is a new approach to virtual

fabrication in that rectangular finite solid

strips, represented by the voxels built around

the dexels, are laid to form a layer, which is

subsequently stacked up to form a virtual

prototype. Figure 2 shows the process of

dexel-based VP of a gearbox housing.

2.2 Virtual prototypes for design

validation

Dexel-based virtual prototypes may represent

physical ones relatively more 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, RP may be a

convenient tool for direct production of

customized products, provided it can

fabricate prototypes of the required accuracy

and of appropriate materials. Indeed, some

researchers (Greul et al., 1995; Jeng et al.,

2000) 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 customized

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.

2.3 Visualisation of RP

The proposed VP system facilitates design

validation through visualisation of the RP

process and the resultant physical prototype.

Indeed, visualisation also helps the designer

understand what will possibly happen to a

particular part of the prototype. It is common

that not all features in the model are required

for a specific analysis (Gadh and Sonthi,

1998). 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.

2.3.1 Superimposition of product model on virtual

prototype

For this purpose, the proposed VP system can

display the 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.

Figure 1 Rectangular finite strip of solid built around a dexel




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2.3.2 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 spacing,

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 spacing. In fact, a curved surface can be

accurately produced only if the layer thickness

and the hatch spacing 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 spacing, 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 clearly the

effects on the prototype quality by changing

the process parameters. Subsequently, an

optimal set of process parameters may be

chosen quickly for efficient production of

prototypes.

3. The proposed dexel-based VP 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 on a CAD package, the system

simulates the characteristics of an RP process

to perform virtual fabrication of the product

prototype. The virtual prototype may then be

used in various analyses. As shown in Figure 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.

Figure 2 Dexel-based VP of a gearbox housing




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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 and the Part Fabricator.

The Model Viewer module 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 (Choi and Kwok, 2002). The Slicer

module slices the STL model to produce the

contour information of each layer, while the

Hatcher module performs hatching of all

layers to generate the laser/binder path.

Subsequently, the Part Fabricator module

reads in the hatch information and simulates

the fabrication process to form a virtual

prototype. It allows the designer to visualize

the process by displaying the modelled results,

such as the surface quality, with respect to the

process parameters. Indeed, it is vital to show

Figure 3 Flow of the proposed VP system




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the effect of different process parameters on

the prototype in real-time.

3.3 Visualisation and tuning of process

parameters

Once the simulation process is completed, the

designer can manipulate the virtual prototype

using the utilities provided to visualize 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 model 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 orientation of

the model, the layer thickness or the hatch

spacing may be changed accordingly.

3.4 Colour prototypes

It is well known that it is not easy to make

colour prototypes on RP machines. Recently,

a 3DP machine capable of producing colour

prototypes has been made commercially

available. During fabrication, binders in red,

yellow and blue are mixed to produce colours.

However, the colour mixing mechanism is

quite complicated in that each binder is

supplied by its own nozzle and tube. Most

importantly, the control software for the

machine has yet to be perfected to handle the

colours properly.

In comparison with physical prototyping, it

is relatively easy for the proposed VP system

to fabricate colour virtual prototypes. Indeed,

decorative patterns and designs may also be

achieved with dexel-based virtual prototypes.

To produce colour prototypes, the VP system

simply assigns a colour to each dexel.

However, the STL format is neutral in terms

of material properties and does not carry any

colour attribute. Therefore, a modified STL

format has recently been proposed to take

advantage of the two pad bytes that are

originally wasted in each facet. These two pad

bytes are now used in the modified STL

format to store colour indexes. The pad bytes

are divided into 16 bits. The first bit indicates

whether the model is mono-coloured. If it is,

then no colour will be assigned to individual

facets. Otherwise, each facet will carry a

colour represented by the blue, green and red

indexes stored in the remaining 15 bits. The

first five bits store the blue index; the next five

bits store the green index; and the last five bits

store the red index. Since each colour index

has 32 grey levels, ranging from 0 to 31, a

virtual prototype may have a maximum of

323 ¼ 32; 768 colours.

Although a valid format for the colour

prototypes is available, no modelling software

is found to be able to export a colour model to

the modified STL format. Fortunately, two

commercially available software packages,

namely the Magics RP (Magics) and the TNO

STL Painter, may convert a normal STL file

to the modified STL format. These packages

allow the user to selectively paint each feature

or facet of the STL model with a colour

desired, and subsequently saved as a colour

STL model.

The colour STL model is then sliced and

hatched to generate layer contours for virtual

fabrication. Each line segment of the layer

contours will carry the colour attribute of the

corresponding facet in the colour STL model,

which will subsequently be assigned to the

dexels to fabricate a colour virtual prototype.

4. Implementation

The proposed VP system has been

implemented in C++ language, with the

World Tool Kit (WTK) VR support libraries.

4.1 The Model Viewer

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. If 3D viewing is expected, the stereo

mode must be chosen. The system adopts a

semi-immersive VP interface (Weyrich and

Drews, 1999), 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 a 2D prototype is enough




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for analysis, the normal mode can be chosen,

and no emitter and glasses are needed.

4.2 The Slicer

The Slicer module consists of a slicing

algorithm 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 Figure 4(a) and (b).

The model in Figure 4(a) is sliced with the

normal un-offset approach, which is generally

adopted in most commercial RP systems. It

can be noticed that when the surface

converged in the upward direction, there is

excessive material at the edges in each layer.

Consequently, it leads to deformation and the

original round shape of the prototype

becomes oval, as shown in Figure 4(c). To

solve this problem, the offset slicing shown in

Figure 4(b) 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

Figure 4(d).

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 Hatcher

The Hatcher module processes the layer

contours in CLI format to determine the

coordinates of 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

located 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

Figure 4 The normal (un-offset) and offset slicing approaches




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contour is within the tolerance zone around

the hatch line.

