Right chaps I thought I would walk you through what I have done to create 2 part moulds.
First off I used Lego to create a frame and backed the lower half with play-doh.
Into this I pushed the parts I want to mould halfway in making sure there are no gaps to allow the blu-stuff underneath them.
I then use paper clips to create channels for the resin to run down.
The way I have set it up here is I will inject resin in the top down a central channel that will then go off in either direction at the bottom and then fill the cavity up from the bottom pushing air out the top. this seemed to work well on my first test run.
Mix and pour the blu-stuff on top and allow it to harden. (I found mix of 20ml was enough for each side of this mould.)
Once the blu-stuff is dry in about 20 minutes then you pull the Lego apart and remove the play-doh. play-doh is very good here as it is not to sticky and should not pull the master parts out of the blu-stuff. If it does just put them back as best you can.
Then flip the blu-stuff so it is on the bottom and coat it in something that will create a barrier else the stuff you pour in will bond with the bottom half (I found Aqueous cream worked) just don’t get it on your master parts as it might lessen the detail the blu-stuff picks up but I have heard some people use dusting spray as it contains wax and not sure if this will lose detail or not. build the Lego back round to it till you have a well and then fill with blue stuff till it is cured.
Once cured pull the Lego apart and then separate the 2 halfs. It might take a little bit of effort and sharp knife.
You might need to cut away some parts to make sure the channels are clear for the resin to run down but you should now have a mould to cast into.
In the new year I will try and cover using the resin to cast with.