Vacuum bagging is the process of applying vacuum pressure to materials to remove all air bubbles and voids. After being heat-sealed, a large sheet of material can be pulled into a smaller size or sealed completely flat. The latter method is used when vacuum-forming plastic to create a replica of an object. Here are various essential things you should know about vacuum bagging:
Benefits Of Using a Vacuum Bag
There are three primary benefits of using a vacuum bag:
- The first is the pressure applied to the part via the film and the vacuum, which can be molded accurately. This leads to a smoother and more accurate finished product than what can be achieved with hand lay-up.
- The second benefit is that it produces parts that are free of voids. During pressure application, air cannot enter or escape through the film, causing any volume of air trapped in part to be forced out.
- The third benefit is that it allows for parts with mirror-smooth surfaces because it evenly applies pressure over the entire surface of the part while evacuating air from behind the film.
Vacuum Bagging Processes
Vacuum Bagging utilizes various supplies to be successful. A good example is Vacuum Bagging Supplies – Fiberglass & Composite Supplies. There are two primary processes used for applying film to a part:
The first process is called hand lay-up, where the film is applied manually to the part by laying it directly on top of the part. A frame is then placed over the film and sealed around its edges with tape, wrapped with a wet towel or other wet media. Once it is sealed, a vacuum is applied to the bag, which pulls the film tight against the part and forces out any air pockets trapped between the film and the part.
The second process is called resin transfer molding (RTM). It involves mixing resin with a catalyst, laying down that mixture onto one side of a pre-cut film, and then sandwiching the part between the two films. A frame is placed over that and sealed around its edges with tape wrapped with a wet towel or other wet media. The bag is then evacuated of air by applying a vacuum, forcing the resin mixture against the part and filling any voids in the mold created by the part.
Layer Selection and Application
The layer selection process is different between bagging and hand lay-up. Vacuum bagging requires fewer layers than hand lay-up because the part does not need to be sanded and fair after each layer is applied. This means building up a part with many different layers is easier to create specific features without worrying about sanding and fairing after every layer.
- The application process consists of creating a female mold [mold with pull-tabs instead of pins] and using it as a female side to create a male mold by laying the part into place and then placing the female mold on top of it. Next, you will want to cover your male mold with a release agent [such as wax] and then coat both sides of the part with a primer surfacer. One important thing to note is that if you use a vacuum bagging system, you will want to spray the primer surfacer into a shallow pan and then use a paintbrush or roller to apply it to the part. It is important not to use an airless sprayer because any trapped air in the bag can be forced out while spraying, and this would cause your part [that has been coated with release agent and primer surfacer] to stick inside of the bag.
- Once primed, it is time to apply your first layer [B-staging]. This process involves spraying the part with a mixture of resin and catalyzer [resin/hardener combination], immediately curing when sprayed onto the part. After applying the appropriate amount [brush or roller application], you will place the part into a bag and evacuate air from inside it using either a hand pump or a vacuum pump. Once evacuated, seal the bag with tape [making sure to leave an opening for air to escape] and let cure for 48 hours.
- Once cured, remove from the bag and apply your next layer of B-staging. The only difference is that this layer will be a hybrid mixture of resin and hardener [no catalyst added]. This reduces cure times to as low as 15 minutes under ideal conditions.
- After curing the B-staged part, you can apply your final surface coats, allowing spray application into a bag. If you are laying multiple layers of surface coats, don’t forget to brush or roll the first coat and spray the second and third.
- Once all desired layers have been applied, it is time for B-staging again. This important step prevents the part from inhibiting the resin/catalyzer mixture [or any other surface coating] on its second trip through the bagging process. However, no priming surfacer will be involved because all layers should contain a primer surfacer after the first one.
- Once all layers have been applied, you are ready to apply your final topcoats of paint or clear coat. You will still involve some curing time, so don’t rush right into the painting process. Bagging a part is an important step to provide strength and stiffness to complex molded parts.
In conclusion, vacuum bagging is a very important process that can significantly increase your part’s life, performance, and durability. At the same time, it may seem like a complicated process. However, by following the steps outlined above, you should have no problem applying vacuum bagging to any molded parts.