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Water-based coatings for 3D printed parts.

Abstract Three-dimensional (3D) printing or additive manufacturing is an emerging technology that enables the exploration of new ideas and designs. The process converts a 3D digital model into a 3D object, allowing end users to hold and easily visualize their designs. Since 3D printed parts are built through layer-by-layer deposition, they exhibit limited surface smoothness and surface quality. By applying water-based coatings onto 3D printed parts, surface quality can be improved by reducing surface roughness and by sealing surface gaps without deforming the parts themselves. In this study, water-based coatings were applied to 3D printed parts using a computer-controlled dip coater. Surface profiles of 3D printed parts before and after dip coating were measured using a profilometer and parts were examined with optical microscopy. Two commercial water-based coating systems were evaluated, and the effect of coating speed, drying conditions, and number of coated layers on surface roughness of 3D printed parts was studied. The effect of printing with different print tips and in different orientations on surface roughness before and after coating was also studied. Coating liquids with higher solids loading were found to be most effective at smoothing or planarizing the 3D printed part surface.

Keywords 3D Printing, Dip coating, Water-based coating, Surface characterization, Planarization

Introduction

Three-dimensional (3D) printing or additive manufacturing (AM) has attracted tremendous attention. (1-3) This process has revolutionized the world of new product design by allowing designers to create 3D digital models on a computer or via a 3D scanner and obtain the actual 3D parts within hours. By reviewing the actual 3D parts, designers can easily make changes based on desired shapes, sizes, functions, and applications before they finalize a new product design. Exploratory and industrial applications of 3D printing or AM technologies can be found in many fields, including automotive and aerospace, (4-6) dental implants and prosthetics, (7-9) medical devices, (10,11) education, (12,13) fashion, (14) footwear, jewelry, and geographic information systems.

In this work, we use the fused deposition modeling (FDM) 3D printing technique, which involves the extrusion of a polymer melt through a narrow circular tip. The extruder rasters in the X-Y plane and the build plate moves in the Z direction to create 3D parts. To print a 3D part based on a digital model, the computer begins by slicing the 3D digital model into a series of thin layers and generating codes for the specified printer to deposit successive layers of modeling material at precise locations on the printing platform. These printed layers fuse together to create the final object. The primary advantage of this technique is its ability to create almost any shape or geometric feature according to the 3D digital model. One downside of this technique, however, is the limited surface quality of the printed parts due to the limited print resolution, which is defined by the layer thickness.

To improve the surface quality of 3D printed parts, there are two main types of post-printing treatments: (i) mechanical deformation via sanding, polishing, or mass finishing with vibratory or centrifugal barrel machines, and (ii) sealing via chemical vaporization or coating applications. Surface smoothing by mechanical deformation or chemical vaporization has been well developed and is widely used. However, these processes require specialized equipment, which is commercially available but may not be very convenient or economical. Conversely, surface smoothing by applying a coating is relatively simple as coatings can be readily applied by spraying, brushing, or dipping. However, coatings have not been widely implemented due to the lack of understanding of the process requirements and the interactions between coatings and 3D printed parts. Mireles et al. (15) studied the sealing of 3D printed parts with different types of coatings and demonstrated that applying coatings on 3D printed parts can eliminate voids and reinforce bonds between printed layers to prevent leakage and to improve part strength.

Applying coatings onto 3D printed parts can also introduce desirable surface properties that are made possible by the wide range of commercially available or specialty coatings. For example, a commercial, water-based, antimicrobial coating for 3D printed parts was evaluated by Zhu (16,17) and was shown to give a more than 5 logio reduction in bacterial growth as compared to nontreated 3D printed parts. This makes it possible to 3D print medical devices without modifying existing 3D printers by adding antimicrobial surface properties to 3D printed medical devices in a post-printing stage.

In this study, two commercial water-based coatings were applied onto 3D printed parts using a computer-controlled dip coater. The effects of coating speed, drying conditions, and number of coated layers on surface roughness of 3D printed parts were studied by comparing surface profiles of 3D printed parts before and after coating as measured using a profilometer and optical microscopy. Surface roughness of 3D parts, which were printed with different print tips and in different orientations, was also studied.

