A method (600) for sealing together a first section (132, 136) and a second section (134, 138) of a piece (100) of packaging material (200) is presented. The method comprises arranging (602) the piece (100) of packaging material such that the first (132,136) and second section (134,138) lie against each other, melting (604) at least one plastic layer (210, 214, 216) in the first (132, 136) and second section (134, 138) by feeding the piece (100) of packaging material through a heating station (302), wherein the heating station (302) comprises at least one ultra-sonic horn (306) placed on a first side of the piece (100) and a first group (308) of anvil elements placed on a second side of the piece (100), wherein the first side is opposite to the second side, wherein the first group (308) of anvil elements is fixedly positioned with respect to the at least one ultra-sonic horn (306), and cooling (606) down the piece (100) of packaging material such that the at least one plastic layer (210, 214, 216) is solidified.
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