Fiberglass brings a lot of advantages into boat building, however, the material is made at the same time as the structure of the product which means that the material isn’t complete until the cure cycle has finished and from there comes the need to control all the conditions that are crutial for the process. The three most important ingredients that are needed for the cure are heat, pressure, and vacuum. All three are mandatory for prepreg layups. Pressure and vacuum are known to work to always result in a better laminate. Pressure compacts the laminate, and so provides good consolidation and interlaminar bonds. Vacuum draws out volatiles and trapped air, and so resulting in a low void content. Both help to improve resin flow. The best fiberglassing method is a vacuum bag which is able to provide both pressure (up to 14.7 psi) and vacuum.
Bagging a Laminate
All laminates are bagged in essentially the same way. Once you know the basics, you can bag just about any structure. There are some tricks to handle complicated geometry, but the bags all look alike. You should follow the steps bellow to make a nice looking and functional product.
- Release Agent – Before you even start laying up the part, you apply a release agent to the tool. This may be a liquid release coating, a wax, or even a solid barrier such as Teflon tape.
- Bag Sealant Tape – is a putty-like material which comes in rolls with a release paper on one side. You press the tape against the tool, leaving the release paper on until you are ready to apply the bag itself.
- Peel Ply – when the laminate is in place, it’s time to apply the bag. The first item to go down is a peel ply. Peel plies are a tightly woven fabric, often nylon, and impregnated with some type of release agent. The peel ply will stick to the laminate, but it will pull away without too much difficulty. Peel ply is optional. Most often it is used to give the laminate a rough, rather than smooth, finish.
- Release Film – after the peel ply comes a layer of release film. This is a thin plastic which has been treated so it won’t bond to the laminate. It is highly stretchable so it can conform to complex geometries.
- Bleeder and Breather – at least one layer of bleeder cloth goes above the release film. Bleeder is a thick, felt-like cloth. Its purpose is to absorb excess resin. The bleeder also acts as a breather, providing a continuous air path for pulling the vacuum. If the bag wrinkles against the hard laminate, it will trap air. The breather prevents this from happening. The breather must be thick enough so that it doesn’t become fully saturated with resin.
- Bag – the bag is the last item to be placed. It’s a relatively thick plastic layer, available in different amounts of conformability. The bag is usually applied along one edge at a time. Start at one corner and press the bag into the BST, removing the release paper from the tape as you move along the edge. Be careful not to get any wrinkles in the bag or it will leak. Make sure you remember to attach the vacuum port (not shown in the figure) before closing the bag. The base of the port goes inside the bag; cut a small cross in the bag for the attachment flange to fit through. If the tool has an area for the port, make sure there is a breather path from the port to the part. If the port goes on the part itself, put several layers of breather under the port to prevent print-through.