LCP: Skin-Core Structure and Anisotropy
LCP appears in connectors, precision structural parts and high-frequency components, chosen for heat resistance, dimensional stability and the ability to mould extremely thin walls. Every one of those advantages is an obstacle to ultrasonic welding.
Three characteristics, three difficulties
Very low melt viscosity. Molten LCP flows like water, so once an energy director opens it spreads or escapes instantly, without dwelling at the interface long enough to bond. A part can be given exactly the right energy and still fail to hold, while producing plenty of flash.
Almost instant crystallisation. It solidifies as soon as it leaves the melting point, so the melt has a very short life. Hold must follow the end of ultrasound with no gap, or the melt sets first and no amount of force will close it.
Strong molecular orientation. This is what makes LCP distinctive. During moulding the molecules align with the flow, producing a marked skin-core structure: a highly oriented, strong skin over a looser core. Strength along the flow direction and across it differ by a factor of several.
Orientation decides joint strength
A weld cuts across the material and severs exactly those oriented chains. LCP joint strength is therefore far below parent-material strength by nature, and that is anisotropy rather than poor process work.
Design accordingly: place the joint where the load is lower, and calculate from the joint's real strength rather than the material datasheet. Expecting parameters to bring the weld up to parent level is the wrong direction.
What to do
Use a shear joint with modest interference. An energy director depends on melt forming and staying in place, and low viscosity prevents that. A shear joint contains the melt between the mating faces. Keep the interference small; LCP is brittle and will not carry it.
Err low on energy. Low viscosity plus fast crystallisation makes the window very narrow. Use energy or depth mode, not time mode.
Hold immediately. No delay between the end of ultrasound and the hold phase — more critical here than with other materials.
Make the fixture rigid. A narrow window demands repeatability; if the part can move, the result will drift.
Consider other joining methods first
Honestly, many LCP parts suit ultrasonic insertion or staking better than welding. Neither requires the two sides to fuse, which sidesteps the hardest part, and the captured component need not be compatible.
LCP parts in connectors are usually combined with metal terminals and PCBs, which cannot rely on fusion anyway, so staking is the natural choice.
What we can do
Welding a material like LCP does not rest on some proprietary parameter set. It rests on three things together: enough amplitude headroom, the right joint form chosen at the drawing stage, and process monitoring that watches every part. With a narrow window, "this part welded well" and "every part welds well" are different statements.
The actual values have to come from trials, because geometry, filler, orientation and fixture condition all move the result. We work the same way ourselves: trial first, then fix the parameters, then lock them in with monitoring.
Difficulty ratings and the compatibility matrix are on the downloads page. Amplitude ranges and multi-dimensional monitoring are on the ultrasonic welding machine pages.
Send the grade, filler and drawings marked with the flow direction to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.