Wednesday, March 14, 2012

Some Basics of creating carbon fiber parts

What is Carbon Fiber?

Raw carbon fiber is a composed to extremely thin fibers that are twisted together to form a “yarn”, which to the naked eye looks like one single strand of fiber. These yarns, or strands, are then woven together to form the carbon fiber fabric you will see in the DIYCarbon kits – they are often woven together in a criss-cross over-and-under weave style, which is known as “plain weave” although variations such as the twill weave (over-two, under-two) and basket weave are common as well.  This fabric of interwoven fibers is called raw carbon fiber, and can be used to create automotive parts by applying an epoxy resin which hardens the fabric to create a finished piece of carbon fiber reinforced plastic.

Finished Carbon Fiber is a Composite Material

Finished Carbon Fiber is a composite – that is, a material made up of two dissimilar materials, that when combined, yield inherent advantages greater than either material individually. In the case of carbon fiber, those materials are the raw carbon fiber fabric itself, and the epoxy resin it is combined with to create the final product, known as Carbon Fiber Reinforced Plastics (CFRP). Other composites include fiberglass, Kevlar, and graphite reinforced polymers – these composite materials are used in many applications over plastic, wood or metal due to their ability to be easily molded and advantages in weight savings and strength over those more common materials.

For the purposes of this overview, we will be focusing solely on carbon fiber, although in many cases the advice and concepts apply to other composites as well. As you likely know, Carbon Fiber is used in many automotive applications for its great strength to weight ratio, yielding a lightweight yet strong material for body & interior panels. For racing purposes, carbon fiber is generally used for larger parts, as the weight savings becomes negligible with smaller parts, and the production of carbon fiber can often become expensive. However, the allure of carbon fiber attracts all sorts of automotive enthusiasts for a variety of purposes, both functional and non-functional, due to both the allure of it being a very advanced material, and the brilliant depth and shine of a carefully finished carbon fiber product.

Raw Carbon Fiber versus Carbon Fiber Reinforced Plastics

Carbon fiber in raw form is a soft, pliable fabric much like any other fabric (think of a blanket), and can be easily draped or wrapped around just about anything (such as interior trim, a body panel, or even a person). Once introduced to a “matrix” such as an epoxy resin, it then becomes a composite material known as Carbon Fiber Reinforced Plastic. In this state, the carbon fiber is no longer a soft, pliable fabric but it becomes a hardened, strong, and unpliable material, much like the finished carbon fiber products you see on race cars. Simply put, raw carbon fiber (fabric) is the before, and carbon fiber reinforced plastics is the after. For purposes of this website, we’ll use “carbon fiber” and “carbon fiber reinforced plastic” somewhat interchangeably, but it’s important to note that technically speaking, the former is referring to just the raw, unfinished fabric, whereas the latter is referring to the composite material after epoxy resin has been applied and hardened.

One big advantage of working with raw carbon fiber, especially for DIY projects, is that it is very easy to mold into the shape you want, and can be wrapped over just about anything as an overlay. The disadvantage is that once the epoxy resin has been applied and it becomes a carbon fiber reinforced plastic, there is no flexibility whatsoever, and you’re pretty much stuck with the shape it hardened in. Carbon Fiber is not a flexible material, and there are no “do-overs” once the epoxy has set.

Types of Carbon Fiber

There are many types of carbon fiber, which we’ll explore now. Carbon Fiber can vary by the style the yarns are woven together (weave patterns), the weight of the carbon fiber, and the size of each yarn in the carbon fiber fabric.

Weave Patterns

Plain Weave (1×1) – the most common style of weave. Each yarn (or strand) is woven in a simple over-and-under weave style creating little squares. It is a simple over one strand, under one strand pattern, as shown in the picture.
Twill Weave (2×2) – With a twill weave, each yarn travels over-two, and then under-two other yarns, as opposed to a over-one, under-one ratio for plain wave. This creates a slightly different texture .
Harness/Satin Weave (Anythingx1) – Each yarn travels over two or more yarns, then under one. For example, if it travels over 3 and then under 1, it would be a 3×1, or 4HS. Harness Satin is commonly abbreviated as HS, and the number is derived by adding the numbers before it. An 8HS weave then, would be a 7×1 weave, where each yarn travels over 7 other yarns, and then under one yarn.
Basket Weave – very similar to the plain weave, but with two yarns under, and then two yarns over, almost like a double plain weave.
There are certain functional advantages to each type of weave, although for most purposes outside of aerospace use, it really comes down to aesthetics and personal preference as to which you weave you should choose.

