RE: Vacuum form a clear canopy!

RE: Vacuum form a clear canopy!
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RE: Vacuum form a clear canopy! - 11/22/2006 7:23 PM

Jim Messer

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I sure do appreciate all the information given here. I am designing and building a 40% VANS RV-6 and will need a big canopy. The fuselage is 17 inches wide and the canopy will be at least 24 inches long. Wish me luck.

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RE: Vacuum form a clear canopy! - 12/6/2006 10:30 AM

drcrash

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quote:

ORIGINAL: Jim Messer

I sure do appreciate all the information given here. I am designing and building a 40% VANS RV-6 and will need a big canopy. The fuselage is 17 inches wide and the canopy will be at least 24 inches long. Wish me luck.

It sounds like you'll need a vacuum former with its own oven, wider than a typical kitchen oven.

Plans to do that can be found in Thurston James' book The Prop Builder's Molding and Casting Handbook, using inexpensive nichrome wire coil for heating elements. There's an article on tk560.com about actually building it, with cost-saving substitutions. You should be able to build a two-foot-square oven for about $60. (You don't actually need the ceramic insulators Thurston James specifies; you can wire the nichrome coil directly to the backing cement board with baling wire. Jim Egner doesn't discuss that in his article, which is a bit old now.)

http://tk560.com/vactable4.html

You should also check out the vacuum forming discussion forum on tk560, where people talk about vacuum former (and oven) designs and variations; other people have adapted the James plans in various ways---different sizes (e.g. to 4 x 4 feet), different aspect ratios (e.g., 24 x 32 inches, which might be good for your purposes), different voltage power supplies, compact over-and-under oven-and-table arrangements, different vacuum sources, etc. There's also an applet to help you adjust the coil layout for different size/shape ovens.

There are also vacuum forming discussion groups on hobbymolding.com and CNCzone.com:

http://www.hobbymolding.com
http://www.cnczone.com/forums/forumdisplay.php?f=291

Paul

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RE: Vacuum form a clear canopy! - 12/7/2006 3:07 AM

lilhachet

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Where do you get your petg? Thanks, guy

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RE: Vacuum form a clear canopy! - 12/7/2006 2:35 PM

drcrash

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quote:

ORIGINAL: lilhachet

Where do you get your petg?

The best place is usually a local plastics supplier, to avoid shipping charges.

Different plastics suppliers have different minimum orders; for some it's $100 or so to keep riffraff like us out, but others are happy to sell you a single 4 x 8 foot sheet, or part of a sheet. (There may be a $5-10 cutting fee for partial sheets. For thin PETG, you probably wouldn't save much over buying a whole sheet.) Unfortunately, you generally have to call around to figure out which suppliers are happy to sell to little guys.

The ones that cater to smaller customers often don't have as big a selection, but can special order lots of things they don't normally stock, and have it within a day or three.

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RE: Vacuum form a clear canopy! - 12/7/2006 4:17 PM

drcrash

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I thought the discussion of webbing deserved its own thread, so I started one:

EDIT: wrong link for the webbing thread. No wonder not many people went over there

That should have been: http://www.rcuniverse.com/forum/m_5086453/anchors_5086453/mpage_1/key_/anchor/tm.htm#5086453

< Message edited by drcrash -- 12/19/2006 4:06 AM >

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RE: Vacuum form a clear canopy! - 12/19/2006 4:19 AM

drcrash

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quote:

ORIGINAL: drcrash

I thought the discussion of webbing deserved its own thread, so I started one:
[...]
http://www.rcuniverse.com/forum/m_5086453/anchors_5086453/mpage_1/key_/anchor/tm.htm#5086453

OK, over in the webbing thread I suggested using free blowing to create nice smooth canopies without carving and sanding, or actually making a mold at all, and then turning around and using a free-blown canopy as a mold.

allanflowers suggested using a rubber sheet as a proxy for hot plastic, to visualize how the plastic will stretch and fold, so that you can figure out and fix webbing problems.

I just put two and two together. Instead of using actual free blowing to create canopy shapes with hot plastic, use a rubber sheet full of wet plaster. That would be stable, and give you lots of time to adjust the tension on the rubber sheet in various ways, change the angle of the cutout it's sagging through to change front-to-back proportions, etc. The weight of the plaster would let you choose mostly-gravity-formed shapes over mostly vacuum-flared shapes. (But you can still use vacuum, and control the proportions, like in full-size canopy free blowing.)

Then when you're happy with the shape, just leave it alone and let the plaster set. Voila, you have a bubble canopy mold that is as smooth and graceful as a free-blown canopy, plus you have much easier control of the process. For example, you could try different thicknesses of rubber to get the degree of bubbliness you want, given the weight of the plaster mix, and you could adjust the tension of the rubber around the cutout to make subtle changes to the shape. (Just tack the rubber to a wire hoop shaped around the fuselage-join cutout, and distort the hoop.)

The extra control also makes this kind of technique more appealing for things like fuselage halves. Getting the halves to match well enough would be much easier with several minutes to adjust the local tension of the rubber around the profile cutout.

Has anybody else tried anything like this?

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RE: Vacuum form a clear canopy! - 1/10/2007 3:35 AM

gsmith6879

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I got in on this forum a little late but have tried my hand at forming some canopy parts from .030 PETG. In spite of all my efforts I am still getting a lot of dust specks on the finished canopy. What is the secret in eliminating the static electricity that causes the parts to attract dust?

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RE: Vacuum form a clear canopy! - 1/10/2007 2:26 PM

drcrash

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quote:

ORIGINAL: gsmith6879

I got in on this forum a little late but have tried my hand at forming some canopy parts from .030 PETG. In spite of all my efforts I am still getting a lot of dust specks on the finished canopy. What is the secret in eliminating the static electricity that causes the parts to attract dust?

