The apparatus includes a container (20) having a base (22) and sides (32) adapted to receive a plurality of plastic gel cassettes. An inlet port (12) is positioned in the base of the container and in fluid communication with the chamber and a baffle (11) is positioned over the inlet port, such that, in use, when gel forming fluid passes through the inlet port into the chamber, the baffle substantially reduces fluid turbulence and vertical fluid movement in the vicinity of the inlet port during flow of the fluid into the chamber. Pretreatment of the plastic cassette to remove polymerisation inhibitors prior to filling the same with fluid is by exhaustive vacuum treatment, optionally with nitrogen gas purging. This can be achieved conveniently using a vacuum chamber in which one or more plastic cassettes are placed. Optionally the vacuum chamber may be the container in which the cassettes are filled with fluid. No barrier films or chemical scavengers are required.