| Ernie
(aka RacerX) Please note that the information
I have provided came from "my research & experimentation, working
experience with people knowledgeable on building homebuilt aircraft's,
speaking with tech reps of fiberglass product manufacturers and of course
technical info sheets gathered from anywhere and everywhere I could obtain
them". Also, to advise everyone that they should gather as much info
as they feel they need to make their decision - not base it on one persons'
info. As I already mentioned I know very little but am always glad to
assist if I can.
Fiber Glassing
Methods -
by Ernie
Bonding
Fiber Glass Doors Skins Together:
The question was asked about how and what type of materials do you need
to bond the fiberglass door skins together…
The glass door skins aren't that heavy but require
substantial adhesion just the same.
Epoxy/polyester resins, both are brittle without
a supporting matrix. Epoxy resin allows for more flex than polyester.
Supporting matrix are fillers, such as glass balloons, phenolic powder,
chopped fiberglass, etc.. Use these as fillers not as an adhesive!
Here's a few products you might look into for bonding
your 'glass door skins over your stock doors:
3M Automix SMC/fiberglass panel adhesive part
#8219; This is a 2-part urethane adhesive use to bond FRP ('glass)
body panels to each other and to metal frames - though the metal needs
to be primed with a good 2-part primer (DP!!) 30 minute work time - handle
the part in 2 hours and cure time is 24 hours
Duramix Universal structural adhesive part #4125;
This is a flexible urethane adhesive for bonding plastic add-on parts
like spoilers, flares, ground effects. It's provides a tough, flexible
bond that resists cracking. 10-minute work time sets up in 30 minutes
and cures in 4 hours. "Duramix" had been purchased by 3M in
August of this year 2002. I haven't heard if 3M plans on keeping Duramix
products separate from their 3M products.
SEM makes a multipurpose adhesive, #39957
that bonds different composites, 'glass, aluminum and many types of rubber.
I don't know how thick this material is.
As with these suggestions, I ask that you please
do further research by asking the appropriate tech rep, gathering the
tech sheet data etc., before making your decisions.
Also, inspect the door skin to check consistency
between 'glass skin and steel door skin. There may be larger gaps that
require a thicker type of adhesive or a non-sag type. Never bond 'glass
using adhesive to a bare steel panel!!!
To comment
about DynaGlass
This is a fiberglass reinforced "body filler"
not to be used as an adhesive!!! It will crack, the same as plastic fillers.
Don't use these as adhesives.
You don't need to drill holes into the steel for
better adhesion - you'll be losing surface area -which you need as much
surface area as possible to aid the adhesion factor. Just sand the metal
with #80 or #180 grit sandpaper (dry - don't ever sand bare metal using
water!) Preferably with a DA sander but hand sanding is satisfactory -
just make sure there are no shiny spots left on the steel. Wipe clean
with lacquer thinner a few times to ensure its contaminant free.
As for the bonded 'glass panels surviving - I don't
believe it will last in the long run due to the different expansion characteristics
- eventually you will start seeing fatigue cracking and stuff. This has
been my experience. Lightweight stuff like hood scoops seem to last a
bit longer because they're lighter - though the hood vibration could cause
the bond to prematurely crack if a poor adhesive was used - such as plastic
filler employed as the bonding agent. Of course, being on the top surface
it gets the most heat from the direct sunlight causing it to expand and
contract more so than glued on 'glass door skins. Color of the car makes
a difference also. Very dark colors are not recommended - lighter colors
are better.
Check with a 3M tech rep for their suggested 2-part
primer. I like to use PPG's DP epoxy primer!
Can vibration
cause the glass body cap to crack prematurely?
My findings on the stress cracking of fiberglass
caps that are used on rebodying cars are not due to vibration primarily
- especially if one uses an adhesive designed for the part and its usage.
It comes from the lack of torsional rigidity. For
example, on a convertible, you can brace, triangulate, etc., a frame structure
to virtually eliminate linear load stress; meaning if you inspect the
gaps at the door and then apply force on the front end or rear, if the
reinforcement was fabricated correctly you wouldn't find a door gap change
weather loaded or not.
