36" Mobile Cast Neapolitan

[william]

Hi
Is there any benefit in having a more radius transition from the sidewall to the top slope, seems like hard angles like that would be vulnerable to cracking.
Bill

[david s]

That looks ok. Make sure you use a release agent. I use 50/50 motor oil and kero, applied thinly. This produces a thin mixture, you don't want lots of oil on the surface of the casting as it can create problems with stuff not sticking to it and degreasing the castings is laborious.
Regarding the water content I found that the manufacturers recommended water amount resulted in to dry a mix and have always had to add more. Too much water though will result in a weaker casting. You can make the mix more fluid for the lower section where it is contained by the outer wall. But thicker for the upper section where it needs to stand on its own. Are you adding any ss needles? Handle the mix with care if you do, they're not called needles for nothing.

[mefornaio]

Thanks David I have always used dish soap or liquid laundry soap as a release agent for small formed concrete jobs it works as good as most mixes and as cheap as u can get. the key is to let it dry before the pour. We also use straight diesel on large jobs... foundation,basements, ect where there is a lot of form surface. Never tried your mix but thank you.
I did the first section a few min ago. The suggested water was a joke. I wanted it kinda on the stiff side yet just enough to vibrate but not too much water.
There was NO way I could even do that with the suggested H2O thanks for the heads up.
I did add the needles thanks for the tip. I'm going to wait a bit then take the wall form off. This stuff is already pretty hard.

Hey Bill you asked "Is there any benefit in having a more radius transition from the sidewall to the top slope, seems like hard angles like that would be vulnerable to cracking"
There is certainly going to be more outward force on the vertical wall than with a different radius but I guess we will see. Im sure someone here will answer that for you. . I can say that I have seen masonry structures from walkways to parking structures crack in such odd areas that you just look at them and go blank.

[david s]

Calcium aluminate castable attains it's full strength in 24 hrs. You won't increase strength by holding the moisture in like a Portland cement casting, but if you need to fill any voids do it before the casting has fully dried. 24 hrs after removing from the mould is a good time for this I've found.

[mefornaio]

Thanks again David.
I held off on the rest of the casting yesterday I had to get more materials and I wanted to see how well the mix turned out. I think the mix is where it needs to be Im happy with it.
Im concerned about the section joints bonding to well with each other. My plan is to coat the existing edge with melted wax in the revel area then using the luan board to create a gap (1/8") against the face. So each section will interlock at the revel but on the face have a slight gap. Im assuming removing this thing from the mold EVEN IF everything releases well is going to be a chore. Honestly I haven't even really thought this through very well yet.
I pulled the forms pretty quick so I have no reference on actual shrinkage. It does not look like it is going to shrink enough NOT to form in some type of gap. Could you or some one advise me if my reasoning/solution is in the ballpark on this issue?
Oh also the guys at the yard told me that about 2 handfuls of needles per bag is a what they use is this consistent with these ovens? I used a full ramekin cup (like au jus would come in) for the first section 1 per bag. Not quite 2 handfuls but they seemed to be even distributed.
Thanks again for your continued help

[david s]

It sounds like you have gone way under the recommended addition of fibres. I think it is a min of 2% by weight of dry castable. But any is better than none and it's done now. Be careful handling the castings because there will be the odd needle protruding slightly (it hurts)
There should be no shrinkage at all unless you've added too much water. Removing the first casting from new mould is always difficult. Subsequent castings come out easier. You will probably think you'll never get it to release, but keep at it and you should be successful.

[Gulf]

If you have access to compressed air, a dust blower attachment for a flexible air hose may help. Jetting a little compressed air between the cast and the form should help it release. I've never done an oven cast, but it has worked for me on other types of concrete castings.

[mefornaio]

Thank you.
Bummer on the needle content now I have 2 sections the same. I don't know if that fiber in the bag burns out or stays but if it does there is lots in there.
In my experience with reg concrete containing fiber is it doesnt really help big cracks but it works great for hairlines.
Removing concrete with fiber in it is great because the pieces tend to not break into tiny pieces.

