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We used 100 psi, but more importantly our compressor was held 80 gallons and is capable of replenishing at 11.5 cfm at 90 psi. CFM is more critical than pressure in this case since you need to fill a large volume quickly before the Plexi cools beyond its ability to remain flexible.

3/4" plywood.

It's 40" in diameter and once it cooled, it stood 19" deep.

Thanks Kougerat. For our first few domes, trust me, we were all wearing face shields, safety gloves, the works. But this was somewhere around the 8th dome we blew so we perhaps had become complacent about safety, but by then we had also learned what the primary and secondary failure modes were, and neither were of the sort you would call catastrophic. The first mode of failure was the our putty seal (between the top of the base plate and the bottom of the Plexiglas) blowing out once the Plexiglas had cooled and hardened enough to no longer expand. Were we lucky that the seal would let go before the pressure reached the maximum strength of the Plexi? Yes. But over 16 bubbles later, it became a fail-safe mechanism we relied on, and became comfortable enough not to wear all the safety gear. The second failure mode was not actually during the expansion phase, but during the cool down. We learned that if we left the freshly blown Plexiglas bolted between the two pieces of plywood while it cooled, it would contract against the bolts holding the sandwich together, and break inwards with a sudden contraction when the stresses around the bolt holes overcame the material strength. The one and only time that happened, it scared the bejeesus out of us because it popped five minutes after blowing it.

Google "Vacuum Sealing Putty" for source near you. You need to stick it around the entire circumference of the shape you're blowing. If you have through-bolts holding the form together, make sure you run the putty around the inner edge of the holes so that the holes are outside the putty line. None is needed between the Plexiglas and the plate with the large form-specific hole, only between the Plexi and the plate with the air hole. You can raise the temp up to 350 deg to give you more time to form the bubble.

@@sososaul8721 thanks, i was allready considering high temperature resistant silicone kit but rather dont start a mess. I hope I can find a putty that is temp resistant enough as I know rubber melts below form temperatures allready. any advise welcome and I could source something available on this side of the dome (globe). What compressor did you use for this lancaster dome (tanksize and air-rate). I was afraid exeeding the 350degrees because of surface bubble risk.

@@dashcamholland6784 Our compressor is 4 HP with a 60 gallon tank delivering 11.5 CFM continuously at 90 psi. Our blowing pressure was set at 100 psi.

@@sososaul8721 thanks thats what i estimated would be necessary as i got actually 2 50liter compressors which blow the dome just half way up but mainly because of the major airleak around bolts and edge. I still dont know which heat resistent vacuum putty (double sided tape or just strip?) could be used. Any example maybe so i could source that on this site of the dome. I presume it should be of poly or acrylic content? Cheers from the netherlands

Ours was 4 HP delivering 11.5 CFM at 90 psi. Our blowing pressure was set at 100 psi.

We used three heater elements from old kitchen ovens, one on the bottom, one up top, and one on the back wall. We also used a convection oven circulating fan inside but in the end, we got the best results using only the bottom element with a heat shield to prevent localized hot spots. We cycled it on and off manually while watching the internal temperature.

@@sososaul8721 great, thank you for explanations! :-)

@@sososaul8721 thanks! What do you think about using halogen lamps 1400W like this: allegro.pl/oferta/lampa-halogenowa-do-piekarnika-kombiwaru-halogen-7811927969 ? Do you think it would be appriopriate ?

@@PablitoSroczynski That's interesting. I'd use a thin metal heat shield placed a couple inches above the element to prevent direct, localized heating.

@@sososaul8721 thanks

See my reply to "Leeds8" below for most of your questions. We tried 1/2" (12mm) thick Plexi blowing a 40" dia (100mm) bubble on our very first attempt and we were only able to blow it about 12" deep before it cooled too much and we blew out the seal causing a leak. We used 1/4" thick Plexi from that point onwards with much better results, so we don't have any useful experience with 1/2" thick Plexi. I can suggest that you're going to have to heat soak the part for at least 60 minutes at the upper end of the heat range to ensure an even temperature throughout, and to give you the time to blow it while still hot. Blow it downwards to let gravity help, but beyond that, it's going to be a trial and error process for you. Good Luck!

