How to thermo-form a tube from PVC plate?

[MarkS]

I need to build three large collection cups for a few test skimmers I will be making. Due to the sizes, I cannot afford to use preformed acrylic tubing. The first tube will be 12.75" OD x 18" tall. The second will be 18" OD x 18" tall and the third will be 24" OD x 24" tall.

What I have in mind is to build three cylindrical jigs out of 3/4" plywood and sheet them in 1/6" aluminum so the outside diameter equals the inside diameter of the tubes. I figured that for the 12.75" tube, I'll need a 38.375" length of clear PVC plate, 1/4" thick. This will wrap around, leaving me with an approx. 1/8" gap to weld the piece together. The 18" tube will need a 54.812" (54 13/16") length and the 24" tube will need a 73.6875" (73 11/16") length. After the PVC cools and the tube is formed, I'll use my plastic welder to fill the gap.

I am pretty satisfied with my jig design and details, however, I need help with creating something to heat the PVC so it can be formed. I figure I can tear apart a large electric heater and use the element connected to a oven thermostat with some blowers to distribute the heat. However, what should I use to build the case of the oven? Is this even the best way? How about heat lamps?

I'm hoping to hear from someone that has done this before. Honestly, I would like replies to come from members that have some experience in the area. If you are interested in this, but do not know how, feel free to tag along, but do not derail the thread. I WANT AND NEED to get this up and running. I am not interested in alternatives. Sorry to be blunt, but I've had threads derailed in the past because well meaning members that thought the idea was too complicated and started throwing out alternatives. This is one of my pet peeves and is not the direction I want this thread to go.

If anyone has any ideas related to the above, please let me know.

TIA,
Mark

[orlenz]

I have made tubes out of acrylic sheet, at work we have an oven for forming acrylic, it uses heat lamps to give even heating, I cut the peices 1/4" long to allow for shrinkage and wrapped the soft acrylic around a cardboard carpet tube, I'm sure the same would apply to pvc, the trick is to get even heating, and to work fast before the material starts to stiffen up. (you will need to work extra fast with your jig because the aluminum on your jig will suck the heat out of the pvc)

[Mind_nl]

oke so you need a big oven for even heating, we have large ovens at work to melt paint (powder coating) on the end there is a cool down chambre to allow the coated parts to cool down slowly to prevent cracks in the paint. The cool down zone is very hot but you can work in it for about 20 minutes. Maybe you can find a company in your arrea with a big oven like that.

[Acrylics]

Hi Mark,

Go find a pizza oven Actually, many shops do use these.

Heat lamps do work well as do many other types of elements. Even the 220V elements from an actual oven. Just make a box frame, line it with drywall or some other heat resistant material and you're good to go though not exactly OSHA approved nor the *best* method but workable in a pinch.
You'll want to make the jig radius bigger than the actual size do to uneven cooling though. (jig will retain heat, ambient air will not). So if you want a 12" radius, make the jig 13" radius (or so) to allow for this. You *probably* won't get it perfect the first few times so be prepared for this if this is critical. After the piece is formed, throw a bunch of heavy blankets on top to help it cool evenly. Oh and remember to line the outside of the form with high thread count cotton sheet or flannel to keep the material from sticking to the form and reducing "mark-off".

HTH,
James

[Kim G in Ks]

My only experience is forming a 12" tube out of acrylic. It was done in 2 pieces, both formed in an ordinary household oven. The pieces you are wanting will take a lot of room and a lot of heat. Do you know the temperature you need to heat it to to soften it up enough to be able to form it? What are you making your jigs out of? If I remember correctly, PVC has a much lower melting point than acrylic - somewhere in the neighborhood of 200-225 degrees. I would think that a collection cup could also be square or a 6 to 16 sided design, much easier to make on a good table saw. I would imagine that for the costs of properly setting up an oven to do what you want to do would be cost prohibitive, unless you plan on making LOTS of them and selling them on the open market. Good luck in whatever direction you choose to go with. I will tag along to see if this comes to fruitation.

Hope this helps

Kim

[BeanAnimal]

I think the trial and error part is going to be as costly as purchasing the needed tubes. Remember the material will want to stretch on the outer surface and compress on the inner surface. Try to wrap a ream of paper around a beer mug... you will get the idea.

As already stated, a simple wooden box lined with drywall will suffice to make an "oven". You could also use fiberglass insulation to make it retain the heat better. Pre-heating your form (not as hot as the melting point) will also help.

