My method of casting a lead nose weight for a RC plane. Once you have made the plasticine plug there are a number of ways to make the mould. I've tried plaster but there was some residual moisture in it which wasn't visible and it destroyed the cast. You can mould straight into sand but that's quite a delicate operation. This is my preferred method. The main thing to remember with any method is that the mould must be bone dry. Any water instantly turns to steam when you pour in the molten metal which causes it to spatter making a mess of the cast and possibly a mess of you too. It's quite a dangerous process so do some research and take the necessary precautions.