I added two model parts, a command cupola and an external fuel tank, to a sprue.
Step two: I built the mold box using perspex.
the instructions I was following called for the sprue to be placed in a bed of modelling clay, like plasticene or blu-tack and when I tried this, the pressures involved snapped the glued joints of the sprue. I decided to adopt a different approach and put a foot on the sprue so it could stand on a 1 cm bed of silicone.
tep three: Having laid a foundation of silicone, which I left to cure over night, the sprue was placed on top.
You can see on the image how the accumulation of release agent in the keys has softened the edges of the lego bricks. It is noticable that inside the depressions made the model parts, there is no such softening.
My improvised mold making work station in the attic
My improvised gas mask - I've ordered a real filter mask, the chemicals are pretty nasty, but it hasn't arrived yet.