Developer crossmixing apparatus

A crossmixing device for ensuring adequate mixing of developer in an electrostatic printing machine that includes a sump for holding said developer comprising developer moving means external to said sump for removing developer from said sump and shifting it in opposite directions above two adjacent portions of said sump in a direction transverse to the path at which the developer is removed from the sump and returning developer to said sump, said moving means comprising a screw conveyor external to said two sump portions for moving said developer, said screw conveyor having a left-hand thread on one portion thereof adjacent to one of said sump portions and a right-hand thread on another portion thereof adjacent to the other of said sump portions so that said developer moves in opposite directions along said screw conveyor, the movement of developer in each screw conveyor portion being opposite to said shifting movement of developer above the adjacent sump portion, said screw conveyor including an inlet to and an outlet from each portion thereof, the inlet and outlet of each screw conveyor portion being spaced from each other in a lengthwise direction along said conveyor, each inlet receiving developer material at the terminus of said shifting movement above an individual one of said sump portions, the outlet from each screw conveyor portion directing developer into a region of said sump adjacent the beginning of said shifting movement along the other one of said sump portions.