In order to improve the productivity of injection molding, the screw rotation speed is usually increased to shorten the plastication time and molding cycle. Under these conditions, molding defects may occur due to the non-uniformity of molten resin temperature, and clarification of this phenomenon and countermeasures are required. In this study, the reciprocating plastication process using a full-flight screw was observed in a visualization heating cylinder with the screw rotation speed increased to 350 rpm. From the obtained video images, extended images of the screw channels were created to clarify the solid-bed (SB) shape and to quantify the width of the SB formed. At the high rotation speed of 350 rpm, gaps were formed between the pellets in the screw feed channel, causing the SB subsequently formed in the latter part of the metering zone to split into several SBs with low cohesive force and density. In the compression zone, a SB appeared at the start of the charge process and gradually expanded. After that, it split into several SBs and the total width of these SBs decreased temporarily. At the end of the charge process, the total width increased to the full width of the channel. At 350 rpm, the number of SBs in the compression zone slightly increased and a small number of SBs were observed at the metering zone when the cycle time was 46s compared to 90 rpm. Although the mechanical heat generated increased due to the high-speed rotation, the melting promotion effect by heat transfer decreased due to the shortened rotation time. Furthermore, when the cycle time was shortened to 23s, the time of heating by heat transfer to plasticize the required amount of resin also shortened, resulting in more unmelted resin in each screw zone. This caused the melt to flow out into the reservoir without being melted completely. As described above, the plasticization process during high-speed screw rotation was clarified in detail, and useful knowledge was obtained for countermeasures against molding defects.