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#1
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Help understanding how to work with prepreg
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 |
#2
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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 |
#4
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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 . 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 |
#5
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"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 . 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 |
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