Experimental methods and materials

Materials

Two water-based polyurethane coatings were evaluated: Pro Finisher (PU1), a clear gloss polyurethane (Rust-Oleum[R], Vernon Hills, IL) designed primarily for wood floor finishing and JetFlex[R] (PU2), a water-reducible polyurethane dispersion designed for coatings on plastic in aircraft interiors (Sherwin Williams, Andover, KS). The solids loading for the PU1 coating liquid is 28 wt% and ~26 vol%, based on manufacturer data. PU2 has a higher solids loading, 40.5-52.6 wt% and 37.3-40.1 vol%, based on manufacturer data. These coating materials were selected to represent examples of commercially available, water-based coating systems that are suitable for improving the smoothness and appearance of the parts. They are not marketed as specifically for coating 3D printed parts.

Substrates for coating evaluation were 3D printed rectangular parts (3 cm x 13 cm x 75 cm). These substrates were printed with an acrylonitrile butadiene styrene (ABS) model material under default conditions using a Fortus 400 me 3D printer (Stratasys, Eden Prairie, MN). Parts were printed with three different print tips: T10, T16, and T20, which print with layer thicknesses of 127, 254, and 330 [micro]m, respectively. Parts were printed in two different orientations: a side orientation and a vertical orientation, as illustrated in Fig. 1. The 13 cm x 75 cm face is the primary face for this study.

Coating preparation

Coatings were applied to the 3D printed rectangular parts using a computer-controlled dip coater with EasiV software (v3.1, Parker Hannifin Corp, Rohnert Park, CA). Each part was attached to the dip coater with its long axis vertical, and the coating deposited on the primary face (13 cm x 75 cm) was evaluated. Therefore, the dip coating direction was parallel to the printed layers for parts printed with a side orientation and perpendicular to the printed layers for parts printed with a vertical orientation.

Each part was submerged in the coating solution for 2 min and then pulled out at a constant withdrawal rate (2, 10, or 20 mm/s). Unless otherwise specified, parts with multiple coating layers were air-dried for ~2 h before applying another layer of coating. For comparison, some coatings were dried using a heat gun with gently blown air using the low setting on the heat gun. All coated parts were oven-dried at 80[degrees]C for 2 h after the last layer of coating had been air-dried for ~2 h and were stored at room temperature for at least 48 h before coating characterization.

Viscosity measurements

Coating solution viscosities were measured using an AR-G2 stress-controlled rheometer (TA Instruments, New Castle, DE) with a 40 mm 2[degrees] stainless steel cone and plate geometry. A Peltier plate was used to control the temperature and all measurements were taken at 23[degrees]C. A plastic guard was used to cover the geometry to minimize evaporation. Measurements were performed by stepping the shear rate from 100 to 0.01 [s.sup.-1]. The measurement time was approximately 25 s for each point.

Surface characterization

Surface topography of the 3D printed parts before and after coating was characterized using a stylus profilometer (P10, KLA-Tencor, Milpitas, CA) with a 5 [micro]m radius tip. Measurements were taken under a 50 mg load force over a distance of 2000 [micro]m on the primary face of the 3D parts and perpendicular to the printed layers. Figure 2a shows an example of a surface profile of a 3D part printed with tip T16. Figure 2b is an optical micrograph of the part that shows the origin of the height variation due to the curved protrusions from the individual printed layers. It should be noted that the layer thickness in a 3D printed parts is not identical due to micromechanical vibrations present during printing. In this paper, we will use this average peak height (APH) as a measure of roughness. Based on the values of APH measured before and after coating, the degree of planarization (DOP) can be defined as:

DOP(%) = 100[1 - [APH.sub.After coating]/[APH.sub.Before coating]. (1)

This measure of planarization is similar to that used in the microelectronics industry. (18-20) Surface topography of 3D printed parts was also evaluated using a digital optical microscope (VHX-2000, Keyence, Itasca, IL).

