A portable system provides a high-strength joint between two portions of radiant energy transmitting thermoplastic strap having an opaque radiant energy absorbing area on one surface thereof wherein the strap portions are overlapped so that the opaque area is at the interface therebetween. The overlapped strap portions are urged together by being tensioned over an arcuate guide surface. Infrared energy is directed through a free uncontacted outer surface area of one of the strap portions and onto the opaque area for melting the adjacent strap surfaces to form a joint.