1. INTRODUCTION
Prototyping or model making is one of the important steps to finalize a product design. It
helps in conceptualization of a design. Before the start of full production a prototype is
usually fabricated and tested. Manual prototyping by a skilled craftsman has been an ageold
practice for many centuries. Second phase of prototyping started around mid-1970s,
when a soft prototype modeled by 3D curves and surfaces could be stressed in virtual
environment, simulated and tested with exact material and other properties. Third and the
latest trend of prototyping, i.e., Rapid Prototyping (RP) by layer-by-layer material
deposition, started during early 1980s with the enormous growth in Computer Aided
Design and Manufacturing (CAD/CAM) technologies when almost unambiguous solid
models with knitted information of edges and surfaces could define a product and also
manufacture it by CNC machining. The historical development of RP and related
technologies is presented in table 1.
Year of inception Technology
1770 Mechanization
1946 First computer
1952 First Numerical Control (NC) machine tool
1960 First commercial laser
1961 First commercial Robot
1963 First interactive graphics system (early version of Computer
Aided Design)
1988 First commercial Rapid Prototyping system
Table 1: Historical development of Rapid Prototyping and related technologies
(after Chua and Leong, 2000)
2. BASIC PRINCIPLE OF RAPID PROTOTYPING PROCESSES
RP process belong to the generative (or additive) production processes unlike subtractive
or forming processes such as lathing, milling, grinding or coining etc. in which form is
shaped by material removal or plastic deformation. In all commercial RP processes, the
part is fabricated by deposition of layers contoured in a (x-y) plane two dimensionally. The
third dimension (z) results from single layers being stacked up on top of each other, but not
as a continuous z-coordinate. Therefore, the prototypes are very exact on the x-y plane
but have stair-stepping effect in z-direction. If model is deposited with very fine layers,
i.e., smaller z-stepping, model looks like original. RP can be classified into two
fundamental process steps namely generation of mathematical layer information and
2
generation of physical layer model. Typical process chain of various RP systems is shown
in figure 1.
Figure 1: RP process chain showing fundamental process steps
It can be seen from figure 1 that process starts with 3D modeling of the product and then
STL file is exported by tessellating the geometric 3D model. In tessellation various
surfaces of a CAD model are piecewise approximated by a series of triangles (figure 2) and
co-ordinate of vertices of triangles and their surface normals are listed. The number and
size of triangles are decided by facet deviation or chordal error as shown in figure 2. These
STL files are checked for defects like flip triangles, missing facets, overlapping facets,
dangling edges or faces etc. and are repaired if found faulty. Defect free STL files are used
as an input to various slicing softwares. At this stage choice of part deposition orientation
is the most important factor as part building time, surface quality, amount of support
structures, cost etc. are influenced. Once part deposition orientation is decided and slice
thickness is selected, tessellated model is sliced and the generated data in standard data
formats like SLC (stereolithography contour) or CLI (common layer interface) is stored.
This information is used to move to step 2, i.e., generation of physical model. The software
that operates RP systems generates laser-scanning paths (in processes like
Stereolithography, Selective Laser Sintering etc.) or material deposition paths (in processes
like Fused Deposition Modeling). This step is different for different processes and depends
on the basic deposition principle used in RP machine. Information computed here is used
to deposit the part layer-by-layer on RP system platform. The generalized data flow in RP
is given in figure 3.
3
3D CAD
2D CAD Drawing
/ Manual outline/
Lattice Data
Point cloud data Data acquired from
MRI or CT scan
Reverse Engineering
3D reconstruction
2.5D reconstruction
3D CAD
STL (3D)
Layer information, SCL or CLI
Geometric data
Auxiliary geometry (supports etc.)
Process parameters
Machine parameters
Machine data set
Specification of
machine layer
information
Figure 2: Tessellation of a typical surface of CAD model (after Pandey et al. 2003b)
The final step in the process chain is the post-processing task. At this stage, generally some
manual operations are necessary therefore skilled operator is required. In cleaning, excess
elements adhered with the part or support structures are removed. Sometimes the surface of
the model is finished by sanding, polishing or painting for better surface finish or aesthetic
appearance. Prototype is then tested or verified and suggested engineering changes are
once again incorporated during the solid modeling stage.
