(Continued from this thread on vacuum former boxes/platens:
https://www.rcgroups.com/forums/showthread.php?t=613530)

If you have a 2-burner hot plate and are trying to use it as a heat source for vacuum forming, you can turn it into a decent oven with $10 worth of disposable aluminum pans and grill toppers, plus some binder clips to hold it together. It should take well under an hour to make the oven.

(Even if you have to buy a hot plate, this is a very cheap oven; you can find $20 two-burner hot plates on sale for $15 or even less if you're not in a hurry. I got mine for $10 at a thrift store, and had the binder clips lying around, so the total cost was about $20.)

The resulting oven can heat plastic up to 12 x 20 inches. That's big enough to make objects up to about 10 x 16 inches.

The problem with a two-burner hot plate is that the heating elements are the wrong shape to heat rectangular plastic. The upside is that they are good infrared emitters, and infrared is easy to redirect.

We'll do that in two ways. One is simply to move the plastic away from the burners, so that the infrared has a chance to spread out from the hot spots and fill in the cool spots, so we'll make a deep oven. For that to work, we need reflective walls, so that most of the IR doesn't just escape out the sides. It will be reflected back by thin aluminum. Aluminum is a great reflector for infrared, and very thin, cheap aluminum will do the job just fine.

The second way we'll spread the "heat" (infrared) out is with a perforated aluminum reflector near the burners, made from a disposable grill topper. The holes let some of the infrared through directly, but most of the infrared is bounces off, and spreads out before making it out around the edges or through the holes.

Here's what the oven will look like:


The plastic-holding frame will sit up on the top edge.

Here's how to make it:

First get four 12 x 20 disposable pans from a restaurant supply store, and two disposable grill toppers from a grocery store or dollar store, and some medium binder clips from an office supply store.

Then cut holes for the burners in the bottom of one of the pans. The holes should be just big enough that the pan sits flat on top of the hot plate, with both the burners and the metal reflectors under them poking up through the holes. You may need to notch the side walls a little bit to accomodate the endmost edges of the metal reflectors.

If your hot plate has end caps that prevent the pan from sitting flat, take them off. If that leaves it without feet, put it up on some small metal objects.

Here's mine, sitting on a couple of pieces of windowscreen frame aluminum:



And here it is with the two-holed bottom pan in place:



On top of this, we'll put two more pans, with their bottoms cut out, to give the oven height. I'll call these the "middle" and "top" pans. The middle pan goes upside down, so that its top edge mates with the bottom pan's top edge. The top one goes right-side-up, so that its bottom mates with the one's bottom.

Rather than simply cutting out the bottoms, we'll slit them in sort of an H shape.

Cut the middle pan's bottom so that there's a full-width tab on each end a few inches wide. Slit the rest lengthwise and bend it back to the side walls. The tabs will stick up into the top pan, and be bent outward against the top pan's end walls.

The top pan should be slit lengthwise, and the ends of the bottom pieces cut free, making a very wide H shape. The lengthwise tabs that result should be bent down, inserted into the middle pan's bottom hole, and bent outward against the middle pan's side walls.

This gives us a stable interlocking shape, so that the top pan can't slide off the middle pan. (We'll binder-clip the bottom and middle pans together at their top lips, so that joint will be stable, too, but not yet.)

Take a grill topper and make its side walls vertical, rather than angled. To do this, you'll have to crimp the edges inward at the corners, crumpling it a bit. Cut away most of the aluminum on the side walls, leaving a few tabs connecting the bottom and the lip, to hold it together.

Test-fit the grill topper into the oven, upside down, inserting it down through the pans so that it's just inside the bottom pan. Its edges should be against the bent-down-bottom tabs of the middle pan, so that it's about three inches above the burners.

Now look straight down into the oven, and make sure the grill topper is centered over the burners.

