Reduction of polymerization inhibitor irregularity on additive manufacturing windows

A method of reducing irregularity of an oxygen gradient across the window of a bottom-up additive manufacturing apparatus during production of a three-dimensional object thereon, comprising:

• (a) producing said object from a polymerizable liquid on a window, said window comprising:
• (i) an optically transparent semipermeable member, on which semipermeable member said three-dimensional object is produced;
• (ii) a fluid supply bed in or adjacent said semipermeable member, from which supply bed a polymerization inhibitor is fed through said semipermeable member;
• (iii) at least a first and second fluid orifice connected to said supply bed on opposite sides thereof and in fluid communication with one another through said supply bed;
• while
• (b) feeding a fluid comprising said polymerization inhibitor through one of said orifices, optionally while drawing a vacuum through the other of said orifices;
• and
• (c) periodically reversing the flow of the fluid through said supply bed.