Fixture Venting: Trapped Air Pushes the Part Back

The better the nest fits and the more evenly it supports, the better — up to a point. Beyond that a reverse effect appears: air gets trapped between the nest and the part.

Symptoms

Typically the weld height becomes unstable, jumping randomly within a range at the same settings; the part sometimes bounces before it settles; and taking it out produces a faint sucking sound.

The mechanism is simple. As the horn descends the part is pushed down, the air in the nest cannot escape in time, and it forms a cushion. That cushion holds the part up, the real contact state differs from what you assume, and since the trapped volume differs part to part, the result drifts.

Deep nests, soft materials and thin walls show it most, because they are more easily moved by air pressure.

How to vent

Cut vent grooves. Fine grooves in the base and side walls leading outward give the air a path. They need not be large, only connected. The most direct measure.

Drill vent holes. Through holes in the base, especially at the centre of a deep nest. The diameter matters less than the position.

Do not make the nest fully conformal. Leave local clearances and break a continuous contact face into sections, so the gaps between them become the path. Support stiffness is kept and air is not trapped.

Slow the approach. A slower descent gives the air time to escape. A parameter-level mitigation, less reliable than structural venting but available immediately.

The opposite case: vacuum

Some thin-walled and soft parts need negative pressure to hold them in place, and then the nest is connected to vacuum. This does not contradict venting: venting removes unwanted air, vacuum actively holds the part. Both amount to controlling the pressure in the nest rather than leaving it to chance.

With vacuum, watch the port positions: a port under a thin wall pulls the part out of shape, so place them where the part is supported.

An adjacent problem

Poor venting brings another symptom: the part will not come out. After welding the nest holds a partial vacuum, the operator has to prise the part free, the cycle stretches and edges get damaged. On an automated line it is worse, because the gripper cannot lift it.

The remedy is the same venting, or an ejector in the base of the nest.

How to diagnose

When venting is suspected, run a crude but effective trial: drill a temporary hole in the side wall, or shim one corner of the part so air can escape, then weld ten parts and see whether the height scatter shrinks. If it does, cut proper grooves in the fixture.

Another signal: the scatter varies with how quickly parts are loaded, worse when the operator works fast and better when parts are placed slowly. That is almost certainly an air cushion.

Fixture options, approach speed control and depth modes are on the ultrasonic welding machine pages. The joint and tooling design-review checklist is on the downloads page.

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