Advances in Polymer Materials for Additive Manufacturing: Producing Polymer Composites via SLS Additive Manufacturing

Additive manufacturing is a production technique that creates three-dimensional geometries by applying the additive design principle, in which materials are joined to each other layer by layer. The terms direct digital manufacturing, freeform manufacturing, rapid prototyping, and layer-additive manufacturing are also used to describe additive manufacturing. This production technique is subdivided into different groups within polymer-based material processing. Different methods have been developed to process polymers in various forms using the additive manufacturing method, ranging from polymers in the solid state to those in liquid form [1-3]. When considering industry-standard criteria such as mechanical properties, thermal stabilization, component precision, surface quality, and long-term stability, selective laser sintering (SLS), a powder-bed additive manufacturing technique, is one of the most advanced additive manufacturing methods in recent years. The SLS method is a thermal process in which material integration, powder melting, and subsequent solidification of the melt into the desired shape occur [4], [5]. One of the greatest advantages of Selective Laser Sintering (SLS) is that many different parts can be manufactured together in a single production run at the same time. Unlike many other additive manufacturing techniques, in laser sintering, parts to be sintered can be freely placed throughout the entire production area (x-y-z directions) without requiring any support structures, and their production can be carried out [5]. Figure 1 shows different schematic representations of the SLS technique. In this technique, laser-sintered parts are created layer by layer at layer thicknesses ranging from 0.06 mm to 0.18 mm [6]. In additive manufacturing methods such as selective laser sintering and selective laser melting, unlike conventional plastic processing techniques such as injection molding or extrusion, laser-sintered parts are produced under pressureless conditions. In laser sintering, the driving force is the temperature increase resulting from the interaction of the polymer in the powder bed with the laser, along with the temperature provided by the heating system. As a consequence, the properties of the produced parts depend on the interaction of energy input, melting temperature, and the melt viscosity of the materials [6]. In the laser sintering process, the sintering window is one of the most important criteria for the usability of a polymer powder in the laser sintering (LS) process. The sintering window covers the range between the start of crystallization temperature (Tc onset) and the start of melting temperature (Tm onset). Differential scanning calorimetry (DSC) evaluation of powders enables the determination of temperatures that can be selected in the process [4], [5]. Figure 2 shows an example sintering window area in a DSC thermogram.

Figure 2. DSC Thermogram – Laser Sintering (LS) temperature range as "Sintering Window" [4]

For the SLS method, the thermoplastic polymers currently known and most widely used are polyamide (most often PA 12, followed by PA 11, PA 6), and some polymers other than polyamide include polystyrene, polypropylene, polyethylene, polyetheretherketone, and thermoplastic urethane [3], [9], [10]. To improve the mechanical and physical properties of laser-sintered polymer parts, some studies in the literature reinforce polymers with fillings such as glass microspheres, silicon carbide, aluminum powder, hydroxyapatite, nanosilica, nanoclay, and nano-Al2O3 in micron and/or nanoscale sizes [11]. On the other hand, many other filler materials and combinations thereof, and their use in polymer composite structure production via the SLS additive manufacturing method, investigating the distribution of fillers within the polymer structure and improving the interface compatibility properties of filler materials with the polymer are current topics. For this reason, some different-quality fillers such as hollow glass microspheres can be used to obtain lightweight polymer composite structures using the SLS method. Weight reduction is particularly important for the automotive and aerospace industries to increase fuel efficiency and reduce CO2 emissions. One of our research efforts in this field concerns the investigation of the effects of hollow glass microspheres (HGM) and their quantities in the polyamide 12 (PA 12) matrix on the final properties of the PA 12-HGM composite structure manufactured using the SLS method.

The main objective of the research is to achieve weight reduction in additive manufacturing products manufactured by SLS with HGM addition to PA 12 without sacrificing much in terms of mechanical properties.

Based on the results of physical and mechanical tests conducted, with the increase in the quantity of hollow glass microspheres in the structure, in addition to a significant increase in E-modulus, the weight of the composite structures was significantly reduced, up to 20% [12]. We are continuing our research and development work on developing these and similar types of new polymer composite structures using the SLS technique, a powder-bed additive manufacturing method.

References [1] International Organization for Standardization (ISO)/ASTM International. ISO/ASTM 52900:2015 [ASTM F2792], Additive manufacturing-General principles-Terminology. https://www.iso.org/standard/69669.html. Publication date: December 2015. Access date: 18.01.2020. [2] Forster A. M., Materials Testing Standards for Additive Manufacturing of Polymer Materials: State of the Art and Standards Applicability, National Institute of Standards and Technology, Gaithersburg, (2015). [3] Dizon, J. R. C., Espera, Jr. A. H. , Chena, Q., Advincula, R. C., "Mechanical Characterization of 3D-Printed Polymers", Additive Manufacturing, 20: 44-67, (2018). [4] Schmid, M., Wegener, K., "Additive Manufacturing: Polymers Applicable for Laser Sintering (LS)", ICMEM 2016, Nový Smokovec, (2016). [5] Schmid, M., Laser Sintering- with Plastics Technology, Processes, and Materials", Carl Hanser Verlag, Münih, (2018). [6] Mielicki, C., Wegner, A., Gronhoff, B., Wortberg, J. , Witt, G., "Prediction of PA 12 Melt Viscosity in Laser Sintering by A Time and Temperature Dependent Rheological Model", RTejournal, 9, (2012). [7] Wang, X., Jiang, M., Zhou, Z., Gou, J., Hui, D., "3D Printing of Polymer Matrix Composites: A Review and Prospective", Composites Part B, 110: 442-458, (2017). [8] Stansbury, J., Idacavage, M., "3D Printing with Polymers: Challenges Among Expanding Options and Opportunities, Dent. Mater., 32: 54-64, (2016). [9] Kumar, S., "Selective Laser Sintering: Recent Advances", Journal of Laser Applications, 607: 1-8, (2010). [10] Yan, C., Hao, L., Xu, L., Shi, Y., "Preparation, Characterisation and Processing of Carbon Fibre/Polyamide-12 Composites for Selective Laser Sintering", Composites Science and Technology 71: 1834-1841, (2011). [11] Mousa, A. A., "The Effects of Content and Surface Modification of Filler on the Mechanical Properties of Selective Laser Sintered Polyamide12 Composites", Jordan Journal of Mechanical and Industrial Engineering, 8 (5): 265-274, (2014). [12] Özbay, B., Serhatlı, İ. E., Bulduk, M. E., "Manufacturing and Characterization of Filled Polyamide 12 Composite Structure by Selective Laser Sintering Method", 5th Edition of International Conference on Polymer Science and Technology, Berlin, (2019).

  Prof. Dr. Ersin Serhatlı Istanbul Technical University Department of Polymer Science and Technology         Burçin Özbay Metallurgical and Materials Engineer / Researcher Fatih Sultan Mehmet Vakıf University ALUTEAM