Stereolithography or otherwise known as SLA was first developed by Dr Hideo Kodama. He used a UV light source to cure a photoreactive polymer. Chuck Hull later coined the term SLA and developed and patented the first SLA machine as we see them today. Due to expiring patents, there are now multiple companies who are developing SLA machines. They all use the same basic principles though. They have a photo polymer called resin. A light source that will selectively cure the resin and a build plate that will move up and down in the Z access to allow the machine to build layer upon layer. There are two major types of SLA machines, right side up and inverted. Right side up is typically used in industrial applications. It allows for large build volumes and highly accurate parts. These machines are large and bulky due to the volume of resin they must contain to print a part. As you can see in this graphic, the right side up machines set the build platform into the resin and submerge the part down into the resin tank as it builds apart layer by layer. If you were to build an object about the size of a can of pop in a right-side up machine you would require about seven gallons of resin in the tank at that time. This greatly increases the operational costs of these machines. Inverted SLA is more commonly seen in desktop printers. This is because the printer is more compact. The inverted design does introduce some trade-offs which we will discuss later. You can see from this illustration that there is much less resin being held in the tank. This innovation allows us to have desktop SLA machines. To start us off here is an example of a desktop SLA 3D printing set up. You have a printer, a wash station and a cure station. In the bottom of the printer, there's a light source, controls, motors and interface panel. On top of the machine you have a UV protective cover. Since the resin is photosensitive, we need to block any external UV light from entering the build space. As we open the cover, we see our resin tank, resin, a build platform and here we have a printed part. If you look at the part, you'll see a lot of uncured resin on this part. This all needs to be washed away to finish this piece. The part is currently in what's called a green state? It's fully formed yet not fully cured. You can see the supports here. They're very thin and frail, but play a critical role in SLA printing. Supports and part orientation in an inverted SLA are very different from that of fused filament fabrication. Having poor part orientation or not enough supports will lead to deform parts or poor part tolerance. As previously mentioned, the part now needs to have any excess resin washed away. To do this, we use an isopropyl alcohol bath. Not all machines have an automated wash system, many of them use simple plastic tanks that you have to manually agitate and time the washing process. Once washed, you can now see that there is no liquid resin remaining on the part. The part is still in a green state. It is still not fully cured. To fully cure the part we place the part in a UV curing chamber. Much like the wash station, not all machines have a UV curing chamber to automate this process. Many people simply put their parts out in the sun to cure or use a UV lamp to do the same. So with that high level overview of the SLA process, let's look closer now at the subsystems of an SLA printer. Let's start with the motion system. There are very few moving parts in an SLA machine. The build plate moves along the z-axis to allow the part to build layer upon layer. The image shown here shows a motion system controlling movement and distance to the resin tank during printing. This allows for new resin to flow underneath each layer to be cured. The resin tank not only acts as a container but as a window to pass UV light through to cure the layers. The bottom of the tank is typically made from a semi flexible material. As you print the resin cures not only to the build platform, but also to the resin tank. Over time, this will reduce the optical clarity of the window and the tank or film must be replaced. The peel mechanism is sort of a combination of resin tank and motion system. This form lab system uses a sliding peel. Whereas other systems simply use the Z Motor to pull up on the build plate. Since the film on the bottom of the resin tank is flexible the part will peel away from the resin tank. This puts a lot of force on the part, which is one of the reasons why supports are very ornate and complex in SLA. There are three major types of light sources used in desktop SLA, laser SLA, DLP SLA, and MSLA. Laser systems use a galvanometer. This is a series of mirrors and motors that will move the laser beam around the build surface to cure the layer. DLP or digital projection projects a high-power UV light image of the cross section for each layer to cure. You can see this process in the video as it shows the layers progression and the flash of each cross section for each layer being cured. MSLA or masked stereolithography uses an LCD screen to mask and block out light and areas you do not want to cure. Then a UV LED will shine through the unmasked areas to cure the layer. Now, let's look at a print in action. As the print starts the printer has already received its G-Code instructions from the slicer. The build plate lowers itself into the tank of resin and the light source will selectively cure the model. As each layer cures, the newly cured layer is slightly fused to that tank. So the next step is to peel that part from the tank. The tank slides over which separates the tank from the part. The build plate raises and resets itself for the next layer. Take a close look at the light source beneath the resin tank. Is this laser, DLP or MSLA? So the question still stands. When is SLA the most appropriate technology to use? You must consider the trade-offs of printing and post-processing the part as compared to fused filament fabrication, but also consider the advantages of SLA. SLA is known for achieving very high resolution. You can see this in these examples. What's impressive is how small these objects actually are and how detailed they can be. The resolution of the prints is truly impressive but SLA has limitations the materials. While this is a challenge, many companies are pouring a lot of energy into developing new resins that will open up more opportunity for SLA. There is no definitive guide that will tell you what technology to use for what you're trying to make. You must consider the implications and trade-offs that that technology affords. With your new knowledge of SLA you can now better understand the landscape of desktop 3D printing hardware.
