Investigating the Dimensional Accuracy and Surface Roughness for 3D Printed Parts Using a Multi-jet Printer

The bonding of the layers in FDM happens due to the polymer-based filament heating up to the plastic stage under gravity's effect. The bonding area between the two layers is less, leading to the formation of staircase defects and high thermal contraction. In MJP, however, the liquid material is used for both the primary and support material. Due to fluid layer interaction, the chances of thermal contraction and slow cooling rate require attention for exploring dimensional accuracy and surface properties.

A substantial research contribution has been made toward the benchmarking of various AM processes. Singh et al. (Ref 22) in this context investigated the effect of FDM process parameters on the dimensional accuracy of the parts produced. They examined the volume, density, and a number of coatings in the investment casting (IC). They proposed an alternative method to develop nylon-6 from waste and tailored properties for aluminum matrix composite (AMC). Beniak et al. (Ref 23) considered the FDM process to know the effect of layer thickness and printing temperature on the shape and dimensional accuracy of the fabricated parts. They concluded that the printing temperature was most significant to control the variations in the shape and the dimensions. Hyndhavi et al. (Ref 24) studied the effect of process parameters (layer thickness, raster angle, and build orientation) and their interaction on the dimensional accuracy of the specimen fabricated through the FDM process with acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) materials. They applied Grey relational analysis (GRA) method to optimize the parameters, which yielded a minimum percentage change in the parts' length, width, and thickness. Kozak et al. (Ref 25) considered the selective laser sintering / selective laser melting (SLS/SLM) process to investigate the effect of process parameters (laser power, spot diameter, and exposure time) on the shape and dimensional accuracy and surface quality of manufactured parts. A new dimensional thermal model was developed for the estimation of process parameters along with a sensitivity analysis to validate the results. Sahu et al. (Ref 26) presented a fuzzy decision-making logic in integration with the Taguchi method to improve the dimensional accuracy of FDM processed ABS parts. The process parameters used in this study were: layer thickness, orientation, raster angle, raster width, and air gap to observe variation in dimensions of length, width, and thickness of the parts. It was reported that shrinkage was pre-dominant in length and width directions, but dimensions expanded in the thickness direction. Noriega et al. (Ref 27) proposed the neural network approach to reduce dimensional error for the parallel face of the ABS prismatic part fabricated on FDM. The method increased the accuracy of manufactured parts. Tronvoll et al. (Ref 28) investigated the dimensional accuracy of the thread manufactured through the FDM process. They proposed the narrowing of thread profiles which increased the performance of printed threads. Mou et al. (Ref 29) considered three different AM processes to fabricate parts with protruded surfaces (like holes, flat surfaces, micro-scale ribs, and long channels). They used non-contact methods to record the dimensional accuracy and surface roughness. It was found that the FDM process was the least accurate in terms of dimensional accuracy and surface roughness, whereas MJM (ProJet MJP 3600 3D printer used) produced the most precise parts. Garg et al. (Ref 30) considered the FDM process to investigate the effect of orientation-based deposition on the surface finish and dimensional accuracy. The post-processing was done using cold vapour treatment and investigated. It was reported that minimal variations were observed in dimensional accuracy. Yang et al. (Ref 31) investigated three groups of 3D printed scaffolds printed with an FDM Ultimaker original printer. The chosen material was pure PLA1, PLA2, and composite PLA/ hydroxyapatite (HAp). The surface morphology was studied using SEM, micro-CT, and superimposition techniques. The results suggested the layer height of PLA1 varied the most, whereas PLA/(10% HAp) printed with fused filament fabrication (FFF) produces better accuracy compared to pure PLA (PLA1 or PLA2).

After going through the literature, some of the important points are as follows. Much study has been done on the influence of parametric variation, various materials, different material combinations, and fabrication orientations on FDM parts' dimensional deviation and surface behavior. However, there has been less research on the effect of component orientations on surface behavior in the case of multi-jet printers. As a result, the present research investigates the impact of part orientation on dimensional accuracy and surface roughness behavior. Figure 1 depicts a schematic representation of the study's approach.

