Experimental investigation and Taguchi optimization of FDM process parameters for the enhancement of tensile properties of Bi-layered printed PLA-ABS

Additive manufacturing (AM) technology has gained significant popularity, among which Fused Deposition Modeling (FDM) has emerged as the predominant technique for 3D printing. FDM offers the unique ability to achieve the desired and tailored engineering properties required for specific applications. This experimental study investigates the influence of varying FDM process parameters on the mechanical properties and highlights the optimal set of parameters for better tensile strength for a bi-layered composite of PLA-ABS (polylactic acid and acrylonitrile butadiene styrene). Also, it investigates the most-to-least influential printing parameters. Four process parameters were played out i.e., Infill density (50%, 75%, and 100%), number of layers (20,25 and 30), printing speed (20 mm sec−1, 40 mm sec−1, and 60 mm sec−1), and bed temperature (90 °C, 95 °C, and 100 °C), while keeping other parameters constant. Taguchi optimization technique was used for optimization and experiments were designed according to Taguchi orthogonal array L9 (3 4). After printing, the samples were evaluated for tensile properties and the results were analyzed. It is found that the infill density is the most influential parameter while bed temperature is the least influential. Infill density of 75%, 30 layers per part, a printing speed of 20 mm sec−1, and a bed temperature of 100 °C are the optimal set of parameters for better tensile strength. Alongside, percent elongation, printing time, and strength-to-weight ratio were also analyzed and correlated.


Introduction
Unlike subtractive manufacturing, additive manufacturing (AM) technologies are among the acceptable and well-suited methods for making rapid prototypes [1]. AM is considered as one of the growing methods across the globe for modern manufacturing applications, especially in marine and aerospace industries. Some well-known techniques of AM in use are Direct metal laser sintering (DMLS), Selective laser sintering (SLS), Electron beam melting, Selective heat sintering, Selective laser melting, Ultrasonic additive manufacturing, Laminated object manufacturing, Stereolithography (SLA), Digital light processing and Continuous liquid interface production [2]. Almost all the available methods are not yet fully matured and still facing challenges for their industrial and commercial applicability [3]. Among the available methods, fused deposition modeling (FDM) is a widely used technique of additive manufacturing. Fused deposition modeling offers various advantages over other manufacturing processes. It is comparatively cost-effective and easy to manufacture [4]. Products made by 3D blend exhibited improved tensile strength, while a 50% PLA and 50% ABS blend achieved the highest flexural testing result.
The literature detailed the various aspects of FDM regarding the impact of process parameters on the mechanical properties of different printable materials. However, to the best of the author's knowledge from the literature study, the research investigations lack the study of the bi-layered printed composite where alternative layers of poly lactic acid (PLA) and acrylonitrile-butadiene-Styrene (ABS) are printed. Therefore, the aim of this research is to study the impact of processing parameters i.e., (infill percentage, number of layers, printing speed, and bed temperature) on mechanical properties i.e., (tensile strength, percent strain, and strength-to-weight ratio) and printing time of bi-layered composite of PLA and ABS fabricated through FDM. Variations in the properties are investigated as the function of process parameters. Moreover, optimized values of parameters were investigated for achieving parts with better mechanical properties and printing time.

Materials and methods
The materials used, methodology, design of experiment (DOE), experimental setup, process parameters, optimization technique, and tensile testing are detailed in this section.

Process overview
A dog bone shaped specimen was modeled using SolidWorks V. 2018. To perform tensile testing, specimens were printed as per software generated STL file. The dimensions of CAD file are shown in figure 1. Standardize ASTM D638 specimen was used for finding tensile strength for experimentation results. The CAD file is interfaced with a 3D printing machine (CreatBot DX 3D Printer) which translates the file into relevant STL (Stereo Lithography) file format. The CAD model of the specimen is shown in figure 1. The infill pattern used in experiments was grid style which is shown in figure 3. This choice aligns with existing literature, which suggests that the grid pattern is highly suitable for bearing tensile loads [27]. Its inherent characteristic of providing maximum resistance to such loads makes it a preferred option.

