Evaluation of Polypropylene Powder Grades in

Transcrição

Nicht zur Verwendung in Intranet- und Internet-Angeboten sowie elektronischen Verteilern.
www.kunststofftech.com
© 2007 Carl Hanser Verlag, München
Wissenschaftlicher
Arbeitskreis der
UniversitätsProfessoren der
Kunststofftechnik
Zeitschrift Kunststofftechnik
Journal of Plastics Technology
archivierte, rezensierte Internetzeitschrift des Wissenschaftlichen Arbeitskreises Kunststofftechnik (WAK)
archival, reviewed online Journal of the Scientific Alliance of Polymer Technology
www.kunststofftech.com; www.plasticseng.com
eingereicht/handed in: 24.01.2007
angenommen/accepted: 10.06.2007
Dr.-Ing. Lothar Fiedler, Luis Osvaldo Garcia Correa, M.Sc.,
Prof. Dr.-Ing. Hans-Joachim Radusch,
Martin Luther Universität Halle-Wittenberg
Dr.-Ing. André Wutzler,
Polymer Service GmbH, Merseburg
Dr.-Ing. Jörg Gerken,
rpm GmbH, Helmstedt
Consideration of the Laser Sintering
Processability
The paper deals with the evaluation of polypropylene (PP) powder grades in consideration of the laser
sintering processibility. It is reported on experimental investigations for comparison and ranking between the polymer grades analysed. Although strong differences in the basic materials behavior exist,
the general applicability of PP powder grades for laser sintering could be stated. Proposals were made to adapt PP to the laser sintering process for optimal processability and high property level.
Autor/author
Dr.-Ing Lothar Fiedler, Luis Osvaldo Garcia Correa, M.Sc.,
Martin Luther Universität Halle-Wittenberg
Center of Engineering Sciences
06099 Halle (Saale)
E-Mail-Adresse: [email protected]
Webseite: http://www.kunststofftechnik.uni-halle.de
Tel.: +49 (3461) 46 27 38
Fax: +49 (3461) 46 38 91
Polymer Service GmbH Merseburg
Geusaer Str., Geb. 131
06217 Merseburg
Webseite: http://www2.iw.uni-halle.de/ww/psm/
Dr.-Ing. Joerg Gerken,
rpm - rapid product manufacturing GmbH
Dieselstrasse 15
38350 Helmstedt
Webseite: http://www.rpm-factories.de
Carl Hanser Verlag
Zeitschrift Kunststofftechnik/Journal of Plastics Technology 3 (2007) 4
L. Fiedler et al.
Evaluation of Polypropylene Powder Grades
Consideration of the Laser Sintering
Processabiliy
L. Fiedler1, L. O. Garcia Correa1, H.-J. Radusch1, A. Wutzler2, Jörg Gerken3,
Martin Luther Universität Halle-Wittenberg1,
Polymer Service GmbH, Merseburg2,
rpm GmbH, Helmstedt
In comparison to polyamide (PA) the application of polypropylene (PP) for laser
sintering does not lead to satisfying results until now. Therefore, the goal of this
work was to identify and to evaluate the properties of PP being essential for the
laser sintering processability. For this purpose thermal and rheological analysis,
FTIR spectroscopy, and granulometric experiments were performed. The majority of the PP investigated turned out to be potential materials for laser sintering.
Strong differences in the materials behavior influencing the laser sinter processability have been found concerning the degree of crystallinity, the capability
to absorb the laser energy, and in the particle size distribution. In the result of
the investigation strategies for materials modification of PP grades for adapting
to laser sintering were proposed.
Der Artikel befasst sich mit der Untersuchung von Polypropylen-Pulversorten
hinsichtlich ihrer Lasersinter-Verarbeitbarkeit. Es wird über experimentelle Ergebnisse berichtet, die einen Vergleich und ein Ranking der untersuchten Pulversorten gestatten. Obwohl deutliche Unterschiede im grundlegenden Materialverhalten der untersuchten Proben existieren, konnte eine grundsätzliche
Eignung der Polypropylene für das Lasersintern festgestellt werden. Es werden
Vorschläge unterbreitet, die Materialien für verbessertes Sinterverhalten und ein
gutes Eigenschaftsprofil zu modifizieren.
