Consolidation of Polymer Powders by Selective Laser Sintering
J.-P. Kruth1, G. Levy2, R. Schindel2, T. Craeghs1, E. Yasa1
1
2
K.U. Leuven, Division PMA, Heverlee, Belgium
Inspire AG, IRPD Institute for Rapid Product Development, St. Gallen, Switzerland
ABSTRACT: Selective Laser Sintering of polymers has found widespread applications for rapid prototyping
and rapid manufacturing. Well known manufacturing applications are the production of hearing aids, by
thousands a day, and the series production of design lamp covers. However, the range of polymer materials
that can be processed by SLS today is still very limited, restricting most SLS parts to be made from nylon
(polyamide). This paper tries to understand the basics of selective laser sintering of polymers. It gives an
overview of the materials that were yet successfully processed by SLS or that are being investigated, including
reinforced or filled polymers, degradable polymers, biocompatible polymers, etc. It also compares the
properties of current SLS polymers to that of injection molded ones.
1 INTRODUCTION
Selective Laser Sintering (SLS) is one of the popular
Rapid Prototyping and Manufacturing (RP&M)
processes that emerged in the last two decades [39,
45]. It builds 3D objects by applying subsequent
layers of powder material on top of each other and
by consolidating each layer with a laser beam
scanning the powder layer in accordance with the
CAD geometry of the part, before the next layer is
applied and consolidated (Figure 1).
The main advantages of SLS over other RP&M
methods are:
- SLS allows building parts in a wide range of
materials, including polymers, metals and various
types of composites (e.g. metal-metal, metalpolymer,
polymer-ceramic,
metal-ceramic
composites) [8, 41]. Research is also going on to
process pure ceramic parts using SLS [41].
- Parts produced by SLS demonstrates material
properties that are very close to the material
properties obtained by other manufacturing
processes, like injection molded plastic
components, or cast and machined metallic parts.
The SLS process is indeed not limited to the use
of dedicated materials, like photopolymers in
stereolithography or wax-like materials in ink-jet
printing processes. Theoretically, any material
that can be provided in powder form and that
melts when rising the temperature (which means
almost any material) can be processed by SLS,
and as the final part density approaches full
density, their properties may reach those of
similar bulk material.
Figure 1 : Basic layout of SLS
Even though SLS theoretically allows to process
almost any material and to reach normal bulk
material properties, practice is still far from this
ideal situation. Depending on the type of laser
consolidation applied, the powder may not always be
consolidated to full density or may even not be
processable. Consolidation mechanisms that could
be induced during laser heating are: solid state
sintering, liquid phase sintering, partial melting, full
melting (often referred to as Selective Laser Melting
or SLM) or chemically induced binding [41]. Solid
state sintering is today hardly used, because it would
require a very slow scanning process to keep the
powder particles at high temperature for a sufficient
long time to allow diffusion of atoms to occur,
which mainly applies to metals and ceramics, but not
at speeds compatible with the demand for economic
RP&M. Chemical induced binding is also hardly
applied today, leaving mainly two consolidation
principles over for current practice: liquid phase
sintering or partial melting (commonly referred to as
SLS) and full melting (commonly referred to as
SLM).
Laser consolidation of polymers normally
involves melting of thermoplastics (partial-SLS or
full-SLM). A clear distinction should be made
between (Figure 2):
• (semi-)crystalline thermoplastics
• amorphous thermoplastics.
When heated up from very low to very high
temperatures, all those thermoplastic materials will
change from a hard (solid and glassy) structure to a
softer (tough leathery or rubbery, solid or nonpourable) structure and finally turn into a viscous
flowing melt: see Figure 3.
Figure 3 : Phases and transition temperatures of some
polymers
Figure 2: Thermoplastic polymers (Red = material used in
SLS)
Today, only few powder materials are available
on the market for SLS or SLM processing.
SLS/SLM of polymers mostly (±95%) involves
polyamide-12, i.e. a typical nylon grade. Only few
other polymer powders are commercially available
today for SLS: limited success was achieved with
polycarbonate and polystyrene (see below). SLS and
SLM of metals is today also restricted to mainly
stainless steel and a few grades of tool steel, while
titanium (mainly Ti6Al4V) and aluminum start to
reach industrial applications.
This paper focuses on the consolidation of
polymers by SLS and SLM. It will also give some
examples of other consolidation mechanisms like
chemical induced binding or cross linking of
polymers. It will describe the fundamentals of
polymer processing and describes the various classes
of polymers that can be laser processed starting from
deposited powder layers.
2
POLYMER CLASSES AND BASIC
CONSOLIDATION
Even though polymer is the most processed type of
material in SLS/SLM, the consolidation phenomena
invoked for polymers are probably still amongst the
least understood or at least the least described in
literature [6, 54, 64].
Materials however differ in the way and the
temperatures at which those transitions occur. Semicrystalline polymers have a glass transition
temperature Tg that is below or around room
temperature (-100° to 50°C) and a distinct melting
temperature Tm that is above 100°C (between 100°
and 400°C) at which a significant volume change
happens: see Figure 4. Amorphous polymers do not
depict a clear melting temperature range. They have
a glass transition temperature Tg that lies around
100°C and above which the material will gradually
evolve to a leathery, rubbery and finally liquid state
as temperature increases, without clear transitions.
