Electronic Thesis and Dissertation Repository

Thesis Format



Master of Engineering Science


Chemical and Biochemical Engineering


Hrymak, Andrew N.


Microinjection molding (µIM) exhibits significantly higher shear rates and faster cooling rates, as compared to conventional injection molding, which affect the characteristics of its final products. The effect of carbon black (CB), carbon nanotubes (CNT), and graphene nanoplatelet (GNP) fillers on the electrical conductivity properties of microinjection-molded polypropylene (PP) nanocomposites was systematically studied. Results showed the electrical conductivity properties of PP/CNT and PP/CB microparts were significantly influenced by mold geometry. PP/CNT/CB hybrid filler microparts demonstrated synergistic increases in electrical conductivity and crystallization temperature with higher CNT loading. Morphological observations indicated significant CB and CNT phase separation. Powder-PP/GNP composites exhibited higher electrical conductivity compared to pellet-PP/GNP due to the similar particle size between PP powder and GNP flakes promoting more uniform microscopic dispersion. Moreover, ‘precoated’ samples exhibited smaller GNP flake size, better microscopic dispersion and exfoliation of GNP fillers.

Summary for Lay Audience

There is a significant demand for small scale microinjection molded parts in the fields of electronics, automotive, and medical industries, with a special focus on microelectromechanical systems (MEMS). Microinjection molding (µIM) is a promising technology with many advantages such as high efficiency for mass production, low cost, excellent process control, and a wide selection of thermoplastics that can be used to manufacture micro-components. Since microinjection molding is significantly different than many existing processes, there is a need to study how the process affects the material properties of the finished microparts produced through microinjection molding. Recent studies have characterized the properties of carbon-filled polymer composite microparts. However, further studies are required to understand more complex systems. The following results were obtained in the present study:

First, it was found that the mold geometry can significantly affect the distribution of carbon fillers within the microparts, and consequently, the electrical and thermal properties of the entire microparts.

Second, adding two types of carbon fillers concurrently (e.g., carbon black and carbon nanotubes) to polypropylene plastic can result in synergistic improvements to electrical conductivity and thermal properties of the microparts under certain conditions.

Third, using various pretreatment methods to prepare the composite materials before microinjection molding can significantly improve the properties of produced microparts. For example, ‘precoating’ the fillers on the surface of polypropylene powder through solvent blending and ultrasonication resulted in better filler distribution, smaller particle size and more numerous filler particles.

As a result of findings from this research, future commercially produced microparts can incorporate some of the aforementioned strategies to improve the electrical, thermal, and morphological properties of microinjection molded polymer composites.