Printing materials
In the study, PLA and ABS filaments are used for the 3D printing of the standard specimen. The detail of the materials are as follows: 2.2.1. Acrylonitrile butadiene styrene (ABS) Acrylonitrile, Butadiene, and Styrene are the three monomers that make up the polymer ABS, which is a material of choice for many applications due to its rigidity and resilience to abrasion, impact, and strain. It can be used in a variety of products, including Lego toys, pipe fittings, consumer goods, and home electronics. Due to its many physical characteristics, including high stiffness, good impact resistance, especially at low temperatures, and good insulating properties, ABS is an appropriate material of choice for a variety of structural applications [28].
ABS has weak resilience to weather; ordinary grades burn readily and remain burning after the flame has been extinguished. Poor resistance to solvents, especially for aromatics, ketones, and esters, can experience stress cracking when certain greases are present, It has a weak dielectric strength with a minimal constant service temperature [29].

Poly lactic acid (PLA)
PLA has established itself as a possible replacement for petroleum-based polymers since it is bio-based, biodegradable, and biocompatible. Its qualities are comparable to those of currently popular polymers such as PET, PVC, etc High-performance materials are a great alternative to polystyrene (PS), ABS and PP in even more critical applications. However, in the preceding year, the high manufacturing costs of PLA in comparison to its petroleum-based competitors hindered its commercial feasibility [30].
Currently, the price of PLA can be decreased by streamlining the Lactic Acid & PLA production processes and increasing demand for PLA. As PLA must contain a crystalline component to improve the quality of the product, most commercially available L-PLA products were semi-crystalline polymeric materials with a significant melting point of around 180°C and their glass transition temperature within the range of 55°C-60°C [31]. PLA is a thermoplastic with good aesthetics, strong strength, and high modulus [32]. At standard room temperature, it exhibits excellent rigidity and good strength in comparison to PS. Compared to other available plastics, less energy is needed in its production, and it shows improved thermal processing. The qualities and possible uses of PLA are growing due to further advancements in composites, nanocomposites, and bio-composites. PLA has a low glass transition temperature of (T g 55°C) [31]. Its applications are constrained as compared to those of other thermoplastic polymers like ABS because of its poor ductility, low impact resistance, and brittleness. It rarely crystallizes and It's processing mostly yields amorphous products. PLA (polyester aromatic) is substantially more sensitive to both biological and chemical degradation than PET (aromatic polyester). It performs poorly as a gas barrier and has inconsistent thermal behavior. Low flexibility and lengthy mold cycles are required. It has a low hydrophobicity. It deteriorates slowly [33]. Mechanical properties of PLA & ABS materials [34] are shown in table 1.
The two most common materials for FDM are PLA and ABS. There are benefits and drawbacks to using these materials for 3D printing. Although ABS is strong, heat resistant, and water resistant, and has got the feasibility to be acetone treated, however, it falls short when it comes to strength. PLA, on the other hand, has got better tensile strength and allows more detailed and clearer prints but has got some limitations like bad heat resistance, medium water resistance, etc In this study, instead of using one material, parts are printed with alternate layers of both ABS and PLA to get rid of the limitations of these materials and get products having desired properties of both materials.

Printing parameters
For the purpose of improving mechanical properties and getting the compatibility of PLA and ABS composite, suitable FDM process parameters were selected based on previous studies and were varied in acceptable ranges as per literature. Four significant parameters based on the study are infill percentage, number of layers, printed speed, and printing bed temperature. The parameters levels were decided to be varied levels 1, 2, and 3 from lower to higher i.e., infill percentage of 50, 75, and 100, layer numbers of 20, 25, and 30, printing speed of 20, 40, and 60 while printing bed temperature of 90, 95, and 100°C were selected for level 1, 2, and 3 respectively. Based on the literature review and printer constraints, four parameters i.e., printing speed, infill percentage, bed temperature, and the number of layers were selected from a large set which was influential. Printing speed is the speed with which the extruder moves along X-Y axes relative to the bed while extruding filament to make the products. The range of printing speed depends on the printer and printing technology. The printer we have used can reach a maximum printing speed of 120 mm sec −1 . Infill percentage is the fraction of the inside of the printed part filled with material. In other words, it can be described as the solidity of the printed part. It ranges from 0% being hollow to 100% being filled from the inside. Research shows that there is a significant effect of infill percentage on the tensile strength of the product. The number of layers is the number of material layers needed to make the product. The lower number of layers results in a greater layer height while a higher number of layers results in a smaller layer height. As the number of layers increased, diffusion between them also increases resulting in parts with higher tensile strength. Bed temperature is the temperature of the printing bed needed for the first layer of material to stick. Apart from the four parameters mentioned above, there are other parameters as well. All these parameters have been kept constant and all the experiments are conducted in the same environment. Extrusion Temperature is the temperature, up to which the filament material is heated, so that it can be extruded out of a nozzle. As we have used two filaments (ABS and PLA) and two nozzles, so extrusion temperature of 255°C is used for ABS, and 215°C is used for PLA. The interior of the part can be filled by using various angles relative to the build platform which is known as a raster angle that can be varied from 0°t o 90°. In this study, the raster angle was kept at 45°). The number of Contours is the number of outer boundaries that enclose the inner printed part. Usually, it ranges from one to six, and we have used four number of contours. Bead Width is the thickness of the bead or layer that is deposited by the FDM nozzle in one pass. Normally, it ranges from 0.3 mm to 1 mm. We have used a bead width of 0.4 mm. A complete table of all the values of constant parameters is given in table 2.
The bi-layered composite was developed using slicing software and printers' capability of having multiple nozzle (only 2 nozzles were utilized as we were using 2 materials PLA & ABS) the process is discussed in section 2.4 methodology printing process setup.

