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  #11  
Old 01-24-2014, 06:23 PM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

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Originally Posted by stonecutter View Post
Don't bother with poly fiber in your mix design, they will burn out.
Precisely, that's exactly what you want. Read #2
If you omit the fibres you run a greater risk of steam explosions and cracks. They are added not for reinforcement, the stainless steel needles do that.
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  #12  
Old 01-24-2014, 06:40 PM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

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Precisely, that's exactly what you want. Read #2
If you omit the fibres you run a greater risk of steam explosions and cracks. They are added not for reinforcement, the stainless steel needles do that.
I did read that.

Networks of open capillaries may assist more steam to escape, but that also makes for a weaker casting, even with SS needles. There is nothing better than drying out the oven very slowly....after the material has cured. Explosions and cracking will happen with or without poly fiber, because of rapid firing, poor casting technique or drying the material before full strength is achieved.

Also, a porous material is a lot more susceptible to absorbing moisture into itself after it has been dried, and is going to be weaker than one that is dense during thermal cycling.

I respect your knowledge of casting ovens and refractory material, but I disagree with that logic and technique, because that practice creates weaker material and goes against what I know about masonry and strong castings.

EDIT: I have been reading different papers regarding the addition of PA fiber to refractory, and they support your point as to greater resistance]e to damage during the dry cycles. They also support what I am saying with regard to mechanical strength.

So I have to agree, David, after reading several research/engineering papers, that adding the poly fiber is a good idea for general home oven building. As with any mix design, there are things to consider...here is an excerpt from one of the papers...

Fiber reinforcement can offer advantages provided selection takes into account the following
factors:
• The fibers must have a minimum length to provide anchoring in the castable matrix.
• The unit volume and volumetric ratio of the fibers should be designed so that the distance
between them is compatible with the spatial scale of the mechanical loads to which the mate-
rial may be subjected.
• The mechanical strength and elasticity of the reinforcing particles (i.e., fibers) should be kept
within a suitable range at the temperature at which the stress is at its greatest (in the case of
castable drying, in the range 150–200C, considering the surface temperature).

Another great point:

The correct design of the geometrical parameters of the polymeric fibers (length and diameter) is important in the optimization of the castable permeability after firing. Suitable selected
values of fiber length and diameter maximize the permeability level. This geometrical dependence is associated with the ability that the used fibers have to generate an efficient connection among the different regions of the castable structure.

For the short fibers (length <3 mm and diameter of 15 μm), despite their large number, the
length of the permeable paths is not sufficient to generate connections.

For the thicker fibers
(diameter >50 to 100 mm), the same effect is observed and is associated to the fewer fibers
per volume of castable.
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Last edited by stonecutter; 01-24-2014 at 07:46 PM.
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  #13  
Old 01-24-2014, 07:42 PM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

As an aside, this is what I appreciate most about discussions like this. This one forced me to look into another aspect of refractory casting I had not considered to be of any value, and as a result, I will make a better cast oven in the future. Thanks David.
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  #14  
Old 01-24-2014, 08:32 PM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

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Originally Posted by stonecutter View Post
I did read that.

Networks of open capillaries may assist more steam to escape, but that also makes for a weaker casting, even with SS needles. There is nothing better than drying out the oven very slowly....after the material has cured. Explosions and cracking will happen with or without poly fiber, because of rapid firing, poor casting technique or drying the material before full strength is achieved.

Also, a porous material is a lot more susceptible to absorbing moisture into itself after it has been dried, and is going to be weaker than one that is dense during thermal cycling.

I respect your knowledge of casting ovens and refractory material, but I disagree with that logic and technique, because that practice creates weaker material and goes against what I know about masonry and strong castings.

EDIT: I have been reading different papers regarding the addition of PA fiber to refractory, and they support your point as to greater resistance]e to damage during the dry cycles. They also support what I am saying with regard to mechanical strength.
The amount of fibres in the proprietary castable is minimal as they are not added for strength, so I would doubt mechanical strength would be effected. Presumably exhaustive testing has been done on the product and any loss of strength would be more than compensated for by the benefits the fibres provide in safer drying, otherwise they wouldn't add them to their product. Unfortunately the manufacturers keep their recipes secret so finding out the optimum quantity is not possible. Making your own castable mix is fraught with problems and is one reason I stick with proprietary product. But if I were making a one off for myself I'd be more than willing to risk a "try it and see" recipe which would certainly include polypropylene fibres
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  #15  
Old 01-24-2014, 10:21 PM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

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• The mechanical strength and elasticity of the reinforcing particles (i.e., fibers) should be kept
within a suitable range at the temperature at which the stress is at its greatest (in the case of
castable drying, in the range 150–200C, considering the surface temperature).
In the case of the polypropylene fibres, as they are not added for strength this point is immaterial. In fact they melt at 160 C anyway. The stainless fibres do the strengthening instead and should be added to the mix at 2% min. by weight.
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Old 01-25-2014, 03:56 AM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

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In the case of the polypropylene fibres, as they are not added for strength this point is immaterial. In fact they melt at 160 C anyway. The stainless fibres do the strengthening instead and should be added to the mix at 2% min. by weight.
You're off on this one.

