Vacuum Bagging & Form Bracing

[mainr]

I hope this is not too "off topic" - it relates to composite molding, rather than composites themselves.
It is not clear to me if (and if so, WHY) bracing is necessary on a composite form when used with single sided vacuum bagging , but I'm told the mold will deflect if not braced. I'm told this can be resolved by envelope bagging. When single-sided bagging, does not the ambient pressure on the top surface of the bag equalize that on the back surface of the mold, in effect negating each other?
Thanks for any input.
Cheers.

 

[Compositepro]

Bracing is not required. There may be some small forces introduced from bag tension if the bagging is not done right. Your tool needs to be stiff enough for your application.

 

[RPstress]

Single sided bagging usually means that the bag extends over the part and maybe over the mold. The bag is then sealed along its edges, often to a table with the mold sitting on it.

Providing that the mold is solid and is evenly supported by the table (or whatever), it will not deform greatly.

If the mold is hollow and/or sat on a support structure of some sort then the bag will press down on the mold and deform it where it is less well supported in some places than others.

If the bag is sealed to the mold itself around the part, then there should not be an issue provided the back side of the mold is vented (in an autoclave - under vacuum only the mold is just sat there).

Envelope bagging (where the bag extends around the mold on all surfaces) has to be done carefully, to make sure that the bag doesn't bridge and split. It also mustn't fret or tear where the mold sits on it.

 

[CompositeGeek]

Another reason to brace is to control the thermal changes in the mold during cure. If the geometry is highly critical I have found some bracing reduces the impactof CTE or "directs" the tool expansion and contraction during cure to yield the geometry required.





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