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JRo 05-07-2007 12:43 PM

Casting geodesic dome pieces
 
I know there have been a ton of posts regarding refractory mortar, refractory cement, etc. Sorry, but I'm still confused about some of the differences.

My plan is use a CNC router to cut molds for the hexagon and pentagon pieces used in making the geodesic dome. I would then pour in the castable and end up with hexagon and pentagon bricks. In theory, all angles, etc. would be perfect so the dome should be able to be assembled completely without mortar.

What material should I be looking for to cast these bricks? Can I use the Refmix? Is there a brand name, or something I should be looking for?

Also, my understanding is that there are two types of castable: heat holding and insulative. Do you think you could make 6" thick bricks with 3" of the insulative on the outside, and 3" of the heat holding on the inside? Or would differences in expansion/contraction rates tear them apart?

Main reason for doing this is that if you could add the two types together into a single brick, and assemble the dome without mortar, you could maybe have an oven that could be easily built, torn down and moved, etc.

dmun 05-07-2007 01:05 PM

Re: Casting geodesic dome pieces
 
As one of only a few people to have built a geodesic oven, I think that the no-mortar idea is not a good one. There are considerable side stresses, and I was not able to dry assemble my pieces even for a test. Even a geometrically perfect oven is going to move as it heats up and cools down. A mortared oven will grow and shrink as a unit, a loose one will creak into a new allignment as some parts get hotter than others. You don't want it falling in on your dinner.

As far as castable refractory material, one member cast the geodesic triangles, but I don't have any personal experience with the stuff.

JRo 05-07-2007 01:24 PM

Re: Casting geodesic dome pieces
 
Hi dmun, I'm sorry to hear that you don't think it will work. It was your adventures that inspired me down this path. (Beautiful work by the way!)

How much expansion/contraction is there in these type of castable materials? I would think that 6" thick bricks would have enough angle/surface area between them to keep things in place.

What about machining dowel pin holes in the connecting edges of the bricks? You could insert a couple of 1" thick 'pins' between bricks to help keep things aligned and help distribute the side stresses.

Maybe this is getting to be more trouble than it would be worth, but I'm hoping to be able to take the oven with me when I move. Plus, like you, I have fun doing things a little bit differently.

edschmidt 05-07-2007 06:26 PM

Re: Casting geodesic dome pieces
 
I cast my dome just as you are talking about without mortar with one caviat, I added an additional 2" of mesh reinforced cladding to the exterior of the dome in order to keep everything in place while still allowing movement of the pieces. I personally like the non-mortar approach because it allows the pieces to expand and contract at their own rate without effecting the overall dome. My main concern in the conventional approach is the temperature differential between the top of the dome, and the bottom durring firing. In your scenario with insulative and non insulative materials I would place a "stressed skin" exterior of stucco, or if dissassembily is a major factor a metal dome in order to keep movement under control. You have to remember the strength of the geodesic dome comes from tension, if constructed without using mortar the "tensegrity" to quote buckminster fuller is not present so it must be replaced with a stressed skin. I purchased my castable refractory premade in 50# bags locally and it is rated at 2500deg.
Hope this helps.

JRo 05-07-2007 08:13 PM

Re: Casting geodesic dome pieces
 
Ed, thanks for the feedback. The idea of a stressed skin makes sense. I was thinking that with only the 3/8 height dome, that gravity would hold everything in place. Looking at the model again, it does look like there are some outward forces to consider.

I'm still kicking around the pin idea. If you left the holes large enough to allow for expansion of your pin, it could be sloppy enough that things could move as necessary, but still provide some resistance to the outward forces. I'll have to try modeling it up and see how it looks.

enz 05-16-2007 11:56 AM

Re: Casting geodesic dome pieces
 
JRo,
I'm doing a cast oven, and also am doing things differently. I'm going to cast the oven in quarters on a multi-piece water ballasted (so it won't move) male mold. While I haven't begun casting, you might be interested in my material search.

I will be using Mizzou Castable Plus from Harbison Walker. It has the same thermal conductivity and specific heat as fire brick. The plus designation is for fast heating schedule, they say that after a 24 hour cure at 70-90 F you can ramp up at 100 F/hour right up to your maximum. I got it at ANH refractories in Richmond, CA, but they have other distribution centers around the country (http://www.hwr.com/). It runs $.66/lb and will require right around 1000# for the oven, base and chimney with a 3" wall.

It's important to note here, most of the refractory castables out there are INSULATIVE! It took a bit to convince the guy on the phone that I did not want insulation, but rather thermal mass so be careful.

Enz


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