please explain pre-preg
Dave Martin
I've been following this group for over a year. I don't own an
experimental (yet), but I'm fascinated by the information shared in this
newsgroup (well, most of it). While reading about composite aircraft, I
often see the construction process termed 'pre-preg fiberglass layup'.
What exactly does this involve? I have done simple fiberglass cloth/resin
and catalyst layups before. How does pre-preg differ from this?
Thanks,
Dave Martin
St. Cloud, MN
(No, I'm not the editor of Kitplanes.) :-)
Peter Lert
: Hi all,
OK, in a nutshell (which Nature makes from her own prepregs): The
way we homebuilders generally make composite (or, as my father-in-law
persists in calling it, "compost") structures is by a method called "wet
layup." This is not an underwater basketball term. Rather, it means that
(initially dry) glass fiber material is put in placed, then saturated with
epoxy or vinylester resin...just the way you're used to doing it.
"Prepreg" is a contraction for "pre-impregnated." The material
comes from the manufacturer already saturated with the (temporarily
inactive) resin system. The most common prepregs are heat-cured in
autoclaves (basically, ovens big enough to accommodate the entire mold).
Many prepregs have to be shipped and stored at low (freezer) temperatures
to prevent them "kicking" before they're used.
The advantages of prepregs include, in addition to the obvious
reduction in labor, much more accurate control of the ratio of resin to
fibgerglass (they're also available in "exotics" like Kevlar or carbon).
Moreover, prepreg structures are usually vacuum-bagged into their molds,
providing further weight control and very accurate structure.
In addition, since most prepregs are high-temperature cures, their
creep point is higher than that of room-temperature cure materials. Thus,
prepreg structure can tolerate higher temperatures--that's why people can
get away with things like fire engine red Lancairs instead of the usual
Frigidaire white you seen on Long-Ezes and the like.
Downside of prepregs? They're expen$ive, require special storage
and handling, they're expen$ive, you need an autoclave to cook most of
them, they're expen$ive, assembly personnel need special training, and
they're expen$ive.
Roger D. Mellema
(Dave Martin) wrote:
~Hi all,
~I've been following this group for over a year. I don't own an
~experimental (yet), but I'm fascinated by the information shared in this
~newsgroup (well, most of it). While reading about composite aircraft, I
~often see the construction process termed 'pre-preg fiberglass layup'.
~What exactly does this involve? I have done simple fiberglass cloth/resin
~and catalyst layups before. How does pre-preg differ from this?
~
~Thanks,
~Dave Martin
~St. Cloud, MN
~(No, I'm not the editor of Kitplanes.) :-)
Dave,
"pre-preg" stands for "Pre-impregnated" cloth. It is wetted with
catalyzed resin, squeezed between rollers so that just the right amount of
resin remains in the cloth, and then frozen so that the resin will not
set. You just take it out of the freezer and put it in the mould, vacuum
bag it (or not) and let it set up. Of course it can also be put into an
autoclave if you have one.
Pre-preg is nice but you will end up with a part that has *millions* of
"pin-holes". I keep thinking I have discovered the answer for filling
them ~ but it seems a lot of work no matter how you attack it. These pin
holes can be devastating is you have "wet" wings drip drip
drip drip drip seep seep aaaaargh!!
--
Roger Mellema
BD-4 Builder Newsletter
Ford V-6 powered BD-4, N76VR
ber...@loop.com
> Downside of prepregs? They're expen$ive, require special storage
>and handling, they're expen$ive, you need an autoclave to cook most of
>them, they're expen$ive, assembly personnel need special training, and
>they're expen$ive.
There is a trade off that's not obvious. The kitplane industry is
cyclical - most of the sales in the summer, few in January. If you use
(labor intensive) wet layup you end up having to pay workers with nothing
to do in the winter, just to have a trained work force in the summer. The
alternative is to have a smaller workforce and average the orders,
resulting in long wait times for kits at peak times of the year.
With pre-pregs, you don't have as big a problem since your work force is
smaller to begin with. Instead you just leave some pre-preg in the
freezer longer.
A subtle difference, but (to some of us) significant.
RR
scott rider
I
>
>Dave Martin
"Pre-Preg" is just "preimpregnated" shortened up.
