I have a vinyl figure that is not restoring to natural shape after heating. I wanted to simply put something inside to help it keep its correct shape, but I am afraid of having the material react with the vinyl, as I've see rubber and styrofoam do. Does anyone have any experience with this, or suggestions that might be safe? Thanks in advance.

Plain old sand?
The clean quartz sand you can get at a pet store that sells aquarium stuff (natural color).

I stuffed newspaper in a toy once and it did the trick.

Kingboy, did you want a short term solution or a long term one?

just wondering, how high is the heat you are appying to the figure?

I've messed up resin toys with ink from newspaper before so i'd avoid that. Plain brown paper should be fine though.

Actually it was just plain packing paper I don't know why I said newspaper. Probably because I remember pulling it right out of the box the toy came in and fixing the problem, but what do people normally pack with? Newspaper.

Anyhow if it's a vinyl toy I doubt it would make a difference. Resin is soluble to the oils in newspaper ink, hence why your toy was messed up. Unless your Tetrahydrofuran it's not going to mess up your vinyl toy.

Thanks for the suggestions.

I want a long-term solution. And in terms of heat, I used a blow dryer for a bit of time. I know some people swear by near-boiling water, but I never tried that. Maybe it would work better.

Not sure about sand - I don't want it too heavy. And I was considering paper, but I thought it would make crinkling sound every time I handled it. I really prefer something that would be essentially undetectable, like a foam/sponge-like substance. But in truth, I will rarely ever handle it once it goes up on the shelf, so paper is probably fine.

I had a couple of really badly warped figures, and used the hairdrier trick on them, but they tended to rewarp pretty soon after.

Near-boiling water then a plunge into iced water was much more effective. The only negative point was that some of the metallic paints oxidised... a quick buff bought back the original colour though and the figures are still standing fine. That's just my personal experience, can't guarantee it will work for you. (that was a disclaimer, in case you were wondering!)

Although I have read that some people use the high heat followed by a rapid quench before, as a general recommendation - not necessarily for this thread discussion - let me just say that I would avoid this simply from a materials point of view. As soft as vinyl can be, that is some harsh treatment you are putting it though, and despite what you may commonly think of from clips of blacksmithing, is not a good method even for annealing most metals. Slow, gentle cooling is far better to allow the material to set and avoid fracture. Not saying that this would necessarily happen to your toys, you might even get away with it a few times over, but strictly speaking if you are patient and can wait/hold and set the figure, there is no advantage to rapid quenching.

You should have no reaction with a can of expanda foam, they used that stuff to make the old 3 foot store display sofbi more solid and I have never seen one show damage due to that, but I would cross reference any odd chemicals they may have listed as an ingredient as the formulas have likely changed over the years.

...Man that is a GREAT question.

Personally i have worked out 2 options for getting toys to stand/fix up shape issues:

1) easy - add rice to weigh down the bottom/fill out the toy

(rice is non reactive, lasts for millenia/is archival and is damn cheap and easy to find)

2) hard - add a stand. THEN add magnets to the stand, and to the bottom of the toy.

(only issue with this is the toy will always need to be on it's stand)