1. An injection moulding machine for injection moulding plastic material and the like, said moulding machine comprising a fixed mould portion, a cooperating movable mould member, a movable piston-operated injection system for injecting moulding material into said mould, a hydraulic system including a variable output pump and adapted to move said movable mould member, means for continuously monitoring the actual velocity of displacement of said movable mould member, means for continuously comparing the monitored velocity of displacement of said movable mould member with a predetermined velocity displacement program for the movable mould member, and means for adjusting the velocity of displacement of the movable mould member to maintain said actual velocity thereof at a value substantially equal to said predetermined velocity of displacement thereof, said movable mould member displacement velocity adjusting means including a servo-valve for controlling the output of said pump, a second hydraulic system including a second variable output pump and adapted to move said movable piston, means for continuously monitoring the actual velocity of displacement of said movable piston, means for continuously comparing the monitored velocity of displacement of said movable piston with a predetermined velocity displacement program for the movable piston, and means for adjusting the velocity of displacement of the movable piston to maintain its actual velocity at a value substantially equal to said predetermined velocity of displacement thereof, said movable piston displacement velocity adjusting means including a second servo-valve for controlling the output of said second pump.