Help understanding how to work with prepreg

Chris,

The "big boys" will typically cure the outer skin first, at high temp
and pressure (for say.. a carbonfiber indycar chassis), then follow up
with a honeycomb (nomex or alu) and inner skin at a lower pressure.
The pressure chosen depends on quite a few factors; honeycomb crush
strength, cell size, quality of fittment. Basically honecomb is a
real pain from a manufacturigng standpoint. If you accidentally
expose the edge of the honeycomb to pressure, you will 'walk' the
honeycomb sideways like an accordian; not good. People I've worked
with in the racecar world avoid foam, as it's got no real shear
strenght or stiffness compared to honeycomb so it would be kind of a
waste as a core in a carbon composite structure if stiffness is a
concern. If you were to do a foam core, i'm sure you could find a
resin that cures at a temp below that which the foam can handle, while
keeping the pressure really low. The part would be a low temp part
however.

-Alex
<a style='text-decoration: underline;' href="http://www.nextstepcad.com" target="_blank">www.nextstepcad.com</a>


On 30 Mar 2004 09:41:25 -0800, cziom2521.TakeThisOut@yahoo.com (Chris) wrote:

 >Hi everyone,
 >
 >I'm just trying to educate myself about composite construction
 >techniques, and I'm a little confused on the process of using prepreg
 >materials in an autoclave. Could someone who has done this help me
 >understand the process?
 >
 >Assuming I have prepreg cloth and access to a large autoclave and I
 >want to make a part, say a seat for lack of a better example, what
 >would I use for core material, and how do I keep it from getting
 >crushed in the autoclave? Typical foam that I am familiar with will
 >melt under the 250F curing temperatures, and it seems that a core like
 >Nomex honeycomb will be crushed when the pressure is applied. I can
 >imagine how to do a mold, but it seems like any core material will
 >come out a molten, flattened mess.
 >
 >Can someone help out a novice understand how this is done? What do the
 >big boys do when they want to make a high temperature part? I'm having
 >trouble visulizing how this actually works.
 >
 >Thanks for educating me. This is just for my own enjoyment and
 >edification, but alot of people I've asked have the same question.
 >
 >Chris<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Help understanding how to work with prepreg 

I have a little professional composite aircraft experience, so here goes.

First of all, prepregs do not have to be autoclave cured. There have been a
lot of prepreg resin systems developed in the last 10 years that work very
well under just vacuum pressure and oven cure. This is ussually a MUCH
cheaper way to process the material, with nearly the same performance.

Now to actually answer your question. Honeycomb holds up just fine in an
autoclave. Honeycomb is VERY stiff in the thickness direction., and not very
stiff n the edgewise direction. The key to getting autoclaved honeycomb
parts turn out good it to put a very gradual core bevel on the edges. (3 to
1 or more) This keeps the edges stable and keeps them from pushing in. If
you can keep the edges stable, the core won't crush. The crushing stress of
core is somthing like several thousand psi. Most autoclaves only apply 50 or
100 psi.

By the way, there are many high temp foams available as well that are oven
or autoclave compatible. Foams are generally less stiff in the thickness
direction (for the same density core), but they are homogenous, so they have
much better in plane stiffness. So they are a trade off. They are easier to
get a good part without the edges pushing in, but they are less stiff in the
directions you really care about once the part is cures, which means they
are not quite as strong for a given weight.

I hope this helps.