Results and discussion

Rheology of the coating solutions

Measured viscosities for the two coating solutions as a function of shear rate are shown in Fig. 3. PU1 is a relatively low viscosity, Newtonian solution, whereas PU2 is a higher viscosity, shear thinning solution. The higher viscosity is due in part to the higher solids content as compared to that of PU1. This high solids content also likely contributes to the shear thinning behavior of this particular dispersion. The effect of dilution on the rheology of PU2 was also explored. The as-received coating liquid was diluted with isopropanol (IPA). The viscosity decreases with increased dilution, as expected, and retains the same shear thinning behavior. The same general behavior is expected for dilution with water, the system that we chose in the coating study.

Surface morphology of 3D printed parts before coating

Surface roughness of 3D printed parts is related to the selection of a print tip. 3D parts printed with a fine tip have a lower surface roughness (i.e., APH), but take a longer time to print. Figure 4 shows surface profiles of 3D parts printed with tips T10, T16, and T20, respectively. The measured APH values from the surface profile were 22, 61, and 87 [micro]m for parts printed with tips T10, T16, and T20, respectively. Standard deviations were [+ or -] 1 [micro]m. Estimated print times from print tips T10, T16, and T20 for a side oriented part, as shown in Fig. 1, were 33, 11, and 8 min, respectively.

Surface morphology of 3D printed parts after coating

Table 1 lists APH values measured from surface profiles for 3D printed samples with tip T16 in the side orientation before and after coating with the PU1 coating system. In the side orientation, the dip coating direction is parallel to the direction of the printed layers. Each sample was measured with a profilometer on the primary face at three locations: (i) approximately 5 mm from the top, (ii) at the middle, and (iii) approximately 5 mm from the bottom. Note that 'Sample T16' refers to the noncoated 3D part printed with tip T16, while Samples 1-9 are 3D parts coated with PU1 under different coating conditions, as indicated.

The data in Table 1 show that both the application of a coating as well as the specific conditions under which it is applied influence the surface morphology. First, the APH drops from 61 pm for an uncoated part to ~50 [micro]m or less for a coated part. Hence, the coating is planarizing or smoothing out the corrugations on the surface. For samples coated with a single layer but with varying dip coating speeds (e.g., Samples 1-3), the DOP values are similar but increase slightly with increased coating speed. This increase in coating speed is expected to be associated with an increased coating thickness (21,22) and increased thicknesses have been shown to yield larger DOP values. (19) Applying a second coat (e.g., in Samples 4 and 5) resulted in a significant reduction in measured APH and an increase in DOP, indicating that adding additional layers of coatings results in better surface smoothness on coated 3D parts. However, no further increase in smoothness was observed by adding more than two layers.

The drying treatment also played a role in the ability of the coating to planarize the 3D part. For Samples 1-5, which were dried by hanging in air for 2 h after dip coating, the standard deviations from the measured APH values at top, middle, and bottom locations of the coated sample were about five times higher than that of a noncoated sample. Further, the values of measured APHs followed as: top > middle > bottom for each coated sample, suggesting that coatings were thinner at the top than at the bottom of coated samples. This result is likely caused by the as-deposited coating layer flowing due to gravity during drying. (23) That flow would result in a thicker coating near the bottom and, combined with leveling, could result in a larger DOP at the bottom.

To minimize variations in coating coverage, a hot air gun was used to gently dry the coated Samples 6-9 after dip coating. The faster drying resulted in significantly lower standard deviation values among APH values measured along the length of the 3D parts, resulting from more uniform coating coverage. However, the shorter drying time and hence shorter time for leveling resulted in an overall lower DOP value (e.g., compare Sample 2 with Sample 6 and Sample 5 with Sample 7). This illustrates the apparent trade-off between overall spatial uniformity, which is enhanced by hot air drying, and planarization, which improves with longer drying times. Drying the sample on a horizontal surface after withdrawal would eliminate the effects of the gravity-driven flow.

