Powder coat oven: user error?

[NikA]

Twice now I have melted batches of bullets while attempting to PC them, and I am wondering if I am doing something stupid. I look forward to comments.

I started out with a used conventional toaster oven with the manufacturer's temperature dial. I became convinced that at some point during my use of this oven, something on it broke such that it was no longer heating properly. In order to rectify this, I wired all the elements to be always on, stuck in a thermocouple, and attempted to use my PID for temp control. This immediately melted a batch of bullets, due to my attempt to use a very short, unobtrusive TC. I wired in a longer TC probe and successfully PCed additional bullets, although the temps would drop rapidly and take a long time to equilibrate when new trays were put in.

Today, I picked up a new convection toaster oven. To check whether the manufacturer's temp dial was any good, I measured with a TC and found the oven wouldn't get much over 340F at the highest setting. As a result, I set the oven to "always on", plugged in the PID, and measured with the PID TC and a second TC to determine that the PID could actually control the oven at 400F. First batch of bullets, seems like the oven is taking quite a while to get up to temp. Come back 10 minutes later, melted. I moved the rack down but it doesn't seem to make a huge difference.

My question is two-fold: 1) What am I missing here? Is it radiative heat transfer to the TC probe vs. the bullets? That seems like the likely culprit but I am surprised in this temp range. 2) Is the manufacturer's temp dial good enough? What are the implications of PCing at ~350F vs 400F?

Thanks in advance for your help.

[Grmps]

My experience with countertop ovens is all the temperature gauges are off. I put 2 oven thermometers in the middle of the shelf I will be baking on and see what oven setting brings those thermometers up to 400*. I believe Occasionally PID units need to be tuned in and it is possible to get a bad one.
I'm glad to hear you got a convection oven, they heat quicker and more evenly due to air circulation. Once you have determined what setting (oven or PID) gets you to 400*, to speed up recovery time from opening the door you can line the bottom of the oven with some heat retention media. I use ceramice BBQ briquetts, others us fire bricks or ?
you stated that originally with the PID @ 400 you melted the bullets, you also mentioned that the new oven set on the highest temperature would not show higher than 340 on the PID. This kind of tells me that there is something wrong with the PID. do the oven thermometer test and note where the oven and PID need to be set for the 2 oven thermometers to read 400 degrees. if the 2 oven thermometers don't match, beg-bum-borrow-or-buy another oven thermometer till you have 2 of them that agree.

[NikA]

My first guess was that there was an error in the PID reading the TC. I also use this PID for my casting pot, and I calibrated the probe for that task so that I am relatively certain it is reading correctly and the PID works. I thought that measuring with two separate TC probes on two separate pieces of equipment was enough to avoid problems with TC measurement error in the toaster oven, but I guess that might not be the case. I'll try a different wiring setup to see if I get different results.

I really suspect the melted bullets are a result of the TC not receiving as much heat as the bullets due to placement or blocking. Both times it's happened in a relatively deep pan, which is nice for recovering the lead but may be part of the problem. I had hoped that the convection feature would help to solve this but evidently there's more to it.

[fredj338]

I just use a conventional oven therm to check my conv pizza oven. The factory setting is off by about 30deg, so just turn it up until it preheats to 400deg +/-. Never melted a bullet, but then I don't leave them in more than 15m.

[MyFlatline]

Haven't melted a bullet yet, toaster oven to wall oven, cook for roughly 20 minutes. Takes about 5 to get the powder flowing and 15 to cure.

[popper]

Did you wire the fan to be on all the time?

[NikA]

Fan is wired such that when PID provides power/heating elements are on, the fan is on. I didn't see a convenient way to wire it such that it was always on, even when the PID was not supplying power to the unit. New oven so I don't want to alter it to much in case it needs to be returned/repurposed.

A coworker suggested a practical test on the thermocouple measurement that I will try out tomorrow and report back on.

[Grmps]

150mm is good for the thermocouple. When you're sure you are keeping the oven, add a second power supply for the fan and leave it on all the time this will extend the life of your fan and keep the heat evenly distributed.

[PBaholic]

I picked up a cheap toaster oven at Goodwill. My boolits melted on my 1st try at 400, so I set temp back to 350 for 20 minutes and tried a few. These baked fine.

Next batch was at 375 for 20 minutes, and these were fine too. I ran this way for a while, then I started experimenting with different alloys, and discovered why my 1st batch melted.

If you are using lower lead alloys, your melting temp could approach 400 degrees. My original alloy was a BHN 17, which was really too hard for my .45 and .38 pistols anyways. I finally settled on BHN 10, which PC's fine at 400 now. This is right around COWW lead, but I don't use COWW's. I use 2 parts pure lead (BHN 5) and 1 part Linotype (BHN 20+).

I did smash tests of the PC at 350, 375 and 400 and could not tell a difference. All bonded extremely well.

I also don't pre-heat, just stick them in at 400 on the dial, set the timer for 25 minutes and let it go. My powder melts in the 1st 5 minutes, so I guess technically I'm sorta pre-heating. My powder is HF red, and have done 10K in the last year.

[KenH]

Something's wrong...... well, you know that {g}. Have you checked your TC/PID setup in a pan of boiling water? Should be right around 212F.

Perhaps that deep pan you mention is holding the heat? For proper operation the TC tip (tip only) must be located right around the bullets themselves.

When you turn your oven on with PID control, how close does the temp stay to setpoint?

