A Theoretical and Experimental Research on the Influence of FDM Parameters on Tensile Strength and Hardness of Parts Made of Polylactic Acid

Fused Deposition Modeling (FDM) is a rapid prototyping method, widely used in the manufacture of plastic parts with complex geometric shapes. The quality of the parts manufactured by this process depends on the plastic material used and the FDM parameters. In this context, this paper will present the results of a theoretical and experimental research on how FDM parameters influence the tensile strength and hardness of samples made of PLA (Polylactic Acid). Keywords-3D printing; experimental tests


INTRODUCTION
With the increasing complexity of industrial products and the requirements related to environmental protection and resource conservation, new manufacturing technologies are needed. FDM is the most significant technique in Additive Manufacturing (AM) that refers to the process in which successive layers of material are stored in a computercontrolled environment to create a three-dimensional (3D) object [1]. Since 2009 the demand for FDM has grown steadily and many experts believe that this technology has the potential to revolutionize production in many sectors [2]. The main advantages of FDM are [2]: the manufacturing process is simple, allowing the making of parts with complex geometries and cavities, the dimensional accuracy is good, and it is a more cost-effective method compared to other 3D printing techniques. Additive technologies represent a field that is suitable for the needs of today's society through specific interdisciplinary and application possibilities in various areas such as: medicine, engineering, aeronautics, automotive, architecture, etc. However, there are still some limitations and disadvantages, especially in terms of the lower mechanical properties of FDM parts compared to products by conventional methods, such as injection and compression techniques [2,3].
Since mechanical characteristics are extremely important for functional parts, it is necessary to investigate the influence of process parameters on the mechanical performance and geometrical qualification [3,4]. Further research is also needed to determine the printing parameters, such as: deposition orientation, layer thickness, and deposition speed, mainly because the relevant information in the literature is diverse in terms of the values obtained by the mechanical characteristics of parts made with 3D printers. This applies to PLA material, which unlike ABS has not been extensively analyzed [5][6][7][8][9][10]. The novelty of this paper consists in the comparison of different printing parameters (layer thickness and infill percentage), which can be used in order to optimize the mechanical characteristics obtained for different applications.

II. A COMPARISON OF THE MATERIALS USED FOR 3D
PRINTING IN FDM TECHNOLOGY FDM has the advantage of using a wide range of materials. Table I shows a comparison between the physical and mechanical characteristics of various such materials used. The data were collected from the manufacturers' data sheets specifications. These values are indicative and depend heavily on the printing conditions. The mechanical properties considered refer to the orientation of the horizontal test specimen (XY orientation). Under these conditions, this paper analyzes the influence of technological conditions of deposition on the mechanical characteristics of the deposited material. Economic, environmental, and safety challenges have caused scientists and economic agents to partially replace petrochemical with biodegradable polymers [11] such as the PLA.

A. Setting Up Parameters
The mechanical properties of 3D printed parts are important indicators for evaluating print quality. The experimental research program includes tests performed to determine how the technological parameters of the printing influence the mechanical characteristics of the deposited material. In these conditions, the working parameters of the printer used must be taken into account. The initial data targeted in the experimental determinations were grouped into two categories (Table II). The first category refers to process parameters considered as constant quantities: deposition direction, printing speed, filling pattern, and deposition temperature. The second category is the process parameters considered as variable parameters: layer thickness (g), infill percentage (g u ), flow rate (F r ). Test specimens were made and were grouped on sets of samples, each set of samples consisting of 5 specimens, belonging to a group of process parameters, highlighted in Table III.

B. Experimental Set-Up
The adopted material for making the test specimens was PLA. The recommended melting temperature for this material is between 200-220°C, while the temperature of the heated plate has been set to 60°C. The filament used to obtain the test specimens was Creality PLA, with a diameter of 1.75mm [12]. The specimens were made on the Creality CR-X printer, equipped with 2 extruders, with an extrusion nozzle diameter of 0.4mm. Work methodology was applied as in Figure 1. The shape and main dimensions of the specimen are shown in Figure 2. The specimens used in this study to assess dimensional accuracy (Figure 3), repeatability and mechanical properties are modeled on American Society for Testing and Materials ASTM D638, Type IV standards for tensile testing of plastic [13].  Tensile test specimen's dimesional details [13].
The specimen model was designed using Autodesk Inventor software [14], transformed into STL format, and the G-code was made using the 3D printer software Creality Slicer [15]. In this software the change of parameters was possible.  Electro-mechanical testing machine with a 2.5kN load cell and a high performance extensometer. Each set of samples consisted of 5 specimens, each belonging to a group of process parameters, highlighted in Table III. A total of 60 specimens were manufactured by 3D printing. The specimens were subjected to mechanical tests to determine the strength characteristics of the material: ultimate tensile strength (R m ), elongation at break (ε), tensile Young's modulus (E l ), and hardness (HS). Tensile tests were performed on an electro-mechanical machine with a force cell of 2.5kN, at a speed of 5mm/min (Figure 4). Elongation at break was measured using an axial extensometer (Figure 4). Hardness was measured at the points specified in Figure 3. The values of hardness were determined on 15 specimens, with infill percentage of 50%, 75%, and 100%, in 6 points per specimen (Figure 4), with a Shore D durometer ( Figure 5).

