BY SUDEEP KUMAR SINGH
The Basics
Objects printed through stereolithography. Source:
Stereolithography also known as SLA 3D printing – is considered as very popular additive manufacturing techniques. It uses a high-powered laser to solidify liquid resin contained in a reservoir to create the desired 3D shape. This process converts photosensitive liquid into 3D solid plastics in a layer-by-layer manner using a low-power laser and photo polymerization. It belongs to the resin 3D printing category.
SLA is one of three primary technologies adopted in 3D printing, together with fused deposition modeling (FDM) and selective laser sintering (SLS). 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.
History
A figure of the patent filed by Hull. Source: dewyseng.com
1. SLA is the oldest additive manufacturing technique.
2. The technology and the term were created in 1986 by Chuck Hull, founder of 3D printing company 3D Systems. According to him, SLA is a method of creating 3D objects by successively “printing” layers, by which he meant a photosensitive material.
3. 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.
4. SLA was the first entry into the rapid prototyping field during the 1980s and has emerged into a widely used technology.
Components
SLA components (Source: Manufactur3DMag.com)
Every standard SLA 3D printer is generally composed of four primary sections:
1. A tank filled with the liquid photopolymer: The liquid resin is usually a clear and liquid plastic.
2. 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.
3. A high-powered, ultraviolet laser
4. A computer interface, which manages both the platform and the laser movements
Working
Process of stereolithography. Source: http://www.thagiwara.jp/rp-resin/IUPAC/iupac2000.html
Software
1. Designing a 3D model through CAD software.
2. The CAD files must be converted into STL files. Standard tessellation language (STL), or “standard triangle language”. STL files describe the surface geometry of the 3D object, neglecting other common CAD model attributes, such as color and texture.
3. The pre-printer step is to feed an STL file into a 3D slicer software, such as Cura, which generates 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
The platform is raised out of the tank after printing and the excess resin is drained. The model is removed from the platform, washed to remove excess resin, and placed in a UV oven for final curing which helps objects to reach the highest possible strength and become more stable.
Advantages/Disadvantages
Pros | Cons |
High Precision | Longer time for printing |
High quality prototype generation | High printing cost due to higher machine & material cost |
Detailed features and complex geometrical shapes are well handled | Resin used are not suitable for functional prototypes and mechanical testing due to its fragility |
Minimum layer thickness 25 μm | Limited material and color choices available (black, white, grey and clear material) |
Minimum feature size can be achieved from 50 to 250 μm | Support structures are needed for printing steep slopes and overhangs which may cause problems |
Can achieve tightest dimensional tolerance | |
Smooth surface output | |
Build volumes can be as high as 50 x 50 x 60 cm³ without sacrificing precision |
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