I've been using my knife forge to heat up dart shafts (turning it off and using the thermal mass of the refractory in the forge), which has speeded things up, but there is always the problem of it being too hot, and then cooling down enough it's not faster than using the heat gun. The challenge is hitting the curve with how long to leave it on before straightening some shafts, and how long to keep straightening before turning it back on (and going to work on something else while it heats up). Other than that, it really works great.
Well, this morning in the shower I decided to weld up a metal trough and fill it with sand, then put a long burner under it to heat the sand. I could then put in a bunch of darts, and keep the temperature right where I want it, replacing shafts in the sand as I take them out and straighten them. I already have some 18" diameter metal pipe, courtesy of my brother, who bought a bunch for the fence he built across his front yard a few years ago. I was going to cut a third or so out of the cylinder and close off the ends. Then I started thinking it might take too long to heat up that much sand! Also, did I really need the sand open to the air? Maybe a smaller pipe filled with sand would work even better.
I checked on the price for a 6" diameter pipe, and it was about $40 before tax. Looking around a different metal yard on the way to get that, I found a "rem" (remnant piece, already cut, and therefore discounted) that was a six inch square tube, five feet long. I got it for about $32 after tax, and didn't have to wait for somebody to cut it.
I already had a six inch square of 3/16" thick steel plate (same thickness as the tubing), and I started to weld it together tonight; but unfortunately my MIG welder trigger started going out and I'll have to try and fix it tomorrow. I was barely able to tack it together.
I'm taking photos as I go, and I'll post them later. I plan on making a stand for it that holds it at an angle, with the open end up, and welding a lip on the open end, to help keep the sand in when I pull out shafts. I've already got some burners started from when I was making them for special fire effects at the photo studio I used to work at. I'll have to borrow my forge's regulator until I get another one, which I will probably want to, if this works as good as I think it will. I bought 200 lbs of clean sand, and I hope I'll only use about 150 lbs or so to fill it. That much mass will take some time to heat up to where I want it (which I don't know the number yet but will establish by experimentation and monitor with a baking thermometer), but should also hold it for a long time.
I was worried about getting too much of the dart hot, and not being able to straighten bends right next to each other, but the forge has a fairly long hot area (over a foot), and it didn't seem to be a problem for most bends. For those that are a problem, I can finish off with localized heat (heat gun or whatever).
I am making it longer than half the longest dart I plan to make, so I can do one end at a time and flip them over. I was thinking 4.5', but if 5' works I won't have to cut it down…