Jul 7th 2021
In the first part of our series on bagging films we introduced what they are and the various advantages they have in creating composites. In this next edition we’re going to get into more of the nitty gritty of bagging films and what to characteristics they have.
Flexibility & Elongation
Elongation, also referred to as flexibility, elongation at break and tear strength are characteristics of a bagging film that allow the bagging film to conform to tight radii and changing contours in a part’s surface. Bagging films that are more flexible are less likely to bridge a part. Elongation properties are added to certain bagging films to make processes more efficient and to mitigate issues that come with complicated bagging schemes. In short, bagging films with higher elongations may offer a higher degree of forgiveness vs standard films. Especially when considering the high number of pleats a bag may need for a large complicated part. For someone just getting into composite bagging processes, a bag with higher elongation properties will aid in overall part quality. In corners or female radii along the tool, it takes only a small bridge in a vacuum bag to result in the part being resin rich, having issues with porosity within the parts radii areas, or worse, blowing a bag during cure which could result in a scrap part.
Illustration for reference only as may not represent entire bagging scheme
When performing work with prepregs, resin infusion, or traditional style layups, the thickness of the bagging film plays a role in bagging processibility and the part’s cure. Thicker bagging films usually offer an “easier” degree for workability than that of thinner bags in that they do not fold over or crease as easily as thinner selections. Although most bagging materials carry a standard 2 mil thickness, Prepregs and traditional layup’s using vacuum will benefit most by using thick, soft, and flexible bagging for most complicated layups and bagging schemes. However, when performing resin infusion processes, using thinner or stretchy bagging films may slow the rate of resin flow vs a standard film. This is also relative to sizing of bags and projects that may be applied to.
A bagging film’s width should always be taken in consideration for economical purposes. For instance, if a project is a small part with a tool width of ~12”, the need for using a bagging film with the width of 120” would not be economical. On the other hand, for a boat hull or fuselage of an airplane, wider bagging material widths are integral for processing effectively and choosing the correct bagging material is of utmost importance. It is always better to have a bagging width that is too much than not enough. Generally, choosing a film with a width that is 30-40% greater than that of the tooling surface should provide an adequate amount of material for pleating around the edge of the tool. For more complex contours of the part, the bag should widen considerably as more pleats should be added around the edge of the part. The exact figure of bagging widths is not usually done in one part. Repeating processes usually gives better and more efficient numbers.
Vacuum bagging film can also be affected by the humidity conditions present in the area it is stored and exposed to. In areas on low relative humidity some bagging films will become more harder to work with and may feel as if they are brittle. When exposed to areas of higher humidity, these films usually become softer and easier to work with. Some bagging films, such as Airtech’s Stretchlon series films will keep the same properties in lower humidity conditions and provide consistency throughout the bagging process.
Next week we'll look at Bagging Film Applications & what may or may not work best for your particular use case or project.