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Wednesday, March 14, 2012

Production of Carbon FIber Pieces


Part 2: Carbon Fiber Production Basics

There are several ways to produce finished carbon fiber reinforced plastic (CFRP) parts – the most common for hobbyists is a wetlay (or sometime referred to as an overlay), although many other options exist but are generally only used for projects of considerable scale or quantity. In this section we will give a brief overview of each, although if you are planning on using a DIYCarbon kit, you will be using a Wet lay-up/Overlay and skip the rest of this section (unless of course you are interested, then please read on!).

Wet lay-up or Overlay – A basic Wet-layup or overlay is the most common method for hobbyist or DIYers looking to create carbon fiber parts as it does not require any specialty equipment. In this process you will overlay the carbon fiber over an existing part. You will want to make sure the part you overlay has smooth, non-porous surfaces that the wet carbon fiber fabric can be laid against. In this process you will combine your epoxy resin & hardener, then soak the carbon fiber fabric in the resin/hardener mix. You will then take the soaked carbon fiber fabric and lay it up over the part. You may choose to lay-up several layers of carbon fiber fabric at once, but the resin & layers will only bond to each other if they resin hasn’t cured yet. After allowing the resin to cure, you will need to remove the mold, sand and then finish the carbon fiber reinforced plastic in the method you prefer (be it paint, clear-coat, or any other kind of surface finishing). The downside to this method is that the part you overlay is part of the final product, so it does not save any weight and in fact adds a marginal amount of weight. This method is perfect for aesthetic applications of carbon fiber like automotive interior trim, as you can ensure an OEM-like fit and finish and reuse the OEM mounting points, while only gaining a trivial amount of weight.

Molds & Patterns – This process is much like an overlay, only instead of overlaying an existing part, you create a mold (also known as a plug, a pattern, a master, or a buck) that is used to create the part. The mold is then removed from the final product, requiring no core or base product that gets overlaid. There are several ways to create a mold – commonly materials include wood, metal, or plastic. The materials for the mold should be easily shapeable, resistant to the materials being used, non-porous, and strong enough to uphold their original shape over multiple uses (poorly made molds will warp or become distorted over time, causing the final product to be misshaped). The mold needs to be perfect in cases of creating replica automotive parts like hoods or interior parts, otherwise the final product will not fit correctly or possibly not fit at all.

Vacuum Bagging – In this process, the wet-laid carbon fiber and its mold are sealed in a plastic bag, of which all air is removed, thus pressing together the wet carbon fiber against the mold to ensure a void-free covering.  The advantage is that the fiber is vacuum pressed against the mold, preventing any wrinkles, bubbles, or irregular shaping – the final product will take on the exact shape of the mold. The disadvantage of this method is that it requires extra equipment including the vacuum bagging, a pump to create the vacuum, breather cloth, peel-ply, and many other materials. There are several resources around the internet on how to create a simple vacuum bagging system using a tire pump for a bicycle, although obviously there is more advanced equipment available as well. This system is great for very complicated parts that have lots of uneven contours, odd shapes, or need to be extremely precise.

Vacuum Infusion – In this process, carbon fiber is laid dry onto the mold, and the vacuum is created before the resin is introduced (in Vacuum Bagging, the resin is applied before the vacuum pressure). Once a complete vacuum is achieved between the carbon fiber and the mold, the resin is then injected into the vacuum where it covers the carbon fiber, then the extra resin is then sucked out of the vacuum sealed mold.  There are several advantages to this method over Vacuum Bagging, most notably that less resin is used (minimizing weight) and the procedure is much less messy than vacuum bagging where the carbon fiber still has to be wet-laid. The disadvantages are again cost & equipment – much more specialty equipment is required to create a vacuum infusion system, and maintenance on the equipment is more involved since resin (which will harden & cure) is injected through it.

Pre-Pregs (Pre-Impregnated Fabrics) – unlike other methods, this method is entirely “dry” and when you hear of “dry carbon fiber” this is how it is created. With Pre-pregs, the fabric is pre-impregnated with a resin system that will cure when heat is applied to it. Generally the pre-preg material is frozen and stored in very cool temperatures until it is ready to be cured, and then baked in an oven at extremely hot temperatures. This process gives you much more control over the process and allows you to use stronger resins than a wet lay-up, and will be 20-30% stiffer and stronger than an equivalent-thickness wet laminate. The disadvantage of this method is again cost, as well as equipment to bake the material. Applications such as (high quality) mass produced carbon fiber hoods, F1 race parts, and airplane/aerospace parts will use pre-pregs to create a highly controlled, extremely high quality end part on a large scale.

Part 2 Conclusion

There are several ways to create parts, all of which depend on your budget, the scale of the project, and the complexity of the part being created. For the purposes of the rest of this guide, we will be concentrating on wet-laid carbon fiber projects that use overlays or simple molds which will be ideal for most small projects done by DIYers and hobbyists. Many of the principles throughout this guide can easily be expanded to vacuum bagging and vacuum infusion as well.

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