Additive manufacturing of carbon fiber-reinforced thermoplastic composites
Additive manufacturing, or 3D printing, encompasses manufacturing processes that construct a geometry by depositing or solidifying material only where it is needed in the absence of a mold. The ability to manufacture complex geometries on demand directly from a digital file, as well as the decreasing equipment costs due to increased competition in the market, have resulted in the AM industry experiencing rapid growth in the past decade. Many companies have emerged with novel technologies well suited to improve products and/or save costs in various industries. ^ Until recently, the applications of polymer additive manufacturing have been mainly limited to prototyping. This can be attributed to multiple factors, namely the high cost of the machines and materials, long print times, and anisotropy of printed parts. In addition, the low unit cost and cycle time of competing processes such as injection molding further skew the economics in favor of other processes. The addition of fiber-reinforcement into polymers used in additive manufacturing processes significantly increases the strength of parts, and also allows larger parts to be manufactured. In 2014, large-scale additive manufacturing of fiber-reinforced polymers was pioneered, and has generated significant attention from both academia and industry. Commercial machines that incorporate high throughput extruders on gantry systems are now available. New applications that require high temperature polymers with low coefficients of thermal expansion and high stiffness are being targeted, for example tooling used in the manufacturing of composite components. The state of the art of this new paradigm in additive manufacturing as well as the target applications will be discussed in detail. ^ Many new challenges arise as AM scales and reinforced polymers are incorporated. One of the most notable challenges is the presence of large temperature gradients induced in parts during the manufacturing process, which lead to residual stresses and sometimes detrimental warpage. The current solution to this problem has been to print faster in order to lessen the temperature gradients, however very high extrusion speeds are likely not ideal for achieving optimal material properties. The high shear rates induce further damage to fibers, and entrapped air during the extrusion process may not escape, leading to high void content. Another significant challenge is overcoming the anisotropy in printed parts, which arises due to the stiff reinforcing fibers orienting primarily in the print direction. This complicates the use in demanding applications such as composite tooling, where high stiffness and low CTE are desirable in all directions. ^ In 2014, a group of graduate students at Purdue University was formed to develop a better understanding of large-scale additive manufacturing processes incorporating high temperature and high fiber content polymer composites. The team spent more than one year designing, developing, and optimizing a lab-scale system that offers full control over all processing parameters, and has begun studying the relevant phenomena and developing models to predict the outcome of printing processes. ^ This thesis will summarizes the system development process, printing process, composite tooling applications, as well as the mechanical, structural, and viscoelastic properties of printed materials, making it one of the most comprehensive documents written in large-scale additive manufacturing of fiber-reinforced polymers to date. The properties of 50 weight percent carbon fiber-reinforced PPS, a material of high interest in the field, will be presented in detail. The viscoelastic properties will be measured and discussed in the context of both stress relaxation during the printing process and the required performance metrics of composite tooling. A summary of the major results and recommendations can be found in chapter 7.^
R. Byron Pipes, Purdue University.
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