Which plastics weld ultrasonically, and which will fight you

The short version: nearly every rigid thermoplastic can be ultrasonically welded to itself. How hard it is varies enormously, and the difference comes down to one thing — whether the plastic is amorphous or semi-crystalline.

Amorphous plastics: the easy ones

Amorphous plastics have no orderly crystal structure. They soften gradually across a wide temperature range, which means the melt at the joint stays workable long enough to flow and fuse. They also transmit vibration efficiently, so the energy actually reaches the joint.

With these you can usually weld far-field — joint 40 mm or more from the horn face — and still get a clean result. That gives the part designer a lot of freedom.

Semi-crystalline plastics: the ones that need care

PP, PE, nylon, acetal, PET and PBT have an ordered crystal structure with a sharp melting point. Two things follow from that, and both make life harder.

First, they damp vibration. Energy that leaves the horn gets absorbed on the way down, so a joint far from the horn face may never reach melt temperature. Near-field design — joint within about 6 mm of the horn — becomes important rather than optional.

Second, the melt window is narrow. The material goes from solid to liquid quickly and re-solidifies just as fast, so the melt has very little time to flow into the joint. You need more amplitude to get it there in the first place.

Moisture: the problem nobody expects

Nylon and PET pull water out of the air. When the joint melts, that water turns to steam and foams the weld. What you get looks like a weld and breaks like a biscuit.

This matters more here than in a lot of places. During monsoon, a tray of nylon mouldings left open on the shop floor overnight can pick up enough moisture to wreck a whole shift's welding. The fixes are simple and not optional:

• Weld as soon after moulding as you can. "As-moulded" parts are dry parts.
• If you must store them, seal them in poly bags with desiccant.
• If a batch has already been sitting, dry it before welding. Follow the resin supplier's schedule, not a guess.

Polycarbonate is milder about it but still benefits from the same discipline.

Fillers, regrind and release agents

What is in the resin changes the weld as much as the resin itself.

• Glass fill up to about 20 per cent often helps — the part is stiffer, so it transmits vibration better.
• Glass fill above 30 per cent starts to hurt. There is less polymer at the joint face to actually melt, and the glass abrades the horn. An aluminium horn on 40 per cent glass-filled nylon will wear visibly in weeks. This is where a titanium horn, or a carbide-faced one, pays for itself.
• Talc, calcium carbonate and other mineral fill behave similarly — more filler, weaker weld.
• Regrind is inconsistent by nature. If your weld quality drifts through the day and nothing on the machine has changed, check whether the moulding shop changed the regrind ratio.
• Mould release on the joint face is a straightforward contaminant. Silicone release is the worst. If parts arrive shiny and slippery, that is your problem.
• Plasticisers in flexible PVC migrate to the surface over time and interfere with welding. Weld flexible PVC fresh.

Welding two different plastics

People ask this a lot, usually hoping for a yes. The honest answer is: sometimes, and only if two conditions hold.

1. The two resins have to be chemically compatible — similar enough that the molten polymers actually mix rather than just sitting against each other.
2. Their melting temperatures have to be close, within roughly 20 degrees Celsius. Otherwise one is still solid when the other has already degraded.

Pairs that generally work: ABS to SAN, ABS to acrylic, ABS to PC in some grades, PS to SAN. Pairs that do not, no matter how much power you throw at them: PP to PE, PP to ABS, nylon to almost anything else, acetal to almost anything else.

PP to PE catches people out because both are polyolefins and both feel similar. They will not fuse.

Non-woven fabric is a separate case

Non-woven PP fabric — masks, bags, scrubbers, wipes — welds beautifully, but by a different mechanism. There is no energy director. The fibres themselves fuse under the horn against a patterned anvil, and the anvil pattern determines the strength and the look of the seal. We cover that properly in the non-woven guide.

When in doubt, weld a sample

Compatibility charts are a starting point, not an answer. Grade matters, filler matters, colourant matters, and the same nominal resin from two suppliers can behave differently.

Send us the actual parts you intend to weld, in the actual material, and we will run them. It takes a day and it tells you more than any chart.

Questions we get asked