Enhanced Solubility of the Support in an FDM-Based 3 D Printed Structure Using Hydrogen Peroxide under Ultrasonication

Micro/Nano Scale Manufacturing R&D Group, Korea Institute of Industrial Technology, 143 Hanggaulro, Sangnok-gu, Gyeonggi-do 15588, Republic of Korea Department of Mechanical System Design Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Republic of Korea Chemical Research Center, Samyang Co., Hwoam-dong, Yuseong-gu, Daejeon 34055, Republic of Korea


Introduction
For conventional manufacturing technologies, such as machining and molding processes, there has been enormous effort to improve their productivity by reducing lead times, costs, and material consumption.Recently, considering such manufacturing issues, the three-dimensional (3D) printing process has received much interest as an alternative.On the basis of its characteristic procedure of layer-by-layer integration, it enables the production of 3D freeform objects on the basis of digital CAD data while minimizing material consumption and maximizing the freedom of product design [1][2][3][4][5].Moreover, as the trends in manufacturing technology have changed from mass production to mass customization in small batches, the applications of 3D printing techniques have exponentially increased in a variety of industries, including aerospace, automobile, medical device, construction, and electronics [6][7][8].
In ASTM F2792-12a [9], the types of 3D printing were separated into seven classes according to their process characteristics: binder jetting, direct energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photopolymerization.Fused deposition modeling (FDM), which was originally invented in the infancy of 3D printing, is categorized into the group of material extrusion techniques that utilize a nozzle or orifice for selectively dispensing material.Owing to its various advantages, such as the low cost of the material/printer, simple working mechanism, and universality of material use, FDM has been one of the most commonly and widely used systems not only in general industry but also among the research community.
A typical FDM process necessarily involves a set of build and support parts.e support structure has a role in preventing the overhung or undercut shape of the build structure from sagging or collapsing during the layer-by-layer printing process.As shown in Figure 1, there can be internal and external support structures depending on the structural morphology of the build structure.In the case of a singlenozzle FDM system, because both the support and build parts are printed by the same material, the support parts have to be stripped o manually, which unavoidably leaves some defective burrs on the surface of build part.Furthermore, in the manual process, it is not possible to perfectly remove the internal support parts that are embedded inside the build structure, including those in a void or cavity.To overcome these problems, dissolvable materials have been utilized as the support materials in dual-nozzle printing systems.Among the various dissolvable materials, polyvinyl alcohol (PVA) has attracted much attention as the support material owing to its water solubility, biocompatibility, mechanical exibility, and low cost [10][11][12][13].Even though PVA has shown suitability for serving as the FDM support part, there is still the limitation of the removal process with respect to dissolution time and possibility of residues.Especially for the hollow-shaped build structures such as the one in Figure 1(a), it essentially takes a considerable amount of time to remove the embedded internal support structure.To enhance the removal e ciency of PVA supports in a 3D printed part, several studies have attempted adjusting process parameters such as solvent temperature and applying agitation to improve solvent dissolution [14,15].
Here, we introduce an e cient physicochemical method to promote the solubility of PVA support parts by using hydrogen peroxide (H 2 O 2 ) and ultrasonication.
e oxidative degradation e ect of PVA in H 2 O 2 has been investigated in the elds of textiles and pharmaceuticals and detergent-or adhesive-based industries [16,17].e main purpose of this study was to investigate the feasibility of H 2 O 2 for removal of PVA supports in a 3D printed part.We rst experimentally compared the solubility of PVA in water and H 2 O 2 by applying each solvent to two di erent 3D printed parts: a PVA-only part and a build part with a PVA support.
e superior dissolution rate in H 2 O 2 was conrmed and was further enhanced by applying ultrasonication to the solvent.In addition to the fast dissolution rate, we investigate the residues of support material left on the build parts after the dissolution process.Additionally, by mechanical testing of H 2 O 2 -treated specimens and nontreated ones, we con rmed that there was no deterioration or degradation of build parts in terms of mechanical properties and shape geometries.Overall, we demonstrated the superior capability of a H 2 O 2 -based reagent in conjunction with ultrasonication with respect to e ciency of PVA support removal.
ese experimental results are expected to be useful for the postprocessing of 3D printed parts, particularly for a large number of support parts or di cult-to-remove parts with very complex shapes.

