Effect of the Process Parameters on the Mechanical Properties of 3D-Printed Specimens Fabricated by Material Extrusion 3D Printing ^†

Abstract

This work aims to study the influences of nozzle temperature, layer thickness and raster angles on the mechanical properties of acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) specimens fabricated by material extrusion 3D printing. Tensile tests were carried out in order to evaluate the mechanical properties of ABS and PLA specimens. The results showed that tensile strength decreased at a higher nozzle temperature for ABS, while an increase followed by a decrease in tensile strength occurred for PLA, with the maximum value obtained at 250 °C. Scanning electronic microscopy was used to analyze the surface fracture after tensile tests of specimens fabricated with different nozzle temperatures. Moreover, the highest tensile strength values for both ABS and PLA were achieved with a raster angle of [0°], the same direction as the applied tensile load. Additionally, a higher tensile strength was obtained for both ABS and PLA at a lower layer thickness. Based on these results, the process parameters used to manufacture a 3D object influence its mechanical properties.

1. Introduction

Additive manufacturing (AM), also commonly known as 3D printing (3DP), is defined as “a process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies” [1]. This technology has been widely used in both industrial and academic field in recent years due to its advantages compared to conventional manufacturing processes such as injection molding or machining technology. 3DP does not require expensive molds and cutting tools; however, 3DP cannot be used in high-volume production processes [2,3].

Material extrusion 3D printing (ME3DP) has a similar AM principle to the famous trademarked Fused Deposition Modeling (FDM) by Stratasys, Inc. On the market, the most popular feedstock materials in ME3DP are ABS and PLA; both are polymers that can be printed using this method and have properties such as dimensional stability and a low transition temperature [4]. The components fabricated by ME3DP have certain drawbacks, such as anisotropy and low mechanical properties. In order to solve these problems, many investigations have been conducted aiming to find the process parameter values that allow for the production of parts with better mechanical properties [5,6,7,8,9].

One of the process parameters that should be considered is the orientation of the components of the build platform, since it was observed that the position of an object on the build platform affects the mechanical properties of 3D-printed ABS components [10]. Also, it was found that the tensile specimen of polycarbonate has a higher tensile strength when the side with a larger area is placed on the build platform [11]. Another process parameter related to orientation that has been investigated is the raster angle. A raster angle in the same direction as the load application produces the highest tensile strength [12,13]; this raster angle was found to obtain the maximum flexion strength values [14].

One way to determine the reduction in the mechanical properties of 3D-printed components is to compare them with components made using another type of thermoplastic molding process. It was found that, with suitable printing parameters, the tensile strength reaches 91% of that achieved using injection molding and the flexural strength is 86% of that achieved using injection molding [15]. In addition, it was observed that the ABS impact specimen fabricated by FDM only achieved almost half of the impact strength of the specimen manufactured via injection molding [16].

Therefore, knowledge of the influence the process parameters have on the mechanical properties of 3D-printed components is critical. Thus, the aim of this work is to study the influences of nozzle temperature, layer thickness and raster angle on the mechanical properties of ABS and PLA specimens fabricated via material extrusion 3D printing.

2. Materials and Methods

In this section, the filaments, 3D printer and process parameters are described, the specimens are characterized and the experimental plan is described in detail.

2.1. Filaments and 3D Printer

Gray ABS and white PLA filaments were purchased from MakerBot Industries, Brooklyn, NY, USA. Both filaments had a diameter of 1.75 mm and desiccant gel silica was contained within the packaging of each filament.

A 3D model of the test specimen was created using Autodesk Inventor, which also exported the 3D model to .stl files to upload to Simplify3D v.4.0.0 for slicing. The test samples were fabricated on a MakerBot Replicator 2X 3D Printer (MakerBot Industries, Brooklyn, NY, USA). The ABS and PLA filaments were dried at 80 °C and 50 °C, respectively, in an air-forced oven, before printing the samples.

2.2. Process Parameters

Many process parameters are involved in ME3DP and the values of these parameters depend on the properties of the thermoplastic filament that feeds the 3D printer. The main process parameters are the nozzle temperature, extrusion width, raster angle, infill, layer thickness, number of shells, number of skirts, platform temperature and printing speed.

