Motorsport Composite Fundamentals: Vacuum Bagging
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Vacuum Bagging
03.14
| 00:00 | There are alternatives to the standard hand layup open cure we discussed in the previous module, each in the interest of a better final product. |
| 00:07 | The first and most accessible is vacuum bagging. |
| 00:10 | This involves using a range of products and consumables, most importantly a sealed bag and a vacuum pump. |
| 00:16 | With the layup sealed inside the bag, we use the pump to pull a vacuum on the bag, removing all the air from inside. |
| 00:23 | With the reduction in pressure inside the bag, we get the atmospheric pressure outside the bag bearing down on it. |
| 00:29 | Put simply, this allows us to place the part under very strong and evenly distributed mechanical pressure while the resin cures. |
| 00:36 | This pressure serves a few different functions, one of which is to remove trapped air pockets, if there are any, consolidating the layup against the mold and reducing the chance of voids and pinholes. |
| 00:46 | It also clamps layers together so they don't shift during the curing process. |
| 00:50 | And the surface finish on the open side is generally smoother and more consistent due to the contact with the bag, or more accurately an intermediate layout called peel ply. |
| 00:59 | Having the part in a vacuum also reduces the humidity. |
| 01:02 | This is a big advantage as too high a humidity has a negative effect on the resin as it cures and will cause issues with the strength and appearance of the finished part. |
| 01:11 | Most important though is how the vacuum bagging helps to improve the fiber to resin ratio. |
| 01:15 | This is the key to achieving the prized strength and stiffness to weight ratios that composites are well known for. |
| 01:21 | Of course, the reinforcement and resin matrix aren't particularly strong or stiff on their own, so it makes sense that when combined to form the composite, this needs to be in the correct ratio for the best results. |
| 01:32 | For a perfect hand layup and standard open cure, we can measure out an appropriate amount of resin for our weight and reinforcement, usually being about in equal parts. |
| 01:42 | For example, 200 grams of carbon with 200 grams of resin, giving us a 50-50 fiber to resin ratio by weight. |
| 01:49 | However, it's difficult to get a perfect distribution by hand, meaning in some areas, we might end up with more resin than we need and other areas might not be wet through properly. |
| 01:59 | So, what we'll find is that with a wet layup, we'll often require more resin and generally end up closer to 60% resin content by weight. |
| 02:07 | With vacuum bagging, as we mentioned, any air pockets are removed and any excess resin is squeezed out into the empty space in the vacuum bag or a layer of called the bleeder. |
| 02:16 | The result is a reduced resin content, bringing us closer to this 50-50 fiber to resin ratio, which is starting to get into the realm of what's considered high performance. |
| 02:26 | Although in saying that, some refined composites using prepreg materials can see this as low as 40 % or even 30% resin content, but we'll discuss prepregs in more detail just shortly. |
| 02:36 | The alternative curing process to vacuum bagging, or rather the extra step in the process, is to use an autoclave. |
| 02:42 | Autoclaves are a considerably less accessible option and are typically used with prepreg materials. |
| 02:47 | So, again, we'll come back to discuss them together with prepregs in a coming module. |
| 02:51 | In summary, vacuum bagging allows us to apply mechanical pressure to a layup during the curing process in the interest of preventing voids and achieving a better fiber to resin ratio. |
| 03:01 | While a standard atmospheric cure can achieve great results, generally speaking, a part constructed with the use of vacuum bagging process will provide better mechanical and acidic properties. |
