Perhaps there is one thing to consider, but trial and error will probably sort that out. The thing I wonder about is how are you going to avoid an air bubble in the mold when pressing the heated sprue into the mold? Such an air bubble will be trapped and cause a disformed part.
When using rubber moulds and pouring resin into it, the bubbles will be there too. With casting of brass, bronze or white metal there are always air shafts in the moulds to release any air trapped inside.
For small bolt heads the problem may not occur.
regards,
Ivo