Why Flame-Retardant PC/ABS Will Not Reach the Pull Figure

Charger housings, power adapters and appliance internals usually have to meet a flammability rating. A customer carries the same joint design over to a flame-retardant grade, the pull figure drops, and the machine comes under suspicion.

The problem is the material, not the machine

Flame retardant takes no part in fusion. The loading is usually substantial, so it displaces resin at the interface and fewer molecular chains are left to diffuse across and entangle. Flame-retardant grades therefore weld to noticeably lower strength than the non-FR version of the same grade.

That is not a tuning failure but a material ceiling. Adding energy scorches the surface first and leaves the interface as it was.

Three things that do work

One: spread the strength requirement over geometry. If strength per unit length cannot rise, lengthen the joint or choose a more effective form. Shear joint area grows with engagement depth, which builds total strength more readily than a butt-style energy director. That decision belongs on the drawing.

Two: amplitude first, time last. Flame retardant attenuates the wave only modestly, but with less resin available at the interface the energy density required goes up. More amplitude helps; more time only sends heat into the part body.

Three: control flash. Melt flow of an FR grade often differs from the non-FR version, so at the same director height the flash volume can change noticeably. Re-check the position and capacity of the flash trap rather than copying the non-FR tool.

Two things not to expect

A higher-power machine. Power determines how much energy is available; amplitude determines the density at which it enters the interface. FR parts lack the latter, and power does not substitute.

A longer weld time. The process window on an FR grade is usually narrower than the non-FR version, not wider. Extending the cycle produces surface damage and yellowing first.

Re-validate after a batch change

Different retardant systems behave very differently. Two grades both rated V-0 may use different chemistries at different loadings, with different effects on welding. Re-validate parameters and run destructive sampling after any change of supplier or grade rather than assuming they carry over.

This matters especially in electronics, where FR material changes are often driven by cost or supply and are not announced to process engineering. Folding material changes into change control is the least troublesome approach.

A note on ratings

A flammability rating certifies the material, not the welded joint. The material state at the weld differs from the bulk, so where a product's safety assessment involves the weld location it has to follow the assessment process for the assembly rather than citing the resin rating directly.

Additive effects on weldability and joint selection are in the weldability handbook and the joint design guide on the downloads page. Amplitude ranges and process monitoring are on the ultrasonic welding machine pages.

Send the FR grade, the rating required and drawings to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.