Abstract: A lost core process for removing an overmoulded lost core from within a hollow product which has been overmoulded over the core, wherein the core is melted and removed by a vapor contact method. Heat is transferred to the core by condensation of vapor of a heat transfer fluid, which has a boiling point higher than the melting point of the core and lower than the melting point of the product. The heat of condensation transferred to the core by the vaporized heat transfer fluid melts the core which then flows from the hollow part. This lost core process allows the manufacture of high quality surfaces on the insides of hollow parts, while permitting close wall tolerances to be maintained, and may be used to manufacture complex parts such as water pump housings and intake manifolds.