See where the burners are, through the holes in the grill topper reflector. You want to cut away the aluminum in the middle of each sides, around but not over the burner. Mark the aluminum with a felt-tip pin, remove the grill topper, and cut away the extra aluminum there.

Test-fit it again.

Now take the interlocked middle and top pans off, put the grill-topper reflector in the same place between the bent-down tabs of the middle pan's bottom, and binder-clip it to those tabs. Now you should be able to lower the interlocked middle and top pans onto the bottom pan, and the reflector should go slightly into the bottom pan and stay in position, about three inches above the burners. That's enough room for the IR to spread out sideways significantly between bounces.

The result should look like this:



(I left the binder clips off when assembling it to take pictures, sorry.)

Here's another picture looking more downward into the oven:



Here's a picture showing some plastic holding frames made of aluminum windowscreen framing, sitting on the top edge of the oven. In actual use, these are binder-clipped around a sheet of plastic. (But other kinds of frames should work fine.)



Like most bottom-heating ovens, this one works better with a lid on it. That's what the fourth pan is for. You can sit it on top to help trap the heat, so that the plastic doesn't cool so much from the top, by convection:



This helps the oven heat more evenly.

Paul

EDIT 7/12/07:

I set up a new site for vacuum former plans, etc.: www.VacuumFormerPlans.com

Here's the starter "instructable" showing how to build a good, cheap, upgradeable vacuum former: http://www.instructables.com/id/E8RW98YF3C4XLCQ/

It's a good match to this oven.

--

Tired of buying cheap plastic crap? Now you can make your own!
www.VacuumFormerPlans.com

That looks like a nice, inexpensive way to get the plastic out of the kitchen oven. Let us know how it works out after you're able to test it some more. I might like to do something liek that.

Randy

Yes, please! More infor when it becomes available. Very interested in how this works out.

BTW, I can't seem to find perforated oven trays easily. I expected to find them at Wallyworld Mart but they didnt carry them in my local. Any suggestions?

Originally Posted by ToxicToast

Yes, please! More infor when it becomes available. Very interested in how this works out.

BTW, I can't seem to find perforated oven trays easily. I expected to find them at Wallyworld Mart but they didnt carry them in my local. Any suggestions?

Try everything's-a-dollar type stores.

This is a somewhat seasonal item; more places have them in the warm months than in cold months, because they're used for outdoor grilling. (They may be in whatever section they keep disposable aluminum pans in, or they may be with the outdoor grilling stuff.)

Since Christmas, I've seen them at H.E.B. grocery stores. (Here in Texas, that is. In cold places, they may show up later.)

At my Lowe's or Home Depot (I forget which) they've got a different kind, not disposable, made of somewhat thicker aluminum and completely flat (no bent up and rolled edges), in the section with the barbecue grilling accessories. Those cost a few dollars, I forget how much exactly, for a two-pack. I'm sure they could be adapted for this, but you'd have to figure out how. (E.g., maybe using both of them crosswise so that you can bend down the ends to binder-clip them to the tabs sticking down from the middle pan.)

Paul

Originally Posted by Randy Due

That looks like a nice, inexpensive way to get the plastic out of the kitchen oven. Let us know how it works out after you're able to test it some more. I might like to do something liek that.

Randy

I haven't done extensive testing, but I have done several tests, including sag tests, glow tests, and eleven-spot checks with a touchless infrared thermometer.

(BTW, a "glow test" is where you turn out the lights and look at the red glow through a sheet of frosted glass or translucent diffuser plastic. The red glow is a good proxy for the infrared glow you can't see. It tells you whether the IR is being distributed pretty evenly, and it's how I found the hot spots in my over-and-under so I could fix them.)

The new oven heats the plastic more evenly than Ralis Kahn's popular "over-and-under" design using a portable electric grill over the plastic, facing down. (If you build it "straight" according to Ralis's plans, without my improvements.)