Table 2 lists APH values measured for 3D parts printed in a side orientation with print tips T10 and T20 before and after coating with PU1 under different conditions. Similar results were observed here as were observed for parts printed with tip T16, namely, a slight increase in DOP with increasing coating speeds. Additionally, the results show that applying one or two layers of coating onto parts printed with a T20 tip results in surface profiles that are comparable to or better than those of noncoated parts printed with a T16 tip. This result suggests that one can reduce print time by selecting a print tip with thicker layers and then achieve comparable surface quality (and potentially add color or other functional features) by applying a coating. Using a hot air gun to gently blow-dry samples after dip coating also resulted in significantly reduced variations in coating coverage, as shown by reduced standard deviations among top, middle, and bottom APH values for Samples 16-19.

The above discussion pertains to coatings applied onto 3D parts printed in a side orientation, such that the direction of withdrawal from the coating solution was parallel to the direction of the printed layers. For parts printed in a vertical orientation, as shown in Fig. 1, the dip coating direction was perpendicular to the printed layers on the part surface. Table 3 lists measured APH values for 3D parts printed with print tip T16 in a vertical orientation. Comparing the two orientations under identical conditions, there is little difference in the DOP and overall APH across the sample. This result indicates that the effectiveness of this coating in smoothing the surface is insensitive to the direction of the surface corrugations. However, coatings applied to parts printed in a vertical orientation show significantly less variation in measured APH values between top, middle, and bottom positions (e.g., compare Samples 20-24 with Samples 1-5 in Table 1). This result suggests that flow after deposition is less when the direction of dip coating (and gravity) is perpendicular to the printed layers on 3D parts.

Figure 5 shows optical micrographs of cross sections of 3D parts that were coated with PU1 at a coating speed of 10 mm/s. As discussed above, these optical micrographs show that the valleys in the layer-by-layer construction of the parts with two layers of coatings appear to be more filled in, resulting in a smoother surface than those with only one layer of coating.

When 3D printed parts were coated with PU2, a much higher degree of planarization was achieved. Table 4 lists the APH and DOP for 3D parts printed with a T16 print tip before and after coating with PU2. Figures 6 and 7 show the profilometery data and optical micrographs, respectively, for the uncoated part ('Sample T16') as well as Samples 31, 33, and 35.

When 3D parts were coated with undiluted or lightly diluted (10 vol% water) PU2, the measured APH values were less than 10 pm and DOP values were over 90%. 3D parts coated with highly diluted (30 vol% water) PU2 had DOP values similar to those obtained using PU1. In the surface profiles shown in Fig. 6, the peak locations for Samples 31 and 33 did not match those for Sample T16, suggesting that the minor undulations on the surfaces of these samples were not directly related to the underlying part structure. As shown in Fig. 7, these coatings were considerably thicker than those created using the highly diluted PU2 and PU1.

The results presented can be understood based on the literature on planarization of features in microelectronic devices. (19,20) Figure 8 shows a general sequence of events as an example. The as-deposited liquid coating in its initial state is leveled and thicker than the feature size (Fig. 8a). A flat coating surface is achieved by surface tension-driven leveling. While some drying occurs during dip coating, it is not unreasonable to expect the as-deposited layer to level completely, creating a coating that is thicker over the valleys than over the peaks in the corrugated surface. Roughly from this starting point, the coating dries. While the viscosity of the coating is still low enough, the level surface is retained as volume is lost during drying. However, at some point during drying (Fig. 8 b), the coating solidifies and volume loss from continued drying is accommodated by shrinkage of the solid. If diffusion due to concentration gradients set up over the peaks and valleys is ignored, then the local shrinkage is based on the solids content at solidification. In the example, the thickness of the coating over the peak is 30 [micro]m at the solidification point (Fig. 8b) and 15 [micro]m after complete drying (Fig. 8c); therefore the APH drops from 50 pm at initial state to 25 [micro]m with the coating, resulting in a more planar surface and a DOP of 50%. Using a coating with a higher solids loading and lower shrinkage after solidification would increase planarization. Diffusion would enhance the planarization over this baseline. Since drying occurs at a uniform rate from the free surface, the polymer solution over the peaks has a higher concentration, prompting diffusion of polymer to the region of the coating over the valleys, which has a lower polymer concentration. This simple example also can be used to understand why multilayer deposition results in improved planarization.