Figure 3 indicates the glass transition range (∆Tg)
and melting range (∆Tm) for the three most
important semi-crystalline polymers (PE, PP, PA 6)
and indicates for three amorphous polymers (PS, PC,
PMMA) the glass transition temperature ranges
(∆Tg) and the temperature range in which the
transition to leathery material goes quickly (∆Tf).
This last range (∆Tf) can physically not be defined
exactly.
Notice also that the Tg and Tm values largely
depend on the molecular weight (MW) of the
polymer. This explains why there might be a
significant difference in SLS processability between
a low and a high molecular weight PA or PE.
Figure 4 : Comparison of relative volume between amorphous
and semi-crystalline polymers [6].
The temperature transitions and melting range of
polymers can be observed by recording a DSC plot
by Differential Scanning Calorimetry analysis. It
measures the difference in the amount of heat
required to increase the temperature of a sample of
the given polymer and of a reference piece or pan as
a function of temperature (Figure 5). Both the
sample and reference are maintained at very nearly
the same temperature throughout the experiment.
Figure 6 shows a typical DSC plot of PA 12. It
clearly shows a melting peak during the heating
cycle and a re-crystallization peak during the
cooling cycle of the DSC test. Figure 7 gives the
DSC plots of Duraform PA 12 and PA 6 during the
heating cycle [58]. Notice that the sign of the
differential heat flow may be inverted in the DSC
plots. Figure 7 illustrates the much smaller melting
range (small width of peak around 187°C) and the
smaller melting temperature (Tm=187°C) of PA 12
as compared to PA 6 (Tm=223°C). This explains,
besides viscosity differences (see §3), why PA 12 is
more prone to SLS than PA 6 (or PA 66 having Tm=
262°C and a wider peak than PA 12).
rapidly change into a viscous liquid. The molten
polymer flows in between the powder particles,
forming sintering necks. With enough heat, the
complete layer is fully molten and overlaps to the
previous layer. As the molten polymer cools down
below Tm, polymer crystals nucleate and grow,
recreating regions of ordered molecular chains
(crystallites) mixed up with disordered amorphous
regions. Above Tm, the polymer depicts a relatively
low viscosity, as compared to amorphous polymers,
which favors the rate and amount of consolidation.
The densities obtained will be close to full density
and the mechanical properties will be close to those
of molded polymers. However, the freezing of the
polymer at Tm coincides with an important shrinkage
(phenomenon not occurring with amorphous
polymers), that may induce geometrical inaccuracies
and distortion of the part (Figure 4). A good way to
prevent this is to preheat the polymer powder to a
temperature slightly below its melting temperature
and keep it there for a certain time after
consolidation [66]. Ideally the powder should be preheated and post-heated to a temperature between the
crystallization and melting temperature. In such a
way, no major shrinkage will occur during laser
melting, and localized shrinkage (and hence part
distortion) can be avoided: the shrinkage will occur
in a uniform way when the part will be slowly
cooled down in a uniform way after the whole build
is finished. To facilitate the control of pre- and postheating and of shrinkage, it is recommended to have
a certain temperature range between the
crystallization and melting peaks in the DSC plot, as
depicted in Figure 6 for PA 12 (see further §3).
Figure 6 : DSC-curve for melting and crystallization of
PA12 [4,74]
Figure 5 : Differential Scanning Calorimetry analysis [72]
The laser consolidation of (semi-)crystalline
polymer powders will happen by heating above
their melting temperature Tm. Semi-crystalline
materials have a highly ordered molecular structure
with sharp melt points. They do not gradually soften
with a temperature increase but rather remain hard
until a given quantity of heat is absorbed and then
The consolidation of amorphous polymer
powder occurs by laser heating above the glass
transition temperature, at which the polymer is in a
much more viscous state than semi-crystalline
polymers at similar temperature. Amorphous
polymers have a randomly ordered molecular
structure. They do not have a sharp melt point but
instead soften gradually as the temperature rises. The
viscosity of these materials changes when heated,
but they seldom are as easy flowing as semi-
crystalline materials. The flow and sintering rate will
be less, resulting in a lower degree of consolidation,
higher porosity, less strength, but also a lower
shrinkage that is favorable in cases of SLS of
patterns for producing mould for molding or casting
(i.e. Indirect Rapid Tooling, investment casting
patterns, etc.). The higher porosity is also favorable
to avoid breaking the ceramic investment casting
shell when heating the SLS pattern during the
debinding cycle intended to release the mold cavity.
In several applications post-infiltration of the pores
of the SLS part is applied to consolidate it.
polymer powder may provide essential additional
information to the DSC plots in order to characterize
it SLS processability (see below).
a
b
Figure 8 : Powder particle shapes REM, a) precipitation, b)
Cryogenic milling
Figure 7 : DSC plot of PA 12 (Duraform) and PA 6
In conclusion for semi-crystalline polymers we
rather approach full melting consolidation (even
though the parts may still depict some porosity),
whereas for amorphous polymers we rather see a
partial melting binding mechanism. The former can
be called Selective Laser Melting, while the latter
coincides with liquid phase Selective Laser
Sintering. However, unlike metal powder processing
[40], this terminology is rarely used for laser
processing of polymer powders: one generally refers
all polymer variants as Selective Laser Sintering.
