Infiltration Behavior of Thermosets for Use in a Combined Selective Laser Sintering Process of Polymers
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Selective laser sintering (SLS) of polymers is an additive manufacturing process that enables the production of functional technical components. Unfortunately, the SLS process is restricted regarding the materials that can be processed, and thus the resulting component properties are limited. The investigation in this study illustrates a new additive manufacturing process, which combines reactive liquids such as thermoset resins and thermoplastics to generate multi-material SLS parts. To introduce thermoset resins into the SLS process, the time-temperature-dependent curing behavior of the thermoset and the infiltration have to be understood to assess the process behavior. The curing properties were analyzed using a rotational viscometer. Furthermore, the fundamental infiltration behavior was analyzed with micro-dosing infiltration experiments. Additionally, the infiltration behavior was calculated successfully by using the Washburn equation. Finally, a thermoset resin in combination with a dosing system was chosen for integration in a laser sintering system.
Additive manufacturing technologies, such as selective laser sintering (SLS) of thermoplastic powders, generate components directly from a CAD data set without needing a form or a mold. Thus, the layer-wise process of selective laser sintering allows the generation of individualized complex serial parts. Contrary to conventional manufacturing techniques such as injection molding, which are optimized for high-volume production and low unit costs, the costs for additive manufactured parts are almost independent from the degree of complexity and quantity.1 The advantages of additive technologies are beneficial for products with a high level of customization and complexity as well as small lot sizes, known as “mass customization”.2
State of the Art
high stability at building chamber temperature
specific curing behavior under the temperature occurring in SLS
high infiltration rate, which goes along with a low viscosity and a defined surface tension between the solid and liquid
separation of the two components may not occur
Thus, introducing a material that cures during the SLS process presents specific challenges. To develop thermosets for the SLS process, the curing under a specific temperature profile, determined by the SLS process itself, must be monitored precisely. The thermal household during SLS will be influenced for example by the energy input (laser power, scan speed, etc.), number of exposed components, and chamber and feeder temperature. Besides the curing of the resin, the infiltration of the liquid in the powder bed is of main importance because it ensures a sufficient connection between the two layers. The speed of infiltration and wetting of a powder layer by the used liquid does not just depend on the viscosity, but also on the surface tension σ and other characteristics.12
Holman et al.12 studied the spreading and infiltration behavior of small droplets (approximately 60 μm) of aqueous polymer solutions on high-green-density porous beds. The spreading and infiltration occur in this investigation at the same time scale, precluding their treatment as separate phenomena.12 Tan13 studied the pL impact with porous media with computational fluid dynamics (CFD) and validation via high-speed imaging and used a similar setup as described in this article. Nevertheless, the named study focuses on the inkjet process with aqueous inks and paper as substrate.
The aim of this investigation is to analyze the spreading and infiltration behavior with a specific test setup and determine whether spreading and infiltration can be separated. Furthermore, the influence of the pore size or rather bulk density as well as the surface tension of the used liquid on the infiltrations behavior is of main interest.
For the following investigations, an unmodified PrimePart ST PEBA 2301 polyether block polyamide powder (PEBA) from the supplier EOS GmbH, Germany, is used. PEBA is a thermoplastic elastomer consisting of polyamide and polyether backbone blocks. Two different aging states of the powder, virgin and used powder, were implemented to analyze the infiltration behavior dependent on the bulk density of the material.
As reactive liquid, two epoxy resins, Araldite GY 764 and Araldite GY 793, purchased from Huntsman Advanced Materials, Switzerland, were used. Araldite GY 764 is based on bisphenol A, whereas GY 793 is based on bisphenol A/F. The used curing agent, Hardener XB 3473, was also purchased from Huntsman Advanced Materials. The used mixing ratios were 100:23 parts per weight for GY 764 and 100:22 parts per weight for GY 793. The epoxy resins were selected because of their low viscosity and high reaction temperatures, which made them compatible with the sought processing through a micro-valve in the hot building chamber. GY 764 has a viscosity of 350–550 mPas, GY 793 650–750 mPas and the Hardener XB 3473 80–125 mPas.16 For the further investigations, the mixtures of Araldite and Hardener are abbreviated with the pure resin names Araldite GY 764 and Araldite GY 793.
Characterization of Material Properties
The surface tension of the used resins was analyzed with the pendant drop method and the surface tension measurement setup from DataPhysics. At least ten drops were dosed and evaluated according to their dimensions. The Laplace-Young equation was used to calculate the surface tension of the liquid.
