High temp epoxy?

[Lewie]

Have you tried "Particle board" in place of Plywood ???
lewie

 

[stephen thomas]

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

Cost. rapid iteration. Excessive expansion.
It certainly would be possible to simply use an aluminum plate for the top, of course, but so far the above line explains were the experimental stage is.
It was also explained in my early post that something is needed to consolidate, fair, and smooth surfaces that will contact or form the acrylic.

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?

There is not likely to be a complete female form.
Current iterative phase considers the possibilty of a male plug - if necessary.
Most of these canopies are free-blown to a non-marking witness/indicator. It is the easiest way to get nearly perfect optical clarity.
Since my initiative is to form a canopy for which the height is intended to exceed the 1/2 width limit, i have to consider options.
One is to accept some extra "bulging" aka a Malcolm hood. If a Malcolm hood was not such a drag penalty, building a replica would be a perfect solution, and gorgeous evocative look. You can afford the drag for the extra visibility when the engine is making some 1500 or 2000HP. Harder when the engine might make 80 HP on a standard day.
The part i plan to experiment with is to simply make the ring taller and faired to the shape, to contain the bulge while blowing.

Have you considered using IR lamp heating rather than oven heating to minimize the operating temperature of the plywood mold?

Of course.
Or a pizza oven.
Canopies have even been formed in boxes over campfires.

The problem comes when you need to anneal it.
So it does not craze in a few years time.
At that point, you need an oven big enough to contain the part.
So, given the overall convenience, i chose to use an oven.

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.

right now, i don't necessarily see that as a disadvantage. It means more of the plastic is migrating into the form.
I also see a means to let the sheet slip as pressure is applied at lower temps. What i've experienced is that the plastic behaves more as if it were being draped (more thickness in the part). What i have not solved is how to let it slip in the early stages, and then "grab" & stick it for the final blow.

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.

If i make a plug, I'll have to reconsider the process.
But thousands of canopies have been free blown since the 1930's.
The place in Elgin, IL where i was first exposed to the process during a long weekend seminar in the mid-1970's used a plywood box with fiberglass insulation roughly/approximate stapled around it, a black pipe with holes drilled in it for a gas jet, and a simple ring form. They manufactured quite a few of the smaller canopies. I don't know how or if they annealed them - might have been a factor in abandoning that part of the supply business.

smt

 

[stephen thomas]

Have you tried "Particle board" in place of Plywood ???

Particle board is notoriously, uniformly porous - it is used for vacuum spoilboards on routers to take advantage of that effect.

It's not strong enough for a ring form.

As a material, it could be cheap if a plug is necessary - I'll have to throw a few pieces in to bake next oven cycle & see how it holds up.

smt

 

[EmGo]

Umm, if you just need a sealer, how about a coat or three of those hi-temp paints they have at Tractor Supply or any auto parts store ?

 

[stephen thomas]

Umm, if you just need a sealer, how about a coat or three of those hi-temp paints they have at Tractor Supply or any auto parts store ?

Maybe you did not read the first post and a few that follow?

Bond.
Laminate.
Consolidate
Fill.
Fair.
Smooth.
Seal.

But yes, i even dug out a leftover 4 gal carton of NOS Flamebar Ocean 441 left over from doing a job for Barb Bush back in the early 90's.
All the exterior woodwork/redwood was spec'd to be sealed with flame retardent & that was the product.
After looking the can over and trying to find any data on the 'net, i decided it, or high temp flat black Tractor Supply paint could be a cheap expedient that would probably complicate further iterations of the tooling.

As reported in post #19: for now the problem is solved.
I ordered the epoxy mentioned by TRboatworks:

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)

 

[dgfoster]

SMT,

I use Sodium Silicate R/U on an ongoing basis binding sand to make cores for my iron castings. (You may have also back in the day.). It is very heat resistant, compared to epoxy dirt cheap, is about the same viscosity as West Sytem laminating epoxy, and very benign to handle. ”water glass” purchased in hardware stores can vary some in formulation whereas R/U is specific. It would, I am pretty sure, provide an impervious layer for your mold and also act as an adhesive.

I suspect you will not chose to try it, but if you did I would send you a pint of the ”good stuff.” Most foundry supply houses should be able to provide it.

Denis

 

[CalG]

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.

Suggestions?

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?

Thanks!
smt

Back in the day....

Our four man aviation cabinet shop made forms for vacuum thermo-forming kydex and Lexan panels from MDF and HDF (Medium- High Density Fiberboard}

Features where built up, screwed and glued, filled with bondo , smoothed by sanding, painted with epoxy floor paint.
We home built a tool that heated the work via radiant heater coils at one station, then transported the hot, ready to form panels over to the waiting vacuum station with the mould all ready.

Many hundreds of aircraft window panels were produced this way. Might not have held up for thousands.

We might have been just young and dumb enought to get by with thinking "hey! that's not that hot" ;-)

 

[stephen thomas]

dgfoster -
message indicates email from me is blocked?

smt

 

[dgfoster]

Gee, Stephen, you are not such a bad sort that I would block you! ;-)

I PM’d you my current email. I suspect you might have an address that is no longer functioning.

