Casting Cement Elements

[MarvinG]

Dear community,

Over the last winter I built my first brick oven. Since this may, its constantly hot and in commercial use. We have opened our small take away and have sold hundreds of pizzas since then. The feedback has been extremely positive which gives me a lot of energy.
-> my build:

Meanwhile I have a "new" Idea, however I will not be the first one to try this. Therefore I'm writing here.
I would like to build the same dome oven, slightly bigger, around 42", but this time. But instead of bricks, I would like to build it out of refractory cement. Basically I would like to make 4-5 casted cement elements. The result should be a precast oven kit in a way. Something like this:

My questions to the forum: Did someone experiment with casting elements for an oven? How would you build the form? I was thinking of XPS and Epoxy, but this topic is new to me.

The reason I don't want to buy a precast kit is simply because its fun building. And it will allow me to grow my business here as well.

best,
Marvin

[fox]

I am working on my second wood oven mold, it is not an easy DIY project and you can expect to invest a lot of time and money to produce a multi component mold!
I work with basic tools in a basic workshop without hi tech machines but I do have a life times experience with mold building and composite materials.
I would say that building a take apart multi use, multi component mold is most definitely a difficult and complex job but given enough thought and time it is obviously possible.

[MarvinG]

Hi Fox

Thank you for your reply. I'm not as experienced as you in this regard, but I bring plenty of experience in other topics and have good hands. The principle is clear, the youtube videos how to do are also available. In my case the material sourcing (I live in Georgia, Caucasus) is pretty difficult. Yesterday I bought some glass fibres, however considering the epoxy prices I might switch to the polyester alternative. Let's see.
Can you give me a tip how you would/did bild the positive to then apply the mould? I'll give xps a try and then see how it goes. What I'm struggeling with, is that Gelcoat and mold release are not available. any easy substitutes you know of?
Thanks!
Marvin

ps: it would be great to see some fotos of your work!

[fox]

Well the first mold I built worked really well and the ovens are very crack resilient but I did not really take many build photos of the mold building.
I do have a video of casting one in the mold and there are some more of my ovens on my youtube channel.
You will need gelcoat, any marine chandler should sell polyester products or maybe roofing specialist.
Epoxy is far more difficult to work with in this case and far more expensive too!
The easy method is to find two inflatable yoga balls and make you basic dome shape fro them as the gel wont stick to the silicone balls.

[MarvinG]

Great job! I think I stumbled across your video the other day when I was researching on youtube.
So, you would recommend me to work with polyester and specific resins or hardeners. Yeah, i'll see if I find the Gelcoat, I mean I can order everything online abroad, but from the components you need some quantities thats not very cool due to high shipping prices.
Do you think I could also use silicone Gel just to brush on my model, if I have to decide against the balls and build something with xps?

Another question: When you used the balls for moulding. Did you cover the whole ball first, and then cut rings and added the pieces where they rings are attached together?

[fox]

Well like I have said there is an awful lot of work involved much of that work you just wont anticipate until you get going!
Those ovens are actually made up from 7 components , the bottom and middle ring each produce three separate parts with half lap joint, it really does take a lot of time to do but that is OK if you are enjoying yourself!
I dont really have the patience or vocabulary to explain in detail how it all fits together but each component had a 2mm gap but the inner half lap joint sits together tight …
Very difficult for me to explain in words …
So basically stage one, in my case, I inflated a ball to 32”, cut a 32” hole in a sheet of ply and set in the ball to cast an exact half.
I used three coats of gel and three layers of 2oz fiberglass mating.
I found this picture

[fox]

So that casting was pulled off the ball and then used as a mold, so the inside of the casting is then lined with gel and fibergalss to produce the inner form for the finished dome, so ultimately that first casting is thrown away! So you can see how things add up !

[MarvinG]

Yeah, I guess it's like always. 70% of the work is hidden somewhere you cannot see it, until you have it in front of you...
Great pictures. It gives me a good impression. And I can totally understand that it's not really explainable through words here.