Several researchers tried to analyze and model the
complex behavior of polymers during laser sintering,
including aspects like viscosity, sintering (neck
formation, viscous flow), laser absorption,
temperature distribution, thermal conductivity, etc.
[8, 13, 34, 42].
The complex thermal behavior of polymers is
amplified in SLS processing due to several factors.
Among others, the costly pulverization either by
cryogenic milling or precipitation-process (Figure 8)
limits the availability of polymer powders. The
pulverization can influence also the consolidation
process: the powder layer’s thermal characteristic
differs. Moreover, the molecular weight of the
polymer will influence the melt viscosity.
Characterizing the melt flow index (MFI) of a
The material is in powder form with a certain
particle size distribution (Figure 9) and the induced
energy, heating the fine powder particles, may lead
to evaporation or disintegration which in turn
disturbs the process. It shields off the laser window
and the IR heating elements. In short, this has a
negative influence on the process consistency and
the economy of the process. Further the fine particles
may create either a dusty atmosphere or coagulations
clustering preventing the homogeneous deposition of
layers, thus preventing good layer consolidation.
Figure 9 : Typical particle size distribution of Duraform PA
12 (Source FHSG St. Gallen)
A variety of commercially available technical
polymers with excellent properties for SLS have
been tuned by blending or by special design of the
chain length (analogy to molecular weight ranges).
Often they result in a multi-peak DSC diagram. SLS
can handle only one fixed controlled consolidation
temperature or several very close peaks (typically 510oC difference), which means that polymers with
more than one tight melting range are not
processable with SLS. Figure 10 demonstrates the
DSC-curve of a commercial pulverized blend with
no real full melting SLS processability, due to two
different melting points at 1780ºC and 1850ºC.
We can associate seven main SLS polymer
powder materials with different application fields.
The four main thermoplastic SLS applications
are:
a. SLS of polymer parts: semi-crystalline polymers
(§3)
b. SLS of investment casting patterns: amorphous
polymers (§4)
c. SLS of metal or ceramic parts using a sacrificial
polymer
binder:
thermally
degradable
amorphous polymers (§5)
d. SLS of reinforced or filled semi-crystalline
polymers for highly loaded parts (§6)
SLS today also allows production of parts using:
e. Flexible elastomeric material parts (§7)
f. Polymer-polymer blends (used in the racing
industry) (§8)
g. Thermosetting materials (§9)
Those different material categories are discussed
in more detail in following sections.
Cooling curve
polymers is not totally clear yet. Some semicrystalline polymers are able to crystallize much
faster than others, as a result of their chain structure.
The rate of crystallization is minimum near Tg and
Tm, and reaches a maximum between those two
temperatures. Crystallization rate should however be
kept relatively slow (relative to the vertical build
rate) in order to avoid part distortion due to gradient
freezing shrinkage which would be localized in
upper layers. Good crystallization and uniform
shrinkage is then obtained by keeping the powder
bed at high temperature for a sufficient long time
after sintering.
Figure 11 : Example of PA 12 part produced by SLS
As already introduced in §2, the DSC plot of a
well processable thermoplastic material for SLS
should depict:
- a narrow melting range (narrow melting peak in
heating curve): Figure 7
- non-overlapping melting and re-crystallization
peaks: Figure 12, Figure 13, Figure 14
- and should avoid double melting peaks: Figure
10.
Figure 7 already illustrated the difference of PA
12 and PA 6 in terms of melting range.
Heating curve
Figure 10 : Commercial pulverized blend with no real SLS
processability: melt curve (left) and re-crystallization curve
(right)
3 SEMI-CRYSTALLINE POLYMERS
Today, the production of prototypes (RP) and
functional parts (RM) by SLS is basically limited to
nylon, i.e. polyamide (PA) [2, 11, 12, 13, 36, 78]:
Figure 11. Quite some research is going on using
other (semi-)crystalline thermoplastics: polyethylene
(PE) [55, 57], POM [54, 74], PEEK [52, 53], PCL
[70, 71, 75], HDPE [29]. PEEK and PCL are of
special interest as biocompatible polymers [52, 65,
75].
The fundamental reason why PA sinters well,
while more difficulties are experienced with other
Figure 12 : DSC plots (heating and cooling) of new
reinforced PA 12 powder containing elongated rather than
spherical beads for better strength
A powder with slow re-crystallization rate, as
depicted by non-overlapping or slightly overlapping
endothermic and exothermic peaks during the
heating and respective cooling phases of the DSC
analysis (Figure 12), might result in nearly fully
dense parts with minimal distortion, while highly
overlapping peaks will yield bad results (Figure 13).
