Injection moulders typically have a whole bank of heater elements, and though the machine is 3 phase the elements are all single phase with individual controllers. Thus the meter must measure each phase of the 3 phase supply both current and voltage wise and then sum the power for the heaters. As well you will have a large motor that either drives a hydraulic pump or direct drives the extruder auger, and then you might have either air or hydraulic linear actuators that move the moulds into position and operate the auger to inject.
Thus you typically will want to measure power during start up as the machine warms up, then the idle power with it hot, then the power in between cycles when it has a running auger and then the power during the injection stroke. The start up power is likely to be the highest as all heaters are on and warming up, and then the power use during injection will be slightly smaller as the heaters cycle randomly while the motor and actuators are running. If you want total power do not forget the energy used by the air compressor, as this is often neglected, and is quite expensive.