A pagewidth printhead assembly that includes a support beam assembly having a composite shell of metals selected so that the shell has a coefficient of thermal expansion substantially the same as that of silicon and a core located within the shell and defining a number of longitudinally extending ink reservoirs for containing respective inks. At least one printhead is mounted on the support beam assembly to receive inks from the ink reservoirs the, or each, printhead including a silicon-based printhead integrated circuit.