I want to make an oval shape from a 28" length of 2.1" OD, .050" wall, grey PVC drainpipe, flattening it to about half it's original diameter. It's too big for the oven, and I don't know anyone in the pizza business (let alone someone cool enough to let me contaminate their pizzas with dioxin!) so I'm wondering if I could do this with a heat gun and weighted boards. Any tips, or means I haven't thought of? Robobass
A heat gun will work fine. For that size tube, just aim the heat gun through the tube, maybe swapping ends every couple of minutes to get even heating. The typical 1500 watt heat gun will warm the tube up to the proper temp in a few minutes.
I might use someth> I want to make an oval shape from a 28" length of 2.1" OD, .050" wall,
I *think* that melts not much above 122 degrees F. Once it starts softening it will "melt" pretty quickly and get very floppy. You'll need gloves and it wouldn't hurt to have a "form" approximately like the shape you want it to finish in. Once it start getting hot you may want to heat it very gently and start pressing it into the shape you want.
Not sure on melting point of PVC but the 2008 NEC won't allow it to be used in ambient air above 122 F and the wire which may be rated above that temp has to be derated (load wise) to 122F.
The 122F temp is where the PVC starts to soften or deflect ie lose it's structural strength. Fully melted processing temp is 310F to 390F depending on exact composition. The temp the OP needs to soften the tube enough to form the tube and take a set is in the 200 to 250 degree range. Much hotter than you can handle with bare hands or even thin gloves.
I'd agree that a good form and slow heat> I *think* that melts not much above 122 degrees F. Once it starts
We used a propane weed burner to heat some 2" electrical PVC and make bends. It works great, but it's tricky. For the ovals you want to do, I can see it would be very difficult to keep it pliable, yet avoid collapse.
If it collapses to a Figure-8, the save would be to armor up like a gladiator (full face helmet and goggles underneath, heavy jacket, two pairs of pants, etc) just in case, plug the ends of the pipe and apply MODEST air pressure (like 5 to 10 PSI), and heat it again just enough for the dents to pop out.
Cool!!! Thanks for the tips! I hadn't thought about the inversion/figure 8 problem. I really don't have the tools to make a form, but could I buffer the clamping boards with some foam rubber to soften this effect? Or perhaps blow some heat to the exposed sides and not just through the inside, so that the area that I want to bend the most gets more heat? I'll report bach after I've experimented. Robobass
I originally thought hot air gun, but to limit the temperature and keep the heating much more even, how about passing steam through it, e.g. from a wallpaper stripper if you have one, otherwise fix a hose to a kettle? Board plus weight sounds fine, but to prevent the figure of eight problem you need to have "stops" of the right height each side (bits of wood, or something else the right size) to stop the board from flattening it too far.
That foam rubber *may* melt sooner than the pipe. If most or all of your heating were from the outside I'd say slide a piece of 3/4" or 1" pipe inside this to prevent a total collapse.....may pay to lube it, but then you may ignite the lube also. You'd need to be able to pull the inner pipe out.
I thought about that one too. Good in theory, sand packing works great for bending steel pipe and tubing without collapse, but sand will not work very well for heat bending plastics.
The sand will have a lot of unwanted thermal mass - it will suck up a lot of the heat from the inside of the plastic, and as the outer layers of the pipe come up to bending temperature the inner layers will be chilled as the sand sucks up the heat. And by the time the inside (and the sand) is finally warm enough and ready to bend, the outside is either soupy and losing it's basic shape, or on fire.
And once you get the bend in and cool off the outside with a cold water hose, the heat from the sand is going to keep radiating back out from inside. If you try dumping the sand you'll have to handle the pipe to get it to flow out - and you'll mess up your nice neat bends.
Air doesn't have nearly the thermal mass as sand, and can be chilled down far easier But you do have to be prepared to control the pressure inside the pipe being bent - too much will cause a bulge and/or a rupture of the pipe.
And cold PVC pipe under high pressure can shatter when ruptured and will do a nice impression of a Chicago Pineapple, so you keep the pressures low and wear eye and face protection.
Too little air pressure in the pipe (as in a partial vacuum) from chilling, and you'll suck that pipe flatter than a soda straw on a thick milkshake.
It also seems that some of that sand could become embedded in the molten pvc and serve to scratch up whatever is pulled through it. Even if it is only a drain it could help to cause clogs, unless it is re-smoothed.
A more serious problem is that filling the pipe with sand will prevent the OP from doing what he wanted in the first place: squashing the pipe into an oval. The volume of the sand is fixed and the circumference of the pipe is fixed. Squashing the tube decreases the volume of the pipe, but the sand won't compress. Which is why packing a tube with sand in order to bend it helps prevent distortion of its cross section.