In a heat sealing operation for environmentally isolating joints and connections, a spring actuated clamp is utilized to provide positive engagement of the heat shrinkable material in the area between the members being connected. The clamp has two juxtaposed clamp arms with opposed convex clamping jaws. The clamp arms are resiliently loaded by a coil spring so the convex surfaces on the clamping jaws are placed into a clamping relationship on the heat shrinkable material.