Figure 14 [74] illustrates that PA2200™ (i.e. a
PA 12 powder specially developed for SLS) has
more separated melting and re-crystallization peaks,
than normal milled PA 12 powder. A larger dT range
between those peaks allows for a larger process
window in term of fluctuations of temperature in
time and space within the processing chamber and
build part [54].
Figure 13 : DSC plots (heating and cooling) of IP60 powder
showing overlap between melting and re-crystallization
peaks [US patent 5,648,450]
Figure 15 : Transmission light microscopy images of
microtome sections of PA (left) and POM (right) [54]
To obtain fully dense parts, the melt viscosity
should be low enough to allow complete
consolidation within the time scale of the process.
Pure polymers with melt viscosity in the range of
few tens to few thousands poise (e.g. nylon and
waxes) can be processed successfully and reach near
full density [6]. However, even for PA 12, the SLS
part still contains small amounts of porosity and the
process is something between partial and full
melting. The densities obtained in SLS of PA 12 are
just slightly below those of compression molded
parts (0.95-1.00 versus 1.04 g/cm3), yielding very
similar mechanical properties under compression,
but somewhat lower tensile strength and notched
Izod values that are sensitive to small voids.
(
b)
a
Figure 14 : DSC plots of PA and POM
The Chair of Polymer Technology (LKT) of the
University of Erlangen successfully produced
polyoxymethylene powders (POM) by cryogenically
milling for application in SLS. Although the interval
between the melting and re-crystallization peaks is
smaller than for PA 12/PA2200 (Figure 14), they
were able to produce parts in POM and aluminum
filled POM (30% Al by volume) that demonstrated
good mechanical properties and even better surface
quality than commercial PA 12 SLS powders (i.e.
PA2200). Figure 15 illustrates the improved surface
roughness of POM parts as compared to PA2200
[54]. This figure also illustrates the higher crystalline
structure of POM as compared to PA2200, that
results in a slightly higher strength (47 vs. 45
N/mm²), higher E-modulus (3071 vs. 1980 N/mm²),
but a lower elongation at break (about 3% for POM,
compared to 9% for PA2200 and 20% for injection
molded POM).
b
Figure 16 : a) The rise in viscosity of re-used PA 12
material, b) resulting orange peel texture on SLS part
The melt viscosity is linearly related to the
molecular weight (MW). For good interfusion of the
polymer chains, sufficient to provide particle
necking and layer to layer adhesion, a low melt
viscosity is desirable. However, low viscosity results
in high shrinkage and poor part accuracy. Thus an
optimum MW range exists but is not easily
controllable, neither at the polymer production nor
during aging in process.
In contrast to SLS or SLM of metals, one has to
be aware of the thermal degradation of polymer
powders over time when re-using the same powder
repeatedly. The long exposure to heat leads to chain
growth, a rise in MW and so a rise in viscosity. This
causes non-constant consolidation conditions and a
shift in the melting temperature. The empirical
melting temperature ramps up as a function of build
height and age of the powder. This results in the
decay of the MFI (Mold Flow Index) hence a drop in
powder flowability and a rise in melt viscosity
preventing proper sintering with reasonable quality.
In the limit this can lead to creation of unsmooth
patterns on the surface known as “orange peel”
texture (Figure 16b) after a certain number of runs
(Figure 16a). A drop in mechanical properties can be
observed as well.
The consolidation temperature may be raised to
compensate for those phenomena, but only in a
narrow range and with limited effectiveness. A
better way is to limit those problems by always using
a mixture of virgin and re-used powder (typically
70% virgin – 30% reused): see below.
Virgin
QA
MFI
Linear (MFI)
80
Recycled 60
MFI
difficulty in placing and forming the material
because of the high viscosities, but it can reduce
shrinkage and thus improve the dimensional
accuracy.
When polymer powder is reused in SLS, the MFI
is already about three times lower than the virgin
powder (Figure 17). This leads to higher viscosity,
and may deteriorate the quality of the product as
described above.
With PA 12, process stability and repeatability in
the consolidation process can be obtained by keeping
the MFI within a constant range by working with an
appropriate mix of virgin and recycled powder [43].
All those phenomena are amplified for glass filled
PA 12 material, leading to a rapid decline in
mechanical properties, a clear evidence of different
consolidation conditions (Figure 18).
The aging and the resulting change in
consolidation phenomena and final component
properties are very minor with amorphous materials
like PS and elastomeric polyester based materials
that are discussed in sections 4 and 7.
40
20
0
Interior
quality
1 6 11 16 21 26 31 36 41 46 51 56 61 66
-20
Build (run)
Figure 17 : Melt flow index of powder as a function of
number of builds in which powder is used (Source FHSG St.
Gallen)
Melt Flow Index (MFI) is a critical characteristic.
It is the flow in grams that occurs in 10 minutes
through a standard die of 2.095 ± 0.005 mm
diameter and 8.000 ± 0.025mm in length when a
fixed pressure is applied to the melt via a piston and
a load of total mass of 2.16 kg at a temperature of
190°C (some polymers are measured at a higher
temperature, some use different weights and some
even different orifice sizes). MFI is an assessment of
average molecular mass and is an inverse measure of
the melt viscosity; in other words, the higher a MFI,
the more polymer flows under given test conditions.