Weave Weights

The weave weight is the actual weight of 1 square yard of raw carbon fiber fabric. The heavier the weight, the more fibers are woven into each individual yarn (known as the size), generally creating thicker strands that will have an effect both visually and to the strength of the finished carbon fiber reinforced plastic.

The most common weights for automotive applications are:

8 oz – the vast majority of aftermarket car parts on the market
12 oz – the minimum standard DIYCarbon or oCarbon sells (unless a colored weave…)
20 oz – the rarest of the bunch
Weave Sizes

The size is referring to how many pieces of fiber are in each yarn (or strand). A size of 3K means that there are 3,000 fibers in each yarn. Obviously, the greater the size, the thicker the yarn, and the greater the weight of 1 square yard of fabric as well. The greater the size, the greater the weight.

It is more common to purchase carbon fiber by weight than size, although they pretty much go hand in hand, and by knowing the weight you can often deduce the size. For instance, an 8oz weave generally has a yarn size of 6k, meaning 6,000 fibers per yarn (or strand). A 20oz weight is often around a 12k size, or 12,000 fibers per yarn.

Colors (Carbon-Kevlar Hybrids)

A common misconception is that carbon fiber comes in different colors – it doesn’t. Each yarn of carbon fiber is black – although it may look silver to the naked eye based on how the light reflects on the weave, it isn’t – there is no silver yarn, carbon fiber only comes in black.

Often you will see different colors available such as red, blue, or yellow. This colored carbon fiber isn’t really carbon fiber, it is a Carbon-Kevlar hybrid. The Kevlar can come in different colors and is woven together with the (black) carbon fiber yarns to create the multi-colored look.

What is (Epoxy) Resin?

To turn raw carbon fiber into a finished piece of carbon fiber reinforced plastic, you must add the magical ingredient of resin to harden the fiber. If you’ve never worked with epoxy resin before, it is a gel-like substance that you will brush over the fiber, and once it hardens the fiber and resin will become a finished piece of carbon fiber reinforced plastic. It can be very messy & sticky, especially if you’ve never worked with it before, which is why mixing sticks, gloves, and special mixing containers are recommended, as well as a clean & adequate work-space that you are okay with getting dirty (do not try this in your living room).

Epoxy resin works in a pretty simple way – you have a resin and a hardener. The hardener (or sometimes referred to as a catalyst) is added to the resin in a pre-set ratio, such as two parts resin to one part hardener. Separately, both the resin & hardener are gel-like liquids, but once the resin & hardener have been mixed together, a chemical reaction will occur and the liquids will become a solid. Sometimes, particularly with fast-drying resins, a noticeable amount of heat will be generated during this exothermic reaction, so be sure to read the safety & warning labels of all of the products you are using before mixing the resin & hardener. You generally have a certain window of time before the chemical reaction completes and the resin becomes too solid to work with (known as the “working time” of a resin), which means that once you have mixed the resin & hardener together you need to work efficiently, and only mix small batches of resin at a time that you can fully use up within the resin’s stated working time.

Generally the working time of a resin, and the time for it to fully cure are vastly different. You may only have 20 minutes of working time before the resin is too hard to work with anymore, but it may take a few days for the resin to fully cure and the chemical reactions to fully complete before you can begin working with the piece again – this would be called the “curing time.” You can accelerate the curing time applying moderate heat to the resin, but it’s important to allow the resin to fully cure before moving to the next stage. Another important note is that once the resin has fully cured, no more resin should be added to the part – the cured resin and the uncured resin will be unable to bond correctly, and the second layer of resin will be limited in its performance and may peel off or decompose rather quickly. Any and all resin you intend to add to a part should be done before the resin has been able to cure completely.

Choosing a Resin

There are several commonly used resins you can choose from – all of which have their pros and cons. At oCarbon and DIYCarbon all kits & parts we sell use professional grade UV treated epoxy resins as we have found this to be the best for automotive applications in terms of strength and minimizing yellowing & fading to producing a high quality finished product that will last a very long time.

One common misconception that is perpetuated by those who make low-cost carbon fiber parts is that certain resins will “never yellow or fade.” This is 100% false – all resins will fade or yellow when exposed to ultra-violet (UV) light, as the UV light breaks the chemical bonds in the resin’s molecules.  The speed & severity of yellowing/fading can be minimized substantially by which resin you choose, as well as introducing UV inhibiting chemicals into the resin to help minimize UV damage. Even OEM carbon fiber parts from manufacturers such as Lamborghini, Pagani, Ferrari or Audi will yellow or fade over time, especially parts exposed to extreme heat such as engine covers, or parts exposed to a lot of sunlight such as roofs and hoods in hot climates such as deserts.