1
INTRODUCTION
Although laser sintering is a well established technology in the field of Rapid
Prototyping and Rapid Manufacturing, the catalogue of successfully used polymeric materials for these methods is surprisingly narrow [1]. The most common
materials used in this technology are polyamide, polystyrene, and a new grade
of a thermoplastic elastomer [1,2,3]. The application of special high performance plastics like PEEK is in development [4,5]. At the other hand, more and
more customers ask for laser sintered parts made from commodity polymers
like PE or PP, but the application of such polyolefine powders for laser sintering
does not lead to satisfying results at present. Several problems and failures like
Zeitschrift Kunststofftechnik 3 (2007) 4
1
L. Fiedler et al.
extended shrinkage and curl, blocking of the powder bed and cracks in the most
upper powder layer during the pre-heating process have been observed. If it
was possible to get sintered parts at all, strength and toughness of the parts
were absolutely nonsatisfying. Therefore it is of particular interest to discover
the mechanisms associated with the laser sintering process, as well as the related properties of the applied plastics powders. A proper understanding of the
laser sintering process and an adequate modification of the materials to be sintered may lead to a successful result in this process. Research is in progress by
different scientists. An approach for modeling of the quasi isothermal melting
and coalescence was introduced by Alscher [6]. Seul [7] and Schmachtenberg
[8] discussed the role of surface energy and roughness for the sinterability of
powders and propose functional coatings.
The goal of our investigation was to identify and to evaluate the properties of
polypropylenes, being essential for the laser sintering processability, and to discover feasibilities for modification of commercially available PP materials.
2
MECHANISMS OF LASER SINTERING AND RELATED
PROPERTIES
Laser sintering is an inovative production process in which parts are produced
layer by layer using the effect of local melting by an infrared laser beam. The
advantage of this technology is the feature to produce functional parts directly
from computer models without application of any tools [9]. A scheme of a sinter
station and the main steps of the sinter process are shown in figure 1.
Laser
Powder
Leveling
Optic s
Scanning
Mirrors
Interaction
Laser - Powder
Powder Leveling Roller
Platform
Overflow
Cartridge
Melting and
Coalescence
Elevator
Crystallization,
Shrinkage and
Curl
Figure 1: Scheme of a sinter station and the main steps of the sinter process
2
L. Fiedler et al.
The first step of the process is the powder leveling, i.e. the preparation of a new
powder layer on the platform of the machine. The powder is delivered by a cartridge, and a leveling roller or a sweeper, respectively, moves the powder to the
lowered platform and spreads it to a thin layer. Particle size distribution and particle shape are essential properties for this procedure. The accuracy of the sintered part in vertical direction depends on the layer thickness and is limited by
the maximum size of the powder particles. Very fine and non-spherical particles,
at the other hand, tend to agglomeration and will disturb the leveling procedure.
The second step is characterized by the interaction between laser and powder.
The laser beam scans the particular geometry and is responsible for the local
melting process. There should be a sharp drawing of the borderline of the part,
a close hatching of the inner area as well as a sufficient bonding of the drawn
pattern to the previous layer [1]. A good absorbance of the laser radiation is important for the accuracy of the sintered part. If there is a good interaction between laser and powder, the local melting is easy to control and the laser power
as well as the temperature differences in the processing chamber can be minimized.
If the powder particles are molten, the fused material has to join to melt strands
or areas. This procedure of coalescence is the main problem in the third step of
the sintering process. The driving force for the coalescence is the surface energy of the plastics [7]. The melt flow is determined by the viscosity of the melt.
The zero shear viscosity of the materials used is an important parameter to describe this process step.
The last step is the cooling of the part. It is associated with crystallization,
shrinkage and sometimes with curl. To avoid curl it is necessary to realize such
processing conditions, that the crystallization does not start during the sintering
process [10]. Generally, a low degree of crystallization, a narrow melting peak
and a wide temperature difference between melting and crystallization are recommended [6]. All these properties may be checked efficiently by means of differential scanning calorimetry.