Hence the MFI of a polymer is vital to anticipating
and controlling its processing. Generally, higher MFI
polymers are used in injection molding, and lower
MFI polymers are used with blow molding or
extrusion processes.
To achieve functionally strong SLS parts,
polymers that have high molecular weights should
be favored. High molecular weight can lead to
Figure 18 : Tensile strength and elongation of PA-GF
polymers as function of number of build in which nonrefreshed powder is used
PA polymer powders for SLS are normally
supplied without additives. Some additives may be
added to favor the powder fluidity and powder
spreading, but those have no influence on viscosity
and sinterability. Some SLS tests have been
performed with POM, PE, PBT and PPS having
been modified by addition of stabilizers and fillers
[54]. The University of Erlangen is also testing PEUHMW powders that are modified with absorbers
and fillers [74]. The use of fillers will be further
discussed in §6.
Alternatively, SLS parts in PA 12 and other
polymers may be strengthened applying crosslinking agents. Those cross-linking agents are often
applied on the finished "green" SLS part by
impregnation and by applying high level radiation
[74]. The University of Erlangen demonstrated an
increase of E-modulus (from 1650 to 1935 N/mm²)
and tensile strength (from 38 to 46 N/mm²), with a
drop in elongation at break (from 9 to 4%) [74].
Atmospheric control in the SLS chamber is very
critical in terms of temperature (control of sintering
and shrinkage), humidity (powder fluidity) and
oxygen content (to avoid thermal degradation of the
polymer
by
oxidation
resulting
in
depolymerization). A nitrogen atmosphere is normally
used, together with a careful control of the amount
of rest oxygen. Typically polymers like PE and PP
are more prone to rapid oxidation than aromatic
backbone polymers like PET (semi-crystalline) or
PC (amorphous).
Powder particle morphology also influences the
laser consolidation. Figure 19a shows powder (PA
12) with a near to spherical shape. This yields a
lower density during layering because air voids
remain (Figure 19b). When no pressure is applied
during consolidation (as the one occurring during
injection molding), the voids and air traps remain in
the solid part. This reduces the effective cross
section and the tensile strength compared to molded
parts from the same polymer (10-20% reduction).
Figure 19b (PA 12) and Figure 20b (PS) depict the
tensile break cross section demonstrating a partial
melting regime, which is more pronounced in case of
the amorphous PS (Figure 20b).
Comparison between SLS and molded parts
concerning strength is discussed in §10.
a
a
b
Figure 19 : a) PA 12 Powder, b) tensile break cross section
showing some air voids (Source FHSG-St. Gallen)
Several researchers investigated SLS of
biocompatible semi-crystalline thermoplastics like
PEEK and PCL (Poly-ε-caprolactone) [5, 51, 53, 60,
65, 71, 75]. PCL is a biodegradable linear polyester,
while PEEK is not biodegradable. Their respective
melting temperatures lie around 62°C for PCL and
340°C for PEEK. A polymer has never one specific
molecular weight, but consists of a mixture of
molecules with different chain lengths, and so,
different molecular weights. The mean molecular
weight and the molecular weight distribution are
important properties of a polymer. Typically SLS of
PCL with MW of 50.000 g/mol demonstrated good
laser sinterability [75], while PCL with a MW of
40.000 g/mol could not be laser sintered well [70,
71] due to big distortions caused by shrinkage. This
shrinkage is due to the lower molecular weight of the
second PCL powder, and due to the purity
differences compared to the first powder.
b
Figure 20 : a) PS Powder (CastForm), b) Tensile break
cross section demonstrating partial melting regime
4 AMORPHOUS POLYMERS
Amorphous polymers with strongly temperaturedependent viscosities (or high activation energy for
viscous flow) have been readily processed using SLS
[6, 7, 24, 32, 49]. Typical examples are
polycarbonate (PC) and polystyrene (PS). Depending
on the powder size and MW, they are generally preheated to a temperature near or even above the glass
transition temperature Tg and will melt as the
temperature further rises above Tg. They mainly
have the advantage that their volumetric shrinkage,
when cooling down, does not show a jump, as it
does for semi-crystalline polymers (Figure 4). This
fact provides high accuracy with little distortion.
However, amorphous polymers show a lower level
of consolidation (higher residual porosity) and hence
do not reach the strength of their molded equivalent
(Figure 20b). The higher degree of porosity, low
thermal shrinkage/expansion and high accuracy
makes them well suited for investment casting
patterns.
5 THERMALLY DEGRADABLE AMORPHOUS
POLYMERS FOR SLS OF METAL OR
CERAMIC PARTS
Various SLS applications make use of a powder
combining a degradable polymer binder and a
structural material (metal or ceramic) that produces a
“green part” in which the structural particles are
bound into a degradable polymer matrix [9]. These
‘green parts’ are further processed in a furnace to
burn out the polymer (“de-binding”) and to
consolidate the remaining polymer-free metal or
ceramic part by post-sintering, infiltration, or
HIPing. The polymers used as the sacrificial binder
in those applications are different from the polymers
described in the previous and subsequent sections.
