So you want to work with thermoplastics?
Just be warned the lower the temp required the more likely you will end up with this sort of issue. That is a set of medical splints made from one of the many medical grade thermoplastics (thermoplastics have been used for splints for a long time and cost about the same as the stuff marketed at cosplayers- I was looking into them a decade ago for my Galadriel breastplate). On the left we have the left hand as left in the car over night and on the right the one brought inside. Thumb to the outside in both cases.
Yes it is summer but this was not even in direct sunlight just in the car. Overnight. Last night. This was not staged or deliberately done (these are needed medical devices!)
The easier a method is to use the easier it is to destroy. That said these are super fun to heat back into shape as they are super easy to form and reform and flatten and use over and over again. Part of why they were developed is the ease of remodeling.
Not saying don’t use the it, just saying there is no perfect material to work with. So think about storage and transport before choosing ease of use- there is nothing wrong with the stuff but you will need more care to look after it especially post painting/surface detail if something like this happens…
This is why I have plastic storage boxes for my armour (fibreglass- but it has paint that could potential transfer) and wire racks for my fabric and leather componants (in the shade and I switch the racks around every so often) and am open wire wardrobe in the same spot that also moves around for airflow.
I want my stuff to last as long as possible, so I build using methods used in the originals. Tiny ridiculously tiny machine stitches for corsets and Victorian- flatlined and much use of glazed cotton in areas not seen. Lots of hand finishing and interlining of facings and lining in my Renaissance gear. Use of high qualitly slip latex for my appliances (I want to do foam latex and silicon so bad but foam is super fragile and silicon is actually also fragile when encapulated in stuff that is easy to paint- think of how chicken fillets wear out, ya’ll know what I mean here!)
I think of the thermoplastics marketed to cosplayers fosshape is the one I most want to try. It’s been around for decades and the way it shrinks when worked suggests bonds form that are less easy to break.
I already use foam extruded PVC (generic term for Sintra et al), pipe PVC and styrene. The foam board is brittle but so yummy to form. PVC already formed into pipes seems to be a little softer to work with than in sheets, not quite sure why, it may be just different purposes as you need flex with plumbing! And Styrene is really prone to warping with direct heat.
And fibreglass requires hundreds of hours of sanding and bogging and sanding to get a good surface.
See, nothing is perfect, no bias. Just I do get annoyed when the cons are not given for hyped materials.
I still can’t find my own splints, which I desperately need, so may be buying a small sheet of stuff to redo them 😉 Mine are for trigger finger so wrap around index and middle fingers up to the first joint and extend on to the palm to thumb. I was aware of the melty issues so always kept mine out of the sun. Sigh. But I have lost them somehow.