The viscosity of the resins was determined with a Discovery HR-2 rotational viscometer from TA Instruments. As measurement geometry, a plate–plate setup was used. The frequency was set to 0.1 Hertz, and the normal force was 10 Pa. The uncured mixture was applied between the two plates at a starting temperature of 25°C, and the specimen was heated at a rate of 2 K/min up to 200°C. Finally, the complex viscosity was analyzed depending on the temperature.
Besides the influence of the temperature, the influence of the bulk density on the infiltration behavior was investigated. Therefore, the bulk density was analyzed according to DIN EN ISO 60. The bulk density is defined as the mass of the polymer particles divided by the total volume they occupy. For the analysis, an ADP bulk density tester from Emmeran Karg Industrietechnik was used. The average of at least five measurements was calculated.
Design of experiments
Bulk density powder (g/cm3)
Araldite GY 764
20, 40, 60
20, 40, 60
Araldite GY 793
20, 40, 60
20, 40, 60
Results and Discussion
First, the bulk density of the used substrate materials, a thermoplastic elastomer with a volumetric median particle size of 77 μm, was analyzed. As substrate material, virgin powder and powder that had passed through one processing cycle were chosen. The virgin powder shows an average bulk density of 0.39 g/cm3. With increasing numbers of processing cycles, the bulk density decreases to a value of 0.36 g/cm3. The packing density of the bulk material can be calculated by dividing the bulk density and the density of the solid material. Thus, the packing density of the used material is 41% and 37% for a solid material density of 0.95 g/cm3.
Besides the influence of the porosity of the substrate material, the influence of the used liquid resin and thus the surface tension and its viscosity on the infiltration behavior was determined. Therefore, the surface tension of the resins Araldite GY 764 and Araldite GY 793 was measured via the pendant drop method and calculated according the Laplace-Young equation. The resin with the trade name Araldite GY 764 shows a value of ± 37 Nm/m higher surface tension than the resin with the trade name Araldite GY 793 (± 22 Nm/m). Nevertheless, the surface tension cannot be linked directly to the infiltration speed because of the influence of the viscosity and contact angle on the infiltrated volume expressed in Eq. 3.
The infiltration behavior of the resins in a substrate shows a higher porosity or rather lower bulk density of 0.36 g/cm3 (Fig. 5, right). The bulk density of the substrate changes from 0.39 g/cm3 to 0.36 g/cm3. This increase of porosity leads to an increase of infiltration speed. At room temperature, the resin with the Araldite GY764 trademark infiltrates the porous structure in < 60 s, whereas for a substrate bulk density of 0.39 g/cm3 after 80 s a droplet height of 0.5 mm is still measurable. For the substrate with the bulk density of 0.36 g/cm3, the deviation between the minimum and maximum values is larger than for the powder with a bulk density of 0.39 g/cm3. This may be a result of inhomogeneous packing and thus an irregular infiltration. Comparison of the two resins demonstrates a slightly faster infiltration for Araldite GY 793. As shown before, the infiltration speed increases with rising temperature because of the reduced viscosity of the resins. For temperatures > 60°C, the absorption of the liquid is even faster and the droplet height cannot be detected any more.
This article introduces a new hybrid additive manufacturing technique that will overcome the hurdle to manufacturing multi-material parts in one additive process. With selective laser sintering of plastic, only one material can be processed up to now. The implementation of reactive liquids, such as epoxy resins, in the laser sintering process allows the generation of multi-material parts. Therefore, the curing and infiltration behavior of the used resins has to be understood.
The interaction of the resin with the porous powder bed is investigated with a new test setup. The development of droplet height between the powder bed and liquid was determined and linked to the infiltration behavior. For both analyzed epoxy resins Araldite 764 and Araldite 793, Hardener XB 3473 was used. With increasing temperature, the infiltration process is accelerated for both resins. This goes along with the reduced viscosity with increased temperature. At 20°C, the resin does not totally infiltrate the powder bed. For the highest analyzed temperature of 60°C, the infiltration takes < 2 s. This means for layer times in selective laser sintering of 40 s and processing temperature of the powder bed surface of 120°C, the infiltration process will not determine the processing time. Thus, the analyzed systems can be used in the combined laser sintering process. Nevertheless, the infiltration depth and parallel curing must be analyzed in further investigations. Furthermore, the Washburn equation was used to predict the infiltration behavior of the used combination of powders and liquids. Taking the simplifications into account, good agreement of the calculated and measured values could be shown.
The authors thank the German Research Foundation (DFG) for funding Collaborative Research Centre 814 – Additive Manufacturing.
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