Stereolithography: Everything You Need To Know About SLA 3D Printing

stereolithography

Stereolithography, or SLA, was the first 3D printing process, the first patent having been filed in 1984 by Charles Hull and subsequently granted two years later in 1986. He wasn’t the first however –  Alain Le Mehaute, Olivier de Witte and Jean Claude André in France applied for the patent just weeks before Hull, though it was abandoned before it was granted. Stereolithography is the most widely known of the vat photo-polymerization processes, and has spawned similar 3D printing technologies such as DLP (Digital Light Processing).

View our guides to the other 3D printing technologies here.

Stereolithography: Explained

Reading time: Approx 5 mins.

Stereolithography 3D printing uses photo-polymerization to produce 3D models using an ultraviolet (UV) resin. A laser is used to solidify layers of resin in a similar layer-by-layer process to FDM. These liquid resins are the printing material of SLA 3D printers and the equivalent of filaments in fused deposition modeling.

Setting up an SLA 3D Printer

Stereolithography, like all 3D printing technologies, required a 3D CAD file from a CAD software before printing. This is then sent to a slicer, which slices the CAD file into an .STL or .OBJ file to be 3D printed through stereolithography. These STL files are not to be confused with SLA, they are the 3D files which have been sliced so the 3D printer knows which layers to print.

Resin 3D printers are equipped with a resin tray to hold the UV resin, a mobile platform that functions as the Z axis which is lowered into the tank, a scraping system that functions as the X axis, a UV laser, focusing optics and a mirrors called galvanometrics on the X and Y axes to aim the laser beam.

Here’s a video on the SLA 3D printing process by students at Loughborough University, UK:

Bottom-up vs Top-down SLA 3D Printers

There are two types of stereolithography 3D printers, bottom-up and top-down. Most SLA 3D printers use a top-down method, though Formlabs use bottom-up. Each way has its advantages and disadvantages which we have outlined below:

Bottom-up Advantages

  • Requires less resin as the part is pulled out of the vat. Also means machine can be smaller.
  • Easier to control the thickness of each layer.

Bottom-up Disadvantages

  • Requires the resin vat to replaced more often to avoid losing print quality.
  • Increased chance of the print failing due to the part’s weight.
  • SLA 3D prints must be printed at an angle.

Top-down Advantages

  • Faster 3D printing as no need to separate from the build plate after each layer is printed.
  • Less force on 3D part being produced so less chance of print failure.
  • Less supports are needed as the part doesn’t need to be printed at an angle.
  • Generally more reliable.

Top-down Disadvantages

  • Requires a larger machine and requires more resin.
  • Changing resin is difficult and replacing resin tanks is expensive.
  • Thickness of the resin between the surface and the top of the 3D model must be controlled carefully.

Overall, it is up to you to decide which method works better with your goals with 3D printing. This video below explains the debate in terms of micro 3D printing, which may perhaps provide more information.

Stereolithography 3D printing

Layer sizes in stereolithography printing with a photopolymer resin are usually between 130-150 microns. This layer is exposed just above the platform, ready for the UV laser to hit it.

When 3D printing, the UV laser hits the platform, which hardens the liquid resin and forms the first layer of the object being 3D printed. The laser hardens the resin based on the STL file sent to the 3D printer.

When 1 layer has been fully solidified, the platform descends so the next layer can start. The next layer is then solidified which continues until the whole object has been 3D printed and the model is submerged in the tank. After this, the platform is raised, pulling the 3D printed object back out of the tank.

stereolithography 3D printing process
The Stereolithography 3D printing process, involving the platform, mirrors, resin tank and UV laser.

It is worth noting that sometimes in SLA 3D printers the process is reversed, such as in Formlabs 3D printers. In this case, the platform is submerged inside the resin tray, whilst the laser moves from the top to the bottom rather than vice versa.

Stereolithography Post-printing

Stereolithography, unlike Selective Laser Sintering, uses supports. These supports require a solvent to remove excess non-solidified resin, such as isopropanol. Unlike FDM, these supports are always made of the same material as the object being 3D printed. This is similar to FDM in that any parts that overhang require supports.

Unlike Selective Laser Sintering or FDM, stereolithography 3D printing requires a post-treatment to strengthen the model. This involves the part being cured under a UV light after being 3D printed which further strengthens the model and allows the material to achieve its optimal properties.

