JEC COMPOSITES MAGAZINE - Issue #36 - October 2007 - 59

Focus History of SL materials development First introduced in the late 1980s, stereolithography is the most widely used method for rapid prototyping from CAD data. The basic process utilizes finely focused ultraviolet laser to cure successive cross sections of liquid photopolymer to form a solid part (see figures a anb). Photopolymers are complex mixtures containing reactive monomers and oligomers, photoinitiators, stabilizers, and other additives. Components are usually multifunctional and highly reactive to ensure fast cure and high strength of the cured layers. The first photopolymers developed for SL by Huntsman (formerly Ciba-Geigy) were based on acrylate resins that generated highly reticulated, rigid networks for the production of solid three-dimensional parts used as concept and form-and-fit models. Initial SL parts based on acrylate chemistries were quite brittle, having surfaces that required significant secondary finishing for removal of the “stair/step” pattern produced as the photopolymer cross sections cured in the vat. Through the mid-1990s, epoxy resins were incorporated into the acrylate network. The epoxies expanded as they polymerized to compensate for the shrinkage of the acrylates in the formulation. With this change, models built with these initial hybrid photopolymers exhibited improved accuracy allowing production of durable master patterns. With the wide range of epoxy building blocks available, formulators were able to develop many different SL resins to meet the diverse requirements of a broad spectrum of applications. Among these new epoxy/acrylate resins were materials featuring high temperature resistance (up to 250°C) for automotive under-the-hood applications and flexible polypropylene-like materials for snap-fit applications. In 2002, while previous SL resins produced ambercoloured parts, as hybrid formulations continued to evolve, Huntsman introduced first-generation white ABS-like photopolymers to its line, for the production of parts that could be used for limited functional testing. In addition, selectively colourable resins were introduced for building medical models. These unique photopolymers could be exposed to a second curing cycle to produce red-tinted areas within predefined sections of a model. Fig. b: SL process Fig. a: SL curing principle When the laser beam with a Gaussian energy distribution hits the surface of the liquid photopolymer, the curing process is initiated. The liquid changes to a highly reticulated gel close to the solid state within seconds. This polymerization occurs where the energy delivered by the Gaussian laser beam is superior to a resin-dependent parameter called “critical exposure”. The resin is cured through the cure depth, another resin-dependent parameter. The depth to which the laser penetrates inside the resin is deeper than the layer thickness, allowing stitching to the previously solidified layer. This is called the overcure. In absence of overcure, layers would simply be stacked on top of each other, and delamination could occur. Layer thickness varies from 0.025 mm to 0.15 mm depending on the systems. From 2003 to 2006, the line of RenShape® SL epoxyacrylate hybrids expanded to include clear materials for building high accuracy parts and secondgeneration white ABS-like photopolymers with improved functional characteristics. In 2006, Huntsman introduced the first major change in SL chemistries in nearly a decade with its advanced epoxy hybrid photopolymers. These next generation materials, based on novel photo-chemistries, are expanding SL resins capabilities into a new performance arena. Specifically, the reticulated network contains less residual stress and is less prone to ageing and dark cure, yielding SL parts with previously unattainable toughness and durability. No36 October - November 2007 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #36 - October 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #36 - October 2007