Ultrasonic sealing of non-woven fabric: it is all in the anvil

Non-woven work behaves so differently from rigid plastic welding that people who are good at one are often surprised by the other. There is no energy director. There is barely a joint design. Almost everything that decides whether it works sits in the anvil.

What is actually happening

Non-woven fabric — the material in surgical masks, shopping bags, wipes and scrubber pads — is nearly always polypropylene fibre, spunbonded or meltblown into a web.

Put that web between a vibrating horn and a patterned anvil and the fibres at the raised points of the anvil pattern get worked hard. They heat, melt and fuse into one another. Everywhere the anvil is not touching, the fabric stays fabric.

So the seal is a field of tiny fused dots, in whatever pattern the anvil has. That is the whole process, and it explains why the anvil is the important tool and the horn is often just a flat bar.

Continuous versus plunge

Continuous (rotary) sealing

The fabric runs between a stationary horn and a rotating knurled anvil roll. The seam is endless, limited only by how much fabric you feed. Line speeds of 10 metres a minute and up are ordinary.

Used for:

• Side and bottom seams on non-woven carry bags.
• The long body seams on flat masks.
• Slitting and sealing in one pass — the horn cuts the web and seals both new edges at the same time.
• Quilting and pattern bonding on wadding and pads.

Our 150 mm non-woven horn is built for this kind of work — a long flat working face with a matched anvil roll behind it.

Plunge sealing

A shaped horn comes down onto a static anvil, welds, and lifts. One defined weld at a time, but you can shape it however you like.

Used for:

• Ear loop attachment on masks — four small welds per mask, and the horn face profile decides whether the loop tears out.
• Nose wire tacking.
• C-type and cup mask body forming, where the horn has to follow a three-dimensional shape.
• Scrubber pad edge sealing and the rosette pattern in the middle.

The anvil pattern, properly

This is where the experience is. A few rules that hold up:

• Fine, dense pattern: strong seal, close to continuous, stiffer to the touch. Good for load-bearing bag seams.
• Coarse, widely spaced pattern: softer, more flexible seal with more unmelted fabric between the dots. Better where the seal has to drape or sit against skin.
• Too much contact area: the energy spreads, nothing melts properly, and you get a weak seal that pulls apart in fluff.
• Too little contact area: the energy concentrates so hard that it cuts instead of sealing. You get a perforated line rather than a seam.
• Pattern direction matters relative to the direction of pull. A diamond pattern resists tearing in both axes better than parallel lines do.

Anvil rolls take a beating over millions of metres of fabric, so they are made in hardened steel. The pattern has to hold its edges — once the peaks round off, seal quality drops and no machine setting brings it back.

Fabric variables that will change your settings

• GSM. Heavier fabric needs more energy or a slower line. A setup dialled in for 25 GSM mask fabric will not run 70 GSM bag fabric.
• Number of layers. A three-layer mask is a different job from a single-layer bag, even at the same total GSM.
• Meltblown versus spunbond. Meltblown is finer and more delicate. Push it and you will burn through.
• Additives and finishes. Hydrophilic treatments, antistatic finishes and heavy colourants all shift the process window.
• Any polyester content. PET in the blend raises the melt temperature and can stop the seal working with PP settings.

Common problems on a non-woven line

Fabric cutting instead of sealing. Too much amplitude, too little anvil contact area, or too slow a line speed. Speed up before you turn anything down.

Seal pulls apart into fluff. Not enough energy reaching the fibres. Check line speed, check the anvil pattern has not worn flat, check the pressure.

Seal strong in patches. Nearly always the horn and anvil are not parallel, or the anvil roll is running with some play in its bearings. Fix the mechanics, not the settings.

Fabric wandering. A web tension problem, not a welding problem, but it shows up as bad seals so it gets blamed on the machine.

Horn getting hot on a continuous line. Continuous rotary work is a real duty cycle. Check the machine is specified for it and that cooling is adequate.

What we build for this

Mask-line horns including C-type and disposable-mask profiles, 150 mm non-woven horns for bag and quilting work, ear-loop welders, fabric sealing machines and scrubber making machines from semi-automatic up. The horns are cut to the pattern and profile the job needs — this is not a shelf item, and anyone selling it as one has not thought about your fabric.

Everything currently in stock is listed under non-woven and sealing and horns and fixtures. Send us a metre of your actual fabric and tell us the seam you need; we will seal it and post it back.

Questions we get asked