Wedge and membrane welding: leak-tight joints in geomembranes and sheets

The process in brief

Wedge welding and hot gas welding of geomembranes and films are performed by overlapping two layers of film and feeding heat into the joint via a tool while a roller presses the layers together. Heat is supplied by a heated wedge or hot air, and the joint is formed once the film is plasticised and pressed together during cooling.

These methods are used for sealing membranes, reservoirs, landfills, ponds and site shelters—in other words, large surfaces where a rigid joint is not feasible. The material is typically PE, PP or PVC sheet with thicknesses ranging from a few tenths of a millimetre up to a couple of millimetres.

If the same materials are to be joined as pipes or pressure lines, film welding is not the appropriate method. Instead, socket fusion is used for small diameters, and butt fusion or electrofusion for larger ones.

When working with PVC-U, different conditions apply: lower heating element temperature, short changeover time and extraction. Read how in butt fusion of PVC.

Wedge or hot air

Wedge welding provides high speed and a very uniform seam because heat is transferred directly through the wedge. The temperature is controlled on the machine and typically lies in the range of 400-450 degrees on the wedge, while the travel speed is adjusted to suit the film thickness. This method is suitable for long, straight seams and thicker membranes.

Hot air welding is slower but more flexible and is used for corners, details, repairs and areas where the automatic welding machine cannot reach. Hand-held hot air welding places greater demands on the operator because temperature, pressure and speed must be maintained manually.

For thin films, double seams with a test channel are also used: an automatic machine lays down two seams with an open channel between them so that the channel can be pressure tested after welding. This is an effective inspection method, as any leak in one of the seams will register in the channel.

Preparation and parameters

The surface must be clean and dry. Dust, sand, moisture and grease cause channels in the seam which typically only become apparent during testing. The overlap width is adapted to the film thickness, and the film must lie flat and without tension so that the seam is not distorted during cooling.

The welding parameters are temperature, speed, roller pressure and overlap. They are interdependent: if the speed is increased, the temperature must be raised, and vice versa. Therefore, a test weld must always be performed prior to starting production, and the test must be carried out using the exact same settings as the production run.

Weather and wind affect the outcome. Working in freezing temperatures, rain or strong winds requires either screening or postponement. Read about the conditions in weather and environmental conditions.

Inspection of the seam

Visual inspection identifies open edges, folds and burnt areas. A sharp, uniform seam with a distinct, slightly bulging bead is the desired result. Folded or wrinkled seams must be remade, regardless of the test results.

Vacuum testing, compressed air testing in the test channel and tensile testing are the standard methods. For membranes beneath reservoirs and landfills, leak-tightness is a critical requirement, and testing is documented per seam or per field with corresponding positional data.

Tensile testing of film seams complies with EN 12814-2 for bend testing and EN 12814-1 for tensile testing, depending on what has been agreed. The results are evaluated in conjunction with the fracture pattern: failure within the seam is unacceptable, whereas failure in the parent material outside the seam demonstrates that the seam is stronger than the material itself. See DVS 2203 and testing of welds.

Safety and working environment

Wedge and hot air tools operate at temperatures far above the melting point of the film, and the equipment must be treated as hot tooling: place it on its designated stand and keep the power cable clear of the working area.

When welding PVC film, hydrochloric acid is released, necessitating extraction. When welding in confined spaces, tents and containers, adequate ventilation must be provided, both due to the heat of the equipment and the fumes generated when the plastic is heated.