Question about Low-Vac and weight

[TMann]

... well said. It's all good.

[trans4md]

if you want highly conformal parts where conformal of layups to parts may be dificult, then LoVac -may- be helpful provided you use the same techniques, materials and tools.

This is good information for me; I failed to give the "conformity" benefit significant consideration.

 

From a total-cost-of-time standpoint, is LoVac significantly affecting construction time for parts (either increasing or reducing)?

[TMann]

From a total-cost-of-time standpoint, is LoVac significantly affecting construction time for parts (either increasing or reducing)?

Time is of no consequence. An additional 10 minutes is of little importance when you have to wait overnight for the part to cure.

 

Add to that the time you save by not having to fill defects such as bubbles.

 

Again, until you make a commitment to try these techniques, you won't know. You need to invest some effort and see if the results work for you.

[Cozy Girrrl]

Lets just say Randi is frugal, so she'd not be an advocate of this if it were not LoCost

[chasingmars]

Well, there are hand layups and there are hand layups. For NON vacuum bagged wet layups, you should be happy with 50% epoxy, 50% glass by weight. Most builders are probably closer to 60% epoxy by weight. So if you have a layup that should weigh 10 lb. with the 50/50 ratio, it'll weigh 12.5 lb with a 60/40 ratio - 25% more. Sounds pretty crappy. And it is

It's not trivial to get to 40% even with vac. bagged wet layups. To get to 35% - 40%, there is no choice but to go to pre-pregs.

 

The numbers we're talking about of 50% (or worse) and 40% by weight for hand and vacuum layup concern me relative to my own experience. I'm seeing substantially lower numbers based on tests I did early in the build process.

 

Basically, what I did when trying to learn the vacuum bag (not lovac) technique is build the six-ply flat sample piece in chapter three about a half dozen times. One was a hand layup for reference, the others were at various vacuum levels. I weighed the cloth after cutting to ensure that I was accounting as best I could for the actual vice nominal cloth weight (this came out at 8.5 oz/sq yd for the BID), then after completion trimmed the edges (for a consistant piece) down to 16 x 10 inches. Six plies means 6.3 oz of glass in each sample, and then I weighed the sample to find out how much resin was in each.

 

My hand layup weighed 10.35 oz, which is 61% glass / 39% resin by weight. A moderate vacuum (10.5inHg) drew down to only 8.85 oz, for 71% glass / 29% resin, unless my math is terribly off.

 

These were done with MGS 285, which I find fairly low viscosity, though I haven't worked with any of the other approved systems for comparison, and of course this sample has no core so there's no micro or epoxy in an interface layer.

 

Still, the "achievable numbers" being talked about here, what is that based on? Have these been measured, are they perpetuated conventional wisdom, or based on previous research? Specifically I'm wondering if we aren't getting volume fraction and mass fraction mixed up here. My copy of Composite Materials for Aircraft Structures uses volume rather than mass fractions nearly exclusively with Vf of ~60% for unidirectional reinforcements (we won't see close to that if only because BID won't lie that flat, but the point remains). As a further sanity check, with E glass at a density of 2.6 (from the same ref) and MGS 285 at 1.2 (from datasheet), my Volume fractions on those two test samples are 0.42 (glass) for the hand layup and 0.53 (glass) on the vacuum layup.

 

Either my samples are lying to me (or are hugely dry, which they don't appear to be), or the mass fractions stated of 50%+ epoxy best effort for hand layups and 40% for vac bagging are in error. I think we're mixing mass fraction with volume fraction, and thereby coming to wrong conclusions.

 

My thought is that good hand layup is in the Vf 0.40-0.45 (glass) range (i.e. 55-60% resin by volume or only 36-41% resin by weight), good vac woven reinforcement bagging is in the Vf 0.50-0.55 range (i.e. 45-50% resin by volume or 28-32% by weight), and prepreg uni or filiment gets down into that unobtainable "ideal" (for uni/filiment) Vf 0.60 range. OF course the respective mass fractions are valid only for E-glass (and approximately for S-glass) as both carbon and kevlar are much less dense than glass driving their respective epoxy mass fractions substantially higher.

 

This would mean your vacuum bagged portions might have about 1/3 less epoxy weight in them than your hand laid parts... and that's still not going to save that much weight... personally I think we scrap a lot more than 25% of the 17 gallons, but even if we do, that's still only 43 lbs if *everything* is perfectly bagged, and that's not acheivable. In the end, although I disagree with his ratios, Marc is right that aggressive, thorough vacuum bagging of the whole plane can at best save you about thirty pounds. Probably less... some of the largest areal layups are hard to bag effectively, and there are uses of epoxy that simply aren't part of a composite glass layup where there's no saving (e.g. flox, fairing micro, etc)

 

Vacuum bagging is not a weight cure all... it helps greatly for part quality control in my opinion, but that's about it.

[Marc Zeitlin]

Still, the "achievable numbers" being talked about here, what is that based on?

Industry (read, the company I work for, and other similar ones) standards.

