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TPU is often described as "PVC without the problems" — no plasticiser to migrate, better low-temperature flexibility, far better abrasion resistance. That is true of the finished product, but it is not true of the welding process. Polyurethane responds to radio frequency energy differently from PVC: it needs more energy to reach welding temperature, it demands considerably higher clamp pressure, and it retains an elastic memory that will pull a hot seam apart the moment the press opens. This guide covers the material families, the process parameters and the equipment requirements for TPU high frequency welding, so that a seam which looks correct on the day of production is still correct a month later.
Why TPU Behaves Differently From PVC
Many processors approach TPU with a PVC recipe and adjust only the weld time. The result is usually a seam that welds, then fails days later. The reason lies in four material differences that each change the process, not just the settings.
Lower Dielectric Response
Flexible PVC has one of the highest dielectric loss factors of any common thermoplastic, which is why it welds so easily. TPU sits lower, so more power density and more time are needed to generate the same heat at the interface.
Higher Softening Range
Depending on grade, TPU softens and flows well above the range at which flexible PVC does. Reaching a true interfacial weld therefore takes longer, and the heat has more time to spread sideways.
Elastic Recovery
TPU is an elastomer. If a seam is still hot when the press opens, the material springs back and the freshly formed weld is peeled apart from the inside. This is the single most common cause of TPU seam failure.
Moisture Affinity
TPU absorbs atmospheric moisture, and polyester-based grades can hydrolyse at welding temperature. Water in the interface converts part of the RF energy into steam and leaves voids in the seam.
| Property | Flexible PVC | TPU | Effect on the Process |
|---|---|---|---|
| Dielectric response | Very high | Moderate | Higher power density and longer cycles for the same seam area |
| Softening range | Lower | Higher, grade dependent | More energy delivered per unit area; greater risk of heat spreading into the part |
| Clamp pressure required | Low to moderate | High | Press tonnage becomes a specification limit, not a convenience |
| Behaviour on press release | Stays put | Springs back | Mandatory cooling under pressure before the press opens |
| Moisture sensitivity | Low | High, especially polyester grades | Drying and preheating move from optional to essential |
| Fumes if overheated | Corrosive hydrogen chloride | Irritant, but not corrosive to tooling | Extraction still required; corrosion-resistant plating less critical |
| Surface embossing | Holds pattern readily | Recovers and dulls the pattern | Pattern must be pressed deeper, or held with a cooled tool |
None of these differences is a barrier to high frequency welding — TPU is welded in very large volumes for medical, footwear, sports and outdoor products. The point is that the recipe and the machine must be specified for polyurethane from the start. For a wider comparison of how these two polymers and PU differ at the molecular level, see the guide on how PVC, TPU and PU material properties affect the welding process.
Material Forms Used in TPU Welding
TPU reaches the welding press in several different forms, and the form determines how the energy couples into the joint. The stack, rather than the polymer name, is what the recipe is written against.
| Form | Typical Thickness | Welding Notes |
|---|---|---|
| TPU film | 0.05–1.0 mm | The most predictable form. Thin films weld quickly but tolerate very little excess energy — power resolution matters more than power rating |
| TPU-coated fabric | 0.3–2.0 mm coating | Only the TPU faces fuse. The textile carrier does not heat, so the seam is a coating-to-coating bond and the fabric must be oriented correctly |
| TPU film over foam or board | Laminated | Compressible core absorbs clamping force; higher tonnage and longer cooling under pressure are required |
| TPU to TPU laminate | Multi-layer | The best case. Symmetrical stacks heat symmetrically and give the widest process window |
| TPU with textile or mesh insert | Mixed | Non-polar inserts do not heat and create a heat sink; weld depth has to be verified by cross-section rather than assumed |
| TPU tube and hose | Extruded | Geometry concentrates pressure inconsistently around the circumference; dedicated holding fixtures are normally required |
Grade Selection Before Process Selection
Two TPU sheets of the same thickness can behave completely differently in the press. The grade family governs both the welding window and the service life of the finished seam, so it should be fixed before process development begins.
