Volume element printing system with printhead groups of varying vertical displacement from substrate

1. A volume element (voxel) printing system for printing a three-dimensional object, the system comprising:

a first printhead group having a plurality of first printheads configured to print a first voxel layer on a substrate;

at least one second printhead group having a plurality of second printheads downstream from the first group, the second printhead group configured to print a subsequent voxel layer on at least part of the first layer, said second printheads being reconfigurable as backup printheads for the first printheads in case of failure;

a conveyor configured to convey the substrate past the printhead groups; and

a control system configured to control and monitor the printhead groups, the object insertion device, and the conveyor, and dynamically to reconfigure the second printheads upon detection of failure of a first printhead, wherein

the at least one second printhead group is vertically spaced from the substrate by a greater distance than the first printhead group.

2. The printing system of claim 1 , wherein each printhead includes a plurality of micro-electromechanical nozzle arrangements each configured to eject a voxel, said voxel selected from the group consisting of a polymer, a metal, a metal alloy, a wax, an evaporative drying material, and a compound.

3. The printing system of claim 1 , wherein the printhead groups are configured to print simultaneously such that a plurality of different layers are simultaneously printed as the substrate is conveyed past the printheads.

4. The printing system of claim 1 , wherein the printhead groups are configured not to print on predetermined parts of a preceding layer to define a cavity for receiving a pre-formed object.

5. The printing system of claim 4 , further comprising an object insertion device configured to insert an object into the cavity.

6. The printing system of claim 1 , further comprising at least one curing assembly interspersed between the printheads to perform a curing process.

7. The printing system of claim 6 , wherein the curing process is selected from the group consisting of evaporative drying, freezing of voxels, ultra violet curing, printing of reagents, printing of catalysts, and printing of polymerization initiators.