A computer-controlled system for forming composition-controlled objects using 3D printing includes two or more liquid reactant reservoirs, and a mixing sub-system for mixing the two or more liquid reactant compositions, which in turn includes a flow control sub-system to control a mass ratio of the mixed two or more liquid reactant compositions. The computer-controlled system further includes a scanning sub-system that, under control of the computer, causes relative motion of a mixed liquid reactants nozzle over a substrate； thereby depositing the mixed liquid reactant compositions onto the substrate. The system still further includes an illuminations system, operated under control of the computer, to polymerize the deposited mixed liquid reactant compositions.
