Regrind, Pigment and Flame Retardant: Change the Formula, Redo the Parameters
A familiar scenario: parameters that ran well for six months suddenly produce weak welds or heavy flash. Machine checked, tooling checked, and the answer turns out to be a change of material supplier or regrind ratio.
The same grade is not the same material.
Regrind
Every melt cycle degrades polymer chains to some degree and lowers molecular weight. Blending regrind into virgin material means:
Lower melt viscosity — melt flows more readily at the same energy, flash increases and the joint thins.
Faster start-up — a slightly lower softening temperature melts sooner at the same settings, inviting over-welding.
Batch-to-batch variation — and this is the real problem, since regrind sources and histories differ. However wide the window, incoming material wandering across it defeats the process.
In practice, fix the regrind ratio and write it into the work standard rather than running 10 % one day and 30 % the next. Tighten it further for sealed or structural parts.
Pigment and masterbatch
Pigment takes no part in fusion and acts as an impurity at the interface. At low loadings the effect is limited, but two points matter:
Dark colours generally need more energy. Black with a high carbon black loading rarely shares parameters with natural material. Where one product is made in several colours, validate each colour separately rather than assuming one recipe covers all.
Poor pigment dispersion creates local weak spots. Masterbatch is more consistent than powder.
Flame retardant
This has the largest effect. Loadings are often substantial, and the way flame retardants work makes them unhelpful to interfacial bonding — they take no part in fusion, and at high loading they amount to a barrier layer at the interface.
The result is that flame-retardant grades typically weld to noticeably lower strength than the non-FR version of the same grade. This is set by the material, and more energy will not recover it — more energy scorches the surface first.
In practice, where the product must be flame retardant, design joint strength around what the FR grade actually achieves by lengthening the joint or choosing a more effective joint form at the design stage, rather than forcing it in process.
Glass and mineral fillers too
As filler content rises, less resin is available to fuse at the interface and strength falls, for the same reason as with flame retardant.
Glass fibre also wears horns noticeably; aluminium horns have much shorter lives on highly filled material, so consider surface hardening.
What to do when material changes
1. Record the change. Supplier, grade, lot, regrind ratio, colour — log any of them that moves. Without a record there is nothing to investigate later.
2. Run a trial. Weld the new material at existing parameters and confirm fusion by destructive test, not by appearance.
3. Re-establish the window if needed. Trial results close to a window edge mean the new material narrowed it, and the centre point needs finding again.
4. Review monitoring limits. Normal ranges for energy, power and post-weld depth may shift as a whole, and the limits should follow.
Weldability guidance by material and filler system is in the handbooks on the downloads page. Process monitoring and parameter memory are described on the ultrasonic welding machine pages. Send old and new grades with trial results to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.