There is an extension of Pro/Engineer called Mold Design. Do you have access to it?
When you choose Applications-->Mold/Cavity design, it should bring up some new options if you have this.
While it would not be impossible to make your core and cavity in regular Pro/E, it is certainly much easier if you have access to the mold design package.
If you are designing a production tool, you should probably design your part to take plastic shrinkage into account.
This is one of the benefits of the mold design package, since it lets you focus on your desired design and not the details that change for manufacturing.
I'm tooling mold designer.
If you don't have the module "mold Design" yo must to do next:
-Make a copy of your model and apply the Shrinkcage value scaling the part, if we talking about PC plastic you must to scale 1.006.
-Create an assamble and insert your model shrinkaged.
-create inside the assy a new part empty and activate.
-Copy the main surfaces for the cavity side or the core side
and complete with another surfaces merged that you can create after that.