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Post by bombadyl on Jan 25, 2021 7:19:48 GMT -8
I shall also add : there is never more than 3cm thick of cement on top of the foam (never less either). I try to keep the mass as a low as possible.
I have other pieces of foams to complete the mould from the top during the process to avoid more than 3cm thick. These are not visible on pictures (not made yet) but can be seen in purple in my sketchup.
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Post by Vortex on Jan 25, 2021 11:29:06 GMT -8
I use silicone mastic to seal the joints then when it's set linseed oil on the whole wooden molds. I screw the molds together so they can be taken apart which is a lot easier than trying to get a whole cast out of a mold in one go. Ramming can force pockets of air into the casting, nothing beats vibrating. I use a homemade vibrating table that has an old black & Decker drill screwed to the bottom with a large U bolt in the chuck. I also go around and tap the sides of the wooden molds with a hammer to help free the bubbles and get them to rise up while it's vibrating. You can see all the bubbles coming to the surface and popping. I find it's best to make the molds so the parts that will be the inside surface of your stove are the bottom of the mold, that's where you get the best finish. An old picture of the molds from my original stove:  |
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Post by pigbuttons on Jan 27, 2021 15:40:09 GMT -8
I do have a sort of vibrating table : there is a motor with excentric bolted under the wood board in the middle of the car tyre you can see on my pictures. But pigbuttons was suggesting I would get better results without vibrating, only ramming. I guess I will try a combination of both ! Any feedback on the 15min usable time ? Is it fine to make 2 or 3 mix within an hour for the same casted part ? The concrete I bought already contains small fibers made of SECAR 71 ( ? ). They claim it helps getting the water out when drying so it matches your description. You are right, my mould is not the strongest... The translucid boards are quite flexible in the middle so my plan was to add an outer frame. For the inside, i hope the extruded polystyrène will resist the weight. I plan to add cheap acrylic joint to fill the small gaps. Do you think I shall paint the extruded polystyrène to make it waterproof ? I was hoping the mineral oil for unmolding would be sufficient. You are absolutely right about working time. I mixed it in approximately one liter dry and proportional water, place (adding the wet mix into the form) in 20mm lifts (layers), and ram until it stops compressing. You might be able to do slightly larger batches because of the large flat areas of your design. Mine was an inner and outer box with only 25mm gap all the way around. So it was a small narrow area that had to be filled and rammed. You could probably use the end of a hammer handle as a ram and work even faster as the rammed surface area would be larger and easier to see. SECAR71 is good stuff and yes the fibers are included. My advice i don't add anything but water as they have engineered the mix to be stand alone - only add water and mix until just blended, don't over mix or the fibers will bunch together and you'll have weak areas. As far as the ball test, yes it works just as you described, but you only need to do it once and make sure you keep a log of how much water you used. Then if you measure your mix carefully each batch should need the same amount. But once you see and feel the mix with the right water content, you can mix quickly each time with little problem. If you work efficiently you won't have any problem with cold joints between lifts as the product is designed to bond extremely well to itself. I had one area where it got a little soft after two or three firings. I just scraped the soft part out, mixed up another batch, placed it in the hole, rammed it the best I could and it has lasted almost two full seasons without any signs of cracking or failure. I'm the only one that knows it even has a patch. . |
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Post by pigbuttons on Jan 27, 2021 15:53:37 GMT -8
Vortex, I know you have much more experience than I and don't want to diminish your position, But: the kind of refractory cement being used should never have enough water mixed with it to form bubbles. When is has the right amount of water it still looks like a granular solid. It ends up just a little more than damp when mixing is finished. It will hold together in a ball but will not flow at all, it is not a liquid by any stretch of description. It cannot therefore trap air in pockets because air still travels unhindered through the mix when it is being rammed. It is not like the kind that comes pre-mixed in a tub or others that do look more like thin runny putty or a thick liquid portland cement based concrete mix.  I use silicone mastic to seal the joints then when it's set linseed oil on the whole wooden molds. I screw the molds together so they can be taken apart which is a lot easier than trying to get a whole cast out of a mold in one go. Ramming can force pockets of air into the casting, nothing beats vibrating. I use a homemade vibrating table that has an old black & Decker drill screwed to the bottom with a large U bolt in the chuck. I also go around and tap the sides of the wooden molds with a hammer to help free the bubbles and get them to rise up while it's vibrating. You can see all the bubbles coming to the surface and popping. I find it's best to make the molds so the parts that will be the inside surface of your stove are the bottom of the mold, that's where you get the best finish. An old picture of the molds from my original stove : |