Summary

In this study, water-based coatings were applied onto 3D printed parts by dip coating using a coating speed of 10 mm/s followed by air drying for 2 h and oven drying at slightly elevated temperatures (ca., at 80[degrees]C) for 2 h. Surface studies by profilometry and optical microscopy showed that the curved surfaces caused by the layer-by-layer construction of the 3D printed parts were fully covered by the coatings, resulting in smoother surfaces. The viscosity and solids loading of the coating liquids as well as process variables, such as coating speed, number of coated layers, and drying method, influenced coating thickness and surface profiles. Planarization was found to improve with the use of a higher solids loading coating liquid. We also found that employing hot air flow during drying improved coating uniformity. However, in this limited study, this hot air also appeared to lower planarization. More studies on the effects of drying conditions are needed to explore this effect further. The orientation of the surface corrugations relative to the dip coating direction had only a small effect on planarization. PU2, a coating with a relatively high solids loading, was shown to produce smoother surfaces and multiple coats were also shown to improve surface smoothness. The results from this study are a starting point for understanding how to design coating liquids and processes for 3D printed parts. 3D printed parts are inherently more complex than the rectangular bars coated in this study. The complexity leads to interesting questions and challenges related to coating application and drying.

DOI 10.1007/s11998-015-9710-3

J. Zhu ([mail])

Stratasys, Inc., 7665 Commerce Way, Eden Prairie, MN 55344, USA

e-mail: jennyzhu@comcast.net

J. L. Chen, R. K. Lade Jr., Wieslaw J. Suszynski, L. F. Francis

Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE, Minneapolis, MN 55455, USA

Acknowledgments The research presented in this paper was conducted as an Industrial Fellowship project under the Industrial Partnership for the Interfacial and Materials Engineering (IPRIME) program and the Coating Process Fundamentals Program (CPFP) at the University of Minnesota. The authors thank the industrial partners of CPFP and especially Stratasys for their support.

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(8.) Mironov, V, Boland, T, Trusk, T, Forgacs, G, Markwald, RR, "Organ Printing: Computer-Aided Jet-Based 3D Tissue Engineering." Trends Biotechnol., 21 157-161 (2003)

(9.) Leukers, B, Gulkan, H, Irsen, SH, Milz, S, Tille, C, Scheiker, M, Seitz, H, "Hydroxyapatite Scaffolds for Bone Tissue Engineering Made by 3D Printing." J. Mater. Sci., 16 1121-1124 (2005)

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Table 1: Effect of coating conditions on the surface
morphology of 3D printed samples before and after coating
with PU1. All samples were printed in a side orientation
with a T16 tip

                                               Number     Blow-dried
                   Part         Dip coating   of coated      with
Sample-ID      orientation      speed, mm/s     layer      heat gun

T16                N/A              N/A          N/A         N/A
Sample-1     Side orientation        2            1           No
Sample-2                            10
Sample-3                            20
Sample-4                             2            2
Sample-5                            10
Sample-6                            10            1          Yes
Sample-7                                          2
Sample-8                                          3
Sample-9                                          4

                Measured average peak height, [micro]m

                                                    AVg.
Sample-ID    Top   Middle   Bottom   Avg.   Old.   DOP (%)

T16          61      60       62      61    1.1      N/A
Sample-1     50      46       40      45    5.1      26
Sample-2     50      43       39      44    5.6      28
Sample-3     47      42       40      43    3.9      30
Sample-4     43      33       26      34    8.2      44
Sample-5     40      33       28      33    6.2      45
Sample-6     50      49       47      49    1.5      20
Sample-7     42      39       39      40    1.7      34
Sample-8     34      34       32      33    1.2      45
Sample-9     33      35       33      34    1.2      45

Table 2: Effect of 3D printing and coating conditions on the
surface morphology of 3D printed samples after coating with
PU1

