36x60x54 Twin Tandem Mill Engine

[GreenTwin]

I have seen a tree of parts cast using the lost-PLA process, with investment casting, and with gray iron, and those parts do not have hard spots, even though they are thin (I have two of them, and have personally drilled them easily).
See video below.
So I know this method is viable if the right materials and processes are used.
This method used standard PLA filament, and not any special filament.
The holes have a taper to them intentionally, and they came out perfect, (it is a bit of a trick to have perfect cast holes).

I agree with JasonB that the bound sand approach could be tricky and/or expensive at this model engine scale.

It should be noted that the original poster is using a simplified 3D model, which may make it viable in a sand mold scenario.

 

[Jasonb]

The simplified design will be fine with bound sand, most commercial foundries here tend to use one of the airsetting binders for just about everything. I would think it is the same in the US. But whatever sand you use it still needs loose pieces to be able to pull it from the sand.