a multi-material virtual prototyping system

A multi-material virtual prototyping system

S.H. Choi*, H.H. Cheung

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

Received 9 January 2004; received in revised form 30 May 2004; accepted 1 June 2004

Abstract

This paper proposes a multi-material virtual prototyping system for digital fabrication of heterogeneous prototypes. It consists mainly of a

topological hierarchy-sorting algorithm for processing slice contours, and a virtual simulation system for visualisation and optimisation of

multi-material layered manufacturing (MMLM) processes. The topological hierarchy-sorting algorithm processes the hierarchy relationship

of complex slice contours. It builds a parent-and-child list that defines the containment relationship of the slice contours, and subsequently

arranges the contours in an appropriate sequence, which facilitates toolpath planning for MMLM by avoiding redundant tool movements.

The virtual simulation system simulates MMLM processes and provides stereoscopic visualisation of the resulting multi-material prototypes

for quality analysis and optimisation of the processes.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Multi-material; Layered manufacturing; Virtual prototyping; Visualisation; Toolpath planning

1. Introduction

1.1. Layered manufacturing

Layered manufacturing (LM), also called rapid

prototyping (RP), is an additive manufacturing process

that produces a physical prototype from a 3D CAD model

layer-by-layer. The CAD model can be generated from many

sources, including CAD designs and conversion data from a

3D scanner. LM systems are widely adopted in

manufacturing and medical applications to save costs and

time.

Manufacturers use LM technology to produce prototypes

of products for design evaluation, and as master patterns for

production tools. Lopez and Wright [1] studied the impact

of LM on product design and development processes.

The study shows that LM may help drastically to shorten

product development cycles and enable designers to easily

incorporate mechanical and ergonomic features into new

products.

In medical field, surgeons use LM to plan and explain

complex surgical operations, especially craniofacial and

maxillofacial surgeries. D’Urso et al. [2] described the use

of anatomical biomodels fabricated using LM for education

of patients, diagnosis, surgical planning, and navigation.

Brown et al. [3] used life-sized wax stereolithographic

prototypes of a patient’s pelvis and an interpositioning

template for accurate positioning of the fixation plate.

Harrysson et al. [4] used stereolithography apparatus to

build physical biomodels from computed tomography (CT)

data for planning and rehearsal of complex osteotomies of

severely deformed bones.

Some researchers have explored LM for direct

manufacture of biologically active implants. Das et al. [5]

used micro-computed tomography (micro-CT) technique

and selective laser sintering to fabricate Nylon-6 tissue

engineering scaffolds for human proximal femur trabecular

bones. They attempted to develop a multi-material

deposition system to fabricate tissue engineering scaffolds

with functionally graded material composition.

Currently, commercial LM machines can only produce

single-material prototypes [6,7]. However, there is an

increasing need for multi-material prototypes [8]. This is

because high value-added products often involve advanced

and complex designs, while medical operations are

becoming more complicated and delicate. A typical

example is tissue engineering scaffolds that have to be

tailor-made to fit the individual requirements of each patient

[9]. Indeed, a multi-material prototype that can clearly

differentiate one part from another of a product, or tissues

0010-4485//$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cad.2004.06.002

Computer-Aided Design 37 (2005) 123–136

www.elsevier.com/locate/cad

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

E-mail address: [email protected] (S.H. Choi).

http://www.elsevier.com/locate/cad

from blood vessels or bone structure of a human organ,

will be particularly useful for designers or surgeons,

respectively. Therefore, it would be very desirable and

useful to develop multi-material layered manufacturing

(MMLM) technology.

In general, a heterogeneous object may be composed

of either several materials with varying composition or a

collection of discrete materials [10,11]. Effective

representation of multi-material objects is a vital step

in the development of MMLM technology. Pratt et al.

[12] noted that there is an increasing need to transfer

additional types of information for development of LM

technology, especially material information. However,

a critical problem is that the current CAD models do not

contain any material information. Consequently, most

LM systems treat a model as homogeneous during

down-line process planning [13,14].

