Is there a way to glue a knot in thread?

Try a tucked blood knot.

works on fishing line. Or I suppose you could try a dab of super-glue on the knot after you have tied it. I would go for the fishing knot however much surer.

Interesting knot. It looks like it would definitely do the job. The problem is that this is a mobile, so the length of the line is critical. I am afraid that in order to tie the knot, I'd need to leave at least an inch or two of string to be cut off. I am afraid that would mess up the balance of the mobile.

I thought of super glue, but I was afraid that it would harden to the point that the edge where the glue ends would be fragile. It's a mobile. The kids might move it about a bit.

I was hoping that there might be some special kind of sewing or craft glue that would keep the knot from coming apart, but would remain flexible. I did a search for fabric glue and came up with several products. Maybe I'll just try one on some spare string. I thought someone here might have used one before and could make a recommendation.

You need to test a piece first, but what I have done is to strike a match, blow it out, and touch the still hot head of the match to the end of the nylon. It scrunches itself up into a rough ball or nugget. But test on a scrap first, just in case it's not nylon. Also, have a glass of water handy to dip the nylon string into if it catches fire. But by blowing out the match that shouldn't happen.

There are a lot of stopper knots, quite a few directly relevant to fishing line.

The double overhand stopper is an easy one

The Ashley stopper knot is bigger, and also easy to tie

The length of the line will not affect the balance of the mobile: the line weighs next to nothing and the hanging item always positions itself so the center of gravity is directly under the line.

As pogonip recommended below, heat has beneficial effects on plastic in cases like this. Try tying your stopper knot then heating it up a bit - you shouldn't even have to fuse it, just heat it enough to "set" the plastic in its knotted shape.

I don't have any scrap. I got this thing from a garage sale. It's a mobile. The length of each string matters. The string is very small. I am not putting a match anywhere near it. If I ruin one line, then I have to replace them all.

A reel of light fishing line wont cost you much at all to replace these lines. Where I live you can get a reel for a couple of Euros in the supermarket. Then you could replace all the lines

Claire in Montreal FRANCE

Assuming the material is nylon of some sort...

When you re-tie the knots, pull them tight with pliers (but not to the breaking point, of course), then blast them with a hot-air hair-dryer. If you get the heat level just right, it should "set" the knot such that it won't easily come un-tied again.

On nylon rope, I carefully use a propane torch to set knots.

If you have a disposable lighter, you can use that too. Light it, let it flame for a few seconds, then let it go out. Touch the hot metal around where the flame was to the line. That's what I do with all nylon strings, straps, etc.

Sharon

As everyone says, heat will set it. Its called monofilament, and is likely nylon. If you need to you can get more for =A31 from poundland.

NT