In general, it's tricky. Do you actually see the dust specks, or are you seeing tiny bubbles or other flaws that you assume are due to dust specks?

Do you get flaws where the plastic is NOT in contact with the mold, or only where it IS in contact with the mold? If the latter, it's probably due to tiny flaws in the mold. You need to make sure that your mold is very, very smooth. If it's a Bondo mold, it might help to seal it with straight resin, no filler, and then make sure that's very smooth and clean. (Well, not too smooth. About 400 grit is about right, because the micro-channels due to the sanding grooves help keep the plastic from sticking and trapping air. But very, very clean.)

If your PETG has protective film on it, don't remove it until just before you put the plastic in the oven.

You want the mold clean as clean can be. You might try wiping it down (with a very, very lint-free cloth) with Windex; some people say that helps. You might also try wiping it down with one of those anti-static disposable clothes dryer "fabric softener" things that you throw in with a load. (I haven't tried that, but it sounds promising.) Keeping everything in the area as clean as possible is a good idea, too, so that you don't kick up dust.

If you get bubbles where the plastic doesn't pull down to the mold (e.g., at the radius where the mold or riser meets the platen, and the plastic doesn't suck all the way in), you should suspect the plastic. You may be overheating the plastic, causing moisture or other chemicals to make vapor bubbles.

(By the way, many people say that PETG doesn't have moisture problems, but that's not quite always true. It's a little bit hygroscopic (moisture-absorbing), so it's unusual but possible that you need to pre-dry the PETG.)

The manufacturer or distributer may not be keeping things clean enough, or the PETG may have chemical flaws that result in outgassing. You might try another source of PETG and see if the problem goes away.

One source of PETG is 2- and 3-liter soda bottles. If you cut off the ends of a 3-liter, split it at one of the mold marks, and unroll it, you can get a 6 x 13 sheet of PETG with an annoying mold mark right across the middle of it. That may not be big or perfect enough for what you're doing, but might be good enough for an experiment.

Here's an old and not-quite-conclusive thread from RCG about the subject, with many people chiming in. (Including Doug Walsh, author of Do it Yourself Vacuum Forming for the Hobbyist.)

http://www.rcgroups.com/forums/showthread.php?t=334760&highlight=canopy+dust+static+electricity+

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RE: Vacuum form a clear canopy! - 1/10/2007 5:34 PM

gsmith6879

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Great information. A lot of things to think about. I will check out the RCGroups forum and see what I can learn.

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RE: Vacuum form a clear canopy! - 9/9/2007 6:38 PM

drcrash

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Anybody had any luck casting polyester (fiberglassing) resin and bondo?

My understanding is that large solid casts of regular polyester resin can get VERY hot, and maybe even explode. It's not recommended for more than an eighth or a quarter of an inch thickness at a time... just mixing the stuff in large quantities is difficult, because you have to get it mixed before the heat from the exothermic reaction makes it all kick. Oops.

I think a better way would be hollow casting the bondo/resin mix as thin shell, say 1/8", and filling in the middle with polyurethane foam. (2-part stuff, about 4-8 lbs/cu. ft. density. The spray can stuff is a pain and not any cheaper.)

For the time being, though, I'll mostly stick to plaster...

BTW, a good place to get plaster is a local ceramics supply place. You can get 50 lbs of #1 molding plaster (plaster of Paris, more or less) for about $18, and 50 lbs of Ultracal-30 (harder, good for casting) for about $23. Cheaper than at a home improvement store or craft store.

For smallish stuff that you may want to work after molding, water putty from the hardware store is good. (But pricier per pound.) It doesn't set up hard and fast like Ultracal, and is stronger than plaster of Paris. You can work it a bit while it's still "green" (solid but not set hard), and after that you can sand and drill it, and spackle it with more water putty. (Fresh water putty sticks pretty well to old hard water putty.) You can also mix it 1:1 or 2:1 with plain plaster of Paris to get intermediate properties and price.

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RE: Vacuum form a clear canopy! - 9/19/2008 7:46 PM

Deadstik

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I'm getting ready to make a vacuum forming box but I have one question that seems to get skipped over. What do you make the frame to hold the plastic out of? Also, what do you use to hold the plastic to the frame? I'm assuming that you come up with some wood blocks and put the frame on them in the oven till you get the proper "droop" and then pull it out, stick it over the plug/vacuum box and turn on the vac. What temperature do you set the oven to??? or do you just use the broiler setting till the plastic droops correctly?? Maybe I'm just having a brain freeze here but if someone could send me a picture of what they're using to hold the plastic and their method I'd really appreciate it. I don't really have the time right now to build one of the nicer, one piece heater/vacuum former tables that I've seen... but if I can get past this hurdle I'll be able to at least catch up....

Thanks,

Deadstik

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RE: Vacuum form a clear canopy! - 9/20/2008 5:46 PM

grumman-cats

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I don't have any photo's handy right at the moment but I made the a square holder out of 1/2" angle iron and I pinch the plastic between it and a flat top piece of steel. This seems to work well and doesnt' get affected from the heat of the broiler. The rest of what you know is right, You'll get the feel for how much the plastic has to droop with trial and error. different thinkness plastic forms differently so make sure you get a little extra.

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RE: Vacuum form a clear canopy! - 2/11/2009 8:29 AM

dadragon

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dion;I would like to talk to you about your canopy molding,they look realy good,you can email me at dadragon43@hotmail.com george.

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RE: Vacuum form a clear canopy! - 7/13/2009 11:00 PM

airboss45

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CANOPY HELP! i need to replace my global skyraider canopy asap. please is there anyone out there with one to sell? any info at all would be a great help. thanks don "airboss"

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