Stereolithography

History

Stereolithography or "SLA" printing is an early and widely used 3D printing technology. Also known as Rapid Prototyping, 3D printing was invented with the intent of allowing engineers to create prototypes of their designs in a more time effective manner. The technology first appeared as early as the 1970s. Japanese researcher Dr. Hideo Kodama first invented the modern layered approach to stereolithography by using ultraviolet light to cure photosensitive polymers. On July 16, 1984, three weeks before Chuck Hull filed his own patent, Alain Le Mehaute, Olivier de Witte and Jean Claude André filed a patent for the stereolithography process. The French inventors' patent application was abandoned by the French General Electric Company (now Alcatel-Alsthom) and CILAS (The Laser Consortium). Le Mehaute believes that the abandonment reflects a problem with innovation in France.

However, the term “stereolithography” was coined in 1986 by Chuck Hull. 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 UV light beam is focused onto the surface of the liquid photopolymer, creating each layer of the desired 3D object by means of crosslinking (or degrading a polymer). In 1986, Hull founded the world's first 3D printing company, 3D Systems Inc, which is currently based in Rock Hill, SC.

Stereolithography's success in the automotive industry allowed 3D printing to achieve industry status and the technology continues to find innovative uses in many fields of study. Attempts have been made to construct mathematical models of stereolithography processes and to design algorithms to determine whether a proposed object may be constructed using 3D printing.

Technology

Stereolithography is an additive manufacturing process that works by focusing an ultraviolet (UV) laser on to a vat of photopolymer resin. With the help of computer aided manufacturing or computer aided design software (CAM/CAD), the UV laser is used to draw a pre-programmed design or shape on to the surface of the photopolymer vat. Because photopolymers are photosensitive under ultraviolet light, the resin is solidified and forms a single layer of the desired 3D object. This process is repeated for each layer of the design until the 3D object is complete.

In models featuring an elevator apparatus, an elevator platform descends a distance equal to the thickness of a single layer of the design (typically 0.05 mm to 0.15 mm) into the photopolymer vat. Then, a resin-filled blade sweeps across a cross section of the layer, re-coating it with fresh material. The subsequent layer is traced, joining the previous layer. A complete 3D object can be formed using this process. Designs are then immersed in a chemical bath in order to remove any excess resin and cured in an ultraviolet oven.

It is also possible to print objects "bottom up" by using a vat with a somewhat flexible, transparent bottom, and focusing the UV or deep-blue polymerization laser upward through the bottom of the vat. For example, the Form 1 stereolithography machine starts a print by lowering the build platform to touch the bottom of the resin-filled vat, then moving upward the height of one layer. The UV laser then writes the bottom-most layer of the desired part upward through the transparent vat bottom, and the photopolymer hardens selectively where the laser strikes. Then the vat is "rocked", flexing and peeling the bottom of the vat away from the hardened photopolymer; the hardened material detaches from the bottom of the vat and stays attached to the rising build platform, and new liquid photopolymer flows in from the edges of the partially built part. The UV laser then writes the second-from-bottom layer and repeats the process. An advantage of this bottom-up mode is that the build volume can be much bigger than the vat itself, and only enough photopolymer is needed to keep the bottom of the build vat continuously full of photopolymer.

Stereolithography requires the use of supporting structures which attach to the elevator platform to prevent deflection due to gravity and to hold cross sections in place in order to resist lateral pressure from the resin-filled blade or retain newly created sections during the "vat rocking" of bottom-up printing. Supports are created automatically during the preparation of 3D Computer Aided Design models and can also be made manually. With more expensive stereolithography models, these supports must be removed from the finished product manually.

Medical modeling

Stereolithographic models have been used in medicine since the 1990s, for creating accurate 3D models of various anatomical regions of a patient, based on datasets from computer scans. Medical modelling involves first acquiring a 3D CT, MRI, or other scan. This data consists of a series of cross sectional images of the human anatomy. In these images different tissues show up as different levels of grey. Selecting a range of grey values enables specific tissues to be isolated. A region of interest is then selected and all the pixels connected to the target point within that grey value range are selected. This enables a specific organ to be selected. This process is referred to as segmentation. The segmented data may then be translated into a format suitable for stereolithography. While stereolithography is normally accurate, the accuracy of a medical model depends on many factors, especially the operator performing the segmentation correctly. There are potential errors possible when making medical models using stereolithography but these can be avoided with practice and well trained operators.

Stereolithographic models are used as an aid to diagnosis, preoperative planning and implant design and manufacture. This might involve for example planning and rehearsing osteotomies. Surgeons use models to help plan surgeries but prosthetists and technologists also use models as an aid to the design and manufacture of custom-fitting implants. Medical models are frequently used to help in the construction of Cranioplasty plates for example.

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

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. Many 3D printers can produce parts with a maximum size of approximately 50×50×60 cm (20×20×24 in) and some printers, such as the Mammoth stereolithography machine (which has a build platform of 210×70×80 cm), are capable of producing single parts more than 2 meters in length. 3D printed prototypes and designs are strong enough to be machined and can also be used to make master patterns for injection molding, thermoforming, blow molding, and various metal casting processes.

Although stereolithography can be used to produce virtually any synthetic design, it is often costly; common photopolymers can cost about $800 per gallon and SLA machines can cost $250,000. Recently, public interest in 3D printing has inspired the design of several consumer SLA machines which can cost $3,500 or less, such as the Ilios Photon 2 by Ilios 3D, Form 2 by Formlabs or the Nobel 1.0 by XYZPrinting. There has also been a reduction of the cost of photopolymer resins.