Factors Affecting Ultimate Tensile Strength and Impact Toughness of 3D Printed Parts Using Fractional Factorial Design

This paper aims to investigate the mechanical properties of spec- imens printed by 3D open-source printers. It discusses the effect of ve fac- tors (part/print orientation, layer height, extrusion width, nozzle diameter, and lament temperature) on the ultimate tensile strength and the impact toughness of the 3D-printed samples. A 2 6-1 resolution V fractional factorial experiment was run with the 16 samples printed on a Prusa I3 MK3S in PLA. Tensile strength and impact toughness were tested using Instron 3367 and Tinius Olsen 66 testers, respectively. In analyzing the data, a normal proba- bility plot complimented with ANOVA (Analysis Of Variance) revealed that only part/print orientation was statistically signicant at p = 0.05. Regression equations were used to predict the ultimate tensile strength and the impact toughness as a function of the part/print orientation. Both the toughness re- sponse and the tensile strength response are maximized with horizontal part orientation. Verication experiments have been implemented to validate the adopted regression equations’ predictions under different circumstances and the results of those experiments appear to conrm the model.


Introduction
Fused lament fabrication (FFF) is an additive manufacturing technique that builds parts layer by layer using the extruded thermoplastic lament at low cost [1]. FFF parts are widely used in many elds such as food [2], electronics [3], aerospace [4], automobile [5], etc . The lament is heated up to a semi-solid state and extruded through a heated nozzle to form a layer that is adhered to the previously deposited layers. One of the most widely used polymers for 3D printers is Polylactic acid (PLA) extracted from corn starch, cassava, and sugar. PLA has a high hardness, high strength, low toxicity, and good re-newability. PLA is an eco-friendly material that reduces the consumption of petroleum resources. It's generally easy to print with, odorless, readily avail-able, and affordable. It requires less energy than other materials because it has a low melting temperature [6, ?].
Despite the enormous advantages of 3D printing technology in the produc-tion of complicated parts, its use is limited due to the lack of studies on the mechanical properties of these parts. Tensile strength and Izod impact strength are well-known tests to reveal those properties. Tensile strength testing is a fundamental materials science test, where a sample is subjected to a controlled, increasing tension load until failure [7]. The Izod impact strength test is an American Society for Testing and Materials (ASTM) standard method used to measure the impact resistance (toughness) of materials. Impact toughness represents the amount of energy absorbed during the fracture space [8].
In recent years, some researchers reported studies regarding the mechan-ical properties of 3D-printed PLA parts. Tymrak et al. [9], tested different types of RepRap printers and concluded that RepRap printers have similar tensile strength as commercial printers. Bledzki et al. reported that the tensile strength of injection-molded specimens using PLA lament ranges from 30 to 63 MPa [10]. Tymrak et al. studied the tensile strength of 3D printed samples using ABS and PLA and reported that the average tensile strength is 28.5 MPa and 56.5 MPa for ABS and PLA printed samples, respectively. Tankella et al. [11] also tested the tensile strength for vertical and diagonal print orien-tation for eight materials: ABS, Nylon Bridge, T-Glase, HIPS, polycarbonate, NinjaFlex, nylon 618, and SemiFlex. They concluded that maximum tensile strength of 49.08 MPa is obtained with polycarbonate. The part/print orienta-tion is reported, in the literature review, as one of the most signi cant factors of the tensile strength of the 3D printed samples [12, 13, ?,?,?]

Fractional Factorial experiment work ow
Design of Experiment (DOE) methods, such as Fractional factorial experi-ments and full factorial experiments, are used to explore controllable factors on tensile strength and impact toughness of 3D printed parts in this paper. They are statistical methods that can explore the controllable factors on a response e ciently and at a low cost compared to the classical designs [14].
The fractional factorial experiment is recommended over full factorial exper-iment for experiments where high-order interactions can be neglected. If the high-order interactions are negligible, information on the main effects (effect of individual factors) and low-order interactions (low order joint effect of fac-tors may be obtained by considering only a fraction of the original design. Thus, it solves the problem of the exponential increase of the number of ex-perimental runs for a full factorial experiment. Based on the number of factors k in the fractional factorial experiment, 2k−1 runs of random order are imple-mented. For each run, the 3D part is printed, tested using the Izod impact tester and tensile strength test, and the corresponding tensile strength and im-pact toughness values are recorded. Data recorded during the experiment is fed into MINITAB® software to be analyzed. Normal Probability Plot (NPP) and Pareto chart are used to identify signi cant terms (terms that have an effect on the response), whereas Analysis of variance (ANOVA) is used to analyze the effect of those signi cant terms. The mathematical relation between the responses, tensile strength, and impact toughness in our experiment, and their signi cant terms is expressed by regression equations. Finally, plots of residual are analyzed to guarantee the complete randomness of the experiment. After the main experiment is done, a veri cation experiment is then implemented to validate the results of the regression model for unseen combinations of factors' levels [15].

