Pulp Injection Molding (PIM) is a technique for realizing three-dimensional structures made of paper-based material, which is made up mainly of pulp and starch. In this study, we investigated the effects of four types of pulp fiber lengths, the mixture rate of starch and PVA as binder on tensile and bending strength characteristics, and the number-volume-morphology development of internal voids. The results obtained are summarized as follows; (1) The longer the average pulp fiber length, the higher the tensile strength and the lower the bending strength tends to be, (2) Increasing the PVA content was found to resultantly increase the ratio of un-dissolved PVA caused by a lack of water. It was confirmed that tensile and bending strengths peaked around 20 to 30 wt% of PVA content ratio, and decreased thereafter, (3) As for the morphology of the internal voids, short fibers were seen to generate rather spherical voids that were uniformly distributed, while long fibers were seen to generate thin stripe-shaped voids perpendicular to the flow, and (4) The degree of deformation of molded products due to water absorption dropped drastically with increasing PVA content, and under the same PVA content, this dropping tendency became greater in short fibers than in long fibers.

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