This video below explains the differences between the post processing in FDM and SLA:

Stereolithography Materials / Resins

Stereolithography 3D printers use resins, rather than the plastic filaments used in FDM. These resins are more expensive to 3D print with than filaments, and start at around $50 per litre of resin.

These $50 per litre resins are the most basic materials however and are not specialized for highly detailed prints. For high-detail, castable resins, you may be out of pocket by as much as $400 per litre. Moreover, it is important to remember that resins do not last forever, they spoil eventually. Their shelf life is usually around a year.

stereolithography resin
The resin used in stereolithography 3D printing is shown here being poured into the resin tank.

Print Quality

SLA 3D printers have a superior surface finish than models printed using SLS or FDM for the same layer thickness. The layers are usually barely visible due to the high-quality finish offered. However, there are few colors available to resin 3D printers, though Formlabs recently announced a variety of new colors for their SLA 3D printers.

Applications

Stereolithography is often used for rapid prototyping because of its fast speed, accuracy, and part strength. Parts can be made quickly at a fairly low-cost. Furthermore, stereolithography allows for complex shapes to be created which traditional manufacturing techniques simply cannot make. This makes 3D printing a fantastic niche option for oddly shaped models.

In addition, SLA 3D printing can 3D print immediately functional objects. This means no extra time needs to be spent changing the model, it works straight out the print.

Apart from rapid prototyping, Stereolithography has the most applications in industries such as the dentistry and jewelry industry. This is because SLA can be used to quickly create injection molds which are then used to create jewelry pieces such as necklaces or rings. This is sometimes through Lost Wax Casting, which indirectly uses SLA to create jewelry.

Moreover, dental pieces like crowns can be created through SLA due to its high quality.

The biggest achievement of 3D printing in an industry however is in hearing aids. Since its introduction, over 10 million hearing aids have been made using stereolithography, and over 97% of hearing aids now use 3D printing. This is because accurate hearing aids based on each patient’s ears can be created in-house, quickly, and for cheap. This has revolutionized the industry, showing just how powerful 3D printing can be.

Stereolithography has applications in many industries, and also spawned a number of other 3D printing techniques:

DLP (Digital Light Processing)

DLP is similar to SLA, but it uses a video projector instead of the laser used in stereolithography. This enables DLP to scan through entire objects at a quicker pace than SLA, as it can do the entire layer at once, unlike SLA. However, DLP is unable to 3D print with the same level of high-resolution, and are more limited in how many parts that can be printed simultaneously. SLA can print many objects within the build volume concurrently without a problem.

DLP direct light processing stereolithography
The DLP 3D printing process differs from Stereolithography as the projector does not use mirrors like SLA 3D printers do.

CLIP (Continuous Liquid Interface Production)

Made famous by Carbon 3D, their CLIP technology makes printing objects much faster. Most famous for their TED Talk, “What if 3D printing was 100x faster?” Carbon demonstrated the speed of their CLIP technology by printing a full object in the 10 minutes during the talk. This is an upgraded form of stereolithography which we will hear more about in the future.

You can view Carbon 3D’s TED Talk here: (but don’t forget to read the rest of this article where we discuss the Advantages and Disadvantages of stereolithography)!

Advantages and Disadvantages of Stereolithography

Stereolithography Advantages

  • Smooth surface finish of models, especially if this area had supports on it.
  • Parts 3D printed with stereolithography have high accuracy and the layers can barely be seen. In addition, SLA prints have high dimensional accuracy and therefore is ideal for parts where intricate figures are needed (such as the dental, hearing aid and jewelry industries).

Stereolithography Disadvantages / Limitations

  • SLA 3D printing takes longer than DLP. This is because DLP can trace the cross-section of a model in a single frame, whereas SLA 3D printers cannot.
  • Little versatility in printing parameters. When printing with stereolithography, you can only change layer heights, the resin material, and location of supports.
  • Polymer materials create models which can be brittle and are not as strong as other 3D printing technologies.
  • Parts created through stereolithography have a limited lifespan and will start to lose their mechanical properties eventually. Moreover, they will start to degrade in sunlight. These parts require new coatings to extend their usable life.

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