Additive manufacturing, also known as three-dimensional (3D) printing, has found applications in numerous industries. Surgical and dental implants customized to each patient are now produced rapidly using 3D printing, as are many prosthetics. Surgeons also prepare 3D replicas of patient organs to plan complicated surgeries. Some automotive and aerospace components can also be rapidly produced via 3D printing. The potential is only limited by the ability to create “inks” from the required raw materials. Several companies are, in fact, developing 3-D printed biomaterials using inks consisting of cells and growth media.

However, for plastic/polymer parts produced via additive manufacturing, the surface morphology is not smooth like that of parts manufactured via conventional molding processes, for instance. Each part is created via a layer-by-layer deposition process, and the surface consists of hills and valleys that reflect the layers and minute spaces between the layers, and the level of surface roughness depends on the thickness of the layers. With most 3D printers, the print resolution is somewhat limited. Consequently, 3D-printed parts typically do not have a high-gloss appearance.

In many cases, the parts are subjected to sanding or polishing or other mechanical treatment to improve the surface appearance. However, this approach requires access to specialized equipment and adds both processing time and cost. The application of a coating via spraying, brushing, or dipping is an alternative method to smooth the surfaces of 3D-printed parts. This approach is attracting attention, because it is easy and relatively inexpensive, does not require specialized equipment, and provides the potential to not only smooth the surface, but impart added functionality to 3D-printed parts, according to Jiayi Zhu of Stratasys, Inc.

“Coatings haven’t been widely used for 3D printed parts yet, because there is little known about how they may interact with the part surfaces. There are a few examples, though. We have, for instance, shown that when a water-based antimicrobial coating was applied to printed parts, bacterial growth was significantly reduced compared to that observed for non-treated parts. This result certainly suggests that coatings can be applied to printed parts and enhance their performance,” Zhu says.

The application of a coating via spraying, brushing,
or dipping is an alternative method to smooth the surfaces of 3D-printed parts. This approach is attracting attention,
because it is easy and relatively inexpensive . . .

To further explore the potential use of waterborne coatings for improvement of the surface properties of 3D-printed parts, scientists at Stratasys teamed up with researchers in the Department of Chemical Engineering and Materials Science at the University of Minnesota to evaluate the surface morphologies of 3D-printed parts before and after dip-coating using two different commercially available waterborne polyurethane (PU) coatings.

The first coating (PU1) was designed for wood floor finishing and had a solids content of 28 wt%, while the second was designed for the coating of plastic components for airplane interiors and had a solids content of 40.5–52.6 wt% and a higher viscosity.

These coatings were applied to 3D-printed rectangular parts (3 x 13 x 75 cm3)—one that was printed with the longest length horizontal to the base and the other with the longest side vertical to the base. The parts were created using a Fortus 4000 mc 3D printer from Stratasys and three different print tips that varied in fineness. The finest tip provided the longest printing times, but the lowest surface roughness values.

A computer-controlled dip coater was used to apply the coatings to the parts, which were dipped with the long length in the horizontal direction (parallel and perpendicular, respectively, to the printer layers in the horizontal and vertical parts). Most parts were allowed to air dry for about two hours before recoating. A select few were dried with a heat gun. All of the parts were oven-dried at 80°C for at least two hours and then stored at room temperature for at least 48 hours prior to analysis.

The surfaces of the uncoated 3D-printed parts had curved protrusions for each of the individual layers that were observed using an optical micrograph. The surface topography was also determined using a stylus profilometer, and the average peak height for all of the protrusions for a given part was used as a measure of the roughness both before and after coating. The researchers then determined the degree of planarization (DOP) based on the change in APH.

“Coating of the 3D-printed parts clearly reduced the surface roughness by planarizing or smoothing the surface, regardless of printing tip size. However, the application conditions significantly affected the level of reduction,” observes Zhu. The DOP values increased slightly with increasing dipping speed, likely due to the increased coating thickness. A second layer further improved the DOP, but little change was noted for three layers.

How the coated parts were dried also affected the surface morphology. For parts left to hang in air to dry, the coating was thinner near the top and thicker near the bottom due to gravity. This variation was largely prevented by drying the parts immediately with a heat gun, but the shorter time for leveling resulted in lower DOP values. “There was a definite trade-off between achieving uniform coverage with faster drying and planarization for reduced surface roughness. Other drying methods, such as placing the parts on a horizontal surface, should help avoid this issue,” says Zhu.

Applying a coating was also found to be as effective for planarization of vertical parts (the coating was applied perpendicular to the printed layers), but with little variation in the APH values from the top to the bottom of these parts. “Apparently, the flow of the coatings due to gravity was reduced for these parts,” Zhu notes.

Also interesting was the fact that the more viscous, higher-solids PU2 coating provided a higher degree of planarization than PU1. Furthermore, when diluted by 30% with water, PU2 provided results similar to those observed with PU1. “This behavior is similar to what is reported for planarization processes in the microelectronics industry,” Zhu comments.

So what do these results suggest regarding the use of coatings on 3D-printed parts? “Coatings clearly affect the smoothness of 3D-printed parts, and the direction of coating relative to the printed layers does not seem to matter. The coating properties, number of coats, and the drying conditions do, however, significantly influence the planarization performance. More studies are needed to determine the full effects of drying conditions. In addition, studies involving the coating of more complex parts will be necessary given that most 3D-printed parts are not simple rectangular shapes. Even so, these results certainly indicate that coatings can be used to reduce the surface roughness of 3D-printed parts and the insights gained provide a good foundation for further investigations,” Zhu concludes.

For additional information on the research reviewed here, see J. Zhu et al., “Water-based coatings for 3D printed parts,” J. Coat. Technol. Res., 12 (5) 889-897 (September 2015).