Hi Soso Saul, Thank you for your answer. When heating acrylic at 275F - 350F, how long is the heating process? thank you

For our 1/4" thick, 40" diameter bubble, we bring the oven up to temp with the piece inside, then let it heat soak for 45 minutes. If you're planning to use 1/2" thick Plexi I'd let it heat soak at temp for 1.25 hours as a minimum.

Plexiglas remains malleable between 275*F and 350*F but be careful at the upper end since fine bubbles will begin to appear in the glass if you exceed 350. The pressure question isn't as simple to answer. There are several other variables that come into play including whether your compressor can produce the volume of air you need to fill the bubble before it cools below 275, the thickness of the Plexiglas, whether you're blowing up or down, how hot the Plexi is, and how well you can seal the Plexi to the form to prevent leaks. In a nutshell, trial and error is the best way to find out for your specific application. Crank the compressor to max pressure and use an air nozzle with a control valve to control the rate of expansion to achieve the size you want before your part drops below the critical 275 degree point. It will cool very fast... about 50 degrees in 30 seconds or less at ambient temps. Once you reach the size you want, keep supplying just enough air to make up for lost pressure until it drops below 275.

Thank you for the reply, pressure and air availability wont be an issue. I don't want to make anything quite on the scale you have :) I have some small windows to make that have a gentle dome to them and using a mould and using suction just isnt cutting it so your video has given me another idea ;) fingers crossed, once again many thanks.

Hi Charlie, you can indeed use acrylic, and you should be safe blowing 2 mm thick plate to 50 mm diameter. Just bear in mind that the thinnest part will be at the apex and given your dimensions, it will be about 1 mm thick. I'm not sure what temperature you'd want to heat it to though. Trial and error!

We're not giving away all our "secrets" since blowing Plexiglas this size is a lucrative process for us. However, blowing a true hemisphere results in the thickness at the apex becoming half the original thickness of the plate. So work backwards and choose your Plexi sheet twice as thick as the minimum desired thickness at the apex.

Thank you. I am investigating this process for creation of CriCri aircraft canopies. Brilliant looking finished product :-)

Thanks CG. The Plexiglas becomes malleable above 275 deg F, though the closer you get to 350 F, the more likely fine bubbles will start to appear in the plastic.

@@sososaul8721 Great info.

Thanks for the comment, though we have perfected the process with ambient temps. Besides, it's not exactly feasible to raise the temperature inside our warehouse, nor is it particularly desirable to keep the Plexiglas malleable any longer than needed once the shape has been blown to the correct size.

FlyBaby They discover things that modelers were capable to make many years ago.

We never claimed to have "discovered" or invented anything, though making a 40" diameter dome is just a little different than making the same for a scale model.

@@sososaul8721 This particular dome is 40"?? I'm wanting to make somthing and that looks like the perfect size!! 😀😀😀😀

Hi Phil. The dome is indeed 40" in diameter. We blow them to about 19" deep and by the time they cool down, they shrink to about 17"-18" deep.

@@sososaul8721 Can you make something custom for me? For an airplane?

Upside down seems to make more sense...

Thanks for your comment. Indeed our first couple of attempts weren't entirely successful for a variety of reasons, one being the localized cooling effect at the centre as you mentioned. That was easily remedied by placing a radial diffuser on the air outlet.After about five attempts battling such things as humidity in the air source, clamping and sealing methodology, orientation of the form for heating and blowing processes, and even using better temperature monitoring, we are able to create distortion-free, optically clear domes using the pressure method above.

What is your recommendation for dispersing air? I really like your simple get it done design!

We used a large router bit shaped to make quarter rounds to make a bell-shaped orifice in the center of the wood panel, then attached a round, flat metal plate just above the orifice to force the air radially out to the perimeter rather than up vertically.

@@sososaul8721 Awesome... Mind if I asked what your solution to humidity and sealing the rim was?

@@servanttofriend8481 For humidity control, we use the same sort of in-line air dryer used by professional paint shops to keep moisture out of the air flow. For sealing, we purchased a putty-like rope from Aircraft Spruce specially made for vacuum-forming.

The build thread is located here: www.fiero.nl/forum/Forum3/HTML/000116.html Just be aware that it's currently 28 pages long so I've added an index in the first post to help you navigate to the parts that interest you the most.