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High temp epoxy?

Building tools out of plywood ("forms" & rings) to heat mould cast acrylic sheet.
Forming temp is aprox 325 - 350F
So far the plywood holds up ok.

I need a "glue" to bond multiple plywood sheets, and to seal and fair them to hold air pressure or vacuum.


Waiting for pricing from these guys: https://www.masterbond.com/contact-thanks
But tech rep advised "Can cost as much as $600/ounce" for some specs.

This looks more reasonable: https://www.fibreglast.com/product/High-Temp-Tooling-Epoxy

Any easy options i missed?



Jul 11, 2012
I used System 3 laminating epoxy to glue an oil cooler back together for my VW after breaking it in half. It was built like a radiator and I never had a problem with for the next 10 years.

How much load will there be on your glue joints?
Very little.
But need to seal the inner faces and edges - the experimental tool i made out or baltic birch might as well be a piece of screen as far as holding any air pressure.

I sent system 3 an inquiry.
The site has some products that hint at possibility including the ones that require oven heating to final cure.
But it is maddening to try to find any actual info.

Thanks for the suggestion!
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Sep 12, 2007
Northern Illinois

The glue in the plywood is likely resorcinol resin, at least the glue lines have that characteristic dark brown color. Back fifty years ago when I was building wood sash to restore a railroad car, this was the goto for waterproof glue. It does have to be clamped tight as it has poor gap filling properties. I haven't worked with in in years, but googling around, it seems to still be popular with wood boat builders. I see Amazon has one quart kits, but no tech specs. Hope this helps.

I used to use Resorcinal back in the same time frame you did, for exterior doors and windows and such. :)
Then discovered WEST.

The big issue for this app with resorcinal is as you state - not gap filling, and i wonder if it seals holes as well as epoxy with some filler?
Second issue is ease of use (compared to epoxy) = "Not".
3rd is Epoxy tends to be cheaper, by a fair amount. I don't know it that holds going into the higher temp versions.

Might have to explore that, though - thanks for reminding me!



Mar 21, 2017
325F isn't exotic IMHO, that is pretty much the normal cure temperature for higher end prepreg cloth materials, they make tooling using the same product, and cure it at slightly higher (not by much) temperature
but I doubt epoxy alone will be enough to make a good tool surface, I think you'll need to overlay cloth few layers to have a decent chance the tool won't crack/leak after first couple uses
System 3 responded:
Thanks for your interest in System Three. We do not have an epoxy that would meet your needs as 325 to 350F exceeds the glass transition temperature for all of our epoxies.

JZ79 - cloth will be the last layer if it becomes necessary to make a full plug.
But the ply or other material needs both a sealing option, and something to consolidate it for finishing smoothly.
Trying to get away with just blowing - looks just about possible from failed attempt with leaky form. Right now the ply is too porous to work well.

Just got back in from the shop - will search other leads provided in a bit. :)



Cast Iron
Jan 21, 2011
Troy mi
You don’t mention size or other details. Is it possible to substitute a non porous wood like white oak?

Conrad Hoffman

May 10, 2009
Canandaigua, NY, USA
Almost no epoxies have a Tg that high, but it doesn't matter. It's common to use epoxies above their glass transition temperature, which is actually a range, rather than a specific temperature. Also, an epoxy's Tg can never be higher than the temperature it was cured at.
The ring style for blowing a bubble compasses about 72" x 24" in plan & the ring might be up to 4" thick.

If a plug (male mould) becomes necessary, then that x aprox 16" T though it would of course be hollow.

Plywood is necessary for stability in the heat cycles.

The ring + a full size backer go in the oven full time + temp.

A plug "might" go in, or it might be possible to use outside the oven after heating the sheet.
Generally, the plug would be pre-heated to some lower-than-forming temp, though.
In that instance, lots of epoxies are OK to say 140 - 150F.

Should have mentioned size above - a lot of epoxies suggested might be good - except when the only package options for the product are "syringes" it sounds expensive. :) I did send some RFI/RFQ's but the Fiberglast 3300 is looking like the most viable option so far.

Also, an epoxy's Tg can never be higher than the temperature it was cured at
Did not know that - thank you for pointing it out.




Hot Rolled
Jun 23, 2017
I used this for laminating MDF to manufacture large vacuum forming moulds. Never had a glue joint come apart. I don't know if it is available in your location. One disadvantage, it smells like cat piss.


Oct 23, 2010
Maryland- USA
This will get you close (and probably close enough):

And looking about there are more and this one which crossed past the common 300 degree constraint:

And just some vocabulary from the folks whose general purpose epoxy I use:

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Got some useful suggestions, and, for me 2 clear epoxy choices.

RTV won't work due to smoothness issues and ease of use - though i see where it could be a help prototyping - so thanks for that suggestion.
It's on hand for sealing the oven anyway. :)

Waterglass is interesting, but i have no experience and it would be just one more system and product to solve for this project.
I'll keep it in mind, though. You never know what may be useful "later".

It appears that TR's Max HTE checks all the boxes for me: Highest temp rating, appropriate viscosity, and it is even (relatively) inexpensive!
I'm going to order the 40 oz kit, at least. (edited: placed order for 80oz kit)

Back up option would be the Fiberglast 3300.

This is the "practice" form. 12" x 33" aperture.
This is 1/2 the width and less than 1/2 the length of the actual part to experiment with end shapes, and possibly build up to try to blow a deeper form without bulging. ($25 per blank each test, vs $90-ish for full size) Aluminum plate is air diffuser/deflector for pressure through the backer.


The plywood was leaking through the face and cores but it was possible to build just enough pressure to blow one very deflective piece that collapsed when the edges failed to be retained in the form.

After trying again with a wider acrylic sheet and better clamps, the plywood itself simply would not retain any air pressure. Possibly "dried out" from previous test?
Eventually as noted, the ring form will *probably* be built taller as i figure out how to use it to shape the acrylic bubble.
E.G, glue to laminate/bond, smooth and fair edges, and to seal where necessary for air pressure.

Thanks much!
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Robert R

Hot Rolled
Aug 27, 2005
Raymond , CA
Some background describing the thermoforming process:

Stephen could you explain why you are attempting to seal the plywood rather than glue a thin aluminum top sheet to the plywood form?

Are you attempting to form the acrylic panel using air pressure alone or will there be a upper form to control the final shape and wall thickness?

Have you considered using IR lamp heating rather than oven heating to minimize the operating temperature of the plywood mold? The oven approach will heat the mold to the same temperature as the acrylic. That may result in a reduction in wall thickness at the clamped edges.
The ACRYLITE website recommends that the mold temperature be maintained at 190 deg F. At this lower temperature the plywood mold could be surfaced with a low cost formica panel.
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