Plated, Painted and Printed Parts: There Is a Layer in the Way
Trim parts, panels and cosmetic housings are often plated, painted or printed. Put those operations together with ultrasonic welding in the wrong order and trouble follows.
What the problem is
Ultrasonic welding works by melting the resin on both sides so the chains diffuse across and entangle. A layer of plating, paint or ink at the interface takes no part in that, and amounts to a sheet of paper between the two halves.
Two outcomes follow: either nothing joins and the parts open by hand, or — more troublesome — part of the joint fuses, strength is unstable, and sampling passes and fails by turns.
Metal plating adds its own problems: it conducts heat well and has a high modulus, changing the thermal and stiffness distribution near the interface, and flakes of it can enter the joint as foreign matter.
Three routes
One: weld first, finish afterwards. The cleanest sequence. Plating or painting after welding covers the joint and looks tidier. It suits parts finished all over where the finishing process will not damage the weld.
Note that plating pre-treatment is chemical, and acids, alkalis and solvents reach the joint. On resins sensitive to stress cracking such as PC, residual weld stress plus chemical contact readily cracks after finishing, so reduce weld force and extend hold to lower the stress.
Two: mask the weld area. Keep the joint region as bare plastic through the finishing operation. This is the usual approach, at the cost of masking tooling and extra labour.
Leave margin at the masking boundary rather than stopping exactly at the joint. Paint overspray and plating edge effects both reach beyond, so mask a band wider than the joint.
Three: move the joint to an unfinished face. Many parts are finished only on the outer surface. Putting the joint on the inside solves both problems at once, and costs least when decided on the drawing.
Printed parts specifically
Screen and pad printing leave a thin ink film that is easily overlooked, and it interferes with fusion just the same. Solvents in the ink can also trigger stress cracking, with PC and PMMA the ones to watch.
Another overlooked point is incomplete curing: residual solvent in uncured ink volatilises under the heat of welding and forms bubbles in the joint.
How to verify
When a surface layer is suspected, run a comparison: weld a few unfinished parts at the same settings and compare strength and appearance. If the bare parts are clearly better, the layer is confirmed as the cause.
Sectioning helps too: a surface layer often shows as a differently coloured line across the section, interrupting the fused band.
Mould release counts as a layer
More hidden than plating or paint is mould release. It appears on no drawing and no inspection sheet, yet it changes friction at the interface directly. Change the release agent or the supplier and the weld result changes, with nobody thinking of that layer.
Write whether release is used, and which, into the purchasing specification, and watch it on batch changes.
The effects of additives and surface condition on weldability are in the weldability handbook on the downloads page. Force control, hold settings and process monitoring are on the ultrasonic welding machine pages.
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