They should allow de-binding, i.e. thermal depolymerization. Polymers used for this purpose are
PMMA and copolymers MMA-BMA that can be debinded in a furnace at temperatures around 350450°C. They should not be water soluble [6, p. 112]
and should have sufficient strength, even when
heated up in the de-binding furnace, in order to
avoid that the ‘green part’ would collapse before
some solid state sintering can consolidate the
metallic or ceramic particles together. The polymer
must also be fully de-bindable (i.e. de-polymerized
and sublimated) and should not yield excessive
contamination of the remaining part with carbon
residuals (important e.g. for steel parts). If the
strength of the polymer in the green stage is
insufficient, the green part can be consolidated by
cross-linking the binder. This can be done by
impregnating the green part with a water soluble
thermosetting acrylic emulsion and drying it in a
oven at 50°C, before it goes to the de-binding
furnace [6, p. 139]. Alternatively, a cross-linker can
be added into the degradable polymer coating of the
powder [6 p. 139]. This will eliminate the step of
impregnating the green part.
When using sacrificial binders, care should be
taken because de-polymerization could also occur
during SLS [6 p. 108, 35, 37]. Indeed, these
polymers are deliberately designed to de-polymerize
as an aid to their thermal removal in post-SLS
processing. Polymers based on copolymers of nbutyl methacrylate (BMA) and methyl methacrylate
(MMA) have been observed to also de-polymerize
during SLS processing [68, 69].
The polymer binder may be added to the
structural material in two ways: mixing [20, 28, 46]
or coating [61, 62, 67, 77]. Typical volume of binder
in the powder is 5% for mixed powders [20] as well
as coated ones [6, p136]. The latter typically
corresponds to a 5 µm polymer coating on a 55 µm
1080 carbon steel grain [68].
Figure 21 [78] demonstrates three stages in the
SLS process of Apatite-Wollastonite (AW) glass
ceramic with 5 wt% MMA-BMA binder:
a) powder mixture before processing (white
particles are AW glass ceramic, grey particles are
MMA-BMA),
b) highly porous part after SLS processing (The few
polymer connections are just enough to give the
'green' part sufficient strength to be transferred to
the post-treatment furnace)
c) 'brown' part after polymer de-binding and 1 hour
firing at 1150°C. During firing, AW is partly
melted. The part still is not totally dense and may
need further densification to convert the brown
part into a final one.
a
b
c
Figure 21 : SLS of AW glass ceramic with sacrificial binder:
(a) Powder mixture (b) Green part (c) Brown part
Figure 22 : Injection mold made at K.U.Leuven by SLS of
polymer-coated steel powder (LaserForm™) and molded
connector housings
Figure 22 shows a typical mould made by SLS of
a polymer-coated steel powder (LaserForm™). After
building the part, the green part was de-binded in a
furnace and infiltrated with bronze.
6 SEMI-CRYSTALLINE REINFORCED
POLYMER FOR REINFORCED PLASTIC OR
COMPOSITE PARTS
Today there exist several SLS powders aimed at
production of reinforced polymer parts. Applications
include parts made from glass reinforced PA [14,
16], Cu filled PA [10], Al filled PA [1, 22, 47], SiCPA [27, 33], HA-HDPE, i.e. hydroxyapatite
reinforced high density polyethylene [31, 57], HAPA [58, 59] and other filled or reinforced polymers
[3, 15, 18, 19, 25, 26, 30, 38, 73, 80]. Unlike the
sacrificial polymer binders described in §4, the
polymers used here should withstand thermal
degradation and should be durable. Figure 23
illustrates an injection mold made from Cu
reinforced polyamide.
reinforced PA powder having elongated filler beads.
This new powder turned out to be better than the
traditional GF-PA powder: tensile strength x 1.8; Emodulus x 1.3; elongation x 3.3.
Figure 24 : Glass-nylon powder mixture
(Source: K.U.Leuven)
Aluminum filled PA 12 often shows coagulation,
since mixing the aluminum and PA powder particles
isn’t always very successful. This causes segregation
during layering due to the differences in size and
specific weight (Figure 28). This fact is even more
striking in PA 12-Cu. The inhomogeneous solid with
voids creates mechanical defects and deteriorates the
tensile strength. The local heat concentration on the
filler has a negative influence on the binder polymer
causing a very short recycle ability of 2-3 times only
(for PA 6-8 times depending on judgment criteria).
Figure 23 : Injection mold made from Cu-filled Polyamide
and Polypropylene molded parts (injected at 2.76 MPa and
230°C)
The powder for producing such polymer matrix
composite can be provided in 3 different ways:
- The initial powder may be a mixture of polymer
particles and reinforcement beads. Figure 24
shows such a mixture of spherical PA and glass
particles: on the right side of the picture, the glass
particles are colored blue with Prussic ink for
better visibility
- Alternatively, the single powder particles may
already be a composite containing of a polymer
matrix (typically PA or PE) containing filler
particles [57, 58]: see Figure 25.
- A 3rd alternative is to use a polymer-coated metal
or ceramic powder [10, 48].
