Why Vacuum Bag Fiberglass Parts

One way to make light, strong and clean parts is through a technique called vacuum bagging.  In general, parts made with vacuum bagging techniques show better strength and stiffness than simple molding lay-ups and they are smoother to the touch.  The strength and stiffness of vacuum bagged parts obviously comes from  the compression from the vacuum pressure.

In a nut shell, vacuum bagging involves placing your mold and parts, while still in the cure cycle into a flexible plastic bag.  Once in the bag, all the air is sucked out at a pressure of about 15 psi.

The bag must be flexible enough to conform to all the contours of your mold, it must not be susceptible to attack by your resin, it must not interfere with the cure of your resin and it must be able to withstand the temperatures released by the resin during the curing process.

Vacuum bagging can be accomplished either by placing the entire mold into a bag or by attaching a plastic film, usually made of PVC to the flanges of your mold and then applying the vacuum draw.  Note that the vacuum draw is held throughout the resins’ curing time.

Vacuum bagging small parts is easy and can even be accomplished with large zip lock type food storage bags that have been modified with a nipple that will accept the line from your vacuum source.  Larger parts are more complex and difficult to set up.

Bagging larger parts that require a large volume of resin require a bleeder layer as well as catch areas for the excess resin that will be drawn out of the mold from the vacuum.  The extra resin pulled out by this process is responsible for the resulting light weight parts which are such a desirable outcome of vacuum bagging.

There is plenty of information regarding vacuum bagging on the internet.  That being said, I plan to compose a manual covering all aspects of vacuum bagging in the future.

Good Glassing to you,

Steve

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