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Post by pigbuttons on Jan 27, 2021 16:04:30 GMT -8
I shall also add : there is never more than 3cm thick of cement on top of the foam (never less either). I try to keep the mass as a low as possible. I have other pieces of foams to complete the mould from the top during the process to avoid more than 3cm thick. These are not visible on pictures (not made yet) but can be seen in purple in my sketchup. With that thickness in mind, you should be able to work very well in and around all of the features of your form. Understand though that the thickness is two dimensional as well. So it is not just about how thick the part is from inside to outside, but also how deep is it from the bottom of the mold to the point where the mix is being added. For example, the area at the back around the riser, yes the riser thickness is only 3cm, but from the floor of the mold it extends up by half the diameter of the riser itself. You will be ramming straight down and not 'around' or side ways into the vertical features of the mold, though some minor angle technique is necessary to make sure the bottom corners get rammed tight as well. |
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Post by Vortex on Jan 28, 2021 2:05:46 GMT -8
Vortex, I know you have much more experience than I and don't want to diminish your position, But: the kind of refractory cement being used should never have enough water mixed with it to form bubbles. When is has the right amount of water it still looks like a granular solid. It ends up just a little more than damp when mixing is finished. It will hold together in a ball but will not flow at all, it is not a liquid by any stretch of description. It cannot therefore trap air in pockets because air still travels unhindered through the mix when it is being rammed. It is not like the kind that comes pre-mixed in a tub or others that do look more like thin runny putty or a thick liquid portland cement based concrete mix. No problem, I've never even heard of a castable mix used that dry. The usual one we get here has 20 KG of grog, 5Kg of calcium aluminate cement and you mix it with 3.5 to 5 Litres of water. It's like a stiff mortar consistency, only the strong vibration makes it level out in the molds. I cant see how what you describe will not have lots of tiny air pockets in it between the granules, but best of luck with it. |
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Post by martyn on Jan 28, 2021 4:55:37 GMT -8
Yep standard refractory mix is 4lt of water to 25kg of product, I have mixed and vibrated tons and tons of it ....
It is a dry mix and all the more likely to hold air but if you have had success with your method that is great.
For large pieces of 50kg or more i use a poker for smaller forms i use a vibrating table.
I did try one of those off set motors on a rubber tyre and they will work to a fashion with a slightly wetter mix but a proper table is an investment as they are not cheap.
My table cost around £1000 but will settle even dry mixes in seconds.
A poker is a good tool and you can usually hire one for a day, you cant use it in a conventional way but you can hold it against the outside of a ridged strong mold or place a thick piece of ply on a tyre and place the mold on the ply and use the poker on the plyboard, however the poker wont like it and will overheat if you push it!
Good luck with whatever method you choose....
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Post by bombadyl on Jan 28, 2021 11:59:45 GMT -8
Thank you all for sharing your experience. Yes the mix I have requires about 4.25L of water for a 25Kg bag. I have just received it so I will have a try this week-end ! My computations say that each of my 2 pieces need 12.4Kg so I hope I will get it right the first time so I do not have to buy another bag !
I have also received the vermiculite board : I can confirm it is extremely rigid ! So rigid that it even seems fragile in case of shock. The surface is smooth but a bit dusty/sandy and some very small particles are coming off when touching. I intend to use it as the top of the firebox. Here are some pictures:
And here is the setup of my quick and dirty "vibration table". We will see this week-end if it is strong enough to vibrate my mix !
I also have a bottle of linseed oil, so I am ready to cast ! I do not know if it will work but at least I am having fun doing it 
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Post by martyn on Jan 28, 2021 12:12:43 GMT -8
I think you might need to mix it with a little bit more water for that type of vibrator to work, they are quite low frequency and really rock and roll the board.
It will be a compromise but should work ok.
The problem is the refectory mix will contain mainly crushed fire brick and that is very porous and soaks up the water, as the cement is so fast drying it locks in the water and encases it, so now you have more potential for producing steam and cracking!
