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Stereolithography - SLA 3D Printer Simply Explained

What is Stereolithography?

Objects printed through stereolithography.
Objects printed through stereolithography. Source: Engitype.com

Stereolithography – more commonly referred to as SLA – is one of the most popular and widespread techniques in the world of additive manufacturing. It works by using a high-powered laser to harden liquid resin that is contained in a reservoir to create the desired 3D shape. In a nutshell, this process converts photosensitive liquid into 3D solid plastics in a layer-by-layer fashion using a low-power laser and photopolymerization.

SLA 3D Printer is one of three primary technologies adopted in 3D printing, together with fused deposition modeling (FDM) and selective laser sintering (SLS). It belongs to the resin 3D printing category. A similar technique that is usually grouped with SLA is called digital light processing (DLP). It represents a sort of evolution of the SLA process, using a projector screen instead of a laser.

Stereolithography: A History

A figure of the patent filed by Hull.
A figure of the patent filed by Hull. Source: dewyseng.com

Despite being less popular than FDM technology, SLA is actually the oldest additive manufacturing technique.

The word “stereolithography” derives from ancient Greek. The words “stereo” and “(photo)lithography” mean “solid” and “a form of writing with light”, respectively.

As the eldest additive manufacturing technology, SLA is sometimes considered to be “the mother of all 3D printing technologies”. It was developed by the US-based company 3D Systems, which was founded by Chuck Hull in 1986. Hull coined the term “stereolithography” in 1986. He defined this technology as a method of creating 3D objects by successively printing thin layers cured by ultraviolet light.

In 1992, 3D Systems created the world’s first SLA apparatus, which made it possible to fabricate complex parts, layer by layer, in a fraction of the time it would normally take. SLA was the first entry into the rapid prototyping field during the 1980s and has continued to advance itself into a widely used technology.

Stereolithography: Components

SLA components.
SLA components. Source: Manufactur3DMag.com

Every standard SLA 3D printer is generally composed of four primary sections:

  • A tank filled with the liquid photopolymer: The liquid resin is usually a clear and liquid plastic.

  • A perforated platform immersed in a tank: The platform is lowered into the tank and can move up and down according to the printing process.

  • A high-powered, ultraviolet laser

  • A computer interface, which manages both the platform and the laser movements

Stereolithography: How Does It Work?

Process of stereolithography.
Process of stereolithography. Source: http://www.thagiwara.jp/rp-resin/IUPAC/iupac2000.html

Software

As is the case for many additive manufacturing processes, the first step consists of designing a 3D model through CAD software. The resulting CAD files are digitalized representations of the desired object.

If they are not automatically generated as such, the CAD files must be converted into STL files. Standard tessellation language (STL), or “standard triangle language”, is a file format native to the stereolithographic software created by the Abert Consulting Group specifically for 3D Systems back in 1987. STL files describe the surface geometry of the 3D object, neglecting other common CAD model attributes, such as color and texture.

The pre-printer step is to feed an STL file into a 3D slicer software, such as Cura. Such platforms are responsible for generating G-code, the native language of 3D printers.

SLA 3D Printing

When the process starts, the laser “draws” the first layer of the print into the photosensitive resin. Wherever the laser hits, the liquid solidifies. The laser is directed to the appropriate coordinates by a computer-controlled mirror.

At this point, it’s worth mentioning that most desktop SLA printers work upside-down. That is, the laser is pointed up to the build platform, which starts low and is incrementally raised.

After the first layer, the platform is raised according to the layer thickness (typically about 0.1 mm) and the additional resin is allowed to flow below the already-printed portion. The laser then solidifies the next cross-section, and the process is repeated until the whole part is complete. The resin that is not touched by the laser remains in the vat and can be reused.

Post-Processing

After finishing the material polymerization, the platform rises out of the tank and the excess resin is drained. At the end of the process, the model is removed from the platform, washed of excess resin, and then placed in a UV oven for final curing. Post-print curing enables objects to reach the highest possible strength and become more stable.

Alternative Process: Digital Light Processing

As we mentioned before, one descendant of SLA is digital light processing (DLP). Unlike SLA, DLP uses a digital projector screen to flash a single image of each layer across the entire platform. As the projector is a digital screen, each layer will be composed of square pixels. Thus, the resolution of a DLP printer corresponds to pixel size, whereas with SLA, it’s the laser spot size.


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