Welding PVC: Low Temperature, Additives and Corrosion

Rigid PVC is amorphous with a broad softening range, so weldability itself is not the difficulty. What matters is its thermal stability and what comes out of it once overheated.

Overheating costs more than appearance

Heated far enough, PVC decomposes and the products are corrosive. That is a different order of problem from a resin simply scorching: discolouration affects one part, while the decomposition products attack the horn, the fixture and nearby metal.

Typically the horn's working face pits or darkens, locating parts in the fixture corrode, and in time dimensions move as well. By the time weld quality is seen to drift, the corrosion has usually been running for a while.

So the parameter principle for PVC runs opposite to most resins: err low rather than high. Give enough energy to fuse and stop, without adding a margin for safety.

Setting parameters

Prefer energy mode. Time mode compensates for nothing, and PVC does not tolerate surplus energy; together those two invite overshoot. Energy mode caps the total input.

Keep the cycle short. PVC melts quickly and needs fast fusion rather than prolonged heating. A long cycle spreads heat into the part body and raises the risk of decomposition.

Moderate to low amplitude. Amorphous resins do not need the high amplitude that semi-crystalline ones do, and PVC less than most. Too much amplitude melts too fast and drives local temperature past the limit.

Give it enough hold. PVC shrinks appreciably, so force must stay applied until the fused zone solidifies, or voids remain inside and sealed parts leak.

Protecting equipment and tooling

Extract at the station. Local extraction carries the decomposition products away and is the most direct measure.

Treat the horn surface. Aluminium horns are more vulnerable in a PVC environment; consider surface hardening and put the horn on a scheduled inspection for pitting on the working face.

Choose fixture materials accordingly. Locating parts and clamps should be corrosion resistant or protected.

Clean at end of shift. Wipe horn, anvil and fixture down; do not leave residue overnight.

Flexible PVC is a separate case

Flexible PVC carries a high plasticiser content, with a low modulus and high internal damping, so it behaves much like a soft elastomer: the wave attenuates quickly and the part deforms under force. Follow the soft-material approach — near field, a larger contact area, a contoured nest that supports rather than clamps, and a longer hold.

Plasticiser also migrates to the surface, adding a layer at the interface that works against fusion. That affects the result more than parameters do, so incoming storage time and surface condition belong under control.

Compatibility

Rigid PVC is partially compatible with ABS and can be welded, but the window is narrower than for like-to-like and the strength does not reach the same level. Use it with care where the joint carries load or must seal, and re-validate after a batch change.

The compatibility matrix and difficulty ratings are on the downloads page. Energy mode, amplitude adjustment and hold settings are on the ultrasonic welding machine pages.

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