pressure on the injection unit in the injecting process

[mohammdyou]

most of injection machines use some kind of valve in the injection unit (I mean in the carriage, the whole part that moves cylinder forward and backward) that the oil pressure should keep the unit in the time of injection process. is there any reason that they don't use a valve to close the oil route instead of steadily use the oil to push forward the carriage?
I hope that I explain it correctly

[rickbatey]

It really depends on how much oil the system can pump and how much the press demands to inject at the speed you've set. Some machines bleed some injection pressure off to carriage cylinder during injection. Others pressurize the cylinder and then lock it in by closing the valve and using the pump(s) solely for injection and plasticizing. Again it depends on OEM and the type and options ordered on a press.
Rick.