Contribution
Since there is no extensive data available about the mechanical properties of parts printed by Prusa I3 MK3S and most home users can not test the mechanical properties of their samples, our experiment comes to satisfy the need to study these properties. A half fractional factorial experiment of ve two level factors is used to study their effect on the ultimate tensile strength and impact toughness of samples printed with Prusa I3 MK3S printer using PLA lament. This is the rst time that this Design of Experiments strategy has been used in the 3D printing eld to determine the choices of the signif-icant parameters and optimize the tensile strength and impact toughness of 3D printed parts. The ve factors considered in this experiment are part/print orientation, layer height, extrusion width, nozzle diameter, and lament tem-perature. This paper proposes a regression model to describe tensile strength and impact toughness mathematically as a function of their signi cant terms and predict their values for untested conditions. Veri cation experiments are also implemented to validate the regression model resulting from this experi-ment.
The remainder of this paper is organized as follows: Section II describes the test sample design, the required physical instruments, the setup of the tensile strength and the impact toughness testers, and the fractional factorial exper-imental design. Section III shows and analyzes the results of this experiment and presents the implementation of veri cation experiments to validate the prediction of the proposed regression models and nally section IV provides a conclusion and discusses potential future work.

Sample Design
The test specimen is designed to allow the implementation of the impact tough-ness and tensile strength testing using the same specimen. Figure 1 shows the engineering drawing of the test specimen with all dimensions in mm. As shown in Figure 1, the left part of the specimen has a standard 45°notch with 1.6 mm depth including a 0.25 mm radius for Izod impact testing. This geometry results in a nominal cross-sectional area of 84 mm2 for the impact section.
The right part of the specimen shows a narrow section that has a nominal 70 mm2 cross-sectional area used for the tensile testing. Solidworks is the software used to model the specimen as shown in Figure 2 and saved as ".STL" le. PrusaSlicer software takes the generated STL les as an input and produces the G-code that instructed the printer to make the 3D parts. This software has been used to control the printer settings, such as part orientation, layer height, scan speed, extrusion width, lament temperature, etc.

Experimental fractional factorial design
In this investigation, a fractional factorial design was applied to evaluate the effect of the variation in part orientation (A), layer height (B), extrusion width (C), nozzle diameter (D) and lament temperature (E) on the tensile strength and impact toughness of the 3D printed samples. Those factors are chosen based on our knowledge and experience in 3D printing in addition to literature studies. This experiment comprises ve factors with the two levels represented by "-1" and "+1" for each factor. Table 1 describes the experiment's factors and their levels in detail. This experiment tests the tensile strength and the impact toughness of each of the 16 printed samples.  The fractional factorial experimental design is a 1 fraction of a 25 (32) full factorial design by conducting only 16 runs in this experiment.
Since only half of the total full-factorial runs is used in a half-fractional factorial experiment, a design generator is used to choose this fraction with some desirable proper-ties. The desirable properties are the balance and independence between the factors. Balance in a factor refers to an equivalent number of the two levels (each factor has 8 of the two levels '-1' and '+1' in our experiment), whereas the independence between factors refers to zero dot product between any two factors as shown in Table 2. The best design generator in half fractional fac-torial is the highest-order one to manage aliasing terms. In this experiment, the levels of the rst four factors (A, B, C and D) are constructed as the all possible combinations of them (full factorial design of four factors) and the last factor (E)'s levels are generated using the design generator E = ABCD as shown in Table 2 to guarantee the desirable properties. This construction refers to resolution V where resolution V designs provide estimates of all main effects which are not aliased with any terms containing less than four fac-tors. In resolution V designs, two-factor interactions are not aliased with main effects or other two-factor interactions but may be aliased with three-factor interactions. The resolution of an experiment identi es the degree of terms to which the main effects are (aliased) and is expressed with Roman letters (I, II, III, IV, V, VI ,...). Based on the sparsity of effects principle, we will assume three factors and higherorder interactions are negligible.