Materials and Methods
For the 3D printing of the build and support structures, two di erent thermoplastic laments of polycarbonate (PC; Samyang Corp., South Korea) and polyvinyl alcohol (PVA; ESUN, China) were used, respectively.ese laments had a common standard diameter of 1.75 mm for an FDMbased 3D printer.e printing machine was a customized system equipped with two separate extruders that could separately dispense the build and support materials.For stable printing of materials, the temperature parameters of the PVA nozzle, PC nozzle, and chamber in the system could be set up to 220 °C, 240 °C, and 100 °C, respectively.Under optimal conditions, prism shape samples of PVAonly and PVA/PC-combined parts were fabricated as representative structures with external and internal support parts, respectively.
For the dissolution process of printed parts, H 2 O (distilled water) and H 2 O 2 (30 wt% in H 2 O, Sigma-Aldrich) were used and compared with each other in terms of dissolution rate.e printed parts were immersed into 500 ml of each solvent in a beaker that was loaded into a constant-temperature sonicator (TI-H10, Germany), which was set at a solvent temperature of 80 °C and an applied frequency of 25 kHz.For analysis of PVA residues on the surface of build part, energy dispersive X-ray microanalysis (EDX, X-act, United Kingdom) was carried out using four types of 3D printed samples: one each of PVA and PC and two PC-PVA complex samples.
e PVA and PC samples were prepared as reference controls.e two PC-PVA complex samples were prepared and dissolved in H 2 O and H 2 O 2 separately.To evaluate the change in mechanical properties after solvent treatment, the tensile test specimens were prepared according to the standard ASTM D638-1 and set onto a universal testing machine (EZ20, Instruments, United Kingdom).e tensile tests were performed at a rate of 5 mm/min.2 Advances in Materials Science and Engineering equipped with dual extruder modules that could print two di erent materials for a single part.Speci cally, the 3D printer included several thermostatic control systems to modulate the environmental conditions of printing, such as the temperature of the surrounding air in the printing chamber.e use of temperature-controlled conditions in not only the nozzle but also in the chamber instead of room temperature was necessary in order to stably print and integrate 3D structures without defects or delamination in the nal product.Table 1 shows the optimal conditions for the 3D printing of PC and PVA laments, which were used for the build and support materials, respectively.With these process parameters, defect-free parts could be fabricated in the developed FDM machine.For solubility tests, the 3D printed cubic samples with PVA-only and PC/PVA-combined composition were prepared as shown in Figures 2(b) and 2(c), respectively.We selected two di erent types of support structures, that is, external and internal ones, as shown in Figure 1. e external supports are needed below build structures that have an undercut or overhung shape (e.g., a "T" shape).Typically, since most surfaces of external supports could be exposed to solvent, they are relatively easy to remove by dissolution.4

Results and Discussion
Advances in Materials Science and Engineering could be fabricated with good reproducibility using the process conditions in Table 1.