I believe I made a mistake with my gap idea. Now I've created a 1/2" lip (the reveal) with no backing that Im assuming is going to be tough to keep intact during removal.

Created this with the Masonite board.

I've been stressing about un- molding. I took a pry bar to the edge of yesterday's section kinda nudged it gently and saw the edge along the mold move..I feel so much better now.
I wonder if I should just follow suit with the rest of the sections (needle content) or increase it? I'm assuming its not going to make much difference in a thermal sense. By thermal sense I mean the other sections heating at a different rate than those with lower needle content.
I appreciate your input.

Thanks Joe that sound like a good idea.

[david s]

The fibres (in the bagged mix) are not designed for strength, at least not in castable refractory I use. They are designed to burn away at low temp leaving a network of pipes that water can escape through. Try taking some out of the bag if you have any left and put them on a plate and place in your kitchen oven. They should melt at around 160 C.

[mefornaio]

Sorry David my above statement about the fiber was misleading. What I was referring to were the fibers contained in the bagged KS4 mix.

[david s]

The rate of needle addition shouldn't affect the thermal characteristics. I often add extra ss needles to more vulnerable areas of the castings as I'm laying up the mix in the mould.I think this is general practice.

[mefornaio]

LOL. I see you DID mean the ones in the mix.
I adjusted this next batch with your suggested 2%. I weighed out 4 good handfuls and it seems the guys at the yard were pretty darn close (depending on hand size I guess) it was off by about 4 oz from your suggestion. Oh well we shall see what happens sooner or later.
That sounds like good system adding the extra needles to certain areas.
I'm casting this next section and calling it a day. I'm hoping to cast the last section tomorrow.

The guys I purchased the oven materials from have something like the Cal sil board I see mentioned on here. It appears rigid but is more like an older version of a drop ceiling tile and once wet turns to mush. What seems to be the standard material for insulating the hearth for a mobile application? It looks like that board may compress under the oven weight but then again it feels very firm I have done some reading on here about the more ridged version but can not think of the name.
I really need to decide on an insulating material for the hearth. To get started on fabricating the stand for this oven to mount on the trailer and the finished height of the hearth is crucial for pan..
I'm still playing with the idea of making it rotate. I believe I solved the rotating dilemma relatively easily at least the main pivot point. I will share later.

Thanks again

[mefornaio]

Tragedy has struck.


The first section broke when I was removing it. Im not sure if it was from me prying up or simply from the lack of needles. I was prying up to free it from the form but only slightly. Im getting REALLY worried about the rest of the sections now. I'm thinking as Bill mentioned above, my straight wall design is part of the problem. That radius may not be able to handle the weight of the dome section not supported.
I have another section that I went light on the needles and I have no reason to think it will not do the same thing. Its either recast both sections or weld up some sort of support cage around that section to move it. I realize I should have not exerted ANY pressure in an upward direction on the dome but it seems like there has to be more to the problem.
David in your opinion if I had everything right as to the mix and density should this mix hold it shape or is it just bad design? Please don't hold back.
Thanks I appreciate it.

[william]

Hi
I don't think your straight wall design is the problem it is the hard angle transition to the top., I was referring to a curved or radius transition.(just rounding over that hard angle) I have used cast refractory crucibles to melt glass in that have straight wall sides but have a curved transition to the bottom. I must say that this is only a instinctive opinion from working with refractories and heat in my business
Bill

[david s]

Removing castings from moulds, especially the first casting, is difficult. The thinner and therefore weaker the casting is the more likely it is to break. For my flue gallery design I made it as thin as I dare to save weight because a lot of thermal mass there is unneeded IMO. After breaking a couple of casting, both where sharpish bends occurred, I designed some buttressing which I add to the outside of the cast after I've laid it up. Works pretty well.