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Heat treating by radiation

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scp:
DC: We probably need to start a new thread, as using a radiant heater is a good topic by itself. My heating pad comes with a thermostat at a similar price. I just used it twice. If you use it following the instruction (I didn't bother), it should last indefinitely. I don't trust its thermostat. Better use an instant or laser thermometer and keep it under or around 170C (338F) for less than 20 minutes. If you see smoke, you better turn it off. Good luck.

willie:

--- Quote from: Beba on September 27, 2019, 07:44:47 am ---Willie, those heaters are simple cal-rod heaters. Same as the cooking rings on the top of an electric stove. They will not handle much of a bend before breaking the internal element. They are designed for primarily for conductive (in contact) heat. Used solely in a convection application they will burn out quickly.

Using radiant heat is an option I hadn’t considered. Radiant heat waves heat only the objects the waves come in contact with adding an element of control to the process. so I’ll be keeping track of this thread for solutions to the raised questions.

--- End quote ---

Beba,
those heaters are a bit long for the rated voltage, and don't get red hot like the stove type. a buddy that installs them reports they can be bent using a tubing bender, at least before they are fired up. he said he never tried to rework one that had been in service.
If you have one it might be worth trying, but after giving some thought to the requirements for bow heating put forth in some of the posts above, I have been leaning towards re purposing a milk house heater. the coil could be removed and hung from something like a piece of stiff but malleable wire or some sort of support that could be easily be formed  into the shape to follow the contours of the limb

DC:

--- Quote from: willie on September 27, 2019, 03:41:18 pm --- I have been leaning towards re purposing a milk house heater. the coil could be removed and hung from something like a piece of stiff but malleable wire or some sort of support that could be easily be formed  into the shape to follow the contours of the limb

--- End quote ---

That's where I've been. I got stalled trying to find ceramic insulators of the right configuration and fabbing reflector/s. I like your idea of hanging the coil from a rod so what we need is insulators with two holes, one for the coil and the other for the rod. Could probably braze reflectors on in between the insulators. Hmmmm :D

willie:
Don
keep us posted about the insulators you find. I am looking also. some sort of ceramic filament might be useful.

3d printing can do ceramic parts

thinking about a piece of flexible metal conduit for a support

Santanasaur:
Maybe I’m wrong about this but I think it might be better not to use  insulation at all. My thinking is that radiative heating hits the belly and absorbs or bounces off before affecting the back.With convection heating, hot air has a chance to go around and scorch the back. This is why I also like using the heat gun close up where you get more radiation to less hot air. If im thinking about this correctly, to get a deeper heat treat before the back starts scorching, air temp should be minimized while as much of the heat as possible comes from direct radiation and not hot air.
in cooking terms I think we want to broil the bow as opposed to baking it, for similar reasons you broil or grill a steak rather than baking it. This is why a salamander broiler in restaurants is open so that it doesn’t heat up and act like an oven. Insulation would make the rig a better oven but worse of a broiler.

On a separate note, if a good way to thermally insulate the back from the belly was found (badgers heat tape post) then convection would work without the issue of scorching the back.

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