| Grade Family | Strengths | Welding Consideration | Typical Products |
|---|---|---|---|
| Polyester-based | Higher mechanical strength, oil and abrasion resistance | Most sensitive to moisture; drying before welding is essential to avoid hydrolytic loss of strength | Footwear, wheels, industrial covers, mechanical parts |
| Polyether-based | Better hydrolysis and low-temperature performance | Wider process window; the safer default for humid plants and long-life medical products | Medical bags, inflatables, outdoor and marine products |
| Aliphatic | Non-yellowing, optically clear | Generally lower mechanical performance; weld parameters are often tighter because the film is thin | Clear medical devices, transparent windows, decorative films |
| Shore A 70–82 (soft) | High flexibility and drape | Compresses under load, so clamping pressure is partly consumed by deformation; higher press force is needed | Waterproof bags, apparel, liners, air cushions |
| Shore A 85–95 (firm) | Structural rigidity, dimensional stability | Welds well and holds embossing; needs more energy and longer cooling | Insoles, shoe uppers, sports equipment, bladders |
| Shore D (rigid) | Engineering stiffness | Poor dielectric response and little flow; normally joined by ultrasonic or thermal methods instead | Structural mouldings, mechanical components |
Process Parameters by Thickness
The bands below are starting points for developing a recipe on a machine at normal working temperature. They are deliberately expressed as ranges, because TPU grade, coating weight and lamination all shift the optimum substantially. The most useful thing to take from the table is the direction of change: as the stack thickens, weld time grows, pressure grows, and cooling under pressure grows faster than either.
| Total Stack Thickness | Preheating | Weld Time | Cool Under Pressure | Pressure |
|---|---|---|---|---|
| 0.1–0.3 mm (thin film) | Not normally required | Short, at reduced power | Approximately half of weld time | Lowest usable; watch for flash |
| 0.3–0.8 mm | Beneficial in humid conditions | Moderate | Comparable to weld time | Raised until full weld depth is reached |
| 0.8–2.0 mm | Recommended | Longer | Longer than weld time | High |
| 2.0–4.0 mm, or laminated with foam | Essential | Long, or split into a staged cycle | Extended; often the longest part of the cycle | Very high; consider hydraulically actuated press |
| Above 4 mm | Essential, sometimes both sides | Multi-stage or multi-pass | Extended; tool may need active cooling | Highest; press capacity is normally the limit |
The Four Parameters and How They Interact
Power
Raise power only after pressure has been optimised. On TPU, insufficient clamp force is often misread as insufficient power, and the operator compensates by turning up the generator — which burns the seam edge without improving weld depth.
Weld Time
Longer weld time is not a substitute for higher pressure. Excess time allows heat to migrate into the body of the part, producing distortion and a hard, brittle seam line rather than a stronger one.
Pressure
Pressure holds the two interfaces in intimate contact while the polymer chains diffuse across the boundary. Because TPU compresses and recovers, usable contact pressure is lower than the gauge reading — expect to need noticeably more than an equivalent PVC part.
Cooling
The parameter most often left at zero. The seam must be held under full pressure until it has cooled below the material's elastic recovery range, otherwise the weld peels from the inside as the press opens.
Moisture, Drying and Preheating
Water is the most under-rated variable in TPU welding. In a humid plant, film that has been stored in the open can pick up enough surface moisture to produce foaming, pinholes and a measurably weaker seam without any change to the machine settings.
| Symptom | Moisture-Related Cause | Action |
|---|---|---|
| Bubbles or a milky haze inside the seam | Interfacial moisture flashing to steam | Dry the film and reseal in a low-humidity area |
| Pinholes that appear only after leak testing | Steam paths escaping through the hot seam | Add preheating ahead of the press to drive off surface water |
| Strength drops after weeks in service | Hydrolysis of polyester-based TPU around the seam | Dry the material; consider a polyether grade for humid end uses |
| Cycle time unachievable on thick stacks | Energy being spent on water rather than on the polymer | Preheat to remove the water load from the weld cycle |
For thick, laminated or moisture-prone stacks, a high frequency preheater installed ahead of the welding press removes surface moisture and lifts the material closer to welding temperature, which shortens the weld cycle and widens the window between an incomplete weld and a burnt one.
