Scholarly article on topic 'Selective Laser Sintering of Filled Polymer Systems: Bulk Properties and Laser Beam Material Interaction'

Selective Laser Sintering of Filled Polymer Systems: Bulk Properties and Laser Beam Material Interaction Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Katrin Wudy, Lydia Lanzl, Dietmar Drummer

Abstract Additive manufacturing techniques, such as selective laser melting of plastics, generate components directly from a CAD data set without using a specific mold. The range of materials commercially available for selective laser sintering merely includes some semi crystalline polymers mainly polyamides, which leads to an absence of realizable component properties. The presented investigations are concerned with the manufacturing and analysis of components made from filled polymer systems by means of selective laser sintering. The test specimens were generated at varied filler concentration, filler types and manufacturing parameter like laser power or scan speed. In addition to the characterization of the mixed powders, resulting melt depth were analyzed in order to investigate the beam material interaction. The basic understanding of the influence of different fillers, filler concentration and manufacturing parameters on resulting component properties will lead to new realizable component properties and thus fields of application of selective laser sintering.

Academic research paper on topic "Selective Laser Sintering of Filled Polymer Systems: Bulk Properties and Laser Beam Material Interaction"

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Physics Procedia 83 (2016) 991 - 1002

9th International Conference on Photonic Technologies - LANE 2016

Selective laser sintering of filled polymer systems: Bulk properties and laser beam material interaction

Katrin Wudya'b'*? Lydia Lanzla'b, Dietmar Drammera'b

aInstitute of Polymer Technology, Am Weichselgarten 9, 91058 Erlangen bCollaborative Research Center 814 - Additive Manufacturing, Am Weichselgarten 9, 91058 Erlangen

Abstract

Additive manufacturing techniques, such as selective laser melting of plastics, generate components directly from a CAD data set without using a specific mold. The range of materials commercially available for selective laser sintering merely includes some semi crystalline polymers mainly polyamides, which leads to an absence of realizable component properties. The presented investigations are concerned with the manufacturing and analysis of components made from filled polymer systems by means of selective laser sintering. The test specimens were generated at varied filler concentration, filler types and manufacturing parameter like laser power or scan speed. In addition to the characterization of the mixed powders, resulting melt depth were analyzed in order to investigate the beam material interaction. The basic understanding of the influence of different fillers, filler concentration and manufacturing parameters on resulting component properties will lead to new realizable component properties and thus fields of application of selective laser sintering. © 2016PublishedbyElsevierB.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Bayerisches Laserzentrum GmbH

Keywords: Selective Laser Sintering; composite material; filled polymers; bulk properties

1. Introduction and motivation

Production in the future has to fulfill certain requirements like a high degree of individualization and personalization by simultaneous reduced durability. The trend towards individualized serial products steps up the need for the respective manufacturing techniques to be more and more flexible. Selective laser sintering, an additive manufacturing technique, which allows the manufacturing of components directly from CAD data without any specific mold or form, may comply with this heightened demands on products. [1]

To fulfill the exemplified requirements a wide available material spectrum for selective laser sintering is necessary. Up to now only a small number of semi-crystalline polymers can be processed via selective laser sintering

* Corresponding author. Tel.: +49-9131-85-29717 ; fax: +49-9131-85-29709 . E-mail address: wudy@lkt.uni-erlangen.de

1875-3892 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Bayerisches Laserzentrum GmbH

doi:10.1016/j.phpro.2016.08.104

[2]. However, new component properties can be reached with filled systems. The aim of this work is to set up a feasibility study for manufacturing composite materials consisting of glass spheres and polyamide 12 through selective laser sintering. Bulk, optical and thermal properties of the powder mixture will be analyzed to investigate the influence of the filler concentration and filler size on process relevant characteristics. Furthermore, the influence of energy input parameters on single layer specimens was investigated.

