Membrane presses use a sealable bag and some form of vacuum generator. By placing materials in the bag, sealing it and then sucking out most of the air, a pressure differential is created between the reduced pressure inside the bag and atmosphere on the outside. It is this pressure differential that is used in vacuum pressing. The second style of vacuum press uses a membrane fitted to a hinged frame attached to a rigid vacuum bed through which the air is removed. A FramePress can be a more convenient solution if you are regularly pressing large panels.