Tooling and Electrode Design for TPU
An electrode designed for PVC rarely performs well on TPU, because the two materials respond to pressure and to the electric field edge in different ways. Tooling for polyurethane is generally designed around three principles.
Generous Contact and Support
Because TPU needs higher pressure, the tool and its backing platen must support the load. Thin unsupported electrode sections deflect under clamping force and produce an uneven seam.
Controlled Edge Radius
The electric field concentrates at sharp edges. With TPU's tighter tolerance for excess energy, an over-sharp radius produces a burnt line exactly at the seam boundary instead of a clean cut.
Surface Finish for Release
TPU softens and can stick to a hot tool. Tool surfaces and release films should be chosen so that the part lifts cleanly without stretch marks or surface transfer.
Embossing Depth
Elastomeric recovery dulls a shallow pattern. Where a lasting texture is required, the engraving has to be deeper than the equivalent PVC tool, and the pattern should be pressed while the material is at temperature.
Tooling is manufactured to the geometry and surface requirement of the specific part, and the same machine can be fitted with different electrodes for different products. Construction details, electrode materials and the working gap are set out on the high frequency mould page.
Equipment Selection for TPU
Equipment for TPU is specified on three numbers rather than one: generator rating, clamp force and cooling capability. A machine with adequate power but insufficient tonnage cannot weld TPU reliably, no matter how the electrical settings are adjusted.
| Seam Area | Generator Rating | Clamp Force Guide | Typical TPU Products |
|---|---|---|---|
| Under 150 cm² | 3–5 kW | 2–4 tonnes | Small medical bags, tubing sets, sensor pockets |
| 150–600 cm² | 5–8 kW | 4–8 tonnes | Drainage and infusion bags, insoles, shoe uppers, small bladders |
| 600–2,000 cm² | 8–15 kW | 8–18 tonnes | Waterproof bags and backpacks, sports bladders, covers, seat pads |
| 2,000–6,000 cm² | 15–25 kW | 18–35 tonnes | Large inflatables, tents, tarpaulins, air ducts |
| Above 6,000 cm² | 25–60 kW, or multi-pass | Hydraulic press; 35 tonnes and above | Industrial covers, flood barriers, large air structures |
Machine Configurations
| Configuration | When It Fits TPU Work | Limitations |
|---|---|---|
| Single-head pneumatic | Thin film and light-coated fabric where clamp force demand is modest | Tonnage ceiling; long cooling times reduce output |
| Double-head | Medium volume where the long cooling phase can be hidden behind a second loading station | Needs consistent part geometry across both stations |
| Hydraulically actuated press | Thick stacks, laminated foam, footwear and any part needing high clamp force held through cooling | Higher capital cost; requires hydraulic maintenance competence |
| Rotary table | High-volume single-product runs where cooling dominates the cycle | Low flexibility between product variants |
| Integrated weld and cut line | Continuous production of large parts where the outline must be trimmed in the same cycle | Requires stable material supply and accurate web guidance |
Chenghao builds the full range of high frequency machines used for polyurethane work, from small single-station presses to hydraulically actuated and rotary systems, and produces the electrodes and moulds each application requires.