2. State of the art

In selective laser sintering a polymer powder is selectively fused by a CO2 laser and thus a component is build up layer by layer in a surrounding powder bed. In a first step the polymer powder is applied into a building chamber, which is preheated on a temperature between the melting and crystallization point of the semi crystalline polymer, via a coating system (a knife or a roller). The layer thickness is set between 80 and 150 ^m at a median particle size of approximately 60 ^m. The theory of quasi-isothermal laser sintering signifies that polymer melt and powder coexists during the whole building process. Semi crystalline polymers fulfill these requirements due to a hysteresis between the melting and crystallization temperature. After material coating/ the cross section of the component is fused through a CO2 laser controlled by a scanning system. In contrast to conventional plastics processing techniques, like injection molding or extrusion the polymer remains in the place where it is fused and no pressure is applied during laser sintering. The particles coalescence is mainly driven by viscosity and surface tension of the melt. The surrounding powder acts as support structure for the fused components. After the energy input the building chamber is lowered down by the thickness of one layer. Hence, layer by layer a component is generated. A schematic diagram of the selective laser melting process is shown in Figure 1. [1, 3]

Fig. 1. Process scheme of selective laser sintering. [4, 5].

Selective laser sintering can process any polymer materials that tend to fuse or melt when heat is applied [6]. However, the material spectrum suitable for SLS is limited to few thermoplastic polymers like polycarbonate (PC), thermoplastic elastomers (thermoplastic urethane TPU), polyetherketone (PEK), polyamide 12 (PA12) and polyamide 11 (PA11). Lately, great efforts were made to overcome this disadvantage and develop new noncommercial materials, like polybuthylene terephthalate (PBT) for selective laser sintering [7-10]. However, available materials cannot completely meet the needs of different functional end use parts. Composite materials can overcome this lack of properties available with laser sintered parts.

There are several approaches in literature, which try to manufacture composite materials via laser sintering. Mazzoli et al. [11] characterized bulk properties, particles size distribution and several part properties of a composite material consisting of polyamide 12 and aluminum particles. The authors suggest using the developed material for stiff parts, with a metallic appearance in prototype construction [11]. Nevertheless, there was neither a systematic change of material properties like filler concentration nor a variation of processing parameters to detect beam powder interaction.

Forderhase et al. [12] analyzed the dependence of particle size, as well as filler form (spheres and fibers) of a glass filled polyamide 12 powder on mechanical properties. According to the investigation, optimal glass filled material for SLS applications is a mixture with 29 volume percent glass beads with a median particle size of 35 ^m [12]. Experimental in combination with theoretical modeling and numerical analysis were conduct in [13] to derive

optimal process parameters for a composite material consisting of polyamide 11 and glass beads (volume fraction 10 to30 percent).

Kruth et al. gives in [14] and [15] an overview of commercial filled powders for selective laser sintering and describes a new glass filled polyamide 12 powder, which have a better performance than commercial GF-PA powder. Furthermore, the investigations reveal in the case of a commercial glass filled PA12 powder a poor connection between the glass spheres and the polymer matrix.

3. Experiments

3.1. Material

For the experiments, a polyamide 12 glass sphere composite is used. In table 1 all used materials, suppliers, trademarks and some material characteristics are listed. The composition varies between 10, 30 and 50 volume percentage glass spheres. The two types of glass spheres only differ in the particle size distribution, so the influence of the filler size on bulk and resulting part properties can be analyzed. The average particle size of the glass spheres Spheriglass 3000 is 27 ^m and 11 ^m for the Spheriglass 5000. To ensure to prepare a homogeneous mixtures of the polyamide 12 particles and glass spheres the composite is mixed in a rotary mixer for one hour at 400 rpm.

Table l.Used materials.

Material Polyamide 12 Glass spheres Glass spheres

Supplier EOS GmbH Velox GmbH Velox GmbH

Type PA2200 Spheriglass 3000 Spheriglass 5000

Dosage form Powder Powder Powder

Median particle size dso>3 (^m) 1 60 27 11

Bulk density (g/cm3)1 0.44 1.36 0.99

Density (g/cm3)b 1.01 2.5 2.5

Melting temperature (0C)1 dT/dt = 10K/min 185 0C

Crystallization temperature (0C)1 dT/dt = 10K/min 150 0C

Refraction indexb 1.51 1.51

1 own measurements b material data sheet

3.2. Powder Characterization

To get an idea of the shape and size of the polyamide 12 powder scanning electron microscopy (SEM) images are taken. Therefore, a scanning electron microscope (type: Ultra Plus) form the supplier Carl Zeiss AG is used.

