SLA - Stereolithography

Stereolithography (SLA) was invented by French and American scientists in the early 1980s and patented by American Charles Hull in 1983. SLA is a type of additive manufacturing – also known as 3D printing – applied in fields ranging from manufacturing to biomedicine.

It is predominantly used for professional and industrial applications, though the emergence of low-cost resin 3D printers has made the technology available to consumers and hobbyists.

Stereolithography is a “vat polymerization” 3D printing process; liquid, photosensitive resin is poured into a vat (or tank), and UV light interacts with the resin to selectively polymerize (i.e., cure, solidify) it.

The UV light cures the resin layer by layer until the final object is complete. The thickness of each layer is called layer thickness or height. In stereolithography, layer height is typically around 50µm – about as thin as a human hair – but can be as small as 10µm. In general, the thinner the layers, the better the quality, and the longer the print times.

 

SLA 3D printing is highly versatile, accurate, and produces smooth surfaces, making it ideal for precise jewelry-making and dental implants, among other applications. One drawback of the process is that parts can deteriorate when regularly exposed to sunlight, due to the photosensitive nature of 3D printing resins.

Another thing to note is that it is necessary to add support structures during the build process so that overhangs have something to hold onto. Resin 3D printing also implies several post-processing steps, namely:

• resin washing (to wash away any excess resin dripping off the part),
• support removal (to break away the support structures),
• and post-curing (it is necessary to expose the part to more UV light for further solidification).

SLA 3D printing was initially designed as a “bottom-up” printing method, where the light source shines on the resin from below the vat. The first layer is at the bottom of the tank, and the build plate moves upward as each layer solidifies. Objects emerge upside down from the vat.

Most resin 3D printers work that way, but a few “top-down” systems exist as well. In a top-down setup, the light source sits above the resin tank in a top-down setup, therefore curing the surface instead of the bottom. The build plate moves down to leave room for new layers atop the previous ones until the object is complete and appears upright.

The term “SLA” generally refers to laser stereolithography, and as initially developed, the light source exploited in resin 3D printing came from lasers reflected off mirrors. Printers using lasers are very precise, and very expensive to acquire and maintain. Today, stereolithography comprises other technologies such as DLP (digital light processing) and MSLA (masked stereolithography).

In DLP, the light source is a projector instead of a laser. Whereas laser SLA printers trace out print layers point by point, DLP printers cure each layer all at once in a single flash of light. This makes them quite fast compared to traditional, laser-based resin 3D printers. Similar to DLP, MSLA solidifies entire layers at once. Instead of a projector, however, MSLA printers leverage an array of LEDs as a light source. The LED lights shine through an LCD screen, which selectively masks the light by illuminating or turning off specific pixels. An MSLA printer’s resolution hence depends on its LCD screen’s resolution.

Some companies have even designed 3D printers that use a mobile phone’s LCD screen as the light source. Such designs are innovative and illustrative of just how accessible SLA 3D printing is becoming.

 

SLA printers turn liquid resin, a.k.a. photopolymer, into solid plastic figures. As a general rule, resin 3D printed parts boast high resolution and levels of detail but have a limited lifespan due to their light sensitivity. That said, many different 3D printing photopolymers exist, and each has its own thermal and mechanical properties. 

Standard resins produce plastic parts that can be very thin and highly detailed but very brittle. Polycarbonate-, polypropylene-, and ABS-like plastics are more durable, though they may be more limited in precision.

Some less common materials include “filled” resins, which are resins filled with metal or ceramic particles. The resulting parts are “green” and, like pottery, must undergo heat treatment after printing. Photopolymers can be solid in color or as clear as glass.

 

SLA 3D printing has applications in any situation requiring objects with smooth surfaces and high precision. This can range from architectural models to sonar submersibles and marketing props though its main industries have historically been dentistry and jewelry. 

In jewelry-making, the primary use case is creating inexpensive, castable molds to pour metal into. For example, jewelers can quickly produce prototypes to test sizing for custom ring orders.

Resin 3D printing is a fast and easy way to fabricate dental models, casting patterns, restorations, and more from a dentist’s standpoint. Companies like market leader Formlabs have even developed specialty resins for dental use cases, such as digital dentures.

Stereolithography has been available for dozens of years, and while it already satisfies many users and their applications, there is always room for improvement.

As mentioned earlier, resin 3D printed parts tend to be brittle and break down over time. However, companies in the industry are working to create more durable and elastic materials.

There is also the issue of sustainability: SLA 3D printing generates a large amount of new plastic, much of which winds up in landfills, leading to experiments in producing SLA resins from renewable feedstocks or even waste cooking oil.

As SLA 3D printing becomes more and more accessible to users in a wide range of fields, innovations will only continue. Stereolithography 3D printing has been in use for almost 40 years, and it is not going away anytime soon.