Lo-fi way to get even heat in vacuum former heating element

June 26, 2021 3:48 PM

I built a 16" x 10" hobby vacuum former and so far it's working great except for one problem. The plastic is being overheated & thinned in the center.

I know nothing of thermodynamics!

I built my former heater using an inverted stainless steel roasting pan with aluminum sides. The plastic heats in about 90 seconds.

See here for diagrams

I'd like to even out the temperature & eliminate the hot spot, ideally without having to buy new heating tubes. I am willing to make a different shape/style of enclosure.

I tried heating the plastic further away from the heat source. This passably reduced the hot spot but increased the heating time to 4 minutes.

Would drilling some holes just above the hot spot keep the center a bit cooler?

I am so far out of my element here. :)

Any ideas would be greatly appreciated!

posted by i_mean_come_on_now to Science & Nature (6 answers total)

Response by poster: Sorry, title should read:
Lo-fi way to get even heat in vacuum former heating enclosure, not element!
posted by i_mean_come_on_now at 3:51 PM on June 26, 2021

Are the four elements currently wired in parallel? If so I'd try wiring the two elements in the centre in series and then that assembly in parallel with the outer two elements. This will reduce the heat output of the centre two elements by 75%.

You could also check to see if all the elements draw the same amperage. If there is a significant difference you could try moving the lowest amp draw elements to the centre.
posted by Mitheral at 4:47 PM on June 26, 2021

I think you might want to NOT drill holes where the hotspot is, but still drill them elsewhere. That way the hotspot will only re-radiate lower energy deep infrared instead of the higher energy, directly-radiated orange and yellow which comes directly from the heating elements.

Or maybe not drill holes at all and just follow Mithreal's advice.

I learned the re-radiation lesson the hard way 30 years ago when I destroyed some high temperature circuit boards by exposing them directly to heating elements instead of shielding the heating elements.
posted by ZenMasterThis at 6:04 PM on June 26, 2021

Best answer: I need to preface this with an assumption that your plastic is primarily being heated by radiant heat.

I have a crude spreadsheet that might be useful if I can remember how to use it. Our problem was simpler: we had to evenly heat a 600 mm wide 'belt' as it passed 40mm in front of a bank of 700mm long tubular heaters in a vacuum (no convection). The heater axis was perpendicular to belt travel. Our solution was to use two types of tubes... one radiating its full length and the other that only radiated from its ends. The heater vendor could make the end heated ones to our spec. The model predicted for our application with a 1000W full length heater we'd need custom heater radiated 656W over its 96 mm heated ends --> 1.11% uniformity. Unfortunately the project was canceled before we tested it.

For you, the luxury of custom heaters is out but blocking some of the radiant energy is an option... maybe with an oval shield of metal screen? You are welcome to a copy of the spread sheet, it may give you a gut feel for what is needed. It was written to be used with excel's solver (in the analysis pack add-in) for uniformity optimization but works with hunt and peck inputs too.
posted by tinker at 6:45 PM on June 26, 2021 [2 favorites]

Agreed you want some sort of shield between the elements and the hottest spot. If you're lucky a small oval piece of tin foil (much smaller than the overheating area) right in the center will do, otherwise a larger screen of some sort will add tinker says.

You could rewind your elements yourself with nichrome wire to make custom elements. Slightly thinner wire on the outer sections and slightly thicker wire in the middle. But the shield idea is way easier.
posted by flimflam at 10:43 AM on June 27, 2021

Best answer: While tuning it, try browning some paper in it to identify hotspots.

(Since paper is cheaper than plastic.)
posted by sebastienbailard at 10:40 AM on June 28, 2021 [1 favorite]