Methodology and printing process setup
This subsection details the methodology used for making specimens. PLA/ABS composite was made using bilayered slicing, using two extruders. As shown in figure 2 During dual extruder printing, extruder 1 fed the PLA, and extruder 2 fed the ABS alternatively to print the bi-layered samples. It means that at a time, one extruder will extrude the relevant material. Figure 3 depicts the various views and alternate printed layer of ABS/PLA of the specimen. The printer used in the printing process was the 'CreatBot Printer' which was a tri-nozzle 3d printer with specifications given in table 3 and the picture of the 3D printer is given in figure 4.

Design of experiment
Taguchi L9-34 design of experiment was used, according to which 9 experiments were designed. Slicing of the CAD model of the specimen was done using CreatBot 3D software and then sliced files were fed to CreatBot DX 3D Printer for printing.
Taguchi design was used to build the design of the experiment for the parameters given in table 4. Taguchi technique provides a way of effective optimal design [35,36]. The advantage of Taguchi's design includes minimizing the number of experiments and simplifying the test plan. According to this method, the minimum number of experiments should be equal to or greater than the sum of the degree of freedom (DOFs). The degree of freedom is very important for the selection of the proper array in Taguchi design. For a parameter, DOF can be defined as its number of levels varied. As there are three levels of each parameter, the DOFs for each parameter are two with a combined DOFs of eight for all four parameters. In that matter, a suitable array of L9 is selected. Time and cost are highly reduced due to the Taguchi method as the number of experiments was reduced. If we use the full factorial method, we will have to do a total of 81 experiments, which is very time-consuming and costly. Taguchi DOE provides the same result with only 9 experiments [37].
A total of 27 specimens were printed with a set of 9 experiments, repeating each one thrice to eliminate errors and getting better results.

Levels of process parameters
Each run of experiment is run at different values, called levels. Our experimental plane has three levels for each parameter. Levels of chosen parameters were selected while keeping printer constraints and previous studies. The complete description of the low, medium, and high levels of all four parameters is given in table 5.        that, the test results were further analyzed using Minitab software which gives the SN-ratio, Analysis of variance, probability graph, and the SN ranks. The tensile testing setup is shown in figure 5 while the specifications of UTM are given in table 6.

Results and discussions
Optimization, Tensile strength, strain, young's modulus, printing time, strength-to-weight ratio, and dimensional accuracies are discussed in this section.
The tensile strength is expected to be 53.175 MPa based on the regression model. For validation or confirmation, three specimens were printed at the optimized parameters given above and their average tensile strength was found to be 51.92 MPa. Their stress-strain curve is given in figure 6.
The experimental value of tensile strength was compared with that obtained from the regression model, and an error of 2.3% was found.
A comparison from the regression model and validation experiment for tensile strength is given in table 7. There is a difference of 1.255 MPa between the predicted and experimental value, which indicates the accuracy of the analysis.   (27) specimens were printed. To analyze the young's modulus and tensile strength of printed samples, each of the samples was properly clamped and pulled to rupture. Before the tensile test, the weight of each sample is measured using digital balance for calculating the strength-to-weight ratio. During the tensile test, strain is also noted to calculate young's modulus for each sample. Table 8 depicts the results of tensile strength, strain, and young's modulus for samples printed at varying printing parameters. Using Microsoft Excel, the average values of the test were considered for a more accurate stress-strain plot as shown in figure 7. Experiment 6 shows the highest Ts while experiment 7 shows the lowest Ts. The printing parameters for experiment 6 were infill density of 75%, 30 number of layers, printing speed of 20 mm sec −1 , and bed temperature of 95°C. While for experiment 7 the infill density of 100%, 20 number of layers, a printing speed of 60 mm sec −1 , and a bed temperature of 95°C. Figure 7 shows the stress-strain graph of all experiments.