Torsional rigidity is something else. I've found
the only way to completely eliminate the twisting action of the vehicle
is to build a complete roll cage such as you see in a "real race
cars" - thus you also take away from the appearance and practicality
of the car for street usage. It is the twisting motion of the vehicle
when you go in and out of driveways that is the culprit of the stress
fractures and sometimes complete delimitation of the body cap!
Even on factory built convertibles you will sometimes
find stress fractures in the lead seams on the tops of the quarter panels.
Even the GTO convertible that our shop recently finished restoring had
the tops of the quarter panels where we releaded the seams stress fractured
due to the going in and out of our shop for delivery to the upholsterer
and just getting the mechanical problems solved. We repaired it once already
and has been holding up but it will fracture again!
You can duplicate the efforts of the aerospace
industry by applying the adhesive with rivets, which will obviously be
a stronger bond and most likely last longer than just the adhesive bond
alone. One mistake some people make is when using rivets you must be sure
to use the same material rivet as the anchor material is made of e.g.,
steel body=steel rivets, aluminum body=aluminum rivets.
When looking to purchase adhesives I suggest you
look at the tech sheet for the product to ensure the adhesive has the
strength properties required for your usage. There are three basic strengths
you must observe:
Peel strength = just as the name implies, peeling
the bonded panels apart at one end
Shear strength = pull and push loading. Imagine
sliding bonded panels in opposite directions.
Tensile strength = the pulling apart of two bonded
panels
I'll give you an example of an adhesive that I
use. You've probably heard of the "Stud Gun, Spitznegal" etc..
This is a device that will weld a metal stud onto the metal panel of a
car to assist in the pulling out of a dent. Then you cut off the stud
and grind the remaining head of the stud from the panel. This is an improvement
over the drill a hole and then screw in the dent puller, then weld up
the holes.
I use an adhesive that has tremendous tensile strength
- which I apply a drop of the adhesive onto the panel I need to pull on
- wait a few moments, stick the metal stud into the drop of adhesive -
wait a few more moments. I then take my stud puller and attach it to the
stud and pull out the panel. Then I take a metal spreader or scraper and
just peel/scrape the adhesive drop and stud off the panel. Nothing to
grind off (which is a bad thing to do to metal panels) or holes to weld
up.
This is a great adhesive for this but definitely
wouldn't use it for bonding flares and stuff!
So, take your time and research the material you
need.
If I may offer
a primer on how adhesives works (I feel
this may help in your search for the appropriate adhesive)
Adhesives need to perform two contradictory functions
during their operating cycle. When the adhesive is applied it needs to
make essential contact with the surface or using the industries term "wet"
the surface.
The adhesive needs to be mobile enough to flow
into all the crevices and corners of the substrate. If the adhesive doesn't
wet well onto the substrate, poor adhesion is the corollary. A good example
of poor wetting is the way beads of water run off on a freshly waxed car.
Once you achieve good wetting, the adhesive needs
to cease flowing and become solid. This is called setting or cure. There
are a variety of ways to achieve this. Heat catalyzed adhesives solidify
when they cool down. Structural adhesives used for permanent assembly
of composites cure through chemical reaction (cross linking). The final
cured adhesive becomes a solid, tough, stable medium that tenaciously
bonds two surfaces together.
But you must understand that even the best adhesive
may fail if placed in unworkable joint design or if applied 2 inches or
more away from the intended bonding area. The most important rule is to
have adequate surface contact area.
One other note is that in the real world the loads
placed on the bonded panels hardly ever consist of one stress type, but
rather two or more stress load: Peel, shear, compression and tensile loads.
But the design of the application can be optimized to minimize unwanted
loads.
Types of Adhesives
One part urethanes are usually used as sealants.
These products cure by ambient moisture absorption and their cure rates
can vary in relation to humidity. These cure slower than two part urethanes.
Common applications for one part urethanes include
marine, bath and shower products and in construction.
Acrylic adhesives are less common than other adhesive
families, at least in my experience. Acrylic adhesives do offer the fastest
ambient room temp cure while at the same time offering a suitable work
time. They will also bond difficult substrates with the minimum amount
of surface preparation. They're easy to use and will work just as well
even if there is a slight variation in the mix ratio. These products are
highly flammable and emit a certain odor.