Although MMLM technology has attracted much

research interest, practical and viable MMLM machines

have yet to be developed. A MMLM machine may

include mainly two sub-systems, namely a hardware

material-depositing mechanism and a computer control

software system that can process multi-material slice

contours to control the material-depositing mechanism,

as shown in Fig. 1.

The development of MMLM technology is largely a

software issue. The material-depositing mechanism may

consist of an array of nozzles or tools, each of which

deposits a type of material on specific areas of a layer.

The tools may be bundled together or controlled to move

independently. They may be adapted from those used in the

existing single-material LM machines. However, a viable

computer software system for MMLM processes has yet to

be developed. The system should be able to generate

multi-material slice contours, which are subsequently sorted

to relate specific contours to a particular tool, such that the

area is appropriately deposited with the desired material.

Hence, processing of complex multi-material slice contours

and planning of multi-toolpaths are particularly important

issues. They facilitate control of tools to deposit selected

materials at the appropriate contours, and at the same time

to avoid redundant movements and potential collisions of

the tools which move otherwise independently to increase

efficiency.

This is, however, a daunting task because slice contours

are generally random in nature with no explicit topological

hierarchy relationship. As a result, it is very difficult to

identify one contour from another within a slice as required

for multi-toolpath planning. Therefore, it can be seen that

the difficulty of processing slice contours and the lack of

suitable control software largely hinder the development of

MMLM technology.

1.2. Virtual prototyping

Virtual prototyping (VP) is a process of using a digital

prototype, in lieu of a physical one, for 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 3D stereoscopic

simulation with innovative input and output. The purpose of

VP is to reduce the iterations of design-prototype-test

cycles. Integration of VP and LM allows a designer to

analyse and visualise the influence of process parameters on

the part quality. Once a VP process is finished, the resulting

digital prototype can be sent via the Internet to customers to

solicit comments, or the process parameters can be used for

optimal fabrication of the physical prototype. This reduces

the number of physical iterations and thereby the

associated manufacturing overheads, leading to faster and

cost-effective product development. Since digital prototypes

are mostly used in VP, there is no need to worry about the

costs and the quality of physical prototypes. However,

few researchers have exploited the strengths of VR to

facilitate LM. Gibson et al. [15] investigated the

contributions of VP and LM towards product development.

They suggested the use of VR as a complementary tool to

LM. Morvan and Fadel [16] linked VR with LM to visualize

the support structures of a part and to help a designer

identify improper support structures. Jee and Sachs [17]

Fig. 1. A layer of a multi-material part being fabricated.

S.H. Choi, H.H. Cheung / Computer-Aided Design 37 (2005) 123–136124

developed a visual simulation system for 3D printing.

However, their system was aimed at providing a visual tool

to examine surface texture only.

The authors have recently developed a VP system for

digital fabrication and visualisation of product prototypes in

a virtual environment [18]. The system allows design

validation and parameter optimisation of a LM process prior

to physical fabrication. By providing 3D stereoscopic

visualisation of the LM process and the resulting digital

prototype, a designer can analyse its quality more efficiently

and take prompt action to improve both the LM process and

the product design. Currently, this system can only simulate

single-material LM processes. As there is now an increasing

need for multi-material prototypes, it is natural and timely to

enhance this system for MMLM processes. The paper,

therefore, presents a multi-material virtual prototyping

(MMVP) system for digital fabrication of heterogeneous

prototypes. The MMVP system may be subsequently

adapted and integrated with a material deposition

mechanism to form a practical MMLM machine.

2. Review of the related works

The benefits of MMLM technology have been widely

recognised. Researchers have recently started working on

MMLM and they have made important contributions.

Their work has been categorically focused on a few areas,

particularly CAD representation of heterogeneous objects

for MMLM, development of experimental systems for

simple objects, and virtual simulation of MMLM processes.

