what are size limits for thin cast iron (huge wok?) mold for polycarbonate sheets?

I'd like basically dome shape about 2ftx3ft polycarbonate or plexiglass where the dome is a ball about 5ft diameter. Thing is they should be as close to perfect dome as possible since one will slide over the other with minimal clearance, with some guide rails on the sides (got most of all that worked out in Solidworks).

Some YouTubes show "domed" skylights being blown from flat material with heat and they make it look fast and easy, but while they might get fairly consistent shapes using same tightly controlled methods, it doesn't really matter what semi-organic shape a skylight has, as long as it looks kinda cool on roof top from 100ft away on the ground.

What would be the chances of me calling a mold-maker/cast-iron guy and having their CAM crank out a Lost Wax or whatever mold and being able to produce a Cast Iron fairly smooth and accurate 5ft diameter dome section of 2'x3' with a bit past that to help the plastic sheet settle smooth for later trimming, and the Cast Iron would be maybe 1/8" to 1/4" thick?
What other ways could a One Off fairly precise 5' diameter dome shaped piece of steel be produced?
What about those ship hull plate pounding guys? If I bring them some nice 5' diameter templates out of finely finished plywood with nice hand holds, of both inner and outer dome shape a 1/4" bigger for outside will they be able to make me a 1/4" thick fairly accurate dome of steel a bit bigger than 2'x3'?

What sort of money are we talking?



Reason for Steel or CI is I'd be able to (no joke) recoup some of the cost by selling or renting it out as a mega-wok. My wok experts say the trick is a very hot wok surface so once you get a mega-wok heated up you will have lots of very hot surface to move food to new hot-spots for that correct "breath of wok" that only high-power restaurant setups can produce. Might be able to place it upside down over a fire and use for that Middle Easter flat bread where the slap pizza shaped dough on hot surface.

I think I've got the molding oven figured out. Bunch of flat steel plates with similar interlocking so it stacks flat, and just enough room for a couple big propane burners under, and I know a guy with a stack of old "Fire Blankets" to help insulate.https://www.amazon.com/Lightweight-...72&sprefix=camping stove wood,aps,161&sr=8-14
I guess the mega oven could also have secondary food baking application. lol.

 

How many thousands of dollars is your budget, or 10's of thousands ? This is a huge job, and if you want 2 domes sliding over each other, it means precision. Send your request to engineering works for quotes. Maybe search internet for available domes, and maybe you only need to have one made. The sliding aspect will cause both inner and outer to scratch each other, so they won't be 'clear' for long.
Why ?

 

A cast iron mold would have to be machined post casting to achieve the desired finish and accuracy.
Anyway, you don't need one, the way to go is female, heated concrete mold for vacuum forming the polycarbonate. Making one isn't complicated, you need the wooden sphere mirror polished, black iron pipe and concrete with very fine aggregate (or UHPC for best accuracy and finish).
The mold is heated with a steam generator (you can rent one or build your own) until the desired surface temperature is reached, then you heat and vacuum form the polycarbonate sheet.

But, the whole exercise isn't really necessary, you can order such a custom dome from people who actually know how to do it. I think you can order one from china for less then DIY.

 

Oh, here we go, sort of. At least someone makes 60" DIAMETER domes. If an American firm is saying "diameter" I'm guessing that means pretty true spherical but I'd need to ask to make sure, maybe even visit.
60"dia. x up to 30" high
plus 1/4" up to 1" wide flange
Email us for a complete quote with S&H ******
3/16" thick
Clear $655.95 ea.
White $701.95 ea.
Bronze $724.95 ea.


1/4" thick
Clear $721.95 ea.
White $774.95 ea.
Bronze $802.95 ea.

I'm thinking maybe use one of their Factory domes to make a couple concrete molds, maybe a male and female, and use those to form my parts.
I was originally thinking of just a male mold and covering it with soft temp resistant but still stretchy fabric, and maybe attaching rows of weights around the flat sheet to be formed to help it drape, so it would drape enough to take the shape but not get so soft it messes with optics. Then again, I known some auto-glass guys that polish out scratches to "you can't find where they did it" quality.

