Hi Pham,
I’ve looked into various welding techniques a bit, but not really in regards to strength of bond. Heat sealing creates a bond of more than adequate strength for the way I design things – when reinforced properly, the fabrics fail before the heat welds do – so I haven’t needed to explore other options.
For the typical DIYer, RF/HF/ultrasonic welding is well beyond their price point (~$600 for a “cheap” hand-held model direct from China to about $6000 and up). I think that unless you’re setting up a manufacturing business or have a bunch of money to spend on a fancy machines, heat sealing is the way to go. Even most outdoors gear manufacturers I know of use heat welding (inflatable boat manufacturers tend to use expensive foot-pedal operated heat welding machines).
If there’s an inexpensive handheld RF/HF/ultrasonic welder available somewhere, I’d love to hear about it, but I haven’t been able to find one.
Cheers!