Apparatus and method for making welded honeycomb core

A structural honeycomb core (10,10′) made of crystalline thermoplastic to provide corrosion-resistant, heat-resistant, high-impact structure is fabricated by use of welding heads (40) to inter-welding strips (20-27; 20′-27′) at spaced locations along their lengths, release substrates (30,50) being interposed between the strips to prevent any one weld from joining more than two of the strips. In a first embodiment, the strips are welded while the crystalline thermoplastic is in its flexible state; and the welded web is expanded and heat-treated to convert the material to its rigid state. In a second embodiment, the strips are corrugated and rigidified prior to welding.