Figure 26 shows the fracture surface of a part
made with a popular commercial glass filled nylon
powder. One can see that the connection between the
glass beads and PA binder was not perfect, that
many glass beads have been pulled out and that the
PA was stretched during breakage, resulting in a
rough spongy surface. Figure 27 shows a new
Figure 25 : Cross section of a glass filled nylon part
(Source: K.U.Leuven)
Figure 26 : PA-12 GF One of the most popular glass beads
reinforced HTD materials (Source FHSG St. Gallen)
a
Figure 27 : New glass filled PA powder (Source FHSG St.Gallen)
b
a
Figure 29 : Polyester based elastomer a) Green part after
sintering at low hardness b) Infiltrated with polyurethane
(Source FHSG St. Gallen)
The influence of laser processing parameters on
Shore A hardness and ductility is shown by Figure
32. Clearly the parts processed with higher laser
power become more ductile and have a higher Shore
A hardness.
The influence of laser processing parameters on
elastic modulus E and the tensile strength Rb at
rupture is shown by Figure 33.
b
Figure 28 : a) PA 12 and 30% Al blend; b) a tensile break
cross section with defects due to inhomogeneity (Source
FHSG St. Gallen)
7 ELASTOMERIC MATERIALS
Elastomeric polymers have a structure with long
chains and only few cross-links between them.
Below the glass-transition temperature they are
brittle, above they are very elastic. When the density
of cross-links enlarges, strength and stiffness also
enlarge, and ductility lowers.
A new elastomer powder material for SLS was
recently released [44]. Figure 29 demonstrates this
polyester based elastomer (patent pending). A typical
part is depicted in Figure 30. The most significant
properties in the thermoplastic material selection
were considered and compared with reference to
elastomeric materials as Neoprene, EPDM and
natural rubber. Figure 31 gives a comparison of the
different elastomeric materials for Shore A hardness
and elongation at break: materials 1 to 4 are SLS
materials.
Figure 30 : Example of a flexible shoe made by SLS of
polyester-based powder
8 POLYMER BLENDS
Polymeric blends offer an alternative mean to obtain
SLS parts with specific structure and properties,
permitting the development of new applications.
Most polymeric blends are multiphase systems and,
therefore, their properties largely depend on their
microstructure [17, 76]. Salmoria, for instance, used
blends of PA and HDPE (blend ratio’s of 80/20,
50/50 and 20/80 wt%) to achieve dedicated
properties. Depending on the blend ratio, different
phases and micro-structures were observed using
SEM, EDX and XRD analysis [56]. Others are
mixing PS and PA [63].
E Modulus [MPa]
Shore A (high)
Elongation %
100
1000
75
750
50
500
25
250
Elongation %
10
5
not infiltrated
infiltrated
0
0
5
10
Laser Power [W]
a
15
20
Sintaflex: Rupture at break Rb vs. E Module
6
0
0
0
1
2
3
4
5
6
7
Rb
Rb (inf.)
8
Rb [Mpa]
Shore A Hardness
Shore A (low)
Sintaflex: E module vs. LS Laser Power
15
Material
4
2
0
Figure 31 : Developed elastomeric material for SLS in
relation to commonly used elastomers (Source FHSG St.
Gallen)
Sintaflex not infiltrated
Shore A Hardness
100
80
60
7W 9W
40
11W
13W
15W
5W
20
0
100
150
200
250
Elongation (%)
300
350
Shore A Hardness
Sintaflex infiltrated
100
80
60
5W
40
7W
9W 11W 13W 15W
20
0
100
150
200
250
300
350
Elongation (%)
Figure 32 : Hardness Shore A vs. Elongation [%] for
infiltrated and not infiltrated parts processed with different
laser powers (Source FHSG St. Gallen)
In a US patent [21] the following is claimed:
A particle for use in selective laser sintering
(SLS), including a core (1) formed from at least one
first material, and at least partial coating (2) of the
core (1) with a second material (further components
are optional), the second material having a lower
softening point than the first material, wherein the
softening point of the second material is lower than
approximately 70°C.
0
b
2
4
6
8
E Modulus [MPa]
10
12
Figure 33 : (a) The elastic modulus [Mpa] against laser
power for infiltrated and not infiltrated parts
(b) The tensile strength Rb [MPa] at ruptures against elastic
modulus (Source FHSG St. Gallen)
The coating (2) generally contains a polymer,
preferably a thermoplastic polymer, e.g. a polyvinyl
acetal, preferably a polyvinyl butyral. It may consist
of alloys with a low softening point which are used
e.g. in fuses. Moreover saturated linear carboxylic
acids with a chain length of ≥16 (e.g. heptadecanoic
acid, melting point 60-63°C.) or polymers in the
broadest sense may also be suitable. The softening
point of the 2nd material of approximately 70°C or
below, allows laser sintering to be carried out at
significantly lower temperatures compared to
particles which have been used hitherto, and
therefore also allows a significantly lower
temperature difference between irradiated particles
and standard room temperature. Tests have shown
that the lower maximum temperature difference also
improves the temperature homogeneity of the
building space as a whole.
