Title: Extrusion Welding

Geomembrane Extrusion Welding

Extrusion welding is the main finishing technique for HDPE and other polyolefin geomembrane materials.

Extrusion welding is a finishing technique where a bead of molten plastic is used to weld thermoplastic geomembrane materials. The quality of an extrusion weld is dependant on the skills of the welder operator. Layfield’s staff are uniquely skilled to provide your extrusion welding requirements on all types of thermoplastic geomembranes.

Extrusion welding machines use a heated barrel with a plasticating screw to deliver molten plastic to a specially shaped shoe. The shoe is shaped to make a bead of plastic suitable for the material type and thickness being welded. A hot air blower is attached to the extrusion welder to preheat the weld area. The welder uses its own weight to create the pressure required for welding and the operator steers the welder manually. Extrusion welders come in a number of sizes with different output rates.

Extrusion welding is most commonly performed on HDPE sheet materials. Extrusion welding is also commonly used on Enviro Liner and Polypropylene. Extrusion welding of other thermoplastic materials is not common.


1 Oct 2010 Extrusion (Repair) Welding
Material Styles
High Density Polyethylene (HDPE) 40 mil, 60 mil, 80 mil, 100 mil
40T1 mil, 60T1 mil, 80T1 mil, 100T1 mil
Polyvinyl Chloride (PVC) Solvent or hot air welds
Enviro Liner® 30 mil, 40 mil
Polyproplyene (PP) 36(S)2 mil, 45(S)2 mil
Reinforced Polyethylene (RPE®) Not applicable
Geosynthetic Clay Liner (GCL) Not applicable
HAZGARD® HAZ535, rest of the series can be solvent or hot air welds
Arctic Liner® Solvent or hot air welds
XR-5® Hot Air or special adhesive
Specialty Liners Hot air welds
Note1: T refers to a textured geomembrane material (one side or two sided).
Note2: S refers to a supported fabric scrim in the material.

Extrusion welding is a thickness limited process. The amount of heated plastic required to make a weld may melt through thin sheets. Extrusion welding requires a balance between temperature and the thickness of the welding bead. If the bead is too thick, or the welding temperature is too hot then distortion of the sheet can occur. Generally speaking, the thicker the material the less distortion will occur and the better the extrusion weld. Some materials, such as PP, do not require as high a temperature for welding. This allows extrusion welding to take place in thinner materials. Welding shoe profiles are also adapted to the thickness of the material being welding with an almost flat shoe being used for thin sheet. Special welding equipment that produces a small welding bead may be required to weld thin film materials.

Extrusion welding around a pipe penetration

Extrusion welding is a slow process and is normally limited to finishing work, cross seams, and tie-ins. An extrusion welder is required on all HDPE projects in order to do the finishing work that a wedge welder cannot perform. Normally wedge welding would be used for all production welds and extrusion welds would be used for pipe penetrations and finish welding. An extrusion welder requires a minimum clearance in order to operate. In pipe penetrations and sumps there needs to be a minimum of 600 mm (24 inches) of clearance underneath the pipe in order for welding to take place.

Before extrusion welding begins a trial weld is prepared on separate pieces of project material under site conditions. This qualification weld is used to confirm the welder’s settings. A short section of weld is produced (about 1m) and coupons are taken and destructively tested. If the testing meets project specifications then welding can proceed. If the tests fail the welder is adjusted and re-qualified.

Extrusion welds are also routinely tested using destructive testing. Layfield’s crews are equipped with field portable tensiometers for field destructive testing. Wedge welds are tested in shear and in peel. Shear testing is the basic test of strength across a weld while peel testing is used to confirm weld quality. Due to the arrangement of an extrusion weld peel testing can only be done from one side. Specifications for shear and peel tests are contained in the geomembrane specifications booklet and in this section in the HDPE specification section.

Completed extrusion welds are normally tested with a vacuum box. The seam is flooded with soapy water and a clear box placed over the top and a vacuum applied. Defects will show up as a stream of bubbles in the vacuum box. For details on vacuum box testing please see the Layfield tech note on testing.

Field welding requires that the surfaces to be welded are clean and dry. There can also be limits due to temperature and other environmental factors. Extrusion welders can be sensitive to the presence of moisture in the weld area but the preheat blower tends to keep the weld area dry in most conditions. Extrusion welding in extremely cold weather may be limited; please see the Layfield tech note on cold temperature welding for more details.

Preparation for extrusion welding includes cleaning and grinding the areas to be welded and then tacking the patch or overlap using hot air hand welding. The tack weld holds the material in place during grinding and welding operations. All patches should have rounded corners.

Extrusion welding of HDPE sheet requires grinding prior to welding. HDPE films oxidize at the surface when exposed and the extrusion welding technique requires that this layer of oxidation be removed. Extrusion welds are prepared by grinding with and abrasive wheel immediately before welding. Grinding should remove no more than 10% of the thickness of the liner material. The direction of the grinding marks is not significant, both parallel and perpendicular grinding have been shown to be effective providing that the grinding marks do not extend beyond the weld area. Preparation of the weld area by grinding should take place no more than 45 minutes prior to welding. Surface grinding is not always required on PP or Enviro Liner materials (dependant on site conditions) however roughening with a wire wheel is common practice.

Extrusion welds are used for tie-ins to sumps

Because of the large mass of material that is extruded in the extrusion welding bead there are potential problems with stress concentrations during cooling. To prevent stresses from building up in an extrusion weld there are two restrictions. The first restriction is that extrusion weld beads are not to be artificially cooled with air or water. The second restriction is that no more than two extrusion beads may be placed beside each other.

Extrusion welding is very sensitive to the presence of moisture and a common problem is moisture seeping out of the weld overlap during welding. Since extrusion welds are often repairs, there can be significant water that has seeped into the defect prior to welding. In certain cases it can be very difficult to control this water so that a weld can be made. 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 tacked and welded normally. The sacrificial patch is left under the completed patch.

If you have any questions regarding extrusion welding or field installations please contact your Layfield representative.