Help understanding how to work with prepreg

[Chris]

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

[Joe Wilding]

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" > wrote in message
om...
> 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

[Alex Femec]

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
www.nextstepcad.com


On 30 Mar 2004 09:41:25 -0800, (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

[Alex Femec]

On Wed, 31 Mar 2004 19:32:31 -0700, "Joe Wilding"
> 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" > wrote in message
om...
>> 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
>

[Joe Wilding]

"Alex Femec" > wrote in message
...
> On Wed, 31 Mar 2004 19:32:31 -0700, "Joe Wilding"
> > 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?
>

By inplane stiffness, I meant in-plane compression stiffness. Actually they
have better in-plane shear stiffnesses as well. However, both of these
properties are next to useless, once the prt is cured, they only help to
stabilize the core during layup and cure.

What they are lacking in compared to honeycomb is "transverse" shear
stiffness. (in the 13 and 23 direction, as opposed to 12 direction for
inplane shear.) These transverse values are what are important for a cured
part as they are what predict when a panel will buckle (Which is why you use
core.) For a given density of core, honey comb will almost always have
higher transverse shear stiffeness, which is why you don't see foam much for
high performance applications.

I beleive rohacell & divinycell both have a high temp capable foam product
line. I don't know the particulars. It has been years since i have used
either of them.





> >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" > wrote in message
> om...
> >> 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
> >
>