Patent ID: 8640881

Claim:
A screen changer for apparatus that processes a flowable plastic material, the screen changer comprising: a movable element in the form of a portafilter constructed as a slide plate that is movable along a path between two positions; a hydraulic cylinder for moving the movable element along the path; a supply line for providing hydraulic fluid to the hydraulic cylinder; a first hydraulic pump disposed within a reservoir containing hydraulic fluid, designated a volume pump and having a first volume capacity and a first pressure capacity, for pumping hydraulic fluid from the reservoir through the supply line; a second hydraulic pump disposed within the reservoir, designated a high pressure pump and having a second volume capacity that is lower than the first volume capacity and a second pressure capacity that is higher than the first pressure capacity, for pumping hydraulic fluid from the reservoir into the supply line; a non-return valve in the supply line, the non-return valve selectively opening or interrupting a hydraulic flow path between the volume pump and the hydraulic cylinder; a high pressure line which bypasses the non-return valve and feeds hydraulic fluid from the high pressure pump into the supply line downstream from the non-return valve; a relief valve connected to the supply line upstream of the non-return valve, so that when the non-return valve is closed, the first hydraulic pump delivers hydraulic fluid in a closed bypass loop back to the reservoir, without the hydraulic fluid reaching the hydraulic cylinder, wherein the relief valve is configured to open at a switching pressure, which is adjustable, and the switching pressure prevails in the supply line when the non-return valve is closed and the first hydraulic pump is switched on, and wherein the relief valve is configured to close when the non-return valve opens and pressure in the supply line falls below the switching pressure; two feed and return lines connected to the hydraulic cylinder; and a reversing valve downstream from the non-return valve and fed by the supply line for selectively feeding flow volume created by the first and second hydraulic pumps into one or the other of the two feed and return lines, such that one of the two feed and return lines can selectively serve as the feed line and the other as the return line, whereby the direction of movement of the hydraulic cylinder is determined by selecting the feed line and the return line, respectively.