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I finally had time to get some photos of my trimmer line "oven" uploaded.

Here is a picture of the line before & after cooking:

Line_Before_n_After.jpg (26.22 KiB) Viewed 2735 times

The pieces I'm making now are actually considerably straighter. The weights I used are clamp on electrical connectors.

Here's a picture of the oven:

Oven_sm.jpg (47.85 KiB) Viewed 2735 times

Inside the insulation jacket, there is a 24" piece of 4" stove pipe, with a 4" to 3" reducing section at the bottom. The pipe is attached to two horizontal rods through a pair of teflon insulating spacers to cut down on heat loss. The support rod & clamps are chemistry ring stand hardware. The heat gun is variable, and I've been running it at the 950F setting. Due to room air sucked in around the bottom, it's nowhere near that hot inside. I have a thermocouple temperature sensor that I use to measure the internal temperature.

Here's a photo of a line loaded in the oven, and one of the "baffles" I have to put on the top to get the temperature up to about 400F. The black wire near the top of the 1st photo is the thermocouple cable.

Baffles_sm.jpg (32.85 KiB) Viewed 2735 times

It takes just a minute to cut a piece of line & load it into the oven. Cooked for about 15 minutes, they come out nice & straight, with no discoloration or bubbles from overheating. The longest time is waiting until the oven cools off enough to handle the baffles & remove the line.

The next step is to come up with a better clamping arrangement & baffles so I can try to do a couple lines at once. It would also be nice to come up with a system so I can safely swap lines in & out while the oven & hardware are hot.

I'm currently using Home Depot 0.095" diameter line, which is very stiff. It's not very easy to snake in & out of the action of many air pistols. I'm investigating a couple vendors who make 0.080" diameter round line in orange. It's still winter here, so the stores aren't exactly stocked up with weed whacker line, and I'll probably have to order it. In addition to being easier to use in the pistols, it should cook considerably quicker.

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I've made about a dozen strings in my "oven", and the first couple came out a tiny bit on the short side. I'd cut the string to at least 24" long (~ 60 cm), and then have trouble getting 50 cm (20") of straight line when I was done. There's a bit on each end that needs cutting off, but only ~ 2" worth.

I measured everything carefully, and it turns out the line is shrinking by about 15%! It fattens up a bit, but by less than 10%. This was cooking it at around 400F. I backed the temperature down to around 350F, and that reduced the problem to the point where I can get the length I need, but it's still shrinking quite a bit.

I suspect they wind it up on a spool under considerable tension when it's made, and that when it anneals, all that internal stress gets relieved.