3
EXPERIMENTAL
3.1
Investigated Materials
Commercial polypropylenes, which may be potential materials for laser sintering, should show a low viscosity and should be delivered, if possible, as powders. A series of polypropylenes assumed to be appropriate for laser sintering
was assembled for the evaluation and ranking procedure. The chosen materials
and their codes are listed in table 1. Among them grades for rotational molding,
for high speed injection molding, and some ultrafine polypropylene powders
recommended for use as additives were included. Most of the investigated
polypropylenes were homopolymers in order to meet the condition of narrow
melting peaks, as discussed in [6] and [11].
3
L. Fiedler et al.
Furthermore a commercial sinter powder (polyamide) and some polyamide
based substitutes already successfully applied in laser sintering have been
used as reference materials. They are listed in table 2.
Nr. PP Type
MFI
Melting
Basic Application and
[g/10min] temperature
Code
Properties
(190°C, 2.16kg) [°C]
1
Copolymer
12.2
166
White powder for rotational molding
P-1
2
Homopolymer 14.4
166
Powder for rotational
molding with black pigments
P-4
Ultra fine powder, round
particles, produced by
precipitation
P-5
Injection molding grade,
high melt flow, nucleated and antistatic, pellets
P-6
Powder for masterbatch
application with anticaking additive
P-7
3
4
5
Homopolymer 24.6
Homopolymer 27.6
Homopolymer 23.1
163
170
163
6
Homopolymer 28.8
164
Powder for masterbatch
application
P-8
7
Homopolymer 15.3
169
Injection molding grade,
medium melt flow, nucleated and antistatic,
pellets
P-9
Table 1:
List of polypropylenes included in the investigation
Number
PA type
Basic Application and Properties
1
PA12
Original powder for laser sintering
A-1
2
PA11 and PA12
Dry blend of A1 and A3, recycled
A-2
3
PA11
Coating powder with high melt flow
A-3
4
PA11 and PA12
Dry blend of A1 and A3
A-4
Table 2:
List of polyamides used as reference materials
Code
4
3.2
Thermal Analysis
3.2.1 Differential Scanning Calorimetry (DSC)
DSC is a powerful method to characterize the melting and crystallization behavior of semi-crystalline plastics. There are two parameters, which are of special
interest for the laser sintering process. Both will be explained by means of an
example of a typical DSC curve given in figure 2:
endo
3
2
onset
1st heating
cooling
2nd heating
1
Corr. Heat Flow (W/g)
L. Fiedler et al.
TC
onset
TM
0
-1
curl
-2
caking
window of
sinterability
-3
-4
20
40
60
80
100
120
140
160
180
200
220
Temperature (°C)
Figure 2:
DSC plot of polypropylene P-4
onset
TC
onset
= 126°C, TM
= 153°C, ∆T = 27°C
The first important parameter - the window of sinterability - is the difference between the onset temperatures of the melting (TMonset) and crystallization peak
(TConset). This window delimits the range of the powder bed temperature. Too
low temperatures lead to premature crystallization of the sintered layers, that
causes curl, and too high temperatures lead to growing of the part or to caking
of the powder bed. Generally is valid, the wider the window the easier the control of the sintering process.
The second parameter - the degree of crystallinity of the used material - is represented by the area below the melting peak of the sample. Here is valid, the
lower the crystallinity the lower the shrinkage and the better the dimensional accuracy of the part.
The experimental results regarding the width of the window of sinterability of the
materials investigated are presented in figure 3:
5
35
Window of Sinterability (K)
30
25
20
15
10
5
0
A-1
A-2
A-3
Figure 3:
A-4
P-1
P-4
P-5
Samples
PA
P-6
P-7
P-8
P-9
PP
Width of the windows of sinterability of the materials according to
table 1 and 2
Comparing the windows of sinterability, it becomes obvious that all PPs are in
the same range or sometimes even better than the PA references. The materials with the largest window of sinterability are the polypropylenes P-1 and P-5,
but the windows of sinterability of all PP samples are big enough, thus no problems are expected regarding the temperature control of the sintering process.
