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Stereolithography

Stereolithography is used when resolution and fine detail is paramount. It is the oldest method of 3D printing, first developed in 1983 by Chuck Hull.

SLA machines are used for prototype parts, surgical guides and practice models, end-use limited run production parts, miniatures and more. The resins used can be tweaked to have a wide range of properties so while they are not exactly the same as mass-manufactured materials, they can be made to simulate them. They are best used for parts where detail and a smooth surface finish is the most important factor.

All SLA printers work on the same basic principles; a photoreactive liquid resin is exposed to light and undergoes a chemical change to become a solid layer. Prior to 2014, SLA machines were large, bulky objects about the size of a refrigerator. This was because after each layer, the entire vat of resin would drop down so that the next layer of resin could fill in and start again.

Image Credit: Formlabs

The SLA Process

In 2014 Formlabs literally turned the industry on its head by flipping the printer upside down. This is now the most common method of desktop SLA printing and the largest reason is because the resin tank no longer has to be the size of the entire build area. In fact, resin tanks are typically only an inch high, or 25mm.

At the start of the print the build platform lowers itself and presses into the bottom of the resin tank, leaving only a small gap of resin. A light source shines up from below and cures the resin, however it is now cured to both the build platform and the bottom of the resin tank. The layer must be pried away from the resin tank, usually by tilting the tank down and away from the build platform. Then the platform raises up so a new layer of resin can fill in the gap and the process repeats.

Image Credit: Formlabs

Below is an explanatory video about SLA (and one of their products, the Form 2) from Formlabs.

Stereolithography vs Digital Light Projection

Both SLA and DLP use a similar operating principal: they expose a UV-reactive resin to light to induce a chemical change. How they do that is very different, but in practice they are quite similar. In this article unless otherwise noted everything applies to both DLP and SLA.

SLA

In this method, a UV laser is steered by galvanometers and then reflected off of a mirror up into the bottom of the resin tank. This is the traditional way of creating high accuracy parts. Because it is using a laser it is theoretically slower than DLP machines since it has to trace the entire layer line by line. However, SLA’s layer lines are usually less noticeable.

DLP

DLP instead uses an LCD screen and hundreds of digital micromirrors to create an image of the entire layer all at once. DLP is theoretically faster than SLA machines because it is able to flash the entire layer at once. Because it is using pixels, parts can sometimes appear like they are made of voxels.

Supports

SLA machines have characteristic supports that look a bit like trees growing from the build platform. They are very thin and small to minimize the surface blemishes they leave behind when removed.

Image Credit: Micro3D

When is SLA a good idea?

  1. Detail detail detail. Nothing will give better small results than SLA.
  2. You need a specialty material or behavior, such as true flexibility and compression. Resins can be fine-tuned to mimic many other material properties.
  3. A smooth finish is required.
  4. Printed part accuracy to CAD is required.
Image Credit: Thomas Sanladerer
Image Credit: Elegoo

When should you try something else?

  1. You don’t want that much detail. FDM is cheaper if it’s not required.
  2. If the production run is large. Resin is expensive compared to FDM (but not to SLS).
  3. The printer will be in an area where there can’t be a mess. Resin is sticky and requires gloves to handle. The parts must be cleaned and cured after printing as well.
  4. Your part has to be made out of the same material as the final production run.
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