When two or mold parts must be used a parting line must be selected. The parting line must remove all undercut. In the example below the parting line is along the sharp edges. Note a mold form will be made for each of the mold part. Mold release will be used also for each mold part to keep the parts separate.
To keep the soft mold parts from moving while casting, registration marks are use. The marks in the mold form base are registrations for the top to bottom. Larger marks are needed for "thick" molding materials. Air bubbles will be trapped in small marks. Softer rubbers need more care in registration placing close to the parting line or leaks will happen.
Pour cup, sprues and venting
A pour cup is a funnel built into the mold to to get the casting material in while minimizing the impact to the shape. In the spear example the pour cup is in at the shaft end. Sprues are channels from the pour cup that give a way for the casting material to enter hard to reach areas. Vents are like sprue channel that allow air to escape the mold. Viscous materials and metals require special detail to these process to get the best quality casting.
Soft rubber molds need a way to retain their shape. A plaster or hard plastic backer are used.