 

Have these been measured, are they perpetuated conventional wisdom, or based on previous research?

Measured. On actual parts and test coupons.

 

Specifically I'm wondering if we aren't getting volume fraction and mass fraction mixed up here.

Nope. See something like:

 

http://www.jdlincoln.com/L_501_adhesive_prepreg_1.html

 

this is a glass prepreg (chosen at random) with a 42% resin content BY WEIGHT. We can get prepregs with resin contents in the 35% range, but they tend to be very difficult to use and get voidless.

 

My copy of Composite Materials for Aircraft Structures uses volume rather than mass fractions nearly exclusively ...

I can't imagine how one would measure the volume of a dry, woven glass cloth, except by measuring the weight and then using the density of the glass fibers to determine volume. Why bother, if one has already measured the weight, and weight doesn't change with temperature, as volume (for the epoxy) does?

 

Either my samples are lying to me (or are hugely dry, which they don't appear to be), or the mass fractions stated of 50%+ epoxy best effort for hand layups and 40% for vac bagging are in error. I think we're mixing mass fraction with volume fraction, and thereby coming to wrong conclusions.

Nope. From Sport Aviation, 5/1988 (20 years old, but the basic concepts haven't changed):

 

"If you will check some of the advanced articles on fiberglass components, you will see that many of the early articles and textbooks on fiberglass show... "a good ratio by weight of epoxy to fiberglass cloth to be about 50/50." However, with today's material the best for strength is approximately 40% epoxy and 60% cloth. This is very hard to do with a hand lay-up procedure without vacuum bagging."

 

Even the hucksters at:

 

http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=220359727406

 

indicate 35% resin weight content for high performance prepregs, and 40% by weight for "general" layups.

 

I won't argue with what you've measured - I'll only tell you what the industry says in it's product literature and in the testing that's been done. Prepregs, be they glass or carbon, generally have epoxy weight contents in the 35% - 45% range, with hand layups being higher.

 

Strength is compromised if the epoxy content is too low and if the fabric is compressed too much - there IS such a thing as too much vacuum. If we saw epoxy weight ratios in the 25% - 30% range, we'd be extremely suspicious of the layup quality.

 

My $0.02.

[brainfart]

In all (mostly non-English) language publications I have seen so far, volume fractions are being used. Yes, many people indeed confuse mass and volume fractions.

[Marc Zeitlin]

In all (mostly non-English) language publications I have seen so far, volume fractions are being used. Yes, many people indeed confuse mass and volume fractions.

Volume fractions are used when mixing the two (or more) EPOXY components. Mass fractions are used when determining fiber/mixed epoxy ratios.

 

Again - how would one determine the volume of woven cloth?

[chasingmars]

Again - how would one determine the volume of woven cloth?

What's wrong with dividing measured weight by published density? We're only talking two significant figures here.

 

My understanding is that volume fractions are used because it"s far less sensitive to fibre type when speaking generally (because it's insensitive to fibre density)

[TMann]

Again - how would one determine the volume of woven cloth?

.... you mean like a 'Loud' weave?

[brainfart]

> Volume fractions are used when mixing the two (or more) EPOXY components.

 

Sometimes, by some people...

 

> Mass fractions are used when determining fiber/mixed epoxy ratios.

 

I guess we disagree on the volume fraction stuff then.

Strange world, there are quite some differences between the two sides of the pond. Americans measure fuel consumption in miles per gallon, we use liters per 100km. Most epoxies are weighted here, rarely mixed by volume. And for fiber content comparisons of laminates we use volume fractions.

 

> Again - how would one determine the volume of woven cloth?

 

Density of the raw material is known. Yarn diameter is known. Weight per lenght is known (1 tex = 1g/km), grammage (weight per area) is known. Thickness of the cloth and the laminate is known. More than enough data to come up with a volume ratio, don't you think?

[aviator_edb]

.... you mean like a 'Loud' weave?

Ours goes to 11....

[Marc Zeitlin]

Density of the raw material is known. Yarn diameter is known. Weight per lenght is known (1 tex = 1g/km), grammage (weight per area) is known. Thickness of the cloth and the laminate is known. More than enough data to come up with a volume ratio, don't you think?

Yeah, thanks for the primer on the relationship between volume and mass - I'll write that down.

 

My point, which was obviously too subtly made, was that if one has to weigh the cloth in order to figure out it's volume, why bother calculating the volume, and then trying to inaccurately determine the volume of the epoxy as well?

 

You have the weight of the cloth (even using your methodology) - it's easier to weigh the epoxy than to measure it's volume (which is temperature dependent, while the weight is not), so the rational thing to do is use the easily determined, more accurate values of weight, rather than the secondarily derived, less accurate values of volume, to determine epoxy fractions in a layup.

 

At any rate, confirmation from the M&P engineer indicates that the general numbers for wet and prepreg layups that I stated were approximately correct, within a couple %age points.

