Instructions | Plastics Blow Oven

1

Warning

There are always dangers when working with mains voltage, propane and plastic fumes. Take precautions when building and operating the oven. Do not leave the oven unattended. Make sure you have a fire extinguisher, good ventilation and a respirator. If you are at all uncertain about any of this, then don't do it, or do it with an older, wiser friend :)
2

Build Oven Body

Lay eight CMUs on the floor together to make a pad. Make sure that the two CMUs in the center are on their side, so that their solid faces are pointing up.
Build up the walls of the oven with more CMUs. Use six CMUs for each layer of wall. If you are using Kanthal wire as the heating element, then you can lay just two layers. If you are using a propane burner as the heating element, then lay at least three layers of CMUs for the walls. Make sure that one CMU on the bottom wall layer is on it's side, so that the cores allow access to the interior of the oven. This is necessary for air flow and for running the gas line to the burner.
Lay bricks on the top layer of CMUs. These cover the open cores of the CMUs, and also can be moved around to accommodate the clamps and the lids.
3

Installing the Heating Element

If you are using a propane burning as the heating element, then put the burner inside the oven. Run the gas line through the opening in the CMU. Lay a flat cast iron pan on top of the burner. Lay eight hollow core bricks on the flat cast iron pan.
If you are using Kanthal wire as the heating element, measure the resistance of the wire and calculate the length necessary to draw only 10A at 120V. You can run more current, but make sure that your circuit can handle it. If you make the Kanthal wire longer than necessary, you can always move the connections on the wire to lower the resistance.
Coil the length of Kanthal wire on a pipe. Cut short lengths of copper wire from Romex and remove insulation. These wires are used to suspend the heating element inside the oven. Stick the copper wire between the layers of CMUs to hold them in place, and make sure they are spaced out around the interior. Connect the coiled Kanthal wire to the copper wires with nuts and bolts.
Install the 20A light switch in the junction box and connect to a power cord and length of Romex. Run the Romex to the oven. Remove the outer insulation from the Romex to allow the hot and neutral to fit between different cracks in the CMUs. Connect hot to one end of the heating element, and connect neutral to the other end of the heating element, with nuts and bolts.
4

Make Oven Lid

Cut MDF to two feet by two feet. Drill a hole in the center for the air fitting. Plug the hole and pour a silicone pad. Place a 12" x 12" piece of acrylic sheet over the silicone while it is curing, to make a smooth surface. If you are making a bigger oven, and will only be blowing one size of plastic, then you can save silicone by making a silicone seal around the edges, instead of pouring a pad. Cut slots for clamps to fit into lid. Install air fitting in hole.
5

Make Aperture

Cut aperture for bubble. This will dictate the shape of the bubble. You can use a hand router, CNC router, or keyhole saw, etc. Finish the interior edges of the aperture, so that it is smooth. Find square metal rod to be used by the clamps to hold the aperture down.
6

Prepare Oven

Run the heating element with lid on oven. Do not load any plastic or connect air fittings yet. Use an oven thermometer or surface thermometer to measure temperature of oven. Once the oven is around 320F, take off lid and load plastic. Put plastic sheet on silicone pad. Put aperture over sheet. Clamp metal rods down. If using propane, turn off burner. Put Lid back on oven.
7

Blow Bubble

Rotate the lid periodically. Measure temperature of plastic. When plastic is around 270-300F, attach air fitting and turn on air compressor. Turn off electric heating element. Blow bubble until it is around six inches or more. Maintain air pressure and let bubble sit and cool down. Add additional air if bubble contracts. For better quality bubble, let anneal overnight. Remove bubble and unload from lid.

Discussions

MRMAINT62 wrote 04/05/2018 at 06:55

Thank you for responding. How much does it usually cost you to purchase all the parts to create the heat for your set-up.

I understand the need for even heating, and I was wondering if you think the element from the oven would do a proper job if used in the bottom of the oven you build with the concrete blocks?

