Textured Surfaces and Overmoulded Soft Touch: Where to Put the Load

Textured surfaces and soft overmoulded parts share one property in ultrasonic welding: their appearance cannot be restored. A light mark on rigid plastic can sometimes be polished out or viewed away; a flattened grain or a crushed soft layer is scrap.

Texture: flattening is plastic deformation

Grain depth is typically tens of microns. When the horn presses on the grain and ultrasonic motion softens the material locally, the peaks flatten. That is plastic deformation, not elastic recovery, and releasing the force does not bring it back.

The order of remedies:

One: move the contact onto a smooth area. Design a smooth ring or several smooth pads into the textured surface specifically for the horn. This is the easiest route, provided it is raised while the tool is still being made.

Two: weld from the other side. The horn presses on the non-cosmetic back face while the textured surface rests in the fixture. The support face must match the geometry completely and be as soft as possible, because a fixture flattens grain just as well as a horn.

Three: add an interlayer where contact on the grain is unavoidable. A soft pad distributes pressure, but it only reduces the damage rather than removing it, and it absorbs energy, so the parameters have to be reset.

Four: reduce force to the minimum that still couples the energy. On textured parts, prefer high amplitude and low force.

Soft overmould: a compliant layer in the way

TPE or TPU over a rigid substrate creates two problems.

Soft material does not transmit energy. Ultrasound travels through rigid material, while a soft layer absorbs a large share of the vibration as heat, so the energy never reaches the joint interface and instead softens and distorts the overmould. The horn must never press on soft material.

Soft material takes a set. Even without vibration, sustained high force leaves a permanent impression.

There is only one main answer: separate the welding surface from the overmoulded surface structurally. The horn contacts exposed rigid substrate, the ultrasonic path runs entirely through rigid material, and the soft layer sits outside the load path. This is decided at design and cannot be recovered later.

Where soft material is unavoidably near the load path, consider another joining method — adhesive, snap fits, screws — or move the overmoulding operation after welding.

The fixture is half the job

On cosmetic parts the fixture matters as much as the horn. Three requirements.

Full contact. A part supported on a few points concentrates force at those points, and that is where the marks appear.

A face that is not hard. A hard fixture face is itself a source of marking. The usual approach is a soft lining on the support face, or a non-metallic support material.

No forced constraint. A part straightened in the fixture keeps the stress after release, and both appearance and dimensions can shift.

Full contact without over-constraint is the hardest balance in fixture design and usually has to be found on real parts.

Confirm feasibility first

Feasibility here is highly sensitive to the specific material and grain depth. Assess weldability before the tool is cut and avoid modifying it later. Putting the textured areas, the overmould extent and the cosmetic standard on the drawing beats adjusting parameters afterwards.

Fixture design and cosmetic-part processing are on the downloads page. Amplitude and force control and energy transfer through compliant materials are on the ultrasonic welding machine pages.

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