Long and Short Glass Fibre: Same 30%, Different Result
The drawing says PA66+GF30, the supplier changes, and welding starts to fail. The loading has not changed; the fibre length has.
What length changes
The distribution at the interface. Short fibre disperses fairly evenly in the melt, so resin and fibre alternate across the joint face. Long fibre forms bundles, producing large fibre-occupied regions alongside relatively resin-rich ones. The result is a weld whose strength varies more along its length than with short fibre.
The attenuation. Longer fibre means the wave crosses fewer interfaces, but each impedance step affects a wider region. In practice far-field welding on long-fibre parts is less stable: at the same amplitude some locations fuse and others do not.
The abrasion of the horn. Long fibre slides further across the working face and wears it more locally. Aluminium horns last noticeably less on long-fibre material than on short.
What to do
Put fibre length in the purchasing specification. This is the root fix. Giving only a percentage leaves a variable free to move. Re-confirm after any change of supplier or grade.
Move the joint towards shear. An energy director relies on instantaneous melting at a tip, and where fibre bundles, that tip may itself be a fibre-occupied region that will not melt. A shear joint fuses progressively along the mating face and is less sensitive to local fibre concentration.
Set amplitude for the worst location. Do not set parameters from an average; set them from the hardest section of the joint, or a few places will always be left unfused.
Manage the horn as a consumable. Replace by piece count rather than by calendar on long-fibre material. Photograph the working face and watch the trend, which is more reliable than a single judgement.
The ceiling on strength
Long or short, fibre takes no part in fusion. A 30% loading means roughly that share of the resin at the interface is missing, and joint strength cannot reach parent-material level. That is a material limit, and equipment does not recover it.
Design to the real strength of the filled material, usually by lengthening the joint or choosing a more effective joint form, rather than searching in the parameters.
Signals of a batch change
Suspect incoming material before the machine when any of these appear: yield steps down after a particular date; flash suddenly increases or decreases; post-weld height shifts as a whole at the same settings; horn wear accelerates. Compare the date yield changed with the date the batch changed — faster than any machine adjustment.
Fibre thresholds, additive effects and joint selection are in the weldability handbook and the joint design guide on the downloads page. Amplitude ranges and horn selection are on the ultrasonic welding machine pages.
Send the grade, fibre length and drawings to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.