UNDERSTANDING CARBON FIBER
Composites are created from two or more dissimilar
materials that act together as one. While concrete
and plywood are technically composites, the term
composite in industry has come to refer to reinforcement fibers held together in a resin matrix
and formed in a mold. Carbon fiber is one of several
textiles used in this class of materials. When joined
together through a procedure called a layup, the
fiber and the resin form a material with properties
that exceed those of either constituent material.
As a rule of thumb, composites offer their greatest
strength in the direction the fibers run — similar to
how wood is strongest along the grain. Because of
this, you can “tune” a composite’s strength characteristics by controlling and combining the directions
of the fibers. If you want strength over the length of
a part, simply align the fibers lengthwise. Likewise, if
you’re making a tube that needs torsional (twisting)
strength, it’s best to arrange the fibers helically,
like springs, weaving them together with opposite
The table below describes the three most common
forms of manufactured fibers: woven, unidirectional,
and filament. Each general type can be produced
from carbon, fiberglass, aramid (Kevlar), boron,
basalt, and several other materials, which are chosen
according to their particular physical properties.
Comes in rolls and resembles the
thick nylon fabric used in trampolines.
Weave styles vary according to the
fibers’ directional alignment, drape
(how well the cloth conforms to mold
surfaces), and wet-out (how easily the
fibers can be infiltrated by resin).
Comes in rolls and resembles a very
fine, wide paintbrush. Composed of
parallel fibers that are intermittently
joined over their width by thin resin-coated fibers that keep the strands
aligned into an easily usable form.
Comes on reels as continuous strands
of fibers, loosely gathered into a
thread. It can be easily unwound and
placed wherever necessary in a layup.
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Resin is available in hundreds of different types,
each with its own chemical and physical characteristics. In general, polyester, epoxy, and vinylester
liquid resins are the most widely used in composite
You can also buy pre-preg fiber, which already has
the resin mixed in. Pre-preg is easier and less messy
to use, and it doesn’t waste resin, but it’s also more
expensive, is more difficult to obtain, has a limited
shelf life, and comes in limited resin types. Most pre-preg comes shipped cold and must be used immediately, or can be frozen (to retard curing) but must
be used soon. Another type, elevated-temperature
cure pre-preg, ships normally and is cured in an
oven or autoclave. Molding a composite using pre-preg is known as a form of dry layup, while starting
with separate fiber and resin is called wet layup.
When constructing high-performance composites, designers attempt to do several things at
once: orient the fibers for highest strength, inhibit
delamination (the peeling apart of fibers), and
ensure dimensional accuracy. In addition, they try
to minimize voids and bubbles in the composite,
which weaken the structure, and keep the resin-to-fiber ratio down to somewhere between 50/50
and 40/60, which is the optimal range for strength
and lightness. Many automotive enthusiasts who
purchase carbon fiber hoods for their cars complain
that they often weigh more than the original steel.
This arises from too much resin, either in a thick,
glossy top coating called a flood coat, or in a
generally poor layup.
MAKING THE CASE
We’re going to create our own composite iPod cover
using wet layup, and compression-mold it by using
C-clamps. This relatively low-tech method proves
successful for making flat shapes out of sheets of
material. For shallow rounded forms, you can compress the shapes under plastic bags filled with sand,
clamped between wooden boards. Since the iPod
has a simple rectangular shape, we can create a
nicely consolidated, smooth-surfaced shell by using
an internal mold (or mandrel) with blocks clamped
on its large sides. Although this demonstration explains how to make a case for an iPod mini, you can
use this same method to make a hard, lightweight
case for practically any device that has a uniform
cross-section over its length.