The surface's don't knit because all of those rectangles (created by Surface Plane 5 and 6) appear to coincide.
Hope that helps. I'm not sure what your end goal is so I can't offer much more help!
After my original post I noticed the cavity and core surfaces were not in the correct folder, but if you looked at the jpeg screen shot that I corrected that. As for being a simple part yes, but running it through SW standard mold tools process does work with those end openings and under cuts on the side of the part.....please take a shot at this and show me, I would like to see how you would accomplish this and I myself learns something. Yes the rectangle opening is a shut off area created by a cam slide (not yet created) coming in as the mold closes meeting up with the cavity as the mold closes.
Not sure why this surfaces won't knit, because I don't know exactly how the "Tooling split" command handles the surfaces. I tried to follow the complete procedure for mold tool functions by using the "shut-off" surfaces function first, so Solidworks makes the core and cavity surface automatically instead of manually as you did it. The result in the end is the same, surfaces will not knit...
It's nothing wrong with your surfaces, if you make a copy of the parting surface and then knit core and cavity to one parting surface each, SW has no problem to do that. It's also possible to use these surfaces to "Cut With Surface". (see attached file). I hope that somebody that understands what the "tooling split" does with the surfaces can answer your question. I don't use the mold function to create core and cavity because it has limitations on "real" products with complicated parting lines, shut-offs and side cores.
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