One of the primary benefits of 3D printing is the capacity to manufacture complicated forms and geometries. But accuracy and repeatability are required for interchangeability. So in the present investigation for the MJP-based 3D printer, precision and repeatability are investigated. A part is placed in different orientations to check the precision and reparability. The objective of arranging comparable parts in various orientations is to maximize the base plates’s limited space utilization. The cost of each unit will increase if the whole surface of the base plate is not used for similar components. However, to optimally use the base plate, dimensional deviations due to different orientations of components on the base plate will not be accepted to fulfill industry requirements for interchangeability.

In order to examine the geometries, dimensional accuracy, and surface roughness of MJP-based produced components (SR), the part is designed which is a combination of a rectangle, cylinder, and fillets. These shapes provide an opportunity to examine the linear and radial dimensions related to different orientations of the part. In orientations A, B, C, and D, the product is aligned with the faces II, IV, and I and inclined at 45° to the face I and II, as shown in Fig. 2. The references for the dimensions for comparison between the actual and nominal values are represented in Fig. 3. Four parts subjected to different orientations as illustrated in Fig. 3(a), (b) and (c) were fabricated by using MJP (MJP-2500 modeled). All parts are fabricated with the same material, that is, VisiJet M2R-WT (3D systems). The properties of the material are given in Table 1 (Ref 32).

The MJP 3D printer was used for the fabrication of the 3D part. In this model of the MJP-3D printer, there is no such provision for changing the operating parameters. Some technical specifications of the printer are printing mode—high definition (HD), net build volume (XYZ) 294 × 211 × 144 mm, resolution (XYZ) 800 × 900 × 790 DPI (Dots Per Inches), 32-μ (micron) layers. Support material (VisiJet-M2 –SUP) is the only completely compatible support material for Project MJP2500, for all the range of primary materials that can be used on the printer (Ref 32). After fabricating the part on the MJP 3D printer, the post-processing of the printed part was done. In the post-processing, the part was put into the oil bath and preheated at 35–45 °C for ten minutes. Due to post-processing, the support material will dissolve in the oil bath, and the required part will be left behind. After taking out part from the oil bath, the part was washed with tap water and is ready to use. The printed parts were measured by the Coordinate Measuring Machine (CMM) (Quick Vision WLI Series 363-CNC video measuring system with white light interferometry), which can measure the product dimensions without contact (measuring range of 300 × 200 × 190 mm). Here, the height (Z-print axis) is measured at four different points (H₁, H₂, H₃, and H₄). Two points are examined along the X-axis (length), namely (L5 and L6). Six points are considered along the Y-Axis (width) (W₁, W₄, W₅, W₇, W₈ and W₁₀). The radius (R₁, R₂ and R₃) and diameter (D₄, D₅ and D₆) were also measured. End-to-end measurements were taken for height, length, breadth, and diameter. The radial distance from the geometric center to the outer surface was considered, and the average of five observations was regarded as a single observation. Table 2 reflects the nominal values of various parameters considered in the investigation.

The SR value for the fabricated parts on different faces was measured by the roughness tester (IN-SIZE-ISR-S400 model). It can measure R_a, R_q, R_t, R_z, R_c, R_max, R_rsm, R_pc. The device has a range of R_a 0–100 µm, accuracy ± 3%, resolution of R_a is 0.001 µm (Micro Meter), diamond type of the probe is used, and measuring force 0.75 MN (Mega Newtons) is exerted on the part and traveling distance during measurement 0.8 mm (Ref 33).

Qanta FEG 250, the quanta line scanning electron microscope, was used. It is a versatile, high-performance instrument with three modes (high vacuum, low vacuum, and field emission scanning electron microscope). The samples for the SEM were taken out from fabricated parts, and the top surfaces of all the four samples were investigated.

