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Hello again

I'm just at the start of molding my diy carbon foil mast.
The mold will be in 2 pieces.
I was wondering if I could lay up 7mm of cabon in one hit I think it will be probably 30 layers or more? Would the epoxy still go off ? Or is it not recommended ?
I know with polyester resin it's not recommended from a heat perspective.

Thanks for you time.

Steve

It depends on the epoxy. The manufacturer should state what the max thickness recommended is. Most manufacturers have slow and fast setting epoxies, usually it just depends on the hardener used.

unless you have a press with heat I wouldn't suggest you do it in one shot. it will most likely have a soft center. if you have a oven / or heated press you can get it fully cured all the way through with the higher temps.

The epoxy I use doesn't set until 180 degrees, at room temp after 4 days it goes like rubber ( will not set at room temp)

One the second note... you almost need a over or a heat box if you are doing a carbon lay up on a mast to get the epoxy and carbon to bond and work 100% for strength and such if you are trying to keep it thin like 14mm thick.

If you are worried do two half and press them for the third time to make them one. I would just build a heat box that you can put your whole vacuum bag and everything in to do a post cure with the epoxy specs for the post cure.

just my 2 cent
Terrie
www.jellyfishboards.com

good luck and keep us posted on your project

Yes absolutely it will go off,
It's an exothermic reaction so as the epoxy kicks it heats itself kicking more epoxy heating more. The more epoxy the greater this exotherm.

Dependng on the epoxy it's exotherm and the amount of it this could get out of hand.
I've had a full cup of epoxy start to smoke when I was doing a lay up and got delayed screwing around with a rail wrap that went bad.

However I've done race fins (30 some layers) in exactly the construction method you were describing and potted tuttle fin heads and the exotherm was no more than warmish using system three general purpose epoxy with "medium" hardner

I'd keep an eye on it during the "kick" phase. and if you have a choice with your epoxy I wouldn't start my first one with the "FAST" curing hardener or any accelerators.

Warpage is another concern. uneven heat can cause the thing to cure wonky I made relatively thin mold halves (10-15 layers of 6oz glass) and vacuumed them down to a mirror to prevent any warping. Parts came out true. Generally the mold is supposed to be 5 to 10 times stiffer than the part to prevent warpage if not using any external pressure. Which is not likely practical unless you CNC'd your mold. So vacuum or clamp the crap out of it. In addition mildly flexible molds make removing the part MUCH easier.

Be sure to post cure the part (WHILE STILL CLAMPED) If you don't it WILL warp the moment it meets mister golden sunshine.

My most important tip:
As for mold release I cannot recommend locktite freekote mold release enough, however it is not a barrier nor is wax. If your mold isn't fully cured an epoxy based mold may still chemically bond to your part on the first few pulls. speaking from experience I'd DEFINITELY do my first two pulls from the mold using Wax and PVA otherwise it's a gamble.
PVA provides a mechanical barrier that wax and other mold releases do not. The surface finish will not be perfect but a little sanding is way easier than throwing away a part AND mold. After the first 2 pulls Loctite freekote and FMS sealer and the part will just fall off most times.

given your part will need sanding for a good surface finish on a PVA pull it may be worthwhile to put on a carbon sock over the whole thing after you press the 2 halves together to prevent splitting due to shear.
If I were building a thin mast I would build both halves out of unidirectional fiber and then use the external carbon sock to hold it together and take the torsional load.

And remember the better the mold surface finish the better the part. Go nuts sanding the mold, it will pay back 10 fold.

