Spray on coating to stop heat transfer? 2

[leebr]

We use metal frames to hold the edges of plastic sheets while they are heating in large ovens prior to a vacuum forming process. The heat is absorbed into the plastic and the plastic gets heated faster near the frame and is thus thinner at the end of the heating cycle. This thickness variation is a problem. Is anyone aware of a spray on coating that will stop or slow the transfer? Or perhaps a material to make the frames out of? The max heat is 550 F. Thanks very much.

 

[corus]

My guess is that the frame gets heated faster than the plastic and the heat is transferred into the plastic near the edges by conduction. A spray on substance might not reduce the conduction if it's relatively thin. Reducing the area of contact to reduce the conduction might be beneficial. If the frame edges at the contact point are flat then try a different shape to reduce the area.

corus

 

[MintJulep]

Do the metal racks sit directly on the floor of the oven?

Heat transfer from oven floor to metal frame via conduction, and then from frame into plastic via conduction is probably much higher than the convective heat transfer at the center of the plastic sheet.


Cheap and simple first thing to try is get the racks out of direct contact with the oven. Just some simple spacers from any suitable insulating material would do the trick.

If that doesn't work, then some sort of ceramic for a frame probably would. I found this with Google, but there are many others:

http://www.zircarceramics.com/index2.htm

 

[IRstuff]

You could make the frames out of a number of materials such as quartz or alumina. You need to find a low thermal conductivity material, in addition to be high melting point.

This problem is that the portion that is in contact with the frame has a much higher thermal transfer coefficient to the oven that the rest of the material.

You could also try to design the frame to minimize contact area with the material being processed, e.g., use needle-like projections to minimize the contact area and increase thermal resistance.

TTFN