Designing for SLA Resin 3D Printing
Stereolithography, otherwise known as SLA printing, is the process of making three-dimensional (3D) parts or models by curing photosensitive liquid resin with ultraviolet (UV) light. Traditionally the UV light source was a laser, but today’s UV light sources include UV projectors, digital light processing (DLP) projectors and LED solid state light focused through a liquid crystal display (LCD). Regardless of the UV light source, the liquid resin will harden or cure layer by layer until a solid 3D plastic component is produced.
The differences between SLA and FFF / FDM (fused filament fabrication / fused deposition modeling) printing often dictate the use case for each technology. SLA is an excellent choice when printing organic shapes, highly detailed models and lattice structures. The level of detail an SLA printer can produce is dependent on the laser spot integrity / UV image projection integrity and resin properties. This detail is far superior to traditional FFF / FDM results, especially in the less than 100 micron range. The purpose of this article is to communicate best practices and design techniques that will facilitate success in SLA printing. For more information on SLA 3D printing, please refer to Chapter 12 of The 3D Printing Handbook.
Designing for Print
Supported walls are walls that are connected to other structures on at least two sides. This mitigates the chance of warping. Walls of this nature should be designed at a minimum of 0.5 mm thick (see Figure 1).
Unsupported walls are walls connected to the rest of the model on less than two sides. These walls have a high chance of warping or detaching, especially if the height in the z-direction is large. These walls should be at least 1.0 mm thick and designed with filleted bases (where the wall connects to the rest of the print) to reduce stress along the joint (see Figure 2).
Overhangs are the bane of FFF / FDM printing, but pose little issue with SLA printing, as long as the model is printed with adequate internal and external support structures. Printing without supports often leads to warping of the print, but if printing without supports is necessary, any unsupported overhangs must be kept less than 1.0 mm in length and at least 19 degrees from the horizontal (see Figure 3).
Embossed details to include text are features on the model that are raised above the surface. These must be at least 0.1 mm in height above the surface of the print to ensure the details will be visible.
Engraved details to include text are features that are recessed into the model surface. These details risk fusing with the rest of the model if they are too small. As a result, these details must be at least 0.4 mm wide and at least 0.4 mm from the surface of the model to the recessed detail (see Figure 4).
Horizontal bridges between two points on a model may be successfully printed, but the wider the bridge, the shorter the span (less than 21 mm). Wide bridges have a greater z-axis area of contact, thereby increasing the chance of print failure during peeling (see Figure 5).
Holes with a diameter less than 0.5mm in the x, y, and z axes may close off during printing. It is better to keep hole diameters greater than 1.0mm and best if greater than 2.00mm (see Figure 6).
Upon orientation of a part for SLA printing, the biggest concern is the Z axis cross-sectional area. The forces involved with adhering a print to the build head and not peeling off or sticking to the PDMS or FEP surface are directly related to the cross-sectional area of the print. With this in mind, parts are normally printed at an angle (usually 45 degrees) to the build head to maximize the footprint and minimize the density in geometries along the z-axis. This often calls for the use of more support structures when compared to FDM/FFF printing. As a result, minimizing support is not the objective or limiting factor in SLA printing but should be used liberally during the printing process.
SLA printed parts are uniform with the same mechanical properties in all orientations. Since the layers chemically bond to each other as they print, the results are nearly identical physical and mechanical properties in all directions. Whether the part is printed parallel or perpendicular to the build head, the resulting material properties are not noticeably impacted.
The SLA process achieves a much higher resolution than the FFF / FDM process since it uses a laser / UV light source to cure the resin. Resolution in the horizontal or X-Y plane depends on laser spot size or clarity of the UV image and can range anywhere from 30 to 140 microns. Contrary to manufacturer claims, this is a parameter that cannot be adjusted.
The vertical resolution varies from 25 to 200 microns. This is controlled by the layer height setting in the slicer and is a trade-off between print speed and quality. For a part that has minimal radii and fine details, there is little visual difference between a print at 25 microns versus 100 microns. This is in contrast to a traditional FFF / FDM machine with typical layer heights of between 100 to 350 microns.
Hollowing and Cupping
The SLA process results in a solid, dense model. However, if the print is not to be used as a functional part, hollowing the model significantly reduces the amount of material needed as well as the print time. It is recommended that the walls of a hollowed print be at least 2 mm thick to reduce the risk of failure during printing.
When printing a hollow part, drainage holes should be added to prevent retention of uncured resin in the final print. Uncured resin creates pressure imbalances inside the hollow chamber resulting in what is known as cupping. With cupping, small failures including cracks and holes propagate throughout the print. If not corrected the result is a print / part failure. As a result resin drainage holes should be modeled in the CAD or slicer software. These holes should be at least 4.0 mm in diameter, with a minimum of one hole per hollow section.
Getting the correct clearance for assembly of static or moving components is always a challenge in 3D printing. For the best chance of success, use a 0.5mm clearance between moving parts, a 0.2mm clearance for assembly connections and a 0.1mm clearance for push connections in order to have a snug fit.
Every manufacturing method has its design criteria and limitations, and SLA 3D printing is no different. Hopefully with the help of this article you are well on your way to creating the perfect SLA 3D models. If you have any questions or feel like there's something that you consider important to your design process that hasn't been mentioned here, we'd love to hear about it in the comments down below.
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