2.1. CAD representation of heterogeneous objects

for MMLM

To fabricate multi-material prototypes with MMLM

technology, both geometric and material information must

be made available. Although STL is now a de facto

industrial standard file format for LM, it contains only

geometric information. Researchers have recently proposed

CAD formats for heterogeneous objects [10,19]. Chiu and

Tan [20] proposed a modified STL format in which a tree

structure was used to represent materials. Pratt [21]

conducted a survey on several methods, including voxel-

based modelling, mesh-based decomposition, and set-based

heterogeneous solid modelling approach, for representation

of material macrostructure. Patil et al. [14] reported the

possible use of R-function for representation of material

composition, and they proposed the rm-object model for

representation of heterogeneous solids. Biswas et al. [22]

proposed a representation scheme for fields defined over

geometric domains. Such proposed formats, when fully

developed and widely adopted, will be useful for MMLM.

However, there are still some major problems to solve.

Indeed, a significant proportion of complex objects,

particularly human organs and bone structures, etc. are not

designed using CAD systems. Instead, they are captured by

laser digitisers, or CT and magnetic resonance imaging

scanners. The digitised images are normally processed to

form a model in STL format or to extract slice contours,

which are random in nature without any explicit topological

hierarchy relationship. Hence, processing of such slice

contours for multi-toolpath planning remains a challenging

obstacle that has yet to be surmounted for the development

of MMLM.

2.2. Development of experimental MMLM machines

for simple objects

A few researchers have recently attempted to develop

experimental MMLM machines for simple objects. Weiss

et al. [23] described building multi-material structures with

pre-fabricated parts, such as advanced tooling and

embedded electronic devices. Qiu and Langrana [11]

reported the development of a MMLM machine for

fabrication of multi-phase electromechanical components

for naval and other military applications. Simple hollowed

circular objects were fabricated. Bondi et al. [24] developed

a laser chemical vapor deposition system, which was used

primarily to deposit carbon to form small 3D and laminated

structures. It could be extended to fabricate multi-material

objects if advanced control software for the system was

available. These systems were among the pioneering

work, although they could only handle relatively simple or

small parts.

2.3. Virtual simulation of MMLM

Cheng and Langrana [13] and Qiu et al. [6] described

a virtual simulator used to simulate fabrication of

multi-material prototypes. Using computer graphics

technology, the simulator could detect and remove errors

of MMLM processes quickly. Zajtchuk and Satava [30]

used VR technology for medical applications. At the US

Naval Research Laboratory (NRL), VR visualisation was

used in a wide range of projects [25]. Although the

applications are focused on relatively simple objects, they

had highlighted the usefulness of virtual simulation of

MMLM.

3. The proposed multi-material virtual prototyping

system

The above pioneering work has made significant

contributions to the development of MMLM technology.

However, there is a lack of integrated effort to tackle the

core software problems of slice contour processing and

multi-toolpath planning for fabrication of complex objects.

Therefore, a multi-material virtual prototyping (MMVP)

system is proposed to fabricate digital multi-material

prototypes. The system consists mainly of a topological

S.H. Choi, H.H. Cheung / Computer-Aided Design 37 (2005) 123–136 125

hierarchy-sorting algorithm for processing slice contours,

and a virtual simulation system for stereoscopic

visualisation and optimisation of MMLM processes.

The topological hierarchy-sorting algorithm constructs the

hierarchy relationship of complex slice contours for

subsequent fabrication of multi-material prototypes [26].

In particular, the explicit hierarchy relationship

facilitates optimisation of toolpaths by avoiding redundant

tool movements. The virtual simulation system uses a

dexel-based approach for digital fabrication of prototypes

[18,27]. A dexel is a hatch vector representing the path that a

tool has to follow within a contour to build a portion of a

layer. By building a volume of a specific height and

width around a dexel, a strip of material may be represented.

Hence, during a digital fabrication process, rectangular

solid strips are laid to form a layer, which is subsequently

stacked up to form a prototype.

Fig. 2 shows the flow of the proposed system. It includes

colouring and slicing the STL model of a multi-material

object, processing and sorting of slice contours,

Fig. 2. Flow of the proposed MMVP system.


generation of multi-toolpaths, and digital fabrication of

multi-material prototypes.

