Wissenschaftlicher Zeitschrift Kunststofftechnik · Carl Hanser Verlag Zeitschrift Kunststofftechnik/Journal of Plastics Technology 3 (2007) 4 ... Laser Optic s Scanning Mirrors Powder

Carl Hanser Verlag Zeitschrift Kunststofftechnik/Journal of Plastics Technology 3 (2007) 4

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

Evaluation of Polypropylene Powder Grades in 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 bet-

ween 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 ma-

de 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., Prof. Dr.-Ing. Hans-Joachim Radusch, Martin Luther Universität Halle-Wittenberg Center of Engineering Sciences 06099 Halle (Saale) Dr.-Ing. André Wutzler, Polymer Service GmbH Merseburg Geusaer Str., Geb. 131 06217 Merseburg Dr.-Ing. Joerg Gerken, rpm - rapid product manufacturing GmbH Dieselstrasse 15 38350 Helmstedt

E-Mail-Adresse: [email protected] Webseite: http://www.kunststofftechnik.uni-halle.de Tel.: +49 (3461) 46 27 38 Fax: +49 (3461) 46 38 91 Webseite: http://www2.iw.uni-halle.de/ww/psm/ Webseite: http://www.rpm-factories.de

Zeitschrift Kunststofftechnik Wissenschaftlicher

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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

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L. Fiedler et al. Evaluation of Polypropylene Powder Grades

Zeitschrift Kunststofftechnik 3 (2007) 4 1

Evaluation of Polypropylene Powder Grades in 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 major-ity of the PP investigated turned out to be potential materials for laser sintering. Strong differences in the materials behavior influencing the laser sinter proc-essability 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 Er-gebnisse berichtet, die einen Vergleich und ein Ranking der untersuchten Pul-versorten gestatten. Obwohl deutliche Unterschiede im grundlegenden Materi-alverhalten 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 poly-meric 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 perform-ance 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

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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 re-lated properties of the applied plastics powders. A proper understanding of the laser sintering process and an adequate modification of the materials to be sin-tered 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 dis-cover 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 OpticsScanningMirrors

Powder Leveling Roller

Platform

Overflow Cartridge Elevator

InteractionLaser - Powder

PowderLeveling

Melting andCoalescence

Crystallization,Shrinkage and

Curl Figure 1: Scheme of a sinter station and the main steps of the sinter process

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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 car-tridge, 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 par-ticle shape are essential properties for this procedure. The accuracy of the sin-tered 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 im-portant for the accuracy of the sintered part. If there is a good interaction be-tween 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 mini-mized. 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 en-ergy 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 de-scribe 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 rec-ommended [6]. All these properties may be checked efficiently by means of dif-ferential scanning calorimetry.

3 EXPERIMENTAL

3.1 Investigated Materials

Commercial polypropylenes, which may be potential materials for laser sinter-ing, should show a low viscosity and should be delivered, if possible, as pow-ders. 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].

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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 [g/10min] (190°C, 2.16kg)

Melting temperature [°C]

Basic Application and Properties

Code

1 Copolymer 12.2 166 White powder for rota-tional molding P-1

2 Homopolymer 14.4 166 Powder for rotational molding with black pig-ments

P-4

3 Homopolymer 24.6 163 Ultra fine powder, round particles, produced by precipitation

P-5

4 Homopolymer 27.6 170 Injection molding grade, high melt flow, nucle-ated and antistatic, pel-lets

P-6

5 Homopolymer 23.1 163 Powder for masterbatch application with anticak-ing additive

P-7

6 Homopolymer 28.8 164 Powder for masterbatch application P-8

7 Homopolymer 15.3 169 Injection molding grade, medium melt flow, nu-cleated and antistatic, pellets

P-9

Table 1: List of polypropylenes included in the investigation

Number PA type Basic Application and Properties Code

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

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3.2 Thermal Analysis

3.2.1 Differential Scanning Calorimetry (DSC)

DSC is a powerful method to characterize the melting and crystallization behav-ior 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:

20 40 60 80 100 120 140 160 180 200 220

-4

-3

-2

-1

0

1

2

3

endo

window ofsinterability

TMonset

Temperature (°C)

C

orr.

