With respect to the fabrication of metal parts carrying surface relief microstructures using composite reaction molding as a rapid prototyping variant of micrometal injection molding, the influence of microsized iron powder on the viscosity of different unsaturated polyester resins applied as polymer binder has been investigated systematically. The initial binder viscosity determines the viscosity of the composite and hence the accessible critical filler load. A further dilution of the unsaturated polyester resin with styrene allows a significant increase of the filler load at constant shear rate and temperature from 36 up to 57 vol% iron content. A successful estimation of the critical filler load using different established empirical models initially developed for ceramic-filled solutions or dispersions is possible. A pronounced flow activation energy increase at high iron loads was also observed.
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