3.1. Representation of multi-material objects

by colour STL models

To build a multi-material prototype, both geometric

and material information should be provided. In the

proposed system, colour STL models are used to

represent multi-material objects. A colour STL model

takes advantage of the two pad bytes that are spared in

each facet in the original monochrome STL format to

store colour indices. The two pad bytes are divided into

16 bits. The first bit indicates whether or not the model

is monochrome. 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 indices

stored in the remaining 15 bits. The first 5 bits store the

blue index; the next 5 bits store the green index; the last

5 bits store the red index. Since each colour index has 32

grey levels, ranging from 0 to 31, a digital prototype

may have a maximum of 323 ¼ 32,768 colours, each of

which represents a type of material. Thus, a colour STL

model contains both geometric and material information.

A software package for colouring monochrome STL

models to associate material information has been

developed as an integral part of the proposed system.

Commercially available software systems, such as

Magics RP [28] and FlashTL [29] can also be used to

paint colour STL models. When a colour STL model is

completed, it is sliced to generate multi-material slice

contours that are outputted in a modified common

layered interface file format [26]. A colour representing

a particular type of material is assigned to the related

layer contours.

3.2. Multi-toolpath planning using topological hierarchy

relationship

To fabricate a layer of a multi-material prototype, a set of

tools or nozzles are controlled to deposit specific materials

on the appropriate slice contours. For each tool to

effectively and efficiently deposit a specific material on

the related contours, it is necessary to: (i) identify and relate

specific contours of a slice to a particular tool; and

(ii) arrange the toolpaths of each tool in an appropriate

sequence to shorten build time and to avoid possible

collisions. Therefore, multi-toolpath planning is particularly

important for the subsequent control of tools to deposit

selected materials at the appropriate contours, and at the

same time to detect and avoid possible collisions of the tools,

which move otherwise independently to increase efficiency.

However, the geometry of most STL models tend to be very

complex, and the slice contours do not possess any explicit

topological hierarchy relationship. As a result, it is very

difficult to associate specific contours with a particular tool.

Hence, it is very important to establish the hierarchy

relationship of slice contours. Therefore, this paper proposes

a virtual system for MMLM, which uses a topological

hierarchy-sorting algorithm to process complex slice

contours for multi-toolpath planning. The algorithm first

establishes the hierarchy relationship representing the

connectivity of solid contours and the related cavities,

and then arranges the contours in an appropriate sequence

for subsequent planning of multi-toolpaths. The planning

process is based on two major issues.

1. Identification of slice contours for association with the

related tools.

2. Sequential control of tools to avoid redundant

movements and possible collisions.

A layer of a mechanical gearbox is shown in Fig. 3a.

It has nine contours of four kinds of materials. Four nozzles,

N1 to N4, are assigned to deposit blue, red, yellow, and green

materials on the related contours, respectively. Originally,

the contours are random with no explicit topological

hierarchy relationship. Toolpath planning based on random

contours will result in a considerable amount of redundant

tool movements, as shown in Fig. 3b. In order to optimise

the toolpaths, the contours of the same material should be

appropriately arranged such that the toolpaths will be the

shortest possible. This requires establishment of topological

hierarchy relationship of the slice contours. To do so,

the proposed algorithm first builds a parent-and-child list

that defines the containment relationship of the slice

contours, and subsequently arranges the contours in an

appropriate sequence.

Referring to Fig. 3c, the contours are sorted into five

hierarchy levels. The level number of a contour indicates its

number of parents. For instance, C1, C2, C3, C4, and C6

are Level 0 contours; they have no parent and are therefore

not contained in any other contours. Similarly, a Level 1

contour has one and only one parent, although a parent

can have multiple children. Hence, according to the

parent-and-child list, there are five contour entities, namely

C1, C2, C3, C4, and C6 ! C7 ! C8 ! C5 ! C9. Based on

the relationship of external (solid) and internal (void)

contours, the contour entities can be further grouped into

seven contour families, namely C1 (Family 1), C2 (Family

2), C3 (Family 3), C4 (Family 4), C6 ! C7 (Family 5),

C8 ! C5 (Family 7), and C9 (Family 9). Each contour

family has a specific material property.