Good luck - I'll be watching this thread to see how it goes.

[Beagle333]

You need to experiment more with your PID and probe. Something appears to be amiss there. And it might be just probe placement.

Also, the fan should be wired direct. My toaster oven has two plugs and wires, one for the heat and one for the fan, because a PID sends power to heat in short quick pulses once it gets close to the set temperature. It doesn't use a 10 or 20 second "on" cycle then stay off for 10 or 20 seconds like a thermostat. This quick pulsing won't let your fan ever get to full speed and move the air properly.

My probe placement:

[NikA]

The experiment suggested by my colleague was along the lines suggested here: take a temperature measurement with a TC located where the boolits would be located while the PC was baking. This was a very instructive measurement: I quit measuring when the TC measured 500F in the deep pan, and the PID TC read ~325F. Even with a flat pan the discrepancy was over 100F. This tells me that the convective element of the toaster oven is really not relevant to the PC curing process, and that a significant portion of the heating is happening by secondary radiative emission (or reflection) from the pan.

Interestingly, when I set the thermostat to 450F, the oven maintained a temperature that was only 25F above the set temperature when measured on the pan, which tells me that manufacturers have designed this into the oven. This is interesting, because it means (to me) that the convection element on these ovens is probably just a gimmick. It certainly is not reducing the temperature gradient from the side of the oven to the top of the pan.

For the time being, I think I'm going to go with the manufacturer's thermostat since it seems to be fairly well calibrated. I have one more idea about how to design an oven for ideal secondary emission for boolit cooking purposes which I will try out in the next couple days. I will report back when I have a chance to get to that.

[NikA]

Someone here also suggested the alloy might be relevant. The melted bullets were undersized commercial casts that I was trying to coat up to size, so this is certainly a possibility. I have retained the melted lead for future casting, and since the boolits were not useful to me anyway in their current form, I don't feel particularly bad about having melted them since worst case scenario was recycling them.

[popper]

convection element on these ovens is probably just a gimmick.

Nope, try again. I ? your 'deep' pan as the problem + not letting the fan run all the time. You are radient 'broiling' your boolits. he convection is supposed to cook with hot air.

[KenH]

Thanks for the update NikA - I can't think of any allow that would allow melting in toaster oven at 400F. Pure tin doesn't melt that low.

If you oven will maintain a 475F temp (25 above 450 setpoint?), that may or may not be indicative of what temp it will hold at a lower temp. Test to see how well it maintains temp by setting at 375F, and see what range temp it holds around 400F - that's where you're interested at. Using PID control my toaster oven holds 399 to 401F pretty solid.

You mention:

I quit measuring when the TC measured 500F in the deep pan, and the PID TC read ~325F.

Is this two different Thermocouples you're using with two different meters? It sure sounds like one of them is pretty far off. Have you checked both in boiling water to check accuracy at 212F?

Ken H>

[NikA]

If you read my OP, both TCs measured the same temp at the same place in the oven before any pan was inserted. One of the TCs is new for this project, the other was calibrated to within a couple degrees previously.

I understand that a convection oven is supposed to be cooking with hot air. My claim here is that, based upon the data I have collected, this is not really the case. Clearly, heat transfer is occuring via some other effect. My options are conduction, convection, and radiation. Conduction in a non-condensed fluid like air is typically very small. Convection should not be affected by my placement of a shield between the TC and the heating elements provided there is still circulation. That leaves radiation. Given that the radiative transfer from the elements is basically the same (their distance to the measurement point hasn't changed), my options are either reflected radiation from the pan or secondary emission in the IR range from the pan. I don't really have the tools to measure that at this time, but it does tell me that the convection part of the convection toaster oven is not nearly as important as manufacturers would like us to think.

[popper]

Technically, there is no convection cooking. It is by conduction or radiation, both are absorption of energy. Your pan absorbs both and conducts to the boolits as well as radiating to the convection air. Convection merely tends to keep the air temp consistent. Actually I've been using a hot plate to cook my PC, works quite well. Conduction to the boolits and loss to the air is reduced as the PC is an insulator. Your oven is used like the hot plate but then you have to control the plate temp, not air temp.

[Grmps]

I would guess that the fan is not working properly or the pan is too big, the air can not circulate properly. I do all my baking off of screens/mesh and can do 2 screens at ounce in my larger countertop covection oven.
LASC
Common Bullet Metal Melting Temperatures
Metal Melting Temp. Notes:
Lead (Pb) 621 degrees Weight, ductility, obturation.
Tin (Sn) 429 degrees
Antimony (Sb) 1,166 degrees Hardens lead alloys, extremely brittle.
Linotype 464 degrees - solid 465 degrees - liquid
Foundry Type 619 degrees Extremely brittle. NO expansion on game, shatters on steel targets.
WW (clip-on) 463 degrees slushy To 505 degrees molten

[KenH]

Isn't the "convection" portion of an oven nothing but a small fan that runs to cause the heat to be more evenly distributed around the oven to prevent hot spots from the heating elements?

[bangerjim]

AGREE......wire the fan on ALL THE TiME. Shutting off the fan with the elements causes hot spots and defeats the purpose of a convection oven.

I have used 4 different ovens and have settled on the BIG Oster totally digital one WalMart sells. Perfect temp control within 2F or so. Being convection, you need to set 25F higher than the 400 you want. They are all that way......even you kitchen oven.


Play with you PID settings, position of the T/C and wiring the fan ON all the time.