Questions regarding resin coverage vs fabric weight

General Composites Discussions Composites Talk
#1

I was trying to do some costings but realized that I may need to consult the knowledgeable people on this site for help…

The answer is probably an obvious ‘yes’, but does, for example, 3k 300gsm carbon fabric (10.5 oz in American) need more resin to wet out than 3k 200gsm (7oz)?

If so, is there some sort of rough guide to work out how much more resin is needed as the weight of the fabric increases?

Also, would 6k 200gsm fabric need double the resin 3k 200gsm would need?

(I’m struggling to word this last question properly) If I were to take a dry 1meter squared section of 3k 200gsm carbon, what would be the weight of resin needed to get it to weigh the same as a meter squared section of prepreg carbon?

Thanks in advance for any answers

Regards,

Kyle.

#2

A useful quote from another site…

"Pre Preg Vf…

The ratio of fibre to resin is a critical parameter and must be controlled carefully to ensure a strong and reliable end product. This ratio is expressed as Vf, the fibre volume fraction, and it determines what percentage of the volume of the finished part is fibre and what is resin. If there is too much resin, the applied forces are carried by the weaker matrix or resin and the part fails earlier than expected. Too little resin, and the part loses strength where the fibres are not supported. For high strength parts Vf can be as high a s 70% (i.e 70% fibre and 30% resin) but is typically around 55%"

So to answer your last question, If you wanted to produce a part with a Vf of 55% using your 1 sqm of 200gsm carbon you would need to use 163grams of resin.

In answer to your other questions… Yes, it would take more resin to wet out 300gsm carbon as opposed to 200gsm. I usually mix up an equal weight of resin to the carbon I am using.

#3

Excellent. That’s spot on. Thanks very much Marc!!

#4

Marc is right:

This question is all about the Vf. To get very reliable calculations, you need to know the Vf for every fabric you use, in the process you are using. Vf is connected to thickness of the layer. Which means that if you do not have burn-out equipment, or are using aramides, you can calculate Vf from laminating 10 layers of the desired fabric on top of each other.

Keep in mind though that interaction between layers can also have an influence on Vf. (nesting of layers)

The fun thing is that also the density of the fiber plays a part. 200 gr/m2 glass fabric needs less resin than 200 gr/m2 carbon or aramide. Although the fabric weighs the same, due to the higher density you have less volume of fiber.

As for the 3K vs 6K: If the bundles are not spread (spread tow fabric) then indeed, the resulting laminate is thicker, so Vf goes down, and resin usage goes up. This is not neccesarily true for multiaxials.

For prepregs: The Vf should be in the specification. For fabrics usually 45-48%

It would be really nice to have a table of Vf’s per fabric:

fabric - Vf (hand) - Vf (infusion @ 60%) - Vf (infusion @ 95%) - Vf (wet bagging)

#5

I’ve had a brain meltdown today with numbers and figures - how did you get 163g?

#6

Attached is a XLS sheet, which, although half in Dutch, half in English, might be a help.

I use it to quickly see the weight of different laminates, both in hand laminate and infusion.

It also contains some Vfs for different materials. Disregard the Vfs of foam, but these ARE helpful in the sheet to add the resin weight and thickness.

#7

Hi Kyle,

This is how I worked out the numbers, it’s probably not the easiest way to do it and my maths are not great but this is the method that works for me :slight_smile:

You have you 1sqm of 200gsm of carbon, therefore it should weigh 200gm. Now you want it to be 55% of the finished article so you need to know what the other 45% of resin needs to weigh…

200/55 = 3.63(therefore 1% equals 3.63gm) and 45% would be 45 x 3.63 = 163.35

Hope this helps.

#8

Oh oh, it is volume fraction, not weight fraction.

So you should take into account the density of resin (cured) and carbon.

#9

Oh pee on my fire Herman… I thought I had something right for a change lol

Where do we find find the density of the cured resin? I have never seen that mentioned in any datasheet of any resin.

How would you work out the example above?

#10

Some figures:

Carbon 200 gr/m2
Weight: 200 gr/m2
Vf (hand) 40%
Vf (infusion) 55%
Thickness (hand) 0,29mm
Thickness (infusion) 0,21mm
Resin usage (hand) 212 gr/m2
Resin usage (infusion) 116 gr/m2

Glass fabric 200 gr/m2
Weight: 200 gr/m2
Vf (hand) 40%
Vf (infusion) 55%
Thickness (hand) 0,19mm
Thickness (infusion) 0,14mm
Resin usage (hand) 138 gr/m2
Resin usage (infusion) 76 gr/m2

Also see updated excelsheet. (check the library, you can play with the figures.)

#11

Thanks very much for the replies Herman and Marc.