Thus, with the hierarchy relationship, it is no longer

necessary to identify and relate contours to a particular

nozzle one-by-one for multi-toolpath planning. Indeed,

only grouping of the outermost contours is required.

For instance, the seven contour families of the gearbox

layer can be treated as seven independent inputs to generate

hatch vectors that define the multi-toolpaths. Hence, seven

toolpaths (PC1, PC2, PC3, PC4, PC6,7, PC8,5, and PC9) can be

generated accordingly. However, these seven toolpaths


should be further arranged in accordance with the specific

geometric and material information of each contour family,

in order to avoid redundant tool movements. In other words,

each nozzle should deposit the required material on all

related contours before returning to its datum position.

Referring to Fig. 3c and d, the seven toolpaths can be easily

grouped. PC1, PC2, and PC3 are grouped into a toolpath-set

{S1 (PC1 ! PC2 ! PC3)}, which is associated with nozzle

N1 because contour families 1, 2, and 3 are of blue material

property. In the same way, PC6,7 and PC8,5 are grouped into a

toolpath-set {S3 (PC6,7 ! PC8,5)} for association with

nozzle N3. Finally, PC4 and PC9 are grouped into

Fig. 3. (a) A multi-material slice of a gearbox with multiple-inclusion contours. (b) Multi-toolpaths with redundant tool movements based on unsorted slice

contours. (c) Topological hierarchy relationship of the gearbox slice contours in (a). (d) Multi-toolpath planning using topological hierarchy relationship.

(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


{S2 (PC4)} and {S4 (PC9)}, which are associated with

nozzles N2 and N4, respectively, as shown in Fig. 3d.

This process considerably simplifies the association of

toolpaths with the appropriate nozzles.

However, the toolpaths in each toolpath-set are still

random, because the contours at the same hierarchy level

are random in nature. This may cause redundant tool

movements when the associated nozzle moves from the exit

point of one toolpath to the entry point of another.

Therefore, based on the bounding-box criterion,

the algorithm further sorts the contours at the same

hierarchy level into an appropriate sequence. Contours

with the same level number are arranged according to their

minimum and maximum ðx; yÞ values. The sorted sequence

facilitates optimisation of multi-toolpaths by avoiding

redundant movements. For example, the three toolpaths in

the toolpath-set {S1 (PC1 ! PC2 ! PC3)} are generated for

contours C1, C2, and C3, respectively. They are all at the

same level (Level 0) and the sequence is sorted as

C1 ! C3 ! C2. Accordingly, the toolpaths in S1 are

arranged and linked as {S1 (PC1 ! PC3 ! PC2)}. If the

contours in a toolpath-set are at different hierarchy levels,

the contours at the highest level (with the smallest level

number) are deposited first. In other words, material

deposition starts inwards from the outermost contours.

For example, the two contour families (C6 ! C7) and

(C8 ! C5) in the toolpath-set S3 are at different hierarchy

levels. The contour family (C6 ! C7) is deposited before

(C8 ! C5) because the level of (C6 ! C7) is higher than that

of (C8 ! C5). Hence, the toolpaths in S3 are arranged and

linked as {S3 (PC6,7 ! PC8,5)}. Similarly, the toolpaths in

the remaining toolpath-sets are arranged and linked

as shown in Fig. 3d. According to the hierarchy levels, the

toolpath-sets are subsequently arranged in a sequence of

S1 ! S2 ! S3 ! S4; the movement sequence of the nozzles

is given as N1 ! N2 ! N3 ! N4. Therefore, practical

sequential multi-toolpaths for MMLM can be easily

generated. Fig. 4a shows, in comparison with the

unsorted toolpaths, how these multi-toolpaths can reduce

build time by avoiding redundant tool movements.