Scuba,
Sounds like J R has done lay ups and I agree with all, except for using mostly uni.
You need bibias for sure, and normal weave, and uni.
If you search the french forum you can find layup schedules, see frankites posts.
I recommend thick glass mold back to ensure a flat strut!
And remember that once epoxy is fully cured you can laminate on it but it will be a mechanical not chemical bond, so best if strut and fuse are made at the same time so you can join the next day.
or better yet one piece.
And I do super simple, cheap mold release.
thick greese applied with GOOD light, as thin as possible with finger tips, EVERYWHERE!!
then gently press seran wrap (the ultra thin fridge plastic). It sticks nice to the greese but can be moved easy, and is mechanical barrier. I do it twice, never had a problem just my method, not at all normal, but works well for me.
Employ vacuum system if at all possible, if not, sand in plastic bags makes for nice maniable weight.
hope this helps
revhed

by now you've probably guessed that different resins cure a different temperatures.
many of the higher strength resins cure at elevated temps.
room temperature curing resins can be modified with different hardeners to change the cure temperature and/or the working time.
some (not all) have improved material properties with elevated post cures.

its relatively easy to mix a batch of room temp curing epoxy and have it exotherm in the pot to the point that things are melting and smoldering.

spreading the mixed resin out on a cookie sheet after mixing will dissipate much of the heat if necessary.

the fact that you intend to 'dilute' the epoxy by at least 50% with your carbon fiber will also reduce heat build up.

-your mileage may vary.

one worry i have at the moment is how you intend to wet out the carbon.
all plys at once or one at a time?

regards,
-bill

I would not try to wet too much carbon at once unless you are becoming an infusion master.
Why not take a room temp or elevated temp epoxy and effectively create your own prepreg?

You could wet a few layers at a time on plastic film on a table, cover with another plastic sheet, roll and store in refrigerator (or cold outside within reason, it's still winter) until all your layers are ready, then prep the mold and lay the reinforcements all at once, so they cure at once.
You would want to test how your material behaves first with small batches, and even better if you let the saturated cloth cool some before rolling to make sure the reaction is in "suspended animation"
Or store flat if you lots of space.
Do take care to deal with any condensation issues when rewarming if you try this though, i.e. let it warm back up before exposing the cloth to air.
Just an idea, YMMV.
I have used a similar approach only on the small scale for ding repair, just so I can do filler plus reinforcement, hot coat and gloss coat all out of one cup of epoxy, with one brush.
Real prepreg will hold for months in the fridge, but even room temp epoxies will hold for 12h or maybe a few days, if kept around 3-5C.

EDIT:
BTW most seem to recommend at least 50% fibers at +/-45, torsion seems the main concern with these thin masts.

revhed wrote:
And I do super simple, cheap mold release.
thick greese applied with GOOD light, as thin as possible with finger tips, EVERYWHERE!!
then gently press seran wrap (the ultra thin fridge plastic). It sticks nice to the greese but can be moved easy, and is mechanical barrier. I do it twice, never had a problem just my method, not at all normal, but works well for me.

cool idea revhead,

you able to get an A grade finish with this? or do you end up with wrinkles from the plastic?
I tried the same with spray glue as the "grease" and release film but seemed to always leave some wrinkles. then again release film is pretty unstretchy and a lot thicker so saran wrap might do a better job.

Hay a big thanks
To all you guys for pointing me in the right direction,
The heat thing may be a bit of a problem, living in WA it's roughly around 38c to 30c degrees day time temp, so will have to lay up in the morning when temp is around 15c.
I was thinking of just using 3k plain weave or twill at around 200grams P meter
West system 105 with slow hardener.

Just thinking wether I should just us carbon or cabon,glass carbon,glass etc glass of the same weight.?

The other thing if I just use solid carbon or the other sandwich, the thing would sink, would it be better to put some thin piece of balsa in the centre of the mast of about +/-4 mm so the thing would float? Or would I loose to much stiffness?

Thanks guys

Scuba

Johnny Rotten, gotta love your name and what about Sid?
As you noted because the "seran wrap" or other clingy kitchen wrap stretches slightly, if you GENTLY using your CLEAN finger tips you can work out almost all of the wrinkels, depending on your mold of course. Just go slow with good light so you can see well! If you get a tear STOP, START OVER!
If you have someone to help the initial laying in of the seran wrap it can help.
For wings we get sometimes SUPER clean parts but as always needing some fine finishing sanding.
Scuba,
I am not sure if so little balsa will over come weight to float, I think not?
And imho the more carbon, not f glass, in the strut the better.
You need uni for sure! 45 bibias also and normal cloth.