resin to fibre ratio during vacuum bagging process

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#1

I am using a LN9169 glass fibre along with LY5052 and HY5052 resin/hardener system for vacuum bagging of components.

The recommended weight fraction of (resin+hardener) to the fibreglass cloth is 35:65 for the finally cured component.

Is there any thumb rule to achieve the target weight fraction during the vacuum bagging process? What quantity of additional resin and hardener should be added to compensate for the quantity sucked by the breather cloth? What gel times are recommended for the above epoxy system that affect the final weight fraction of the component? (Currently I am using one and a half hours gel time) before starting the vacuum bagging process that lasts for another three hours. Is there any established relationship between the geltime, the time spent during vacuum bagging and the level of vacuum that determines the final resin/fibre ratio of the finished product?

Request help and suggestions on the above. Thanks a million in advance.
m_s_kamalesh@hotmail.com

#2

Hi,

It’s obvious, but I’m going to state it anyway, that it’s impossible to achieve the ratio you have mentioned above using the wet layup technique. You are on the right track by vacuum bagging your piece. The resin/fabric weight ratio will depend on how strong of a vacuum you are pulling. I believe the optimal vacuum is obtained at 27 to 30 inches of mercury. It’s nearly impossible to account for how much excess resin is being soaked up by your breather. Either way the best you can do with a vacuum bag is 50/50. Of course you can obtain better ratio by applying only the desired amount of resin to your layup. This can be done using the peel ply instead of the release ply under your breather. The peel ply is not breatheable and does not allow for any excess resin to pass through. It’s important to understand that this will only work if you can pull strong enough of a vacuum to make sure that all your resin is soaked into the fabric.

Another way is through vacuum infusion. There isn’t a whole lot of free information available for this process since it’s patented. People who do know about it tend to not like to share. Probably because they paid good money to learn it. Regardless, this process is highly affective and can yield up to a 30/100 resin to fabric ratio. There are also many other advantages to vac infusion as well but I will not go into that.

Anyway, you posted a couple of product names/numbers? Is there a manufacturer that goes along with that? because I am not familiar with those names/labels.

Evan

#3

I forgot your other questions.

You should start pulling your part as soon as you place it in the bag. Waiting for it to gell before pulling makes it harder to suck up excess resin. That means worse ratio. In that aspect they are related, but other than that you don’t need to pull any longer than the time that it needs to pre-cure. Once your resin reaches that stage, your ratio will not improve by pulling your part longer nor will it get worse. I usually pull it out of the bag as soon as I can. I like to do my trimmings before the part gets completely cured.

Evan

#4

First, there’s no way to ever guarantee the fabric/resin ratio without using pre-preg materials. You can try to come close, and there are some excellent methods available, but they’re not perfect. And to be honest, nothing in composite work is perfect. Those ratios represent a virtually unreachable ideal. You can get close, but that’s where skill and experience come into play. In order to determine how much epoxy is being sucked up by your breather fabric, determine the weight of the breather fabric before vaccuming, and then determine the weight after curing to determine how much resin you’re pulling into the fabric. I know that with my current epoxy system I’m losing 7-10% of the epoxy I’m using for the pieces I can put on a scale, pulling vacuum as soon as the piece is bagged. I’m stumped why you’re waiting until green stage to pull vaccum. I guess it’s because you want to ensure your resin/fabric ratio, but it kind of does away with the benefits of bagging to wait until the green stage to pull vaccum.

As Evan mentioned, you can measure out your epoxy and apply it to your fabric to get a good wet out along the desired ratio. My method is to measure out the cloth on the scale, measure out the appropriate amount of epoxy, and then place the cloth between two sheets of 4mil or higher mylar cut 6 inches bigger than the fabric on all sides. Lift up the top piece of mylar, pour your epoxy in, and then use a squeegee or bondo spreader to evenly apply the epoxy and get out the bubbles. Lay up the piece, apply vaccum, and hopefully you come out with the appropriate ratio of resin to fabric. If you want, experiment with some smaller pieces before doing a full size piece to figure out how much resin you’re pulling out of the piece.

As far as vacuum infusion, the process isn’t that secret anymore. Check out http://www.fibreglast.com/documents/361.pdf for an intro and discussion of the basic techniques.