The situation is changed, considering the degree of crystallinity demonstrated in
Figure 4:
60
Degree of Crystallinity (%)
L. Fiedler et al.
50
40
30
20
10
0
A-1
A-2
A-3
PA
Figure 4:
A-4
P-1
Samples
P-4
P-5
P-6
P-7
P-8
P-9
PP
Comparison of degrees of crystallinity of the materials according
to table 1 and 2
6
L. Fiedler et al.
A huge difference occurs between the polyamides and polypropylenes investigated. The crystallinity of the PPs exceeds the values for PAs of about 100%. A
high crystallinity will cause a high amount of shrinkage and includes the risk of
distortion of the sintered part. It is to conclude, that the polypropylenes for laser
sintering should be modified. Two measures can be taken into consideration to
avoid excessive shrinkage: Either one modifies the chemical structure of the
material to decrease the crystallinity, or one fills the material with a reinforcing
filler to reduce the overall shrinkage of the compound. Both measures should
improve the sinterability of the PP powders.
3.2.2 Thermo-Gravimetric Analysis (TGA)
Materials for laser sintering have to stand a high thermal load during the preheating and the laser treatment. It is recommended to check the thermal stability of the potential materials. Figure 5 shows the onset temperatures of thermal
destruction, measured with a thermo-gravimetric run under nitrogen atmosphere.
All polypropylene materials used were stabilized sufficiently. The thermal degradation in nitrogen atmosphere begins at relatively high temperatures, thus no
degradation should occur during the sintering process. The thermal stability of
the polypropylenes is markedly better than that of the investigated polyamides,
as displayed in figure 5. Similar tendencies have been obtained from measurements under the much harsher conditions of an oxygen atmosphere, i.e. from
the measurement of the oxygen induction time (OIT).
460
Onset Temperature (°C)
450
440
430
420
410
400
A-1
A-2
A-3
PA
Figure 5:
A-4
P-1
Samples
P-4
P-5
P-6
P-7
P-8
P-9
PP
Onset temperature of thermal destruction in nitrogen atmosphere
of the materials according to table 1 and 2
7
3.3
Rheological Measurements
The laser beam scans a given area during the part building process. The powder particles are transformed into melt droplets by this way. A closed area can
only be obtained by coalescence of adjacent melt droplets. There is no acting
pressure in difference to conventional sintering processes, and the influence of
gravity on the melt flow can be neglected. This process of coalescence is forced
by a high surface energy of the melt droplets, and it is supported by the increase of the specific volume due to the melting of the crystallites. High viscosity at the other hand would act disadvantageously for coalescence. The existing
melt deformation rates are very low due to the small mechanical driving forces.
Therefore, the zero shear viscosity is a useful parameter to describe the flow
behavior during the laser sintering. The viscosity can be extrapolated to zero
shear stress or strain, respectively, if it is possible to quantify the plateau region
of the flow curve. The lower the viscosity the more complicated is this measurement. Unfortunately, the cone-plate rheometer used in these investigations
reached the limits of its performance capacity. So, we decided to compare the
statistically firmed results at adequate low shear rate of 0.1 s-1. Three temperatures just above the melting temperature were chosen. The results are given in
figure 6.
Viscosity at 0.1 Hz (Pa s)
3500
L. Fiedler et al.
3000
180°C
200°C
230°C
2500
2000
1500
1000
500
0
P-1
P-4
P-5
P-6
P-7
P-8
P-9
Samples
Figure 6:
Viscosity of the investigated materials at low shear rate and
different temperatures
Here the results of the measurements using PA are not shown, because they
are not comparable to PP. The measured viscosity of the sinter powder A1 has
been increased during the long duration of the measurement. This is an effect
of crosslinking, which is in agreement with the observed fact, that recycled PA is
of worse quality and must be mixed with virgin powder to maintain the sinterability.
8
L. Fiedler et al.
Comparing the viscosity values of the polypropylenes, it becomes obvious, that
big differences between the different grades exist. The materials P-5, P-6 and
P-8 show the best results, whereas P1 seems to be not applicable for laser sintering technology. This behavior is confirmed by optical micrographs of solid
particles and molten droplets on cold and hot stage, respectively, , as shown in
figure 7.
Figure 7: Optical micrographs from hot stage melting at 180°C
a) Solid polypropylene particles P-1, b) Molten polypropylene droplets P-1
c) Solid polypropylene particles P-5, d) Molten polypropylene droplets P-5
The melt droplets of polypropylene P-1 are not able to coalesce completely.
