All About Stereolithography (SLA) 3D Printing

SLA is a 3D printing process that uses a scanning UV laser to cure the surface layer of photosensitive resin. The resin is supplied in a bath, and, in the vast majority of SLA machines, the part is built upside-down. With each layer, the build plate will move upward, making it appear as if the part grows out of the liquid polymer. The machine must also print necessary support structures to support overhangs within the design. 

The UV-sensitive photopolymers used in the process are collectively referred to as ‘resins.’ They are photo-catalyzed acrylic monomers that become crosslinked when exposed to UV laser light. This principle allows the machine to create details as small as the laser beam’s width. 

SLA models are sometimes printed in a partially cured state. These models require post-processing in the form of extra UV exposure to complete the cross-linking process. This additional process step helps eliminate partially solidified resin that didn’t fully cure due to back-scatter and diffraction of the UV beam. Whether or not post-curing is performed, all parts must be washed after printing is complete to remove the surface resin. Washing is generally done in an isopropyl alcohol bath. The removal of the printed support scaffold takes place afterward.

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The SLA 3D printing light source is a UV laser that acts as the stereolithography machine’s curing mechanism. This light source is precisely tuned to the catalyst used in the resin. However, different manufacturers use different wavelengths. The most common SLA laser is a 395 µm wavelength laser diode system. It produces 300-500 mW of power in the beam that is collimated to a diameter of around 300 µm. A variety of other laser light sources can be found in some equipment, with catalysts to suit their frequency range. Other types of UV light sources are used in whole-layer stereolithography. These lamps employ either a projector made of microscopic mirrors (in the case of digital light processing or DLP printers) or an LCD mask (usually referred to as masked stereolithography or MSLA).

SLA 3D Printing is used for applications such as:

• Prototyping: Since they can include fine details, SLA-printed parts can be used as engineering test models.
• Manufacturing: SLA creates functional parts for situations that don’t demand much stress resilience.
• Engineering and product designing: SLA parts can be hand-finished and painted to create quality pre-tooling prototypes.
• Jewelry: SLA machines can build cosmetic test articles for jewelers.
• Dental works: SLA can create various dentistry products, including soft tissue, tooth, bone-implant materials, and casting cavities for polyurethane and silicone molding.
• Healthcare: SLA processes can manufacture medical implants using specialized materials.

SLA 3D Printers can print using these materials:

1. General-purpose acrylic resins: These materials are available in various toughnesses and transparencies.
2. Flexible polyurethane elastomers: Used for flexible parts.
3. Rigid polyurethanes: These have good cosmetic value, are more durable than general-purpose materials, and are well suited for product-trial or prototype pieces.
4. Rigid resins: These are chemically and thermally stable and suited to engineering test parts.
5. Dental and medical resins: These resins are medically safe and make for faster builds, quality finishes, and transparent items like mouthguards, splints, etc.
6. ESD resins: These resins are suited to making electrostatically safe jigs for manufacturing.

SLA 3D printing works by moving a UV laser in the X-Y plane. The UV light triggers catalysts in the liquid monomer resin. The print plate begins at the surface of the resin pool, and regions where the laser strikes both resin and the solid plate surface then get polymerized and affixed to the build plate. With that ‘layer’ complete, the build plate moves up, allowing the next layer to affix itself to the previous one. By repeating this process, the part will appear to grow out of the liquid pool. Prints usually begin with the part’s bottom, and the part is printed upside-down. 

Once removed, the part must be washed to remove any uncured resin. Any support scaffolding elements can then be cut away. 

SLA Printing's Print Parameters

An SLA machine’s print parameters are usually fixed by the manufacturers. It is only the part orientation and layer height that can be changed. Table 1 below shows a comparison of the two common SLA printer orientations:

SLA is distinguished from other 3D printing systems and processes through its wide range of materials with very diverse properties and cosmetic qualities. SLA materials have improved and diversified significantly since first appearing in the market. Another distinguishing factor for SLA is its surface finish—one of the highest standards in the industry. The largest SLA machines were designed for the automotive industry and can build whole body panels, dashboards, etc.

SLA Post-Processing Options

SLA post-processing starts by removing uncured ‘wet’ resin. Bottom-up printers must be drained before post-processing while top-down equipment requires no such delay. In both cases, however, parts must be washed to remove any remaining liquid. Though manual spray-booth washing is still common, automatic solutions are marketed for this washing stage. Some resins require additional post-curing under UV radiation. Once complete, support scaffolds are then removed either manually or by automated equipment. At this point, models are usually considered complete. Any further processing such as sanding or painting is typically aimed at improving the part’s cosmetic appearance.

Benefits of SLA 3D Printing

SLA 3D printing offers a wide range of advantages. These are shown in Table 2: