Strength of resins

General Composites Discussions Products Talk
#1

Hello,
I ve roaming at the internet for quite some time trying to find resins for infusion and i have this problem : Some resins have specifications that in papers are superior than the plates samples i am getting .
And let me get this straight - i ve been doing some test samples with 4 different companies - Fibermax Composites , Easy Composites , A. Andreou , Hexcel…(prepreg not infusion) which are not a lot but these are the ones i could get materials from - all the rest companies (most …i havent contacted with everyone in the world yet) .
What i can say is that i am confused . Sorry for the randomness of what is going to follow …

  1. What resin do u use for maximum strength ?
    I am talking infusion resin and prepreg

  2. What is the difference between OOA prepreg and autoclave prepreg?
    And let me make the question more straight - Is it possible to achieve professional results (consumer grade - things that can be sold and bought like bike frames, helmets ,Bike rims -… not F1 cars)
    with OOA prepreg ? with just vacuum ?
    For example the datasheet of a prepreg i am using from Fibermax says that it can be cured with or without autoclave…but the properties table is not different between autoclave or not process … So why use autoclave ???

  3. While infusing is it critical to have full vacuum ? I made a setup with a pressure switch and because i put in between a 3 litre chamber as a buffer i can not get full vacuum …my pump seems to be weak and can get up to -0.9 Bar … not -1 …is the part just a bit more resin reach or are we talking of inferior part.

#2

Resin for maxumum strength? Most strenght comes from fibres and stiffness from thickness, but somethimes I use a resin which can take a bit more elongation if flex is needed. Most plybooks/dwawings only supply fibre wheights, type, and orientations, and resin type.

OoA prepreg vs autoclave cured prepreg. ,mostly flow characteristics. out of autoclave use will probably give some lesser resin flow and interply consolidation, therfore more voids and lesser surface quality. Autoclave curing povides better quality.
But if OoA prepregs give consumer grade finish? I guess so. Ive build sportscars allmost comepletely from out of autoclave prepregs, with quite some clear coated parts. Right now I’m building on a 40 foot cruising catamaran, mostly from OoA prepregs. It’s a big market.
Full vacuum? you will never reach full vacuum(here on earth). The more the better. Same with OoA prepregs. You can infuse with 60% vacuum, but I can’t say if the quality will be good enough for your application. Wouldn’t reccomend it on non gelcoated parts, and only if you have to at others.

#3

Susho, thank you for the reply … you are always around when it comes for advise …
I have a question… If the strength comes from fibers and not from matrix then why i see so big difference in prepreg compared to hand lay up … where the amount of fiber used is the same? Why big companies have so big range of different prepregs anr desins for every situation ?

#4

When you are comparing different processes, you make it difficult to compare the resins/fibres being used. Fact is, even a properly made wet lay part is almost certainly bound to have more voids than a properly infused or pre-preg part. It’s also more difficult to get a precise resin-to-fibre ratio.

Different resins will also have their various advantages and disadvantages. Tg and HDT are 2 of the biggest ones, but you can get big differences in strength too. I take the TDS’ with a bit of a punch of salt to be honest, most manufacturers like to be generous with their data… Price is a good indicator. And real world tests, such as you see doing.

Realistically, you can compare resins until the cows come home. It would be better to conduct your tests, find the resins that give you the properties you are aiming for and that you like to use.

#5

Hand-lay up parts will typically contain more resin and therefore the mechanical properties will be down compared to prepregs where high volume fraction is achieved for structural parts.

OoA prepregs normally have higher void contents than autoclave parts and therefore lower mechanical properties. However if the OoA process is very good then parts with comparable to autoclave / fit-for purpose properties are achieved, e.g. some Airbus wings are made OoA.

Structural prepreg resins have properties that translate the applied loads well to the fibres and maximize the strength available from the fibres i.e. they have a good interfacial bond with the fibres and are also tough.

They have usually been subjected to typical in-service conditions, e.g. temperature and moisture saturation and then have their mechanical performance proven through extensive mechanical test programmes.

As Hanaldo said - ultimately the best way is to do your own tests using your process

#6

Oh, that will become a nice discussion.
The Airbus OOA parts are all made from UD with a tape layer, that can not be compared with the OOA prepregs you can buy!
The resin is not as unimportant as many may think. Some resins stay more flexible and do not give tge part the stiffnes than another resin may do.
The price is a good indicator, but you should always look what you need. I have about 20 resin systems for infusion in my company. Some have the same properties then high temp autoclave prepreg resins. But they are quite expensive.

Til now the tests with OOA prepregs have not been satisfying. Quite high void content and pinholes every time. Then more complex the shape then worse it was.
With an autoclave you can minimize voids and pinholes, but you will nearly always have them.
With infusion you can get a void and pinhole free part with the same properties or better than a prepreg part. But this all depends on the parameter. If you make a mistake you will mess up the part, while a prepreg forgives more. An autoclaved part will always have 80% quality.
No process is perfect for everything. For bigger parts I would always use infusion, for smaller complex shaped parts prepregs are easier to work with.

#7

With autoclave it’s lot easier to acquire pinhole free surface than OOA. Of course OOA can also acquire good surface finish which mostly depends on one’s technique, type of mold you use and the prepreg itself.
Again, we also have prepregs with about 15 different resin systems. The resin systems vary in terms of curing temperature, storage period/temperature and strength too.
I think full vacuum is critical for the prepregs as well.

#8

If the amount of fibre is the same, the resin ratio is not. It is very hard to achieve the same ratio’s in hand layup, while wetting everything out. In handlayup there is no compaction either, so the fibres float more freely in the resin, where in vacuum/pressure systems they are forced together.
Then there is the resin quality itself. with prepregs the resin itself has a good quality already, for one because they need to be cured with elevated temperatures. If you have a good hand-layup resin it needs to be cured too, before that, the cross linking process isn’t done yet, so the strength and stiffness of the resin isn’t optimal.

I do not agree comepletely. The cytek(acg) body panel system is very easy to get pinhole free. As is Gurit’s sprint material. Both are dry fibre systems though, with a resin film applied. Basically it infuses the fibres when the resin starts to flow, without the tubing and stuff involved. Both OoA use.
While a VTM 264 prepreg alone can have surface imperfections in both in or out of autoclave use a lot quicker.

Infusion however can give perfect surface quality, but I found it easier to teach people how to work with prepregs than with infusion or wet layup.(and implement in production environment) (like DD-compound said, more forgiving too)

And why the choice of resin? Snowboard builders like flexy resin with fast cycle times, car builders like perfect surface quality (less prep for paint) so low print through and no pinholes, boat builders like forgiving resins with a long out time, racecar builders like strong resins, etc. etc.

And like DD-compound said, no process is perfect for everything. How and what is something no-one will completely agree on, haha.