A dual platen hat-tool welding machine, in which the temperatures of the two hot-tool surfaces can be independently controlled, was used to study the weldability of bisphenol-A polycarbonate. In these experiments, the outflow in the melting phase was controlled by means of stops, the thickness of the molten film was controlled by the heating time, and the outflow during the final joining phase was also controlled by displacement stops. Strength data for butt welds are reported for a series of tests - both on dried and undried specimens -in which the hot-tool surface temperatures, the heating times, and the displacement stop positions were varied, but the pressure was not. Weld strength data are reported for three specimen thicknesses. It is shown that very high weld strengths can be achieved, even in the undried material.