The parting plane is the plane where the gates will be modeled, and is parallel to the X-Y plane.
Click New Layer to create a new layer for the parting plane. Name this new layer with an appropriate name, and make sure that all other layers are deactivated.
Click (Geometry tab > Local Coordinate System panel) and enter coordinates in the First box. Click Apply then Close.
Select the local coordinate system that you have just created, then click (Geometry tab > Local Coordinate System > Modeling Plane).
Using the modeling plane as a reference, create regions which represent the plane(s) where the mold cavity and core meet, using (Geometry tab > Create panel > Regions). The region(s) should completely surround the part and have a hole where the part is.
Select all the regions you just created.
Click (Geometry tab > Properties panel > Assign).
Click New > Parting surface. The Parting Surface dialog appears.
Specify the interface conductance across the parting plane and click OK. The interface conductance depends on the mold material, the contact pressure, whether there is air or grease between the mold cavity and core, and the thickness of the gap (if any).
Mesh the new regions by clicking (Mesh tab > Mesh panel > Generate Mesh) followed by Mesh Now.
If you have not done so already, model the external mold surface. Leave a small gap between the edge of the parting plane and the mold surface to avoid element intersection problems.