Resin choice is just as important as your choice in raw carbon fiber – it takes both pieces to make a finished part after all. There are three categories of commonly used resins – Polyester Resin (PER), Vinylester Resin (VER), and Epoxy Resin. Within each category there are also varying levels of quality at corresponding levels of price, but for now we’ll focus purely on the types of resin to choose from.

Polyester Resin (PER)

Inexpensive, but the lowest in strength & performance – Polyester Resin (PER) is the most commonly used resin in mass-production due to its low cost and adjustable cure time. Polyester is very resistant to UV light and can be used in applications that are subjected to prolonged exposure to direct sunlight. PER is mixed with a catalyst/hardner called MEKP (methyl ethyl keytone peroxide), and you can adjust the ratio of polyester resin to MEKP to alter the working & cure times, depending on your needs.

The main drawback of PER is its low strength – as PER cures, it shrinks considerably, compromising the strength of the finished product. It is generally not recommended to use it with high-performance fabrics such as carbon fiber. It is also very sticky & tacky when working with it, which can make it unusually messy for novices. A final drawback is that PER releases higher levels of Volatile Organic Compounds (VOCs) which can be harmful to your health & respitory system, so be sure to work in a well ventilated area wearing a mask if you choose to use PER as your resin.

Vinylester Resin (VER)

Middle of the pack – VER is a little more expensive than Polyester Resin with improvements in strength & chemical properties, but not as strong as Epoxy Resin. VER is best described as an “upgraded version of PER” as you will achieve slight increases in strength & performance but otherwise many of the properties are the same (they both use MEKP as a hardener, for instance). Because of this, it is used in projects where PER would otherwise be used, but the builder once a slightly more strong product, such as boat hulls and storage tanks. Like PER, Vinylester also shrinks considerably during curing, causing decreases in strength, and VER also emits VOCs which may be harmful to your health.

Epoxy Resin

The highest cost & the highest quality – unlike PER and VER, the hardener used with epoxy resin actually bonds the resin & hardener together. This creates superior strength although it requires very precise measuring of resin to hardener, unlike PER and VER where you can mix resin to hardener in different ratios. Epoxy resin has much lower amounts of shrinkage while curing, creating superior strength over PER and VER. Because of its superior strength, Epoxy Resin is generally recommended for any high performance application such as aircraft, automotive, or structural uses. Because carbon fiber is a high performance (and exotic) fabric, it is generally recommended that you use epoxy resin with carbon fiber – using PER or VER with carbon fiber is like filling up your “premium fuel only” car with 87 or 89 octane – it can be done, but if you care about performance, it isn’t recommended.

Epoxy Resins tend to have very low VOC emissions, making them more safe to work with, especially for the hobbyist who may not have the proper safety equipment for PER or VER otherwise. The one disadvantage of epoxy resins that is worth mentioning is that they are more vulnerable to UV light damage relative to PER or VER, and require additional protection methods such as mixing in UV treatments to help prevent premature yellowing or fading. All kits & parts sold through DIYCarbon & oCarbon have UV inhibitors mixed in.

Part 1 Conclusion

In part 1 of our guide we have given you an overview of what makes carbon fiber unique, the basics of how a finished piece of carbon fiber reinforced plastic (CFRP) is produced, and the various choices in carbon fiber fabric and resins that exist. Both DIYCarbon & oCarbon allow you to pick the carbon fiber fabric of your choice according to weave pattern, weight, and other choices, although for all carbon fiber applications we recommend using epoxy resin for its superior strength and quality. Now that we have given you the necessary information to pick the right materials, it is time to discuss the basics of how to produce carbon fiber.

Part 1 Definitions/Glossary:

·         Raw Carbon Fiber – carbon fiber fabric before an epoxy resin is applied

·         Carbon Fiber Reinforced Plastics – carbon fiber after epoxy resin has been applied & hardened

·         Composite Materials – any product made of two or more different materials, such as raw carbon fiber and epoxy resin

·         Yarn – to the naked eye, the yarn looks like one single strand of fiber, although it is really several thousand fibers all twisted together to form one yarn. Also known as a pic, or bundle.

·         Weave weight – how much 1 square yard of carbon fiber fabric weighs, in ounces.

·         Weave size – how many fibers are in one yarn.

·         Weave pattern – the pattern in which yarns are woven together, such as a plain weave (1×1), a twill weave (2×2), or a harness satin (HS) weave (anythingx1).

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