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Introduction

Stereolithography (SLA or SL; also known as stereolithography apparatus, optical fabrication, photo-solidification, or resin printing) is a form of 3D printing technology used for creating models, prototypes, patterns, and production parts in a layer by layer fashion using photochemical processes by which light causes chemical monomers and oligomers to cross-link together to form polymers. Those polymers then make up the body of a three-dimensional solid. Research in the area had been conducted during the 1970s, but the term was coined by Chuck Hull in 1984 when he applied for a patent on the process, which was granted in 1986. Stereolithography can be used to create prototypes for products in development, medical models, and computer hardware, as well as in many other applications. While stereolithography is fast and can produce almost any design, it can be expensive.


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History

Stereolithography or “SLA” printing is an early and widely used 3D printing technology. In the early 1980s, Japanese researcher Hideo Kodama first invented the modern layered approach to stereolithography by using ultraviolet light to cure photosensitive polymers.
However, the term “stereolithography” was coined in 1984 by Chuck Hull when he filed his patent for the process.
Chuck Hull patented stereolithography as a method of creating 3D objects by successively “printing” thin layers of an object using a medium curable by ultraviolet light, starting from the bottom layer to the top layer. Hull’s patent described a concentrated beam of ultraviolet light focused onto the surface of a vat filled with a liquid photopolymer. The beam is focused onto the surface of the liquid photopolymer, creating each layer of the desired 3D object by means of crosslinking (generation of intermolecular bonds in polymers). It was invented with the intent of allowing engineers to create prototypes of their designs in a more time effective manner.

Technology

Stereolithography (SLA) is an additive manufacturing process that belongs to the Vat Photopolymerization family. In SLA, an object is created by selectively curing a polymer resin layer-by-layer using an ultraviolet (UV) laser beam. The materials used in SLA are photosensitive thermoset polymers that come in a liquid form.
I. The build platform is first positioned in the tank of liquid photopolymer, at a distance of one layer height for the surface of the liquid.
II. Then a UV laser creates the next layer by selectively curing and solidifying the photopolymer resin. The laser beam is focused in the predetermined path using a set of mirrors, called galvos. The whole cross sectional area of the model is scanned, so the produced part is fully solid.
III. When a layer is finished, the platform moves at a safe distance and the sweeper blade re-coats the surface. The process then repeats until the part is complete.
IV. After printing, the part is in a green, no-fully-cured state and requires further post processing under UV light if very high mechanical and thermal properties are required.
The liquid resin is solidified through a process called photopolymerization: during solidification, the monomer carbon chains that compose the liquid resin are activated by the light of the UV laser and become solid, creating strong unbreakable bonds between each other. The photopolymerization process is irreversible and there is no way to convert the SLA parts back to their liquid form: when heated, they will burn instead of melting. This is because the materials that are produced with SLA are made of thermoset polymers, as opposed to the thermoplastics that FDM uses.


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Application

Medical modeling

SLA offers a variety of real world applications in diverse fields from manufacturing to biomedicine. Before taking a design into full scale production, stereolithography allows businesses to evaluate their design for feasibility, manufacturability, ergonomics, aesthetics without slow and costly prototyping. Models can also be tested using Optical Stress Analysis (OSA) to study the effects of external loadings, torsion, tension, pressure and placed in wind tunnels to measure aerodynamics. Manufacturing firms also use stereolithography models for casting, tooling and production line design. Businesses also use these models as a marketing tool since it enables them to present a physical representation of the product before production is complete. Stereolithography prototyping lowers the cost of production while increasing product durability, benefiting both industry and consumers.

Stereolithography is also becoming prevalent in the field of bio-medical engineering. Surgeons are able to make models of patients and implants to practice delicate routines in advance. These models are also being used to educate families, patients, and students.


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Prototyping

Stereolithography is often used for prototyping parts. For a relatively low price, Stereolithography can produce accurate prototypes, even of irregular shapes.
Businesses can use those prototypes to assess the design of their product or as publicity for the final product.

Advantages and disadvantages

Advantages

One of the advantages of stereolithography is its speed; functional parts can be manufactured within a day. The length of time it takes to produce a single part depends upon the complexity of the design and the size. Printing time can last anywhere from hours to more than a day. Prototypes and designs made with stereolithography are strong enough to be machined and can also be used to make master patterns for injection molding or various metal casting processes.

Disadvantages

Although stereolithography can be used to produce virtually any synthetic design, it is often costly, though the price is coming down. Common photopolymers that once cost about US$200 per liter, are now US$60 per liter, and professional SLA machines can cost US$250,000.
SLA parts are generally brittle and not suitable for functional prototypes.
The mechanical properties and visual appearance of SLA parts will degrade overtime when the parts are exposed to sunlight.