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.
EFI USA has welded millions of linear feet of HDPE seams across 500+ installations since 1993. Our welding protocols are based on three decades of field experience, ASTM standards, and continuous improvement driven by destructive test data. This guide explains the welding methods we use, when each is appropriate, and the quality control protocols that ensure every seam meets performance requirements.
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 range: 350-450 degrees Fahrenheit at the wedge surface, depending on ambient temperature, liner thickness, and travel speed.
- Travel speed: 6-12 feet per minute for standard 60 mil HDPE. Speed is adjusted to maintain proper heat input -- too fast creates cold seams with insufficient fusion, too slow creates overheated seams with degraded material properties.
- Overlap: 4-6 inches of panel overlap is standard. The inner 3-4 inches are welded, with 1-2 inches of exposed overlap beyond the seam.
- 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: Must be the same resin type and formulation as the liner material. Cross-contamination between different HDPE formulations causes weak seams.
- Surface preparation: The seam area must be ground (abraded) immediately before welding to remove surface oxidation. HDPE oxidizes within minutes of grinding, so welding must follow grinding without delay.
- 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: The extruded bead should be 1-1.5 inches wide with a smooth, convex profile and visible squeeze-out on both edges. An undercut or concave profile indicates insufficient heat or material.
- 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 cut into standard 1-inch-wide strips and tested in a field tensiometer for both peel and shear strength. Acceptable results must show film-tearing bond (FTB) -- meaning the base material fails before the seam does. If trial welds do not achieve FTB, machine settings are adjusted and new trial welds are performed until acceptable results are obtained. No production welding proceeds until trial welds pass.
- Frequency: At the start of each welding shift, after each break, whenever ambient temperature changes by more than 10 degrees Fahrenheit, and whenever a machine is adjusted or serviced.
- Testing: Minimum 5 peel specimens and 5 shear specimens per trial weld set. All must achieve FTB.
- 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
Every linear foot of weld in an EFI installation is non-destructively tested. The method depends on the weld type.
- Air pressure testing (dual-track seams): The air channel between the two weld tracks of a hot wedge seam is pressurized to 25-30 psi using a needle inserted at one end, with the far end sealed. Pressure is monitored for 5 minutes. Any decay exceeding 2 psi indicates a defect that must be located, repaired, and retested.
- Vacuum box testing (extrusion welds): A transparent box with a gasket seal is placed over the extrusion weld. Soapy water is applied, and a vacuum of 5 psi is drawn inside the box. Any bubbles indicate a leak. The entire length of every extrusion weld is vacuum box tested in overlapping sections.
- Spark testing (thin liners): For liners under 40 mil, a high-voltage spark tester can detect pinholes and thin spots in seams. A conductive wire is run along the seam, and any arc from the electrode to the wire indicates a defect.
Destructive Testing
Destructive testing provides the ultimate verification of seam strength. Samples are physically cut from production seams and tested to failure in a calibrated tensiometer. EFI's protocol requires destructive test samples at intervals of every 150 linear feet of hot wedge seam and at designated locations on extrusion welds.
Each sample is cut into 10 specimens -- 5 for peel testing and 5 for shear testing. Results must meet the requirements of ASTM D6392 (formerly ASTM D4437). Minimum acceptable values depend on liner thickness and material specification but generally require peel strength of 62 ppi and shear strength of 90 ppi for standard 60 mil HDPE. All specimens must achieve film-tearing bond.
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.
EFI USA's welding crews are IAGI-certified with a minimum of 5 years field experience. Every installation includes a comprehensive CQA program with independent third-party oversight. Contact us to learn more about our welding and quality assurance capabilities.
