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Are autoclaves and vacuum bags necessary?

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Are autoclaves and vacuum bags necessary?

Old 08-12-2008, 04:56 AM
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Default Are autoclaves and vacuum bags necessary?

This is a general interest question.Having watched a lot of "How it's made" type programs ,I notice that a lot of composite construction requires the use of autoclaves,vacuum bags and steel moulds. Are they necessary for modellers? Are the epoxies the same we generally use ,or they are special industrial varieties that need baking, similar to U.V activated adhesives?
Old 08-12-2008, 06:42 AM
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Default RE: Are autoclaves and vacuum bags necessary?

The vacuum bagging gives stronger lighter builds, but is not necessary for our stuff unless you are really looking for the lightest structure. The vacuum also helps hold the layup into tight corners.

Autoclaves are only used if your workspace is too cold for a good cure, less than 70*F, or you're using post cured epoxy which I think needs to go to about 240*F for maximum strength.
Old 08-12-2008, 10:51 AM
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Default RE: Are autoclaves and vacuum bags necessary?

Vacuum bagging pushes the laminates in together as thight as possable, and squeezes out excess risen. Strenght in laminated comes from the proper material (Carbon, FG, kevlar ) and resin. Generally the higher the material content by weight the better. With vacuum bagging you have to be sure you do not wick away to much resin as this leads to a resin starved part that is usually brittle and weak.
Autoclaves as stated above help curing parts in a cold shop. But the major us is in curing pre-pregs (Materials that are prewetted with the resin or epoxy) that have no catalast in them, thusly you can store them and use as you wish, there is a shelf life to these products. The heat of the autoclave causes the resin to kick and cure.
Old 08-12-2008, 03:43 PM
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Default RE: Are autoclaves and vacuum bags necessary?

In addition to what has already been stated here, the autoclave not only elevates the temperature but most also pressurize internally to further increase the compression on the part increasing the fiber to resin ratio. For instance, with vacuum bagging, generally the most pressure you can apply to a lamination is around 14psi because you are limited by atmospheric pressure. This relatively low figure is still a considerable amount of force. For instance, if you calculate the tonnage over any given area of a one foot X one foot laminate, it would be under compressive pressure of 2,016 lbs when in vacuum with a 14psi atmospheric pressure. Many autoclaves are capable of increasing their pressure to 90psi. If you put that same 1'X1' lamination into a vacuum bag, then insert it into an autoclave under 90psi, you now are applying almost 15,000 lbs of pressure to the surface of the lamination. Another thing the vacuum does for you is that it eliminates any moisture in or around the laminate since water will vaporize at room temperature in a -14psi environment.

What does all this mean? In my rough estimate, you can get 75% of the available strength out of a carbon fiber layup without all this fuss. However, the variation in the strength of the laminate from day to day will be relatively high because of inconsistent resin ratios, humidity, gas voids, and cure rates. If you want to be able to depend on the engineering behind a laminate, you do have to go to all of the pre-preg, vacuum bagging, autoclave to ensure that you achieve a reliable consistency in the mechanical properties of the lamination from day to day and week to week.
Old 08-12-2008, 03:47 PM
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Default RE: Are autoclaves and vacuum bags necessary?

fun link:

http://www.hallspars.com/Categories....c-352cd8272c85

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