Vacuum Bagging: Basics
When manufacturing a traditional metal structure, you start with the material in its final form. Some heat treatment or stress relieving might be needed, but the material is essentially complete.
With composites, however, the material is made at the same time as the structure. The material is not complete until the cure cycle has finished, and therefore it is critical to carefully control conditions during cure.
The three ingredients necessary for cure are heat, pressure, and vacuum. All three are mandatory for prepreg layups; only heat is required for wet layups, but pressure and vacuum always result in a better laminate.
Pressure compacts the laminate, providing good consolidation and interlaminar bonds. Vacuum draws out volatiles and trapped air, resulting in a low void content. Both help to improve resin flow.
A vacuum bag provides both pressure--up to 14.7 psi, depending on your altitude--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.
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. When appyling the release, be sure to mask off the edges of the tool so the bag sealant tape (BST) will stick to the tool.
Bag Sealant Tape
BST is a putty-like material which comes in rolls, usually 1/2 inch wide, 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. The tape usually goes on after the part is layed up, especially if it is a wet layup.
One 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. Many engineers consider this a bondable finish, and it usually passes a wet-out test. I've seen test, however, which show that sanding still gives a much better bond.
If peel ply is used, it will absorb a small amount of resin, and this must be accounted for. A net-resin prepreg may end up too dry. Peel ply specs should say how much resin will be absorbed, in ounces per square foot, or grams per square meter.
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.
Peel ply can be either a solid sheet, or it can have perforations (in the latter case, it is often called perf ply). The perforations might be like pin-pricks, or they might be small holes which are punched out. The spacing can also vary from two inches to eight inches. Choose spacing based on the amount of resin that needs to be bled out: wet layups can use close spacing; prepreg manufacturers can recommend spacing for their particular products; and net-resin systems of course use unperforated release films.
Not all release films are compatible with every resin system. A few years ago, I was preparing some cyanate-ester test coupons, and the release film we normally used for epoxies bonded to the coupons. You can also get release film treated so it will bond to the laminate (bondable one side, BOS, or bondable both sides, BBS). BOS can be used to create a permanent release layer on composite tools, or as a moisture barrier on laminates.
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. A thick breather is also desirable to keep resin from coming in contact with the bag. It doesn't hurt anything if that happens, but preventing it makes the bag easier to remove.
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. Pleats will be required for anything but flat or simply curved structures. (I'll cover pleats in a future article.)
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.
In addition to making sure all materials are compatible with the resin system, also make sure they can handle the cure temperature. Also remember that oven or autoclave temperatures will likely approach 375-400 F during a 350 F cure--bagging materials must be compatible with these higher temperatures.
I'm sometimes asked how much vacuum is too much. In general, you can't get too much vacuum: always try for a full 14.7 psi. The only exception would be if you are worried about crushing a core or dimpling facesheets over honeycomb.
Bagging materials are available from many sources. The following pages list distributors of these materials:
- Material Suppliers: Consumables
- Material Suppliers: Distributors
- Material Suppliers: Catalogs
- Equipment: Cure
For additional references on vacuum bagging, see the How To's page.Previous Features