Stereolithography
Stereolithography (abbreviated SL or SLA ) (composed of the words stereo , after the ancient Greek στερεός stereos , German 'hard, firm, physical' , also in the sense of 'spatial' and lithography , the printing technique according to the λίθος lithos 'stone' and γράφειν graphein 'write') is the oldest patented additive manufacturing process in which a workpiece is built up in layers using (grid) points that materialize freely in space. The production of one part or several parts at the same time usually takes place fully automatically from CAD data created on the computer .
The patent application was made in 1984 by the US physicist Chuck Hull .
Principle using the example of laser stereolithography for individual components
A light-curing plastic ( photopolymer ), for example acrylic , epoxy or vinyl ester resin , is cured by a laser in thin layers (standard layer thickness in the range 0.05–0.25 mm, with micro-stereolithography also up to 1 micrometer layers). The procedure takes place in a bath that is filled with the base monomers of the light-sensitive (photosensitive) plastic. After each step, the workpiece is lowered a few millimeters into the liquid and moved back to a position that is one layer thickness below the previous one. The liquid plastic over the part is then evenly distributed using a wiper or squeegee . Then a laser, which is controlled by a computer via movable mirrors, moves on the new layer over the surfaces to be cured. After curing, the next step takes place so that a three-dimensional model is gradually created.
No support structures are required in micro-stereolithography, and post-curing is also not required in many cases. This is different in the case of stereolithography processes for large components, since the resin hardened by the laser is still relatively soft and certain shaped elements (e.g. overhangs) must also be securely fixed during the construction process. For this purpose, support structures are also built during production. After the construction process, the platform with the part (s) is moved out of the container. After the uncured resin has dripped off, the model is removed from the platform, freed from the support structures, washed with solvents and completely cured in a cabinet under UV light.
Another method that also uses photopolymerization to manufacture physical objects is “Solid Ground Curing” (SGC). Each layer is cured by UV light, whereby a light mask has to be printed out in a photoplotter for each layer . However, this process, which was used particularly in the plants of the Cubital company (Israel), has lost a lot of its importance in recent years.
features
- Existing 3D CAD data is converted into STL format . These data are sent to a stereolithography service provider or transferred to their own SLA printer via USB cable, Bluetooth, memory card, etc. On its execution program, downward overhangs of the object to be printed must be intercepted by means of supporting structures (so-called supports) to be inserted. After printing, these are removed mechanically before final curing.
- Once the construction position has been determined, the geometric control data required for the system is generated, known as "slicing".
- This data is sent to the manufacturing system and forms the basis for controlling the laser beam on the bath surface.
- Within a few hours you get a real model of the virtual parts in the CAD.
- Stereolithography enables high precision (typically 0.1 mm; with RMPD processes also significantly lower, down to 1 micrometer per layer) with fine structures and low wall thicknesses.
- Since a model is built in a liquid, support structures are required for overhanging parts of large components that have to be removed again. In contrast to other rapid prototyping processes, however, the support structure here consists of the same material as the component and must therefore be removed mechanically (since a connection to the component cannot be avoided).
- Usually, the model created by means of stereolithography has to be cured in a UV light cabinet after it has been removed from the machine.
In the last few years technical developments have taken place that combine multi-jet modeling with the basic principles of stereolithography. A wax material that is liquefied by heating serves as the support material. The component itself is produced from a photopolymer in a similar way to stereolithography. Both materials are applied using a modified printhead (similar to inkjet printers). In addition, a light source ensures the exposure and thus the curing of the photopolymer. In contrast to RP stereolithography systems, these systems can also be used in the office and are significantly cheaper with prices starting at around 50,000 euros.
Another new technical development is Continuous Liquid Interface Production (CLIP) .
With two-photon lithography , a UV focus with a diameter of 100 nanometers is directed three-dimensionally through the volume of the liquid resin at 5 meters per second. So it is not cured starting from the surface of the liquid resin. So that the UV focus only has a small extent in the direction of propagation of the radiation, a small depth of field and a large aperture are required. Because the two-photon absorption has a quadratic dependence on the intensity of the light, the hardening area of the resin is sharply delineated.
application
Compared to models that are produced using other generative manufacturing processes such as selective laser melting , a stereolithography model is brittle, which is why the areas of application are limited. The geometry of the component is also limited by the necessary support structures for undercuts. The stereolithography process is therefore widespread in product development when creating prototypes (concept, geometry, illustrative, functional models) in mechanical engineering, especially in automotive engineering and medicine . An increasing trend is expected in the next few years in the direct production of end products with the help of stereolithography systems ( rapid manufacturing ). Application examples that already play a role in everyday life are the production of individual housings for hearing aids with the help of stereolithography and the lab-on-chip systems manufactured by microTEC .
Further application examples are architectural models and, above all, casting models for the vacuum casting process . With this, smaller series of test parts can be produced, whereby the material properties of the later series parts can be simulated through the selection of the casting resins.
medicine
Stereolithography models have been used in medicine since the 1990s to produce physical models from CT image data.
Stereolithography models are used to get a three-dimensional overview of the anatomical situation of a patient. However, since the quality of the processing of three-dimensional image data on a computer has increased, this indication has played a much less important role. Operations can also be planned on these models , for example by sawing a model and repositioning the bone parts in advance of conversion osteotomies . Transplanted bone segments or osteosynthesis plates can be preformed on a stereolithography model.
Stereolithography models have lost importance because of their high cost and because of the increasing relocation of surgical planning to the computer.
Because the plastic of a stereolithography model shrinks during polymerization , the model from the CT data set must initially be made somewhat larger.
literature
- L. Klimek, HM Klein, W. Schneider, R. Mosges, B. Schmelzer, ED Voy: Stereolithographic modeling for reconstructive head surgery. In: Acta Oto-Rhino-Laryngologica Belgica. 47 (3), 1993, pp. 329-334.
- JF Bouyssie, S. Bouyssie, P. Sharrock, D. Duran: Stereolithographic models derived from x-ray computed tomography. Reproduction accuracy. In: Surgical & Radiologic Anatomy. 19 (3), 1997, pp. 193-199.
See also
Individual evidence
- ↑ Andreas Gebhardt: Additive manufacturing processes . 5th edition. Carl Hanser Verlag, Munich 2016, ISBN 978-3-446-44401-0 , pp. 48 .
- ↑ Andreas Gebhardt: Additive manufacturing processes . 5th edition. Carl Hanser Verlag, Munich 2016, ISBN 978-3-446-44401-0 , pp. 121 [4] : "[4] SLA = 'StereoLithography Apparatus' is a protected name of 3D Systems Inc., Rock Hill, South Carolina, USA"
- ↑ Handelsblatt : One printer for the whole world. (Patent title: "Method and apparatus for production of three-dimensional objects by stereolithography".) , Dated June 3, 2014, accessed on November 22, 2016.
- ↑ File: Venus vom Hohle Fels-01.JPG
- ↑ Christian Bonten: Plastics Technology - Introduction and basics. Carl Hanser Verlag, 2014, ISBN 978-3-446-44093-7 .
- ↑ 3D printer with nano-precision. In: tuwien.ac.at. Retrieved June 24, 2018 .
- ^ The method of stereolithography. In: protiq.com. Retrieved March 5, 2018 .
