Welding PPS: Heat Resistant Does Not Mean Easy to Weld

PPS turns up in pump bodies, sensor housings and automotive electronics, chosen for its heat and chemical resistance. Neither of those properties is good news for ultrasonic welding.

Where the difficulty lies

A high melting point over a narrow range. PPS is semi-crystalline and melts around 280°C, so opening the interface needs a noticeably higher energy density than amorphous resins such as ABS or PC. The narrow range means too little energy leaves it unfused and too much degrades it, with little room in between.

The base material is brittle. PPS is not tough, and glass reinforcement makes it more brittle still. Weld force, fixture contact points and sharp corners on the joint all become stress concentrations, and what cracks first is often the parent material beside the weld rather than the weld itself.

It is almost always glass filled. Unfilled PPS is rarely used for structural parts; 40% glass is common. The fibre attenuates the acoustic wave and removes roughly that share of the resin available to fuse, so joint strength cannot reach parent-material level. Accept that at the design stage rather than hoping parameters recover it.

What to do

Use a shear joint. An energy director relies on instantaneous melting at a tip followed by flow, and the narrow melting range of PPS makes that instant hard to reproduce, so within one batch some parts fuse fully and others fall short. A shear joint generates heat through sustained friction along an interference fit, and melting advances progressively along the mating face.

Give it enough amplitude. This is the first variable, not the last. Where amplitude is short, extending the weld time achieves nothing but spreading heat into the part body.

Take interference at the low end. PPS is brittle, and excessive interference holds the side walls splayed as a permanent tensile stress. Sealing comes from a continuous fused band, not from pressing harder.

Use soft contact in the fixture. Hard locating points bearing directly on PPS readily press in micro-cracks. A larger contact area and soft pads beat any parameter change.

Treat the horn as a consumable. 40% glass abrades the working face continuously; aluminium horns have short lives here, so consider titanium or surface hardening and carry the horn in the spares plan.

A narrow window has to be monitored

With a narrow window the risk is not "this part welded well" but "every part welds well". PPS is a classic case where the eye cannot tell: appearance is fine, and only a section shows the interface was merely pressed together.

Use energy or depth mode, not time mode. Time mode compensates for nothing, and the PPS window cannot absorb incoming variation.

Monitor several dimensions. Set limits on energy, power and post-weld height together and alarm on any deviation. A single dimension misses cases: a crushed part and a fused part can share the same final height.

Keep periodic destructive sampling. Sectioning a sample to see whether the fused band is continuous says more than a pull figure.

On heat resistance

One point is easily missed: the temperature capability of the weld is not that of the parent material. The crystalline structure in the fused zone differs from the bulk and the fibre is interrupted across the interface, so under sustained load at temperature the weld is usually where failure starts. Design that section for lower stress.

Difficulty ratings and the compatibility matrix are in the handbooks on the downloads page. Amplitude ranges and multi-dimensional monitoring are on the ultrasonic welding machine pages.

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