HO Scale Rock Negative Form for Mould Making

The idea is to make durable moulds, which hopefully last for many reuses. Well, that was the idea. The print was made on my backup printer (a trusty but old Monoprice Select mini 2) with some standard PLA, using a 0.4mm Nozzle at 0.0875 mm layer height with the exception of the first layer (0.2188 mm). 

The print is full of holes, which suggests putting more than 3 massive layers at the top to enclose the model near these slopes and crevices. Also I seem to have some underextrusion in this print. Well other than that, it seems to be a feasible approach at form making. More on that at a later time, as soon as I got to put some more love into this. 

Brushing the interior with a grease-like seperating agent - I use vaseline (vaseline oil or petroleum jelly)  - patched those holes mentioned above. The form takes just short of 100 ml or 3.75 oz of fluid when scaled at 0.96 for my 120 mm printbed. To determine the amount of silicon needed for your form, fill the form with water and pour that into a measuring cup. That should give you a pretty accurate number.

The latest photos show the silicone mould right after taking it out of the mould making form, as well as filled with plaster and the cast removed from the form. The form was still glistening from the vaseline coating, but already hardened. I have a few bubble holes in the material. These could end up as tiny balls dotted over the surface of the cast. They can be easily popped away using an Xacto knife or similar.

The last photos show the plaster cast rocks and the form underneath under different lighting conditions. I'd say this was a successful project.

Lesson learned: The silicone mould doesn't have a uniform perimeter height, which causes plaster to flow out of the mould. A strip of painters tape around the mould fixed that for me.

This model is provided "as is". Feedback welcome. Please try the original mould, its really nice and most of the work is to be credited to the original creator and his model H0 rock moldby Jakub Kočí.

Scale by a factor of 0.96 to fit onto a 120mm Mini-Bed. 
Scale by a factor of 0.54 for N-Scale (I wouldn't go further down, as one will loose a lot of detail). 

As these are rocks and rockfaces in nature don't really abide to model railroading scales, scale as you wish for desired results ;-)