The actual behavior of any manufactured part with the environment is the function of its surface roughness. The parts having high surface roughness will wear out more and have a high coefficient of friction. The life of the mechanical part is also a function of its tribology. If the surface roughness is high, the chances of the formation of cracks will also be more. The high roughness values are undesirable in the field of 3D printing because subsequent operations are required to reduce SR which will lead to the high part cost. The present work measures the SR for the fabricated parts on eleven faces of all four orientations (A, B, C & D). The eleven faces are face I, face II, face III, face IV, face V, face VI, solid curved face, cylindrical part, inclined part, top rectangular part-I, and top rectangular part-II as shown in Fig. 3(d) and (e). These measurements were carried out at four different locations along X-axis and four locations along Y-axis. This process was repeated five times on each face, and the final SR obtained was the average of all the readings. The final SR shown in the graphs is average of all these readings.

This work explores the differences in nominal and actual heights values for various orientations, H₁, H₂, H_3, and H₄. Nominal height represents the part height given by the design software, whereas actual height is the measured height of the fabricated part (refer to Table 3). Figure 4(a), (b), (c) and (d) shows how the height values change by changing orientations.

Figure 4 (a) and (b) depicts the height H₁ and H₂ variation. The minimal divergence between the measured and actual values is discovered for orientation A. In contrast, the largest deviation is found for orientation D.

The fluctuation in height H₃ is shown in Fig. 4(c). The most negligible difference between measured and actual values is observed for orientation B. On the other side, ientation D has the most significant variation.

Figure 4(d) illustrates the variability in height H₄. The variation between measured and actual values is the smallest for orientation D. On the other hand, orientation B has the most diversity. When the mean average deviation is taken into account, the ideal orientation is A, whereas the least optimal orientation is D (see Table 14 for further information).

The variations in heights and standard deviation of heights are shown in Fig. 4 and Table 4. It can be established here that, in the case of height, the best orientation is A, B, and D, and the worst is C (refer to Fig. 4a, b, c, and d). It is suggested for fabricating the part, it should not be placed such that the part is resting on the short edge. It is related to the rate of heat transmission. Heat transfer from the outermost layer to the base plate is not consistent from layer to layer. This is because fluids (the primary material and the supporting material (wax)) are incompressible. Because of incompressible fluids, they do not mix along their boundaries; voids emerge when the wax is removed from the manufactured component. Additionally, due to the high temperature during post-processing, the rate of heat transmission becomes non-uniform, resulting in oversizing and under-sizing parts (Ref 34). The 3D printed parts are post-processed at 30–35 °C in an oil bath. The voids are left behind as the support material is removed from the 3D printed item and dissolved in the oil bath during post-processing. Simultaneously, the primary material will attempt to fill the gaps, while the printed part will become undersized and distorted (Ref 34).

Figure 5(a) illustrates the variability in length L₅. The variation between measured and actual values is the smallest for orientation D. On the other hand, orientations B and C have the most diversity.

Figure 5(b) illustrates the variability in length L₆. The variation between measured and actual values is the smallest for orientation A. On the other hand, Orientation C has the most diversity. When the mean average deviation is taken into account, the ideal orientation is A, whereas the least optimal orientation is D (see Table 14 for details).

Tables 5 and 6 represent variation for various lengths and standard deviation. From Fig. 5, it was found that if the profile of the 3D printed part has maximum surfaces parallel to the base plate, the dimensional deviation will be less and the best orientation is A and B, and C will be the worst. On the other hand, if the inclined plane dimensions are more important, then the parts should be fabricated in such a manner that the inclined plane will remain parallel to the base plate.

Figure 6(a) depicts the variation in width W₁. The inequality between measured and actual values is the smallest for orientation D. Orientations A, B, and C, on the other hand, have the greatest variety.

Figure 6(b) represents the variation in width W₄. The disparity between measured and actual values is the smallest for orientation C. Orientations A, B, and D, on the other hand, have the greatest variety.

Figure 6(c) shows the variation in width W₅. The difference between measured and actual values is the smallest for orientation B. Orientation D, on the other hand, has the greatest variety.

The fluctuation in width W₇ is shown in Fig. 6(d). The divergence between measured and actual values is the lowest for orientation A. Orientation B, on the other hand, is the most diverse.

