Natural Fiber Composite: Vacuum Infusion

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#1

Hello everyone,

I am a research student working on natural (jute) fiber composites and was hoping for some advice/assistance with the vacuum infusion process. I have run several trial castings with the following parameters:
-6 layers woven jute fabric (0/90)
-Fiberglast 2000/2120 series epoxy (de-gassed in vacuum chamber)
-Enkafusion nylon flow media

My layup order is as follows:
Glass plate->laminate stack->peel-ply->flow media->spiral tubing connected to resin feed on one side and another spiral tubing connected to vacuum line on other side->bagging film

I have infused at -30in Hg, -25in Hg, and -20in Hg, and in each case I am left with air pockets in the final casting as shown in the attached pictures.
In one case I was able to make a part with no air pockets, but the fiber volume fraction was very low (~28%)

For my most recent failed casting, this was my procedure:
-Set up lay up as described above
-slowly draw vacuum to -20in Hg
-check for leaks
-De-gas resin in vacuum chamber
-unclamp resin inlet tube to start infusion
-at ~2/3 of the way through infusion clamp resin inlet
-after allowing sufficient time for infusion to finish and resin to gel/cure, clamp vacuum tube and shut off pump

In this case the infusion itself too about an hour and the vacuum was left running for an additional 2 hours before clamping.

Can anyone offer any advice on how to improve my composites? I’d really appreciate any help

Images can be found here: http://s192.photobucket.com/albums/z67/MikePinto_photo/Natural%20Fiber%20Composites/

#2

That fiberglast resin/hardener has a mixed viscosity of 925 – 975 cps which if I’m not mistaken is rather high for an infusion process.

#3

Here are some suggestions:

  1. Use a lower viscosity resin (100 - 300 cP)
  2. end the flow media 1-2" before the end of your laminate stack
  3. Add a 3-4" brake zone between the laminate stack and the vacuum line with only peel ply in between the laminate stack and the vacuum line.
  4. don’t clamp off the resin feed line until the entire laminate stack is wet out
  5. infuse at the maximum vacuum you can achieve (29.9 inHg)
#4

Thank you very much for your input, if you wouldn’t mind clarifying:
2. What is the purpose of ending the flow media before the end of the stack?
3. Is the brake zone just meant to slow the infusion speed?

Thanks again for your help, I will be trying these suggestions today.

#5
  1. The purpose of ending the flow media before the end of the stack is to slow down the infusion speed to ensure full wet out.
  2. The brake, or break, zone helps reduce the amount of resin that enters the vacuum line (waste).
#6

water vapor could be your problem as well. Natural fibers are hydroscopic. I had the same problem like you.

I let the lay-up stack under vacuum for a couple of hours. After that voidless part.

I used resin infusion resin, 11mbar absolute pressure in the bag, braking zone, let the water vapor boil off = success :slight_smile:

#7

I greatly appreciate all the advice posted so far, thank you all.

I just made another casting today with the recommendations above, except that I reduced the pressure to -25in Hg after the infusion was completed and clamped it at that pressure for curing.

My sample had zero voids but had a fiber volume fraction of only 24%. Any other tips on increasing this volume fraction? Should I leave the pressure at -30in Hg during curing?

Thanks again for any help

#8

As soon as the entire laminate stack is wet out, or slightly before, clamp off the resin feed line. Leaving the vacuum at 30 inHg until the resin gels won’t hurt anything.

#9

How are you calculating the volume fraction? Are you using a density of 1.45 g/cc to do the calculation?

#10

Looking at your pictures, your volume fraction could be reasonably right. The fabric is very course.

#11

Unfortunately I am forced to use density approximations to calculate volume fractions, using 1.46g/cc for jute and 1.12g/cc for my epoxy. I know this isn’t the most accurate way to measure the volume fractions but I can’t burn out or dissolve the matrix without destroying my fibers in the process.

#12

It is indeed pretty coarse, believe it or not that was the finest jute fabric I could find, and I tried finding finer jute yarn to weave finer fabric myself but that couldn’t be reasonably attained either.

Would it be possible for me to determine the maximum fiber volume fraction of m composites using fabric parameters such as cover factor, yarn diameter, etc.?

Thanks again everyone for all your advice

#13

Your calculations are probably correct. That is some hefty fabric.

#14

Being a natural product density varies, not only per batch, but possibly even per inch. Fiber diameter will also not be constant. Which makes things more complicated.

try and aim for a perfect (void free) infusion, and measure the thickness.
Do a casting with some resin, so you can measure the (cured) density.

These number should help you determining a density for the fiber used.

#15

I have tried implementing the brake zone and used a shorter flow media, and you can find pictures of my setup in the link below.

However, following my last infusion, I had strange “air” pockets emerge shortly after I clamped my resin feed line. These pockets were located at the end of the laminate stack, were quite large, and only occurred after the infusion was complete. Any ideas what this might have been besides a random leak springing up following infusion? Pictures below:

http://s192.beta.photobucket.com/user/MikePinto_photo/library/Resin%20Infusion%2010_9_12

#16

How long did you keep the part under vacuum, before infusing?

#17

herman,

After drawing vacuum to -30in Hg, I left the laminate stack under vacuum for 1.5 hours to boil off any moisture in the fabric. After that, I rechecked the seal and began infusing (after degassing the resin of course)

#18

OK, that is good. The Soric needs about 30 minutes to reach an equilibrium.

It can still be moisture, or a component of the resin boiling off. Try reducing vacuum somewhat, to for instance 29 Hg

#19

place your gauge at the resin inlet to get an accurate reading of the part.