In Proceeding of
International Conference on Recent Anvances in Materials and Processing
Dec. 15-16, 2006, PSG-tech. Coimbatore, INDIA
ADVANCES IN MATERIALS FOR POWDER BASED RAPID PROTOTYPING
Prashant K. Jain1 K. Senthilkumaran1 Pulak M. Pandey2 P. V. M. Rao3
1
2
Research Scholar
Assistant Professor
3
Associate Professor
Mechanical Engineering Department
Indian Institute of Technology Delhi, New Delhi
2
pmpandey@mech.iitd.ac.in
Abstract: Rapid Prototyping (RP) is two decade old technology to quickly produce tangible
objects directly from a 3D CAD model and is being used to shorten and simplify the product
development cycle for many applications including aerospace, automobile and home appliances
etc., It involves adding material successively, in layers, to create a solid of a predefined shape.
Over the years, RP has evolved from producing prototypes for form, fit and functional testing to
producing final end products for functional use. Since the presentation of first commercial
application in last decade, a large number of processes have been developed. Present day
commercial systems use various materials that range from polymers, metals, metal-polymer
composites, ceramics and sand. As the growing number of new applications constantly motivate in
developing new materials, it is essential to study the feasibility of new materials suitable for
layered manufacturing. The properties of the material, particle size, fusibility of powder particles
and thermal and optical characteristics are the bottlenecks in achieving required mechanical
properties, feature resolution, accuracy and surface quality of the end product. In this paper, recent
developments in materials for powder based layered manufacturing is reviewed and emphasize is
given on issues and challenges in developing new materials and methods to meet high standards of
part quality. Also recent research and accomplishments in processing bio-materials, heterogeneous
materials and direct tools are discussed.
Keywords: Rapid Prototyping, Selective Laser Sintering, Materials
1.0 INTRODUCTION
In 1987, Carl Deckard at University of Texas found
that polymer powders can be selectively sintered
using a laser beam to create complex solid objects.
Also the development of computers and CAD
systems in the past decade made this technology
commercially viable and in the recent past
researchers found this technology suitable for any
type of material which can be pulverized in the form
of powders [1]. Due to the varied material
capabilities, Selective Laser Sintering (SLS) process
now stands as an alternative to conventional
manufacturing techniques. Because of the time
compression between product conceptualization to
realization, these technologies are sometimes referred
to as Rapid Manufacturing [2]. Because of the wide
range of materials it can process, SLS is superior to
other Rapid Manufacturing techniques [3]. The
materials include wax, cermet, ceramics, nylon/glass
composite, metal-polymer powders, metals, alloys,
steels and polymers [4]. A review of materials
processed by SLS has been done by Kruth et al. [5].
Initially Polycarbonate powders (Bisphenol-A
polycarbonate) were used. Later Nylon and nylon
composites have become industry standards for
prototypes and functional models due to high wear
and chemical resistance [6]. Researchers tested the
use of a sacrificial polymer binder and found that any
material can be combined with a low-melting-point
material which will serve as glue/binder in SLS.
Metal systems were studied for laser sintering since
rapid tooling needed accurate metal dies and moulds.
Recent research efforts [7-16] showed the capability
to process high temperature, high performance
materials, making this process comparable to
conventional manufacturing techniques in producing
metal components with almost same mechanical
properties by successfully processing nickel base
1
superalloys, titanium alloys and superalloy cermets
into functional components for automotive and
aerospace applications.
SLS also processes bio-materials for fabricating
scaffolds in tissue engineering scaffolds. Layer-bylayer additive fabrication in SLS allows construction
of scaffolds with complex internal and external
geometries. Moreover, virtually any powdered
biomaterial that will fuse but not decompose under a
laser beam can be used to fabricate scaffolds. SLS
enables fabrication of anatomically shaped scaffolds
with varying internal architectures, thereby allowing
precise control over pore size, porosity, permeability,
and stiffness. Control over these characteristics may
enhance cell infiltration and mass transport of
nutrients and metabolic waste throughout the
scaffold. SLS also allows for the fabrication of
biphasic scaffolds that incorporate multiple
geometries into a single scaffold, allowing for ingrowth of multiple tissues into a single scaffold
structure. Recent advances focus on processing of
Polycaprolactone, hydroxyapatite by SLS for bone
and cartilage tissue engineering [17].