Enhanced Solubility in Hydrogen Peroxide with
Ultrasonication.Using the as-prepared samples, we rst tested the water solubility of the internal-and external-type parts.Figures 3 and 4 present the change in shape and weight of each sample with increasing dissolution time.While the external one was checked every half-hour, the internal one was measured at intervals of four hours due to its relatively low dissolution rate.As a result, it took 246 minutes to totally dissolve out the external-type PVA support with an initial weight of 6.8 g, which corresponded to a dissolution rate of 0.4032%/min.In the case of the internal-type support, it took 1680 minutes for the complete dissolution at a rate of 0.0595%/min, which was slower than that of the external support sample by 6.78 times.ese results were essentially expected because of the di erence in the exposed surface area.Next, the same samples with internal and external support parts were tested in H 2 O 2 .e change in shape and weight was measured in the same manner as the water solubility test.In the test of H 2 O 2 , it took 123 and 830 minutes to completely dissolve the external-and internaltype samples, respectively, corresponding to dissolution rates of 0.8131 and 0.1205%/min, which were overall twice as fast as dissolution in water.Figure 5  O is thought to arise from additional degradation caused by the oxygen ions in the solvent.PVA is well known as a water soluble and hydrophilic polymer.As water molecules penetrate between the polymeric chains of PVA during water uptake, the polymer swells and transitions into a dissolved state due to the weaker hydrogen bonding between the polymer chains (Figure 6(a)) [18][19][20].In addition to the water solubility, the degradation of PVA has been studied using photochemical, electrochemical, and photocatalytic methods for consideration as a waste removal technique [21][22][23][24].ose studies suggested that PVA degradation was accelerated by adding H 2 O 2 into the reaction process, where more reactive species including hydroxyl and peroxyl radicals were generated and the scission of polymer chains occurred more actively [16][17][18]24].We experimentally con rmed that the dissolution of PVA in H 2 O 2 was considerably enhanced compared with the process conducted in water.Such enhancements were thought to be attributable to oxidative degradation induced by the reactive radicals, which would in uence the dissolution synergistically with the weakening of hydrogen bonding (Figure 6(b)).
To further con rm the PVA removal e ciency, EDX analysis was performed using four types of 3D printed samples: one each composed of PVA and PC and two composed of PC-PVA complexes.
ese samples were dissolved separately in H 2 O and H 2 O 2 (Figure 7).Since PVA is a relatively oxygen-rich polymer, as expected, the PVA   Next, we tried to further enhance the dissolution of H 2 O 2 by applying ultrasonication during the dissolution process of PVA. Figure 8 displays the comparison of the dissolution processes depending on whether sonication treatment was employed on the H 2 O 2 solvent.Under ultrasonication, the external-and internal-type PVA supports were completely removed in approximately one and nine hours, respectively.
ese corresponded to dissolution rates of 1.67 and 0.1851%/ min, which were faster by 2.05 and 1.54 times compared to those under static conditions in the same solvent.In the eld of chemical engineering, ultrasonication has proved its advantageous e ects on the degradation or dispersion of substances in some solvent systems [25,26].e results in this experiment were in agreement with the previous works addressing the accelerating e ect of ultrasonication on polymer dissolution or degradation.e high-energy ultrasonic wave was thought to contribute to the aforementioned mechanisms  Advances in Materials Science and Engineering of oxygen-based degradation and water penetration.From the experimental results, we could clearly confirm the feasibility of the physicochemical method for PVA dissolution using H 2 O 2 and sonication treatment.

Sustainable Properties of Support Removal Treatment.
After the support removal process mentioned above, the build part should not be influenced by the reactive dissolution conditions, that is, H 2 O 2 and ultrasonication.To analyze the effect of the dissolution conditions on the build part, two types of PC specimens were prepared for tensile testing as shown in Figure 9.Typically, FDM-printed parts show mechanically anisotropic properties that are dependent on the direction of the printing toolpath.Similarly, Figure  e longitudinal and transverse specimens provided average tensile strengths of 53.28 and 25.92 MPa, respectively.After soaking the specimens for two hours in H 2 O 2 , the tensile strengths of longitudinal and transverse specimens were 52.98 and 26.15 MPa, respectively, which were not changed compared with the values of the untreated specimens.In addition to the intact mechanical properties, there was no weight loss between the specimens before and after the H 2 O 2 treatment, as shown in Figure 9(b).
Figure 10 displays the evaluation of dimensional change before and after H 2 O 2 dissolution using two different buildsupport parts with 80% and 60% of support infill rate.When the build-support part with 80% of inner support was immersed in H 2 O 2 for 9 hours, the build part exhibited a slight distortion (Figure 10(a)), which might be possibly due to support swelling and H 2 O 2 infiltration.When the infill rate of support part was reduced to 60%, the support dissolution was completed in 5 hours, and we could obtain a negligible change of dimensions (Figure 10(b)).We also evaluated the dimensional change using a practical part, as shown in Figure 11.We designed a Lego structure with an internal cavity.For more effective exposure to dissolving agent, the internal support structure was designed as an alternately half-filled shape, thereby enabling facile and fast dissolution (Figure 11(a)).Figure 11(b) shows the resulting printed part according to the dual-part design and its removed PVA part after H 2 O 2 dissolution for 50 minutes.It was confirmed that there was no dimensional change during the support removal process, as shown in Figure 11(c).Owing to the pristine dimensions of the postprocessed part, Figure 11(d) reveals that it could be well assembled with the original Lego part with a reference shape that was identical to the CAD model at the design step.Overall, we demonstrated the feasibility of the removal of a PVA support part in terms of the intactness of the geometry and the mechanical properties.

