1. Prepreg
• Prepreg" is the common term for a reinforcing
fabric which has been pre-impregnated with a
resin system. This resin system (typically
epoxy) already includes the proper curing
agent. As a result, the prepreg is ready to lay
into the mold without the addition of any
more resin. In order for the laminate to cure,
it is necessary to use a combination of
pressure and heat.
2. Advantages of Prepregs
• There are several advantages to using a prepreg
rather than using traditional hand layup:
– Maximum strength properties. In a hand layup, it is
difficult to achieve 50% resin content. This means that
the finished laminate weight is 50% fabric and 50%
resin. Typical hand laminates, even when vacuum
bagged, end up with a significant amount of excess
resin. Excess resin increases brittleness and reduces
overall properties. On the other hand, most prepregs
contain around 35% resin. This is ideal for maximum
cured properties and generally impossible to achieve
in normal hand lamination.
3. • Part uniformity and repeatability. Without the pitfalls
of human lamination techniques, there will be
neither resin-rich areas nor dry spots. Thickness will
be uniform and every part that comes out of the
mold has a theoretical likelihood of being identical.
There is still a margin for error in vacuum bagging
techniques, handling, etc., but prepregs reduce these
problems significantly.
4. Advantages of Prepregs
– Less mess and less waste. Prepregs will bleed
excess resin during the curing process but all of
the excesses of hand layup – cups of resin, messy
rollers, drips – are no longer a problem. Plus, Fibre
Glast prepregs are handled at room temperature
so you are not fighting a clock trying to avoid your
resin setting up before you are ready.
5. – Less curing time. After the heat curing cycle is
completed, the part is ready for service. You do
not have to wait the standard 48 hours to allow a
full cure as in a typical hand lamination.
– Better cosmetics. Mold preparation and mold
release is still required and will directly affect the
cosmetics of the part just like a hand laminate.
However, Fibre Glast prepregs virtually eliminate
air bubbles and a smooth, glossy surface is more
easily attainable.
6. Disadvantages of Prepregs
• Cost. Prepregs are pricey. Even when you add up the cost of
the resin, cure and fabric, prepregs still cost more.
• Shelf life. This is less of a problem since Fibre Glast prepregs
can be stored up to six months at room temperature (75°F).
Nonetheless, heat cures prepreg and storage at warmer
temperatures will reduce the shelf life. Keeping the
material cooler will help and freezing will extend the life
significantly.
• Necessary heat cure. There will have to be a heat source
and vacuum bagging at a minimum. You must be able to
achieve a minimum of 270°F and sustain that temperature
for a minimum of four hours. Many advanced fabricators
use autoclaves, but any source of heat will do.
7. How do Prepregs work?
• Photo credit IStock Photo.Fibre Glast prepreg fabric will be
shipped to you sandwiched between two layers of backing
cloth. This cloth keeps the prepreg from sticking together.
Peeling off the protective backing is easy, and the material
is immediately ready for layup.
• To completely cure the prepreg, heat and pressure are
required. Although the pressure could be achieved using a
press, the more typical method is vacuum bagging. After
the part is laid up, it must be vacuum bagged with all of the
usual elements – a bag to hold the vacuum, peel ply so the
bag does not stick to the part, and bleeder/breather cloth
to absorb the excess resin. Additional information about
vacuum bagging can be found in our white paper Vacuum
Bagging Equipment & Techniques.
8. • The ideal curing environment would be an autoclave
which has both temperature and pressure controls,
although prepregs are commonly cured otherwise.
• The heat requirements generally require an oven in
order to control temperature ramp up, ramp down,
and to sustain a controlled, uniform temperature for
the recommended duration. Ovens used for curing
composites should never be used for cooking food as
there will be contaminants released.
• Heat lamps can be used to create enough heat, but the
trial and error associated with controlling the
temperature will cause problems