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Michael Barton-Sweeney wrote 04/05/2018 at 18:21

I don't know the exact cost because I've had most of the parts lying around. The main costs would be the CMUs, the bricks, the MDF, the silicone, the air compressor and then the heating element that you use and the acrylic. CMUs and bricks are around a dollar or two a piece. The MDF is probably around $20, and you can use silicone caulk I, if you make Oogoo. Kanthal wire is pretty cheap (about $5-10 for 50 feet). Depending upon what you already have, the cost of building the oven could be very inexpensive. You might even be able to get the acrylic for free if you look around and talk to a framer.

I think the heating element in an oven would work fine. Most residential ovens run at 220V, but it shouldn't be a problem to run the element at 110V. I don't know how much current yours will draw, but I would imagine that it would be around 10-15A or below. You also usually have a choice of heating elements in an oven to salvage (bake, broil and convection), so you could pick the one that works for you.

Now that I think about it, convection might even work without taking it out of the oven like your earlier suggestion—I think that most convection elements in residential ovens have air vents on the back panel, so if you did flip the oven upside down, they would be on the bottom and might be able to move the air around to get even heating.

I'm not sure how well it would work, but it's worth looking into, if you already have a residential oven and want to hack around before getting the other parts. I don't think the built-in oven controller will be too helpful if you flip it upside down (the temperature sensor will most likely be on the bottom), and you also have a fixed oven size, but other than that, it seems like a pretty reasonable way to go.

If you have any luck, share your work! I'd love to see what you make.

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MRMAINT62 wrote 04/06/2018 at 05:29

I don't have any immediate projects, but I may just dream one up to have a reason to try this out!

I noticed you have a RC plane in your hand in your pic. How long have you been flying RC? I have a number of planes but seldom get out to fly them. I think I like to build them more than fly them...

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Michael Barton-Sweeney wrote 04/08/2018 at 00:37

Hi Mr. Maint! Yes, I'm into RC gliders. I also spend a lot more time building them than flying, mainly because mine always crash :) I've been making them since I was a kid. Almost all of mine are balsa (tissue or monokote), and a big goal of mine has been to get more sophisticated about plastics, so I can build them better.

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MRMAINT62 wrote 04/05/2018 at 01:12

Since your heat range requirements are that of a standard common oven, why not buy a used one for $50.00 (or get one free on Craig's List) that has a large glass window for viewing. Drill a small hole in the center of the glass. Epoxy your air fitting to the outside and weld (or screw) quick clamps (metal as they will be on the heated inside of the oven) around the edges of the glass. make a metal ring for the blowing window that the clamps can align with, cover the inside of the oven door glass with your silicone mixture, and now you can preheat the oven, open the door to the oven, install the plastic sheet against the glass. Put the frame against the plastic sheet, lock the clamps (using thick gloves), close the door, flip the oven on it's back to have the forces of gravity even across the sheet, wait till the plastic is to temperature, connect the air hose fitting, turn on the air pressure, turn the oven to the anneal temp and have a slick set-up with all the heat regulating hardware already built in! You could remove the burners and use one of those circuits to run an exhaust fan to deal with removing fumes to the outside.

If you like my suggestions, please let me know...

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Michael Barton-Sweeney wrote 04/05/2018 at 02:09

Hi Mr Maint!

I have a hacked residential oven in my shop that I use for other stuff (it's great!). I haven't used it to blow plastic because it seemed harder to solve the problems of getting the air line inside and making it heat evenly, than to just build a simple top-loading oven out of CMUs.

I'm not very good at drilling through glass, so I would probably just replace the oven door with my own lid, if I was going to use the residential oven on it's back. But my hunch is that in either orientation (right side up or on it's back) that it wouldn't heat evenly, because the heating elements wouldn't be symmetrical to the bubble.

Building the oven out of CMUs is also handy, because it means that you can easily change the oven volume and scale it up.

Regardless, that's a good suggestion about trying a residential oven. They are fantastic and cheap. I'd like to see more oven hacks!

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Plastics Blow Oven

Discussions

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