The nozzle temperature is the temperature of the thermistor located in the extruder nozzle, and its value depends on the thermal properties of the thermoplastic filament. All 3D printing filament manufacturers include a recommended range of nozzle temperatures in the filament datasheet.

The extrusion width is the desired width of the deposited thermoplastic filament (see Section A-A, Figure 1). This width cannot be achieved as the expansion that occurs once the material is deposited on the platform depends on the rheological properties of the thermoplastic and is not easily controllable; therefore, it can be said that this width is the separation between the centers of the adjacent deposited filaments.

The raster angle is the angle between the path that the extruder nozzle follows during the printing process and the x-axis. The Omega symbol in Figure 1 represents the raster angle. Different angles can be set for the same printing process; for example, a raster angle of [0/45/90] indicates that the first layer will print an angle of 0°, the next layer will print at an angle of 45° and the next will print at an angle of 90°, before returning to 0°; this will continue successively.

The infill or percentage of infill is the process parameter that indicates the percentage of the model’s volume that is occupied by the deposited material; an object printed with 100% infill is completely solid. When the infill is less than 100%, it is common for the top and bottom part of an object printed with compact layers to look like a solid object.

The layer thickness is the height of each printed layer (see Section A-A, Figure 1); more specifically, it is the distance that the platform drops every time a layer ends. The layer thickness influences the surface finish of the 3D-printed object. Thus, a low layer thickness parameter produces thin layers that improve the surface finish but increase the time of the 3D printing process.

The number of shells is the number of contours on the exterior of each printed layer. This increases the robustness of the object. The minimum value of this parameter is one in most slicing software. The number of skirts is the number of contours outside the first layer of the 3D object, which are used to clean the extruder nozzle. Figure 1 shows both 3D printing parameters in a schematic representation of a 3D-printed tensile specimen.

The platform temperature is an important parameter to consider since this ensures good adhesion between the first layer and the platform. It is also important to keep the first layer of deposited layers at a determined temperature to avoid the shrinking and warping of the 3D object. The value of this parameter depends on the thermal properties of the thermoplastic filament. Finally, the printing speed is the speed at which the nozzle die moves in the XY plan. A low printing speed can produce an accumulation of material (thick thread), while high values can generate thin threads.

2.3. Melt Flow Index

The melt flow index of both the ABS and PLA filaments was measured using a Zwick/Roell Mflow plastometer (Ulm, Germany) at 230 °C. The filaments were manually cut into bars with lengths between 3 and 5 mm and then placed in a heated barrel. ThIt ie weights obtained using the MFI measurements were 5 kg and 2.16 kg for ABS and PLA, respectively.

2.4. Scanning Electronic Microscopy (SEM)

The surface fractures in the specimens after the tensile tests were examined using an FEI-Quanta 650 Scanning Electronic Microscope (Hillsboro, OR, USA) with an acceleration voltage of 15 kV.

2.5. Tensile Testing

To evaluate the tensile properties of 3D-printed specimens and the variability in these properties when different 3D printing parameter values are used, tensile tests were carried out in a Zwick/Roell Z050 tensile machine at a speed of 5 mm/min. The geometry and dimensions of the 3D-printed tensile specimens were type V according to ASTM D638-14 [17]. Also, tensile tests of feedstock filaments were carried out for both ABS and PLA in a Zwick/Roell Z0.5 tensile machine at a speed of 5 mm/min. At least five samples of each filament were tested; each had a length of 100 mm and a gauge length of 75 mm.

2.6. Experimental Plan

The investigated process parameters in this research were nozzle temperature, layer thickness and raster angle. The nozzle temperatures were selected considering the technical specifications of the ABS and PLA filaments, such as the melting temperature and glass transition temperature. Also, an experimental evaluation of the ME3DP process was carried out to determine a range of nozzle temperature values that could ensure the successful 3D printing of both the ABS and PLA. In the same way, the values of the layer thickness were selected based on the lowest and highest resolution observed on the Z-axis of the 3D printer. For the study of the raster angle in the XY plane, combinations of up to four angles were considered in order to evaluate the anisotropy of the specimens. The values of the aforementioned process parameters can be seen in

Table 1. Values of the studied process parameters for the ABS and PLA specimens.

