A thermal ink jet pen including a main ink storage compartment wherein a block of foam is completely sealed off from the surrounding ambient. The pen further includes an ink delivery section which is fluidically coupled at one end to the ink storage compartment and has a thin film printhead mounted on an outer surface thereof. The above main ink storage compartment includes a pair of flexible major side walls which begin to collapse as ink and air are drawn out of the pen during an ink jet printing operation. As this happens, and the collapsible side walls are forced into the opposite surfaces of the compressed foam block, and the inherent spring force of the foam block produces an equal and opposite force in equilibrium with these side walls within the ink storage compartment. This equilibrium condition operates to stabilize the negative backpressure within the ink storage compartment at a relatively constant value and prevents the pen from drooling ink.