In the molding using the vent type injection molding machine, when the melting of the whole amount of the resin carried to the vent port is completed within the first stage, the volatile component can be degassed from the vent port. Therefore, when even a small amount of solid phase resin remains at the time of arrival at the vent port, sufficient degassing cannot be performed, and the characteristics of vent molding are greatly reduced. In injection molding, from the viewpoint of productivity, a predetermined molded article must be molded within a predetermined cycle. For this reason, it is not easy to complete the melting of the entire resin reaching the vent mouth within this fixed time within this restriction. The vent machine needs to set the cylinder temperature exclusively for vent molding, because the cylinder length, the screw shape, the raw material feeding method, and the raw material features are different as compared with the non-empty machine. There is no systematic guideline for the first stage temperature to be set. Therefore, based on experience and skills of individual vent molding technicians, conditions that have been verified and derived every time for each molding environment are now adopted. Therefore, in this study, the influence of various parameters on the plasticizing condition in the first stage was analyzed.