A molding element having a molding surface forming a contact face and a groove forming rib portion forming the groove comprising two opposed rib side faces and a rib top face connecting two rib side faces. The groove forming rib portion provides at least one space opening to at least one of the rib side faces and/or to the rib top face. An insert is received in the space. The insert comprises a cavity for forming a closing device in the groove. The cavity has two opposed cavity faces and opens to at least two faces of the insert corresponding to the faces of the groove forming rib portion. The insert comprises at least one cutting means extending and/or projecting in a direction the groove forming rib portion extends for cutting a material filled in the cavity during demolding.