Heating Blanket

General Composites Discussions Equipment Talk
#1

Hi guys

I am currently building the moulds for the wings of an ultralight aircraft (5,5m x 1,5m).
Still not sure if I will use low temp prepreg or resin infusion.
However I need to postcure the laminate in the molds at least at 80ºC ideally at 120ºC.

I thought about using some aluminium foil to radiate the heat evenly.
On that aluminium foil I paint high temp silicon with an impregnated layer of fiberglass.
When dry, I would lay at 1cm spacing parallel 0,2mm tungsten wire (doesnt shrink with heat)
On the wire I would again paint high temp silicon with an impregnated fiberglass cloth.
On this last layer I would glue some thermally isolating mat.

Questions:
Does silicon bond to dry silicon?
Any alternative idea for the first thermally transmitting layer ? (thought about aluminium, but think it could be very delicate)
Any cheap high temp silicon provider?
The last layer could be rock wool, but rock wool is very dirty. Any suggestions on this?
Any suggestion with the wire?

#2

It sounds like you’re thinking of building your own electric heating blankets - I think that’s way too much work!

Why not build a hot box instead? Simple plywood box that fits over the mold, adjustable heat source and fan(s) to circulate the air inside. Install temperature sensors at multiple locations.

I’ve also heard of wind turbine blades cured using infrared heaters, regulated by thermocouples on the mold surface. Obviously would require some sort of controller between the thermocouples and heater.

#3

Couple of options:

-making silicone heat blankets. Silicone is available in many places, for instance www.brandscomposiet.nl

-painting the mold with heatable paint, for instance www.futurecarbon.de

-placing the whole structure in a hotbox. Simple to make and to do.

-using infrared lamps. However quite tricky, and uneven heating is a big risk.

-making molds with heat tracing.

#4

Hi Herman

I just contacted Future-carbon.
Looks like an amazing product.
I thought about painting it on the back of the Polydur molds to reach 120ºC.
Obviously I have to wait for the price tag.
It looks really easy to do.
Need to think about any insulating heat barrier I can fix on the heat paint to avoid the heat escaping from the mold and heating the iron structure beneath that would leed to the iron expansion and deformation of the mold.
How would you glue the polydur mold to the iron structure?

What do you mean with heat tracing?

Thanks for your helpful tip.

#5

Roxul looks like a valid insulating blanket

#6

I agree with Fan O’Zakk and here are some of the reasons:

[FONT=Calibri]- [/FONT]Typically the ultimate loads on a ultralight wing are well sustained by material cured at 55 °C deg , although the technical sheets of most of resins formulated for high performance composite used in small composite aircrafts and sailplanes ,push the limit up to 80-90 °C degrees. Nevertheless the increase in elastic modulus of resin ( then stiffness mainly) due to cure at higher temperature is so low that it’s not worth to use higher temperature in the oven.
[FONT=Calibri]- [/FONT]Of course for the mould we have to consider that it must be used repeatedly; but its integrity can be guaranteed if its Tg is higher than cure temperature of the wing. Then, if the 55 °C above hold, it is sufficient cure the mould at 80-90 max, as permitted by some resins expressly conceived and certified for ultralight and sailplanes manufacturing.
[FONT=Calibri]- [/FONT]What is behind these reasons is the good compromise between technical and economical issues, that are the most important drivers in the design and manufacturing of sailplanes, ultralights and light sport aircrafts ( a lot of projects were born , but few, very few, survived in the market…!!!)

Conclusion: an oven as conceptually described by with Fan O’Zakk is all what you need…unless money is not a problem .:smiley:

Sorry if my explantion is long but the problem of oven is in many cases overestimated , and it’s not necessary to rent an oven from NASA to cure composites !
During static tests for ultralights certification , a composite wing is cover with a nylon sheet and heated by heat blower until temperature reaches 54 ° C deg, then loaded…nylon sheet… !!!

#7

I got the prices from future carbon. Reasonable but on the high side for my budget as every mold has nearly 8 square meters. The price is 178euros for aprox 4 square meters.
The problem I found is that in order to reach 100ºC ideally 120ºC I would need 18000Watt to heat my molds without insulation.
The question is how well could I insulate the mold skin to get below 13000Watt (max non industrial utility contract) in order to reach 100ºC.

I am building the molds so that I can use low temp prepreg (100ºC-120ºC required) or post curing of certified infusion resins (80-90ºC).

54ºC post curing is dangerous as the wings heat up in the sun easily to 70ºC and the post curing should be at least as high as the service temp.

In case I opt for a gas or gasoline oven do you have any recomendations on the most appropiate heaters? Two Diesel gasoline guns connected at each end to create a hot air circulation inside would heat the box (5,5m x 0,4m x 1,5m) enough?

#8

Actually pre-preg at 100-120 °C are not so " low temp"…

“54ºC post curing is dangerous as the wings heat up in the sun easily to 70ºC and the post curing should be at least as high as the service temp” maybe… but why are composite airplanes colored preferably white ?

#9

I am asking for advice on how to work with composites.
For plane design please visit another specialized forum.

#10

Who are you referring to in this post?

#11

I have a couple of resins which are certified by LBA (German “Luftfahrt Bundes Amt”) and it is specified that for sailplanes, a cure of at least 60 degrees C is used, and for motorplanes 80 degrees C.

Seems pretty clear to me.

For ovens, you can use anything.

Future Carbon
Sauna heater in insulated box which can be large, up to 5x2x2 meter I have see with a single heater)
One of my customers cures 120 degree prepreg masts with an insulated box, and a heatgun…

#12

I am referring to “wings”.
If he likes to postcure his wings at 54ºC he is free to do that, but I have no intention to commit suicide in his plane whether the wings are white black or pink.
In spain we have in summertime 45ºC in the shadow.

Sorry for sounding harsh, but the issue is serious and a personal non trivial safety decision.

Herman: thamks for the sauna tip. I will compare it to heatguns.

#13

Berridos,

if I “offended” you in some way , I can also apologize for that.

But, please, don’t offend me with your affirmation, after that I suggested you some way ( among many ways, of course…) to do things based on real experience in the field ( of composites airplanes, I mean…).

#14

What you are proposing is outright dangerous to my knowledge.

#15

You don’t have to follow what people say Berridos,do your own thing!

#16

fly your plane to Ireland sure and you’ll only need to cure it at 10 degs :slight_smile: - would need to be waterproof though

#17

Lol haha,Just like kid Icarus.