Let me try to understand it this way:
- For the outer layer you put first Gelcoat, then your layers of fibre.
- For the inner part you use the same ball but smaller, and put your fibre layers first, and then add your gelcoat on top I guess.
- Now you have the inner and outer layer of the mold - but need to cut in parts, add the overlaps to make then strong and be able to connect them.
Is this half way right?

Did you ever consider to make it like this? what do you think about it? It gets interesting at around 7 minutes:
https://www.youtube.com/watch?v=N2rrf0iuULQ

[fox]

Your video shows how the vast majority of DIY pizza ovens are made over a sand dome, there are hundreds of builds on the forum that use the sand pile method.

l wanted to build a multi part mold that produces inter locking components that can be cast with vibrated refractory cement.
Anyway your description of how I did it is not quite right as you cant just paint on gel as a finish top coat, so I make a mold from the ball as in the picture, then pull it off the ball, now I have to paint the inside of that mold with gelcoat and add the fiberglass… all on the inside of the mold. Then you can pull out the fished mold and throw away the first outer mold as it is of no use to you!

[fox]

I found the original thread ….. https://community.fornobravo.com/for...the-dome/page2
Like I said that particular mold is no more and I am building a new version but unfortunately I have just had major surgery so I am laid up at the moment..

[MarvinG]

Again great and very helpful information. I will need a day or two to fully dygest your last thread. So in the pic above we see the new (easier to produce) form. Less single parts. And as I can see the inner part is one piece and on the outer one you do not have rings either?

How did you manage that the vent receives enough concrete?
Is that what we see the outer layer which is put over the dome? The vent is kinda missing, no?

Yes, got you for the inner wall. So gelcoat must always be first (inside) and then the fibre layers. I though its a shortcut to apply it later since it would be very nice to save the time for the inner layer, since the ball already has the right shape.

Thanks, Marvin

[fox]

Sorry those pictures are just the early stage of construction and dont represent the finished components.
Basically you need two yoga balls based on the size of dome you require, you may get away with one if you are brave enough to over inflate the ball by 4.5” !?

It is possible to apply gel as a top coat with the addition of a liquid wax that forms a skin oven the gel and allows it to cure but the finish will require a lot of sanding and rubbing down, it is more practical to form a mold and then cast the fished component from that mold.


certainly you could simply cast two halves and fill them with cement but you would be left with a 600lb dome of concrete that is very difficult to manovoure without a crane and would most certainly crack when heated up.
You could cut the one one piece casting up into four with a cutting disk and re ensemble it in place but the joints would be straight cuts with no gap for expansion…

[MarvinG]

Hi Fox

I hope you are getting better from your operation meanwhile. Thanks for the input.
Yeah, I'll def. buy two balls. I think its worth the money ;-).

But in your case, you also end up with a 300 kg dome? Correct? or did you make 4 moulds which are pieces of the whole dome (like cake pieces)? This is what I originally intended to do, but cutting sounds tempting too. How thick should it be, 8cm? or even 10cm? that would be 4"?
It would also be nice to have 3 different moulds. One for piece one and two (the back of the oven) then one for the front right, and one for the front left. So you assemble them all together. I think that is exaclty what you do in your second attempt, no? If I judge from the pictures correctly?
I think If I find a way to do so in a practical manner, I will go for this version. but first things first, I need balls!

[fox]

My first mould produced three rings of roughly the same mass of around 75kg each.
I them modified the mold so the bottom rings were in three pieces and the top piece in two, so they were more manageable to handle.
My new mould will be more conventional with 4 segments (like a cake) and one top cone piece.
The first mold had a 55mm thick casting space, the new one will be 65mm.
so the inner mold just stays as it is but the outer mold has to have all the work done to it so the separate components can be removed.
I have another four weeks recovery before I can lift anything more than 5kg so still stuck at home getting bored!
you can see the rings in this video
edit… in that video I have used fiberglass as the final finish so the oven is very durable and used matting first and top coat of gel mixed with terracota dust.

[hillscp]

Bellissimo forno!