Abstract: A three?plate injection mold was designed for the two?color injection molding process of thermometer shell plastic parts. In this mold, there are two functions in the three?plate die holder. One is for meeting the requirement for multi?point pouring of the mould cavity, and the other is for driving a variety of core?pulling mechanisms through the opening of the template to complete the demolding of plastic parts. Aiming at the pouring difficulty in the first and second forming die cavity, a point gate runner along with a side gate runner was set up to meet the requirement for multi?point pouring formation of the die cavity. Aiming at the demolding difficulty in plastic parts, five types of mechanisms were set up to realize the automatic demolding of plastic parts. The first one is a composite mechanism composed of first core pulling of Harvard slider, which is used for the side core?pulling demolding of the outer wall of plastic parts. The mechanism can complete the interference core pulling of Harvard slider by driving the first and second opening of the three?plate die and then driving four Harvard sliders through the third opening of the template. The slider mechanism can complete the automatic demolding of the outer wall of plastic parts twice. The second one is a core?pulling mechanism consisting of the bending pin, which is used for core pulling and demolding in the local area of the upper and outer walls of plastic parts. The third one is a cylinder?driven side core?pulling mechanism. The fourth one is a core?pulling mechanism of double bread glue oblique slider driven by moving template. The fifth one is a reinforced core?pulling mechanism with inclined ejector. The overall structure of the die is reasonable, and the mechanism is set up properly, which can well meet the requirement for automatic production of plastic parts.