4.4 The Part Fabricator

The Part Fabricator module simulates the

virtual fabrication process. It requires hatch

information to display the volume represented

by dexels. Rectangular solid strips will be

displayed one by one at an appropriate

z-height to simulate the solid material

solidified by the laser/binder head. As shown

in Figure 1, the length of a hatch line is the

distance of its end-points, while its height is

the layer thickness layers and its width is the

hatch spacing. When the virtual fabrication is

completed, the virtual prototype is displayed

and/or superimposed on the product model

for the visual inspection. Similar to the Model

Viewer module, the designer may choose the

stereo mode to view the virtual prototype in a

VR environment.

5. Case studies

Examples are now presented to demonstrate

how the system facilitates quality analyses and

parameter optimisation for RP process

planning. For clarification purposes,

parameters that produce relatively rough

texture were adopted for the fabrications.

5.1 A toy spider

Toy industry is one of the mainstream

industries today. Since it is essential for toy

industry to response quickly to market trends,

short time-to-market is a critical factor for

their success, and thereby, the VP system is

particularly useful for toy industry.

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. Figure 5 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 Figure 6, the surface texture and the

dimensional deviations were clearly

illustrated. The prisms indicated the excessive

material in fabrication, which is located

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 was excessive material at the

edges in 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 mm 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. Figure 7 shows the same

spiders with some pins on them. The pins

indicated the facets 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

whereas the green ones pointed to the

unacceptable deviations. If unsatisfactory

deviations were located at important parts of

the model, 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.

Here, a small layer thickness was used in the

second simulation to reduce the cusp heights.

Figure 8 shows a comparison of the two

virtual prototypes of different layer thickness.

It can be clearly shown that the excessive

material was less than before and at the same

time, the average and maximum cusp heights

were dropped to 0.241 mm and 0.511 mm,

respectively.

5.2 A dolphin

Figure 9 shows a STL model of a dolphin and

its virtual prototype. The model was sliced

with the offset slicing approach, and the

excessive materials were evenly distributed, as

shown in Figure 10. The average and the

maximum cusp heights of the dolphin were

0.151 mm and 0.301 mm, respectively.

Figure 10 also shows the superimposition of

the dolphin on its prototype, with deviations

beyond 0.190 mm highlighted. In order to

obtain a better accuracy, both the layer

thickness and the hatch spacing were reduced

in the second iteration.




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Figure 11 shows a comparison of the virtual

prototypes fabricated in the first and the

second iterations. It can be seen that the

excessive material was less than before and the

average and maximum cusp heights were

decreased to 0.100 mm and 0.199 mm,

respectively. The pins pointing at the

unacceptable deviations were also much lesser

in the second fabrication than that of the first

one. However, it was expected that the layer

thickness and the hatch spacing had to be

further reduced. This was because the fins of

the dolphin were too thin, as shown in

Figure 10. The fins might possibly break if

such parameters were adopted in the

subsequent physical fabrication. Figure 12

shows the virtual prototype fabricated in the

third iteration.

Figure 7 Areas of the spider with dimensional deviations beyond design limits

Figure 5 Spider and a dexel-based virtual prototype

Figure 6 Superimposition of the spider on its virtual prototype




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5.3 A hand skeleton

There are widespread applications of RP in

the medical field for making the prototypes of

human skeletons and organs. A hand

skeleton, as shown in Figure 13, was chosen as

an example, which was sliced with the offset

slicing approach. For a surgery application, a

patient’s hand may be scanned to produce a

prototype such that doctors may study the

injury or deformity more clearly. Suppose a

prosthesis has to be put into the patient’s

hand, the prototype helps the doctor choose

the size that would fit best to reduce the

possibility of mismatch. Figure 14 shows the

superimposition of the hand skeleton on its

virtual prototype. The average and maximum

cusp heights are 0.072 mm and 0.141 mm this

time and those deviations more than

0.140 mm are highlighted in Figure 15. It

seemed that there was a considerable amount

of unacceptable deviations. However, as the

total number of facets of the hand model was

over 110,000, the highlighted deviations

represent only a very small part of the

prototype. It appeared that most of the

deviations were located at the back of the

hand. Since the main part of the hand for the

surgery was the middle finger, this prototype

might be deemed good enough and physical

fabrication could be carried out using the

process parameters. In general, it may not be

necessary to aim at producing a perfect

prototype. Instead, an optimal prototype with

good accuracy at selected areas will be more

practical and economical.

5.4 Colour prototypes

Figure 16 shows two colour spiders from

different views. The one on the left was

formed from the modified STL file while the

other one was formed by the VP system. In

this illustration, it can be seen that the VP

Figure 8 Comparison of virtual prototypes with different layer thickness and

hatch spacing of a spider model

Figure 9 Dolphin and a dexel-based virtual prototype




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Figure 10 Comparison of dolphin and its virtual prototype

Figure 11 Comparison of virtual prototypes of the dolphin with different layer thickness and hatch spacing




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Figure 15 Most of the unacceptable deviations are not located at the fingersFigure 12 Dolphin prototype fabricated in the third iteration

Figure 13 Hand skeleton viewed from different angles

Figure 14 Superimposition of hand skeleton on its virtual prototype




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system performed very satisfactorily. Indeed,

something that may be difficult to achieve in

the real world may be relatively a simple task

in the VW.

6. Further development of the VP system

For RP processes that employ heat energy to

solidify/sinter the material, the subsequent

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, the

individual 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. Therefore, by

incorporating such finite element analysis

technique when 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.

Furthermore, to represent different RP

systems more closely, 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.

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Figure 16 Colour virtual prototype of spider




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Documents

A dexel‐based virtual prototyping system for product development