Resin to Fiberglass ratio with vacuum infusion?

Can anyone please explain how you control the resin to fiberglass ratio when you are vacuum infusing, or point me towards literature to read? I have found several writeups on the vacuum infusion process but none so far address controlling the resin to fiberglass ratio.

My guess is, it is a combination of factors including: The fiberglass material choice and how the resin flows thru it. The viscosity of the resin. The working time of the resin. The vacuum pressure used. And, methods to wick off excess resin. If I am correct, how do you select among these factors to get to a target ratio?

 

You don't "control" the ratio. You aim for a thorough wet out, and the thickness of the glass controls the height of the layup, as it resists atmospheric pressure.
If you are infusing, you just let the resin infiltrate the glass. If you are laying up and then applying a vacuum, you just collect any excess resin as the pressure is applied.

 

rwatson, thank you for your response. I think we are kind of saying the same thing that the fiberglass material selection and the pressure determines how much resin fits under the bag. So do specs exist to help you select the material and pressure?

 

Well, yeah, the Layup Schedule selects the materials.
Too many variables for some sort of exact reliable formula for wet out
The volume of resin will never be more than the Area x Average Glass thickness, so make sure you have at least that much resin on hand, then multiply that by some magic percentage, usually less than 60%
You pull as much vacuum as you can, and aim for around 50% of saturation
A few small test runs might give you a better idea for your particular project.

 

Here are the results based on experimental data by Schoefield, a renowned NA. It is expressed in volume ratio rather than weight as it is more accurate. The ratio is determined largely by the directionality factor of the fiber. Other factors such as resin viscosity and pressure affects it but should be within range.

 

And here is what the fiber directionality factor is about. The more fuzzy and random the fiber is, the more resin it has.

 

Interesting figures, thanks. Autoclaving is about 10% less volume than infusion.

Infusion, assuming 100% vacuum, removes all the air and pushes the incompressible fibres together. Then fills the spaces with resin. How does an autoclave/more pressure improve on this?

 

With infusion only the air inside the bag is removed creating vacuum to pull the resin. The atmospheric pressure of 14.7 psi remains the same. With autoclave, vacuum is drawn + the machine is pressurized to about 100 PSI and heated. The raw material is prepreg, or pre impregnated fiber with a premeasured amount of resin. Very little resin remains to be squeezed out.

An "out of autoclave" process uses prepreg + vacuum bagging + heat. Atmospheric pressure remains the same. Not as good as autoclave but definitely better than infusion.

 

Thanks. I understand the process and the materials, but not the physics. How is it possible to get more air out of the laminate than a vacuum? How is it possible to compress the fibres?
Or is it that the better results are because the prepreg fibres are better aligned and/or have no need for filler threads to hold them together?

 

Yes. With vacuum process, the laminate is relying only on atmospheric pressure of 14.7 lbs/in2. With autoclave, there is vacuum inside the bag + additional pressure outside pressing in the laminate. More pressure= more squeeze.

In infusion, the liquid resin fills up all voids leaving a perfectly flat laminate, top and bottom. In prepreg, a precise amount of resin is deposited to start with. Just enough to fill up all the voids between the fibers.

There are two ways of doing it.

A thin film of adhesive is placed on top of the dry fibers/cloth, heated and passes thru a roller to infuse the fibers. It is so precise because the film adhesive thickness x the area is only deposited. I have read somewhere that the film adhesive is only 2 mils thick or so. Only enough heat is applied so that it melts only the adhesive to liquid but not enough to cure it. The B stage.

The other process is passing the fiber/cloth into a "hot vat" of resin then pulling it thru several pinch rollers, squeezing out the excess resin.

If you will examine it under a microscope, infused laminate is shiny, the resin almost level with the top. With prepreg, it is almost "dry" that you can see the weave and all the undulations of the weave. Meaning, there is very little resin in it. Like comparing a "damp" and a "wet" rag.

 

rxcomposite, thank you for the responses. That is the detail I was looking for. Can we talk directly? Email maybe? I am looking for help with a new boat project.

Another question for you, I have seen the prepreg system of passing the material thru rollers, laying it in the mold and than vacuum bagging. In that case they were using epoxy resin because it gave them 9 hours of working time. Are there any other types of resins that have the working time to make prepreg and vacuum bagging possible?

 

That is a not a new system. Making product AS NEEDED, just slow the curing time. Saves you the cold storage facility needed in prepreg. It has been featured in PBB magazine some 15 years back and was discussed in this forum.

Another method we use for small off cut materials is that we place the material over a plastic backing sheet, pour/spread/squeegee the resin, place another backing sheet then squeegee some more. You now have a prepreg. Sort of.

It would be just as good as wet bagging because this is just a variation of filament winding technique. The tows (fibers) are immersed in a resin bath then passed thru a roller to squeeze out excess resin.

PM me so I can send you my email account.

 

The system of passing resin through rollers to impregnate it has sometimes been referred to as "wet-preg",although it could be described as pre-laminating which is an alternative term for wetting out the reinforcement away from the mould.The more usual pre-preg is wetted out very accurately and consistently at the factory and then stored in freezers to inhibit curing.In practice it is taken from the freezer half an hour or so before use so that it becomes a little more pliant and returned to the freezer as soon as possible.There is frequently a large walk in freezer for long term storage and a selection of smaller freezers that are drawn on for every day use as this means less work for the big freezers and less deterioration of the stock in them.One thing to be aware of is that for the same amount of laminate,the extra compression provided by an autoclave can result in a thinner and slightly lighter laminate,but due to being a bit thinner,the laminate may flex more if a load is applied to the surface.There is a steady supply of pre-preg from businesses that absolutely have to use top quality material at all times as they sell of date expired stock for a fraction of the original cost to businesses that can make use of the marketing advantage of selling high tech composite parts for non critical use.You may sometimes see them on modified street cars or boat consoles.The material is fine for such use but has passed the time for being used for commercial aircraft parts for example.

 

Well said wet feet. Once, the insurance had to reimburse us because the airline logistics messed up. The rolls of prepreg was left "out of freezer" for a specified "out time" and was delivered late.

2nd time was when we had to transfer to a bigger factory and we were not able to ramp up to speed as projected. We had to dump some 50 rolls that were in the commercial cold storage facility. Although it was in cold storage, it was in excess of 3 months. We kept a couple rolls. processed it, had it coupon tested, and passed. The QC and the bosses said NO. It was being used for aircraft parts. That was how strict the rule is in aircraft manufacturing.

 

Resin ratio and edge bridging...

For wet bagging, if the bag is a bit tight, resin can be lost to the edges if the bag does not conform to the corners on a thicker part with core.

For infusion, what happens? Does the entire bridged area fill with resin that must then be trimmed and lost? Or does air flow in the area and mess up the infusion?

Those bridge areas seem like they could add to the usage, but not the ratio.