Applications and Representative Equipment
TPU appears in products where flexibility, transparency, biocompatibility or abrasion resistance are required alongside a sealed, leak-free joint. The machine type follows the product geometry more closely than the material does.
| Application | Material Form | Critical Requirement |
|---|---|---|
| Medical bags and fluid containers | Polyether TPU film, clear or aliphatic | Leak-tight, particle-free, no pinholes. Dedicated medical bag welding equipment uses clean tooling and validated recipes |
| Waterproof bags and backpacks | TPU-coated fabric, soft grades | Coating-to-coating fusion across the full seam width, with folds and corners held without gapping. See the waterproof bag welding machine range |
| Inflatable and air-filled products | TPU film, blown or laminated | Uniform weld depth around curves and internal baffles, followed by pressure testing. Handled on air bag welding and cutting equipment |
| Footwear and insoles | TPU film, foam and fabric laminates | High clamp force to compress foam, plus a tool that welds and trims in one stroke. See the insole welding machine page |
| Sports balls and bladders | TPU film over a carcass | Curved seam geometry with consistent peel strength; air retention over long periods |
| Outdoor gear and tents | TPU-coated or TPU-laminated textile | Long, continuous seams and tape-free waterproofing, with the weld holding at low temperature |
| Automotive interiors and airflow parts | TPU film, laminate with foam | Low-emission joints with no adhesive, dimensional stability and traceable parameters |
| Hoses, ducts and tubing | Extruded TPU tube and profile | Circumferential sealing between dissimilar geometries; dedicated fixtures and often a continuous process |
Quality Control for TPU Seams
TPU seams fail in two distinct ways: immediately, because the weld was never complete; or gradually, because the joint was complete but stressed before it cooled. Testing has to cover both, which means checking strength and not only appearance.
| Check | Method | What It Reveals |
|---|---|---|
| Cross-section weld depth | Microtome or razor section, inspected under low magnification | Whether fusion reached the intended interface, and whether the heat-affected zone is over-wide |
| Peel test | Pull the two layers apart at a controlled rate and angle | The most informative single test on TPU. Failure within the material is good; failure exactly at the interface indicates an incomplete weld |
| Pressure or vacuum decay | Inflate or evacuate the part and monitor the change | Pinholes and micro-leaks that visual inspection misses |
| Burst or hydrostatic test | Pressurise to failure or to a defined multiple of working pressure | Absolute seal capability; required for medical and inflatable products |
| Relaxation check | Re-measure seam width and flatness after a defined rest period | Seams that were released hot and continue to spring back after the press opens |
| Ageing exposure | Humidity or temperature cycling, then repeat the peel test | Hydrolytic weakness in polyester grades and long-term seam degradation |
Six Rules That Decide Whether a TPU Seam Lasts
01 — Dry the Material First
Establish a drying routine and a maximum open-air exposure time before material reaches the press. If the plant humidity swings, seal the film between operations. Moisture is a process variable, not a material fault.
02 — Raise Pressure Before Raising Power
Insufficient clamp force is the most common cause of weak TPU welds and the least often diagnosed. Increase pressure until full weld depth is reached, then set power to the minimum that achieves it within the target cycle time.
03 — Never Release a Hot Seam
Programme cooling under full pressure as a defined stage, not as a delay the operator adds by hand. If output suffers, add a second station rather than shortening the cooling phase.
04 — Develop Recipes at Working Temperature
A recipe tuned on a cold machine drifts as the tool and press warm up through the shift. Verify settings after the machine has reached steady state, and re-check at the end of a production run.
05 — Design the Tool for TPU
Do not copy a PVC electrode and expect it to work. Support the higher load, control the edge radius and set the embossing depth for a material that recovers elastically.
06 — Test the Seam, Not the Sample
A seam that looks correct can still be a surface weld. Peel and section testing at start-up, after material batch changes, and on a fixed interval throughout production is what keeps a TPU line stable.
Frequently Asked Questions
Welding TPU? Match the Machine to the Material
Chenghao Machinery supplies high frequency welding machines from 3 kW to 60 kW with the clamping force, cooling control and tooling that polyurethane requires. Send us your TPU grade, stack thickness and part drawing, and we will recommend the configuration and run a welding test on your own material.
Dongguan Shenghao Machinery Co., Ltd. (Chenghao Machinery) | sales11@chenghaointernational.com | +86 158 1765 5869
Dongguan, Guangdong, China