This paper presents advances in above mentioned
fields and the paper is organized into different
sections based on the different materials. These
materials are namely polymers, wax, cermets,
ceramics, nylon/glass composite, metal-polymer
powders, metals, alloys. Research issues in
processing of bio-materials and functionally graded
material (FGM) for bio –medical applications have
been dealt in last section.
2.0 MATERIALS FOR POWDER BASED RP
SLS can be used to process almost any material,
provided it is available as powder and that the
powder particles tend to fuse or sinter when heat is
applied.
Metal &
Alloys
polymer binder) to the basic powder. Figure 1 shows
the wide range of materials SLS can process.
2.1 Polymers
The initial materials used in SLS are polymers which
are materials made up of long-chain molecules
formed primarily by carbon-to-carbon bonds. Mostly,
three types of polymers are used in engineering:
thermoplastics;
thermosetting
plastics;
and
elastomers. Most polymers used in SLS process are
thermoplastics. Thermoplastics can be recycled in
SLS, thus saving material. Generally, thermoplastic
polymers can be classified into two types: amorphous
and crystalline. Amorphous material has chain
molecules arranged in a random manner like in
polycarbonate (PC). Crystalline material has chain
molecules arranged in an orderly structure like in
nylon.
Amorphous polymers are able to produce parts
with very good dimensional accuracy, feature
resolution and surface finish (depending on the grain
size). However, they are only partially dense parts.
As a consequence, these parts are only useful for
applications that do not require part strength and
durability. Typical applications are SLS masters used
for manufacturing silicone rubber and cast epoxy
moulds [18]. The first sintering model developed for
processing of polycarbonate shows the effect of
energy density on the sinterability of polycarbonate
powder beds [19]. The densification and accuracy of
PC parts are most sensitive to changes in activation
energy and heat capacity of the amorphous polymer
[20]. Also the accuracy of parts depends mostly on
the process parameters as well [21].
Semi-crystalline polymers on the contrary, can be
sintered to fully dense parts with mechanical
properties comparable to injection moulded parts [5].
Prototypes made by these materials widely employed
where strength and wear resistance is the main
consideration. Typical applications of these materials
are fully functional prototypes and sometimes as the
final product. Figure 2 shows some of the nylon
parts.
Single Component
Two Component
Alumide
Cu PA
Amorphous
Semi Crystalline
With Glass &
Carbon
reinforcement
Cermets
Alumina
+
Binders
Polymer
Alumina
SiC
Ceramic
Figure 1 Materials for SLS
Powders that depict low fusion or sintering
properties can be laser sintered by adding a low
melting temperature binder material (typically a
Figure 2 Some polyamide parts produced by SLS
2
Shrinkage of these semi-crystalline polymers
during processing is typically 3-4 per cent [22] and
depends on the process parameters, which
complicates production of accurate parts. New
grades of nylon powders (i.e. Duraform PA12, Fine
Polyamide, PA2200) even yield a resolution and
surface roughness close to those of PC, making PA
also suited for casting silicone rubber and epoxy
moulds. Other polymer-based materials available
commercially are acrylic styrene for investment
casting and an elastomer for rubber-like applications
[23]. Shi et al. [24] studied the relationship between
the crystallinity of the polymer material (Nylon 12)
and the accuracy of the SLS part. They found the
crystallization rate, which is closely correlated with
crystallinity, greatly affects the accuracy and
precision of the SLS part. Tontowi and Childs [25]
measured density of commercially supplied powders,
known as Duraform (nylon-12_ & Protoform (glass
filled nylon-11) and studied the effect of varying bed
temperature on the density of sintered parts produced
by the SLS process. They developed simulation
model for density prediction based on experimental
results. Gibson and Shi [26] comprehensively
analysed the relationship between powder properties,
fabrication parameters and the mechanical properties
of nylon SLS parts. Mechanical properties of SLS
parts are influenced by powder properties and
fabrication parameters. Recently, Ajoku et al. [27]
compared compressive strength of laser sintered and
injection moulded Nylon-12 parts and developed
finite element model for compressive strength. The
modulus of the laser sintered Nylon- 12 is 10% less
than that of the injection moulded Nylon-12
Modulus. This difference is as a result of the porosity
within the laser sintered part which can be inferred
from figures 3 and 4. During polymer laser sintering,
pores can arise as a result of uneven heat distribution
within the build area, inadequate heat supply from
the laser and insufficient process temperatures [27].
These pores do not have any definite size, shape and
location within a sintered component.
