[alfa] Carbon fibre body panels- long....

[Mark Battley <M.Battley@xxxxxxxxxx>]

John, me thinks your contact does exaggerate a little...

Carbon fibre refers to the fibres, you work with these in basically the same
way as you do with any other fibre, be it glass, synthetics such as aramid
(Du Pont's version is Kevlar), exotics lke Boron etc. One point to note is
that there are a huge range of types of carbon fibres with different
strengths and stiffnesses - and cost!

To make a useful material out of them you have to combine them with
(typically) a resin - often polyester, vinylester or epoxy.

For high end applications where good weight control and laminate quality are
required (aerospace, some motorsport, most - although not all- America's cup
yachts, yacht masts, fishing rods etc), prepreg is often used. As you say,
this has the fibres already impregnated with a partially cured resin. This
is stored refrigerated, however there are a lot of different combinations of
fibres and resin types, the period that it can be out of the fridge varies
from hours to weeks. Laying up a large structure like a boat hull can take
weeks, the prepregs that are used for this have resins designed to be okay
at ambient temperatures for the required period. The costs of the prepregs
vary hugely depending on the application and market, and they don't have to
be high for commodity materials. You can also buy prepreg glass fibres.

In general to cure the laminate you want heat and pressure. It depends on
the applications (and hence the resin type) how much heat is required, could
be 80 deg C, 120, 140, or more. The key to operating at high temperatures
(such as a missile) is the resin type, NOT the fibres. Pressure can come
from a vacuum bag (plastic bag over the laminate with a vacuum pump), an
autoclave (sealed chamber with pressure and heat, which also has a vacuum
bag on the laminate), or even shrink wrap tape in the case of fishing rods.

HOWEVER - you can also make laminates out of carbon fibres using wetlayup
(as people are used to for glass fibres)- add liquid resin to the fibres,
roll it or similar to impregnate the resin around the fibres, it may cure at
ambient temperatures or with heating. Depends on the resin you choose.
Sometimes use a vacuum bag for pressure, or a two part mound, or nothing on
the top surface. Or you can use methods like resin infusion where you put a
bag or mould over the part and use pressure to suck of force resin through
dry fibres. The manufacturing process doesn't really care what the fibres
are...

Part of the confusion arises because many people think of fibreglass in its
most simple version of spray up short fibres in a resin. This is relatively
cheap, but not very strong or stiff. However you can also use glass fibres
as cloth, or unidirectional fibres, just as you do with carbon fibres.

Yes there are some versions of resins or glass prepregs that use a black
coloured resin to "look" like carbon fibres. And there are stick on labels
that look like carbon fibre. And John may be right that some products are
not really carbon fibre, or may just have one layer of carbon on the
outside.

However it is possible to make carbon fibre body panels that don't cost
silly amounts of money. Part of the cost is the resin, which will be the
same or similar if it's glass or carbon. As a very crude indication, I did
some quick numbers on the material cost for a kg of laminate made from the
types of standard grade carbon and glass cloths that are used in the marine
industry, with a generic epoxy resin - a kg of laminate (fibre + resin) came
to about $US15/kg for glass fibres and $60/kg for carbon. Carbon prepregs
can be bought for a similar price as my wetlayup example. The difference in
cost will depend on the manufacturing method - what hardware and consumables
are needed, labour required etc, but the manufacturing method may be the
same if it is glass or carbon fibres. Where comparisons get messy is if one
compares low end methods such as chopped strand sprayup (which don't
generally get used for carbon because it's a waste of the good, more
expensive material) to high end methods such as autoclaved prepreg (where
the manufacturing cost is high enough that there is no point in using cheap
materials).

Since many panels may only weigh a few kg, and the material cost is only one
part of the final manufactured product cost, the difference between the cost
of a glass fibre or carbon fibre panel manufactured in the same way NEED not
be huge - for a 3kg panel using my crude numbers the difference in material
cost would be $US135. Doesn't mean that a manufacturer and/or retailer won't
ask a lot more for a black one though...

Phew.

Mark.

Dr Mark Battley
Research Engineer, Composite Materials and Structures
Industrial Research Limited
Auckland, New Zealand

1973 Alfa Romeo 2000 GT Veloce
1998 Alfa Romeo 156 TS
1999 Fiat Marea Wagon, 2.0 20V
1989 Fiat Uno Turbo


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Date: Tue, 27 Jul 2004 16:44:55 +0200
From: "John Fielding" <johnf@domain.elided>
Subject: [alfa] Carbon fibre body panels

Hi All,

I see this thread growing about new bonnets/hoods for Spiders etc.

A little while back I was attending a race meeting at Kyalami race track
near Johannesburg
and I was introduced to a guy who makes the carbon fibre components for the
top slaoon car
teams.  What he told me was very interesting!

True carbon fibre material is normally supplied in a version known as
"pre-preg"
(pre-impregnated) meaning it is already saturated in the special resin that
is used for
bonding.  This pre-preg material is shipped in an insulated styro-foam cool
box and
carried in refrigerated cargo vehicles, the same type as used for hauling
meat carcasses.
In store it needs to be kept at below -10C or it goes off after a few days.
The major
supplier is based in Germany and ships all over the world.  The shipping
costs of a roll
about 1m long is very high because of the special refrigeration airplanes
needed.  The
cost of a square foot of one layer of pre-preg would buy a good quality
tyre!

To form a body component the layers of pre-preg are cut to size and laid
criss-cross
fashion into a mould and heated to around room temperature so that the resin
oozes out and
into the next layer.  Then it is placed in a special vacuum bag and pumped
down to press
the layers together hard.  Then it goes into an autoclave oven to cure it at
high
temperature.  The result is a very light but strong thin wall component.
The company I
used to work for made avionics for missiles nose cones and they bought in
carbon fibre
nacelles because of the very temperatures experienced by a missile
travelling at
supersonic speed.  Fibreglass crazes and burns at this air speed.

He told me the vast majority of components on the market today which sell at
speed shops
aren't carbon fibre at all.  They use a dummy outer layer of special glass
fibre made to
look like carbon fibre.  He showed me some of the spare body panels he was
delivering to a
top team for the V8 class.  One front wing/fender cost about the price of a
small saloon
car!

So I guess the hoods being offered at the prices quoted are not genuine
carbon fibre but
imitations largely with a glass fibre core and made to look like CF.

Just thought you would like to know!

John
Durban
South Africa

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