how i can make an exact medical skull mold ?

Answer 1

Today, the best way would probably be to laser scan or MRI to make a 3D computer model and then "print" that model in plastic using a 3D printer.
The old fashioned way takes a lot of work and you may have to make some choices about missing detail, as it is much easier to mold if you can fill openings like eyes and nose and just cast the outside. Basically, you must make many castings of sections of the skull, which you define with clay boundries so that you can make plaster castings and pull them or make silicone or latex molds and support them. The idea is that you use soft plasticine (oil based clay) to build a boundry dam around a portion of the skull - like the forehead. You then put release if needed on the skull - Vaseline for plaster for example. You then pour a thick mixture of plaster or a thick layer of silicone molding material or build thin layers of latex within the dam. If using soft molding material, you then pour a plaster backing to support the soft material later. Ideally, you want all the parts to fit together, so you leave the first negative in place, remove the clay, build a dam on the next section, treat the edges of the first with release and build a second negative the fits tightly against the first. You may have to put clay into holes (eye sockets, ear canals, etc.) to provide support for outer parts. At times you will pull the negatives and later put them back to work in a different direction on the skull.
When you are finally done, you have a lot of pieces that fit around the skull. If you have done it flawlessly and made all the right choices, you may be able to assemble the pieces and pour a rigid material in the mold. More likely, you will have to build the skull in sections, which you might do with wax or plaster. Then the sections must be assembled.
If you choose one route, you cast all the pieces in casting wax which is quite tough and assemble by heat welding them together. With the wax, you may be able to cast or hand make tiny details that are otherwise extremely difficult to make, like the zygomatic arch and sinus surfaces, and then add these details to the assembled bigger pieces.
Once the wax skull is assembled, if it can't be used for your purpose (it is a model of the skull), you can then add sprues and vents and encase the wax in plaster or investment. After careful heating to drain the wax, you can fill the plaster mold with liquid plastic or heat the investment mold and fill with metal. Break away the surrounding material and clean up the model.
Or you can buy one if you don't need a model of one particular skull.