What it is saying makes sense....the fiber adds strength to the castable during the early drying stage, when there is more water present in the material, and not all poly fiber has the same melting point. Agreed, the SS fiber provides the lasting mechanical strength, but the early firing reinforcement of the poly can't be dismissed as immaterial.
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  #17  
Old 01-25-2014, 04:40 AM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

Each manufacturer designs a number of castables for a range of different applications. There are a number of different fibres used. The product I use contains polypropylene fibres, which melt at 160 C (i know because I've tested them) some others use polyameric fibres and natural fibres. In the case of the polypropylene they are purely designed to reduce explosive spalling.


"Polymeric fibers are an efficient way to enhance castable resistance to spalling upon drying, because they allow more severe heating schedules under a controlled risk. Two mechanisms are responsible for the benefits conferred by these fibers: an increase in permeability caused by fiber melting, thermal degradation or shrinkage,1 as in the case of polypropylene (PP) fibers; and mechanical reinforcement, which results especially from the increased energy dissipated during crack propagation,2 as when polyaramid (PAr) fibers are used."


http://americanceramicsociety.org/bu...iles/Peret.pdf

Last edited by david s; 01-25-2014 at 04:50 AM.
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  #18  
Old 01-25-2014, 05:16 AM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

Quote:
Originally Posted by david s View Post
Each manufacturer designs a number of castables for a range of different applications. There are a number of different fibres used. The product I use contains polypropylene fibres, which melt at 160 C (i know because I've tested them) some others use polyameric fibres and natural fibres. In the case of the polypropylene they are purely designed to reduce explosive spalling.


"Polymeric fibers are an efficient way to enhance castable resistance to spalling upon drying, because they allow more severe heating schedules under a controlled risk. Two mechanisms are responsible for the benefits conferred by these fibers: an increase in permeability caused by fiber melting, thermal degradation or shrinkage,1 as in the case of polypropylene (PP) fibers; and mechanical reinforcement, which results especially from the increased energy dissipated during crack propagation,2 as when polyaramid (PAr) fibers are used."


http://americanceramicsociety.org/bu...iles/Peret.pdf
That was one paper I read last night, it had some good info.

160 C is within the temp range of the later quote I posted about early strength benefits of poly fiber, so I'm not sure if you are disagreeing or what.

Here's another interesting tidbit from this paper...http://americanceramicsociety.org/bu...iles/Peret.pdf

Drying Behavior
Explosive spalling occurred in all the steel-fiber-containing castables tested at 20C/min
(Fig. 8). On the other hand, the material containing PAr fibers survived heating at that rate, attesting to the greater efficiency of these fibers in enhancing refractory resistance to drying and explosion. The thermogravimetric analysis revealed no significant differences among the mass loss rate profiles of the materials. Indeed, the structure of the permeable channels through which the water vapor flows was not greatly modified by the presence of the fibers studied. Neither melting of PAr and steel fibers nor degradation of PAr fiber occurred below the critical temperature range involved in the drying process. This is the key part : The greater resistance to damage on drying displayed by the PAr-fiber-containing material indicates that a more severe heating rate can be applied to the material, thereby lessening the overall time needed for the completion of heat-up.

To summarize, I think the last part sums up everything. The overall quality or performance of the casting is not impaired, but greatly enhanced by adding polyfiber and is beneficial to the whole drying process and speeds it up without the same risk of damage as castable without the fiber.( notice I didn't say cure )

This has been enlightening.
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  #19  
Old 01-25-2014, 05:43 AM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

Gentlemen,

Great discussion!!!
I thoroughly enjoyed the reads. Now to the point of which size fibers to purchase and any recommendations for a site to purchase it from.

Thanks,

Sandro
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  #20  
Old 01-25-2014, 05:49 AM
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Default Re: A castable Barrel Oven with Refractory or Homebrew mix

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Here you go, Greater Boston area, they carry Minute Man Lime....I used it many times when I lived in CT.

TLC Supply Cement

Stonecutter,

Thank you for the info. The place is closed today. I will reach out to them on Monday for a quote and find others around for a price comparison.

Any thoughts on the Whitacre Greer fireclay.


Thanks,

Sandro
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