Figure 3: Generalized illustration of data flow in RP (after Gebhardt, 2003)
3. RAPID PROTOTYPING PROCESSES
The professional literature in RP contains different ways of classifying RP processes.
However, one representation based on German standard of production processes classifies
RP processes according to state of aggregation of their original material and is given in
figure 4.
Figure 4: Classification of RP processes (after Gebhardt, 2003)
Here, few important RP processes namely Stereolithography (SL), Selective Laser
Sintering (SLS), Fused Deposition Modeling (FDM) and Laminated Object Manufacturing
(LOM) are described.
3.1. Stereolithography
In this process photosensitive liquid resin which forms a solid polymer when exposed to
ultraviolet light is used as a fundamental concept. Due to the absorption and scattering of
beam, the reaction only takes place near the surface and voxels of solid polymeric resin are
formed. A SL machine consists of a build platform (substrate), which is mounted in a vat
of resin and a UV Helium-Cadmium or Argon ion laser. The laser scans the first layer and
platform is then lowered equal to one slice thickness and left for short time (dip-delay) so
that liquid polymer settles to a flat and even surface and inhibit bubble formation. The new
Generative Manufacturing Processes
Melting and resolidification
Selective Laser
Sintering
Solidification by
binder
3D Printing
Solid
Cutting and gluing
Layer Laminated
Manufacturing
Cutting and
polymerization
Solid Foil
Polymerization
Wire
One or multicomponent
powder
Foil
Melting and resolidification
Fused Layer Modeling
Ballistic Part
Manufacturing
Paste
Polymerization
Paste
Polymerization
Process
Liquid
Polymerization
Gaseous
Chemical
reaction
LCVD
Heat
Thermal
Polymerization
Lamp
Solid
Ground
Curing
Light of one frequency
Laser beam
Stereolithography
Holography
Holographic Interference
Solidification
slice is then scanned. Schematic diagram of a typical Stereolithography apparatus is shown
in figure 5.
In new SL systems, a blade spreads resin on the part as the blade traverses the vat. This
ensures smoother surface and reduced recoating time. It also reduces trapped volumes
which are sometimes formed due to excessive polymerization at the ends of the slices and
an island of liquid resin having thickness more than slice thickness is formed (Pham and
Demov, 2001). Once the complete part is deposited, it is removed from the vat and then
excess resin is drained. It may take long time due to high viscosity of liquid resin. The
green part is then post-cured in an UV oven after removing support structures.
Figure 5: Stereolithography (after Pham and Demov, 2001)
Overhangs or cantilever walls need support structures as a green layer has relatively low
stability and strength. These overhangs etc. are supported if they exceed a certain size or
angle, i.e., build orientation. The main functions of these structures are to support
projecting parts and also to pull other parts down which due to shrinkage tends to curl up
(Gebhardt, 2003). These support structures are generated during data processing and due to
these data grows heavily specially with STL files, as cuboid shaped support element need
information about at least twelve triangles. A solid support is very difficult to remove later
and may damage the model. Therefore a new support structure called fine point was
developed by 3D Systems (figure 6) and is company s trademark.
Build strategies have been developed to increase build speed and to decrease amount of
resin by depositing the parts with a higher proportion of hollow volume. These strategies
are devised as these models are used for making cavities for precision castings. Here walls
are designed hollow connected by rod-type bridging elements and skin is introduced that
close the model at the top and the bottom. These models require openings to drain out
uncured resin.
Figure 6: Fine point structure for Stereolithography (after Gebhardt, 2003)
3.2. Selective Laser Sintering
In Selective Laser Sintering (SLS) process, fine polymeric powder like polystyrene,
polycarbonate or polyamide etc. (20 to 100 micrometer diameter) is spread on the substrate
using a roller. Before starting CO2 laser scanning for sintering of a slice the temperature of
the entire bed is raised just below its melting point by infrared heating in order to minimize
thermal distortion (curling) and facilitate fusion to the previous layer. The laser is
modulated in such away that only those grains, which are in direct contact with the beam,
are affected (Pham and Demov, 2001). Once laser scanning cures a slice, bed is lowered
and powder feed chamber is raised so that a covering of powder can be spread evenly over
the build area by counter rotating roller. In this process support structures are not required
as the unsintered powder remains at the places of support structure. It is cleaned away and
can be recycled once the model is complete. The schematic diagram of a typical SLS
apparatus is given in figure 7.