Take the fiberglass, (kevlar, carbon, etc.) and pass it through rollers with
pre-mixed resin on them and you end up with pre-preg. Since there is no
real guarantee of how long it will be "unused", the prepreg uses a resin
mixture that has to be heated in an oven to complete the cure.
Benefits:
Very controlled resin/glass ratio for better strength to weight.
Controlled Fiber orientation for better strength.
Resin chemistry *CAN* allow material use in higher temperature
environments - some homebuilders paint their prepreg skins a non-white
using "pre-preg" as their justification.
Drawbacks:
Requires autoclave type equipment to cure
More restricted shelf life/handling
I am sure others will also comment, but as for me, prepreg belongs in
production shops who can handle the requirements, and as a result they
come up with better production control. I am limited to wet layups,
vacuum bagging, etc. with room temp epoxies that must be painted white.
(I can achieve the resin/glass ratios, come close on fiber orientation, but
my epoxies will soften at lower temperatures once cured.)
Scott Rider, BD-5 builder, BD-1 N6296L, Own Opinions,
Bill Pace
.. stuff deleted...
> The advantages of prepregs include, in addition to the obvious
>reduction in labor, much more accurate control of the ratio of resin to
>fibgerglass (they're also available in "exotics" like Kevlar or carbon).
>Moreover, prepreg structures are usually vacuum-bagged into their molds,
>providing further weight control and very accurate structure.
So while we are at it, what does it mean to vacuum-bag? I assume that it has
nothing to do with my Hoover. :-)
-----
Bill Pace Life is not so much an adventure
bi...@doctord.com as it is a paycheck.
Voice: (619) 457-3380
Fax: (619) 457-3068
Boatfly
Peter Lert
: .. stuff deleted...
: > The advantages of prepregs include, in addition to the obvious
: >reduction in labor, much more accurate control of the ratio of resin to
: >fibgerglass (they're also available in "exotics" like Kevlar or carbon).
: >Moreover, prepreg structures are usually vacuum-bagged into their molds,
: >providing further weight control and very accurate structure.
: So while we are at it, what does it mean to vacuum-bag? I assume that it has
: nothing to do with my Hoover. :-)
So, we've gone to the trouble of building a nice mold--possibly
one with various complex and/or re-entrant shapes. How we gonna get our
cloth/resin layup (or prepreg, as the case may be) to fit snugly into each
nook and cranny of the mold?
Well, we could build *another* mold--actually a plug--that would
fit *inside* the first one, leaving enough room for the composite material
in between...but that would be a pain in the ass. What about some
flexible material, with something behind it to push it into the mold? In
fact, that technique is used in making sheet metal parts: it's called
"hydroforming," in which a thick rubber blanket presses sheet metal down
over form blocks at thousands of psi.
We don't need nearly that much oomph for composites, though, so
our "flexible material" need be nothing thicker than plastic film like
"Visqueen"...and for the "something behind it to push with" all we need
is...the atmosphere!
So howzit work? Easy. Drape your layup or prepreg into the mold,
add a vacuum hose, back it up with plastic sheet, seal this to the edge of
the mold (that's why God created Ductape), turn on the pump, and voila`!
The sheet gets sucked down into the mold, compressing the composite
material into it.
Nifty variation: removing excess resin for minimum component
weight. This time, we first back up the composite with a *perforated*
plastic sheet, like those "veggie" Ziploc bags in the TV ads. Next, a
layer of absorbent material like cotton batting; then the (nonperforated)
outer plastic sheet. When the vacuum is applied and the whole "sandwich"
gets pressed into the mold, excess resin gets squoze out of the layup,
migrates through the perforated inner sheet, and gets absorbed by the
batting material. Result: a finished part with exactly the right
(minimum) amount of resin, plus a messy lump of stiff, sticky
resin-impregnated cotton or paper which can be sold to the airlines to
stuff Coach-class seats.
If you want to try vacuum-bagging at home, old refrigerator
compressors (often available free at junkyards) make good vacuum pumps.
Rob Cherney
>So while we are at it, what does it mean to vacuum-bag? I assume that it has
>nothing to do with my Hoover. :-)
Vacuum bagging is a wet layup technique used to obtain a more
favorable ratio of fiber to resin than would be obtained with a more
traditional layup. It involves covering the layup with a peel ply, a
release ply, and a bleeder ply and then bagging the whole mess in a
polyethylene envelope. A vacuum is pulled on the bag and atmospheric
pressure squeezes the excess resin through the peel and release plies
into the bleeder. When all is cured, the peel, release and bleeder is
discarded leaving a nice layup containing a minimal amount of excess
resin. This results in a part that retains all the strength of the
normal wet layup without the weight of the excess resin.
