Stereolithographic 3D Printing of Composites and Graded Materials

Stereolithography (SLA) is an additive manufacturing technology commonly used for printing complex 3D structures due to its low cost, high speed, and excellent resolution. However, standard SLA prints consisting only of one type of polymer resin often lack the properties to be fully functional parts. This thesis focuses on broadening the application range of SLA through high-performance composite build materials and novel processes to enable functionally graded prints.

When fillers are used to modify the properties of the photoresins, different scattering scenarios occur depending on the filler and resin properties. A modified Lambert-Beer equation was developed to unify these different light scattering scenarios, with significant implications on the materials selection and printing parameter optimization for such composite resins.

Using SLA, microfluidic components can be printed directly onto a printed circuit board to realize integrated systems. However, current SLA resins suffer from significant thermal expansion, which leads to adhesion issues during temperature cycling. Photocurable composites with a low coefficient of thermal expansion were developed to address this bonding issue by minimizing the interfacial thermal stress.

Functionally graded materials have attracted great research interest because of their wide applications in biomedical implants, sensors, and soft robots. Traditional standard SLA-printed parts often have limited properties and can't provide multiple complex functions to meet the requirements in these industries. By using a composite feedstock consisting of a photoresin and a polymeric filler, combined with digital control over the light exposure, a novel gradient SLA strategy was developed and used to spatially control the porosity, mechanical properties, and swelling behavior within the SLA prints. This novel strategy will contribute to various applications requiring graded materials, ranging from the biomedical field to soft robotics.

Controlling the exposure parameters for every printing layer is difficult (and sometimes impossible) on many commercial SLA printers. A halftoning-based grayscale exposure was developed to broaden the capability of SLA in graded materials fabrication. 1-Bit SLA slices were converted into 'binary grayscale' (consisting of only pure opaque/transparent pixels) through pixel coding algorithms to mimic 'true' grayscale levels, enabling the printing of functionally graded materials on all projection-based 3D printers.

The photocurable composite formulations and novel gradient SLA processes developed in this thesis increase the functionality of 3D prints, realizing the aim of broadening the application range of SLA.