Stereolithography

Stereolithography is an important additive manufacturing process process that creates three-dimensional solid objects in a multi-layer procedure through the selective photo-initiated cure reaction of a polymer. Substantial progress has been made in the field of stereolithography and this chapter aims at outlining some of the most significant technological changes, discussing novel curing mechanisms, materials and applications.

This chapter describes the history and development of photolithographic systems, explaining the origins of modern stereolithography and photomask system. It also highlight the importance of a modern prototype and summarizes the techniques currently available to produce prototypes.

Infrared laser stereolithography is a new process of stereolithography, substantially different from those already existents; that uses an infrared laser (carbon dioxide laser) to localize the curing reaction of a thermo-sensitive materials building the desired three-dimensional (3D) shape. Being developed by Scarparo and collaborators at the University of Campinas (UNICAMP) in Brazil, since 1992 [1], and patented in 2002, this technique is to have commercial value, with thermo-sensitive material that must have useful properties for applications in rapid manufacturing and tooling, medicine, or other important fields. Certainly, further advances and process optimization are a part of the ongoing activities that are in consonance with a large number of programs around the world of very interesting work towards improvement of stereolithography process.

Microstereolithography is a technology that is based on the same manufacturing principle as stereolithography. Three-dimensional (3D) objects are built by the superimposition of many layers, each being produced by a light-induced space-resolved photopolymerization of a liquid resin. As the resolution of microstereolithography is far better than other rapid prototyping techniques, this technique creates interest in both the rapid prototyping domain, where it can be used to produce high-resolution prototypes, but also in the microengineering field, as it is clearly the microfabrication process that can produce small objects with the most complicated shapes and intricate details.

Rapid Manufacturing also known as solid free-form fabrication is a term describing a range of processes whereby a computer generated design is converted to a three-dimensional (3D) object. This methodology was originally used for the manufacture of models and prototypes (hence the description “Rapid Prototyping”) [1] and has becoming an increasingly important tool for designers and manufacturers [2]; indeed in some cases this approach is being extended to the manufacture of small numbers of complex articles. One particularly exciting development is the use of Rapid Manufacturing for the production of biocompatible components for medical use, where the production of a one-off component is a necessary requirement. Examples include the manufacture of dental prostheses [3] and hip joints [4].

Stereolithography (SL) is a rapid prototyping method for three-dimensional polymer part fabrication [3, 34, 49, 53, 63]. The technique is based on the process of photopolymerization, in which a liquid resin is converted into a solid polymer under laser irradiation [4, 34]. The models are produced by curing successive layers of the resin material until a three-dimensional object is formed. The advantages of stereolithography are its flexibility in manufacturing parts with different geometries and dimensions, its accuracy and its quickness. The challenge is to extend the stereolithography method to directly fabricate parts with complex shapes and good mechanical properties [30, 47, 58]. Recently, polymer/ceramic composite were successively fabricated by stereolithography [29, 46, 52, 62]. The manufacturing process requires the formulation of a photoreactive medium containing a photocurable resin and powders prior to laser exposure. Once polymerized, the photopolymer constitutes a though matrix around ceramic particles.

Hydrogels are cross-linked polymeric structures which are swollen by water [1, 2]. In a more general sense, these polymeric structures can contain solvents other than water, leading to the more general term “gel.” Besides the polymer network and solvents, hydrogels can also contain particulate filler materials, typically ceramic particles. The functional and structural properties of hydrogels can be tailored quite easily, as the network density as well as the solvent content can be varied over a large range. The mechanical properties (especially the stiffness) of hydro(gels) are comparable to many biological tissues. Furthermore, the open network in combination with the mobile solvent molecules facilitates the diffusion of nutrients and dissolved gases, which makes hydrogels a widely used material in biomedicine, e.g. for the use in contact lenses [3], wound-healing bioadhesives, scaffolds for tissue engineering [4], and pharmaceutical hydrogel systems. Hydrogels are also used in a number of sensor applications, as the swelling behavior and diffusion coefficient of hydrogels depend on the ambient conditions [5].

Stereolithography (SL) is a layered, additive manufacturing process in which an ultraviolet (UV) laser is used to selectively cure a liquid photopolymer resin in order to physically fabricate a part. Traditional SL systems use a UV laser with galvanometer-driven mirrors to scan a particular cross-section on the build surface. The limits of the resolution, both theoretical and empirical, need to be established so that accuracy and surface finish of SL-fabricated parts can be predicted.

This chapter presents an integrated thermal-kinetic model to study photo-initiated curing reactions and determine different aspects associated with these reactions, describing both the heat transfer phenomenon all along the reaction and the cure kinetics. This model is sensitive to the resin composition, temperature and light intensity, apart from describing the main events occurring during cure reactions. The kinetic model and the law of the conservation of energy are coupled, while the integrated model is numerically solved.

The use of plastic tooling in injection molding occurs within the field of Rapid Tooling (RT), which provides processes that are capable of producing injection mold tooling for low volume manufacturing at reduced costs and lead times. Such tooling allows the injection molding of parts in the end-use materials for functional prototype evaluation, short series production, and the validation of designs prior to hard tooling commitment. The term Rapid Tooling is somewhat ambiguous – its name suggests a tooling method that is simply produced quickly. However, the term is generically associated with a tooling method that in some form involves rapid prototyping technologies.

Since first experimental demonstration of microstructuring using two-photon polymerization (2PP) [1], the technology has experienced rapid development. The unique capability of this technique to create complex 3D structures with resolution, reproducibility, and speed superior to other approaches paved its way to applications in many areas. Figure 11.1a shows some SEM images of structures fabricated by 2PP for demonstrational purposes. Microvenus statues fabricated from negative photoresist SU8 [2] material are presented in comparison to the human hair. Each statue is about 50 μm tall and 20 μm wide, the overall fabrication time is just few minutes. Figure 11.1b shows an array of microspiders fabricated on a glass slide. Each structure is about 50 μm wide and the spider’s body is supported by eight 2 μm thick legs. Finally, a fragment of a windmill array (Fig. 11.1c), produced by 2PP using Ormocore [3] is shown. Fabricated in a single step, the structure consists of two physically separate parts – windmill body and propeller, which are interlocked in such way that the propeller can be rotated around the shaft. Therefore, using 2PP microfabrication it is possible to produce functional micromechanical components in a single step, without the necessity of tedious assembly procedure. Looking at these images, one can see the strength of 2PP technology and envision many potential applications.

In recent years, additive manufacturing (AM) or rapid prototyping (RP) technologies, initially developed to create prototypes prior to production for the automotive, aerospace, and other industries, have found applications in tissue engineering (TE) and their use is growing rapidly. RP technologies are increasingly demonstrating the potential for fabricating biocompatible 3D structures with precise control of the micro- and macro-scale characteristics. Several comprehensive reviews on the use of RP technologies, also known as solid freeform fabrication, Additive Manufacturing, direct digital manufacturing, and other names, have been published recently [1–4].