Conclusion
To render stable 3D architecture in FDM-based 3D printing, a support structure has to be effectively formed along with the shape of the build part.e present study has indicated our initial effort to use the reactivity of H 2 O 2 in conjunction with ultrasonication for efficient dissolution of PVA structures.PVA has been widely utilized as a water-soluble support material in the field of 3D printing.To the best of our knowledge, the application of a physicochemical method to remove a PVA-based support in 3D printing process has seldom been reported to date.Two different type samples composed of PVA and PVA-PC, which were used as representative samples with external and internal supports, respectively, were tested under several conditions depending upon the application of H 2 O 2 and ultrasonication.When comparing this method with normal water-based dissolution, we confirmed a greater than two times enhancement in the dissolution rate and even a four times enhancement in the case of an external-type support.In addition, the applicability of the reactive method was confirmed based on the unaltered properties in terms of mechanical strength and shape geometry.ese results collectively suggest that the proposed method has the potential to enhance the productivity of the support removal process and overcome obstacles of intractable supports with complicated shapes.On the basis of the results presented here, further studies with other FDM-applicable materials are underway to evaluate the feasibility of the suggested method in 3D printing applications.

3. 1 .
Dual Printing of Build and Support Parts.
Figure 2(a)   shows our developed 3D printing system, which extrudes standard thermoplastic laments in an FDM manner.It was

Figure 2 :
Figure 2: (a) FDM machine equipped with dual extruders and a thermostatic control chamber.3D printed cubic samples for the solubility test of (b) external-and (c) internal-type supports.

Figure 3 :Figure 4 :
Figure 3: Experimental results of the dissolution of PVA supports in distilled water.

Figure 5 :
Figure 5: Dissolution rates of external-and internal-type PVA support samples in water and hydrogen peroxide.

Figure 6 :
Figure 6: Schematic illustration of the mechanism of the dissolution and degradation of PVA materials by (a) water and (b) hydrogen peroxide.

Figure 7 :Figure 8 :
Figure 7: EDX spectra including comparative oxygen peaks on the surface of (a) PVA, (b) PC, and PC-PVA specimens after PVA dissolution in (c) H 2 O and (d) H 2 O 2 .e red boxes in (c) and (d) indicate oxygen peaks.e histogram on the right indicates the weight composition of oxygen in each specimen.

Figure 9 :
Figure 9: Change in (a) tensile strength and (b) weight of direction-varied specimens after H 2 O 2 treatment.In the tensile tests in (a), three specimens were tested in each experiment, and the tensile strength is shown as the mean ± standard deviation.

Figure 10 :
Figure 10: Evaluation of dimensional change before and after dissolution in two different specimens with (a) 80% and (b) 60% of support infill rate.

Figure 11 :
Figure 11: (a) A CAD model of a Lego structure composed of build and support parts.(b) e dual-printed structure including a PC build part and a PVA support part.(c) Comparison of the dimensions of the printed part before and after dissolution of the PVA support.(d) Assembly of the H 2 O 2 -treated part with an original Lego part with an identical shape.

Table 1 :
Experimental conditions of FDM-based 3D printing of PVA and PC laments for the solubility test.
displays the dissolution rate of each PVA support in H 2 O and H 2 O 2 .We con rmed the faster removal of both types of PVA supports in H 2 O 2 than in H 2 O at the same time point.e superior dissolving power of H 2 O 2 compared to H 2