Oven Temps for TJ style?

[jdougn]

As I've searched the internet, it seems to be difficult to find solid information on oven tempuratures. Using a hand-held infra red thermostat, what are typical min/max temps for a properly functioning TJ style oven? What about tempurature variance, especially in the corners? Other information?
thanks,
dn

[cod]

depends on the plastic you are trying to form. a temp near 250-300 degrees is good for some, a little hotter is need for others (like polycarb).


your handheld meter might be misleading if you are reading a very hot spot,
like the coils themselves. You might supplement your readings better with a cheap oven thermostat.

[kayaker43]

Can't use an infra-red meter for something like that, the actual element may be well over 1000 degrees, but half inch away the background may only be 400 and air temp means little in an infra-red oven. The heating efficiency will depend on distance and good design. Getting the proper infra-red wavelength, watt density, and distribution are not so easy, but even when way off will work to a lesser degree.

Even with a good oven design, don't fixate on sheet temperature either, just use sheet sag as an indicator of when its ready to form.

[drcrash]

As I've searched the internet, it seems to be difficult to find solid information on oven tempuratures.

Yes. It doesn't seem that anybody who's built a TJ (Thurston James design) oven has measured it well, or if anybody has, I haven't seen the numbers posted.

Using a hand-held infra red thermostat, what are typical min/max temps for a properly functioning TJ style oven? What about tempurature variance, especially in the corners? Other information?

From what I've seen said, I'd guess TJ oven is typically tens of degrees cooler around the edges than in the center, and especially in the corners. That's mostly me inferring it from how long it takes to get the corners of a sheet of plastic soft compared to the middle. How many tens of degrees? I don't know.

I'm really talking about sheet temperatures there, not so much oven/air temperatures.

One thing to keep in mind is that the oven temperature is not what you ultimately care about. What you care about is the temperature of the plastic. The plastic in an infrared oven may be cooler than the air in the oven, or hotter because it absorbs IR and heats up above the temperature of the air around it.

A touchless IR thermometer IS good for measuring the temperature of the plastic sheet itself.

My general sense is that you want an oven that heats one side of the sheet to take about very roughly a minute per millimeter of thickness to heat a sheet of plastic to forming temperature. If it's much slower than that, your forming cycle will be slow, and if you're heating a thick (say 1/4") sheet of plastic with one-sided heating, it may never really heat through because it loses heat from the other side too fast, and not enough heat ever gets through to the far side.

(I'm no expert on that, and I'd be very interested in hearing from more experienced thermoforming people here.)

For thinner (say 1/8") plastic, you have more wiggle room in the heat level. If you heat a little bit fast, that's okay because the heat will get through the sheet well enough before the heated side gets damaged by overheating. If you heat a little bit slow, that's okay too... a 1/8" sheet doesn't generally insulate so much that the opposite side never gets hot enough.

Most TJ ovens have only ever been used with plastic 1/8" or less thick. (Thurston James designed it for making stage props and scenery, which are usually made from .030" to .060" plastic.) So far as I know, nobody's ever tuned the design to get the heat level just right for heating thick plastic, or to get the heat distribution across the sheet really even for forming difficult plastics.

(I've never heard of a TJ machine user forming anything but HIPS, ABS, and/or PETG. It seems to work fine for 1/8" HIPS or ABS, which the stormtrooper armor guys sometimes use. I suspect that's because the heating rate for TJ's oven was chosen not to heat thin plastic as fast as possible to manimize cycle time... it's a bit on the slow side, so that not-very-trained operators have a bit of slack in the timing. It may also be a bit on the slow side so that a 24" square oven can be run off a single 20 amp circuit.)

BTW, TJ's design is largely based on Nick Bryson's, from his (out of print) book "Thermoplastic Scenery for Theater". Nick's machines were never used for plastic more than .100" thick; the clamping frames wouldn't accept anything thicker.

Nick's machines had an uneven distribution of nichrome in the outer two rows of coil---the coil was stretched more in the middles of the sides, and less at the corners, to avoid cold corners. I don't know why Thurston James left that detail out of his design, but I suspect it was a mistake. I also suspect that TJ didn't go far enough in providing extra heat around the edges; it seems to be the rule that TJ ovens get the plastic soft in the middle well before the corners are soft.

It sounds like you've already built a TJ oven and have an IR thermometer in hand.

It would be great if you'd use it and measure the temperature in the center and near the corners as a sheet of plastic heats, and post the numbers.

I expect to have some numbers for a basically similar design with what I think is an improved coil layout relatively soon... that project's been on the back burner for a while now but I may get off the dime this week or next. It'd be nice to have some numbers for the straight TJ design to compare it to.

As far as checking whether your TJ oven is working "correctly," i.e., as designed, I don't have any measurements for you to compare to. Mainly you need to make sure it's built as designed---each coil segment having the same resistance (when they're NOT all connected in parallel), coils being evenly stretched, etc. Then you can be the guy who posts the first clear numbers on what that really means for speed and evenness of heating plastic.

(Unfortunately, Nick Bryson and Thurston James never had a touchless IR thermometer to actually measure things. They were flying by the seats of their pants, which works okay with a bit of experimentation, but leaves soemthing to be desired when you want to compare things and go beyond what they ever tried to do.)

It would be good if you measure your AC supply voltage, by the way. AC power voltage varies by about 9 percent, resulting in a heat variation of about 19 percent. (Wattage is proportional to the square of the voltage.)

Once you have a baseline, it should be easy to improve your heat distribution. I suspect you'll want to shorten the outer two rows of coils a bit, to get more heat around the edges. (That is, use somewhat less wire but stretch it more, around the same path.)

That's not hard to do; several TJ oven builders have done it, but nobody's posted any good "before and after" measurements as a guide for the next person. (How much to shorten the coils, how that affects the temperature distribution across the plastic.) It would be great if someone did.

You may also want to stretch the coils more in the middle and leave them denser in the corners. (That shouldn't be hard to do, either... ask me how if you're interested... I'm planning to do that for my oven once I get baseline numbers without that feature.)

[jdougn]

Hey, thanks for the latest posts. This week is busy, but hopefully I'll get the chance to graph some tempuratures that are worthy of posting here.

Any suggestions on what areas to measure temps? Certainly the corners, perhaps half-way between the corners and center, then obviously the center temp?
dn