9 THERMOSETTING MATERIALS
Thermosetting polymers can be used in different
steps of the SLS process. These materials can be
used as an infiltrant [23, 50], because it is a
relatively inexpensive path to a fully dense, stiff, net
shape, polymer matrix composite part. In addition
this is a very useful intermediate step for fully
functional materials, as critical surfaces and
tolerances may be achieved easily at this stage. An
example of infiltration with thermosetting polymers
is the production of metal-epoxy molds via SLS
indirect processing [6, p143]. In this process, SLS is
used to form green mold cavity inserts from metal
powder that is coated with fusible thermoplastic
binder. In subsequent steps, the binder is thermally
removed and the metal powder is oxidized to form a
porous metal/ceramic cavity that shows little
shrinkage and generally excellent retention of
geometry, relative to the green part. The cavity is
then strengthened and sealed by infiltration and cure
of an epoxy tooling resin.
(in yellow) in comparison to some molded polymer
materials (in blue): the comparison is shown for
tensile strength, tensile modulus and elongation. We
find some deficits. While some SLS materials have
comparable properties to molded ones, none of them
is able to reach the highest values of tensile modulus
and strength, nor elongation. The low elongation
capability can be explained by the many short
binding necks creating high stiffness with little
elongation.
Injection vs. SLS Materials
It is also possible to produce metal parts by laser
sintering a mixture of metals with thermosetting
polymers [46]. When a thermosetting material is
exposed by a laser source, the thermosetting material
turns into a viscous liquid instantly. When for
instance laser sintering an epoxy resin mixture with
iron powder, the polar groups (e.g. epoxy group) in
the molecule of the resin are activated
simultaneously. The liquid flows penetrate the
‘pipes’ made from pores and wet metal particles. As
such, they make bridges from one particle to another.
The binding effect, depicted in Figure 34, is
dominated by the interfacial characteristic between
the resin and the iron. Iron surfaces commonly attach
some active hydrogen atoms because of its attraction
of some molecules such as H2O and HCl due to its
high polarity. Hydrogen bonds occur between the
electronegative oxygen atoms in polar groups in
resin molecules and the electropositive active
hydrogen ones on iron surfaces. Iron particles are
strongly bonded because hydrogen bond attraction is
more intensive than that of inter-molecular action
existing on the interfaces between iron surfaces and
other non-polar polymers. The resin viscosity is
lowered at higher temperature (i.e., laser energy) and
its viscous liquid can spread easily. Thus many more
iron particle surfaces can be attached by resin.
However, degradation of resin, induced by excessive
laser energy, might occur and reduce the bonding
capability.
10 PROPERTIES OF MOLDED AND SLS
POLYMERS
Although great efforts have been made and some
results were achieved, the reasons for the relative
few SLS material options are many. In future we
would like to have nearly the same choice as e.g. in
injection molding. Today we can only compare the
differences and compensate for deficits. Figure 35
shows all the commercially available SLS materials
Injection
1
12000
SLS
2
3
a
b
8000
4
4000
5 6
de f
c
0
0,0
100,0
7
9
8
200,0
Elongation [%]
300,0
400,0
250
Tensile strength [MPa]
Figure 34 : Binding mechanism for epoxy resin to iron
particles
Tensile modulus [MPa]
16000
200
1
150
2
Injection
3
100
SLS
10
a
4
50
c
0
0,0
50,0
6
9
7
d e f
100,0
8
150,0
200,0
250,0
300,0
350,0
400,0
Elongation [%]
Figure 35 : Commercial available SLS materials (yellow) in
comparison to some injection polymer materials (in blue)
Figure 36 : Boundary between loose powder and solid part
A further interesting finding indicated by
Zarringhalam [78] is the boundary between the loose
powder and the solid part (see Figure 36, which
shows the microstructure of a part cross section).
Under certain conditions unconsolidated particles
may remain in the solid part. This can be overcome
easily by proper energy intake settings, correct scan
strategies like cross scanning (in X, Y, in X&Y and
so on) or multi times scanning of the same layers.
The boundary conditions may be partially improved
by the so called out-line or skip scanning (i.e.
scanning the layer contours separately) imposing a
sharper border line between loose powder and
consolidated part.
To conclude this paper on polymers, Table 1
gives an overview of some commercial available
SLS polymers with their most important properties
concerning the SLS process.
Table 1 : Overview of commercial available polymers with
most important properties for SLS
Tg (Glass
transition
temperature)
(°C)
Tm (Melt
temperature)
(°C)
PA 6
50
230
PA 12
(41)
184
9600
-65 … -60
58 … 60
40000 …
80000
143
340
Material
PCL
PEEK
MW
(molecular
weight)
(g/mol)
7
8
9
10
11
12
13
14
11 CONCLUSIONS
This paper has given a survey of ways of
consolidating polymer powders into solid material
by Selective Laser Sintering. Although a lot has been
achieved so far – making SLS of polymers to one
of the most popular RP&M techniques – a lot
remains to be done to widen the scope of polymers
and polymer blends that can be processed by SLS
and to improve the properties of resulting SLS parts.
15
16
17
18
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