In the first paper of this series, the injection moulding flow of a commercial polybutylene terephthalate (PBT) with high level of short grass fibres was studied in actual processing conditions. Several moulds for tensile or fatigue test pieces were used in order to investigate through the short shots technique how the molten material is filling the cavity. Except when there is no gate, jetting was systematically observed, providing no obstacles within the cavity (wall, insert) break the flow. Jetting is related to the suppression of post-extrusion swelling due to fibres oriented along flow direction.