A dual platen hot-tool welding machine, in which the temperatures of the two hot-tool surfaces can be independently controlled, is used to study the weldability of the three dissimilar thermoplastics-polycarbonate, polyetherimide, and poly(butylene terephthalate) to each other. In these experiments, the outflow in the melting phase is controlled by means of stops, the thickness of the molten him is controlled by the heating time, and the outflow during the final joining phase is also controlled by displacement stops. Strength data for butt welds are reported for a series of tests in which the hot-tool surface temperatures and the melt (heating) times were varied, but the displacement stop positions and the weld pressure were not. High weld strengths are demonstrated for each pair from these three dissimilar thermoplastics.