Microstereolithography

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Microstereolithography

Principal investigator : Serge Monneret

Microstereolithography (SL) corresponds to a layered manufacturing methodology in which objects are built as a stack of horizontal cross sections, each one being formed individually by a light-induced photopolymerisation chemical reaction. Thin layers of liquid material are thus sequentially solidified and stacked from bottom to top to create complicated three dimensional structures. Micro-SL is now one of the most promising manufacturing processes for polymer microparts with complex shapes which has also been used to realise ceramic micro-components (Provin et al. 2003). Another advantage is that it may be used without the need for a cleanroom facility.


Figure 1 : Principle of layer stacking : a metallic plate supporting the object to be manufactured is positioned at a small distance below the liquid surface. The modulated light beam (source : 200 W high pressure mercury-xenon lamp) coming from the dynamic mask (digital micromirror device - DMD) is transferred through the reduction lens, and an image is focused at the free surface of the liquid resin with a reduced feature size. This image defines a 2D binary pattern, which induces a polymerisation reaction solidifying the resin in the illuminated areas, while leaving it liquid in the dark regions (step 1, figure 1). After the first layer is polymerised, the support sinks down one layer thickness in the resin. A scraper then applies new fresh material on top of the existing structure and makes the free surface uniform (step 2, figure 1). Because of the weak layer thickness (20-100 µm depending on the application), supplementary relaxation time is necessary to ensure the surface to be uniform (step 3, figure 1). Finally, a new pattern is displayed on the mask, and so on, following the shape of the object, which thus sinks stepwise in the resin bath as new solid layers are formed. Once fabricated, the object is moved out of the bath and washed with a solvent. The adhesion on the metallic support is weak enough to prevent any damage when the solid micropart is lifted off.

According to the third step of the deposition process, the photocurable material must have a viscosity as low as possible. Indeed, the relaxation time depends mainly on the rheological properties of the resin. The curable system used in this paper consists of 3% photoinitiator dissolved in a photocurable mixing of monomers. With such a material, the fabrication rate is of 2 layers per minute, with a 800 ms exposure time of each frame for layers 50 micrometer thick.

As for the conventional SL technique, the microfabricator can just be considered as a sophisticated output device for computers. The object to be built is generally designed with Computer Aided Design (CAD) tools like solid 3D modellers ; then data transfer is performed using the binary STL encoding, which is the standard data transfer format for additive fabricators like SL devices. In order to build the part, its 3-D shape is then broken down into a sequence of cross sections, like contours on a topographic map. This means that the object is mathematically sectioned by a slicing algorithm into a series of horizontal planes. After that, a special program defines a complete set of binary masks corresponding to the different cross sections. In the following step, the masks are sequentially sent to the automated machine, in order to build the object.


Figure 2 : This figure illustrates the three main steps of the global microfabrication process, from the CAD file to the image of the obtained three dimensional micro-object. The design-to-fabrication time is typically a few hours for fabricating microchambers 10mm of height.

Three-dimensional structures containing reservoirs, outlets, and walls with different shapes have been fabricated with our current DMD-based microSL apparatus. Varying the layer thickness during the fabrication is possible and advised as it allows to optimize the fabrication time by increasing the thickness when simple and regular structures are being built, and decreasing it when details are needed.