                                               Number     Blow-dried
                   Part         Dip coating   of coated      with
Sample-ID      orientation      speed, mm/s     layer      heat gun

T10          N/A                    N/A          N/A         N/A
Sample-10    Side orientation       10            1           No
Sample-11                                         2
T20                                 N/A          N/A         N/A
Sample-12                           10            1           No
Sample-13                           20
Sample-14                           10            2
Sample-15                           20
Sample-16                           10            1          Yes
Sample-17                                         2
Sample-18                                         3
Sample-19                                         4

                Measured average peak height, [micro]m

                                                    AVg.
Sample-ID    Top   Middle   Bottom   Avg.   Old.   DOP (%)

T10          22      22       23      22    0.6      N/A
Sample-10    18      16       16      17    1.2      24%
Sample-11    15      14       11      13    2.1      39%
T20          88      86       86      87    1.2      N/A
Sample-12    73      66       65      68    4.4      22%
Sample-13    70      66       61      66    4.5      25%
Sample-14    60      47       43      50    8.9      43%
Sample-15    56      46       42      48    7.2      45%
Sample-16    71      68       68      69    1.7      21%
Sample-17    56      62       59      59    3.0      32%
Sample-18    57      58       61      59    2.1      33%
Sample-19    49      52       48      50    2.1      43%

Samples were printed in a side orientation with tips T10 or T20

Table 3: Effect of coating conditions on the surface
morphology of 3D printed samples after coating with PU1. All
samples were printed in a vertical orientation with a tip
T16

                                                               Blow-
                                                   Number      dried
                     Part           Dip coating   of coated     with
Sample-ID        orientation        speed, mm/s     layer     heat gun

T16          N/A                        N/A          N/A        N/A
Sample-20    Vertical orientation        2            1          No
Sample-21                               10
Sample-22                               20
Sample-23                                2            2
Sample-24                               10

                Measured average peak height, [micro]m

                                                    AVg.
Sample-ID    Top   Middle   Bottom   Avg.   Old.   DOP (%)

T16          61      60       62      61    1.1      N/A
Sample-20    44      40       40      41    2.2      33%
Sample-21    45      48       43      45    2.6      26%
Sample-22    47      42       40      43    3.9      30%
Sample-23    32      29       27      29    2.6      52%
Sample-24    31      29       27      29    1.8      53%

Table 4: Effect of coating conditions on the surface
morphology of 3D printed samples after coating with PU2.
Samples were printed with a T16 tip

                     Part                     PU2 coating
Sample-ID        orientation                   solutions

T16          N/A                    N/A
Sample-25    Side orientation       Non-diluted
Sample-26
Sample-27                           Diluted with 10% DI-[H.sub.2]O
Sample-28
Sample-29                           Diluted with 30% DI-[H.sub.2]O
Sample-30
Sample-31    Vertical orientation   Non-diluted
Sample-32
Sample-33                           Diluted with 10% DI-[H.sub.2]O
Sample-34
Sample-35                           Diluted with 30% DI-[H.sub.2]O
Sample-36

                           Measured
             Dip coating   avg peak                  AVg.
Sample-ID    speed, mm/s   height, urn               DOP

T16              N/A            61                    N/A
Sample-25         2             0-5      [greater than or equal to] 92%
Sample-26         10            0-5                  >92%
Sample-27         2             0-5                  >92%
Sample-28         10            0-8                  >87%
Sample-29         2             38                    38%
Sample-30         10            37                    39%
Sample-31         2             0-5                  >92%
Sample-32         10            0-5                  >92%
Sample-33         2             0-5                  >92%
Sample-34         10           0-10                  >84%
Sample-35         2             29                    52%
Sample-36         10            24                    61%


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Author:Zhu, Jiayi; Chen, Julia L.; Lade, Robert K., Jr.; Suszynski, Wieslaw J.; Francis, Lorraine F.
Publication:Journal of Coatings Technology and Research
Article Type:Report
Geographic Code:1USA
Date:Sep 1, 2015
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