Digital fabrication of the slice contours in Fig. 3a is

shown in Fig. 4b. As slice contours become more

complicated, time saving from avoiding redundant tool

movements will become more substantial. Thus, the

importance of topological hierarchy relationship for

multi-toolpath planning is prominent. For an ideal case,

concurrent toolpaths should be generated, which will be

studied in the future.

Fig. 3 (continued )


3.3. Digital fabrication of multi-material prototypes

After processing slice contours and multi-toolpath

planning, the proposed system simulates a MMLM process

to fabricate a digital multi-material prototype.

Subsequently, it provides vivid stereoscopic visualisation

of the resulting prototype for quality analysis and

optimisation of the MMLM process. The designer can

Fig. 4. (a) Sequential multi-toolpaths without redundant tool movements. (b) Digital fabrication of gearbox layer using sequential multi-toolpaths.


manipulate the prototype using the utilities provided to

visualise the quality of the product prototype that a MMLM

machine will subsequently deliver. The designer can also

navigate around the internal and opaque structures of the

prototype to investigate the product design. Furthermore,

the digital prototype may be superimposed on its STL model

to 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 orientation of the model,

Fig. 6. A layer of complex slice contours with topological hierarchy relationship.

Fig. 5. Monochrome and colour STL models of the ring. (For interpretation

of the references to colour in this figure legend, the reader is referred to the

web version of this article.)

Fig. 7. Nozzles move sequentially to deposit materials on related contours.


the layer thickness, or the hatch space, may be changed

accordingly.

4. Case studies

4.1. A jewellery product

Hong Kong is a leading exporter of jewellery products.

As these products get more fashion-oriented, innovative

designs become more important. Some jewellery

manufacturers have realised that they should not only rely

on individual skills; they have recently adopted LM

technology to develop high value-added jewellery items.

Hence, the proposed MMVP system will be particularly

useful for building digital multi-material prototypes to help

improve designs and shorten development cycles of

jewellery products at competitive costs. Designers can

view and evaluate their designs with digital prototypes,

instead of physical ones, at minimal costs and time possible.

Fig. 8. Digital fabrication process of a multi-material prototype of the ring.


Furthermore, digital prototypes may be conveniently

transmitted over the Internet to facilitate global

manufacturing.

A ring in Fig. 5 was therefore chosen as an example to

illustrate how the proposed system fabricates digital

multi-material prototypes of jewellery products. The ring

is made of 13 kinds of materials, and its shape is complex.

To produce such multi-material prototypes, an array of

nozzles may be used to extrude materials on the relevant

slice contours in a layer. The topological hierarchy-sorting

algorithm constructs the hierarchy relationship of complex

slice contours for optimisation of multi-toolpaths by

avoiding redundant nozzle movements. The case study

illustrates how the system generates efficient sequential

toolpaths of each nozzle, and subsequently facilitates

quality analysis and optimisation of MMLM processes.

Fig. 9. Ring prototype superimposed on STL model to highlight dimensional deviations. (For interpretation of the references to colour in this figure legend,

the reader is referred to the web version of this article.)

Fig. 10. Colour STL model of a pelvis. (For interpretation of the references

to colour in this figure legend, the reader is referred to the web version of

this article.)


Based on a monochrome STL model of the ring in Fig. 5a,

a colour STL model in Fig. 5b is created by assigning

colours to represent the appropriate materials of the ring.

The colour STL model is sliced to generate slice contours

that contain both geometric and material information.

The layers are relatively rough for clarification purpose.

Fig. 6 shows a layer of the ring. There are totally 52 contours

that are sorted into four hierarchy levels and are grouped

into 34 contour families. Each family has its own material

property. The layer consists of 11 discrete materials.

The contour families with the same material property are

grouped into a set. The 34 contour families are grouped into

11 sets because the layer is made of 11 kinds of materials.

For each set of contour families, toolpaths are generated and

then arranged in an appropriate sequence. Each toolpath-set

is used to control a specific nozzle that extrudes a material

on the related set of contour families. Fig. 7 shows a layer of

the digital fabrication process of the ring prototype.

Each nozzle moves sequentially and deposits a material

on the related set of contour families in the layer by avoiding

redundant movements. More details of the digital

fabrication process of the ring are shown in Fig. 8.

