Silicon mould with FDM 3D printed part

Silicon will pickup the most minute detail from the surface it cures in contact with. Depending on the intricacy of the part it is possible, by a combination of process, to get a really smooth surface without compromising with dimensional accuracy.

_ Firstly, it is obvious that the main focus should be the print, choosing the right settings and material.
Not all materials will give the same surface finish. LOW temperature and speed settings will generally give you the best results.
A well tuned machine is also important. Under extrusion for example will ruin the surface finish. Low settings will lower the risks.

_ Slicing.
FDM leaves obvious layer marks. Printing from @0.05mm to 0.08mm layer height should give you good enough surface finish to work with.
A 400-500% first layer height should ensure a good bed adhesion.
The part in contact with the bed can have a slight dimensional accuracy but can be corrected by hand, like and edge to deburr.