IV. RESULTS AND DISCUSSION
The experimental tests were designed to investigate the behavior of plastic parts (PLA), under the action of traction forces, made by 3D printing, taking into account the manufacturing conditions as input factors of the testing process.

A. Tensile Properties Obtained from the Experimental Tests
The initial information for the tensile tests consisted of the shape and dimensions of the test specimen (Figure 2), the type of plastic used (PLA), 3D printing equipment (electromechanical machine Lloyd LRX Force Tester - Figure 4), as well as the process parameters considered as constant quantities: deposition direction (X-Y), printing speed (80mm/s), filling pattern (lines), and deposition temperature (200°C). The process parameters considered as variables were: the thickness of the deposited layer (g = 0.1; 0.15; 0.20mm) and the infill percentage (g u = 25%; 50%; 75%; 100%). The experimentally determined quantities were: ultimate tensile strength (R m ), elongation at break (ε), tensile Young's modulus (E l ), and hardness (HS). The test methodology was performed in the conditions in which the variation parameters were the thickness of the deposited material layer g and the infill percentage g u . Graphs were drawn that express the dependence of the mechanical characteristics (CM) of the tested specimens according to the infill procentage CM = f(g u ) for different thicknesses of the deposited layer. Under these conditions, the following dependencies were drawn: • Figure 6 shows the dependence R m = f(g u ) for g = 0.1mm and g u = 25%; 50%; 75%; 100%.

B. Shore D Hardness
The Shore D Hardness for 50, 75, and 100% g u can be seen in Figure 15.
V. CONCLUSIONS The objective of the paper was to establish the variation of the mechanical characteristics of the PLA material, deposited by 3D printing, depending on the parameters of the technological process. We have started from the characteristics of the raw material (Table I) which show a wide range of characteristic values: R m varying from 15.5 to 2MPa, and ε = 34.5 ± 8.1%. From the analysis of the obtained results it was found that the values of ultimate tensile strength (R m ), are within the limits of R m, specimen varying from 21.0 to 40.07MPa, and for ε specimen from 6.92 to 16.1%. By comparison with the values from manufacturers' data sheets it is found that R m, specimen represents only 55.6% of the maximum value of R m and for ε the values obtained on specimens are below the recommended lower limit for the raw material ε brut, min = 26.4%. Graphs were drawn that express the dependence of the CM of the tested specimens according to the infill percentage CM = f(g u ), in which g u = 25%; 50%; 75%; 100%, for different thicknesses of the deposited layer (g = 0.1, 0.15, 0.20mm). The analysis of the experimental data showed that the maximum value of R m, test piece = 40.07MPa resulted in the conditions g = 0.15mm and g u = 100%.
Given that the thickness of the layer of the deposited material changed to g = 0.20mm and the infill percentage remained the same g u = 100%, a decrease in ultimate tensile strength was observed, which reached the value R m, specimen = 35.8MPa. This phenomenon can be explained by the low cohesion of the deposited layers, which involves establishing the influence of the deposition temperature. The flow rate of the raw material is directly proportional to the thickness of the layer as can be seen in Table I. Various studies [5,[16][17][18][19] have shown that the ultimate tensile strength increases with the thickness of the deposited layer. In this study, although the maximum value was highlighted for the layer thickness of 0.15mm, a higher elongation at break is observed in the specimens with a layer thickness of 0.1mm. All graphs reveal better performance with the increase of infill percentage and less influence of the layer thickness (similar results). It is well known that despite the fact that results can be less satisfying, surely other criteria can be accomplished by using 3D printing.
In this context, further experiments could be developed to evaluate the effect of other input factors of the manufacturing process on the behavior of the parts in the tensile stresses of the material. Regarding Hardness, there is no correlation between the infill percentage and the Shore D Hardness, because of the shell layers (thickness of 0.8mm) and bottom and top thickness of the layers applied on all infill types of specimens. Further research and possible modification of the shell structure might reveal different results in this direction.