3D printing technology or additive manufacturing is manufacturing by adding materials to objects until it is shaped as expected. This technique is easy and cheap for printing polymer-based materials in the form of filament. Recently, some metal-containing filament has been introduced in the market, one of which is eSteel composed of 45% wt. steel and 55% wt. PLA. Due to its premature introduction, it is a minimal published paper discussing its mechanical properties and dimensional accuracy. This research aims to analyze the dimensional accuracy of 3D printed products of the fused deposition modeling (FDM) technique using eSteel filament. Taguchi method was used to design the experiments with orthogonal array L4 (2³). There were three control parameters with two levels each, namely extruder temperature (220°C, 225°C), layer height (0.3 mm, 0.4 mm), and raster angle (0°/90°, 45°/45°). These parameters were selected based on initial trials. The specimens are in the form of an ASTM D790 flexural test with five replications in each combination. HE3D K200 3D printing machine was used for printing the filament. Analysis of variance indicated that raster angle has the most influence on dimensional accuracy by 32.09%, followed by extruder temperature with a contribution of 31.72% and layer height by 25.53%. The combination of control parameters to produce optimal dimensional accuracy was obtained when combining parameters: extruder temperature of 220°C, layer height of 0.3 mm, and raster angle of 0°/90°.

Keywords: 3D printing; Taguchi Method; Dimensional Accuracy.

The first 3D printer was developed by Charles W. Hull (Chuck Hull) in 1984. It was made for prototype design in rapid prototyping, which means it spent a short time manufacturing product, no need to go through a long process and complicated production [1]. This 3D printing technology is also known as additive manufacturing. Additive manufacturing is manufacturing by adding materials to objects until they are shaped as expected [2-3].

Dimensional accuracy is essential for this technology application in the manufacturing sector. The final form of a product determines the appropriateness of a product function. If it does not match the geometric orientation, the product will automatically be rejected. Consequently, it may lead to a detriment for the company. Most of the previous applications of 3D printing were for rapid prototyping. As its name, the print-ed parts are hopefully in a net or nearnet shape of the actual products to reduce further processes and reduce the cost of post-processing. Therefore, the dimensional accuracy of the 3D printed objects is a must. In recent applications, 3D printing has been used for functional objects, such as in medical products for instance bone implants [4-5], construction, and concrete printing [6].

During the 3D printing process, dimensional inaccuracies may occur because the product undergoes a phase change from liquid to solid, decreasing a specific volume on compaction [7]. As a result, there is a shrinkage of the printed component. The main cause of inhomogeneous shrinkage of some parts in the 3D printing process is the delays in solidification of the layer, the effect of the geometry of the part, and the effect of the boundary on the surface. Another challenge of 3D printed objects is stair-stepping [8] due to layer-by-layer deposition, which also creates dimensional accuracy.

There are several techniques for 3D printing, one of which is the Fused Deposition Modeling (FDM) technique. FDM is one of the most widely used AM techniques for fabricating polymer components [9]. Currently, there are several filaments for 3D printing with the FDM technique from a mixture of plastic and metal, such as eCopper, eBronze, and eSteel. eSteel is a trademark for filaments for 3D printing feeds. Its composition consists of steel (stainless steel) and plastic (PLA). As a new material with a new composition, very few published papers revealed the mechanical properties and accuracy of the printed products made of it. Sakhtivel investigated the PLA-stainless steel composite filament's tensile strength and impact strength as bio-compatible materials using FDM. They revealed that the maximum tensile strength of the printed part was 69 MPa and toughness of 8 kJ/m² [10]. They suggested further observations on dimensional accuracy and surface finish. This research investigates the optimal process parameter settings on 3D printer machines to gain dimensional accuracy using this PLA-steel filament. In addition, a little discussion on flexural strength is presented as a complementary results.

Tools and Materials

The 3D printer machine used HE3D K200 3D. There is no particular reason for utilizing this delta-type printer; it is according to the availability of the machine in the laboratory. In addition, each type of 3D printer should print the object with acceptable precision. A digital caliper of 0.01 mm accuracy was used for measuring the dimension of the product. The filament material used was PLA-stainless steel filament with a composition of 55% wt. PLA and wt. 45% stainless steel, a di-ameter of 1.75 mm (tolerance of 0.05 mm). Some physical properties of this filament: printing temperature of 200°C-220°C, working bed temperature 25°C-70°C, the density of 2.46 g/cm³. The tools and materials presented in Figure 1 are as follows.

Figure 1: (a) HE3D K200 machine (b) digital caliper (c) eSteel filament.

