A leister welder will be much better than that chinese thing.. 20mm seam is common for billboard flex. So it might be your heat and speed settings.. proper welds should tear the coating off the scrim. test each type of material before welding.thanks for the tip. I ordered banner welder from china 2 years ago and it was .. fucking waste of money. It was broken at arrival, we fixed it but .. it was used.. even with best possible seam, it's 20mm wide and will fail. The smell is horrible and very toxic. Oracal's hemming tape works somehow but.. This must be better for heavier loads and bigger banners.