An improved weld is formed between plastic sheets, by performing the steps that include:(a) deforming two joint defining work sheets into a volumetric position substantially defined by two undeformed sample sheets positioned in edge-to-edge position, the work sheets and sample sheets having the same thickness, and(b) heating the deformed work sheets under pressurization at said joint to soften them to form an improved weld therebetween characterized in that the sheet thickness remains the same at and proximate the weld.