 

[PeteM]

Plain old JB Weld is cheap and claims to handle 500F continuous and up to 600F for brief periods of time. While a thicker consistency than you'd probably like, Stephen, I've noticed (when mixing it up on odd plywood pieces and scraping them smooth to use again) that it seems to seal the grain pretty well.

 

[stephen thomas]

Pete -
I was just thinking of you while trying to find that Starrett gear vernier yesterday!
It was stacked in with the more seldom used thread mic's in a office cubby.
Wondered how you were doing & if you got a shop back together.

I briefly considered JB weld but the info i was reading seemed a little bit ambiguous.
Big stopper was the cost and inconvenience of all those little cans or dual syringe dispensers.

smt

 

[dgfoster]

SMT,

I am currently away from my shop and will be for a few days. But upon return I will draw off a pint of SS R/U and put it into a priority USPS box.

Here is a link that may provide clues to your intended use

SS as an Adhesive

This article reports heat resistance to greater than 1000c. I can vouch for that as SS sand cores, not intentionally weakened with sugar, but exposed to molten iron are truly like granite.

Denis

 

[PeteM]

Pete -
I was just thinking of you while trying to find that Starrett gear vernier yesterday!
It was stacked in with the more seldom used thread mic's in a office cubby.
Wondered how you were doing & if you got a shop back together.

I briefly considered JB weld but the info i was reading seemed a little bit ambiguous.
Big stopper was the cost and inconvenience of all those little cans or dual syringe dispensers.

smt

Hi Stephen, Still happily living past my expiration date. Most of the shop is back together, though now I'm mostly repairing scientific instruments and microscopes for a kids' science program.

JB Weld runs around $20 a pint if bought 10 oz twin-packs at a time. Not the way to go if you need gallons. The large tubes can be evacuated pretty quickly if it's pints or quarts at a time. Cheers, Pete

 

[stephen thomas]

Thomas what a great solution-

MAX HTE took a long time to get here, but it worked very well to seal & stabilize the form for my second try.

This item is 1/2 size.
IOW $25 test blank, rather than aprox $95 for a blank to blow a full size canopy for my app.
I'm using this one and some more 1/2 size tests to inform the shape and details of a mould for the "real" one.

watching through the oven window/jockying the airvalve:


Clamps removed and remaining clamps loosened to let the hot plastic shrink.
Size of bubble perimeter is 33"L x 12" w x 8-1/2" tall.



Thank you!

 

[Trboatworks]

Fascinating stuff- that’s quite the project.

 

[stephen thomas]

Anyone have a plastics supplier in the northeast that they are happy with?
About 3 weeks ago, i ordered cast acrylic from a NJ supplier from the cast acrylic page but upon pick-up today, it did not work out. Looking for 3/16" or possibly 1/4" 72" x 96" CAST acrylic sheet. (or 80" x 96" sheet) - Jobber referred by Rohm & Haas does not respond.

Sheets to be cut in 1/3's cross wise (IOW 32: x 80") would consider larger geographic area if shipping did not already cost more than product.

smt

.

 

[surplusjohn]

Some background describing the thermoforming process:

Thermoforming

How to thermoform acrylic: A comprehensive guide about heating & cooling, conditions and possible issues.

www.acrylite.co

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.

 

[surplusjohn]

Phenolic resins would be my first look. You would probably have to bake them. Motor varnish? Sodium silicate is an interesting idea. It would be crazy expensive but you could build the whole thing out of garolite. Lots of vac molds are made from aluminum filled epoxy but those probably dont even see 200 deg except for their surfaces.

 

[surplusjohn]

Phenolic resins would be my first look. You would probably have to bake them. Motor varnish? Sodium silicate is an interesting idea. It would be crazy expensive but you could build the whole thing out of garolite. Lots of vac molds are made from aluminum filled epoxy but those probably dont even see 200 deg except for their surfaces.

More thoughts on sodium silicate. 50 years ago I worked at Upson company. They made wall board from recycled newspapers [mostly screw magazines iirc,] first making a very thick paper then laminating it with sodium silicate. This made a stiff strong and fire resistant board. It was coated and bonded at very high speeds. This was a cheap product made in hugh quantities. They shipped many train cars and trucks out per day.

 

[Mud]

Anyone have a plastics supplier in the northeast that they are happy with?
About 3 weeks ago, i ordered cast acrylic from a NJ supplier from the cast acrylic page but upon pick-up today, it did not work out. Looking for 3/16" or possibly 1/4" 72" x 96" CAST acrylic sheet. (or 80" x 96" sheet) - Jobber referred by Rohm & Haas does not respond.

Sheets to be cut in 1/3's cross wise (IOW 32: x 80") would consider larger geographic area if shipping did not already cost more than product.

smt

.

Have you tried Alro? I think plastics come from MI but because of their network delivery might be economical. I was astounded that they shipped me cut pieces of UHMW for less than I figured I could cut them for.