So if you add more water, you will have to take extra care to slowly cure (heat) the end product.
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Post by pigbuttons on Jan 28, 2021 14:45:43 GMT -8
bombadyl, it all looks interesting to say the least. martyn's post above triggered a thought that needs to be addressed, ( thanks to you martyn ). I don't know if you've also read the curing process of the refractory mix. For a commercial application they have a very strict process that we can never hope to replicate without a large enough oven to fit the whole casting into. Even then it would be so time and energy consuming as to make it impractical.
What I did is build a tiny fire, with three or four charcoal briquets. I did this twice.
Then I built a small wood fire of just kindling and kept adding wood to it slowly until I could just see steam coming off of the casting, then let it die out and cool. I did this three times, each time it takes a little longer to get steam to come out because there is less water still in the casting and it takes higher temps to drive it out.
Once no more steam comes out you have to get it HOT,(this is actually the final curing step as the cement is designed to bond together under high temperatures), but again in stages. I built a regular fire that I estimated would run for twenty minutes at full temp, and then let it die down naturally and cool. The next time I ran it for 40 minutes. Then for an hour and a half.
After that you are good to go with full blast, full load fires.
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Post by martyn on Jan 28, 2021 23:45:28 GMT -8
That is why the tiny plastic fibres are so important, they melt away when the cement reaches a certain temperature (burn out fibres ) and form escape passages for moister or steam and add a little leeway for expansion.
With small castings the curing process can be done in a few days, with big castings like a 500 kg wood oven the curing process can take weeks!
If all possible it is best to cure refractory cement with insulation in place but not a finish coat.
This is because the insulation will distribute the heat but still allow moister to escape.
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Post by bombadyl on Jan 29, 2021 5:34:36 GMT -8
I could not resist giving a try this morning  .
I prepared 3 x ( 4Kg powder + 0.8Kg water) so 20% water as the instructions said between 17 and 20%. In the end I did not put all the water in the mix, so probably around 18 or 19% (equivalent to 4.75L for a 25Kg bag). It was sufficient to successfully pass the ball test. It looked like that:
It was solid enough to use my bare hands to fill in the mould and try to push the concrete in every corner. At the beginning I rammed it with my fingers and then with a wood stick, but it was hard to reach every small corner.
So I tried with the vibration motor and it clearly changed the game. It turned into a self-levelling material and filled in every small corner. I was able to see air bubbles coming out, and the (excess ?) water migrated to the top. That was the end of my first 4Kg mix.
So for the 2 other 4Kg mixes, I just coarsely spread the material over the surface and let the vibrations do their magic. It went approximately well but I clearly have some lessons learnt for the next one...
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Post by Vortex on Jan 29, 2021 6:07:25 GMT -8
Well done. A bit of excess water on top is normal, it enables the mix to flow when vibrated and helps the air bubbles to escape. I usually make the mold the exact height of the cast and then any excess water just flows out over the edges.
Don't bother trying to embed bolts into the cast, the expansion of the metal from the heat will just crack the casting.
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Post by pigbuttons on Jan 29, 2021 19:42:11 GMT -8
Awesome work bombadyl. I'm learning too, thanks. I have a question though. When you have vibrated the mix into the mold, can you make a dent in the top of the material by pressing your thumb into it? Just curious.
Sorry your mould didn't hold up better. It is surprising to see just how dense and therefore powerful the mix is. I learned the hard way too the first time I worked with a standard cement pour. Until you actually see it happen it doesn't seem normal, and therefore it is hard to predict the effect on the mould.
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Post by bombadyl on Jan 29, 2021 22:21:42 GMT -8
Thanks !
I removed the walls and external foam shapes of the mold. It was easy in the end because the material that migrated into undesired places very thin and weak. It looks pretty good actually!
There are marks of air bubbles trapped under the foam shapes but for the external faces it seems not too bad. The vertical faces are perfectly smooth.
The removed parts seem reusable for the next cast !
I will wait a bit more before unmolding the inside faces (bottom of my mould).
To answer your question, yes I could dent the surface with my thumb after vibrating. I even managed to insert the bolts with some efforts but it seems it was a really bad idea. I am glad i did not succeed !
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