Nozzle Temperature (°C)	Layer Thickness (mm)	Raster Angle (°)
200	0.10	[0]
210	0.15	[45]
220	0.20	[90]
230	0.25	[0/90]
240	0.30	[45/−45]
250		[0/45/90]
260		[0/45/90/135]
270

for both the ABS and PLA specimens.

To study the variation in tensile strength with changes in nozzle temperature for both ABS and PLA, the values of the layer thickness and raster angle were fixed at 0.2 mm and 0°, respectively. To investigate the effect of layer thickness on tensile strength, the nozzle temperature was set at 230 °C with a raster angle of 0°. To evaluate the effect of raster angle on the tensile strength, the nozzle temperature and layer thickness were set at 230 °C and 0.2 mm, respectively. It should be noted that the tensile specimens were placed on the build platform with the larger side facing the platform, as shown in Figure 1.

Table 2. Values of the fixed process parameters for the ABS and PLA specimens.

Parameter	Unit	Value
Extrusion width	Mm	0.48
Infill	%	100
Number of shells	Lines	1
Number of skirts	Lines	4
Printing speed	mm/min	6600

shows the fixed process parameter values for both the ABS and PLA specimens. It should be mentioned that the number of shells was set to one in order to avoid the shell contour having any effect on the study of raster angle. Also, the platform temperature was set to 90 °C and 50 °C for ABS and PLA, respectively, to ensure good adhesion between the first deposited layer and the platform.

3. Results and Discussion

In this section, the effect of the process parameters on the mechanical properties of ABS and PLA specimens is assessed, as well as the results of the tensile tests and the melt flow index for both filaments.

3.1. Filaments

The results of the tensile test of ABS and PLA filaments can be observed in

Table 3. Results of the tensile tests and the MFI of ABS and PLA filaments.

Filament	Tensile Strength (MPa)		Young Modulus (MPa)		MFI (g/10 min)
	Average	Std. Dev.	Average	Std. Dev.	Average	Std. Dev.
ABS	34.4	0.5	761	15	5.8	0.1
PLA	55.4	1.0	1499	28	25.5	3.7

. The PLA filament has a higher tensile strength and Young Modulus than the ABS filament. Table 3 also shows the MFI results for both the ABS and PLA filaments. PLA had an MFI that was five times greater than that of ABS at the same temperature of 230 °C, even when using a weight of 5 kg and 2.16 kg for ABS and PLA, respectively, in the measurement of MFI. Thus, during the 3D printing of PLA, the deposited filament flows more easily than ABS.

3.2. Effect of the Nozzle Temperature

From the results of the tensile tests of the printed ABS specimens, it can be seen that, at a higher nozzle temperature, there is a reduction in tensile strength, as seen in Figure 2. The highest tensile strength value was 41.3 MPa on average at a nozzle temperature of 200 °C. It can also be noted that the ME3DP process of ABS at a low nozzle temperature led to complications, such as warped specimens as well as the obstruction of the nozzle die, due to the low fluidity of ABS.

The fracture surface was observed via scanning electronic microscopy after tests using ABS and PLA specimens fabricated with different nozzle temperatures, as shown in Figure 3 and Figure 4. These micrographs can help to describe the mechanical behavior of the specimens.

A brittle fracture and a repetitive pattern of voids originating from the deposited print raster can be seen in the fracture surface of the ABS specimen printed at 200 °C (Figure 3a); these are commonly found in 3D-printed components. At 270 °C, a ductile fracture can be observed, as shown in Figure 3b. Therefore, a gradual change from brittle to ductile fractures with an associated decrease in tensile strength is evidenced when the nozzle temperature for ABS is increased.

In Figure 2, the tensile strength of 3D-printed PLA specimens at different nozzle temperatures can be observed; there was an increase in tensile strength in the range from 200 °C to 250 °C, while at nozzle temperatures between 250 °C and 270 °C, a decrease is observed. The highest tensile strength of 61.2 MPa was obtained, on average, at 250 °C. An increase in tensile strength at a higher nozzle temperature was also reported in tensile specimens from commercial filaments of the LulzBot^® brand; the temperatures studied were 190 °C, 200 °C, 210 °C and 215 °C [18].

