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Refractory cement for roasting chamber parts?
Linnaeus
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Posted on 09/18/2017 9:04 AM
Hi All,

Curious if anyone has used refractory cement (Furnace cement / Aluminide + Silica cements) for parts which may directly contact heating elements or beans.

Reason I ask is that with the advent of 3d printers it has become very easy to prototype parts in plastic. These parts are of course not suitable for any contact with heat as they're thermoformed themselves and the entire apparatus would do a raiders of the lost ark rather quickly. That said, the parts can be used as masters (or a mold may be directly printed) which is suitable for casting.

On casting. Most materials that are typically cast in more detailed molds like this are resins - these resins are also not suitable due to a high volatile content and relatively low melt temperatures.

Some castable materials of interest:
* Iron / Aluminum - Requires a forge - If no forge is available you might as well have them CNC'ed
* Plaster - Very easy to cast and resistant to high heat, however I'm thinking that they'd crack under high heat differential.
* Fire Clay - This is probably the ideal material, but requires a kiln to cure
* Refractory Cements - Which brings us to this one

Where to get it:
Most hardware stores sell "Furnace Cement" - if used alone it may be more likely to crack (based on a little research) and could take up to a month to properly cure. However, it can be mixed with firebrick grog, or even perlite to make a more flexible material under heat stresses.

Use cases:
I was hoping to use this to build the bottom and top of a fluid bed roasting chamber to properly hold the pyrex tube and mount an embedded perf plate.
Additionally, I was curious if this could be used to make a compact enclosure for a heating element.
As a first test I'll be using it to build an air chamber for a heat-gun + flour sifter roaster.

Benefits:
* Friendly to rapid prototyping
* More insulating than steel or aluminum
* Cheap

Downsides:
* Could crack - not yet sure how true this is. Worse, could explode if steam is trapped... Not a risk if properly cured.
* Takes a LONG time to cure - could take 3-14 days (for the size of the parts we'd be casting)
* Rough surface could cause more turbulence than is desired - though turbulence may be a benefit it this is a suitable material to cast the enclosure for a heating element.

Best,
Rob
 
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Brewin_Bruin
Top  #2 Print Post
Posted on 03/07/2018 1:38 PM
I used firebrick + refractory mortar to build my heating chamber, interfaced this to the RC using stainless steel parts (but no machining/welding needed, just drilling and cutting). It's detailed under my recent post "Bake-a-Round build with heat recovery".
Brewin_Bruin attached the following images:
picture_2_4.gif picture_1_4.gif picture_4_1.gif
 
ahardinger
Top  #3 Print Post
Posted on 03/07/2018 8:13 PM
I'll share that I've used DIY refractory cement to make two metal furnaces that I've done cast Iron/Bronze in and it's held up really well. The critical part is the cure and heat-up. Let it air dry for 1-2 days per inch of thickness. Heat it to 230F for 1-2 hours per inch of thickness. Then ramp it up to ~600F slowly (like 50F per hour) and hold it there for 1/2 hour per inch. Ramp @ 100-200 F per hour to normal temperatures.

The 600F part is critical since the cement is holding onto chemically bound water that must be driven off in a controlled manner.

But, with coffee roasting I'm not sure you'll get above 600F, so may then it can just be "cooked" in a normal oven from 230F to 500F slowly over the course of a day or so?

Here is the recipe I used:

Quote

Refractory Cement Recipe #1

What You Will Need:

Portland cement (You can purchase a 94 lb. bag at your local hardware store for less than $10.)
Perlite (Can be purchased for $10 to $25.)
Silica Sand (A 50 lb.bag costs less than $25.)
Fire clay or Well drillers mud (A 50 lb. bag averages less than $10.)

The Formula

1.5 parts Portland cement + 2 parts Perlite + 2 parts silica sand + 2 parts fire clay

What to Do:

Using the portions listed measurements listed above, mix the Portland cement, Perlite, and silica sand together thoroughly.
Combine the mixture with 2 parts fire clay.

Once the mix has the consistency of stiff cookie dough, pack it into the performed form. You may need to add a little bit of water to get the right consistency.
Allow it to dry for several days.
 
BenKeith
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Posted on 03/07/2018 9:28 PM
After reading again, I deleted my post, it didn't apply.
 
sethyrish
Top  #5 Print Post
Posted on 03/07/2018 10:53 PM
I've been looking into this myself as I was going to try casting roast chamber and element chamber for my project but have since changed to stainless construction.

About 15 years ago I decided to buy a bag of castable refractory cement to make a brake drum forge as there was so much conflicting information on whether to use recipes with Portland cement etc and it was so cheap to buy at the time ($11). Now it's $80 for a 25kg bag. This stuff was quite rough in texture but has held up really well especially considering it's in direct contact with steel. I cant seem to find information on it now but only know it didn't contain portland or plaster.

I was thinking some kind of slip cast terra cotta clay would look nice but there doesn't seem to bee much info on the net about it.
https://digitalfire.com/4sight/education/understanding_the_terra_cotta_slip_casting_recipes_in_north_america_132.html

https://www.googl...p;bih=1309

i.imgur.com/w0zlXR0.jpg
Edited by renatoa on 01/21/2023 2:01 AM
 
BenKeith
Top  #6 Print Post
Posted on 03/08/2018 10:38 AM
If I was going to try casting one, it would either be in aluminum or ceramics. Naturally ceramics would be by far the easiest.

I think castable refractory would be the wrong stuff. If you go with hard cast, it would absorb a lot of heat and be very slow on temp changes. If you used insulating castable it would probably crack and would have to be totally protected from being able to get wet. Also, doing either of them in a full circle without some type of expansion joints, there is a chance of cracking. At the extremely low temps (for refractory) you will be running cracking might not become an issue but there is always that chance.
As for the home brew DIY stuff, I wouldn't waste my time. The only benefit it offers is it's cheap. Yes, a 55 pound bag of commercially made refractory is going to cost you $50 - $80 per bag and then you have shipping if you can't get it locally.
I recently went through 20 bags of hard cast and 1 bag of insulating making my wood fired oven. I used the insulating to make the flue with and even at 1" thick and 8" diameter by 30" long, it cracked in several places, some rather large, but the stucco over it hides the cracks. The flue on my oven probably does not get much hotter that what you would be running. When doing pizza's, the dome temp inside is approx. 1,100 degrees but the flue is no where near that. Actually you can put your hand on it when the oven is hot. You won't leave it there long, but it won't burn you to the touch.
Edited by BenKeith on 03/09/2018 3:55 AM
 
renatoa
Top  #7 Print Post
Posted on 12/05/2022 2:00 PM
 
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