DIY: how to copy a 3D-printed object?

Hi everybody

As part of a research project I must make a silicon copy of a 3D-printed object; I have the object (a simple, fist-sized shape) and I have the silicon (which has to be poured in a model), and I have to somehow make a negative of the object. The copy must not be sub-millimiter perfect, and there are no small details or sharp angles. One of the sides of the object is flat and as large as the object.

Since I have some experience with the material, I am planning to make a plaster negative of the object inside a small box, and when it solidifies I pour the silicon in it.

My doubts:

- will the 3D-printed object stick to the plaster? Should I first apply a layer of oil or something?

- will the silicon stick to the plaster? Maybe I can apply again oil, or some powder like talc...?

Any hint from the experts? Thanks in advance!

Have a look if sandcasting is the thing you need.