Same Grade, New Batch: Why the Parameters Move
"The material has not changed, it is still the same grade." That sentence appears constantly in yield investigations, and it usually points in the wrong direction.
The same grade is not the same material
A grade specifies a set of ranges, not fixed values, and several things can legitimately move between batches.
Molecular weight distribution, which governs melt viscosity. Lower viscosity means more flash and a thinner joint at the same energy.
Filler content and distribution. Fibre percentage can move within tolerance, and length distribution and sizing chemistry may change too, so the resin available at the interface changes with it.
Pigment and masterbatch. A colour change alters both loading and dispersion. Dark parts show it most, and a black grade with heavy carbon black rarely shares parameters with the natural grade.
Mould release. The most hidden of the group. Residue on the surface changes friction at the interface directly, and it appears neither in the grade designation nor on the incoming inspection sheet.
Regrind ratio. Where regrind is permitted, the ratio is often decided on the floor, 10% one day and 30% the next.
Why moulding is fine and welding is not
Moulding cares about filling and appearance, and these variations are usually absorbed inside the moulding window. Ultrasonic welding cares about the melting behaviour of a thin layer of resin at the interface, which is a far smaller quantity, so the same variation is proportionally much larger.
Hence the familiar situation: everything is normal in moulding, and welding yield steps down.
How to investigate
The first step is not adjusting parameters, it is comparing dates. Put the date yield fell beside the date the batch changed. That takes minutes and beats half a day at the machine.
Read the shape of the change. A step down usually points to incoming material or the fixture; a slow decline points to horn or tooling wear; random good and bad points to unstable location.
Watch the flash. A sudden increase at the same settings usually means lower viscosity, pointing to molecular weight or regrind. Less flash together with lower strength points to less resin at the interface, meaning filler or additives.
Watch post-weld height. A wholesale shift means incoming dimensions or melt behaviour changed; it is not random scatter.
How to prevent it
Put the variables in the purchasing specification: fibre content and length, regrind ratio, whether mould release is used, and colour. A grade alone leaves five free variables.
Run a first-article check on a batch change — not an appearance check but a destructive one, sectioning the joint or pulling it. Appearance has no resolution against a cold weld.
Fold material changes into change control. Purchasing changes supplier for cost or supply reasons and does not routinely tell process engineering. Getting that information to the welding station is cheaper than investigating afterwards.
Replace after-the-fact sampling with process monitoring. Limits on energy, power and post-weld height move as soon as the material does, without waiting for a sample.
One reminder
Change parameters when you must, but record the current ones first. It sounds obvious, yet many shops cannot, and then cannot go back.
The effects of additives, regrind and pigment are in the weldability handbook on the downloads page. Multi-dimensional monitoring and parameter storage are on the ultrasonic welding machine pages.
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