They separate or they form some small necks only (see figure 7 a and b). The
sintered parts will show an insufficient mechanical strength and a high remaining porosity as a result of this melt behavior. All other polypropylenes are well
coalescing, as to see for P-5 (figure 7 c and d) as an example, and good properties of the sintered part can be expected.
3.4
Granulometry
Particle shape and particle size distribution are very important bulk characteristics for laser sinter materials. The granulometric properties have a strong influence on the processability as well as on the final properties of the sintered
parts. Too coarse particles will limit the accuracy of the sintered part, whereas
9
too fine powders will tend to agglomeration, and an exact powder leveling is not
possible anymore. Some authors proposed a bi-modular distribution to reach
optimal results [10,12].
Round particle shape, as to get from processes like spray drying or precipitation, will be advantageous in comparison to the sharp-edged particles from
grinding processes.
Figure 8 shows a comparison of PA sinter powders with the investigated PP
materials. The particle size distribution has been measured using optical microscopy and image analysis. The vertical lines in the graph demonstrate the
range from minimum to maximum particle size, and the dashes mark the mean
diameter, which is not inevitably the mid-point of the range because of the
asymmetric particle size distribution curves.
1000
maximal value
average value
Particle Size (µm)
L. Fiedler et al.
100
minimal value
10
1
P-1
P-4
P-5
P-7
P-8
A-1
A-2
A-3
A-4
Samples
Figure 8: Minimum, average and maximum particle sizes
The materials P-1 and P-4, designed primarily for a rotational molding process,
are too coarse for the laser sintering process. Even a sieving of the powders
cannot really change the situation, because there is a lack of particles less than
10 µm. Powder P-5, at the other hand, is too fine and will cause problems due
to the tendency of agglomeration.
The particle size distributions of the powders P-7 and P-8 are most similar to
these of the used polyamides, but the range of particle diameters seems to be
too narrow.
It is necessary to develop new technologies of pulverization, which deliver
round particles with a particle size distribution approximately between 1 and 100
µm and a mean diameter equal or bigger than 10 µm.
A combination of two powders of different average particle diameters has additionally a potential to improve the sintering behavior of the blend.
10
3.5
Infrared Spectroscopy
Laser sinter stations are in general equipped with carbon dioxide lasers. These
lasers emit a monochrome infrared light at a wavelength of 10.6 µm, corresponding to a wave number of 943 cm-1. The infrared light hits the surface of
the most upper powder layer and is transformed to heat, which increases the
temperature of the particles and lead to a local melting process. The transformation into heat energy depends on the absorbance of the polymer at 943 cm-1.
It is necessary to increase the laser energy at low absorbance to ensure a melting process, and it may happen, that the depth of penetration of the laser beam
exceeds the layer thickness. As a failure an undefined growing of the part, especially in vertical direction, would result. A high absorbance or a low transmittance, respectively, at the given wavelength to avoid this failure, is recommended.
Figure 9 displays the comparison between the absorbance of the investigated
polyamides and polypropylenes.
25
20
Absorbance (%)
L. Fiedler et al.
15
10
5
0
A-1
A-2
A-3
PA
A-4
P-1
Samples
P-4
P-5
P-6
P-7
P-8
P-9
PP
Figure 9: Infrared absorbance at wave number of 943 cm-1
It is obvious that a huge difference between the two types of materials exists. A
way to change this unsatisfying situation is the addition of an IR-absorber to the
polypropylene.
Such an absorber may be an inorganic filler, for instance a pigment, or an organic material with functional groups, which are activated at the laser wavelength.
11
L. Fiedler et al.
4
CONCLUSIONS AND OUTLOOK
A collection of polypropylene materials has been investigated and compared
with commercial materials for laser sintering. The majority of the investigated
PP materials are potential materials for laser sintering.
Thermal stability, window of sinterability and zero shear viscosity are better or at
least comparable to polyamide properties. Deficits in properties have been
found in the degree of crystallinity, laser energy absorption and particle size distribution. Possibilities to overcome these shortcomings are:
5
•
Preparation of a dry mix of coarse PP with ultrafine powder to
get a flowable material with bimodal or multimodal particle size
distribution for dense sintered parts.
•
Application of new technologies of pulverization to produce fine
powders with rounded particle shape.