But what about those Ship Hull Plate Pounder guys? They seem to be able to fair hulls real good and I hear they can crank out 60 plates a day. Maybe 60" is bit tighter than they normally do, but maybe having it plain simple dome shape that is the same in all directions would help.
TBH, I can't figure out how those guys do what they seem to do. You got some organic water flow hull shape, then you eyeball pound a plate to very close by repeated fitting (or do they got XXXL model of thishttps://www.amazon.com/Duplicator-Precisely-Irregular-Measuring-Widen-Red/dp/B07YB599F1/ref=sr_1_1?crid=24IVBMK3EPSUE&dib=eyJ2IjoiMSJ9.wGcHzzs27Xjmb-TC7x1xDK-wKI_WFmF52rc-6dxws7QLsOAPcfCwpAR9ukqvzgKSryWVl0A8sUm8CZ4Dwa8rmy22Jv-TvreTbt1pB4Q9-VvPQIDzD5rvUU2Mr7VCuBgOIAEhnHLDJ70NmhmUA7JZiv894k9tDbWs5IQhT7mH6zHACitbLsMf7DhK0zuTwMyRCEt7uh8nK-f5sO7llrrybewZAUDHFAtnNJJN9bNpgav8AEj_hUTZrrDMCTBcTgsWqI69J_1FGPd6Oh64BsFG9m16LPh3CKIEGnTnxYlbuijBNIdlhAfVjIKYV4m0ycS78x59A_1eI1aZk97gUVZV--nyCLd5hIWwm9dv-u5FKoSrtzkn7yDQ0CmqwkJo0Ow-oJrUhU7GjveIkrhGSLwX6xVqP6kneqTJK2C4blwDicpNLhd_wJGbFzZousOh7osR.jproScED25iWz1OipujxMuJOIR5LBnpIZ6Cx4S_Zm-c&dib_tag=se&keywords=curve%2Band%2Bline%2Bduplicator&qid=1740606667&sprefix=curve%2Bduplic%2Caps%2C151&sr=8-1&th=1), but then they gotta trim the edges to butt nicely to existing also organic shapes, good enough to make nice welds? I could see doing that on one edge, with some effort but two or three edges sounds crazy for any production schedule. I'd need to be able to botch about 10 plates to sneak up on it.

Looks like I might be able to get my two pieces out of one $721 dome, which would be pretty reasonable.
How well does 3/16" or 1/4" stretched Acrylic cut, drill and otherwise machine?

 

Like I said, there will be rails (and wheels or sliders) on each side. I'd be relying on the inherent strength of 3/16 or 1/4" polycarb dome to not scratch with even fairly small 1/4" to 1/2" standoff between them. I guess I'd stick a few 1/8" or 3/16" little self sticking plastic bumpers or furniture sliders when first operating just to be safe, but I'm pretty sure that wont be an issue.
Top section will be tinted and able to slide over the front. That way at noon you got sun protection, but you could also slide it over the front if sun is low and in your eyes.

 

What about hydroforming? could you hydroform a sphere of the right size and then cut it in half? I have no idea what the typical tolerances are for the process.

 

The part about the polycarbonate not scratching is where the project may come unstuck.It seems to scratch more easily than acrylic unless coated with a protective film.The only company I knew to be doing this normally worked with fighter jet canopies.The other challenge is to form the bowl in such a way that the optical distortion isn't a problem.It may not happen with the first piece and could get expensive during the learning process.

 

I remember one boat with fighter jet canopy.

 

I'm mostly needing a fairly precise spherical shape, at also pretty good strength but I'd be adding perimeter frames.
I guess I'll be contacting the skylight company and asking if they use a real mold or just blow their domes, and what sort of tolerances to exact dome they have.

 

Try looking out of a skylight and the distortion they cause. If it is acceptable to you they may be a viable option.

 

Hemispherical Heads - Tank Components Industries https://www.tankcomponents.com/products/hemispherical-heads/

Try further searches for "dome ends high pressure vessels"

 

Stock Hemisphere Molds - Crossman Manufacturing https://www.crossmanmfg.com/stock-hemisphere-molds/
60” Dia x 30.000” Deep FM60

 

thanks.

"For the 48”, 60”, 72” Dia. Hemispheres – A special set-up charge is required. These are Fiberglass Molds, The surface finish is not smooth. They require matte sanding or sandblasting. These are suitable for painting or projection coatings. They are not suitable for clear, see-thru applications."

kinda more questions that answers. Are the molds made of Fiberglass, or for making Fiberglass, or both?

WTF is "matte sanding" in this context?

They are not suitable for clear, see-thru applications. Ok, but if not for a Fiberglass product and thus understood to be not "see through", WTF material are these molds for? I'm assuming not molten glass.

I guess I'll need to call and sound like an idiot.

 

thx, nice tip

I'd bet if I knew where to look I'd find what I need rusting in a field or junk yard.