Best material for a table-top sized cast concrete mold?

[Bfarn]

Hi all,
I'm looking to make an outdoor table, with the tabletop made of cast concrete resting in a metal frame. The tabletop will have a fairly intricate pattern cut on a CNC router (a series of inch-wide grooves cut about an inch deep), covered with a sheet of glass and caulked into the frame.

What I'm wondering is what material I can use to make the mold, without breaking the bank. I'm not an experienced caster, so I assume I'll be going through a lot of iterations.

The best material I can think of that comes in a large enough size would be a couple sheets of those giant Styrofoam insulation panels from the home depot, glued together into a slab. But that kind of foam doesn't hold a very clean edge against a drill bit. Plus I don't imagine it makes for a very durable mold.

Any other materials I know of are either not sold at that size or would be CRAZY expensive.

Any suggestions? Or maybe an alternative way to cut and/or treat the foam?

Thanks!

 

[robn1]

Any concrete casting I've seen used polyurethane rubber molds, as it's tougher than silicone. Pricey though.

I suppose you could build a negative form from wood, and pour the concrete in that. Just make sure you can release the cast when it's cured.

 

[Bfarn]

You know, that would give it a nice wood grain texture, hadn't thought of that before. Thanks!

I'll look around online for advice on mold-release from wood. With so many deep grooves it might stick in there pretty well. Any advice?

 

[robn1]

Steve Neill gives some great advice in his Enterprise thread http://www.therpf.com/f11/studio-half-scale-tos-enterprise-scratch-build-98400/

You can save money by only making a thin rubber mold, and then back it up with a mother mold made of plaster. In this case you would make a positive form to mold from. The rubber will give you clean details and easy release.

I have no idea how to mold straight from the wood, I've never done anything like that.

 

[Bfarn]

Man, you weren't kidding, what a write up!

It looks like there's a lot of products out there for sealing porous materials prior to casting, so a wood negative seems like the best choice for me for now. I'll come back and report news if I end up succeeding. Now it's time to start spending rnd money...

 

[modellernz]

these guys are right .. a negative mold is the best way to go .. i used to work for a concrete slab company and we used normal plywood which had a smooth surface on one side .. we used ordinary mold release but only very sparingly ..if you check with the local builders ill bet that there is a company specializing in cast concrete slabs for industrial buildings that would probably cast it if you really get stuck .. and with regard to the pattern you can use anything you like so long as it dosent have an undercut of course .. as a signwriter ive been called on to do this many times and we used to cnc ply/mdf/polystyrene .. well any substrate that we knew we could remove from the mold cleanly .. there are alot of vids on youtube for "concrete casting" for gnomes etc

 

[Beaker]

You can get away fairly cheap (comparing cost of material to re-usability of material) with building a thin urethane mold backed up with fiberglass... Our Brush-On 40 gallon kit is $119.25 for 18 lbs of material.. this kit can make a mold that is almost 8 square feet total in size and last for a very long time in production... depends if it is a one off (then maybe not as cost effective) or to produce castings to make money, then it does easily pay for itself.