Ultrasonic, hot plate or spin welding — which process fits your part

We build all three, so we have no reason to push one over another. What follows is how we actually decide when a customer describes a part.

Start with the shape of the joint

This single question eliminates most of the field before you look at anything else.

• Circular joint? Spin welding is probably your answer, and you should be quite reluctant to move away from it.
• Small or medium, any shape, under about 120 mm? Ultrasonic.
• Large, long seam, or needs a serious pressure-tight seal? Hot plate.
• Large and non-circular and needs a seal? Hot plate, or linear vibration welding if the volumes justify it.

Ultrasonic welding

How it works: high-frequency vibration creates friction at the joint, which melts the plastic in a fraction of a second.

Cycle time: 0.2 to 1 second. Nothing else comes close.

Strengths: fast, low energy per part, clean, no consumables beyond the horn, and it is easy to automate on a rotary table or an indexing line. The tooling is a horn and a nest, which is cheap compared with a heated platen tool.

Limits worth knowing before you commit:

• The part needs an energy director or a shear joint. If the mould is already cut with a flat butt joint, ultrasonic will disappoint you.
• Horn size is capped by frequency — around 125 mm at 20 kHz, 170 mm at 15 kHz. Longer seams mean multiple hits or a different process.
• Vibration reaches whatever is inside the part. Delicate electronics, springs and thin internal clips can be damaged.
• Incompatible resins will not join no matter what you do.

Best for: electronics housings, chargers and adapters, battery packs, caps and closures, toys, medical disposables, non-woven fabric, stationery, automotive clips and panels.

Hot plate welding

How it works: a heated platen touches both joint faces until they melt, the platen withdraws, and the parts are pressed together and held while the melt solidifies.

Cycle time: 10 to 60 seconds depending on wall thickness. There is no way to rush the cooling.

Strengths: it melts the whole joint face, so the weld is thick, continuous and genuinely strong. Part size is limited by the size of your platen, not by physics. It copes with warped parts better than any other process because the melt fills the gap. It is the reliable answer for large PP and PE assemblies that have to hold pressure.

Limits:

• Slow. If you need thousands of parts a shift, you need multiple stations.
• PP and PE string when the platen pulls away. Coated platens and careful temperature control reduce it; part design has to allow for it.
• The weld bead is large, so the part needs flash traps if it must look clean.
• Hot tooling means guarding, and a warm-up period at the start of every shift.

Best for: tanks, ducting, large housings, automotive under-bonnet parts, anything big in PP or PE that has to seal.

Spin welding

How it works: one part is spun against the other under pressure. Friction at the circular joint melts both faces, rotation stops, and pressure holds until it sets.

Cycle time: 1 to 3 seconds.

Strengths: excellent hermetic seals, no energy director needed, no ultrasonic vibration to disturb the contents, no consumable tooling wearing out, and the process is very tolerant of resin variation. It handles PP and PE without the difficulty that makes them awkward for ultrasonics.

Limits: the joint must be round. That is the whole limitation, and it is absolute. If you need the two halves to line up in a particular rotational position, you need a servo-position machine that stops the spin at a known angle rather than a simple inertia machine.

Best for: filters, floats, aerosol and pump components, round containers, expansion tanks, anything cylindrical that has to seal.

The comparison table

Two combinations that come up

Ultrasonic plus spin on the same line. A round body spin welded for the seal, then an ultrasonic operation to attach a non-circular fitting. Common on filters and pumps.

Hot plate for the main seam, ultrasonic staking for the internals. The big joint gets the reliable slow process, and the small internal assembly steps get done in under a second each with a hand gun or a small bench unit.

Where people get it wrong

The most expensive mistake we see is buying ultrasonic for a part that should have been spin welded. The joint is round, there is a filter element or a float inside, the customer buys an ultrasonic machine because that is the process they had heard of, and then spends months fighting damaged internals and inconsistent seals.

The second most expensive is buying hot plate for a small part in ABS. It will work perfectly and it will be twenty times slower than it needed to be.

Describe the part and the joint to us before you decide. We make spin and hot plate machines as well as ultrasonic, so there is nothing in it for us to point you at the wrong one.

Questions we get asked