The system also provides stereoscopic visualisation of the

resulting prototype for quality analysis and optimisation of

the MMLM process. Fig. 9 shows the ring prototype being

superimposed on its STL model. The surface texture and the

dimensional deviations are clearly illustrated. In addition, the

system calculates the cusp heights to evaluate the overall

dimensional deviations. In this case, the average and

the maximum cusp heights are 0.097 and 0.176 mm,

Fig. 11. (a) Superimposition of pelvis prototype on its STL model. (b) Pelvis prototype highlighting dimensional deviations beyond accuracy limit.


respectively. Suppose that any deviations more than

0.175 mm are considered unacceptable, the designer may

choose to highlight the areas which are out of the design limit

for subsequent investigation of these important features. The

excessive deviations are highlighted with red or green pins.

The red pins point to the maximum deviations whereas the

green ones point to unacceptable deviations. If unsatisfactory

deviations are located at important parts of the model, the

designer may choose either to change the model orientation

to shift the deviations or to reduce the layer thickness and the

hatch space to improve the accuracy.

4.2. A pelvis

Multi-material prototypes are particularly useful for

visualisation and identification of complex bone structures

to facilitate study and planning of surgical operations.

Therefore, a pelvis model, as shown in Fig. 10, was chosen

to demonstrate possible applications of the MMVP system in

the medical field. For a complex surgery, a patient’s pelvis

was scanned to produce a prototype so that surgeons can

study the bone fractures or injury more clearly. A prothesis or

an inter-positioning template may have to be put into the

patient’s pelvis. A multi-material digital prototype of the

pelvis will be helpful for surgeons to study the bone

structures more clearly. Also, visualisation of the pelvis

prototype and its superimposition on the STL model, as

shown in Fig. 11a, will assist surgeons to select a prothesis

that will fit best to minimise mismatch. It can be seen that the

excessive material is evenly distributed, and the average and

maximum cusp heights are 1.181 and 2.689 mm,

respectively.

Fig. 11b shows the pelvis prototype highlighting devi-

ations over a chosen accuracy limit. Based on the deviation

analysis, surgeons can adjust the process parameters, such as

orientation and layer thickness, to enhance the accuracy of

the target regions. If the prototype is deemed satisfactory,

physical fabrication can be carried out using the process

parameters accordingly. In general, it may not be necessary

to aim at producing a perfect prototype. Instead, an optimal

prototype with good accuracy in target regions will be more

practical and economical.

5. Conclusion

This paper proposes a MMVP system that processes

complex slice contours for planning and validation of

toolpaths for subsequent control of nozzles or tools to

fabricate heterogeneous prototypes. Indeed, multi-toolpath

planning plays a significant role in the development of

practical MMLM machines. The proposed system uses a

topological hierarchy-sorting algorithm to process the

hierarchy relationship of complex slice contours. It builds

a parent-and-child list, and subsequently arranges the

contours in an appropriate sequence. With the topological

hierarchy relationship, sequential multi-toolpaths can be

planned to control nozzles or tools to deposit selected

materials on the appropriate contours by avoiding redundant

movements. Besides, the proposed system simulates a

MMLM process to fabricate digital multi-material proto-

types. Subsequently, it provides vivid stereoscopic

visualisation of the resulting prototypes for quality analysis

and optimisation of the MMLM process. The system can be

adapted and integrated with a material deposition mechan-

ism to form a practical MMLM machine.

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. Thanks are also due to Dr Panos

Diamantopoulos of the University of Sussex for permission

to use the monochrome pelvis model.

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S.H. Choi is an 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 CADCAM, advanced manu-

facturing systems and virtual prototyping

technology.

H.H. Cheung gained his B.Eng. degree from

the IMSE Department at the University of

Hong Kong. He continued his postgraduate

research study in the Department, and

his research interest is in virtual prototyping

technology.


http://www.materialise.com/magics-rp/main_ENG.html

http://www.ind.tno.nl

a multi-material virtual prototyping system

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