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Extrusion welding is the main finishing technology for polyolefin geomembrane materials such as HDPE. Extrusion welding is a finishing technology in which molten plastic beads are used to weld thermoplastic geomembrane materials. The quality of extrusion welds depends on the skill of the welder operator.
Extrusion welders use a heated barrel with a plasticizing screw to deliver molten plastic into a specially shaped welding shoe. The shape of the welding shoe can be made into plastic beads suitable for the type and thickness of the welding material. A hot air blower is connected to the extrusion welder to preheat the welding area. The welder uses its own weight to generate the pressure required for welding, and the operator manually manipulates the welder. Extrusion welders come in various sizes and different output rates. Extrusion welding is often performed on HDPE sheets. Extrusion welding is also commonly used for polypropylene. Extrusion welding of other thermoplastic materials is not common.
Extrusion welding is a thickness-limited process. The amount of heated plastic required to perform the welding may melt through thin sheets. Extrusion welding requires a balance between temperature and weld bead thickness. If the weld bead is too thick, or the welding temperature is too high, sheet deformation may occur. Generally speaking, the thicker the material, the less deformation and the better the extrusion weld. Some materials, such as PP, do not require such high welding temperatures. This allows extrusion welding in thinner materials. Welding shoe profiles are also suitable for the thickness of the welding material, and nearly flat welding shoes are used for thin sheets. Welding film materials may require special welding equipment that produces small welds.
Extrusion welding around pipe penetrations
Extrusion welding is a slow process, usually limited to finishing work, cross seams and connections. All HDPE projects require extrusion welders to complete the finishing work that wedge welders cannot accomplish. Usually wedge welding will be used for all production welds, while extrusion welds will be used for pipe penetrations and completion welding. Extrusion welders require a small gap to operate. In pipe penetrations and sumps, at least 600mm (24 inches) of clearance needs to be left below the pipe for welding.
Before extrusion welding begins, prepare trial welds on different project materials under field conditions. This qualification weld is used to confirm the welder's settings. A short section of weld (about 1m) is produced and a sample is taken and destructively tested. If the test meets the project specifications, welding can continue. If the test fails, adjust the welder and re-qualify.
Extrusion welds are also often tested using destructive testing. Staff are equipped with field portable tension meters for field destructive testing. Wedge welds are tested in shear and peel. Shear testing is the basic test of cross-weld strength, while peel testing is used to confirm weld quality. Due to the arrangement of extrusion welds, peel testing can only be performed from one side. Specifications for shear and peel testing are included in the geomembrane specification manual and this section of the HDPE specification section.
Completed extrusion welds are usually tested using a vacuum box. The seam is filled with soapy water, and a transparent box is placed on top and vacuum is applied. Defects will appear as bubble streams in the vacuum box.
Field welding requires the surfaces being welded to be clean and dry. There may also be limitations due to temperature and other environmental factors. Extrusion welders may be sensitive to moisture present in the welding area, but preheat blowers tend to keep the welding area dry in most cases. Extrusion welding in extremely cold weather may be limited; for details,
Preparation for extrusion welding includes cleaning and grinding the area to be welded, then using hot air hand welding to fix patches or overlaps. During grinding and welding operations, tack welding holds the material in place. All patches should have rounded corners.
Extrusion welding of HDPE sheets requires grinding before welding. HDPE film oxidizes on the surface when exposed, and extrusion welding technology requires removal of this oxide layer. Extrusion welds are prepared by grinding with a grinding wheel immediately before welding. Grinding should remove no more than 10% of the thickness of the liner material. The direction of grinding marks is not important, as long as the grinding marks do not extend beyond the welding area, both parallel and perpendicular grinding have proven effective. Grinding preparation of the welding area should be performed no more than 45 minutes before welding.
Extrusion welds for connection to oil sumps
Due to the large amount of material extruded in the extrusion weld bead, there is a potential stress concentration problem during cooling. To prevent stress from accumulating in extrusion welds, there are two limitations. The first limitation is that extrusion weld beads cannot be artificially cooled with air or water. The second limitation is that no more than two extrusion beads placed side by side with each other.
Extrusion welding is very sensitive to the presence of moisture, and a common problem is moisture seeping from the weld overlap during the welding process. Since extrusion welds are usually repairs, there may be a large amount of water seeping into the defect before welding. In some cases, it is difficult to control this water for welding. One technique is to place a sacrificial patch over the leaking defect and seal it with tape or other sealant. Then a larger patch is normally fixed and welded. The sacrificial patch remains below the completed patch.
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