Test setup
The required physical elements to run this experiment are a FFF 3D printer, PLA material, an Izod impact tester, an Instron tensile tester, and calipers for gauging the actual dimensions for the tested samples. Additionally, MINITAB® software was used in the analysis of the data. All machines used in this paper are provided by the University of Detroit Mercy (UDM), MI, USA. Original Prusa i3 MK3S, Figure 3, is the FFF 3D printer model used to print all the test samples in this experiment. The precision (tolerance) of an original Prusa printer is 0.1 mm on the Z-axis and 0.3 mm on X and Y . The Instron 3367 tensile testing machine, shown in Figure 4, has been used to perform tensile tests in this experiment. This machine was equipped with a 20 KN load cell to measure the tensile strength of the 3D printed parts. The measurement data was controlled, monitored, and recorded using "Series IX/s" Instron software. The Tinius Olsen 66, shown in Figure 5, has been used to implement the impact toughness test in this experiment. It has a pivoting arm that is raised to a speci c height to maintain constant potential energy and then released to hit and break a notched sample. The angle to which the arm swings is indicative of energy lost in breaking the specimen which represents the impact resistance. Original Prusa i3 MK3S used the same ".STL" le with different slicing settings to print all 16 samples in the randomized "Run Order" order shown in Table 2. Some printing settings are kept constant for all samples such as 60°C bed temperature, 100% in ll percentage, and grey PLA lament material. In this experiment, the 3D parts are printed either horizontally or vertically. Fig-ure 6 shows the difference between the horizontal and vertical printed samples. In the vertical printing of the left sample, the square base is rstly printed and other layers are then accumulated vertically, whereas the sophisticated base is printed rst for the horizontal right sample. The mass of every sample is mea-sured using a scale to make sure that there is no interior under-extrusion. All samples weighed approximately 14 grams which suggest that each extrusion is good.
The whole Instron machine with a close up of a sample in the test setup clamped between the jaws of the tensile strength tester is shown in Figure 4. The tensile strength tester, used in this experiment, applied a 20 KN tensile load on each specimen at 5 mm/min cross-head speed. Instron tester is used to measure the maximum force that specimen can withstand before failure in Newtons as shown in Table 3.
This force is then divided by the narrow cross-sectional area to calculate the ultimate tensile strength (UTS) of each specimen. The Tinius Olsen 66 Izod impact resistance tester, Figure 5, is used for this experiment with a pivoting arm raised to a speci c height (to maintain constant potential energy) and swings down to hit and break a notched sample. The energy absorbed by the sample is measured from the height the arm swings after hitting the sample. The impact resistance tester measures inch-pounds (energy) absorbed by breaking the specimen at the notched cross-sectional area. Toughness (J/m2) is then calculated by multiplying the absorbed energy (in-lbs) by 0.112984825 to be converted into joules and then divided by the notched cross-sectional area. The recorded tensile force (N), impact resistance (in-lbs), the printing time for each specimen are shown in Table 3. It also includes the measured length and width in millimeters of the cross-sectional area used in the tensile and notched Izod tests for each sample.

Results And Discussion
Each combination of different levels of factors is used to setup the 3D printer to print the 16 samples. Each 3D printed sample is then exposed to the Izod impact test using the Tinius Olsen tester followed by the tensile strength test using Instron tester in this experiment.
The 3D printed samples' tensile strength was tested using an Instron tensile test machine with a hydraulic linear actuator and a 20 KN load cell. The impact toughness of the parts was measured using an Izod Impact testing machine. Figure 7 shows all the broken samples after the tensile strength and the impact toughness testing. Exper-imental data, obtained from fractional factorial design runs, were analyzed using the statistical software package MINITAB® 19.

