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RESEARCH LINE

New materials for stereolithography

Involved People: Alfonso Maffezzoli, Antonio Licciulli, Antonio Greco, Carola Esposito Corcione, FrancescoMontagna, Christian Demitri, Maurizio Fersini
The need of fast production of prototypes of complex shape in very short time lead to the development from the mid 80's of many additive rapid prototyping (RP) technologies for the production of single objects or of very limited series. The new fabrication concept allowed the construction of complex parts, starting from a 3D-CAD model, without using a mould. In RP, the object building is obtained bonding, layer by layer, its sections by an additive process rather than cutting or «sculpturing» the parts from an homogeneous block. The key point of RP is given by the direct link between the CAD model and the solid object, generated in few hours suggesting that tailor made or single functional parts could be fabricated for example for prosthetic applications in very a short time. Among others stereolithography is by far the most diffused RP techniques. The objects are obtained polymerizing a low viscosity liquid resin section by section. The shape and the dimensions of the parts are directly transferred from a three dimensional CAD system to the stereolithography equipment where a UV laser beam polymerizes the different sections. A computer controlled laser draws the bottom cross section onto the surface of a liquid polymer. The polymer hardens where it is struck by the laser. The part is then lowered to a depth corresponding to the section's thickness. The next cross section is drawn directly on top of the previous one. This is repeated until the part is finished Finally, the part is removed from the vat and completely cured in a special UV post cure apparatus Besides, design, mould fabrication and commercial needs, functional applications of prototypes needs the development of custom made materials for each application. Our group is actually involved in the development of new materials for stereolithography. The research is performed combining offline photo calorimetric measurements and part building on an SLA 250 equipment, modified to be fitted with a small vat containing 500 ml of experimental resin.
The research is essentially focused on:
  • development of ceramic suspensions for the fabrication of complex parts. Stereolithography is used to build a green containing not less than 50% v/v of ceramic powders (Figure 1). The properties obtained are often adequate for investment casting of aluminum as shown in Figure 2. If needed the green is then subjected to burn-out and sintering. The most recent advance indicated that silica based suspension are much easily fabricated.
  • fabrication of hydrogels. Solutions of water soluble monomers are used in the stereolithographic apparatus. As shown in Figure 3 a complex fluid-dynamic pattern can be obtained. Hydrogels capable to mimic the elastic properties of soft animal tissues have been obtained. Hydrogels blocks with microfluidic complex patterns are under development in order to obtain phantoms for the study of complex blood vessel patterns.
  • process modeling. The development of proper models for the UV curing kinetic have been developed. Original models have been applied either to free radical polymerization of acrylic resin either to cationic polymerization of epoxy based resin for STL. A full heat transfer model was also developed to predict the evolution of temperature and degree of reaction of a stereolithographic resin during laser exposition in the SLA 250 vat. Comparison with experimental temperatures, measured by a thermal video system, is satisfactory (Figure 4).
    Actually a rapid prootyoping service with commercial and new materials is available (see: www.prototipazione.unile.it)


Figure 1: Sintered (left) and green (right) silica parts obtained by stereolithography using a proprietary ceramic suspension


Figure 2: Ceramic green mould for investment casting as obtained in a stereolithograpic equipment

Figure 3a and 3b. Hydrogel block (a) with a complex channel pattern(b)



Figure 4: Comparison of experimental temperatures at 18.5 mm (dots), 54.5 mm (E) from the initial laser position, and the numerical model prediction (line)

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University of Salento   Facoltą di Ingengeria    Department of Engineering for Innovation