You may also want to consider building the tube out of (2) halves. This would make measuring less critical. The halves could be trimmed in a table saw. The only drawback would be the possibiluty of being slightly out of round. But, using a form and welding the seem may prove to provide the same problem.

Bean

[MarkS]

Originally posted by Acrylics

Go find a pizza oven Actually, many shops do use these.

Not a bad idea, but have you priced pizza ovens? I know that Middleby Marshal ovens range from $15,000 for a single oven to well over $50,000 for a quad tandem oven.

NOT gonna happen!

Originally posted by Acrylics

Heat lamps do work well as do many other types of elements. Even the 220V elements from an actual oven. Just make a box frame, line it with drywall or some other heat resistant material and you're good to go though not exactly OSHA approved nor the *best* method but workable in a pinch.

That's what I was thinking of doing, but I do not have 220v outlets. I'm probably going to be stuck with heat lamps.

Originally posted by Acrylics

You'll want to make the jig radius bigger than the actual size do to uneven cooling though. (jig will retain heat, ambient air will not). So if you want a 12" radius, make the jig 13" radius (or so) to allow for this. You *probably* won't get it perfect the first few times so be prepared for this if this is critical.

OK, I have to admit that you lost me here. Why would I want to make the tube bigger than I need? I need a gap after it is formed so I can weld it closed. I must not be understanding you here.

Originally posted by Acrylics
You *probably* won't get it perfect the first few times so be prepared for this if this is critical. After the piece is formed, throw a bunch of heavy blankets on top to help it cool evenly. Oh and remember to line the outside of the form with high thread count cotton sheet or flannel to keep the material from sticking to the form and reducing "mark-off".

I'll probably make several test runs with (cheaper) grey PVC plate until I get the hang of it. I was going to line the form with felt. The blankets sound like a good idea.

[Acrylics]

Originally posted by MarkS

OK, I have to admit that you lost me here. Why would I want to make the tube bigger than I need? I need a gap after it is formed so I can weld it closed. I must not be understanding you here.
[/B]

Because drape forming causes the radii to shrink due to uneven cooling. Hmm, how to explain... Let's say you built a form for a 12" radius and formed your piece, when you remove your piece after cooling - the actual radius will be 5-10% smaller so it will probably be ~11" radius. To "pre-accommodate" for this, make your form a little big and your piece will be more apt to be correct. Hope this makes sense.

James

[MarkS]

Originally posted by Acrylics
Because drape forming causes the radii to shrink due to uneven cooling. Hmm, how to explain... Let's say you built a form for a 12" radius and formed your piece, when you remove your piece after cooling - the actual radius will be 5-10% smaller so it will probably be ~11" radius. To "pre-accommodate" for this, make your form a little big and your piece will be more apt to be correct. Hope this makes sense.

James

OK, that makes sense.

[mudder]

Question, where do you get PVC sheet?

[MarkS]

Originally posted by mudder
Question, where do you get PVC sheet?

Your local plastic shop will have it and you can get it from US Plastic.

[SciGuy2]

MarkS, did you ever make the cylinders?

[SA057]

FWIW I have used my kerosene shop blower heater for projects like this. I have made several things frop pvc and other plastics. If you have access to one they get very hot. I use my welding gloves and long sleeves.

[douggiestyle]

dont need 220vac to operate the oven elements. they will heat with 110vac. just more slowly and not as hot. which is what you would want. considering that the elements can get to about 1000F.


i like beans idea do two halves.

that way you could place the mold with sheet on top in the oven and just let the whole thing bake.

its very simple. enough so that we did things like that as sixth grade shop projects. my favorite was blowing acrylic into shapes like bowls and such.

EDIT
ive changed my opinion. drape mold it over a cylinder by baking everything in an oven. then quickley pull the whole thing out of the oven. wrap everything around the cylinder having enough to overlap about an inch. let it cool. then make one saw cut through the overlap. this should give two near perfect mirrored sides to seam.

[SciGuy2]

Originally posted by douggiestyle


...ive changed my opinion. drape mold it over a cylinder by baking everything in an oven. then quickley pull the whole thing out of the oven. wrap everything around the cylinder having enough to overlap about an inch. let it cool. then make one saw cut through the overlap. this should give two near perfect mirrored sides to seam.

I've done that with smaller cylinders with good success.