Working out the amount of moulding rubber you'll need
Here, the best rule of thumb is to fill the area (that you're going to fill) with, say, water, sand or rice (or something similar). Then, pour this amount into a measuring container. The resulting level on the measuring container will give you the amount of liquid you will need. In turn, add an extra 10% to this amount, as this will take into account any losses that occur during during the pouring process.
Can plasticine be used in the silicone rubber mould making process?
The short answer is yes. That said, it mustn't contain sulphur (please ensure you check, as most plasticines do) - this will contaminate the silicone. For optimum results, use Klean Klay.
Why does a part become difficult to remove from a mould?
This typically indicates that the mould has reached the end of its life, so to speak, particularly where small pieces are starting to tear away. In turn, the casting life of a particular mould varies depending on the number of casts that hace been made from it, the type of casting material, as well as the overall complexity of the object and, in turn, the mould, itself.