The variation in width W₁₀ is shown in Fig. 6(e). The divergence between measured and actual values is the lowest for orientation A. Orientation D, on the other hand, is the most diverse. When all dimensions are considered, mean average deviations are taken into account, the optimum orientation is A, whereas the least optimal orientation is D (Table 14 details further).

For the width, the best orientation is A, and the worst orientation is B. The W₁ widths are undersized (refer to Table 7). For width, W₄, W₅, W₇, and W₁₀ dimensions are sometimes oversized and undersized with different orientations (refer to Table 7), and the standard deviation of width is enlisted in Table 8.

The variation in radius is shown in Fig. 7, whereas Tables 9 and 10 contain the actual value, nominal value and standard deviation for the radius, respectively. It can be observed that, for all the orientations A, B, C, and D, as the semicircular part height increases from the base plate to the farthest layer, the dimensional deviation in radius is also increased due to non-uniform heat dissipation to the base plate. Hence, it can be concluded here that, in the case of the radius, orientation B is the best, and the worst is A and C. The actual value of the radius is found to be undersized for all the orientations.

Figure 8(a) illustrates the variation in D₄ diameter. The discrepancy between measured and actual values is the smallest for orientation B. On the other side, orientation D is the most diversified.

The fluctuation in D₅ diameter is shown in Fig. 8(b). The disparity between measured and actual values is the lowest for orientation A. Orientation B, on the other hand, is the most diverse.

The variation in D₆ diameter is shown in Fig. 8(c). The disparity between measured and actual values is the lowest for orientation A. Orientation D, on the other hand, is the most diverse. A is the best orientation when all dimensions are considered, while D is the least optimal orientation (Table 14 details further).

The dimensional variation for various diameters (the comparison of actual values, nominal values, and standard deviation) is shown in Fig. 8, Tables 11, and 12, respectively. Here it can be observed that for the diameter of various circular parts, as the height from the base plate increases, the more dimensional deviation was observed. The reason for this is the non-uniform heat flow rate from different heights of geometries (Ref 34). In brief, it can be inferred here that for various diameters, the best orientation is A and the worst is D. Overall, it can be concluded that for both linear and radial dimensions, orientation A should be preferred to fabricate the 3D part on the MJP. It will be able to provide minimum dimensional variations. All sets of diameters are found to be undersized for all the orientations.

The four figures for the eleven faces and four orientations are discussed in Fig. 9(a), (b), (c), and (d), and Table 13 reflects the measured values of SR. In the case of the orientations A, B, C and D, the value of SR is higher for the faces II, IV, and I. These are those faces (base) from where printing started. While removing the component from the base plates, it was heated from underneath. The heating of these surfaces resulted in an increase in the roughness on the bottom side of the part. In the case of orientation A, face II was at the base, for orientation B, face IV was the base, and for orientations C and D, faces I and II were at the bottom, respectively.

From a set of Fig. 9(a), (b), (c) and (d), it is found that as the layer-by-layer fabrication is moving away from the reference plane X–Y plane (base plate) in the Z-axis, the magnitude of SR is increasing. The roughness of the surfaces is due to the overlapping of the build material and support material. During the 3D printing, printing jets inject both the primary and support materials to make the part dense. These materials are incompressible fluids, so they will not fuse during the overlapping (layering) mode. During the post-processing, the incompressible support material will come out thereby forming pits and voids. As the height of the part is increased, the roller and the plagiarizer will spread the material in an X–Y plane direction; more material will be collected along the walls of the part. That will cause an increase in roughness along the walls of the parts fabricated. Due to high material collection at the ends, the rate of heat dissipation from the top layer to the base plate is decreased (due to the formation of voids) (Ref 34, 35). This may cause a lack of fusion of layers and an increase in roughness. This is observed clearly in Fig. 9(a), (b), (c), and (d) for orientation D. Moreover, for orientation D, the roughness R_a decreases because the part is inclined at an angle, and the machine head had to travel less distance along the Z-axis.