Figure 3 SEM showing porosity within a crosssectional area in a laser sintered part [27].
Figure 4 SEM showing no porosity within a crosssectional area in an injection moulded part [27].
2.2 Reinforced and Filled Polymers
Polymer powders can be easily reinforced with other
materials in order to further improve their mechanical
and thermal properties. Several grades of glass fibre
reinforced PA powders are readily available the
market [28]. The part fabricated from glass filled
polyamide (PA3200 GF) has excellent mechanical
properties and high accuracy. Typical applications of
these materials are housings and thermally stressed
parts. Childs and Tontowi [29] measured density of
glass filled nylon-11 and simulated the effect of
varying bed temperature on the density of sintered
parts. DTM Corporation (Austin, USA) introduced in
mid-1998, copper polyamide, which is a thermally
conductive composite of copper and plastic and can
be used to create tooling for short runs of production
equivalent plastic parts.
Copper polyamide is suitable for injection
moulded inserts to mould around 100–400 parts in
polyethylene (PE), polypropylene (PP), glass filled
PP, polystyrene, ABS, PC/ABS, and other common
plastics. Lower material strengths are the limitation
in application of Copper polyamide moulds.
Recently, Windform XT is introduced into
commercial market which is based on a carbon-filled
polyamide and produces black parts with a smooth
finish and a sparkling appearance [30]. It has a low
density and a high tensile strength and tensile
modulus. Electro Optical Systems (EOS) has also
announced its carbon filled polyamide powder called
CarbonMide having almost same characteristics that
of Windform XT [31].
2.3 Metals and Alloys
In usual practice, SLS allows producing metallic
parts using some kind of sacrificial polymer binder.
Nowadays, direct sintering of metallic powders
without the use of a polymer binder is also
investigated. This further enlarges the range of
powders used in SLS. Early attempts [32] to SLS
process metallic powders and powder blends of
copper, lead, tin, and zinc proved to be unsuccessful
because of balling. As the increase in energy density
causes a larger degree of melting, causing material to
3
form spherical balls whose diameters tend to increase
with further increase in energy density as shown in
figure 5. Since the molten metal is fully contained by
loose powder rather than a fully dense material, the
tensile traction on the melt is not sufficient to confine
it to a layer wise geometry. A two-phase powder
approach was used to overcome balling effects [33].
This was achieved using a pre-alloyed single phase
powder system in which melting occurs over a range
of temperature, or a powder blend of two phases with
different melting temperatures. In the former case,
laser processing parameters are manipulated so that
only partial melting occurs.
Figure 5 Balling effect found on Ni alloy on quartz
substrate [33]
DTM Corporation has developed a process that
applies polymer-coated steel powders (1080 Steel,
316 or 420 Stainless Steel particles coated/mixed
with a thermoplastic /thermoset material) for the SLS
of metal parts. During laser sintering, the polymer
melts and acts as a binder for the steel particles. This
binder needs to be debinded to get the green part.
After debinding, the porous steel part is infiltrated
with copper or bronze. Over the years, DTM
Corporation has developed RapidSteel powder,
composed of 60 percent 420 Stainless Steel and 40
percent Bronze, for which debinding and infiltration
can be done in a single furnace cycle of about 24
hour under pure Nitrogen. These developments give
better material properties of the final SLS parts such
as strength, hardness, machinability, weldability,
wear rate and thermal conductivity [35].
Figure 6 Direct laser sintered 3D metal parts [61].
LaserForm ST-100 (420 Stainless Steel based
powder), is the latest tooling material system offered
to replace RapidSteel 2.0 and Copper Polyamide
powders. LaserForm ST-100 tooling is reported to be
fully dense after LS with surface roughness of 5 μm
Ra. RapidTool moulds have been successfully
employed in both plastic and wax injection moulding
[36]. Reports claim that complex moulds are
produced in less than 2 weeks and are capable of
producing 50,000 to 100,000 parts [37]. EOS avoids
the use of polymer binder and uses direct sintering of
metal powders with a low melting point, i.e. bronzenickel based powders (EOS-Cu 3201 containing CuSn, Cu-P and Ni particles) developed by Electrolux
Co. [38]. After SLS, the part is infiltrated with epoxy
resin to fill in the pores. Hence the final part is a
bronze-epoxy composite, rather than a plain metallic
part and its mechanical and thermal properties are
limited. Infiltration with a metal like copper or
bronze is not possible in this case, since the green
part would melt during infiltration. The direct metal
laser sintering (DMLS) process and a new powder
(EOS-DMLS Steel 50-V1 containing steel, Cu-P and
Ni particles) yielding improved mechanical
properties was introduced in the market by EOS [39].
