The material used to CNC the forms is something I need to consider carefully as it completely determines the cost of machining the forms. Whilst the idea material is pink model board, it is very expensive and would take the CNC machine far, far more time to CNC in comparison to blue Styrofoam. The blue foam costs pennies as well so is by far the cheapest option for the CNC. The problem I face is not only is the the finish on the blue much rougher than the model board, but also blue foam can’t be cast from without sealing it. You can see the rough surface of the foam below-
The ideal would be to write toolpath which provided a very smooth finish on the blue foam and find a way of sealing the foam so it can be cast, getting the same casting quality as one would with the pink model board, at a fraction of the cost. Whilst you can write a toolpath which smooths the surface of the foam a lot, due to the nature of the Styrofoam, it is almost impossible to get a completely smooth surface without any indents. Because I am trying to achieve a polished finish in Bronze and Aluminium, the surface of the pieces being cast should be absolutely as smooth as possible, to minimise the amount of cold work you need to do after casting. So I experimented with how I might be able to finish the surface of the blue foam in a way that would fill in any of those small gaps and indents. My initial test was to simply paint the surface with a couple of layers of thick emulsion paint. However, this didn’t work very well, bringing out the surface defects more than anything else.
The next think I tried was Gesso, which is a mix of chalk, gypsum and binders which can be mixed with PVA or paint. When I applied this to the blue foam surface initially, it didn’t provide the best result-
However, when I sanded this back with a fine grade of wet and dry sandpaper, the surface became very smooth. Extra coats of this could then be applied to fill in any bits missed to provide a surface which was sealed, hard and smooth, perfect for casting. It could also be painted over as well providing a further layer of protection and also helping to see if there are any imperfections left.
This method does take quite a bit of time, but I am saving a lot of money by using blue foam. It will probably take up to 4 days work of applying coats and sanding them back to get the pristine surfaces that I am looking for. But I can work out of hours to do this. I am also considering leaving the surfaces quite rough on some of the planets with lower reflectivity so they have a rougher surface and therefore reflect less light.