Two-part urethanes adhesives have just recently
been given more exposure. These are available in a variety of cure rates
and physical characteristics; from flexible to very hard. Two-part urethane
adhesives are usually best for bonding incompatible substrates (different
materials). They don't have the best heat resistance so keep in mind that
you may not want to use adhesive in your engine compartment applications.
Epoxy adhesives have a wide application usage.
It cures at ambient room temp or with a mild heat bake cycle. Using heat
to aid the epoxy curing significantly improves the bonding properties.
It has low shrinkage during it cure cycle and is resistant to water and
other environmental conditions. The disadvantage of epoxy adhesives is
that it usually requires meticulous surface prep and the mix ratio has
to be exact!!
My understanding of your mount setup is that it
is a joint design. Can it be bolted together somehow? My concern is that
even the best adhesive is likely to perform poorly when used in an application
that is designed for mechanical fastening. Mechanical joining methods
like bolting, welding, riveting are great at carrying tensile and peel
loads but adhesives usually perform their best when loaded in shear and
compression.
If you're interested in converting joints that
use the conventional fasteners to one designed for adhesives - it doesn't
require too much.
You will need to consider stress loads; tensile,
compressive and shear can be easily visualized, but peel loads might not
be too obvious. A peel load is created when bending occurs in one or both
of the joined materials. This generates an extremely concentrated area
of load stress at the bond edge. Tensile loads are often accompanied by
or eventually turn into, peel loads. This is especially true for flexible
substrates.
If I may offer some things to take into consideration
for your adhesive design usage:
- the operating temperature and exposure to the
environment
- the expansion and contraction characteristics
between the dissimilar materials
- the strength of an adhesive in one type of joint
design may be entirely different when the same adhesive is used in different
joint designs
- thinner applications of adhesive layers will
produce stronger bonding joints than thick adhesive layers. There must
be enough of an adhesive layer to wet the intended bonding surface
- make it a point to design the joint for predominantly
shear and compressive loads
- use the maximum bonding area
It is important to test the adhesive for its validity
in a particular application. A common error is to evaluate an adhesive
product using a testing method that is quite different from the actual
usage.
Testing should be done using the same materials
as the real thing. Conduct the testing under the same environment and
temperatures, which it will actually be used.
In reference to your bonding the perforated angle
iron to the fiberglass, it would be advisable to use solid angle iron
because you have more surface bonding area.
Also, if possible, it would make a much stronger
bond if you bonded a sheet of metal or aluminum that would cover at least
twice the surface area of your angle iron, onto the 'glass, then bond
the solid angle iron onto the metal sheet.
Just a thought or perhaps I've misinterpreted your
design?
I just wanted to inform you that you can recoat
polyester resin base 'glass with polyester or epoxy resin. You can recoat
epoxy resin based 'glass with epoxy but "not" with polyester
resin. FYI
Carbon Fiber
Carbon Fiber layup can be composed of a variety
of "weaves" depending upon the part and it's usage etc:
- Carbon Fiber "Plain"
weave usually is employed when fabricating lightweight non structural
parts.
- Carbon Fiber "Twill"
weave is slightly stronger than the plain weave and is commonly used
as the "cosmetic weave" due to the unique appearance desired
by those who wish to display their carbon fiber parts. You'll see this
weave used on hoods, after market interior trim, engine compartment
accessories etc.,
- There's also the Carbon Fiber "basketweave"
which has twice the fiber density of the plain and twill weaves. This
particular fabric weave is superb in structural applications.
I believe the roof and doors on the "real"
Lambos are aluminum whilst the other components are of the composite variety.
Carbon Fiber construction especially the method employed in the construction
of structural components is going to vary from each manufacturer from
hand layup to applying C-fiber over shaped aluminum panels to autclaved
epoxy pre-impregnated fabric to carbon fiber sandwiches; this is a method
where carbon fiber is employed on both sides of a supporting structural
panel, e.g., a fiberglass honeycomb panel, used primarily for maintaining
lightweight but with tremendous rigidity. I do recall a few years ago
that my instructor mentioned that in order for carbon fiber to be impact
resistant, it must be plied with fiberglass (composite?). I don't know
if that is still true due to technological progress.