3.3. Fused Deposition Modeling
In Fused Deposition Modeling (FDM) process a movable (x-y movement) nozzle on to a
substrate deposits thread of molten polymeric material. The build material is heated
slightly above (approximately 0.5 C) its melting temperature so that it solidifies within a
very short time (approximately 0.1 s) after extrusion and cold-welds to the previous layer
as shown in figure 8. Various important factors need to be considered and are steady
nozzle and material extrusion rates, addition of support structures for overhanging features
and speed of the nozzle head, which affects the slice thickness. More recent FDM systems
include two nozzles, one for part material and other for support material. The support
material is relatively of poor quality and can be broken easily once the complete part is
deposited and is removed from substrate. In more recent FDM technology, water-soluble
support structure material is used. Support structure can be deposited with lesser density as
compared to part density by providing air gaps between two consecutive roads.
Figure 7: Selective Laser Sintering System
Figure 8: Fused Deposition Modeling Process (after Pham and Demov, 2001)
3.4. Laminated Object Manufacturing
Typical system of Laminated Object Manufacturing (LOM) has been shown in figure 9. It
can be seen form the figure that the slices are cut in required contour from roll of material
by using a 25-50 watt CO2 laser beam. A new slice is bonded to previously deposited slice
by using a hot roller, which activates a heat sensitive adhesive. Apart from the slice
unwanted material is also hatched in rectangles to facilitate its later removal but remains in
place during the build to act as supports. Once one slice is completed platform can be
lowered and roll of material can be advanced by winding this excess onto a second roller
until a fresh area of the sheet lies over the part. After completion of the part they are sealed
with a urethane lacquer, silicone fluid or epoxy resin to prevent later distortion of the paper
prototype through water absorption.
Figure 9: Laminated Object Manufacturing Process
In this process, materials that are relatively cheaper like paper, plastic roll etc. can be used.
Parts of fiber-reinforced glass ceramics can be produced. Large models can be produced
and the building speed is 5-10 times as compared to other RP processes. The limitation of
the process included fabrication of hollow models with undercuts and reentrant features.
Large amount of scrap is formed. There remains danger of fire hazards and drops of the
molten materials formed during the cutting also need to be removed (Pham and Demov,
2001).
4. APPLICATIONS OF RP TECHNOLOGIES
RP technology has potential to reduce time required from conception to market up to 10-50
percent (Chua and Leong, 2000) as shown in figure 10. It has abilities of enhancing and
improving product development while at the same time reducing costs due to major
breakthrough in manufacturing (Chua and Leong, 2000). Although poor surface finish,
limited strength and accuracy are the limitations of RP models, it can deposit a part of any
degree of complexity theoretically. Therefore, RP technologies are successfully used by
various industries like aerospace, automotive, jewelry, coin making, tableware, saddletrees,
biomedical etc. It is used to fabricate concept models, functional models, patterns for
investment and vacuum casting, medical models and models for engineering analysis
(Pham and Demov, 2001). Various typical applications of RP are summarized in figure 11.
Figure 10: Result of introduction of RP in design cycle (after Chua and Leong, 2001)
Product
design
Part
drawing
Tool
design
Tool
manufacturing
Assembly and
test
Function
testing
Fixtures
special tools
Document
brochure
100 percent
Product
design
Tool
design
Tool
manufacturing
Fixtures special
tools
Document
brochure
Function
testing
RP Model
RP tools
and pattern
Assembly
10 to 50 percent
Small sized production Full production
Time
Cost
Time and cost saving by
New way in model
manufacturing
5. PART DEPOSITION PLANNING
A defect less STL file is used as an input to RP software like QuickSilce or RPTools for
further processing. At this stage, designer has to take an important decision about the part
deposition orientation. The part deposition orientation is important because part accuracy,
surface quality, building time, amount of support structures and hence cost of the part is
highly influenced (Pandey et al., 2004b). In this section various factors influencing
accuracy of RP parts and part deposition orientation are discussed.
