Carbon Fibre Material is Available
in Many Forms.
The shape of the Carbon Fibre itself is determined by the original material and by the manufacturing process. The actual fiber cannot be greatly modified AFTER it becomes carbon fiber. Manufacturing processes vary somewhat but essentially one of several precursor materials is formed into a fibre and carbonized in a number of steps under carefully controlled conditions.
Once the fiber has been made it can then be put together in many ways depending on its end use.
Direction of Fibers VS Properties
The properties of carbon fibre that make it an exceptional material (in particular rigidity) are very different depending on the orientation of the Fibre. Because carbon fibre is much stronger/more rigid in the direction of the fibers than across them the orientation of the fibres in a composite has significant effect on the characteristics of the final construction. That's a long winded way of saying that for maximum rigidity and strength the fibers need to be carefully lined up.
Successful manufacturers and builders will need to analyze stresses and build accordingly. Home builders are at a disadvantage and have little choice but to either overbuild and be willing to live with a heavier or stiffer part than required, or can take a chance and let experience be their guide. Many supplier of carbon fibre will be able to direct you to people who can advise on proper use.
Carbon Fibre Comes in Many Shapes and Forms
After manufacturing, the fibers are then used to make many different products. Carbon fiber is offered as:
- Woven Cloth
- Unidirectional Cloth
- Matting or Felt
- Sleeves (Socks)
- Chops and Powder
- Pre impregnated (PRE PREG)
- Hybrid (combined with other materials
- Coated to modify the characteristics
- Carbon Nanotubes are beginning to be available. Play with carbon from Australia see sidebar offers it.
The type of carbon fiber used depends to a great extent on the type of manufacturing process planned. The choice also depends on what parts need to be reinforced.
1- Woven Cloth
Talking about woven cloth can get very complicated because a great number of variations on this theme.
Cloth comes in many Weight. The weight can be expressed in ounces per square yard or in grams per square metre.
1 ounce per square yard = 33.91 grams per square metre
1 gram per square metre = .02949 ounces per square yard
There is a wide range of weight available. Home builders usually deal with 3.5-12 ounces per square yard. but other weights are possible.
When deciding on a weight of cloth several criteria need to be considered, some are:
- Ease of handling. This can be quite an issue for home builders and it is often a better plan to put 2 layers of lighter material rather than one layer of thicker and less easy to handle weight.
- Shape of the part being built. Thicker heavier cloth is harder to form around tighter curves.
- Amount of matrix needed to fill in the weave properly. Usually epoxy, but sometimes Polyester or vinylester resins.
- Cost, since carbon fiber is often used for "cool value" sometimes it is only used as a thin layer on the visible outside an the inside hidden layers are cheaper fiberglass.
Woven cloth can be made with the same fiber weight running both ways or can be made with stronger fibers lengthwise (warp) and smaller fibers in the width (fill)
There are many types of weave. Most home builders are familiar with the plain weave (left). It is the easiest of weaves to wet out.
Twill weaves (right) are also available. Twill weave usually has the fibers spanning 2 fibers. It makes it easier to form to a shape. Twill weave is reputed to be slightly stronger than plain weave. It is certainly very showy.
Even Showier are the patterned weaves beginning to be offered. See side bar for a sample offered by Fiber Glast
Because the fibers bend up and down as they follow the weave there is a slight decrease in strength when compared with the uniaxial cloth that are made so that all the fibres run one way (except for the few that keep the cloth together.)
Unidirectional cloth is used to reinforce a structure in one direction. Usually lengthwise.
Long strands are laid out and stitched with small thread to keep them in place or are woven with with very small thread in the non important direction.
Unidirectional cloth comes in many weight. It is said to be stronger than woven cloth. Because it concentrates reinforcement in the direction needed, using it often lightens the structure because unnecessary fibers are eliminated. Because the fibers don't ride on each other, as in a woven cloth, there is less chance of sanding off the top layer when finishing, thus severely damaging the strength. It is not as flexible as woven cloth and is sometimes more difficult to place and drape in position. It is also sometimes more difficult to wet out and lay flat. Vacuum bagging is often suggested.
Carbon fiber is offered as bundled fibers. These are not woven but rather are bunches of unidirectional filaments.
