Question about casting in urethane rubber

Hello fellow members!

Simply put, my question is: how is the process to make urethanre rubber castings (ex.: cowls and masks) without seam lines? What material is used as negative mold?

I'm familiar with latex and the use of plaster molds, and I learned how to avoid/correct seam lines. However, urethane rubbers like vytaflex 40 are tixotropic, which poses another difficulty. So, how is the process?

Thanks in advance.

 

Ok, I just got my answer by means of another thread.