Choosing the proper method for welding geomembranes
Choosing the proper method for welding geomembranes often depends on consideration of several factors. This article discusses the most common welding techniques and when they should be used. When considering the process of welding two materials, we must understand how the materials are welded at the molecular level. In order to weld two compatible materials together, their two surfaces must be softened in some way, and to a certain depth so that the two materials mix and fuse as one. For most plastics, and therefore most geomembranes, this can be done by two methods; chemically or thermally.
When plastics are thermally welded, the two surfaces are softened by heat and then brought together, causing the two materials to mix and “weld” together as they cool. When chemically welding the materials together, instead of using heat, a solvent is used to smooth the two surfaces. When pressed together, the two surfaces mix, and instead of cooling, the solvent evaporates leaving the two materials ‘welded’.
While the procedures are quite different, the end result is essentially the same. Many other factors must be considered before selecting the appropriate method, such as ambient temperatures, the similarity of the two materials, and the scope of the project.
Hot air welding
Hot air welders are small, lightweight welders that, as the name implies, use hot air to join materials. This welding technique is suitable for all common geomembranes including PVC, urethane, HDPE, polypropylene, and LLDPE. The two overlapping layers of geomembrane are exposed to hot air that melts their surfaces, then they are pressed together to form a strong bond. Hot air welders are typically used for detail work or welding materials in the field.
Welding Geomembranes Extrusion
Extrusion welders are larger, heavier welders that use a plastic welding rod forced down into a barrel heated by an internal screw. The welding rod is softened and mixed in the heated barrel, and then pushed out as a bead of molten plastic directly onto the geomembrane liner. This bead of molten material is formed by a Teflon shoe on the edge of the overlapping materials. Hot air is blown over the overlap in front of the molten plastic to soften the material and form a stronger weld. The welding rod used by the extrusion welder must be selected to be compatible with the geomembrane as it is often made from the same batch of resin. This style of welding is used extensively on HDPE, polypropylene, and LLDPE materials.
This type of welding is only applicable to geomembranes that can be adequately softened with a solvent, restricting its use primarily to PVC and urethane-based geomembranes. An advantage of chemical welding is its ease of application, requiring little more than a brush and the right solvent. However, it is extremely subject to environmental conditions, low temperatures or high humidity can greatly affect the quality of a chemical weld.
Wedge welders are smaller, hand-held machines that weld two overlapping sheets of material. The welder heats between the two materials with a “wedge” of heated metal and then presses the materials together to form a weld. Thermoplastic materials fuse at the molecular level. Wedge welders run at a relatively high speed between clean and overlay material and are used for long straight welds. More complicated jobs, such as bending or laying pipe sleeves, should not be done with a wedge welder; Detail work like this should be done with extrusion or hot air.
Many geomembrane installations require operators to use several different welding techniques on the same project.