The clear coating that you see is from the epoxy
resin. As with any hand layup it is very difficult to control the resin
saturation onto the fabric, be it carbon fiber or fiberglass but in either
case the resin is what gives you that clear look. You can even sand the
resin after it as cured and clear coat it with automotive 2-part urethane
clear then sand/polish it to achieve that extra glass smooth depth. I've
done it for some customers that had carbon fiber pieces for their race
car.
It is the automotive clear coat that you sand and
polish - of course you will sand the resin prior to topcoating with the
automotive clear! You can also purchase carbon fiber board from Tap plastics
- comes in sheets cut to your specifications - be forewarned - it won't
be cheap! Tap Plastics recommend using spray adhesive for the c-fiber
board to cover your aluminum part - then they recommend going over it
with their resin. I have just clear coated directly on top since the piece
we made was strictly comestic anyway. Im' helping my brother-in-law with
a project of his - which is to cover his aluminum airbox with the c-fiber
board and we're using the Tap Plastics process.
You will have to clear coat the c-fiber coat a few times though - sand
and clear, sand and clear, sand and polish. The reason for this is because
the initial clear coats you apply will not saturate the weave tows (intersecting
fibers) completely and will leave you with what looks like fish eyes -
when actually it is the indentation of the clear filling the gaps between
the weaves. Then you'll have to sand the clear as flat as possible then
reapply the clear again to allow for more film build. Then when you come
back to sand - it will have enough film build that you can then polish
to a nice finish. You'll have to see some of the sample pieces I've done
- not just c-fiber but other painted items. By the way, I've painted many
a unique item - from pearlescent on dishwasher door, oven, refrigerator
and trash compactor for a interior designer doing a custom kitchen, to
custom built railings for a stereo company painted in glow in the dark
neon, and the most unique is the walls of an entire room done in dark
gray metallic and clear coated with neon, candy and pearl music notes
and related symbols for another stereo company - a week later someone
keyed the walls!!! What a shame. I'm doing some R&E on the flexing
limits of the new urethane clears without the flex agent additive - I
don't believe in using flex agents anyway.
"If you are planning on fabricating a lightweight
non-structural part from carbon fiber, perhaps something comestic, I would
suggest that you use epoxy resin. You can use the marine type epoxy resin
for the cosmetic pieces but when it comes to fabricating a structural
part you MUST use a carbon fiber specific epoxy resin and ask a professional
for advice. You will need to understand the layup process of incorporating
the different type of fiber weave such as unidirectional and multi-directional,
when combined can offer substantial strength but again you will also need
to understand the directional stress factors of the structural part you
are fabricating. Again, this is from my own personal research and experimentation.
I make no claim on being an expert on carbon fiber."
Forming Thermoplastic:
For those of you interested, here is basic info, procedure, and techniques
for forming thermoplastic.
Transparent acrylic plastics will get soft and pliable when they are
heated to their forming temperatures and can be formed to almost any shape.
Before you heat any transparent plastic, be sure to remove all of the
protective masking paper and adhesive from the plastic. The plastic should
be free of dust and dirt. If the plastic is dusty or dirty, wash it with
clean cool water and rinse it well. Dry the sheet thoroughly by blotting
it with soft absorbent paper towels. Do not wipe back and forth or up
and down because of the possibility of scratching the plastic if there
is any residual dirt that was missed. I like to use Scott brand blue shop
towels.
A couple of notes:
- Wear cotton gloves when you're handling the plastic to prevent leaving
finger marks on the surface.
- Never use hot water or steam directly on the plastic to heat it because
it will cause the acrylic to become milky or cloudy.
For small forming jobs, such as headlight covers, you can use the kitchen
oven. You can also use infrared heatlamps, like the ones used in paintshops
to fastdry paint spot repairs. I have a single filament infrared lamp
about 28 inches in length. This seems to work well for me.
Heated acrylic plastic will mold with almost no pressure when heated
to its forming temperature. So, the forms used can be made of a variety
of simple constructed materials. For simple curves you can shape forms
from plaster, plywood, or pressed wood. (I may try balsawood as an experiment.)
