3 Plate Plastic Mold
Basically three plate molds has two parting line, and floating plate, floating plate support by support pin, Since the mold has two parting planes, the runner system can be located on one side of floating plate or make special plate that attach in floating plate, we called runner plate, see post about runner plate. Three plate molds are used because of their flexibility in gating location. This types of molds is flexible even use in multiple cavity.
Picture Source: http://www.apiwat.ie.engr.tu.ac.th/molddesign/step2/kouzou.htm