The goal of heat stretching is to introduce heat to vinyl that is under stress so that the vinyl stretches in a mostly permanent way. I only know of being able to do this with vinyl/pvc air dolls. My method involves an iron that can produce steam(the same that you would use on clothes) and a large bath towel. The bigger the towel the better because that is what will be protecting your doll from direct contact with the metal of the iron(which would sadly be fatal for your doll). The towel does not need to be extremely thick but it shouldn't be too thin. The basic procedure is as follows...
1. Fill the iron with water, put it in steam mode, and let it heat up. Blow up your doll.
2. Cover/adjust the doll with the towel especially around the areas you want to enlarge. Make sure all exposed vinyl is far away from your work area (because any direct contact with the iron will pop your doll!!!).
3. Rub the iron along the towel over the area on the doll that you want to enlarge. If your iron has the ability to control the flow of steam manually, engage the steam briefly periodically as you rub the iron around the area to be enlarged. If your iron has to have a continuous flow of steam, set the temperature to the lowest setting for steam and only rub the iron on the towel for small periods of time. In all cases keep pressure on the doll as that will help the area expand. Ideally you want to apply steam and then use the normal dry heat of the iron to maintain the heat of the moisture from the steam at a nice even temperature that isnt scolding hot. Be careful applying heat with the iron in close proximity to seals, seams, glue, joints, and holes as you don't want to wear the vinyl too much as they are the doll components that will likely be taking a lot of pressure come business time
4. Repeat steps 2 and 3 for each area that you want enlarged.
Using this method, I have been able to enlarge both the butts and boobs of my dolls. This method can also be used in many other areas on a doll as desired. I don't think it can be used to remold hard plastic but it wonks great on all inflated vinyl that I have worked with. It takes some getting used to so I would practice on a really cheap low end doll or have extras of the doll that you are trying to modify. The learning curve was really quick for me but I am sure it will vary for everyone.
Anyway I hope this wasn't too long and that someone finds this useful. Enjoy!