Taguchi analysis: tensile strength (MPa) versus parameters
The signals-to-noise (SN) ratio of tensile strength and process parameters is given in table 9. The maximum value of tensile strength was obtained at the 2nd level of infill density, 3rd level of the number of layers, 1st level of printing speed, and 3rd level of bed temperature. As from table 9 infill density is ranked as the most influential parameter on the tensile strength of FDM followed by the number of layers. Printing speed is ranked as the 3rd among influential parameters while the very little effect of bed temperature was observed on tensile strength.
The SN plots are given in figure 8, it is obtained from Taguchi analysis. Response (tensile strength) varies with changing levels differently i.e., for infill density, the tensile strength increases as level value increase from lower (50%) to medium (75%) but when the infill density further increases to the highest level (100%) the strength decreases to even lower than the strength at 50% infill density. Usually, at a low infill density of 50%, the specimen had greater void spaces, and hence no properly filled inner structure for material increases the  probability for the part to break. Similarly, when infill density increased to 100%, the specimen has no void spaces so, a regular structure leads to the crystalized structure which has lower yield strength.
In the case of the number of layers, it has a direct effect on tensile strength, and the strength obtained is highest at the highest number of layers (30 Layers) printed per specimen. While varying numbers of layers inversely affect the dimensions of each layer. i.e., The specimen dimensions decrease as the number of layers increases, and thinner layers forms which leads to greater inter-layer area. The greater inter-layer area increases interlayer bonding, and the thin layer can adjust easily with each other and gives a regular structure to specimen with higher strength. As, previous literature have suggested that improved interlayer bonding strength (IFBS) can be achieved by combining low printing speed and low layer height with a high infill density [9]. Similarly, printing speed has an inverse relation with the strength as from figure 8. At the lower level of 20 mm sec −1 , the strength is maximum and minimum at the highest level of 60 mm sec −1 printing speed. At greater printing speed the layers have less time to cool down and the inner layer has less time to settle down, resulting in lower tensile strength.
Though the bed temperature is the least significant parameter, however, the effect obtained from the analysis shows that strength first decreases and then again increases with bed temperature. Strength is minimum at the medium level of 95°C bed temperature, increasing bed temperature affects the cooling time for printed specimen to solidify while annealing the specimen at little. So, yield strength decreases a little and then again increases.
Our experimentation yielded significantly improved results compared to the previous study. In our bilayered printing tests, we achieved a remarkable strength of 51.92 MPa at optimized parameters(30 no. of layer, 75% infill, 20 mm sec −1 and bed temperature of 100°C), far surpassing the literature's 14.5 MPa for a composite sandwiched structure (50% PLA-50% ABS) at a 50% infill, and using low printing temperatures of 180°C and 220°C for PLA and ABS, respectively [26]. Furthermore, maximum strength of 51.34 MPa was achieved in a CF-PLA/ABS hybrid composite under specific parameters: a print speed of 20 mm/s, an infill density of 67.838%, and a layer height of 0.23 mm [9].
Similarly, figure 9 shows a probability graph for tensile strengths. The probability graph is a graphical method of confirming the data. In the Probability graph, the value of P measures the evidence against the null hypothesis. Our results fall along a straight line. Our normal probability plot is close to the Normal line so, we  can conclude that our data is normal. The value of P plays a significant role in whether the data would be rejected or accepted. P 0.05, data does not follow a normal distribution as a result data would be rejected, and if P> 0.05, data will follow a normal distribution, and hence our whole data would be accepted. In our case, the value of P is 0.876 so our data is acc7urate.

Percent elongation
Percent strain is the percentage elongation of specimen per unit starting gauge length of the specimen during the tensile test. The result and impact of all levels of each parameter on percent strain are given in table 10 and figure 10. From the results, the most influential parameter on the percent strain is bed temperature, followed by the number of layers and printing speed while infill percentage has the least influence on strain percentage. It is also worth mentioning that the effect of infill density on tensile strain of alternate bi-material based 3d printed composite agrees with the tensile strain behaviour of tri-material based 3d printed composite reported in literature [38]. The optimum set of parameters are 75% fill percentage, 20 number of layers, 20 mm sec −1 printing speed, and bed temperature of 95°C.

