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Boss cracking after insert welding

For mobile handset ,we weld nut insert to the plastic panel for assembling the top and bottom panels.The OD of the insert is 2.45 mm and the boss ID is 2.2 mm.The machine used is of 20 khz (2000 W).after welding the bosses are developing horizontal cracks(across the boss) inside.Can somebody give advice?


Hi Alban,

You really need a plastic welding expert for this one - can you ask your equipment supplier?

With my limited knowledge of the subject two thoughts come to mind:
1. If the direction of the cracks is always consistent, this could be caused by distortion of the panel during welding (so that returning to its old shape it stresses the boss). How well does your support match the shape of the panel - could you improve this?
2. The parts seem quite small for a 20kHz system - you might get more control over the weld process at a higher frequency.

Hope that helps.

Hi Chris,
Thank you for the advice.We are also on the same line of changing the machine to 40k.Pleas give some info on the testing of the welding strength of nutinserts and some benchmarks..


Sorry, can't help you with strength figures - as I said it's not really my field. But for that kind of application I would think the geometry of both the insert and the hole would be critical. Can't your suppliers (either equipment or insert suppliers) give you any example data?


Hi Alban,

First of all my name is Abd. Wahab, working in Motorola as Ultrasonic welding engineer. This is my first day joining in the forum.

Regarding the problem that u are facing, maybe i can provide u some help

Boss crack after insertion welding normally happen because we weld them too fast. Normally people will weld the nut in less than 0.3 s. This will result in the nut been force down to the boss hole (cold pressing) the end the nut is been press fit, and the material surrounding the nut is not melted, but was push away. This result in stress surround the boss...and in worst case result in crack. Some of the crack cannot be seen immediately, but can be seen after the unit was used.

My suggestion is, u still remain using the same 20kHz welder, but try to weld the nut slower. This will allow the plastic to melt and reflow and fill up the nut thread, at the same time it will avoid stress on the boss. A good identification of the boss stress is by measuring the "boss OD" before and after welding. If the boss diameter remain the same or slight increase (around 0.05mm) after the welding, it mean the welding is good as the molten plastic already fill up the thread surround the nut. But if the OD increased to about 0.2mm, it show the welding is not good, and the nut was been press fit. Visually, u can see the boss bulge.

Normally we take > 0.6s to weld the nut down. U can reduce the welding parameter to achieve this. The parameter that need to be reduce are:
1. Use low to medium amplitude (total gain of horn/booster should be around 1.5 to 2.5).
2. Low to medium pressure (15 to 40 psig)
3. Low Downstroke speed - to allow melting to occur and prevent cold pressing
4. Use of pre-triggered (if available)

For better setting parameter setup, pls provide me the welder that u been used & the parameter that u been using

I hope this suggestion will help


That's great advice Abd. Welcome to the forum and many thanks for your contribution.

Hello Mr.AWahab,
Thank you for your advice even though I am seeing it after a long time of posting.Now I am with another issue.One of our Mobile phone is having Flat plastic panel fixed upon the main panel.(Both flat).One is made of PC clear(top layer) and the other one (Main panel)is Cycoloy(PC+ABS).If I do Ultrasonic welding,on which component I should provide energydirector.


Hi Mr Alban,

Sorry for the late reply as i was quite busy for the last 3 days.
Regarding welding 2 flat plastic, generally u can place the energy director at either component. Anyway, normally people will place the energy director at the bottom it will help in ease of aligning and top component placement.

By the way, may i know what type of energy director that u plan to used... because normally for mobile phone the tricky part is the wall..normally the wall thickness is around 1.0 to 1.5 mm thickness which result in difficulties and limitation of energy director design.