The plastic injection molding is widely used for the manufacturing of complicated-shaped and high value-added products. The plastic injection molding machine is the most important equipment for the industry. This paper focuses on the mold separation (MS) of the injection molding machines. The characteristics of the MS of injection molding machine with different clamping units are systemically analyzed, the five-hinge joint-double toggle clamping unit and the direct hydraulic pressure clamping unit included. The study points out there are several typical differences between the different clamping units. Meanwhile, it shows there is a linear relation between the MS value and the product weight, and explains why the MS signal can be used to control the injection molding process.