Research Variables

The control variables in this investigation used 60°C bed temperature, 50 mm/s print speed, rectilinear infill pattern, 100 percent infill density, and 0.6 mm nozzle diameter. The dependent variable in this research is the value of the dimensional accuracy of the test specimen. The independent variables of this study can be seen in Table 1.

Table 1: Independent variables.

Research Design

The determination of number and value toward the variable level limits investigating the variables that may affect the process. In this research, the orthogonal array (orthogonal matrix) used was L4 (2³) with three control factors and two levels for each control factor. Five replications were used in each combination. According to [11], the number of replication for a three-factor factorial is two. However, more than the minimum number of replication is preferable to be certain about the experimental error.

Implementation Stage

Before starting the printing process, it should prepare the specimen design using CAD. The design will be printed in the ASTM D790 bending test specimen. This design was chosen because it saves research costs by partnering with a group of pals who undertake flexural test research with the same materials and settings. The material dimensions of the ASTM D790 bending test specimen can be seen in Figure 2. The next step is to slice and set the parameters according to the combinations set in Table 2. After the printing is completed, the next process is to collect data using a digital caliper to measure each printed product's X, Y, and Z dimensions. Each dimension was measured by 5 mm apart. The measurement position of the printed object is presented in Figure 2.

Figure 2: Specimen dimension and measurement position.

Then, the data were processed by using the Taguchi method with an S/N ratio. It aimed to determine the most optimal parameters. ANOVA (Analysis of Variance) was employed to assess the contribution of each control parameter. The response applied is dimensional accuracy deviation; the smaller the deviation, the better it is. Therefore, the response characteristics used are "smaller is better".

The data result of the measurement will be compared with the dimensions of the CAD design results that have been made (based on ASTM D790 standard dimension). The smaller the value of deviation from the nominal dimensions of the 3D printed product indicates the better dimensional accuracy in the experiment. It will take any data processing to obtain the deviation value for each replication. This was conducted three times the deviation value measurement results in each replication, such as on the X, Y, and Z-axis. Therefore, it is necessary to calculate the average for each replication to obtain one deviation value. The results of the average calculation of each replication can be seen in Table 2.

Table 2: Result data of measurement.

eSteel filament is a filament made from a mixture of PLA and stainless steel. The eSteel filament mixture can be seen using a microscope with a magnification of 100 times. The microstructure of the eSteel filament was taken by cross-sectioned and longitudinal views presented in Figures 3a and 3b, respectively.

Figure 3: Microstructure of eSteel filament before extrusion with (a) cross-section view, and (b) longitudinal view.

In Figure 3 above, it can be seen that the composition transmission of the eSteel filament. The grains of stainless steel have various sizes. When it used a 3D printing machine with 220°C and 225°C temperatures, it did not significantly affect the microstructure of the eSteel filament either after or before the printing process. The results of the eSteel filament microstructure after 3D printing with a magnification of 100 times can be seen in Figure 4.

Figure 4: Printing results with extruder temperatures of (a) 220°C and (b) 225°C.

In Figure 4 above, it can be concluded that after the 3D printing process was carried out by using eSteel filaments at 220°C and 225°C temperatures, it did not affect or even change the microstructure. In Figure 4, it can also be observed that stainless steel and PLA were not bonded perfectly (unblended) and only stuck to each other, even though the shape and size of the grains were permanent. The unchanged shape and size of stainless steel material at 220°C and 225°C temperatures can cause defects in dimensional accuracy. Whereby, during experimentation, we found the nozzle clogged with stainless steel grains during the progress of 3D printing.

Defects in the Specimens

Areas with Missing Material

Areas with missing material refer to an area that is empty or not filled by any materials. This defect was caused by the movement of the nozzle motion and the absence of fusion on stainless steel material during the printing process. The poor dimensional control happened because of the encounter of the interlayer, the differences in temperature, and the lack of continuity. Therefore, it created unoccupied surfaces during printing. To overcome this defect, it can change the raster angle according to the shape as expected. This research is also supported by Günaydn K., who states that this kind of defect can be overcome by using better quality filaments to reduce its deficiency [12]. Areas with Missing Material can be seen in Figure 5.

Figure 5: Areas with missing material.

Elephant’s Foot

The elephant's foot refers to the protrusion of the base model. Simply, it happens when it bends or curves at the bottom [13]. Elephant's foot defects occur because the speed of the nozzle movement was too fast when it reached the last point of a layer and moved to the other layer. Consequently, filaments placed in the exact space cause a curve. The way to overcome this defect is to determine the best nozzle speed because the fast pace on the nozzle may involve the filament layer control occurring in the same space, which causes a poor dimension. On the contrary, if the nozzle movement is too slow, it will pile up or create an accumulation. Elephant’s foot defects can be seen in Figure 6.