the best way to seal cushions

The sad truth is that if you sew/stitch the fabric, your sealing options are limited. The best (and I use that term loosely) option you have is running bead of silicone adhesive on the inside of the seams.

Sound hacked?  It is. Manually running a bead on the backside of all your seams is a labor-intensive (read: expensive) process. What's more, the seal isn't even that great and won't hold up for long.

So what are your options? The first thing to do is to get rid of as many seams as possible.  For example: depending on your application/material, a good commercial upholsterer  might talk about thermoforming the fabric to shape.

Assuming you've already done all you could with regards to getting rid of seams, there is really one option for the engineer tasked with making a cushion water/blood/incontinence-tight:

Weld it.

RF vs. ultrasonic welding

The two most popular methods to weld fabrics together are Radio Frequency (RF) and Ultrasonic welding. They both rely on heat and pressure to create the bonds, but go about it in slightly different ways.

Ultrasonic welding creates heat the old-fashioned way. It uses a tool to apply pressure between two sheets of material, and then rubs them together really fast (hence "ultra" and "sonic") to create friction. That friction creates the heat that fuses the two pieces of material together.

Radio Frequency welding is a little more scientific.  Like ultrasonic welding, pressure is applied between two sheets by a die (if you aren't familiar with the term, these guys do good, quick job of explaining it). Instead of creating heat through friction, radio waves are sent through the die and into the material. The waves cause the molecules in the material to vibrate, creating heat.  The heat then fuses the two sheets together.

So in summary, the difference between Ultrasonic vs Radio Frequency welding is the heat mechanism.  Ultrasonic welding creates heat through friction, and RF welding creates heat by moving molecules.

material considerations

You might think that the process that relies on a specific molecular structure (RF welding) works on a smaller number of materials than a process that relies on friction (Ultrasonic welding).

You would be correct.

RF welding is most often used to weld vinyl-based (think PVC medical bags) and polyurethane (popular with healthcare fabrics, and even folding kayaks). Some vendors claim varying levels of comfort in welding other materials like EVA, Nylon, and ABS (but I'd tread carefully).

Ultrasonic welding can be used to weld pretty much any thermoplastic that that creates friction.

weld strength

I have spoken with three different suppliers regarding weld strength. Their responses:

• No difference between Ultrasonic and RF welding
• RF welding can be much stronger
• RF welding is slightly stronger

Based on this and the fact that RF welding is generating heat at a molecular level, I'm inclined to believe that RF welding is superior.  If you can school me to the contrary, I'd love to hear your thoughts.

I intentionally left out cost because it's so variable. Depending on the shape and length of seams, one may be more expensive than the other.

I will say that RF welding equipment is much more expensive, so that might get baked in the price. Your best bet is to go to a reputable upholsterer who has both capabilities (so their unbiased), and work with them to see what's best.