Slab gel cassettes for use in electrophoretic analyses of biologically-derived mixtures are equipped with features that prevent the plastic walls of the cassettes from bowing outward when gels retained within the cassettes swell, as gels typically do during casting, storage, or both. Preferably, the fluoropolymer fiber is present in an amount less than about 25 weight %, and in alternative embodiments less than 20 weight %, 15 weight %, 10 weight %, and 5 weight %. The container file also comprises meta data providing an association between the media source blocks and the respective FEC reservoirs. The apparatus controls the actuator in response to the sensor. The porous area can, for example, be created by dissolving granulate components from a fine-grained granulate blend. Press units equipped with second transfer robots, respectively.