Motorsport Composite Fundamentals: Prepregs & Autoclaves
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Prepregs & Autoclaves
06.42
| 00:00 | Pre-preg is a term used to describe our conventional dry reinforcement materials that have been pre-impregnated with resin. |
| 00:07 | While glass, Kevlar and even flax fibre reinforcements are available as pre-pregs, most of the time we're going to be dealing with carbon fibre, and these could be plainweaves, twills or unidirectional fabrics. |
| 00:20 | The resin already has the hardener in it, and within reason, only begins to cure when exposed to heat in an oven or an autoclave, which we'll discuss in just a moment. |
| 00:30 | I say within reason, because pre-preg material typically needs to be stored in a frozen state and in a sealed package. |
| 00:38 | And when removed from the freezer, it'll have a working time of around a few weeks, depending on the product. |
| 00:44 | Even at room temperature, the resin will begin to cure, and although it won't fully solidify, it'll still become stiff enough that it's extremely difficult to work with. |
| 00:54 | Also, if it's not in a sealed package, the condensation that forms can contaminate the material. |
| 01:00 | Before being laid up in the mould, the material does need to be defrosted, otherwise it'll be too stiff to work with. |
| 01:06 | However, since the reinforcement now contains the uncured resin, it's still significantly less pliable and drapeable, meaning that it's more difficult to lay out in the mould. |
| 01:17 | So, it's typical to cut the material into templates for more complicated forms. |
| 01:22 | The prepreg material is laid into the mould exclusively, meaning no additional resin needs to be applied to the reinforcement. |
| 01:30 | For this reason, the term dry carbon is often used to describe the carbon fibre composite parts made using prepreg materials. |
| 01:38 | The key aspect here is how the reinforcement contains a near optimal amount of resin, and it's distributed almost perfectly throughout the resin, which, as we know, leads to the best strength and stiffness to weight properties. |
| 01:51 | I use the words near and almost here because this really depends on the product. |
| 01:56 | Some products have excess resin content to ensure that the finished part looks great, but this can come at the expense of weight and negate a lot of the advantages of a prepreg, so it's just something to understand that not all prepreg material is created equal. |
| 02:12 | We've mentioned autoclaves a few times now as an alternative or rather extra step to the vacuum bagging process, so let's finally take a closer look at what these are and how they work. |
| 02:23 | Autoclaves are essentially an oven that can pressurize the contents, or in other words, a heated pressure vessel. |
| 02:30 | With vacuum bagging, the pressure on the part is limited by the atmospheric conditions, but if we use an autoclave, we can place the part under much more pressure to further consolidate the laminate and completely eliminate pinholes and air pockets, while providing the heat to initiate the curing process of the prepregs. |
| 02:49 | Prepregs and autoclaves are expensive, but they're really the gold standard for high performance composites. |
| 02:55 | While it's unlikely most of us will get to use them ourselves, autoclaves are the traditional method of curing prepreg materials, so we'll discuss this first before considering another more accessible alternative. |
| 03:07 | Following the initial layup, we have the debulking stage. |
| 03:11 | This is the process of using a vacuum bag to consolidate the reinforcement against the mould, helping to remove any gaps, voids or bridges between the layers, before curing the resin. |
| 03:23 | Sometimes this isn't required for simple layups, with just one or two layers of reinforcement in simple moulds. |
| 03:30 | However, it is critical for multi-layer laminates, particularly those with more complex geometries. |
| 03:36 | In some cases, the initial layers will be debulked, and then the part removed from the bag, before laying up the additional layers, and then repeating the debulk again. |
| 03:46 | Following this, a drop test is performed, also called a drawdown test, which involves pulling a full vacuum on the bag, and then clamping the vacuum line, and monitoring the vacuum in the bag using a vacuum gauge. |
| 03:59 | If there is no drop in the pressure over a time frame of about 30 minutes or so, then we can be confident that there is no leaks in the bag. |
| 04:07 | In which case, the bag layout can be disconnected from the pump, and transferred to the autoclave. |
| 04:13 | The autoclave will likely have its own vacuum lines that can be connected to the bag, as an extra precaution. |
| 04:20 | The autoclave is then sealed, and pressure and temperature are applied and accurately controlled throughout the curing cycle. |
| 04:26 | Again, though, they're expensive and not readily available, but there is a more accessible alternative for curing prepreg materials. |
| 04:34 | With advancements in the engineering of these materials, we now commonly see the use of out of autoclaves, or OOA prepregs. |
| 04:42 | These are laid up and debulked to form laminates, just like they would be when using an autoclave. |
| 04:48 | However, the vacuum bagging provides some mechanical pressure, and an oven provides the heat for the curing process. |
| 04:55 | While conventional home ovens can be used with success, they generally need to be modified to provide accurate enough temperature control to avoid overheating and damaging the materials. |
| 05:06 | Curing ovens specifically designed for this purpose are available, although as you'd guess, they're not exactly cheap. |
| 05:13 | But with that said, they're still a fraction of the price of an autoclave. |
| 05:18 | While these out of autoclave prepregs, paired with vacuum bagging and oven curing, do provide great results in high performance parts, typically better than those of the resin infusion and wet layups, there are some compromises. |
| 05:31 | Generally, the material will have a higher resin to fibre ratio than traditional prepregs, and the pressure during the curing cycle is limited by the vacuum bag. |
| 05:41 | So, to be clear, traditional prepregs cured in an autoclave are still the gold standard. |
| 05:46 | In summary, prepreg refers to reinforcement fabrics that have been pre-impregnated with resin. |
| 05:51 | These need to be stored frozen and sealed, but when defrosted, can be laminated like a typical layup, albeit without the need for wetting out with additional resin, hence the common name, dry carbon. |
| 06:04 | The near ideal resin to fibre ratio and distribution of the resin help achieve great mechanical properties in the finished product. |
| 06:12 | Following the initial layup, the laminate is debulked using a vacuum bag, and then a drop test is performed, and the assembly is transferred to an autoclave, where the part can be cured under elevated pressure and temperature. |
| 06:24 | A more accessible alternative is to use out-of -autoclave prepreg material, which is vacuum bagged for consolidation and cured in an oven. |
| 06:33 | However, there are still some compromises in performance compared to prepregs cured in autoclaves. |
