Method for joining sections of thermoplastic continuous web material

A method for joining multiple pieces of thermoplastic material to form a cellular block, comprising:

• positioning a new piece to be added to the cellular block in opposing face-to-face relation with a last piece most recently previously welded to the cellular block;
• and
• forming multiple spaced apart and elongate welds between the new piece and the last piece while the new piece and the last piece are in the opposing face-to-face relation, so that each of the welds extends in a longitudinal direction and the welds are spaced apart from one another in a lateral direction,
• the forming step comprising positioning a heating device, having a plurality of heating fingers that extend in the longitudinal direction and are spaced apart in the lateral direction, between the last piece on the block and the new piece to be joined to the block and in contact with the opposing surfaces of the new piece and the last piece and causing the heating fingers to heat the new piece and the last piece at locations contacted by the heating fingers, and retracting the heating fingers from between the new piece and the last piece and then pressing the new piece against the last piece such that the new piece is welded to the last piece at said locations.