Brittle fracture was observed in PLA tensile specimens printed at different nozzle temperatures (Figure 4). Similarly to the case of ABS, a repetitive pattern of voids can also be observed in Figure 4a for PLA. Also, the number and size of the voids decreased at a higher nozzle temperature, but this only occurred between 200 °C and 250 °C (Figure 4a–c). This provides an explanation for the increase in the tensile strength, which most likely occurs because the coalescence of the deposited print rasters increases, resulting in a decrease in the size of the voids and causing some voids to disappear, reducing their number. However, at a nozzle temperature above 250 °C, the tensile strength decreases; this is likely caused by the emergence of new voids as a result of the beginning of the degradation process (Figure 4d).

3.3. Effect of the Layer Thickness

A higher tensile strength was achieved at a lower layer thickness for both ABS and PLA tensile specimens, as shown in Figure 5. This is reasonable because there is more compaction between layers in 3D-printed components; similar results were found in the literature [19,20]. However, the decrease in the layer thickness implies that the 3DP process may take longer. A more considerable increase in tensile strength was observed for ABS than PLA when the layer thickness was reduced from 0.3 to 0.1 mm; an increase of 16.5 MPa was achieved for ABS and an increase of 4.3 MPa was achieved for PLA. This indicates that the layer thickness has a greater influence on the tensile strength of 3D-printed parts made of ABS than those made of PLA. Therefore, a reduction in layer thickness does not lead to a remarkable increase in tensile strength for PLA. This means that increasing the length of the 3DP process in exchange for a small increase in the tensile strength of the PLA is not convenient. The probable reason for this is that PLA has a higher MFI than ABS, leading to a high coalescence between the deposited layers at low and high layer thicknesses.

3.4. Effect of the Raster Angle

From Figure 6, it can be seen that the highest tensile strength of the ABS and PLA tensile specimens was achieved with a raster angle of 0° because the print rasters deposited during the 3DP process are oriented in the direction of the load. For a raster angle of 90°, the lowest tensile strength was observed since all tensile forces were subjected to the leak union between the print raster angles. Other research also reported that, with a raster angle of 0°, the highest tensile strength and flexural strength were achieved for ABS specimens [12,13,14]. The ABS and PLA tensile specimens with a raster angle of 45° obtained intermediate values compared to the tensile strength of 3D-printed specimens with angles of 0° and 90°, which shows that when the raster angle increases in the range of 0° to 90°, the tensile strength will tend to decrease.

It is known that one of the problems presented by components printed using ME3DP is the anisotropy of their mechanical properties. For this reason, a raster angle of [0/90] was also studied, which consists of a layer printed at 0° followed by a layer printed at 90°, a pattern that is continued successively until the full thickness is achieved, in a similar way to fiber-reinforced composite. Also, a raster angle of [−45/45], which is [0/90] rotated −45°, was studied. From the results presented in Figure 6, the raster angle of [−45/45] obtained a higher tensile strength than that of [0/90] for the ABS tensile specimen, while for the PLA case there was practically no difference.

Moreover, tensile specimens with raster angles of [0/45/90] and [0/45/90/135] were also studied since functional parts are subjected to loads in different directions. From the results, both ABS and PLA printed with a raster angle of [0/45/90] offer a higher tensile strength than those printed with a raster angle of [0/45/90/135] due to the large number of layers with an angle of 0°.

4. Conclusions

The mechanical properties of ABS and PLA specimens manufactured by ME3DP were evaluated when they were printed with different nozzle temperatures, layer thicknesses and raster angles. For both ABS and PLA, some parameters were fixed, such as a printing speed of 6600 mm/min, an infill of 100%, a shell number of 1 and an extrusion width of 0.48 mm.

The performance of ABS specimens can be improved when using a lower nozzle temperature (200 °C), a lower layer thickness (0.1 mm) and a raster angle of 0° under the aforementioned 3D printing settings. In the same way, the PLA specimen can achieve a better performance with a nozzle temperature of 250 °C, a layer thickness of 0.1 mm and a raster angle of 0° using the aforementioned 3D printing settings. However, a layer thickness of 0.1 mm increases the duration of the 3D printing process. Thus, a layer thickness of 0.2 mm is recommended as it obtains an adequate tensile strength without too large an increase in printing time.