The terms Tow, Roving and Strand is more a measure of how large the bundle of fiber is than of actual differences in the form.
The tow of a bundle of carbon fiber is a measure of its number of filaments per bundle.
Small Tow has 24000 or fewer filaments, Large tow has 48000 to 320000 filaments and sometimes more. IT is sold in large spools.
Often Carbon tow is used in winding tubes and other structures, selectively reinforcing structures and used in compression moulding. It can be used in chop guns but fibeglass can usually be substituted effectively.
5- Matting or Felt
It is possible to find felted Carbon fiber but its use is not for reinforcement of matrix. IT has uses in heat protection and safety applications. Home builders looking for a matted material should use fiberglass.
When a part is to be manufactured with a specific shape, it is sometimes economical to create a dry assembly of carbon fiber in the rough shape of the finished part. This assembly is usually stitched or glued or somehow stiffened so it maintains its shape.
The preform can then be used and impregnated with its matrix without having to worry about the fibers staying in position and thus simplifying manufacturing.
Used for selective reinforcement of carbon fiber parts. It can also be used to wind and join. it's much easier to buy in this form than to cut your own strips. IF you decide to cut then laying out a strip of masking tape and cutting in the middle of the tape saves trouble with unravelling. If used immediately the tape glue dissolves when the carbon fiber is impregnated with epoxy and the tape comes off easily. Test your tape before doing this.
8- Carbon Fiber Veil
Used in making corrosion resistant barriers and for grounding composite structures and minimizing static electricity build-up. Nonwoven structure allows a resin-rich surface that increases chemical stability and reduces micro-crack formation in the composite surface. The weight is minimal, .2 oz.
9-Carbon fiber sleeves or socks
These are available in 2 forms, either biaxially woven, or unidirectional, with minimal stitching to keep the tube together until impregnated. These are very useful if making tubes or round pipes or masts.
In the woven tubes, the fibres end up running diagonally along the tube. Woven tubes are very adjustable and can be used for several sizes simply by compressing lengthwise or stretching. This allows for tapered construction or for curves.
Unidirectional tubing has the fibers running lengthwise only. This allows the composite to be reinforced in the longitudinal direction. Uniaxial tubes need to be bought in the right size since there is no stretch.
Using these tubes simplifies making pipes, tubes and dinghy masts but they are quite expensive.
10- Chopped Carbon Fiber and Powder
Using Chopped carbon fiber is of no particular advantage in a composite since Glass fiber can be used at much better price for similar performance. Chopped carbon fibers are sometimes used to improve conductivity of a material such as plastic, or a coating. It also finds uses to reinforcing concrete.
Carbon Fibre powder finds uses in filling plastics. It is used to change electrical and thermal conductivity as well as increasing dimensional stability.
11- Impregnated Carbon Fibre or Prepreg
Either woven, uniaxial sheet or tape, or roving can be impregnated with a polymer matrix resin. This is referred to a prepreg. Matrices are typically thermosetting resins in a partially cured form. Either the prepreg is kept at room temperature and needs heat to finish setting, or it is shipped refrigerated and requires less heat to set.
Prepred can come in many different forms.
There are many advantages to using prepreg carbon fiber. It is easier to handle with no worry about preparing and getting the epoxy ratio right. It is clean and does not drip. There is usually less worry about the composite setting prematurely since it does not set until heated.
Cost is a deterrent for the amateur as is the need to heat for curing, or keeping the prepreg cold until use. There is a limited shelf life as well.
12- Hybrid or Combinations
Carbon Fibre is sometimes combined with other fibers to modify its characteristics or to reduce cost.
One of the most common combination is with Kevlar. Since Kevlar can be dyed, several companies now offer coloured carbon fibre - Kevlar fabrics.
When combining carbon fiber with metal it is good to remember that it is electrically conductive and can be involved in galvanic corrosion. The so called Silver Carbon Fiber is actually aluminium and fiberglass.
Carbon fiber is sometimes combined with Polyester or with Fiberglass.
13- Coatings and surface treatment
In order to improve adhesion to the resin matrix Carbon Fiber is sometimes surface treated. By partially oxidizing the fiber to make it more chemically active. Other coatings or treatments exist to modify the fiber's handling such as polymer coatings.
email me if you find mistakes, I'll fix them and we'll all benefit: Christine