Trying to design a mold for the above model but keep getting the "cannot knit sheets together" error whenever I attempt to do the tooling split.  I am trying to implement an interlock surface 15mm below the top plane.  Tooling sizes: upper block 80mm, lower block 30mm.  I would prefer to have the slide cores come out the bottom of the mold, hence why I chose the top surface ellipses for the shut off surfaces.  When defining the shutoff surfaces it does not allow me to knit.  Do not understand how to rectify the problem.