Hey there LaxFriedrichs, I am also heavily interested in bladder inflation molding.
I've gotten close to success recently after some frustrating missteps early on. Here's what I can tell you.
Those recommendations on fuse molding some mylar sheet in a specific pattern did not work for me. The bladder would blow at only 30 psi. I've since been told that I should have done my dry-run tests with some dry fiberglass cloth to ensure that no part of the bladder snags at the flashing line.
NO ONE seems to want to make a prototype inflation bladder. No one answers their email save for PolyTech Synergies LLC, and even then their prices are more oriented toward batch processing. They were kind and professional to me, but I just couldn't afford what they were asking.
So - let's talk about how to make your own bladder.
I've tried Smooth On EZ Brush silicone. I probably used too much, frankly. I reduced the volume of my mold, put silicone in one half, rested a near-shaped polystyrene plug in the middle, poured on an excess of silicone over the top half of the plug, and closed the mold. It was messy but silicone rubber that soft is easy to clean up mechanically. It doesn't stick very strong.
After the rubber cured, I melted out the plug with acetone, the deconstructed volume of polysterene foam is amazingly small, about 1/40th reduction, that is not a typo.
Both attempts failed and had to do with poor inflation neck design. If I were to try it again, I'd actually brush it on a plug in multiple coats.
So, speaking of brushing on in multiple coats, here's a cheaper material: latex.
This stuff or, even easier,
this stuff. The former you brush on a plug, the latter you dip the plug into a container. The former can put on a lot of thickness in one coat, but each coat takes a long time for the solvents to evaporate, not to mention the need for wearing a VOC mask. The latter smells less offensive but I wear a mask just to be safe. More interestingly, the latter thinned latex can have its per-layer cure time significantly reduced with a heat gun - you'll know you're getting close when it goes from off white to a pale yellow. So if you want to put in some extra sweat equity in terms of heating each layer carefully, you can get a result faster than with brushing. I did a basic test very recently with the dipping latex and achieved 2mm thickness in 90 minutes, a vast improvement.
Latex, however cheap and relatively easy to fabricate with, will stick to epoxy, so you need to wax the bladder before laying up fiber and epoxy around it.
The basic process (having not actually practiced it yet, mind you) with a finished bladder: pour some sand inside your bladder or
lightly inflate it, lay up your fiber and epoxy, pour the sand out / let the air out, insert into one half of the mold and close the mold. Inflate the bladder, and heat / wait for epoxy cure. Then de mold and hope that both your bladder and mold have been polished and lubricated enough to allow release.
I have not yet produced a BIM part. However, recently, I *did* succeed in blowing my mold apart in one place! After which even a 4mm thick bladder blew up at that location. The second I have a successful part you can bet I'll put up a tutorial.