An infrared light source focuses infrared energy onto a work surface through the open end of a concentrator. The open end of the concentrator includes an elastomeric seal for sealably coupling the end of the concentrator directly to a work piece. Pneumatic pressure is applied through the concentrator and the seal to urge the work pieces together during the heating process to complete the weld. The method of infrared (IR) welding includes the steps of applying concentrated IR energy to at least one thermoplastic member while simultaneously applying pneumatic pressure to the work pieces for welding one member to another.