The performance of any HDPE liner system is ultimately determined by the quality of its seams. The liner sheet itself is manufactured under factory-controlled conditions with consistent material properties and rigorous quality testing. But once that material arrives on a job site, it must be joined together in the field -- exposed to wind, temperature swings, dust, moisture, and the variability inherent in any field operation. This is where installations succeed or fail.
This guide explains hot-wedge and extrusion welding and the tests used to evaluate field seams. The approved project specification and construction quality-assurance plan establish the procedures and acceptance criteria.
Hot Wedge (Fusion) Welding
Hot wedge welding is the primary method for joining HDPE liner panels along long, straight seams. A machine-driven heated metal wedge is inserted between two overlapping panels, melting the inner surfaces of both sheets simultaneously. Trailing pressure rollers immediately press the molten surfaces together, creating a fusion bond as the material cools and re-crystallizes.
The hot wedge machine produces a dual-track seam with an unbonded air channel between the two weld tracks. This air channel is not a defect -- it is an engineered feature that allows 100% non-destructive testing of every inch of seam via air pressure testing. The dual-track design is one of the key advantages of hot wedge welding over single-track methods.
- Temperature and travel speed: Follow the equipment and material manufacturer’s guidance, then qualify settings using trial welds under production conditions.
- Overlap: Use the approved seam geometry and overlap for the equipment and material, with sufficient access for welding and testing.
- Nip pressure: Trailing rollers apply controlled pressure to achieve full contact between the molten surfaces. Pressure is adjusted based on liner thickness and temperature.
- Applications: All long-run field seams, slope seams, base seams, and any seam location accessible by the welding machine.
Extrusion Welding
Extrusion welding is a manual welding process used for detail work, repairs, and locations where the hot wedge machine cannot operate. A handheld extrusion welder feeds HDPE welding rod through a heated barrel, where it is melted and extruded through a die as a continuous bead of molten resin. This bead is deposited over the seam area, fusing with both panels to create a single-track weld.
- Welding rod: Use filler material approved for the geomembrane and welding procedure.
- Surface preparation: Clean, dry and prepare the weld area according to the approved extrusion-welding procedure.
- Preheat: A hot air gun preheats the seam area immediately ahead of the extrusion bead. This ensures the base material reaches fusion temperature before the molten bead is deposited.
- Bead profile: Inspect the completed weld against the approved procedure and repair identified defects.
- Applications: Pipe boots, corner details, patches, repairs, T-joints, cross seams, and any location requiring manual welding.
Trial Welds: The Foundation of Quality
Before any production welding begins, and at regular intervals throughout the day, trial welds are performed on scrap liner material. Trial welds serve as proof that the current machine settings and ambient conditions produce seams that meet strength requirements. They are the most important quality control step in the welding process.
Trial weld specimens are tested in peel and shear before production welding proceeds. Evaluate seam strength, shear elongation, peel separation and acceptable break patterns against GRI-GM19a for the material, thickness and weld type, together with the approved project criteria. Film tearing alone does not establish acceptance. If results fail the approved criteria, adjust the process and repeat the trial welds.
- Frequency: Make trial welds before production and repeat them as required by the approved procedure when conditions, equipment or settings change.
- Testing: Record peel and shear results and the additional acceptance criteria required by the applicable seam specification.
- Documentation: Trial weld results are logged with date, time, machine number, operator, ambient conditions, and test results. This documentation becomes part of the permanent CQA record.
Non-Destructive Testing
The field testing plan establishes nondestructive coverage and procedures for each seam type, including details that cannot be tested with the main production method.
- Air pressure testing (dual-track seams): Test the channel under the approved ASTM D5820 procedure. Use the specified pressure, stabilization period, test duration and permitted pressure change, and verify channel continuity.
- Vacuum box testing: Use the approved ASTM D5641 procedure for accessible seams and repairs. Apply the specified vacuum and observation period, with overlapping test areas. Mark leaks, repair them and retest.
- Spark testing: ASTM D6365 covers suitable extrusion or tape seams and seams that incorporate conductive material. It is useful where other nondestructive methods are impractical, such as tight corners and pipe details. Follow the approved procedure; sparks must not create a hazard.
Destructive Testing
Destructive testing measures seam properties on samples cut from production welds. The approved specification and quality plan set the sampling frequency and locations. Test the specimens using the applicable method and compare results with the specified acceptance criteria.
ASTM D6392 describes destructive peel and shear testing of nonreinforced thermofusion seams. ASTM D4437 covers nondestructive seam testing; it is a separate practice. Evaluate seam strength, peel separation, elongation and failure mode against the approved project criteria and the applicable material and seam specifications. Acceptance values depend on the polymer, thickness and welding method; a single generic peel or shear value is not sufficient.
If any destructive test fails, the seam is considered suspect from the last passing test location to the next passing test location. The suspect seam section is repaired by cap-stripping (welding a new strip of HDPE over the original seam) and retested. The failed test triggers additional destructive tests at closer intervals in adjacent seam areas.
Common Welding Defects and Prevention
- Cold seam: Insufficient heat input results in incomplete fusion. Prevented by proper trial weld verification and continuous monitoring of machine parameters.
- Overheated seam: Excessive heat degrades the HDPE polymer chain, reducing long-term strength. Prevented by controlling temperature and travel speed within the qualified range.
- Moisture contamination: Water trapped in the seam creates voids and weak spots. Prevented by thorough drying of seam surfaces immediately before welding.
- Contamination: Dirt, dust, or debris in the seam area prevents proper fusion. Prevented by cleaning and grinding seam surfaces before welding.
- Stress cracking: Caused by excessive residual stress from welding combined with environmental stress factors. Prevented by proper parameter control and post-weld stress management.
Before installation, agree on installer qualifications, trial welds, production testing, repair procedures, records and any independent quality-assurance oversight required by the project. Contact EFI to review the installation and testing scope.
