injection molding machines for making plastic articles having a stack mold arrangement frequently suffer from the problem of unbalanced forces . various novel techniques for counteracting and / or avoiding the production of these forces are disclosed . these techniques include the use of latches to lock an injection nozzle to a sprue bar , the use of shutters to lock the sprue bar to a stationary platen , the use of a force compensator in a moving stack mold half , and the use of an injection nozzle axially offset from the sprue bar .