A heater chip of a thermal ink jet printer has its entire surface, which is opposite the parallel surface having resistors for heating ink supplied from a cartridge body to nozzles in a nozzle plate, supported by and engaged with a surface of a base of a high thermally conductive radiator. The radiator, which is submerged in the ink in the cartridge body, has fins, which preferably have a surface area greater than the surface area of the base, extending upwardly from the base of the radiator. The cartridge body has surfaces, which are exposed to the ambient, with a surface area preferably greater than the surface area of the fins. Heat is transferred from the heater chip to the radiator base and from the fins of the radiator to the ink. The ink transfers heat to the ambient through the cartridge body.