(Dr. Shreyes Melkote, advisor)
"Machinability of Polymer Composite Materials for Rapid Tooling"
Shrinking timelines from conception to production of new products has necessitated the development of rapid prototyping (RP) technologies. Today there is need for RP techniques that not only rapidly produce physical models of parts for visualization and geometry verification, but also produce parts made out of production-intent materials that can be used in actual testing and evaluation or as prototype tooling for injection molding applications. Recent developments in new materials have enabled the application of conventional and high speed machining for this purpose. Specifically, a class of polymer composite materials (e.g., aluminum-filled epoxy) has been developed that affords high material removal rates in machining and possesses the physical and thermal properties necessary to allow their use as tooling in injection molding/forming applications. Although machining of polymers and polymer composites such as glass/carbon-fiber reinforced plastics has been studied before, there is lack of scientific knowledge about the machinability of the new class of polymer composites such as the aluminum-filled epoxy. This thesis aims to fill the need in this area.
Specifically, the thesis involves investigation of the effects of machining parameters, tool geometry and material properties on the chip formation, cutting forces, surface roughness produced in flat and ball nose end milling of the aluminum-filled polymer matrix composite material (Renshape Express 2000™). The study shows that, even though the material behaves brittle (generating dust-like chips), the machining forces and surface roughness in milling of the polymer composite material are influenced by the same parameters and in similar fashion as in conventional metal machining. Breakout at tool exit was observed due to brittleness of the material. Machined surfaces showed a random texture as opposed to distinct feedmarks. This explains why molds machined out of this material require less polishing. However, the viscoelastic nature of the material plays a very important role in determining the material response to machining. The study showed that the glass transition temperature (Tg) and the degree of cure of the polymer matrix have a significant influence on the material response, e.g. chip formation, to machining. Chips turned from dust-like to continuous when the Tg was lowered (partially cured). The specific cutting energy was also found to be lower and breakout was greatly minimized in the partially cured material. Based on these results, an alternative way of rapidly machining the molds out of the partially cured material and then post-curing to achieve the required mechanical properties is proposed. In addition, comparison of the fully cured composite material with end milling of aluminum 6061-T6 material (commonly used for prototype tooling fabrication) revealed that the specific cutting energy of the composite material is approximately two times lower than that for aluminum 6061-T6. The flatness error and surface roughness of slots machined in the composite material are also found to be significantly lower than in aluminum 6061-T6 for the same material removal rates. Physical explanations for the observed effects are provided.