Stereo Lithography Apparatus or SLA 3D printers are the original types of resin printer. SLA printers use an ultraviolet laser to cure a photosensitive resin. The approach was first proposed in the late 1970s and then patented in the early 80s. While FDM 3D printing was the first technology to break into the home market because it was easier to market at affordable prices, SLA printing has also become popular, primarily because of its unmatched print quality.
How do SLA printers work?
An SLA printer uses an ultraviolet laser to trace the path of a layer over a photosensitive resin. As the laser passes over the resin it cures and solidifies. Once the layer is complete the build platform adjusts so the next layer can be printed. The laser source itself is always stationary. The beam is adjusted across the x- and y-axes with a precisely controlled mirror.
There are two primary types of SLA printers, top-down and bottom-up. Top-down printers are more commonly found in industrial environments, they use a large vat and trace the laser over the surface of the resin in the vat with the build surface just below. Once the layer is finished, the build surface is lowered by the height of one layer, and the next layer is started. These printers are generally limited by the volume of the vat and require the vat and the amount of resin in it.
In bottom-up printers, the laser and mirror system is mounted underneath the vat. Said vat has a transparent underside. The laser cures a layer of resin at the bottom of the vat, and the print bed then peels it off and raises one layer, ready for the next layer to be traced. This type of printer is much more popular in the home-user space. One of its big advantages is that you don’t need a deep or full vat of resin, as the print can be slowly lifted out of the vat. To help peel the layer off the bottom of the vat while keeping it stuck to the print bed, the vat itself generally rocks slightly, this applies an uneven peel force, allowing it to come off smoothly and reliably.
Advantages and disadvantages compared to FDM
The main advantage SLA printing offers over FDM is the significantly increased resolution. Layer heights can be as low as 50 microns, that’s 50 millionths of a meter or 0.05mm. That’s compared to FDM layer heights which are typically on the order of 500 microns or 0.5mm. Being able to print 10 times as many layers per unit of height as an FDM printer means that the resulting prints are literally an order of magnitude smoother. Much finer details can be reproduced.
The main downside of SLA printing is that the resins used are quite toxic and nasty irritants compared to the safe-to-handle filaments used by FDM printers. PPE such as nitrile gloves is necessary when handling resin as skin contact can result in a nasty rash. Prolonged exposure can even result in the development of an allergy. In some cases, this can then be triggered by even the smell of the resin. Resin is also considered hazardous waste and so disposal can be complicated.
Once a print is complete, it needs to be washed in isopropyl alcohol to clean off any excess resin, and then cured under a UV lamp. Only after this can the standard post-processing such as support removal happen.
Running costs for an SLA printer are comparatively high as the resins are more expensive than FDM filaments. In more positive news, the lack of a heating element means that there’s no fire risk.
SLA printing is an amazing technology that can result in fantastically detailed prints. The running costs, however, as well as the safety issues of handling resin can put many off. Have you got an FDM or SLA printer? What was it that sold you on the type that you chose? Let us know down below.