 

I have no explanation for Craig's measurements - I'm not there, and will not attempt to second guess his #'s. All I'll tell you is what the industry uses and expects, and how it gets measured, at least here in the USA.

[JLKnolla]

Ours goes to 11....

Excellent! Still waiting on a reunion tour - talk about mudflaps my girl's got 'em.

[brainfart]

INTRODUCTION TO COMPOSITE MATERIALS

http://ocw.mit.edu/NR/rdonlyres/Materials-Science-and-Engineering/3-11Mechanics-of-MaterialsFall1999/5991E176-AC60-4399-87F0-86AE35963750/0/composites.pdf

 

I guess that's why countless texts like the above mention stuff like "fiber volume fraction" and "matrix volume fraction" all the time, in a subtle kind of way...

 

All I'll tell you is what the industry and academia uses and expects, and how it gets measured, at least outside the USA.

[TMann]

Well it's getting pretty technical here but I think that Craig said it best.

Vacuum bagging is not a weight cure all... it helps greatly for part quality control in my opinion, but that's about it.

My primary motivation for using vacuum bagging is to create quality parts.

Secondary to that is the weight.

 

When you consider the fact that the glass conforms well and that the low pressure eliminates air bubbles that may remain after you squeegee, then it's an improvement. No filling voids with epoxy, flox or micro (less added weight.)

 

This is true with either vacuum bagging or the low-vac method.

[Cozy Girrrl]

.... you mean like a 'Loud' weave?

Yeah TMann, I remember my dad had a jacket with a loud weave on it, left over from the 50's I expect

 

Jeez guys, can you take your Vacuum bagging discussion somewhere else please, what does this mess have to do with the LoVac question in the first place. I think from now on the answer will be:

 

"No, it dos not save any weight at all, if it does it is purely by accident and poor quality control, if you want light weight, go look under headings for Vacuum Bagging I am sure there are plenty of experts over there to weigh in"

[JLKnolla]

Yeah TMann, I remember my dad had a jacket with a loud weave on it, left over from the 50's I expect

 

Jeez guys, can you take your Vacuum bagging discussion somewhere else please, what does this mess have to do with the LoVac question in the first place. I think from now on the answer will be:

 

"No, it dos not save any weight at all, if it does it is purely by accident and poor quality control, if you want light weight, go look under headings for Vacuum Bagging I am sure there are plenty of experts over there to weigh in"

My apologies to Crissi and Randi, I should have asked them the question privately to avoid the swinging unit syndrome - lesson learned.

[chasingmars]

I have no explanation for Craig's measurements - I'm not there, and will not attempt to second guess his #'s. All I'll tell you is what the industry uses and expects, and how it gets measured, at least here in the USA.

Well, I have to find some explaination... so I'm going back to recalculate it all... if I haven't done some dunderheaded math thing, I need to figure out why the numbers are so different and if it's a problem, then it's a big problem because my plane is probably the same.

 

I suspect I'll be back with at least a little more discussion on this, but deferring to C&R, in another thread.

[Cozy Girrrl]

Well, I have to find some explaination... so I'm going back to recalculate it all... if I haven't done some dunderheaded math thing, I need to figure out why the numbers are so different and if it's a problem, then it's a big problem because my plane is probably the same.

 

I suspect I'll be back with at least a little more discussion on this, but deferring to C&R, in another thread.

Thanks Craig,

I was not trying to be ignorant about it, just that every time someone mentions LoVac it degenerates into ------- matches on the merits of Vacuum Bagging, something we know little about nor have any personal interest in. It seriously muddies up the thread like mentioning retracts -anywhere- I can predict how that will go and who all will weigh in on both sides, its tiresome and always comes off like a racial/ethnic/or political argument, people entrench in their camps, nobody gives an inch, nobody learns a thing. Its very much like watching a Civil War renactment.

...and its spelled "Chrissi"

[Jerry Schneider]

. It seriously muddies up the thread like mentioning retracts -anywhere- I can predict how that will go and who all will weigh in on both sides, its tiresome and always comes off like a racial/ethnic/or political argument, people entrench in their camps, nobody gives an inch, nobody learns a thing. Its very much like watching a Civil War renactment.

...and its spelled "Chrissi"

Krisssy,

 

I think the scale is MUCH more betta than the pump.

[Cozy Girrrl]

Yah Geraldo, you know what you can do with your blunt ended electric Ditz Scissors too!

Skipping Osh once again I suppose?

...Chrissi

[DouglasHolub]

"the micro should be put on as dry as possible (peanut butter) it should not be runny at all ( runny is heavy )."

-----------------------------------------------------------------------

Page 3-9 of Rutan's "Moldless Composite Homebuilt Sandwich Aircraft Construction" says:

 

"When laying up over styrofoam, PV foam, or urethane foam, paint a liberal coat of micro slurry on the surface, before laying on the first ply. The slurry can be poured on the foam and spread thin with a squeegee or it can be brushed on with a brush."

 

That's how they taught us to do it at the Velocity builder's classes, too.