Resin

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Resin printing involves curing a liquid resin by exposing it to an energy source which will trigger a hardening process. Usually this energy is a UV light source, though some resin printers use visible light as well.

When compared to FDM printing, resin printers often allow for much finer details because the process doesn't depend on heavy moving parts to deposit the plastic, and because resin solidification can be controlled to a much finer degree. In the case of SLA printing the only moving part used for the imaging is the actuator for the mirror. For DLP, there are no moving parts for the imaging process. Both types of resin printing use a motor for the Z axis.

Resin printers create their prints upside down, as they require a bath of resin from which the print is drawn up. Gravity of course prevents us from having the liquid on top.

Here is a nice collection of resin tutorials.

Types

There are three main types of resin printing: SLA, DLP, and MSLA. All of them accomplish the same thing, but differ in the technology they use to cure the resin.

SLA: Stereolithography, the original resin printing technique invented by 3DSystems in the 1980s. This is a technique that uses a laser spot controlled by galvo mirrors to "draw" the image of each layer into the resin. Because the laser spot moves in roughly a continuous motion, even through curves, SLA has the advantage of not suffering from any sort of voxelation or moire. However, since the laser only cures a spot as large as the laser dot at any given time, print times increase with the volume of the print as opposed to the height. Common printers that use this technique are the Formlabs Form 1/2/3, the Peopoly Moai, and industrial 3DSystems machines.
DLP: Digital Light Processing. This name is a direct carryover from the DLP-based projectors used in the printer's curing system. A projector is focused on the plane of the resin, and the image the projector displays on the resin selectively solidifies one full layer at a time. DLP projectors are used because they use a mechanical micromirror array to control the image. This meets two important requirements that LCD projectors don't: They are light source agnostic, allowing for a UV lamp to be installed in place of the regular lamp, and black pixels have 100% of the light blocked, eliminating the possibility of overcuring. Being a pixel based technology, DLP printers are susceptible to voxelation and moire, though this can be reduced with anti-aliasing. Because they cure one whole layer at a time, the volume of the print is no longer a factor in print times; instead, only the height of the print is. DLP machines include the original B9Creator, as well as many community DIY machines.
MSLA: Masked SLA. This technique is commonly confused with DLP, and has many similarities. An LCD panel is used in place of the projector to "mask" the image onto the resin. Unlike a projector, which is focused at a distance, the LCD panel is placed very close to the plane of the resin (often touching the bottom of the resin vat) and the light source is placed some distance below it. In practice, MSLA printers offer largely the same advantages and disadvantages of DLP machines, also suffering from voxelation and also scaling print time with height rather than volume. They have the added disadvantage that the LCD panel eventually degrades from the UV exposure, making it a consumable item. In return, MSLA machines are also much cheaper and more accessible than DLP machines, starting from under $200 USD. Popular MSLA machines include the Anycubic Photon and the Elegoo Mars.

Safety

UV resin is extremely toxic to touch and via fumes, it will build up in your system the more you are exposed until one day you become allergic, sometimes to extreme degrees (full explanation). It is crucial that all fumes are vented to the outside and gloves are worn when touching the resin before post-cure. Some robust discussion of resin safety

Peripherals

Here's a list of other items you may wish to buy in addition to your resin printer.

Isopropyl alcohol or ResinAway for cleaning your prints and bath
Cotton wool balls for cleaning the FEP film in the bath without scratching it
A big pack of nitrile gloves to keep your hands safe (latex will melt, and contact with the resin will over time make you deathly allergic to it)
An exhaust fan and tubes to pull the fumes away and outside.
A fume mask is good for cleanup, one rated for organic vapours.
Something to protect your tabletop from drips. I do most of my cleaning over a disposable baking dish, and have drawer liner on the counter.
Paint strainers for filtering any chips out of the resin (coffee filters don't work, the resin is too thick)
Some small glass jars for holding left over resin and isopropyl. (Wrap these in al foil to stop light)
A UV light (cheap ones for nail art work well)
Paper towels for spills
And old soft toothbrush and paint brushes for cleaning excess resin off.

Designing with a resin printer in mind

Although resin printing shares many properties with FDM printing, there are a couple of differences.

Supports

Supports are still a requirement when printing with resin. Because of the fine details on parts printed with a resin printer, supports can be a bit more finicky to print. You should keep in mind where you think you want to add supports and how you want the supports to connect to your model. Attaching supports on corners can make it easier to remove the marks they leave later on.

Drain Holes

If your model has cavities, you will have to include drain holes in your design. For each cavity, there should be at least two holes. One to allow for draining and one to allow for air to come in. Without the air hole, the drain will most likely not work unless the drain hole is very large.

Maximizing Resolution

There are a few ways to get greater resolution from your resin printer. The main ways being by optimising pixel density through angles, and the other through anti aliasing.

Optimising Pixel Density

On printers with a pixel size of 0.04725mm (this includes 2K, 115*65mm screens on the Anycubic Photon (and S), Elgoo Mars, Wanhao Duplicator 7, etc) the following angles and layer heights apply:

Layer Height (mm)	Angle (degrees)
0.01	11.950
0.02	22.942
0.03	32.412
0.04	40.250
0.05	46.620

Set your print on the angle corresponding to your layer height on all 3 axes for the greatest pixel density on normally vertical and horizontal surfaces.