There are a number of processes that can realize three-dimensional (3D) shapes such as those stored in the memory of a computer. An example is the use of holographic techniques, but these require many complex calculations to obtain the hologram and there is insufficient accuracy and clarity. A manual or a conventional mechanical process can also make a physical model, but such models require long fabricating times, high cost and excessive labor. To solve these kinds of problems, a new group of techniques called additive manufacturing (AM) technologies have been developed.
Stereolithography is a chemical based method and relies on the combination of light sensitive chemicals and lasers. These chemicals are oriented in such a manner that when they’re exposed to UV laser light they turn from liquid to solid. The 3D printers that use this method are designed to maneuver the UV laser across a thin surface of the chemical liquid in the design of the required object. As each layer solidifes, it is lowered and thinly submerged in the chemical liquid. The UV laser then creates another solid layer by moving across the liquid and so on until the final product is solidifed and complete. The stereolithography method has proven to provide a very high level of detailing and finishing on the surface of the objects created. The entire printing process takes places in the chemical liquid and the finishing involves separating the solidifed 3D object from the pool of chemicals in a single ﬂow.
History and Development of the Photolithographic Systems
Lithography is the art of reproduction of graphic objects and comprises different techniques, such as photographic reproduction, photosculpture, xerography and microlithography. Modern photolithographic AM systems harness the principle of computer generated graphics combined with photosensitive materials to produce 3D objects.
Photosensitive materials have been known at least since the time of the ancient Egyptians and probably long before them. The alchemists of the Middle Ages and Renaissance knew about the phenomena of blackening silver salts by light exposure. However, they did not realize that this phenomenon was due solely to the Sun’s light and not to its heat. In fact, they argued that all changes produced in bodies exposed to sunlight were due to heat and not to light.
In 1775, Schultz discovered that a silver-containing precipitate used to produce phosphorous, turned purple when illuminated by sunlight, whilst the portion turned away from the light remained white. After that, he divided the mixture into two lots, one of which he kept in the dark, exposing the other to sunlight, with a thin cord tied round the bottle, and again a change in the precipitate exposed to the sunlight was observed. He repeated the experiment by covering the bottle with paper from which he had cut out words and entire sentences, this way “writing” the words and sentences in the solution.
Another remarkable achievement was due to Nie´pce (1822), when he made his first successful and permanent copy of an engraving of Pope Pius VII. He dissolved bitumen of Judea in oil of lavender, and spread a thin layer on a glass plate on which he superimposed an engraving of Pope Pius VII made transparent by oiling. After exposure to light, the bitumen under the white parts of the engraving became hard, whilst that under the dark lines remained soluble.
Origins of Modern Stereolithography
Hideo Kodama described an automatic method for fabricating 3D models in layered stepped stages using a photosensitive polymer. Light capable of curing the polymer was directed onto the surface, and the desired shape of a layer was created by using an appropriate mask or an optical fiber manipulated by an X–Y plotter.
A.J. Herbert described the design of two sets of apparatus for producing replicas of solid objects, in a layer-by-layer way, using a photosensitive polymer. The purpose of the first one was only for the construction of solids of revolution, made by rotating a layer of polymer and focusing a spot of light on the layer. The second apparatus constructed solid objects of any desired cross-section.
Hull conceived the idea of modern stereolithography. According to the principles of stereolithography, a 3D object is formed layer by layer in a stepwise fashion out of a material capable of solidification upon exposure to ultraviolet (UV) radiation. Moreover, the non-transformed layers typically adhere to the previously formed layers through the natural adhesive properties of the photosensitive polymer upon solidification. Almost in parallel, Andre´, who prepared different patent applications conducted similar work in France.
The entire process of conceiving a model using stereolithography comprises the following steps :
1. Create a solid or surface model on a CAD system.
2. Export the CAD model.
3. Add support structures.
4. Specify the build style variables and parameters necessary for slicing.
5. Slice the computer model to generate the information that controls the SL apparatus.
6. Build the model using the slice file.
7. Post-process and clean the part.
8. Post-curing to complete the cure process.
Depending on the size and number of objects being created, the laser might take a minute or two for each layer. A typical run might take six to 12 hours. Runs over several days are possible for large objects (maximum size for the machine shown above is an object 10 inches (25 cm) in three dimensions).
You start by creating a 3-D design for your object in a CAD program. This design is tweaked before building with supports that raise it up off the tray slightly and with any internal bracing that is required during building. The SLA then renders the object automatically (and unattended). When the process is complete, the SLA raises the platform and you end up with your 3-D object. If the object is small, you can produce several of them at the same time if you like. They all sit next to each other on the tray.
Once the run is complete, you rinse the objects with a solvent and then “bake” them in an ultraviolet oven that thoroughly cures the plastic.
Stereolithography allows you to create almost any 3-D shape you can imagine. If you can get it into a CAD program, you can probably create it. The only caveat is the need for structural integrity during the building process. In some cases, you need to add internal bracing to a design so that it does not collapse during the printing or curing phases.
The machine has four important parts:
- A tank filled with several gallons of liquid photopolymer. The photopolymer is a clear, liquid plastic.
- A perforated platform immersed in the tank. The platform can move up and down in the tank as the printing process proceeds.
- An ultraviolet laser
- A computer that drives the laser and the platform
The photopolymer is sensitive to ultraviolet light, so when the laser touches the photopolymer, the polymer hardens.
Costs of Stereolithography
Stereolithography is not an inexpensive process. The machines themselves usually cost in excess of $250,000. They have to be vented because of fumes created by the polymer and the solvents. The polymer itself is extremely expensive. CibaTool SL5170 resin, a common photopolymer used in stereolithography, typically costs about $800/gallon. For these reasons, it is uncommon to find stereolithography machines anywhere but in large companies.
However, there are service bureaus that can make the advantages of stereolithography available to smaller shops and individuals. It is also possible that your local makerspace has an SLA printer.
This article builds on the following works :