•
Blending with copolymers and additives to influence viscosity
and crystallinity.
•
Application of inorganic fillers to reduce shrinkage and curl and
to improve IR absorbance.
ACKNOWLEDGMENT
The authors thank the Federal Ministry of Economics and Technology of the
Federal Republic of Germany for funding these investigations in the frame of
the research program PRO INNO II.
6
LITERATURE
[1]
Gebhardt, A.
Rapid Prototyping
Hanser Publishers, Munich, 2003
[2]
-
www.eos.info 2006-11-06
[3]
-
www.3dsystems.com 2006-11-06
[4]
Woicke, N.,
Wagner, T.,
Eyerer, P.
Selective Laser Sintering of High Temperature
Resistant Thermoplastic Polymers
Proceedings of the 21st Annual Meeting of the
Polymer Processing Society, Leipzig, Germany,
June 19-23, 2005
12
L. Fiedler et al.
[5]
Rechtenwald, T.,
Roth, S.,
Pohle, D.
Funktionsprototypen aus PEEK
Kunststoffe, Munich, (2006)11, 62-68
[6]
Alscher, G.
Das Verhalten teilkristalliner Thermoplaste beim
Lasersintern
Essen, Univ.-GH, Diss., 2000
[7]
Seul, T.
Ansätze zur Werkstoffoptimierung beim Lasersintern durch Charakterisierung und Modifizierung
grenzflächenenergetischer Prozesse
IKV – Berichte aus der Kunststoffverarbeitung,
2004
[8]
Schmachtenberg, E., Material optimization of PA12 laser sintering
Schoenfeld, M.
powder to improve surface quality
Annual Technical Conference – Society of Plastics
Engineers (2006)
[9]
Keller, B.
Rapid prototyping: Grundlagen zum selektiven
Lasersintern von Polymerpulver
Stuttgart, Univ., Diss., 1998
[10] Dickens, E.D., Jr.
et al.
Sinterable Semi-Crystalline Powders and
Near-Fully Dense Article Formed Therein
DTM Corp., US Patent 5 648 450, 1997
[11] Schmachtenberg, E., Laser sintering of polyamide
Kunststoffe (1997), 87(6), 773-774, 776
Alscher, G.,
Bruning, S.
[12] McAlea, K.P.,
Forderhase, P.F.,
Booth, R.B.
Polymer Powder of Controlled Particle Size
Distribution
DTM Corp., Internat. Patent WO97/29148, 1997
Stichworte:
Rapid Prototyping, Laser Sintering, Polypropylene, Properties
Kontakt:
Autoren:
Dr.-Ing. Lothar Fiedler,
Luis Osvaldo Garcia Correa, M.Sc.,
Dr.-Ing. Jörg Gerken,
Herausgeber:
Prof. em. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein,
Prof. Dr. Tim Osswald
Erscheinungsdatum:
Juli/August 2007
13
L. Fiedler et al.
Herausgeber/Editor:
Europa/Europe
Prof. Dr.-Ing. Dr. h.c. G. W. Ehrenstein, verantwortlich
Lehrstuhl für Kunststofftechnik
Universität Erlangen-Nürnberg
Am Weichselgarten 9
91058 Erlangen
Deutschland
Phone:
+49/(0)9131/85 - 29703
Fax.:
+49/(0)9131/85 - 29709
Amerika/The Americas
Prof. Dr. Tim A. Osswald, responsible
Polymer Engineering Center, Director
University of Wisconsin-Madison
1513 University Avenue
Madison, WI 53706
USA
Phone:
+1/608 263 9538
Fax.:
+1/608 265 2316
Verlag/Publisher:
Carl-Hanser-Verlag
Jürgen Harth
Ltg. Online-Services & E-Commerce,
Fachbuchanzeigen und Elektronische Lizenzen
Kolbergerstrasse 22
81679 Muenchen
Tel.: 089/99 830 - 300
Fax: 089/99 830 - 156
E-mail: [email protected]
Beirat/Editorial Board:
Professoren des Wissenschaftlichen Arbeitskreises Kunststofftechnik/
Professors of the Scientific Alliance of Polymer Technology
14

Evaluation of Polypropylene Powder Grades in

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