Hi guys,

I'm having some difficulty figuring out the best way to mold a two sided part. I should add that I'm completely new to this!

What I plan to do is machine a master on the CNC out of 100mm Polyurethane board and then make a mold box, placing the master in this, and filling with addition cure silicone rubber. I should add that this is a large part measuring 950x180x100mm. I'm not sure what shore rating silicone to use for this. I'm guessing the 40 Shore A that EC supply isn't too soft for such dimensions, especially since it'll be supported with a box? I'll be needing around 17kg for the mold so want to get it right since it'll be an expensive mistake if not.

The main problem though is figuring out how to get the rear side detail into the casting without trapping lots of air(some air is fine since this face is hidden when assembled) or distorting the flat rear side portion of the cast part(very important as this is the mounting surface). The way I've tackled it at the moment is to level fill the mold with the casting resin and that will form a guide for the rear. I guess I could leave out the rear details but that'll mean throwing it on the CNC and wasting resin. Would much rather do it in one shot to save time and money.

I've attached the front and rear views of the part along with the first half of the mold but I would be grateful for suggestions on how to tackle the second half of the mold.

Build yourself a wooden box that clears the part by 10-15mm all around. Set the part up so that the flat side is level with the rim. You'll need to have a "lid" for the box you can attach the part to. When the part is attached to the lid with double sided tape and all holes sealed fit the lid to the box with screws. Flip the box over and remove the base. You should have the part sitting at the bottom of the box now. Get sheet wax (5mm) and cover the entire part making sure you press the wax into all the tight spots. Apply PVA release to the wax (not neccessary but I do). Lay up a few layers of glass over the part and the walls of the box. When cured turn the box over and remove base and part. Extract the sheet wax. You should have a void of about 5mm between the part and the glass shell. Drill a few 12mm holes in the glass at the highest points, refit the part that's still attached to the base and you can pour the RTV. You'll use a fraction of the silicon compared to filling the box.