Techniques for producing a flat film surface in additive fabrication and related systems and methods

A method of additive fabrication, the method comprising:

• moving, within an additive fabrication device along a first axis, a movable stage laterally beneath a container having a base that comprises an upper flexible film and a lower flexible film, wherein the upper and lower flexible films are suspended over an opening and at least part of the upper and lower flexible films are arranged to sag below a build plane, the container holding a liquid photopolymer, the movable stage comprising an energy source and first and second members arranged on either side of the movable stage, wherein during said movement of the movable stage the first and second members push the upper and lower flexible films upwards to produce a flat region of at least the upper flexible film at the build plane between the first and second members, and wherein the first and second members extend above the energy source to maintain space between the upper and lower flexible films and the energy source;
• and
• directing actinic radiation from the energy source of the movable stage between the first and second members and through the flat region of the upper flexible film at a plurality of selected positions along a second axis, perpendicular to the first axis, to the liquid photopolymer held by the container, thereby forming a layer of solid material that contacts the upper flexible film in addition to the surface of the build platform and/or to a previously formed layer of material.