Understanding the influence of energy-density on the layer dependent part properties in laser-sintering of PA12

As the demand for individualization and complex parts is continuously growing, laser-sintering of polymers is on the edge from a pure prototyping technology to manufacturing parts for applications in series production. The influences on resulting parts and layer depending part properties are well known in the literature but the understanding of the interaction between process parameters and layer dependent properties is missing and limiting the dimensioning. Within this study, tensile bars with different amounts of layers and energy densities were produced and investigated for the resulting mechanical properties, roughness, density and the degree of particle melt. The results showed a strong interaction between the energy density and amount of layers, which results in differences in the fracture behavior as well as the mechanical properties. Therefore, the presented results enable the prediction of necessary part thickness for dimensioning thin parts with lasersintering. Introduction *************************************************************************************** *** Additive manufacturing techniques like laser-sintering of polymers (LS) have gained increasing interest for producing individualized parts and small lots for industrial applications [1]. LS manufactures parts by layer-wise fusing of mainly polyamide powders using a CO2-laser [2]. The resulting part properties are generally influenced by the bonding of the layers and the used process parameters like the energy density (ED) [3] as well as the temperature of the building chamber [4]. The parts possess specific properties like a high surface roughness [5], an anisotropic mechanical behavior and a remaining porosity [2]. Different investigations [6, 7] have been performed to characterize the effect of the used ED on the resulting part properties and how the layer-wise manufacturing influences the development of the part properties [8, 9]. The missing link is the interaction between those influences and thus presented in this paper. State of the Art The laser-sintering process is divided in 3 main steps [2]. First, a layer of powder is spread across the building platform, heated just below the melting temperature of the used material and molten by a CO2-laser. Afterwards the building platform lowers by a layer thickness and the steps repeat until the part is fully generated [10]. Due to the layer-wise processing the resulting parts have specific characteristics which divide them from conventional processes like injection molding [11]. Due to the high temperatures during processing and a slow cooling, the parts have a high degree of crystallinity and a homogenous morphology [2]. The surface structure is determined by attaching or semi molten powder particles and lead to a high roughness [5]. Inside the parts a porosity typically between 3-5 % remains, with pores mainly orientated along the layers, which reduce the cross section of the parts. These pores may lead to increased notch effects inside the parts and therefore influence the resulting mechanical properties [2, 12]. In dependency of the used process parameters, unmelted particles can remain in the parts due to inhomogeneous melting resulting of mostly under dosed energy input [13]. The resulting LS-parts exhibit higher mechanical properties and densities with increasing part thickness [8] as well as influences on the dimensional accuracy, which are also affected by the used hatching strategy [9]. The properties are mainly determined by the used process parameters, which show decreasing anisotropy with increasing energy density [14] until a maximum is reached which is limited by material degradation [15]. Besides the energy the used temperatures inside the process chamber [16], the orientation of the parts during processing as well as the scanning strategy [17] influence the size and shape of the melt pool and therefore the temperature dependent material properties like viscosity [15, 18] and surface tension [19]. The layer attachment mainly influences the resulting part properties and the layer-wise manufacturing of the parts [8, 9]. Still there is a remaining anisotropy mainly for the elongation at break, which leads to higher mechanical properties along the building platform [10]. 835 Solid Freeform Fabrication 2019: Proceedings of the 30th Annual International Solid Freeform Fabrication Symposium – An Additive Manufacturing Conference Reviewed Paper Summing up the state of art, it shows that within the laser-sintering of polymers the influence of the process parameters and especially the ED on the resulting part properties have been investigated in many approaches. Furthermore, the layer-wise manufacturing has exhibited a strong influence on the development of the part properties. The missing link is the interaction between those influences, as they tend to interact and have a great effect on the part properties, which is the aim of the presented results. Experimental Setup The investigations were performed using a polyamide 12 (PA12) powder of the type PA2200 (EOS GmbH, Krailing Germany) with a refreshing rate (ratio between new and used powder) of 50 %. Powder mixing was performed using a mixer of the type MP-20 (Somakon UG, Lünen Germany) for 30 min with a rotational speed of 200 min. To ensure a good powder quality for processing, the viscosity number (Vn) was determined according to DIN EN ISO 307 [20] and showed a value of 61 ml/g. Additionally, the packing density was measured following DIN EN ISO 60 [21] and showed a density of 0.45 g/cm. The used powder has a melting peak temperature of 185 °C and a crystallization peak temperature of 150 °C. The specimen’s geometry of the type 1A according to DIN EN ISO 527 [22] was used with different number of layers (1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40) with constant layer thickness of 100 μm and hence resulting varying part thickness. The parts were produced using a LS-manufacturing system of the type P110 (EOS GmbH, Krailing Germany) with a building chamber temperature of 167 °C and a scanning speed of 2500 mm/s. A hatchdistance (hd) of 0.25 mm was used with 4 different energy densities (ED) (0.25, 0.30, 0.35 and 0.40 J/mm). To ensure a constant thermal history of the parts, 4 building jobs were performed with 8 parts of the same number of layers in the building cross section. Beginning with the highest amount of layers in the first building layer. The part’s dimensions were analyzed using a micrometer for the thickness and a coordinate measuring machine for the width. The characterization of the morphological properties was performed using thin cuts through the cross-section of the specimen’s middle section and analyzed by transmitted light microscopy. The surface roughness was measured using a tactile stylus instrument Waveline 20 (Jenoptik, Jena Germany) with a stylus tip of 2 μm and an attaching force of 0.8 mN. The roughness Ra was determined according to DIN EN ISO 4287 [23] in 3 parallel lines in the middle of the test specimen along the longest axis for the bottom and the top surface with a measuring distance of 15 mm. The density measurement was performed volumetric. To investigate the melting behavior, the degree of particle melt (DPM) was determined according to Zarringhalam [24] by performing DSC-measurement of the specimen’s cross-section following 11357 [25] with a heating ratio of 10 K/s and evaluation the peak areas. The specimens for mechanical testing were conditioned to a humid state until reaching weight consistency. The weight of the specimens was measured repeatedly by means of a precision balance in order to ensure the weight consistency. The mechanical properties were measured following the DIN EN ISO 527 [22] with a crosshead speed of 1 mm/min for the young’s modulus using a testing setup of the type 5948 (Instron, Darmstadt Germany) and 50 mm/min for the elongation at break and the tensile strength with a tensile testing machine of the type 1484 (Zwick Roell, Ulm Germany). The fractured surface was analyzed by scanning electron microscopy to characterize the breaking behavior of the parts. Results and Discussion The dimensional accuracy of the parts is strongly influenced by the interaction of the ED and the number of layers as shown in Figure 1 and exhibits an oversize of the thickness. Mainly the parts with a low amount of layers display a high deviation of the thickness, which increases with a higher ED especially for the first two layers. The resulting thickness consists of two main factors. The surface structure with attaching or semi molten powder particles and the core of the parts, which consist of the molten structure and form in combination with the surface the thickness of the specimens. Higher EDs lead to deeper penetration of the powder bed in the first layers and therefore a higher deviation at the beginning, which is slowly negotiated with increasing layers as a constant layer thickness of 100 μm is added and develops the resulting thickness of the parts.