The peel ply is typically a sheer Dacron fabric. The release, often
made of Tedlar, is a thin perforated plastic sheet; it allows resin to
migrate through it. The bleeder is a polyester batting often sold in
fabric stores. The envelope is made of sheet polyethylene that
encloses the part and is sealed using a tacky butyl sealing tape.
This tape is the type sold in rolls to the glazing industry (i.e.
glass installers).
After a layup is cured, the release ply will allow the bleeder ply to
be stripped off the part. It is difficult to remove the bleeder
without a release layer. The release ply can be reused. It is the
bleeder that soaks up the excess resin during the vacuum process. The
peel ply, which is also stripped off, is not always necessary, but
often offers a more even transfer of resin to the bleeder ply.
Additionally, its use reduces the amount of sanding required when
subsequent bonds are made to the cured piece.
When using the vacuum bagging technique, you usually pull a half
atmosphere of vacuum or better. Over some foam-cored parts, however,
more than a half atmosphere of vacuum will crush the foam. Remember,
a half-atmosphere equates to roughly 1,000 lbs of pressure per square
foot of surface area!
Hope this helps.
Rob-
+--------------------------------------------------------+
|Robert Cherney Home Phone: (410)465-5598 |
|Ellicott City, Maryland e-mail: che...@clark.net |
+--------------------------------------------------------+
scott rider
>So while we are at it, what does it mean to vacuum-bag? I assume that it has
>nothing to do with my Hoover. :-)
>
>
Almost. I use a lab vacuum pump, gets me 29 inches.
This is very simplified: Take your mold, put release compound of your
choice on it, do a fiberglass and resin layup (you can be a little resin
heavy), drape dacron cloth over that, put cotton wadding over that and
cover the entire mess with a plastic sheet. Pump out the air.
What happens is the glass is pushed onto/into the mold with 14 psi, the excess
resin mix squishes through the daron cloth and gets soaked up by the cotton
wadding. The wadding has the additional function of giving the trapped air
a route to the vacuum pump.
After 8 hours, you can peel the dacron and wadding material from the layup,
and what you have left over NICELY fits the mold and has close to the ideal
resin to glass ratio.
The vacuum and bag in this case are a high vacuum pump and plastic tarp
material that you use as dropcloths when you paint.
If you are attaching a wing to ribs and want a little presure to hold down
the skins, your hoover and trash bags give you enough pressure. Tape the
bags over the wing, suck out the air. If the lab pump could extract the air,
you would have far too much pressure holding the skins on. Consider a
50 square foot wing, at only 10 psi, you have 1440 pounds per foot, or 70,000
pounds of pressure holding the wing skin on. Might crunch a rib.
Scott Rider,
scott...@ccm.hf.intel.com Own Opinions, BD-5 builder, yes I know, the BD-5
is aluminum.
scott rider
>scott rider <scott...@ccm.hf.intel.com> wrote:
>> I am limited to wet layups,
>>vacuum bagging, etc. with room temp epoxies
>then are post cured at pretty high temps, like 300.
>RR
>
Me again, (Scott Rider)
I am using Aeropoxy, "not as toxic" as others, but otherwise just another
epoxy, nice wetting characteristics, viscosity easy to control with acetone
in certian cases only. What kind of resin can be postcured as you described,
and what are resulting characteristics (ie. At what temp does it soften up
again? Shear/Tensile limits? Mega-expensive? etc.)
Info appreciated, either posted or mailed.
Scott Rider
BD-5 Builder, BD-1 N6296L, Own opinions, and I know the BD-5 is aluminum...
Dirk Rackley
Hello Scott:
Most all epoxies will show an elevated Tg (glass transition temperature) if they
are post cured at an elevated temp. The maufacturer usually has this info if the
epoxie benefits from it. Or you have to have specimens sent to a test lab.
If you have a speck sheet on the Aeropoxie then look for two figures for Tg.
One for a room temp cure, and the other for an elevated post cure.
Dirk