TPU and Soft Parts: Why Softness Makes Welding Hard

Seals, over-moulded grips, waterproof plugs and flexible boots use TPU or similar soft materials. Customers ask whether they can be welded; they can, but one thing has to be accepted first: soft materials absorb the vibration.

Three physical facts

High internal damping. The molecular chains in a soft material are mobile, so a great deal of the acoustic energy converts to heat as it travels and the wave attenuates quickly. With the horn contacting any distance from the joint, the energy is gone before it arrives.

Low modulus. Apply force and the part itself flattens first. You believe you are loading the interface, while most of the travel is taken up by deformation of the material and the real pressure at the interface is far below the setting.

Large springback. The part recovers once ultrasound stops, and if hold time is short the recovering material tears apart a portion of the freshly formed bond.

What to do

Design for near field. The closer the horn contacts the part to the joint the better, ideally directly above it. Far-field welding on soft materials is essentially unworkable, and this has to be settled on the drawing because there is no later remedy.

Enlarge the horn contact area. At the same force, doubling the area halves the pressure. Soft surfaces mark readily, and more area both reduces marking and spreads the load evenly.

Reduce force and let the fixture carry the part. A soft part cannot be held by clamping harder; more force means more distortion. The nest should follow the part contour and support it by shape rather than by pressure.

Extend hold time. With large springback the force must stay applied until the fused zone has genuinely set. That second cannot be saved.

Consider a higher frequency. Higher frequency means lower amplitude and a gentler mechanical action; the trade-off is less available power and a smaller weld area, which soft parts rarely need anyway.

Hard-to-soft pairings are more common

In real products, soft welded to soft is less common than a soft part joined to a rigid housing, and that raises compatibility. TPU welds to TPU. TPU against amorphous rigid resins such as ABS or PC is immiscible at the molecular level and will not join.

Faced with that pairing, do not spend time on parameters. Change the method: over-mould in one shot, use a mechanical fit, or stake the soft part onto the rigid one. Staking requires no compatibility, since the captured part takes no part in fusion.

One more thing

Soft parts often carry a sealing function, and sealing is judged on whether one continuous leak path exists, not on pull strength. Design the joint for a continuous fused band, then leak test every part; sampling is not enough for seals.

The compatibility matrix and joint dimensions are in the handbooks on the downloads page. Frequency options, force control and hold settings are on the ultrasonic welding machine pages.

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