Hea

t Flo

w (

W/g

)

1st heating cooling 2nd heating

TConset

curl caking

Figure 2: DSC plot of polypropylene P-4

TConset = 126°C, TM

onset = 153°C, ∆T = 27°C

The first important parameter - the window of sinterability - is the difference be-tween the onset temperatures of the melting (TM

onset) and crystallization peak (TC

onset). 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 con-trol of the sintering process. The second parameter - the degree of crystallinity of the used material - is rep-resented 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 ac-curacy of the part. The experimental results regarding the width of the window of sinterability of the materials investigated are presented in figure 3:

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A-1 A-2 A-3 A-4 P-1 P-4 P-5 P-6 P-7 P-8 P-90

5

10

15

20

25

30

35

PPPA

Win

dow

of S

inte

rabi

lity

(K

)

Samples

Figure 3: 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 materi-als 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 prob-lems are expected regarding the temperature control of the sintering process. The situation is changed, considering the degree of crystallinity demonstrated in Figure 4:

A-1 A-2 A-3 A-4 P-1 P-4 P-5 P-6 P-7 P-8 P-90

10

20

30

40

50

60

PPPA

Deg

ree

of C

ryst

allin

ity

(%)

Samples

Figure 4: Comparison of degrees of crystallinity of the materials according

to table 1 and 2

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A huge difference occurs between the polyamides and polypropylenes investi-gated. 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 pre-heating and the laser treatment. It is recommended to check the thermal stabil-ity of the potential materials. Figure 5 shows the onset temperatures of thermal destruction, measured with a thermo-gravimetric run under nitrogen atmos-phere. All polypropylene materials used were stabilized sufficiently. The thermal deg-radation 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 measure-ments under the much harsher conditions of an oxygen atmosphere, i.e. from the measurement of the oxygen induction time (OIT).

A-1 A-2 A-3 A-4 P-1 P-4 P-5 P-6 P-7 P-8 P-9400

410

420

430

440

450

460

PPPA

Ons

et T

empe

ratu

re (°

C)

Samples

Figure 5: Onset temperature of thermal destruction in nitrogen atmosphere

of the materials according to table 1 and 2

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3.3 Rheological Measurements

The laser beam scans a given area during the part building process. The pow-der 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 in-crease of the specific volume due to the melting of the crystallites. High viscos-ity 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 meas-urement. 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 tempera-tures just above the melting temperature were chosen. The results are given in figure 6.

P-1 P-4 P-5 P-6 P-7 P-8 P-9 0

500

1000

1500

2000

2500

3000

3500

Vis

cosi

ty a

t 0.1

Hz

(Pa

s)

Samples

180°C 200°C 230°C

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 sinterabil-ity.

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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 sin-tering 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 remain-ing 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 prop-erties of the sintered part can be expected.

3.4 Granulometry

Particle shape and particle size distribution are very important bulk characteris-tics for laser sinter materials. The granulometric properties have a strong influ-ence 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

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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 precipita-tion, 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 mi-croscopy 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.

P-1 P-4 P-5 P-7 P-8 A-1 A-2 A-3 A-4

1

10

100

1000

average value

minimal value

Parti

cle

Siz

e (µ

m)

Samples

maximal value

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 addi-tionally a potential to improve the sintering behavior of the blend.

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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, corre-sponding 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 transfor-mation 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 melt-ing 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, es-pecially in vertical direction, would result. A high absorbance or a low transmit-tance, respectively, at the given wavelength to avoid this failure, is recom-mended. Figure 9 displays the comparison between the absorbance of the investigated polyamides and polypropylenes.

A-1 A-2 A-3 A-4 P-1 P-4 P-5 P-6 P-7 P-8 P-90

5

10

15

20

25

PPPA

Abs

orba

nce

(%)

Samples

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 or-ganic material with functional groups, which are activated at the laser wave-length.

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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 dis-tribution. Possibilities to overcome these shortcomings are:

• 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.

5 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

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[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 Lasersin-tern durch Charakterisierung und Modifizierung grenzflächenenergetischer Prozesse IKV – Berichte aus der Kunststoffverarbeitung, 2004

[8] Schmachtenberg, E., Schoenfeld, M.

Material optimization of PA12 laser sintering 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., Alscher, G., Bruning, S.

Laser sintering of polyamide Kunststoffe (1997), 87(6), 773-774, 776

[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., Prof. Dr.-Ing. Hans-Joachim Radusch, Dr.-Ing. André Wutzler, Dr.-Ing. Jörg Gerken,

Herausgeber: Prof. em. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein, Prof. Dr. Tim Osswald

Erscheinungsdatum: Juli/August 2007

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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 E-Mail-Adresse: [email protected]

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 E-Mail-Adresse: [email protected]

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

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Documents

Wissenschaftlicher Zeitschrift Kunststofftechnik · Carl Hanser Verlag Zeitschrift Kunststofftechnik/Journal of Plastics Technology 3 (2007) 4 ... Laser Optic s Scanning Mirrors Powder