Is a lip necessary on an HVAC ceiling register boot for use with flexible ducting?

Question

I've installed some new ceiling register boots to use with flexible ducting and realized after install that they have a smooth connection with no lip. I'm assuming they are made for metal ducting, as I think flexible requires a lip for a tight connection. How can I securely attach flexible ducting?

One idea I had was putting five or six screws around the connection to act as a lip. I also thought maybe there is a tool that I could make some dimples that would do the same thing. Is a lip even necessary for a solid connection?

Thoughts?

UPDATE: Since I don't really know the proper names for things, I think pictures might help.

Here is pretty close to what I have:

Here is the lip I'm talking about that is on the current register boxes:

Here is one with no lip:

On my current ducts, there are two zip-ties, one on the inner core and one on the outer insulation. The inner core zip tie is behind the "lip". I assumed this was necessary for a solid connection (Note: I'm using mastic, not tape. Whether one is better than the other is another topic). Here's a picture of the inner core I was referring to that I want to put the zip-tie on.

So I've now changed my question to: Is that lip needed and if so, any way to fake it if not there?

Answer 1

New answer thanks to updated question.

Yes, the lip around the rim of the opening I'd call it a "ridge" but I dunno if that's right) on the boot is there to help ensure a firm connection. It's to keep the zip tie on the inner core from slipping free. This is necessary (the keeping the duct snug, not nessarily the tie) if your ducting is going to be pulling away from the boot due to gravitational stresses - such as when the boot is above the duct and the duct can be pulled down, such as along a wall or below a ceiling or floor. If your boot points down and the duct sits on a horizontal surface, you're good - no extra tension should happen.

Important note - neither mastic nor tape are intended to be structural - they don't serve to hold the ductwork in place, they serve to seal the ductwork so that there are no air leaks.

So the question really is - how do I properly secure my flexible ducting to the boot when there's no lip on the collar? Answer: Self-tapping screws.

Get yourself a box of those (the hardware aisle or the ductwork aisle). When you go to mount your ducting, pull the outer insulation back up away from the core, pull the core over the collar, and use TWO screws on opposite sides of the duct (180 degrees from each other - noon and 6'o clock (or 1 and 7, 3 and 9, you get the idea) to ensure that neither side of the duct can pull free.

Use the inner to snug up the connection (don't over tighten!), seal with mastic (I'd recommend tape though cause of the insulation layer...), slide the insulation up as far as it will go and snug it up with the outer zip.

Answer 2

If I understand your question, you have a boot like this:

And you want to put a crimp like this on it:

So that you can slide some flex duct to it.

If that's the case, then there is a crimp tool to do that:

It's also possible to do using a pair of long needle nose pliers (which saves you the expensive of buying/renting a speciality tool for a one-off job). Just grab on, twist 30 degrees or so, move the plies over a quarter inch, and repeat. It takes a minute or so, and probably doesn't produce as nice results as a professional crimp, but when you put the duct on and seal it with foil tape (as you should be doing anyway), it works perfectly fine.