Just Curious...Why Polyurethane Resin...??

I don't know the answer to this but a quick search seems to indicate polyester resin is harder and therefore more brittle (it has a different molecular chain?), and it may shrink more than polyurethane. Anyway I'm curious too.

 

Polyester resin is more toxic than PU, and not as easy to work with ~ and more brittle, like crowtree says. Plus I think polyester resin is more used for laminating with fiberglass and the likes. When I spoke to the people who were casting Ylisande for Dollfair, they said they used to use polyester resin before poluyrethane became commonly available, and the brittleness and health concerns were definitely reasons why they stopped using it and switched to PU.

All resins that are white or beige to begin with can be tinted to look like a fleshtone.

 

also polyester resin creates much higher heats requiring heat-specific molds - it can also warp if poured in too-large sections. Its main purpose is a bonding agent unless you use it specifically for specimen embedding in which case you would do between 5 and 15 layers with a cure between due to the heat it gives off.... nowdays epoxy resin is also slowly replacing polyester resin for embedding because of the better properties and moisture impervious curing

If you buy large volumes of resin you can have it pre-tinted using a specific formula to ensure the uniformity in colour... kind of like buying coloured house paint - they can add the right amount of pigment for the volume of material..... Barnes Products where I buy my resin from is able to pre-tint resin in smaller volumes which is great especially if you break something and have to replace it there is less guess work....

 

Maybe I know the answer because I workd a bit with resins.
The PU resin can replicate much better the texture and details than the polyester one can. Polyester resins are bad to details, such as the "porcelain" texture of the dolls.
Also, the curing time of the polyester resin is much longer than the PU and it is probably the best reason to manufacters don't use it, or, they would produce much less dolls each day of work. I think it get yellow early than the PU as well.
You can paint over the PU resin much easier than you could over a polyester resin.
Polyester is also is more toxic and is way more difficult to mixture correctly with the catalyst. Easy to get it wrong and your piece get sticky.
Maybe using the PU resin the silicone molds last longer (because it has a limit of copies, you can use the same forever)

 

What about solid surface materials? Is there a reason they aren't used for things other than signs and countertops?

I don't know much about it, mind you, but it seems like it'd be perfect if you wanted to make something that wouldn't degrade over time.

 

I have heard of several other materials being used other than polyurethane resin. Some people used porcelain for their bjd, which from the looks of it the material doesn't have much friction at joints. I came across a mainstream company called Hujoo, they use ABS plastic in some of their lines, which doesn't have much friction at the joints, so they use a locking joints to hold poses.