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04 August 2017 16:22
Stereolithography — the original 3D printing technology
Invented and patented by Chuck Hull — founder of 3D Systems — in 1984, stereolithography (SLA) is the original technology that serves as the foundation of modern 3D printing. It was SLA that originally helped establish 3D printing as a viable tool for manufacturers, and encouraged others to explore new methods of printing and new applications for the technology. It is a testament to Hull’s original vision that SLA is still employed today, across a wide variety of industries, by both seasoned professionals and aspiring hobbyists.
How does it work?
Stereolithography builds up parts one layer at a time by firing an ultraviolet ray into a vat of photopolymer resin. The ray causes the resin to cure and solidify, allowing parts to be built up a layer at a time. Once the whole part is complete, it is removed from the vat and placed in a solvent-based chemical bath to remove any excess material and achieve a smooth finish. Finally, the part is cured in an ultraviolet oven to ensure it has properly hardened.
Applications for SLA
Stereolithography originally established itself as a tool for rapid prototyping, and this remains its primary application today. The speed with which it can generate a highly accurate print of a 3D model means that it is well-suited to a fast-moving prototyping process, where many iterations of a part are produced before production eventually goes ahead. SLA offers a high level of accuracy, with few of the visible layers that other techniques often produce.
At the time of writing, SLA has yet to find its way into production lifecycles. Despite its speed, the photopolymer materials used are quite expensive compared to other 3D printing technologies, which means large-scale production is unlikely to be a cost-effective option. However, it is increasingly common for manufacturers to use SLA printing to generate casts or patterns for other technologies, such as injection moulding or sand casting, which can be used multiple times and can be produced far faster than with traditional methods. It has also been used on occasion for one-off parts that must be tailored to customer’s exact specifications. For example, SLA techniques have successfully been employed to print custom-fitted hearing aids.
Stereolithography relies exclusively on photopolymer materials that will behave in the desired way during printing. However, despite this apparent limitation, the range of printable materials has grown considerably over the years, providing manufacturers with a considerable amount of choice. Resins are available in a wide range of colours, and with mechanical properties that simulate other popular materials, such as ABS. Resins are also available for specialist applications, such as dental parts.
Some practical tips for first-class SLA printing
- Consider your support structures. All SLA parts require support structures during printing to avoid warping, so these must be factored into your design. Avoid any unsupported overhangs of more than 19 degrees. Support structures can be clipped off or snapped away after curing, and any left-over material sanded down to achieve a smooth finish.
- Avoid small holes and thin walls, as these have a high chance of warping, especially during the curing process.
- If you are planning on incorporating interlocking parts into a print, make sure they have proper clearance. As a rule of thumb, use a clearance of 0.5mm for moving parts and 0.1-0.2mm for parts that will need to fit together.
- Consider ways to reduce your material costs. As we mentioned above, SLA materials are quite expensive, so it is a good idea to hollow out your part, where possible, in order to minimise the amount of material used. If you do this, make sure you incorporate holes to allow any excess resin to be drained out prior to curing. If this is not done, the excess material can lead to holes or cracks appearing.
- Wet-sanding is often applied to SLA prints during the post-processing stage to achieve a smoother finish. This can lead to spots on the finish, but a layer of mineral oil after sanding will remedy this. Alternatively, for simple shapes with no fine details, different grades of sandpaper can be used to achieve a sleek, transparent finish.
- Pick the right finish! As with SLS, there are a wide range of finishing options available for SLA printing, including both functional and cosmetic choices. For example, an acrylic finish is often used to protect parts from UV rays and ensure they stay looking new for longer.
Even as new technologies enter the market, SLA shows no sign of going away. We expect new applications for it to continue to reveal themselves as additive manufacturing evolves, material science improves, and confidence in the technology grows, both amongst industry and the general public.