Ultimate tensile strength Data Analysis
The ultimate tensile strength (UTS) is de ned as the maximum tensile load a part can withstand before failure divided by its crosssectional area. During the tensile test, 16 specimens have been stretched till failure and their cor-responding data are recorded in Table 3. Table 3

Impact toughness Data Analysis
Material's toughness measures a material's ability to absorb energy and plas-tically deform without fracturing. Part orientation (A) is the only signi cant factor for impact toughness depending on Normal Probability Plot (NPP) of effects in Figure 8b and Pareto chart in Figure   9b with a 5% signi cance level. In order to investigate some of the next largest effects, the plot was made with a 10% signi cance level.
With 10% signi cance level, A, BE, CE are signif-icant but regression model with those terms and their main effect B, C, E predicts the impact toughness of veri cation samples incorrectly. Thus, part orientation (A) appears to be the only signi cant factor based on several con-rmation runs performed. Other terms may have been dismissed due to the poor resolution of the Tinius tester. The regression equation of impact tough-ness Y2 is equation (2) with constant term and effect of factor A computed in the same way as tensile strength analysis. The maximum toughness of 3D printed part (3629 J/m2) occurs at A=−1 (horizontal part orientation).
From the analysis of Variance of the impact toughness of the 3D-printed samples, pooling insigni cant terms to error, shown in Table 5, part orien-tation is statistically signi cant at 95% con dence interval as p-value is less than 0.05. Residual analysis is done to ensure that the error is normally and independently distributed with constant variance. NPP of residuals in Figure Figure 13a, shows no relationship between the size of the residuals and the tted values indicating that we can assume independence and constant variance. The plot of the impact toughness's residuals versus observation order, Figure 13b, doesn't show any concern about unexpected issues that may affect the validity of the results.

Veri cation experiments
Two nominally identical specimens for the two veri cation experimental runs are built resulting in four test specimens, two for the vertical part orientation and the other two for the horizontal part orientation. Those four samples were used to validate the model's prediction for the ultimate tensile strength and material toughness for an unseen 3D printed specimen. Repetition was used to decrease error variance and to avoid possibly missing data due to periodic unexpected problems in the tensile tester. Sometimes, the tensile strength tester suffers from a sudden increase in tensile load resulting in breaking the sample without recording the associated load. This problem prevented one of the two vertical veri cation samples from being included as its associated   value was not recorded. The response (recorded value) is the average of the two recorded measurements for each experimental run.
The standard order of each experiment and the measurements of length and width in mm for the tensile and impact cross-sectional area are recorded in Table 6. Each sample is subjected to the Izod impact test to measure the impact resistance followed by the tensile test to measure the tensile force with "run" order and experimental data is shown in Table 6. Similarly to main experiment, the tensile force and the impact resistance are used to calculate the ultimate tensile strength and the material impact toughness, respectively. The average of values obtained by practical tensile strength and toughness testing of the two specimens as shown in Table 7 and 8, respectively. The regression equations (discussed earlier) are then used to predict the tensile strength and the impact toughness associated with the speci c orientation as shown in the "Model predicted value" column in Table 7 and 8, respectively. The standard deviation obtained from MINITAB® analysis is used to calculate the low and high limits of the model predicted values. The low and high limits are calculated by the subtraction and addition of the standard deviation value from the model predicted value, respectively.
The summarized veri cation results for the tensile strength and the impact toughness models are shown in Table 7 and Table 8, respectively. The lab measurements lie between the low and high limits of the predicted values by the models for the tensile strength and the material toughness. For example, the average UTS obtained by the veri cation experiment for horizontal part orientation (56.61 MPa) is only 0.39 % greater than the predicted value (56.39 MPa) in Table 7 and well ±2 standard deviation. These results con rm the effectiveness of regression equations to predict experimental results accurately and enhance its validation to predict the ultimate tensile strength and the impact toughness for untested 3D printed parts.

Conclusion
This experiment investigates the mechanical properties of the 3D printed sam-ples by implementing the tensile strength test and the Izod impact test. A frac-tional factorial experiment with ve process factors was implemented. Based on the experimental results, the only signi cant factor identi ed in this exper-iment is the part orientation. Those results emphasize the fact that to get a signi cant change in the tensile strength or toughness of a 3D-printed sample, there is no need to change nozzle diameter, layer size, extrusion width, or lament temperature, just change part orientation. The other parameters may be chosen to minimize the print time with no effect on the In a future study, calibration across the expected range of the load cell should be done to verify its accuracy in the range of the test measurements, or another reliable measurement method should be employed in a future test. Finally, a proper gauge R&R ought to be done before such an experiment to determine how much variance is due to the measurement process rather than the simple variation in response and other noise factors.

Declarations Funding
There are no funding authorities Con ict of interest Figure 1 Engineering drawing of test specimen  Pareto chart of (a) ultimate tensile strength and (b) impact toughness Figure 10 Page 18/19 Normal Probability Plot for UTS's residuals Normal Probability Plot for impact toughness's residuals