(See http://www.halloweenfear.com/vacuumformintro.html for Ralis Kahn's design, and this thread on tk560.com for my mods: http://www.tk560.com/phpBB2/viewtopic.php?t=390)

I improved the Ralis Kahn design with a couple of hacks to even out the heat, and my new hot-plate oven is comparable to my modded Ralis Kahn oven within the same 12 x 18 area. It's actually a little bit better over the main area, because my portable electric grill's heating element heated asymmetrically---part of the heating element was simply hotter than other parts, and I had to tweak the perforated reflector to fix a hot spot on one side of the oven. I needed a third hack to even out my particular grill's heat.

With the new oven I haven't had to do that, but your mileage may vary---some hot plates may have randomly funky burner elements, too. But I wouldn't expect that to matter as much, given how many windings the burner elements have and how much the new deep-oven design randomizes and spreads out the IR.

Like most simple oven designs, the new one is a little weak in the corners. And like most bottom heaters, it's pretty sensitive to whether you have a lid on.

If you have a somewhat higher budget, I'd recommend an over-and-under with a portable electric grill. (The grill costs $30, and the structure to hold it up will likely cost a few bucks more, too.) One thing that I like about over-and-unders is that they work well without a lid, which makes them easier to use consistently---you don't mess up the heating by lifting the lid to peek at the plastic.

But price/performance-wise, the new oven is really hard to beat. It's pretty good, and just vastly better than a lot of things that many modelers try to get by with, such as a plain hot plate or a heat gun. (A lot of kitchen ovens have strange heating patterns, too. Some are quite good, but they're all over the map.)

I cringe every time I read about things like that... which is why I came up with a really cheap design. I've already got a couple of nice vacuum formers, but I thought there was a crying need for a cheap design to keep people from struggling along with their plain hot plates, stove tops, and heat guns.

I may do a third version, and try adding another grill topper between the top two pans, with corner cutouts. (To improve corner heating.) But I probably won't spend a lot more effort on this particular cheapo design, because I think it's getting close to as good as you can expect to do for the money.

(One thing I do intend to experiment with is using this oven for two-sided heating of smaller-sized thick plastic. If you situate a smaller piece of plastic in the middle, a lot of IR should go up past it, bounce off the lid, and heat it from the other side, as though you had a two-sided oven.)

Another thing I may try is a waffle iron or two. You can get old messed-up waffle irons for about $2 at Goodwill Blue Hanger stores, and if you remove the waffling plates, they have two nice 9-inch square 600-watt IR heaters under there. Sweet! I'm thinking a waffle iron might make an even better heater for a high-aspect-ratio oven for modeling purposes. With a $2 waffle iron (and a $6 roll of aluminum flashing and a couple of $1 grill toppers), you could probably make a pretty good 12 x 24 bottom heater for $12-15. With two waffle irons, you could probably do 3- or 4-foot wings or fuses for $20. (Now that would be an oven for serious modeling...)

Originally Posted by Randy Due

That looks like a nice, inexpensive way to get the plastic out of the kitchen oven. Let us know how it works out after you're able to test it some more. I might like to do something liek that.

It works great. My wife's been using it for making masks out of a variety of materials---styrene, acrylic, EVA foam, and PVC, and it works even better than I thought; zero problems. I have two "better" vacuum forming ovens, but my wife tells me not to bother to put this one away and set up another one. She's used to it, likes it just the way it is, and doesn't see how it could work better. (Except for a heat control for really thick plastics, which our other small former needs too... more on that below.)

Turns out the lid isn't necessary. (Or, probably, desirable.) Works fine without it, as long as there isn't a breeze blowing across the plastic. (We turn off the ceiling fan before heating plastic.)

I made another one out of a tabletop portable propane grill, for her/us to use for no-electricity vacuum forming at festivals, and it works fine, too. (I prefer the electric for home use, because you have to adjust the heat of the propane burner as the pressure in the tank goes down. For the electric, I just leave it full on.)