5.1. Factors influencing accuracy
Accuracy of a model is influenced by the errors caused during tessellation and slicing at
data preparation stage. Decision of the designer about part deposition orientation also
affects accuracy of the model.
Errors due to tessellation: In tessellation surfaces of a CAD model are approximated
piecewise by using triangles. It is true that by reducing the size of the triangles, the
deviation between the actual surfaces and approximated triangles can be reduced. In
practice, resolution of the STL file is controlled by a parameter namely chordal error or
facet deviation as shown in figure 2. It has also been suggested that a curve with small
radius (r) should be tessellated if its radius is below a threshold radius (ro) which can be
considered as one tenth of the part size, to achieve a maximum chordal error of (r/ro) .
Value of can be set equal to 0 for no improvement and 1 for maximum improvement.
Here part size is defined as the diagonal of an imaginary box drawn around the part and
is angle control value (Williams et al., 1996).
Errors due to slicing: Real error on slice plane is much more than that is felt, as shown in
figure 12(a). For a spherical model Pham and Demov (2001) proposed that error due to the
replacement of a circular arc with stair-steps can be defined as radius of the arc minus
length up to the corresponding corner of the staircase, i.e., cusp height (figure 12 (b)). Thus
maximum error (cusp height) results along z direction and is equal to slice thickness.
Therefore, cusp height approaches to maximum for surfaces, which are almost parallel
with the x-y plane. Maximum value of cusp height is equal to slice thickness and can be
reduced by reducing it; however this results in drastic improvement in part building time.
Therefore, by using slices of variable thicknesses (popularly known as adaptive slicing, as
shown in figure 13), cusp height can be controlled below a certain value.
Except this, mismatching of height and missing features are two other problems resulting
from the slicing. Although most of the RP systems have facility of slicing with uniform
thickness only, adaptive slicing scheme, which can slice a model with better accuracy and
surface finish without loosing important features must be selected. Review of various
slicing schemes for RP has been done by Pandey et al. (2003a).
5.2. Part building
During part deposition generally two types of errors are observed and are namely curing
errors and control errors. Curing errors are due to over or under curing with respect to
curing line and control errors are caused due to variation in layer thickness or scan position
Figure 11(a): Typical application areas of RP parts (after Chual and Leong, 2000)
(b) SL model with the resection template Silicon implant molded from a tool
(after Pham and Demov, 2001)
Figure 11: Applications of RP processes
Rapid Prototyping
Finishing Processes
Cutting, milling, Lathe, boring, grinding etc.
Sand blasting Coating
Polishing Painting
Applications
Design
CAD model Verification
Visualizing object
Proof of concept
Marketing and presenting
model
Manufacturing and tooling
Plastic mold parts
o Vacuum casting
o Metal spraying
Casting
o Sand casting
o Investment casting
o Die casting
EDM electrodes
Master models
Engineering, Analysis and
planning
Form and fit models
Flow analysis
Stress distribution
Mock-up
Diagnostic and surgical
operation planning
Design and fabrication of
custom prosthesis and
implant
Industries
Aerospace Automotive Biomedical Jewelry
Coin Tableware Consumer electronic
Home appliances etc.
control. Figures 14 illustrate effect of over curing on part geometry and accuracy.
Adjustment of chamber temperature and laser power is needed for proper curing.
Calibration of the system becomes mandatory to minimize control errors. Shrinkage also
causes dimensional inaccuracy and is taken care by choosing proper scaling in x, y and z
directions. Polymers are also designed to have almost negligible shrinkage factors. In SL
and SLS processes problem arises with downward facing layers as these layers do not have
a layer underneath and are slightly thicker, which generate dimensional error. If proper
care is not taken in setting temperatures, curling is frequently observed.