Bulk density, particle size distribution, optical and thermal behavior of the mixtures and raw materials are determined before the laser sintering process, to make a rough estimation of the process behavior.

The particles size distribution is measured via optical imaging. According to this a small amount of powder is distributed on a glass plate. Afterwards with a microscope an image of each particle is taken. This process is automated and approximately 60,000 particles are analyzed to get a particle size distribution.

Bulk density will be measured according to DIN EN ISO 60 [16]. The bulk density is defined as the mass of the powder particles divided by the total volume they occupy. Therefore, a cylinder with a defined volume of 100 ml is filled with powder and the mass of the powder is determined.

Optical properties of the powdery good plays a major role in selective laser sintering. The optical characteristics of powdery goods can be determined with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In DRIFTS, electromagnetic radiation reflected from dull surfaces is collected and analyzed as a function of frequency

(v, usually in wavenumbers, cm-1) or wavelength (X, usually in nanometers, nm) [17]. For the analysis an infrared spectrometer from the supplier Thermo Scientific (Nicolet 6700) with an DRIFTS unit from PIKE Technologies (Diffus IR) is used. The analysis was conducted between wavenumbers of 400 and 4,000 cm"1.

Besides optical, thermal properties of the basic material determine the building chamber temperature and the process window according the model of quasi-isothermal laser sintering. The range of temperatures where the composite material is in a two-phase mixture area must be determined by differential scanning calorimetry (DSC). Thermograms of the composites and pure materials are determined, due to possible changes of the melting and crystallization temperature by the addition of fillers. The DSC measurements are conducted with a device from TA Instruments (Type: Q2000) at a scan rate of 10 K/min under nitrogen atmosphere according to DIN EN ISO 11357 [18]. The mass of the test specimen is approximately 5 mg in all investigations.

3.3. Processing and specimen characterization

3.3.1. Processing

The glass spheres filled polyamide 12 mixtures are processed on a research laser sintering system with the parameters shown in Table 2. The hatch distance (0.2 mm) and the layer thickness were held constant for each building process. The laser sintering system is equipped with a CO2 Laser with a wavelength of 10.6 ^m, a maximum power of 50 watts and a focus diameter of approximately 400 ^m. This laser sintering system was chosen due to homogeneous temperature distribution over the building space, caused by a heater system with 12 heating zones. The processing temperature is estimated according to the model of quasi-isothermal laser sintering starting from the basic materials' DSC curves.

Table 2. Processing parameters.

Material

Composition

Building chamber temperature TB

Laser power PL (W)

Scanning speed vs (mm/s)

Energy density ED (J/mm2)

Polyamide 12 + glass spheres

100/0 90/10 70/30 50/50

4.5 7.9 11.3 9.5

0.025 0.044 0.063 0.105

The energy density is calculated according to the following equation:

Ed =-P- (1)

Vs • K

with PL the laser powder, vs the scanning speed and hs the hatch distance.

3.3.2. Test specimen

To analyze the layer thickness and the filler distribution the test specimens were single layers with the dimensions of 25 mm x 25 mm. These single layers were manufactured in small building chambers shown in Figure 2, to neglect the factors of part position and the coating mechanism. To analyze the influence of energy input on the filler distribution and layer thickness the laser power varies between 4.5 and 11.3W. The varying laser power leads to different area energy densities, which is the most commonly used parameter in laser sintering. The scanning speed was held constant to avoid time dependent effects described in [19, 20].

Fig. 2. Small build chambers forthe exposure of single layers.

3.3.3. Specimen characterization

The layer thickness and the filler distribution are analyzed for the laser sintered single layers on microscopic images of polished cross sections. An Axiolan type microscope by Zeiss serves to scrutinize the specimens in a bright field. The layer thickness is measured at least on 5 positions.

