for joining two faces of thermoplastics in a penetration beam welding method the covered face is multiply scanned and thus incremently heated by a laser beam directed through the covering face . this preheating is repeated until the melting temperature is attained . in the region of the covered face a melt thus materializes simultaneously over the full seam length which wets the covering face and thus also transfers this face into melt by heat conduction . on completion of material softening the faces are mutually moved until abutting . thus the two faces weld in principle as if they had been plasticized simultaneously over the full seam length which significantly enhances the seal and strength of the seam , whilst the expenditure the apparatus is low .