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Carbon Fiber 

Overview

A majority of Carbon Fiber is made from a polymer called Polyacrylonitrile (C3H3N), also known as "PAN". The Carbon Fiber is cleaned, stretched and heated in a process called oxygenation. After this, it is run through a vacuum oven with a reduced oxygen level to remove the Hydrogen and Nitrogen from the plastic at high temperatures, without burning. This process is known as "Carbonization". By aligning the structure this way while carbonizing, it aligns the carbon atoms and bonds them into a crystalline matrix that are aligned parallel to the fiber’s long edge.

This process of aligning the molecular bond and removing impurities makes the carbon very strong along the fiber resulting in very high tensile strength, and good stiffness. The Crystallization process also makes the atoms less dense and therefore lighter. This creates a very high strength to weight ratio. Ten times stronger than mild steel in tensile strength, yet five times lighter.

The carbon fiber strands are then put together in various different ways depending on the application. Then can be wound to make a tube or tank. Woven on a loom to make a sheet or blankets or cut into small pieces to be used in a process called forged carbon fiber. Carbon fiber is then used to reinforce other materials to create carbon fiber composites. Typically, a two-part epoxy resin is used to hold the fibers together for the specific application. 

Carbon Fiber vs Mild Steel and Aluminum

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Carbon fiber has a lot of desirable attributes compared to aluminum and mild steel. The density of continuous carbon fiber tow is about 0.06 pounds per cubic inch. This is less than the density of 3003 aluminum that comes in at approximately 0.10 pounds per cubic inch, and significantly lower than A36 steel that is approximately 0.28 pounds per cubic inch. 

                Tensile Strength is the lineal strength of a material. In other words, how much pressure must be applied to an object to pull it apart. The Ultimate Tensile strength of continuous carbon fiber tow is 600,000 psi with 1.5% elongation at break. For 3003 Aluminum that number falls to 14,500 – 16,000psi with 12% elongation at break, and for A36 steel is measured in at 58,000 – 79,800psi with 20% elongation at break. So not only does Carbon fiber have more than 10 times the Tensile strength than steel it also does not deform near as far while force is being applied.

                Modulus of elasticity is a measurement to determine how stiff a material is over a given load. Because the carbon fiber manufacturing process aligns the molecule along the strand it is not as strong laterally. However, carbon fibers modulus of elasticity is equivalent to mild steel by volume. This is another reason why carbon fiber is more desirable than steel because it is 5 times lighter than steel with similar stiffness.

Manufacturing Process

A majority of carbon fiber is made from Polyacrylonitrile (C3H3N) also known as PAN. First it goes through a process called spinning, where the PAN is extruded into small fibers. During this process it is run through a bath to solidify the fibers then stretched and washed. After this the fibers are put through a process called stabilizing also known as oxygenation. It is stretched over hot rollers till the desired levels of oxygenation are reached. The fibers are ran through an over at over 1000 degrees Celsius in an environment that is low in Oxygen in a process called carbonization. This allows the fibers to release the Hydrogen and Nitrogen atoms without burning the fiber. Once the desired outcome is reached in the carbonizing process the fibers go through a surfacing process. After carbonization the fibers do not bond well with epoxies. Because of this the surfacing process scuffs the fibers and oxidizes to allow for a better bond. The last process before packaging is known as sizing where the fiber is then coated to protect them from further processing. 
After the carbon fiber has been created it is typically rolled onto cylinders called bobbins. Thread counts, also known as K counts, are the number of fibers in a single strand of carbon fiber tow. Typically, this is between three to twenty-four thousand fibers per stand. For instance, if you had 3k carbon fiber tow it would have 3,000 fibers wrapped together, if you have 6K carbon fiber tow it would have 6,000 fibers wrapped together and so on. 
Once we have the carbon fiber tow we can do a few things with it. First it can be wound. In this process you would take your continuous carbon fiber tow, draw the tow through a resin bath then wrap tow around a spool to the desired thickness and length. This can be used to make carbon fiber tubes or even tanks. You can also chop the fiber into little pieces. You would then scatter your chopped carbon fiber in a mold and add resin close the mold and depending on the resin used you could heat or simply just wait till the part is cured. 
Lastly there is carbon fiber weave, also known as carbon fiber mesh. In this process you could take the carbon fiber tow and introduce it to a loom. The fibers would be woven together like a blanket. There are several ways to do this depending on the application you are seeking. There is plain weave where one tow is woven over one tow then under the next in both directions. Similarly there is a twill pattern where, like the plain weave the individual tow is woven over and under a tow in the opposite direction. However, with twill we can request a change in pattern. A 2x2 twill for instance will weave the tow under two tow in the opposite direction then over the next two and repeat in both directions. There is a 4x4 twill that would have the tow weave over four tow then over the next four in both directions, and so on. This allows for the cloth to be more formable but has a tendency to fray and fall apart. There is also harness weave which would give you an imbalance on your weave. In the case of “5 harness satin” the tow would be woven over five tow in the opposite direction then under just one. This would be repeated in both directions. This allows for more stiffness in one direction than the other.
There are also unidirectional mats as well. In this case the fibers all run in the same direction. However, because of this there needs to be some type for stitching or a small amount of resin infused as there is no weave pattern to hold the sheet together. Similarly, there are multiaxial where you have multiple layers of unidirectional that do not necessarily go in the same direction. In the case of plus or minus 45 degrees you could have two layers of unidirectional 90 degrees from each other one at positive 45 degrees the other at negative 45 degrees. 

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Methods for Making Carbon Fiber Parts 

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Forged carbon fiber is a process where the single stand caron fiber tow is cut into small pieces then put onto a rigid object that will hold its shape such as a mold, foam block, or sheet metal. The chopped carbon fiber is then mixed with the appropriate amount of resin epoxy then spread over the rigid object to mimic the shape desired. It is then cured to the proper hardness. This is quite an easy way to make carbon fiber parts. However, you lose some of the properties of the carbon fiber because you have to use the strength of the resin to hold the fibers together opposed to allowing the fiber to carry the force. This can be helped by introducing continuous carbon fiber tow in the direction of the forces intended to be put on the part during the initial phase of the process before curing.

Wound carbon fiber uses a winding machine to create tubes and tanks. Continuous carbon fiber tow is wound onto a mandrel after being drawn though a resin bath. The mandrel is first designed and manufactured for the desired shape. You can use extruded materials like aluminum, PVC, or cardboard as long as the mandrel is rigid enough to withstand the pressure. You could even use a material like shaped foam that you intend to leave inside the tube when the process is complete. You then program the machine for the desired wrap pattern. After the wrap is finished the composite is then cured per the specification of your process.

Mesh Carbon fiber is like the forged carbon fiber process except the continuous carbon fiber to is woven into sheets on a loom. The pattern varies depending on application. This process is the most common way to make carbon fiber sheets and it gives the part the traditional woven carbon fiber look. The sheets are pressed into or draped over a mold or rigid part and resin is infused then cured to make the bond. For smaller parts molds can be made and pressure used to force the air out of the resin and bond it between the carbon fiber stands. Another method used on larger parts is to use a vacuum to draw the resin through the carbon fiber.  The mold with the carbon fiber mesh is placed into a bag then a vacuum is applied to draw the resin through the carbon fiber strands. Depending on the method and resin used will determine if the part needs to be cured at room temperature or an oven is needed to speed up the process. The part is then ejected from its forming material.

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