Resin/Fiber Ratio Math - RC Groups
Dec 02, 2010, 10:51 PM
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Discussion

# Resin/Fiber Ratio Math

Can someone double check my math?

Since Resin/Fiber ratios are done in terms of volume, not weight, a little math is in order. I've always calculated it with the math below, but I just want to make sure that I'm doing it correctly.

Example

I have a completed composite structure that contains only fiber and resin. The total weight of the part is 20 grams. The weight of the E-glass in the structure is 14 grams. This means that the remaining weight is resin=6grams.

The specific gravity of E-glass is 2.55 grams/cc
The specific gravity of the Resin is 1.10 grams/cc

To calculate the volume of each i divide the weight (grams) by the specific gravity.

E-glass is 14 grams/2.55 = 5.49 cc
Resin is 6 grams/1.10 = 5.45 cc

Total volume is 10.94 cc.

Resin volume 5.45cc/by the total volume 10.94cc = 49.81% resin

Is this correct?

 Dec 02, 2010, 11:00 PM Registered User yes, the math looks perfect to me. Are you sure e-glass is that much heavier than resin? A cc of glass weighs more than twice a cc of resin?
Dec 02, 2010, 11:11 PM
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Quote:
 Originally Posted by dmcquinn Are you sure e-glass is that much heavier than resin? A cc of glass weighs more than twice a cc of resin?
Well according to the internet it is.

Specific Gravity - E-Glass
Specific Gravity - E-Glass
Specific Gravity - E-Glass

Most resins seem to range from 1.06 to 1.20 g/cc
Last edited by wyowindworks; Dec 02, 2010 at 11:19 PM.
Dec 03, 2010, 12:25 AM
Registered User
Quote:
 Originally Posted by wyowindworks Can someone double check my math?... The specific gravity of the Resin is 1.10 grams/cc Adam
Looks right to me. A minor point, but assume you've already considered that netting out 'resin' technically means 'resin + hardener' mix at their respective densities. Typically they dont vary much, but for ex:
resin = 1.18 g/cc
hardener = 1.04 g/cc
ratio = 40 PBW
resin 'mix' density = [(1*1.18) + (0.4*1.04)] / 1.4 = 1.14 g/cc
Dec 03, 2010, 12:47 AM
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Quote:
 Originally Posted by ptxman Looks right to me. A minor point, but assume you've already considered that netting out 'resin' technically means 'resin + hardener' mix at their respective densities. Typically they dont vary much, but for ex: resin = 1.18 g/cc hardener = 1.04 g/cc ratio = 40 PBW resin 'mix' density = [(1*1.18) + (0.4*1.04)] / 1.4 = 1.14 g/cc
Yep. Wouldn't you divide by 1.volume mix ratio rather than divide by 1.mix ratio by weight? See bold italic in your qoute.

Last edited by wyowindworks; Dec 03, 2010 at 01:04 AM.
 Dec 03, 2010, 01:10 AM Registered User Adam, I have always used (and seen used) the weight ratio for fiber to resin. When specifying prepreg it is as well done in weight ratio. The accepted "standards" are: 50/50 for glass and/or Kevlar to resin in wet layup: this starts to be on the resin rich side and more adapted/acceptable for mold making. 60/40 for glass and/or kevlar to resin is what one should aim at in a wet layup. For carbon to resin the starting point is at 60/40. Prepreg can go down to 70/30 (glass/Kevlar and carbon) and I have been involved with one manufacturing where a carbon prepreg had a resin content significantly below 30% (in weight). Now the tricky part: the ratios might be different if using a woven fabric or a UD tape (less resin per weight for UD tape). Usually, when establishing a production, you start with the above values and refine the amount of resin to be mixed after the first couple of pull as the resin content is as well dependent of the type material used and the complexity of the molding. You can do an experiment: put a piece of fabric between two perfectly flat and smooth surfaces (glass) and make sure that the fabric is touching both sides without be squashed. Measure how much resin is needed to fill up all the voids (no air bubbles). You will have the optimal fabric to resin ratio for this particular material. In your case you are using 14 grams of glass fabric (I assume it is fabric): wet layup good starting weight ratio 60/40 gives 5.6 grams of resin. In this case, you might be on the "rich" side. But the finish and complexity of the part might require this. When it becomes more critical is when you work on large(r) layups. If you have 1400 grams of material you will require 560 grams of resin. With your method you would be 40 grams heavier. If you use it as a starting point and refine the process after it is all right. But if you stick to this "rule" you might end up with overweight structures.
 Dec 03, 2010, 01:35 AM Just fly it! Thanks, fnev. I feel much better about my parts now. I was really getting worked up when I was calculating my stuff by volume. The kevlar calculations looked alright but my glass ones looked ugly. My figures in post #1 were only an example. If I look at it by weight, my last kevlar/glass part was 73/27 and my last glass part was 69/31. Neither of the parts contained any sploog, but they were painted in the mold. If I take paint (which increased the final weight of the part) into consideration the resin content is even lower. Is the resin content of my parts too low? Maybe the pressure in my bladders is too high? I've been using a really tight and thin H8 weave and it has reduced my resin content but the hoop deflection modulus of the fuse seems lower. Adam Last edited by wyowindworks; Dec 03, 2010 at 01:54 AM.
 Dec 03, 2010, 01:59 AM Registered User You are welcome and I am glad I can help somehow. Ideally the least resin used the better as long as ALL the voids are filled otherwise the fibre won't work properly in the matrix. If the end product is not satisfactory for the loads no increase in resin content will improve its strength/stiffens. It might "feel" stiffer but it is NOT stronger. The layup schedule has to be modified or (at worst) the structure redesigned. This is where the different types of treads, weaving become handy as they have an influence on the resin content and the way the fibers work under load.
Dec 03, 2010, 02:08 AM
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Quote:
 Originally Posted by fnev IIt might "feel" stiffer but it is NOT stronger.
That has been my experience. I've found that the low resin content parts feel a little softer when they are squeezed (hoop modulus), but in failure tests they perform very well and seem a bit more resilient with fewer visible stress zones when being heavily deflected. Do you think that the resin content plays a part in the flexural modulus of a tail boom? How about buckling resistance when under compression?

Dec 03, 2010, 02:13 AM
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Quote:
 Originally Posted by fnev IIt might "feel" stiffer but it is NOT stronger.
That has been my experience. I've found that the low resin content parts feel a little softer when they are squeezed (hoop modulus). In failure tests they perform very well and seem a bit more resilient with fewer visible stress zones when deflected. Do you think that the resin content plays a part in the flexural modulus of a tail boom? Buckling resistance when under compression?

I feel the need to do some more testing......after I tidy up some projects of course.