My infrared thermometer does tell me that the electric has two mild hot spots, one over each burner, that are about 10 degrees hotter than the very middle, and 20 degrees hotter than the rest of the plastic. They don't seem to matter, but I think I know how to fix them---and simplify the design:

Rather than using the perforated aluminum (grill toppers) I should just cut away the bottom of another pan, leaving two solid diamond shaped parts over the burners, and nest that pan in the top pan.

I use a solid piece of aluminum that way in the propane oven (over the middle, because it has a single long burner), and it works like a charm. I'm pretty sure the same thing will work for the electric, but I haven't gotten around to it because we're just not seeing any problems with any materials.

One limitation: the electric oven is too hot for most thick plastics, and the heat controls (infinite switches, like a stove top burner) are not worth using. They switch the heat on and off at too long an interval, and too randomly, and they aren't very precise. That makes it hard to heat both ends of the oven equally.

For typical RC vacuum forming of cowlings and canopies and whatnot, this is not a problem---thin plastics heat all the way through before they get too hot on the heated side---but thick plastics will melt or scorch on heated side before the heat soaks all the way through. For thickish stuff, like 1/8" plastic, we've been flipping the plastic over now and then to heat each side half the time. For even thicker stuff (like 1/4" Sintra PVC, or 8mm EVA foam), we turn the heat on and off to slow the cooking down. That's tedious, but it works.

I just built a off/half/full power gadget using a hefty power diode (which you can get for about $5) and a double-pole toggle switch to avoid having to turn the heat on and off, but don't have much experience with it yet. (It seems to work fine so far.)

any updates on using ???

Looks brilliant, I may try this since I’m often having problems with my vacuum forming oven. Always having to replace maytag parts. Would you absolutely have to use perforated oven trays? I fear I may struggle to find them. Did you experiment yet with the two-sided heating?

I have done some very small items with a home made vac former, by small I mean using the lids from ice cream tubs as the material.
The difficult part is having to eat the ice cream first

I made a simple box with a hinged top and a mesh shelf for the master item. Vacuum cleaner nozzle in the side, and the 'material' clamped in the hinged lid.

It sort of works, but my problem was using a hot air blower for softening the plastic, a bit hit and miss.

Your aluminium heat box is a brilliant way to equalize the heat.

But as I'm too lazy to build a new rig, (I rarely do any vac forming, I built mine more to see if it would work), I think I will experiment with some sort of aluminium container to diffuse the hot air from the heat gun, perhaps a bit like a baffled chimney.

Very good article, thanks for the information and ideas, perhaps one day......

Looks very good.
What is the wattage of the hotplate used.
I tried using a 1600 watt fry pan as a heater.
Using polycarbonate 1.5mm thick and it didn't work well at all.
I am going to try petg with a 1mm thickness and see how that goes as it doesn't require the hot temperatures that polycarbonate does.

DrCrash-
Awesome job! I am curious- how well would a flat skillet work instead of the double burner? I am going to be making one of these ASAP, and seems that I can easily find a hot skillet for cheap. Can you let me know ASAP? I realize I will have to find pans/grate to fit the skillet shape...

Also-
Been reading a TON of other threads on this... gonna go check Walgreens for the Double burner. Any suggestions on where to get the infrared grill grate?

Lastly- I am making a 3d mold of a face out of plaster of paris. What type of plastic do you suggest using? It needs to be thin and clear, but get as much detail as possible...

ok-
I found the double burner at walgreens- so scratch the question about the skillet. I'll go tomorrow to try to find the deep dish aluminum pans to fit this thing...

Any ideas on where to find the perforated grate?
Thanks!!!

Paul-
Another question. Why do you cut the holes out on the sides of the grill topper?
Thanks

Hi there I know this is an old thread .
Really great read and web page.

I have been searching for days, and learned a lot on the actual thermo vacuum forming but what I what I would really like to know is what MATERIAL to use for carving my shape or rc car body from scratch ?

My project is a 1/5 scale 35"x 16" GTP body


thank you so much