Some of the parts produced by DMLS is shown in
figure 6.
Solid state sintering is a thermal process which
occurs at a temperature between its re-crystallization
and melting temperature. The driving force for
binding is a physical diffusion of metal atoms from
one particle to another. This is inherently a slow
phenomenon and illustrates the main drawback of
solid state sintering for DMLS [40]. Studies show
that the average interaction time from the laser beam
with the particles is too much short to initiate
sintering. In this direction, Schueren and Kruth [40]
examined different metal powder mixtures of (Cu,
Fe, Sn) for sinterability. The best results are obtained
with a mixture of Fe and Cu powders. Zhu et al. [41]
demonstrated the feasibility of producing Cu-based
metal parts directly by SLS using various metal
systems such as Cu–Sn, bronze–Ni, Cu-solder and
Cu–Fe. Zhu et al. [42] showed that the ratio of Fe and
Cu significantly influences part distortion in a Fe–Cu
system. Tungsten articles act as an ‘obstacle’ to
affect the accuracy of the part size because the unmelted tungsten particles have a large friction to
impede rearrangement. Wang et al. [43] showed that
the WC-Co system can be processed over a relatively
wide range of processing parameters compared to a
Fe-Cu powder mixture. This is due to the good
absorption characteristics of WC and Co materials
[44]. Simchi and Pohl [45] investigated the effect of
short hatch lengths on warpage in SLS of iron
powder. They have shown that sintered density was
higher when using short vector length. It should be
noted that the scan vector length influences the
development of thermal stresses as well. These
residual internal stresses are responsible for reduced
4
part performance as well as warpage, loss of edge
tolerance and even delamination of layered deposited
parts. On the other hand, the effect of powder bed
temperature on the sintered density was not
conspicuous since the preheated condition was not
high enough. Although it influences the thermal
stresses developed during processing.
2.4 Ceramics
Ceramics are hard, brittle, very high melting points
with low electrical & thermal conductivity, good
chemical and thermal stability, and high compressive
strength. They exhibit both ionic and covalent
bonding. The most common ceramics used in RP are
Al2O3, SiO2, and ZrO2 [46]. Few researchers [47-49]
have attempted to produce directly ceramic parts
without polymer binder material. The absence of any
binder element makes the ceramic laser sintered part
very fragile and viable to breakage. Due to the short
reaction time involved in SLS, solid state sintering is
not feasible. SLS has been used to produce ceramic
parts by using either ceramic particles coated with
binder or a mixture of ceramic and binder particles.
Alumina parts were made using the laser sintering
followed by an infiltration step using an alumina
colloid. After sintering maximum strengths obtained
were around 14 MPa due to the low sintered densities
of about 55% [47].
SLS has been used to produce ceramic investment
casting molds. Partially stabilized Zirconia molds for
Titanium casting were made by SLS of stabilized
Zirconia which was then infiltrated with unstabilized
Zirconia before being sintered [48]. Aluminum with
SiC is light weight, high conductivity and strength,
low thermal expansion coefficient and sufficiently
high wear resistance. Thermal conductivity, as the
next important property, can be changed within a
wide range by addition of different amounts of SiC
particles to the starting powder mixture. Because of
these beneficial properties, several parts are produced
from Al–SiC composites, mainly for the automobile
industry, and for electronic packaging applications
[49].
2.5 Foundry Sand
Now sand powders are commercially available that
can be laser sintered to produce foundry sand
moulds. DTM offers Zirconium and Silicon sand
commercial name SandForm ZrII and SandForm Si.
SandForm Si, used for Al castings, is based on silica,
and has a low density. SandForm ZrII is used for Al
and Fe castings and its binder system matches silica
[5]. The LASERCON coated sand offers by EOS
have a composition of 96.8% quartz sand and 3.2 %
resin. The mean grain dimension is 170 microns, and
density 1550 kg/m3. Figure 7 shows a core made out
of EOSINT machine [28].
