MM SCIENCE JOURNAL I 2020 I MARCH 3774 THE CREEP OF MATERIAL OBTAINED USING SLS TECHNOLOGY JERZY BOCHNIA, SLAWOMIR BLASIAK Kielce University of Technology, Faculty of Mechatronics and Mechanical Engineering, Department of Mechanical Technology and Metrology, Kielce, Poland DOI : 10.17973/MMSJ.2020_03_2019022 e-mail: jbochnia@tu.kielce.pl This paper discusses the results of the creep of material obtained with the use of the SLS additive technology. The author described the creep test, and a selected rheological model was adjusted to the creeping curve obtained in an experiment way. The parameter values of the rheological model were estimated. The anisotropy of the rheological properties of the material with regard to the print direction was determined. KEYWORDS Additive Technologies, Creep of Material, Rheological Model 1 INTRODUCTION The main advantage of the Selective Laser Sintering technology (SLS) is the fact that the manufactured elements after cooling down to the ambient temperature are suitable for use. Applying this method on an industrial scale determines scientific research regarding the mechanical and rheological properties of those materials. This way of building materials and at the same time, creating a material which the model is built from leads to the anisotropy of the mechanical properties. This problem was described in a number of papers, e.g. [Bass 2016] [Adamczak 2017]. With no doubt it is a disadvantage of this technology in relation to e.g. plastic injection molding or mechanical processing [Nowakowski 2017] where the problems of anisotropy are not found or are of lesser significance. The extensive characteristics of the additive technologies (including SLS), the theoretical foundations of those technologies, proper equipment and materials as well as the results of some research might be found in the paper of [Gibson 2010]. Example papers regarding testing powders applied for laser sintering include [Verbelen 2017] [Vasquez 2014] [Kozior 2019]. This paper presents the results of the creep test conducted with the SLS technology using the printer - Formiga P100. The author described how the samples were made and the creep test was conducted as well as the selected rheological model [Schmidt 2017] was adjusted to the creep curve obtained in an experimental way. The parameter values of the rheological model were estimated. The test results might be helpful in various kinds of modelling of engineering calculations [Takosoglu 2014] and construction works [Blasiak 2014] or research papers [Blasiak 2016]. 2 METHODS The samples for the creep test were made of PA 2200 polyamide powder laser-sintered with the use of the Formiga P100 manufactured by EOS. The layer thickness of 0.1 mm and the laser power of 21 W were applied. The diameter of the focused laser beam was approximately 0.42 mm. Cylindrical samples of the following nominal dimensions: D = 15 mm and H = 15 mm were used for the tests. The solid models of samples were drawn in CAD 3D software and saved in a digital file with *.stl extension using the triangulation parameters in export options: resolution – adjusted, deviation – the tolerance of 0.016 mm, angle – the tolerance of 50. Subsequently, with the use of Magic software, the triangulation was checked and the verified *.stl files were imported to Rp Tools software, and then to *.sli file. An example of arranging the samples on the build tray of the printing machine was presented in Figure 1. Figure 1. Sample arrangement on the build tray of the printing machine The creep test was made using the Ispect mini (Hegewald & Peschke MPT GmbH) testing machine with range of 3 kN. Measurement, data acquisition and setting the parameters of the test were performed in the LABMASTER program (Version 2.5.3.21), which is supplied with the Inspect mini machine. In order to grip the cylindrical samples, flat plates were applied (the lower tilting plate was used) The samples were individually vertically placed in the central position of the lower plate, and subsequently, the upper plate attached in the machine handle was pushed back against it so that it could contact the flat surface of the sample. The preload should not exceed 10 N. Then, all indications, i.e. load, distance, time were tarnished (zeroed) and the test was restarted. Figure 2 shows an example sample placed between two plates. Figure 2. The sample prepared for the creep test. 1 – lower tilting plate, 2 – upper plate, 3 – sample In the first block of the program, the force value of 1,400 N was applied, which was an equivalent to the tension of 7,926 MPa at the displacement velocity of the gripping plate v = 0.5 mm/s. The second block of the program consisted in bringing the plate to a stop and maintaining its constant value at the level of 1,400 N (tension 7,926 MPa) for 18,000 seconds. During this time, a slight increase in deformation was detected – the creep of the material recorded as a graph. The third block of the program includes bringing the plate to its initial (zero) position.