Joe Wilding



"Chris" <cziom2521.RemoveThis@yahoo.com> wrote in message
news:9fd6a805.0403300941.7f2f78a1@posting.google.com...
 > Hi everyone,
 >
 > I'm just trying to educate myself about composite construction
 > techniques, and I'm a little confused on the process of using prepreg
 > materials in an autoclave. Could someone who has done this help me
 > understand the process?
 >
 > Assuming I have prepreg cloth and access to a large autoclave and I
 > want to make a part, say a seat for lack of a better example, what
 > would I use for core material, and how do I keep it from getting
 > crushed in the autoclave? Typical foam that I am familiar with will
 > melt under the 250F curing temperatures, and it seems that a core like
 > Nomex honeycomb will be crushed when the pressure is applied. I can
 > imagine how to do a mold, but it seems like any core material will
 > come out a molten, flattened mess.
 >
 > Can someone help out a novice understand how this is done? What do the
 > big boys do when they want to make a high temperature part? I'm having
 > trouble visulizing how this actually works.
 >
 > Thanks for educating me. This is just for my own enjoyment and
 > edification, but alot of people I've asked have the same question.
 >
 > Chris<!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Help understanding how to work with prepreg 

On Wed, 31 Mar 2004 19:32:31 -0700, "Joe Wilding"
<wingnutNOSPAM.RemoveThis@the-wildings.com> wrote:

 >I have a little professional composite aircraft experience, so here goes.
 >
 >First of all, prepregs do not have to be autoclave cured. There have been a
 >lot of prepreg resin systems developed in the last 10 years that work very
 >well under just vacuum pressure and oven cure. This is ussually a MUCH
 >cheaper way to process the material, with nearly the same performance.

very true, just depends on what "performance" means for your
application

 >
 >Now to actually answer your question. Honeycomb holds up just fine in an
 >autoclave. Honeycomb is VERY stiff in the thickness direction., and not very
 >stiff n the edgewise direction. The key to getting autoclaved honeycomb
 >parts turn out good it to put a very gradual core bevel on the edges. (3 to
 >1 or more)

yep, this is done quite often

 > This keeps the edges stable and keeps them from pushing in. If
 >you can keep the edges stable, the core won't crush. The crushing stress of
 >core is somthing like several thousand psi. Most autoclaves only apply 50 or
 >100 psi.
 >
 >By the way, there are many high temp foams available as well that are oven
 >or autoclave compatible. Foams are generally less stiff in the thickness
 >direction (for the same density core), but they are homogenous, so they have
 >much better in plane stiffness. So they are a trade off.

So you're saying there's a foam that gives a better in-plane (shear)
stiffenss than alu honeycomb for a given density? I'd be interested in
knowing what foam this is.... alu honeycomb is great structurally
but it's expensive do you have any links to foam products?

 >They are easier to
 >get a good part without the edges pushing in, but they are less stiff in the
 >directions you really care about once the part is cures, which means they
 >are not quite as strong for a given weight.
 >
 >I hope this helps.
 >
 >Joe Wilding
 >
 >
 >
 >"Chris" <cziom2521.RemoveThis@yahoo.com> wrote in message
 >news:9fd6a805.0403300941.7f2f78a1@posting.google.com...
  >> Hi everyone,
  >>
  >> I'm just trying to educate myself about composite construction
  >> techniques, and I'm a little confused on the process of using prepreg
  >> materials in an autoclave. Could someone who has done this help me
  >> understand the process?
  >>
  >> Assuming I have prepreg cloth and access to a large autoclave and I
  >> want to make a part, say a seat for lack of a better example, what
  >> would I use for core material, and how do I keep it from getting
  >> crushed in the autoclave? Typical foam that I am familiar with will
  >> melt under the 250F curing temperatures, and it seems that a core like
  >> Nomex honeycomb will be crushed when the pressure is applied. I can
  >> imagine how to do a mold, but it seems like any core material will
  >> come out a molten, flattened mess.
  >>
  >> Can someone help out a novice understand how this is done? What do the
  >> big boys do when they want to make a high temperature part? I'm having
  >> trouble visulizing how this actually works.
  >>
  >> Thanks for educating me. This is just for my own enjoyment and
  >> edification, but alot of people I've asked have the same question.
  >>
  >> Chris
 ><!-- ~MESSAGE_AFTER~ -->
 >> Stay informed about: Help understanding how to work with prepreg