The de facto filler content of the single layers is analyzed via thermogravimetric measurements with a device form TA Instruments type Q5000. The employed heating rate was 10 K/min. Only the specimen with a median energy density of 0.063 J/mm2 were tested.

4. Results and Discussion

4.1. Powder Properties

As a first step the basic material characteristics like particle size distribution, bulk density, thermal and optical properties are investigated. The volumetric particle size distribution of the pure polyamide powder and of the two used types of glass spheres is shown in Figure 3. Polyamide 12 powder (PA12, Type: PA2200) exhibits a narrow particles size distribution with a median particle diameter of 60 ^m. In contrast the particle size of the two used glass sphere types is lower. Spheriglass 3000 shows a wide distribution, between 4 ^m and 100 ^m with a maximum at 27 ^m. In the case of the glass sphere type Spheriglass 5000 the particle size distribution is even more shifted to smaller particle sizes compared to Spheriglass 3000. The particle size range varies between 1 ^m and 20 ^m, with a median volumetric particle diameter of 11 ^m. Thus in any case the mixtures of polyamide and glass spheres will have a smaller median particle size than pure polyamide 12.

Fig. 3. Volumetric particle size distribution ofthe basic materials polyamide 12 and two types ofglass spheres (Spheriglass 3000 and Spheriglass 5000).

After characterizing the pure materials, the mixtures are generated by a rotary mixer for one hour at 400 rpm. Bulk material properties, like bulk density, affects the porosity of laser sintered parts. Hence, bulk density is analyzed in dependency of filler concentration and glass sphere type, Figure 4. The bulk density of the mixture with Spheriglass 3000 exhibits a steady growth with increasing filler content due to the higher specific gravity of glass with 2.5 g/cm3 compared to polyamide 12 with 1.01 g/cm3. The bulk density of the mixtures with Spheriglass 5000 increases just as in the case of Spheriglass 3000. Nevertheless, the incline of the straight line is lower, due to a lower bulk density of Spheriglass 3000. The smaller particle tends to agglomerate, because of higher adhesion forces and thus the flowability and bulk density is reduced compared to Spheriglass 3000. A lower bulk density is equivalent to a higher powder porosity, which may lead to lower component densities oflaser sintered parts.

On basis of the specific gravity of polyamide 12 and glass in Table 1 the bulk porosity can be calculated, Figure 4 (right). With increasing filler content, the porosity of the bulk decreases in the case of Spheriglass 3000. Thus the glass particles can fill some gaps between the polyamide 12 particles. Higher filler will lead to a denser fill and may result in components with a higher density. The bulk property stays at a stable level with varying filler content for Spheriglass 5000. The smaller particle size compared to Spheriglass 3000 goes along with higher adhesion forces and thus an affinity to agglomerate.

0 10 30 50 100 0 10 30 50 100

filler concentration (vol.-%) filler concentration (vol,-%)

Fig. 4. Bulk density ofthe raw materials polyamide 12 and glass spheres (Spheriglass 3000 and Spheriglass 5000) as well as of the composite powders.

Beside bulk characteristics, optical behavior of the powdery good is of main importance in selective laser sintering of thermoplastic polymers. Therefore, the optical behavior of the composite material is analyzed with diffuse reflectance spectroscopy. The diffuse reflectance of the mixtures and of the polyamide 12 powder is determined in a wavenumber band between 4,000 cm"1 and 400 cm"1, which is synonymous to a wavelength of 2.5 ^m and 25 ^m, Figure 5. The black graph represents the diffuse reflectance spectra of polyamide 12 and red, blue and green one of the composite powders exemplary for the filler type Spheriglass 3000. At wavenumbers above 2,000 cm"1 the graphs are shifted to higher diffuse reflectance with increasing filler content. Whereas for wavenumbers beneath 1,200 cm the diffuse reflectance is reduced with increasing filler content.

