Powder coating trial run, boolit temps climb too high

[oley55]

Having finally cast my own bullets, I have been busy prepping for powder coating while waiting for my final need, that being the powder. Based on my reading of a number of threads, I have seen numerous mentions of inserting a thermocouple into a bullet so that the "cook time"clock can be started once the boolet reaches 400 degrees. I have tried three different bullets (44, 38, 9mm) and have had similar results were the bullet temperature continues well past the target temperature and pretty much stays there for the duration.

Preheated the Oster toaster oven (with a clay brick heatsink) using a PID and waited until the oven temp was a constant/steady 400 degrees . I have a thermocouple driven thermometer, with the thermocouple crimped into the center of a matching bullet and located within 3/4" of the PID's thermocouple.

These were merely trial runs on bare boolits to get comfortable with how the oven, PID and boolit thermometer work together. The following are the temps and times observed for #102, 249gr RCBS 44-245-KT boolits:


21:17 oven steady at 400
21:18 boolets put in oven
21:23 oven is again reading 400
21:25 bullet thermocouple reaches 400 and the 10 minute clock is running
21:26 oven temp overran set and reads 425
21:27 bullet reads 437, oven reaches it's peak at 433 and starts dropping back down.
21:28 bullet reads 439
21:33 bullet reads 425 and oven is steady at 400
21:35 bullet reads 426, oven 399 and the 10 minute cook time is over.

Will these over temps over cook the powder coating and is it normal for the internal bullet temp to go higher than the oven temp and then stay high? I also wonder if I am confusing my PID temp controller when I use the same PID for my pot and then my oven.(?)

Now that I know exactly where the PID probe sits, first thing tomorrow I am going to reposition the thermocouple bullet to sit within 1/4" of the PID thermocouple and allow it to rest on the silicone baking matt and see if the two temps stay closer together.

[JimB..]

The bullet can’t get hotter than the oven, it’s a science thing. You have some other issue.

I’m new at this and my oven also tends to run over, but only at startup. Tonight I started it at 300 and it ran to 350, then set it to 400 and it doinked around 390 so had to turn the dial a smidge.

I haven’t put a bullet on a probe yet, really just stick them in and cook for 15-20 min depending on how distracted I am. Maybe that extra cook hurts something, but I don’t see what.

[ih772]

OP, you're experiencing an oscillation in temperature. It's pretty normal for your oven to overshoot then undershoot your set-point temperature repeatedly as it adjusts to the new thermal load. It will eventually settle out but that may take longer than your cook time. Don't stress to much over it, your powder coating will be fine at the temps you're seeing.

[slide]

Most pro's get excited if the temp goes 50 degrees over recommended temp. Most powder coats have over bake protection that allows the powders to withstand too high a temp for short periods. If over baked powder coat gets brittle and the color goes off. I have one of the pids that Hatch makes and when I open the door to put in bullets it will overshoot some and then start back down and level out. Generally by that time the bullets have reached the recommended temp and it's off to the races. Tickled to see your using my t-couple bullet method.

[oley55]

I ran some more trial runs this morning with the temp probe right next to the thermocouple bullet and still had the bullet considerably hotter than the PID temp probe.

In the interest of investigation I ran a couple cycles with two thermocouple bullets (124gr/147gr) side by side with the PID probe between them and in contact with the silicone mat. The bullet right of the probe was consistently 50-60 degrees hotter than the bullet on the left. I swapped their plugs into the digital thermometer and the right was still much hotter. So I thought maybe one of the thermocouples was defective. I allowed them to cool for 20 minutes and checked and they within 0.3 degrees of each other. Then I ran another test and swapped the positions of the bullets (left to right) and the bullet on the right side was again 50+ degrees hotter than the left.

These things are 3/4" apart at most. Mind boggling that hot spots could be that different within 3/4". I am reluctant to check other locations as I'm sure to be freaked out.

Here are a series of reading for a 10 minute cook with the oven:
L 406, R 433
L 431, R 480
L 428, R 483
L 433, R 491
L 435, R 496
L 430, R492

[slide]

If you are running a convection oven try turning the fan off. Also try a coated bullet on the end of your thermocouple. My temps are always higher when I use a silicone mat. I prefer parchment paper. Don't get frustrated. You are learning. everybody goes through this.

[Conditor22]

IF the high temp is the PID, the PID may need to be tuned. Some PID's have an "AT" auto-tune function on them. Some have to have inner settings set to run right. Read the directions and see if they tell you anything.

you could test both with a pot of water and see what temperature the water boils at for each of them

[Hick]

You're overthinking this. Do the smash test on some powder coated bullets or shoot them and recover to see if the PC held up. How the PC holds up is the only thing that is important.

[popper]

Thermal lag. Lead heats and cools much slower than air. PC will be OK, just have to watch the bullets don't get so hot they 'slump'. If lead gets too hot you will see it look funny (like crystallized) on the base.

[oley55]

I did some more testing with two thermocouple boolits in different locations this morning and did not find any hot/cold spots and did not have any problems with their temps registering way higher than the PID thermocouple. However I did this testing without a tray of test boolits with them. No idea why/how having a tray full of bullets would create the really high readings I was getting. With bullets, I again got higher temps (+25 degrees) but not like yesterday. Some mysterious thermodynamics going on there I guess.

The powder I ordered from Smokes arrived today and I set about doing some shake n baking PCing and couldn't be more pleased with the results. I did stand the boolits up for these first few runs.

Although I had previously tested and confirmed the accuracy of the timer on my Oster oven, it started hanging up at different points so some of my cook times were not entirely accurate. When in doubt I went for the long side.

Bullets are; 200gr 44mag and 124gr 9mm. Did the hammer test and cubed a round for each. I did use BBs, but despite it raining most of the day I got good coverage must faster/easier than I expected and did not have to try preheating. I used 2:1 white/blue.