Well, if you used the slush method for a molded head that had a neck sculpted onto it, you'd just have to make sure that the resin made it out onto the neck area while you were slushing.
The neck and the head would be hollow, and would just slide over the neck post. You'd probably have to glue it on though, because it's really hard to make a neck perfectly adapted to stay firmly on the post and also just pop off when you want it to..
^^^ Exactly what DA said. Heres a quick tutorial on the slush method with a head with a sculpted neck fitting on to a Hot toys neck.
Step
1. Here is a simple one piece mold made with Smooth on Omoo 25 Silicone. It has a layer of resin inside that has been "slushed", the head inside is hollow but is very thin.
2. This shows the casting resin, here I'm using Smooth cast 300. It sets up in about 4-5 minutes.
3. You need to mix equal parts of "A" and "B" together till you get all the streaks out.
4. Quickly pour it into the mold.
5. Now as the resin begins to "gel" you keep turning/spinning the resin around within the mold especially towards the neck area. You would want to build this area up thick enough to fit over the neck piece.
6. After that sets up around 5 minutes remove the mold from the outer shell. In this example I'm using the yogurt cup I made the mold out of with the bottom cut out.
7. Remove the head.
8. As you can see the bottom of the neck is uneven due to the slushing process so you need to sand it down flat so that it fits flush on the neck post and to the proper height. Also you still need to dremel out the inside of the neck to get that perfect fit.
9. Next simply pop head on to the Hot Toys neck. As DA pointed out it would be a good idea to glue the head onto the neck post for a nice snug fit. I recommend using rubber cement so the head is not permanently glued on.
Again this is one way of doing it, if you are doing a lot of casts I would use the two part mold. No dremeling or sanding for that method.
Sorry about being so long, hope this helps.
