TPU Film Heat Sealing Parameters for Strong Repeatable Seams

Temperature, Dwell, Pressure and the Joint Designs That Hold

TPU film heat sealing is controlled by three parameters, but only one of them is usually measured properly. Temperature, dwell time and pressure all have to be right, and surface temperature at the seam is the one that decides whether the bond forms.

A seam that looks closed and pulls apart at low load was sealed too cold, too fast or with too little pressure, and the failure is invisible until it is tested.

This guide covers what happens at the interface during sealing, how the three parameters interact, the joint designs that distribute load, and the test routine that keeps a production line consistent.

What Happens at the Interface During Sealing

The two surfaces soften, flow into contact and entangle, then cool into one continuous phase. That sequence needs enough heat to reach a flowable state, enough time for the polymer to wet the opposing surface, and enough pressure to displace air.

Trapped air is the most common defect. A bubble between the layers reduces the bonded area and acts as a crack starter, so a seam can measure well on average while failing at the point where the bubble sits.

Surface contamination interferes with wetting in the same way. Dust, mould release, hand grease and condensation all sit between the two layers and prevent polymer-to-polymer contact, which is why a seam made on a humid morning can differ from one made in the afternoon.

Film thickness changes the requirement. A thin film reaches sealing temperature quickly and is easy to overheat, while a thick film needs a longer dwell to bring the interface up to temperature without distorting the outer surfaces.

How the Three Parameters Interact

Temperature, dwell and pressure trade against each other within limits. Raising temperature shortens the dwell needed, and raising pressure can partly compensate for a low temperature, but only up to a point.

Beyond that point the trade breaks down. Excess temperature degrades the polymer and creates a brittle seam that fails by tearing instead of peeling, which looks like a strong bond until it is flexed.

Excess pressure squeezes the softened film out of the seam, thinning it and creating a hard bead at the edge. That bead concentrates stress and is where a seam usually starts to open under load.

Dwell time is set by line speed on most machines, which means production pressure quietly changes the seal. A line run at 20% above its validated speed has a 20% shorter dwell and a weaker seam, with no visible indication on the finished part.

Starting Points by Film Thickness

Treat the figures below as a starting window to be validated, not as settings to copy. Every machine has its own heat transfer behaviour, and the same set point produces different interface temperatures on different equipment.

Film thickness	Typical interface window	Dwell guide	Main risk
50 to 100 micron	Lower end of the grade range	Short, 1 to 2 seconds	Overheating and edge thinning
150 to 250 micron	Mid range, stable	Medium, 2 to 3 seconds	Trapped air and cold spots
300 to 500 micron	Upper end, longer soak	Long, 3 to 6 seconds	Through-thickness distortion
Laminated constructions	Set by the TPU layer, not the carrier	Extended to heat through the carrier	Carrier damage before the TPU flows

The valid method is to measure the interface temperature with a thin thermocouple or an infrared line sensor and build a window around the measured value. Controller set points are useful for repeatability, not for transferring a recipe between machines.

Joint Design That Distributes Load

A seam should fail outside the bond, not inside it. If the bond is the weakest point in the assembly, the joint will open under normal service load no matter how well it was sealed.

Seam width is the simplest way to gain strength. Doubling the width roughly doubles the load the joint can carry, and it costs only a little material. Narrow decorative seams are the ones that fail first.

• Use a lap joint where load is tensile and a double-folded lap where the seam is exposed to abrasion
• Round or reinforce the seam ends, because a square corner concentrates stress
• Keep the seal line away from a fold that will be flexed repeatedly in service
• Stagger seams between layers so that no single line carries the whole load
• Allow for shrinkage in the film so the seam does not carry tension as it cools

Sealing through a coating or a print is a frequent source of weakness. Inks and lacquers sit between the two TPU surfaces and prevent polymer contact, so either the print is kept out of the seam area or the seal is made on the unprinted face.

High Frequency and Ultrasonic Welding

High frequency welding heats the polymer from inside, which produces a clean seam without the surface marking that contact sealing can leave. It suits thicker film and constructions where a smooth outer face matters.

Ultrasonic welding works well for thin film and for seams close to hardware or zips where heat would cause damage. Both methods still depend on clean surfaces and correct pressure, so neither compensates for poor joint design.

Solvent and adhesive bonding remain useful for repairs and for field assembly, but they are slower and less consistent than thermal methods. For a production line, thermal sealing is the default and the alternatives are exceptions.

Testing and Monitoring Production Seams

Test at the start of every shift and after any change to speed, temperature or material batch. A seam that passes at 8 am and fails at 4 pm is a process control problem, and the only way to see it is to test across the shift.

A peel test and a burst test answer different questions. Peel tells you how well the interface bonded, while burst tells you whether the assembly holds pressure. Products that carry air or water need both.

1. Cut a specimen across the seam and inspect the failure mode, not only the force value
2. Record interface temperature alongside the machine settings for every production run
3. Retain failed and passed samples with the batch reference and the date
4. Re-validate whenever the film supplier, the grade or the thickness changes
5. Pressure test finished assemblies rather than relying on seam inspection alone

A cohesive failure — where the film itself tears — is the result you want. An adhesive failure, where the two layers separate cleanly, means the interface never bonded, and no amount of re-testing the same sample will change that.

Specifying Film for a Sealed Product

Tell the film supplier what the seam has to survive, and the grade can be selected for sealing rather than only for mechanical properties. Sealing behaviour is a formulation property, and two films with the same hardness can seal very differently.

The film range used for tents, tarpaulins and outdoor assemblies is documented on the outdoor application page, where seam performance is part of the product requirement rather than an afterthought.

Huanlong produces blown, extruded and foamed TPU film on 44 lines across three manufacturing bases, supported by two R&D centres and more than 30 engineers. Sealing trials are handled as part of the specification work at Huanlong, and Zhejiang Huanlong New Material Science Technology Co.,Ltd holds ISO 9001:2015, ISO 14001 and ISO 45001 certification for its production.

Frequently Asked Questions About TPU Film Heat Sealing