Concrete mold

What is the best way to create a concrete mold for this? My first idea was to make an simple inner and outside mold made of wood (with the inner part a bit lower so it can close the back). But I do not know if the shrinkage of the concrete will be a problem? Can I get it out (will it stay intact) without destroying the inner mold? I need to make a serie of 2. The dimensions are roughly 400150400 mm with a thickness of 10 mm.


I’ve been toying with the idea of doing some of my own concrete things for awhile, but haven’t made the time for it.
Some of my co-workers had some luck a few years ago using SmoothOn or other elastomeric molds with wood forms built around them for structure (might not need it at that scale). Once the concrete begins to set up and harden, you can remove the wood allowing the flex of the mold to lend some forgiveness in removing your parts from the mold. If you design with a generous draft angle and either break up your mold or use appropriate mold release it will certainly help the process.

keep in mind:
draft, mold release, a vibrating table, concrete mix (my co-workers had a mix that included a fair amount of porland cement)

You’re probably going to need to make a few with different times and mixtures to tune your process.

10mm wall thickness? I’m no expert, but that seems awfully thin for concrete.

I’d make the mold from styrofoam insulation sheet. Fast and easy to remake if you need to destroy it. You could make the cavity portion of the mold from MDF - that could be unscrewed to release the part. I think the core portion will be tougher to release, especially with the low tensile strength of concrete. With foam, it is easy to make a new core.

Shrinkage will be a big problem for this. I took a concrete class in college (not sure why, pretty sure it was the only architecture elective available that semester) and we did a bunch of random concrete molding projects. Even with generous draft and mold release on a bunch of the stuff I remember making (it was 10 years ago) almost everything that was a loop required the inside of the mold to be destroyed to take out.

10mm is going to be very thin for that large of a part you’d most likely crack it if you are trying to break the inside of the mold out. Most thick parts it’s easy just to hammer the wood till it pops off, but for a super tall/thin section like that it will probably just shatter. If you can, I would bump it up to at least 20mm at the thinnest section (and have it get thicker due to draft at the bottom)

I’d suggest trying an elastomer mold like IDiot mentioned since that will minimize the shock you need to put into it to get it out and should give you the best surface quality.

Even if you can get it 10mm, you’ll want to reinforce it.

Fiberglass is one way, steel rebar is another but that may prove difficult…

I think if you watch this video, you could take away a lot of info of how they manage to get their concrete furniture really thin and how they mold it. But yea, the 10mm thickness sounds like that is going to be really tough to accomplish, even the furniture in the video is really thin but it still looks thinker than 10mm.

One of the suggested videos from the last one posted had a link to Homemade Modern
He’s got several links to concrete project videos, here is one that is pretty relevant to your problem, the key difference would be that you are probably aiming for multiples off one mold.

Based on your scale, I was also wondering if you had thought of ceramics, slip-cast porcelain might be a way to go. This will probably dictate an even thinner wall section, not sure what the application is either, but it’s a thought.

Wow, thanks everyone for the replies! :slight_smile: I attached a rough planning on how I want to make the concrete mold with the help from tips here.

I have a few questions regarding the planning:

  • I see that 10 mm is going to be hard. Is 20 mm going to be enough (without reinforcement)? Can you guys give a rough estimation what the minimum is with and without reinforcements?
  • Do I need mold release with this setup?
  • I guess I do not need draft angle in this setup? Or do I need a small one for the vertical movement (orientation taken from the picture) of the mold of the outer part (or make those parts screw-able)?
  • Any tips on how to make perfect fillets on the insulation foam?
  • Do I need to watch out when buying for a specific concrete mix?

Thanks for all the help!

On the inside mold. Do you have access to a router? You could always just use the correct sized round over bit and make the inside that way.

Just so you get the lingo correct,

Outside mold = cavity
Inside mold = core

Hard to say about the thickness and reinforcement. You may have to have to do it a couple of times to find out. I would worry about the 400mm span cracking in the middle because concrete does not hold under tension. the 150mm walls are under compression, concrete’s strength.

I would use mold release. Freeman Supply has basically what is a vaseline in a spray can. Should work well.

You probably can skip draft. I would just screw and unscrew the cavity for it to release. If you can, hollow out the core made from insulation. It will make it easier to destroy.

A router should make a good fillet. Use the pink or blue insulation foam as it is denser than the white.

Any big box home center should a mix specific for making concrete counter tops. I’d try that one.

If you have access, This Old House did a concrete counter too recently. It’s on the Roku app an probably online. Talks about materials to release from a mold and density and reinforcement of the mixture, too.

Not internally though. Honestly you’d probably need to do a pre-tensioned design to make it stand at that thickness.

But what have you got to lose? Go for it!

Here is another approach, the exterior surface finish would depend on your skill at finishing. The possibly uneven aesthetic on the outside could add character.

Inner mold only, no exterior mold.

Build your expendable inner core from any of the methods above. Use wire mesh, bent around the corners and cover your core in two or three layers. Wire tie the ends of the mesh together. For optimum concrete strength you need the steel reinforcement as close to surface that is in tension. Fiberglass with a very open weave would also work, but cannot be bent like steel, requiring more elaborate methods to hold it in place.

Useing a thick fairly dry concrete mixture, plaster the mesh and work the mix through the wire mesh. Continue working the surface with steel tools until even thickness and desired surface finish is reached. Personally I think the surface would be more visually interesting than the gravity induced lines of a casting and the likely air defects.

This method is called ferro-cement and was used to build boats. Ferro Basics - | Bill's Boat Works - Ferro Cement Construction | Ken Adkison

I have nothing to add, but wanted to say that I love this thread. Learning alot!

Ask This Old House episode with concrete countertops:

It looks easier than the others and less likely to break afterwards. If you worked with any kind of composites before, it’s kind of the same thing. Maybe the core could be made of two or three parts, so it’s easier to extract them. Here they were using a similar process

That is very cool. The sheet of concrete is a mind expanding image, never thought about concrete in that way. Reminded me of the way clay is de-watered in a filter press. The result is a similar looking sheet, I wonder if that process to make the fiber sheet is similar.

Thanks for the tips! I want to try it out this weekend, my setup is almost ready. How long will it approximately take to cure? If I Google I can only find times for curing floors (with cars etc driving on them). By the way, for buying supplies, I really envy you Americans. It is really hard to find even the simplest things here in The Netherlands, even online.

Edit: Oh, and I found something out when the 15 mm does not work: Just make the front part smaller (where the drawer is) and cheat and the back by making it thicker. What would be a good size for the back (when aesthetics does not matter anymore)? 20 mm? 30 mm?

I made a mix of chopped fiberglass reinforcement and concrete, and it seems to be working :slight_smile: It is recommended to use 12/13 mm in lenght and 0.16 mm in thickness. I only could buy 6 mm, but it seems OK. For the next one I have to make sure all the air gets out in the bottom part, it seems a bit lighter.


Thanks for the tips everyone! I will post a final picture when everything is done :slight_smile:

Looks great Brian, excited to see the final result. Enjoying your explorations on your blog

Looks really cool Brian, I love the concrete texture. How heavy is the thing? Should be pretty light with the fiberglass. Is it strong enough?