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Cast aluminum plate heat transfer 1

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VermontMaker

VermontMaker

Mechanical
May 3, 2024
6
Hi,
I have been making custom composite panels (mostly wood and carbon fiber) for more than 20 years and I am modifying one of my vacuum presses so that the laminate inside can be pressed between two 1" thick cast and ground aluminum plates (for flatness...the plates are also inside the bag) and at the same time increasing the temp around the bag to 120F from 70F in 10F increments over 24 hours, holding at 120F for a day and ramping back down. The bag itself is .03" thick clear vinyl and surrounds the laminate and the two plates. The laminate is .5" thick and the plates are MIC-6. I'm wondering whether I can transfer the ambient heat through the cast aluminum plate into the laminate and, if so, what the time lag is between change in ambient temp and the temp of the cast aluminum. Any help would be greatly appreciated.
 
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IRstuff the panels and plates are roughly 20"x 40" and ideally the plates would reach ambient temp in a few hours. more than 90% of the 20 x 40 surface (although covered by the vinyl bag) is exposed to ambient.
 
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yes, a common composites fabrication technique is to pull a vacuum inside plastic bag, where the atmospheric pressure compresses the laminate layers.
 
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OK, this is one way to figure out the time, but note that as the plates heat up, the delta temperature with the ambient decreases, and you hypothetically never actually reach 120F with a 120F ambient, unless you have loose tolerances, or provide for some ambient temperature margin, say 121 F. Note that I assume both exterior sides of the plates are available for convection and I assumed forced convection.

plate_eyzprn.png


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Not related to heat transfer, but have you really studied how to get a strong, reliable long term bond to aluminum?
 
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Thanks IRstuff. So let me see if I am interpreting your work correctly: the chart and graph seem to show it would take 3 hours (chart) or 20 time steps or 200 minutes (graph) for a roughly 20F change in plate temp. Is that right? And yes both outside faces of the plates are exposed to convection.

MintJulep I'm guessing you have some festivities planned for later today🙂. Thanks for asking about the adhesive, but the MIC-6 plates act only as cauls or flat raferences for the layup, treated with release agent and reused. The composite is squeezed between them.
 
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Sorry, setup error. Much depends on how much convection you can apply. An alternative is a water bath, i.e., just changing the ambient to a circulating water bath. That would substantially increase the heat transfer rate. Corrected sheet below; This is something you can duplicate in Excel.

plate2_ledkuy.png


TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! faq731-376 forum1529 Entire Forum list
 
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So the MIC-6 plates can heat up 50F in three hours with convection? Just want to make sure I understand.
 
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Thanks so much for the analysis IRstuff. Its very helpful to know that the heat transfer through the plates is possible with convection and to have an idea of what to expect before I dive into building the thing.
 
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@TugBoatEngineer: Yes, my industry (Passenger trains) has a long and continuing history of failed bonding of aluminum and stainless steel.
The vehicle structure is typically AL or SS. Manufacturers constantly want to bond floor panels to the structure - because it's faster and cheaper for them - and then in some years the bond fails. Door panels are SS or AL skin, bonded over a honeycomb core; they last maybe 10 years before the skins debond. Various internal panels are AL skin over something core, floor panels are SS bonded over plywood or balsa. Equipment boxes are riveted aluminum, with the joint faying surfaces "sealed so they don't leak" - until the seal bond fails.

But since most of the bonds are good through the end of warrantee the manufacturers don't care, and don't want to hear about proper surface preparation and extra, expensive things like siloxane to make the bonds last longer.
 
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I wish I had a single specific source explaining the problems. Aluminum and stainless are particularly difficult to bond because they both form surface oxides very rapidly. They should be abrasive blasted immediately prior to assembly which usually isn't an option.

 
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