In the case of selective laser sintering the diffuse reflectance at the CO2 laser wavelength of 10.6 ^m and accordingly 943 cm"1 is of main interest. Therefore, the diffuse reflectance is shown in Figure 7 at a wavelength of 10.6 ^m in dependency of filler content for the two types of glass spheres. With increasing glass concentration, the diffuse reflectance is reduced at the wavelength of 10.6 ^m respectively a wavenumber of 943 cm"1. The reduced diffuse reflectance is a hint for a higher absorption with increasing filler content, which leads to lower energy demand during exposure compared to raw polyamide powder. From 30 vol.-% glass spheres on the diffuse reflectance lingers on a stable level near the values of 100 vol.-% glass spheres. At a filler content of 10 vol.-% the

glass spheres with a lower median particle size exhibit a lower diffuse reflectance, which can be tracked back to higher number of interfaces. This effects levels out for higher filler contents.

Fig. 5. left: Diffuse reflectance spectra of polyamide 12 powder as well as ofthe composite powders right: diffuse reflectance in dependency of the filler concentration and glass sphere type (Spheriglass 3000 und Spheriglass 5000).

The melting and crystallization behavior of the polyamide powder and the composite materials is investigated by means of caloric measurements. The effect of Spheriglass 3000 on the melting and crystallization temperature is exemplary diagrammed in Figure 6 (left). With increasing filler content, the crystallization temperature is displaced to higher temperatures. The shift of the crystallization temperature to lower values in an indicator for the foreign particle acting as crystallization nucleus. The materials maximum melting peak is for each mixture roughly the same.

The peak maximum of the crystallization curve against the filler content is shown for both glass sphere types in Figure 6 (right). Again with higher filler content the crystallization temperature is heightened. In context of this investigation there is no dependency of the filler type on the crystallization temperature recognizable. According to the DSC analysis the working processing range is changed due to the addition of fillers, which may lead to a modified processing behavior.

Fig. 6. left: DSC diagrams ofpolyamide 12 powder as well as of the composite powders right: crystallization temperature in dependency offiller concentration and glass sphere type (Spheriglass 3000 und Spheriglass 5000).

4.2. Specimen Properties

In addition to the analysis of raw and composite powders the laser sintered specimens are characterized according to their filler content, layer thickness, filler distribution and adhesion between glass spheres and polyamide matrix.

Figure 7 (left) shows thermogravimetric curves of specimens produced at an energy density of 0.063 J/mm3 exemplary for the filler type Spheriglass 3000. Due to higher filler content the beginning of the degradation is shifted to higher temperatures. The de facto filler content in volume percent is evaluated on the basis of the specific gravities of the raw material and shown in Figure 7 (right) for the two types of glass spheres. The real filler content coincides with the set point in the case of Spheriglass 5000. For Spheriglass 3000 the estimated filler content is slightly lower than the set value.

Fig. 7. left: thermogravimetric analysis ofthe specimen (ED = 0.063 J/mm2) made ofcomposite powders; right: de facto filler concentration in volume percent forthe two glass spheres (Spheriglass 3000 und Spheriglass 5000).

In addition to the filler concentration the layer thickness of the specimens for several energy densities, filler contents and types of glass spheres were investigated, Figure 8. For the analysis of the layer thickness the single layers were embedded in thermoset material and polished vertically to the exposure direction. Subsequently, the evaluation of the layer thickness takes place at at least five positions across the cross-section.

PA12 + Spheriglass 3000

I 600 :

V 500 -

I 400 H

S 300 -I

jg 200 -100 -0

energy density (J/mm2)

■ 0.025 A 0.063

• 0.043 ▼ Ï 0.105 i

Ï W I Ï Î Ï A

n=5,TB=175 °C

800 ■ 700 ■

I 600.

f 500 ■

J 400 ■

'ë 300 •

j? 200 ■

100 ■ 0 ■

PA12 + Spheriglass 5000

energy density (J/mm2)

■ 0.025 ▲ 0.063

• 0.043 T 0.105

X Ï j À

Ï I Ï

n=5, TR=175 °C

filler conentration (vol.-%)

filler concentration (vol.-%)

Fig. 8. Layer thickness ofthe specimen in dependency offiller concentration, fillertype and energy density.

