Process and apparatus for molding polyurethane parts

Production of moulded polyurethane parts by the RIM technique involves subjecting the reaction mixture in the mould cavity to a pressure high enough to prevent the formation of sink marks, pinholes and air bubbles, by feeding a pressurising medium into an overflow cavity connected to the mould cavity or into the gate channel. A method for the production of moulded polyurethane parts by the reaction injection moulding (RIM) technique in which isocyanate and polyol components are fed into a mixing chamber, mixed together and then transferred through a gate into a mould cavity. In this method, the polyurethane reaction mixture in the cavity is subjected to a pressure high enough to close any defects caused by reactive and/or thermal shrinkage and also any air bubbles, by means of a pressurising medium which is fed into an overflow cavity with a hydraulic connection to the mould cavity and/or into the gate channel. An independent claim is also included for apparatus for use in this method, comprising a mixing head (5; 45) with a mixing chamber and a mould (11; 31; 51) with a cavity (9; 29; 49) hydraulically connected to the mixing head by a gate channel (7; 47) and also to an overflow cavity (3; 23; 43) with a valve for injecting a pressurising medium near the overflow cavity and/or the gate channel.