SparePart for 3D Printer 3D System ProJet 1500 The MAC 34 series is a high flow, direct operated 10mm poppetvalve. No small pistons with low shifting force and high minimum operating pressures. The MAC short stroke, direct operated, balanced proppet provides consistent operation. The shifting forces are consistently high, regardless of inlet pressure fluctuations. The balanced, high shifting forces provide very fast and repeatable response time.