Tooling boards are - unfortunately - very expensive for a hobbyist like me, especially if thicker and larger pieces are required. Therefore I wonder if there are alternative materials that have been used by other forum members instead of proper tooling boards to keep the cost low.

The obvious possibility is MDF but I'm not sure if there's a way to get the surface good enough. Does anyone have any suggestions?

My plan is to cnc mill the mould directly without making a plug first, but I could change plans for specific reasons.

Kind regards
Martin

Been looking for that very thing myself Martin ....................  I've spent weeks researching on the Net!
My solution will be to 'mould' my own   

But I'm having great difficulty in finding out what the actual tooling board materials are!

Like you,  my intentions are to CNC my plugs directly,  then apply Pattern-Coat resin  to finish off the plug.
As I'm making headlight prototypes,  CNC'ing the OffSide and NearSide seems obvious.  As opposed to doing them individually by hand as I've been doing.

'Smooth-On' make a wonderful array of expanding polyurethane foams available in various densities, some with a rubbery consistency. Some cure rigid and are quite strong apparently. They tend to use them for back-filling large moulds.  I've got some in stock for testing purposes,  just haven't had the time yet.  And I've also got to build the 'holding' box for it as well.  I think the general idea is to mix the two components,  pour that mixture into the box and secure a lid down, fast!  leaving a few air holes in the top of the box will force the expanding foam out,  the rest of the foam will be compressed to a machinable block of foam.

I just wish I could find a place where I could get my hands on a sample of the tooling board,  as a comparison,  I think it would be quite interesting.





TURK

What kind of temperatures are you looking at curing? As my tooling board is good up to 70degrees. 
You won't find any recipes for tooling block as the suppliers add a lot into the formulation.
I'm working on a project trying to reuse or recycle tooling block as so much is used by the composites industry and 99percent ends up I
 in landfill.
Aluminium billetsare almost comparable in cost to tooling board now. So I'm using more of it.

I'm having good results with gluing up mdf, machine the plug, seal with polyester resin (so duratec doesnt soak right in), sand, shoot with duratec surface primer, sand/polish. Only downside is curing the mold. This part  im still currently trying to perfect. Post curing can't be done on the plug because of the max temp duratec can handle. My first try at this I ended up with dimples in mold after post cure, most likely because I had air bubbles between surface coat and laminate. Working on second try now. About to do final post cure. This time I decided to do a 50c post cure before pulling the mold but even that was a bad idea. Duratec took it fine, but the mdf expands with heat so you can see faint lines on the mold where layers of mdf were glued together. So really, no post cure allowed at all on the mdf/duratec plug. This time when laying up the mold on the plug,  I vacuum bagged it after wet lay to remove all air. Really hoping the post cure goes well this time.  I'll report back when it's done. Buy anyway, for molds that don't need post cure, the mdf/duratec plug is excellent. Soon to find out if it's viable for prepreg molds that are cured off the plug