For complex or compound curves you'll need to use reinforced plastic or
plaster.
Hot acrylic plastic will conform to any waviness or uneveness, so be
sure the form being used is as straight or smooth as possible. To help
alleviate this, sand the form smooth and cover it with a soft cloth like
flannel or felt.
The acrylic plastic should be large enough to extend past the trim line
of the form being made. Also, you must have a method of holding the heated
plastic against the form as it cools.
Here is the procedure I used for making headlight covers for a home built
airplane. This method would work if you wanted to make acrylic headlight
covers for the stock Z headlight buckets.
Fill the headlight bucket with plaster so you have enough material to
start sanding to shape. I use #80 grit sandpaper on a pad and sanded the
plaster until I was close to the desired shape. I then switched to #180
grit sandpaper and did the final sanding to shape.
I then wiped the mold (form) with a damp sponge and blot dried it with
papertowels. Then I took my clear acrylic sheet, which was larger than
the mold and balanced it on the form. My heatlamp was warmed up and I
placed it over the plastic and the mold. After a few minutes I watched
as the plastic started to soften and actually draped itself over the headlight
bucket. Then I shut the heatlamp off and allowed the part to cool. (Do
not force cool, allow to cool naturally.) You can also use a kitchen oven
for this. After the plastic cooled, I scribed my trim line and pulled
the plastic from the form.
Acrylic Plastic:
If you are planning to make a acrylic plastic lens to cover the entire
stock rear expanse after removing the stock tail lights and finish plates
- you wouldn't need to use a form mold for this. There is a subtle curvature
but due to the length of the plastic panel I feel you wouldn't have a
problem with bending the plastic panel to fit unless you're going to use
a 1" thick or thicker piece of plastic.
This insulating foam you mention, is it the stuff you spray out of a
aerosol can and then it expands and then hardens when dry? If so, No,
you cannot use this for form molds. It will melt due to the heat required
to form the acrylic plastic.
Here some heating info:
Acrylic plastics become thermoelastic (stretchy) at about 120 degrees
F -
so watch out where you store your acrylic plastic sheets!
Forming simple curves require approximately 235 degrees F -
while stretch/mold forming require approx 285 degrees F using a 0.125
sheet thickness
If you decide to use 0.250 sheet thickness:
Forming simple curves require approximately 275 degrees F -
while stretch/mold forming require approx 300 degrees F.
Be sure to keep an eye on the plastic sheet as you are forming because
every heat source is different in its displacement of heat. You may have
to move the heat source or the part you're heating further away or closer
to each other depending on how the part is reacting.
Always do a test panel!!
Finally, do not rub plastics (such as your tail light lens) with a dry
cloth - like after you've washed the car and you are wiping it down with
a terrycloth towel. If done over a period of time without dressing the
plastic it will cause scratches and result in a dull finish. Also, you'll
build up an electrostatic charge that will attract dust particles to the
surface.
My suggestion would be to polish the lens with a good plastic cleaner
follwed by a plastic polish if your lens are slightly dull. Use a soft
cloth (used, clean, baby diapers from a diaper service work well) Then
follow up with a good wax or sealant (no glaze!!! - stuff doesn't work
well to me. If you're lens are pretty dull, then you'll need to get out
some sandpaper, sand down, and get some rouge for plastic polishing and
a polishing wheel on a bench grinder and get to polishing! Or another
alternative instead of using a polishing wheel is to actually spray an
acrylic clearcoat on the lens after you've cleaned the part well, then
sanded with a grit appropriate for your clear coat you will apply.
Thanks so much for trusting me in what little I
know. I need to inform you that the information I provide is based solely
on my personal experience, training, observation/opinion and R&D.
Although I make every effort to provide you with accurate info I may not
always fully understand the background to the questions and therefore
not able to promise/guarantee the accuracy of my answers/information.
So, I 'm asking that you please accept my answers/info as a guide and
I urge you to PLEASE make additional inquiries before you pursue a course
of action to resolve your situation and not just based on the information
I provide. Thanks again for allowing me share info with you guys.
Hope this helps,
RacerX - (Ernie) |