Figure 7 Left cylinder head of V6-valve car Sand
Moulding was Core Produced on EOSINT S 700
Direct Croning System [38]
2.6 Functionally graded materials
Functionally graded material (FGM), also called
heterogeneous materials, are a new generation of
engineering materials wherein the micro structural
details are spatially varied through non-uniform
distribution of the reinforcement phase(s), by using
reinforcement with different properties, sizes and
shapes, as well as by interchanging the roles of
reinforcement and matrix phases in a continuous
manner. The result is a microstructure that produces
continuously or discretely changing thermal and
mechanical properties at the macroscopic or
continuum scale. Some primitive FGM objects are
shown in figure 8.
Figure 8 Heterogeneous primitives [63].
Functionally graded material has become a subject
of research in the material science, composites, and
ceramic engineering and metallurgy communities.
Processing methods for FGMs have been extensively
reviewed by Kieback et al [50]. In the specific
context of FGMs involving polymer composites,
processing methods include hot iso-static pressure
casting, compression molding, and centrifugation.
SLS can fabricate such heterogeneous objects. The
material deposited can be varied continuously to
yield a functionally graded material object with
varying material distribution. Some of the studies in
processing of polymer composites by SLS have been
reported by Zhou et al. [51]. Das and Chung [52]
5
discussed the fabrication of FGMs by SLS of Nylon11 composites. They built one dimensional FGM
with varying compositions of glass bead on nylon 11.
They investigated processing of Nylon-based
composites with different volume fractions of glass
fiber and glass bead reinforcements. They also
reported previous attempts of one dimensional FGM
part processed by using blend of tungsten carbide &
cobalt powders and H-13 tool steel & copper
powders.
2.7 Biomaterials
As the powders are subjected to low compaction
forces during their deposition to form new layers,
SLS-fabricated objects are usually porous. This
interconnected porosity is a key property requirement
in biomedical applications, including artificial bones
and tissue engineering scaffolds. Figure 9 shows
some of the complex 3D Scaffold designs. The nature
and extent of this interconnected porosity can be
tailored and controlled effectively to meet different
application criteria through material selection and
physical design, and owing to the additive nature of
the SLS process, control over internal structure is
possible.
Figure 9 complex 3D Scaffold designs [62]
The porosity also offers an opportunity during postprocessing to introduce additional materials into the
object to alter material composition as well as help to
control part stability. Polymethyl methacrylatecoated calcium phosphate powders have been
successfully processed via SLS and subsequent postprocessing enables to produce strong porous
structures [53]. Das et al. [54] investigated the design
and fabrication of scaffolds with periodic cellular and
biomimetic architectures using nylon and built cubes
with 0.8 mm channels and 1.2 mm pillars. Williams
et al. [55] designed and fabricated scaffolds of
polycaprolactone, a bioresorbable polymer, with the
smallest pores being 1.75 mm in diameter. Tan et al.
[56, 57] and Chua et al. [58] found micropores
formed within the scaffold structure produced via
SLS from physically blended Hydroxyapatite
(HA)/polyetheretherketone and HA/polyvinyl alcohol
composites. Internal porosity with 150 mm average
pore size in the SLS-fabricated HA/poly(L-lacide)
specimens are also reported [59] . Das et al. [60]
investigated the development of optimal SLS
processing
parameters
for
CAPA®6501
polycaprolactone powder using systematic factorial
design of experiments. The test scaffolds with
designed porous channels were able to achieve a
dimensional accuracy to within 3%–8% of design
specifications and densities approximately 94%
relative to full density.
3.0 CONCLUSIONS
In this paper current state of the art in processing of
different materials through SLS is presented. Studies
involving developing new materials and improving
the existing materials were discussed. Although
many materials have been developed, there is still a
need for research into new materials. It should be
noted that the SLS process is still a relatively new
process and therefore continued development of the
technology
and
understanding
of
process
fundamentals is needed to carry the technique
forward. The addition of a secondary material to
modify the mechanical properties of polymers is
common practice, to ensure materials meet design
requirements and are suitable for a wide range of
applications. Addition of rigid particles and clay to
polymers can produce a number of desirable effects
on the mechanical properties of parts. The knowledge
of existing materials and the nature of complexity in
processing them by laser will be helpful in achieving
functional requirements of parts for present and
future applications. The future of bio-manufacturing
which combines principles of RP and Bio-science
can form complicated bio tissue scaffolds, is a
potential technology to make artificial organs.
4.0 ACKNOWLEDGMENTS
Authors gratefully acknowledge the financial
assistance provided by Industrial Research and
Development unit of Indian Institute of Technology
Delhi to carry out this work.
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