Sheet welding of PE and PP: butt welding in sheets and components

The same principle as pipe welding

Sheet welding is the butt welding of two sheet edges. The edges are planed flat, heated against a heating element and pressed together while the material is soft. The principle and temperatures are the same as for pipe: PE is typically welded at 200-220 degrees, PP at approx. 210 degrees, depending on grade and manufacturer.

The difference lies in the components. Sheets are wide, heavy and cannot be clamped in a pipe welding machine. They must be secured in a frame or on a welding table so that the edges are precisely aligned opposite each other and do not distort during cooling.

For pipes in PE, PP and PVDF, socket fusion is used instead in the smaller diameters, and butt welding or electrofusion in the larger ones. Sheet welding and socket fusion share temperature and principle, but the tooling is different.

If you are working with PVC-U, other conditions apply: lower heating element temperature, short changeover time and extraction. Read how in butt welding of PVC.

Preparation of the sheets

The edges must be cut clean and straight, and they must be planed or milled so that the surface is free from oxide layers and saw marks. A sheet cut with a circular saw has tool marks from the teeth that do not disappear during heating. Use a planing tool or milling cutter intended for the purpose.

Sheets must lie flat and be supported. Stresses from storage and transport are relieved by letting the sheet acclimatise to room temperature before processing. If the sheet is warped, the seam will be distorted regardless of how precisely the tooling operates.

The thickness dictates the choice: thin sheets below approx. 3-4 mm are difficult to butt weld because the material is easily deformed by the tooling, and they are therefore often welded using hot gas or extrusion instead.

The welding process

The sheets are placed in the clamping frame with the necessary gap, the heating element is inserted, and the edges are heated until an even bead has formed. The element is removed quickly, the edges are brought together, and pressure is maintained until the material has solidified sufficiently to hold its own weight.

Times and pressure follow the same structure as for pipe: heating, changeover, pressure build-up and cooling. For thicker sheets, the cooling time is extended considerably because the heat must dissipate through the entire cross-section. The sheet must not be moved or loaded until the cooling time has elapsed.

Large components such as tanks and ducts require support throughout the entire process, and the joint must cool in the position in which it is to be used. Otherwise, subsequent loading may open the seam.

V-groove and extrusion welding

For repairs and for joints where butt welding is not possible, extrusion welding in a V-groove or a double V-groove is used. The groove is milled or cut, cleaned and preheated, and welding rod of the same material is fed in under pressure. The method requires the same preparation as for pipe, see extrusion welding in practice.

When repairing an existing seam, the old seam must be completely removed in the area being repaired. Otherwise, a zone with incorrect joint profile will remain beneath the new seam, which may subsequently open up.

Inspection of sheet joints

Inspection follows pipe welding: visual inspection of the bead, measurement of bead width and a bend or tensile test from a test joint. The test coupon is cut out across the seam so that the entire seam lies within the coupon.

For tanks and vessels, leak testing is added, either as water filling or as vacuum and pressure testing depending on the component. The testing is documented together with the welding log, see DVS 2203 and testing and testing and documentation.