In the case of Spheriglass 3000 the layer thickness rises with increasing energy density for filler contents between 10 vol.-% and 30 vol.-%, Figure 8 (left). Furthermore, the layer thickness rises with increasing filler content up to 30 vol.-%. In the case of a 50 vol.-% glass sphere concentration the resulting specimen were porous impeding the evaluation of the layer thickness. Up to 30 vol.-% the diffuse reflectance in Figure 5 decreases and thus the absorption of laser energy within the bulk rises, which may lead to higher layer thicknesses. Spheriglass 5000

exhibits an almost similar behavior, with rising energy density the layer thickness is increasing. Contrary to Spheriglass 3000, the layer thickness lingers at a stable level for rising filler content. For high filler contents in combination with low energy densities no specimen could be generated. The particle size of Spheriglass 5000 is with a median particle size of 11 ^m smaller than the particle size of Spheriglass 3000. Hence, Spheriglass 5000 offers a higher surface volume relationship and more boundary surfaces the polymer melt has to embed. A higher content of particle interfaces go along with more beam traps and thus the layer thickness is lower compared to Spheriglass 3000.

Fig. 9. Microscopic images (bright field) of cross sections of single layers for varying filler content and an energy density of 0.063 J/mm2.

In Figure 9 the cross sections of embedded single layers manufactured with an energy density of 0.063 J/mm2 is shown via microscopy in bright field to clarify the filler distribution and the connection between the glass spheres and the polyamide 12 matrix. Like previously described with increasing glass sphere percentage the layer thickness rises slightly, but at the same moment the porosity grows dramatically. For a filler content of 50 vol.-% there is no longer a dense specimen but rather a porous network.

Fig. 10. Scanning electron microscopy image ofanetched polyamide 12 specimen filled with glass spheres (Spheriglass 3000) to visualize the filler adhesion.

The connection between the glass spheres and the polyamide 12 matrix is visualized in Figure 10 via scanning electron microscopy images of an etched single layer specimen (energy density 0.063 J/mm2). The lamellar structure of the polyamide 12 matrix can be emphasized via etching process. Some of the glass spheres are enclosed in the polymer matrix. Nevertheless, the particle in the right corner emphasizes a wide gap between the glass particle and the matrix material, which is a hint for an insufficient connection between the two materials.

5. Summary

Within this paper the process behavior and resulting component properties during laser sintering of glass sphere filled polyamide 12 was investigated. Mixtures of polyamide 12 and two types of glass spheres only differing in in particles size, were generated and analyzed according to theirs bulk density, optical and thermal material behavior. Simple test geometries, single layers, were exposed with different energy densities, accordingly. The filler content, the layer thickness and the filler distribution were investigated by means of the cross section of the single layers.

The addition of glass spheres to a polyamide 12 powder leads to an increase of bulk density depending on bulk densities of the raw materials. The diffuse reflectance decreases with rising filter content. The type of glass spheres and thus the particle size added do not lead to a change of optical bulk behavior. The decline of diffuse reflectance goes along with an increase of absorption at the CO2 laser wavelength of 10.6 ^m. A rising content of glass particles is attended by an increasing crystallization temperature, due to the mode of operation of the foreign particles as nucleating agent. This may cause difficulties like undesirable curling during laser sintering process.

After characterizing the bulk materials single layer specimens were generated with varying laser power. The layer thickness is rising with increasing filler content and energy density in the case of Spheriglass 3000 up to a filler content of 30 vol.-%. For higher glass sphere percentage, the manufacturing of single layers was not feasible. Compared to Spheriglass 3000 with Spheriglass 5000 only porous networks could be generated. This effect can be tracked back to the higher surface volume ratio and thus more interfaces the polymer melt has to enclose due to lower particle size.

On the basis of this paper fundamental interaction between bulk characteristics of a glass filled composite material and laser beam could be derived. These investigations form the basis of further research on the area of functional composite materials in selective laser sintering.

Acknowledgements

The authors would like to extend their thanks to the German Research Association (DFG) for funding the Collaborative Research Centre 814, subproject A6. This support has enabled the investigations in the area of additive manufacturing which have led to the results presented in this article.

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