Flexible Jewelry Molds gives the idea that these are special molds. I chose 'flexible' to let you know that I'm not talking about rigid, metal molds on this site. Molds on this site are heat-cured natural rubber, heat-cured silicone rubber, and room temperature vulcanized (RTV) molds.
These molds are flexible. Some are as soft as a rubber band. On the Shore A scale of hardness they are about 28 durometer. They can also be as hard or even harder than an automobile tire, about 60 durometer. See a graphic here on the Plastics International site to see the range of hardnesses in rubbers.
I try to make molds as hard as possible. The wax still has to come out of the mold without breaking or warping. This means that softer rubber might surround the interior and be supported by harder rubber on the outside. The harder the mold, the less it will deform when it is clamped prior to wax injection. We also want the interior of the mold to hold its shape in the flow of molten wax rushing into the mold cavity. This leads to faithful reproduction of the master in the form of waxes.
'Flexible' also refers to the idea that we need to be adaptable in the materials that we use to make molds. Perhaps a simple putty RTV mold will work on your master? Maybe a heat-cured natural rubber mold is needed to withstand a long production life. Silicone rubbers with different curing temperatures, a range of hardnesses and no need for release spray is probably the best choice for your master. What shape of mold frame will you use? Square, rectangular, round? The more experience you get, the easier it will be to find just the right combination of firmness and shape.
I hope that I can support you in all your mold making information needs!