Strength-to-weight ratio
It is the comparison of its strength in relation to how much it weighs, the values of strength to weight obtained for all the experiments are mentioned in table 11.
From table 11 experiment 3 has got the highest strength/weight ratio of 8.01, which means it has the highest tensile strength as compared to the amount of material being used. Although experiment 6 has the highest tensile strength, it consumes more material to be built and has a lower strength/weight ratio. The strength-to-weight ratio of all the experiments along with their masses are given in table 12. Figure 11 shows that the infill density is ranked as the most influential parameter on the strength-to-weight ratio. It is followed by the number of layers, which is ranked as the second most influential parameter. Printing  speed and bed temperature have almost negligible influence on the strength-to-weight ratio while printing speed has little effect on strength to weight ratio. The optimum set of parameters for the best tensile strength to weight ratio is 50% fill percentage, 30 number of layers, 20 mm s −1 printing speed, and bed temperature of 100°C.

Analysis of printing time
It is the total time taken per specimen to completely print; the processing time was calculated using the stopwatch which is shown in table 13. Printing time is analyzed by using 'smaller the better' tool. Using this tool, we got optimized values of parameters for which the printer gets less time to print the specimen, i.e., we got fast processing time.  Using the smaller the better tool, we determined an optimized value on which our specimens can be printed in the shortest possible time. From table 14 , it can be concluded that the most influential parameter on printing time is printing speed. Higher printing speed decreases printing time but at the same time leads to poor print quality and dimensional accuracy as the material is not given the proper time to solidify. The next layer is added before the previous layer has settled which results in extrusion of more material on the edges. The 2nd most influential parameter infill percentage and the least influential parameter is bed temperature having a negligible impact on printing time. The optimum set of our selected parameters is 50% fill density, 20 layers, 60 mm sec −1 printing speed, and 90°C bed temperature. Less material will be used in 50% infill as compared to 75% and 100% infill. By keeping a smaller number of layers i.e., 20, and by keeping more layer height, printing time decreases. Similarly, with a higher printing speed, printing time can be significantly reduced as specimens will be Figure 11. SN plot and ranks for strength-to-weight ratio using Minitab. printed faster as compared to lower printing speeds. Bed temperature has the very least influence on printing time.
3.6. Dimensional accuracy Some inaccuracy was found in the printed part, and they had a slight difference in dimensions as compared to the CAD diagram provided to the printer. The actual dimensions of the CAD diagram of the specimen as previously mentioned were 5 mm × 7 mm × 25 mm while the dimensions of printed parts for all the experiments are given below in table 15. More inaccuracies were found in thickness as compared to width while length had very small inaccuracies. An increase in thickness was found in all the experiments. There was also a slight inaccuracy in width in all experiments except experiment 6 where there was no inaccuracy found in width. The inaccuracy of all the experiments can be seen in figure 12. As clear from figure 13, the dimensions of most of the printed samples were more than design values from CAD model, however due to very small differences in dimensions of actual model and CAD model, so their effects on properties were neglected.

Conclusions
In this Fused Deposition Modelling, we developed a hybrid composite of PLA and ABS where alternative layers of PLA and ABS were printed, varying four processing parameters and the effect of those parameters was investigated statistically by the Taguchi optimization method. The following conclusion was drawn from our study.  • The optimum FDM processing parameters obtained for greater tensile strength are 75% infill density or infill percentage, printing speed 20 mm sec −1 , 30 number of layers in dog-bone sample, and bed temperature as 100°C.
• The best combination of parameters for improving strength-to-weight ratio was found to be 50% fill density, 30 layers, 20 mm sec −1 , and 100°C bed temperature.
• Up to 90°C, the bed temperature has a direct impact on the tensile strength of PLA-ABS bi-layer composites; however, after that point, PLA in the composites begins to degrade. Therefore, it is not advised to use temperatures exceeding 95°C.
• Printing time is most influenced by printing speed, followed by infill density and number of layers while bed temperature has a negligible effect on printing time. The optimum set of conditions for the lowest time are 50% infill percentage, 20 number of layers, 60 mm sec −1 , and 90°C bed temperature. By keeping these parameters our part quality is also compromised.
• Infill density has the most influence on the filament used by 3D printers.
• This study is primarily focused on the optimization of parameters to enhance tensile properties and reduce printing time. However, there remains a need for further investigation, particularly in the realm of optimizing material proportions, exploring interlayer bonding, and evaluating additional mechanical properties for various material bi-layered printed composites.