(a) Real error slice plane (after Pandey et
al., 2003a)
(b) Error due to replacement of arcs with
stair-steps, cusp height (after Pham
and Demov, 2001)
Figure 12: slicing error
Figure 13: Slicing of a ball, (a) No slicing (b) Thick slicing (c) This slicing
(d) Adaptive slicing (after Pham and Demov, 2001)
5.3. Part finishing
Poor surface quality of RP parts is a major limitation and is primarily due to staircase
effect. Surface roughness can be controlled below a predefined threshold value by using
an adaptive slicing (Pandey et al., 2003b). Further, the situation can be improved by
finding out a part deposition orientation that gives minimum overall average part surface
roughness (Singhal et al., 2005). However, some RP applications like exhibition models,
tooling or master pattern for indirect tool production etc. require additional finishing to
improve the surface appearance of the part. This is generally carried by sanding and
polishing RP models which leads to change in the mathematical definitions of the various
features of the model. The model accuracy is mainly influenced by two factors namely
the varying amount of material removed by the finishing process and the finishing
technique adopted. A skilled operator is required as the amount of material to be removed
from different surfaces may be different and inaccuracies caused due to deposition can be
brought down. A finishing technique selection is important because different processes
have different degrees of dimensional control. For example models finished by
employing milling will have less influence on accuracy than those using manual wet
sanding or sand blasting.
(a) Thicker bottom layer (b) Deformed hole boundary
Figure 14: Over-curing effects on accuracy in Stereolithography
(after Pham and Demov, 2001)
5.4. Selection of part deposition orientation
This is one of the crucial decisions taken before slicing the part and initiating the process
of deposition for a particular RP process. This decision is important because it has
potential to reduce part building time, amount of supports required, part quality in terms
of surface finish or accuracy and cost as well. Selection of part deposition orientation is
process specific where in designer and RP machine operators should consider number of
different process specific constraints. This may be a difficult and time consuming task as
designer has to trade-off among various conflicting objectives or process outcomes. For
example better part surface quality can be obtained but it will lead to increase in the
building time. Pandey et al. (2004b) handled conflicting situation of the abovementioned
two objectives and proposed use of multi-objective genetic algorithm for finding out
optimum part deposition orientations (pareto optimal solutions) for FDM process. In their
work, amount of support structures were also minimized implicitly. Thrimurthullu et al.
(2004) converted multi-objective problem into single objective problem and then solved
by using real coded genetic algorithm. Singhal et al. (2005) made an attempt to find out
optimum part deposition orientation for SL process by using optimization tool box of
MATLAB 6.5 for minimizing overall part surface roughness. Except these, researchers
suggested to find out a suitable part deposition orientation for objectives like maximum
accuracy, minimum building time, support structure or cost. A through review of the
various part deposition orientation studies has been done by Pandey et al. (2004a). Pham
and Demov (2001) discussed guidelines for selection of part deposition orientation for SL
and SLS processes.
6. SUMMARY
This paper provides an overview of RP technology in brief and emphasizes on their
ability to shorten the product design and development process. Classification of RP
processes and details of few important processes is given. The description of various
stages of data preparation and model building has been presented. An attempt has been
made to include some important factors to be considered before starting part deposition
for proper utilization of potentials of RP processes.
REFERENCES
Chua, C.K., Leong, K.F. (2000) Rapid Prototyping: Principles and Applications in
Manufacturing, World Scientific.
Gebhardt, A., (2003) Rapid Prototyping, Hanser Gardner Publications, Inc., Cincinnati.
Pandey, P.M., Reddy N.V., Dhande, S.G. (2003a) Slicing Procedures in Layered
Manufacturing: A Review, Rapid Prototyping Journal, 9(5), pp. 274-288.
Pandey, P.M., Reddy, N.V., Dhande, S.G. (2003b) Real Time Adaptive Slicing for
Fused Deposition Modelling, International Journal of Machine Tools and Manufacture,
43(1), pp 61-71.
Pandey, P.M., Reddy, N.V., Dhande, S.G. (2004a) Part Deposition Orientation Studies
in Layered Manufacturing, Proceeding of International Conference on Advanced
Manufacturing Technology, pp. 907-912.
Pandey, P.M., Thrimurthullu, K., Reddy, N.V. (2004b) Optimal Part Deposition
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Pham, D.T., Dimov, S.S. (2001) Rapid Manufacturing, Springer-Verlag London
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Singhal, S.K., Pandey, A.P., Pandey, P.M., Nagpal, A.K. (2005) Optimum Part
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