Ken Mintz Composite Information pg.6 ==
Page 1 Introduction=================================
Page 2 A Short Description of Fiber Types and Properties=====
Page 3 Advantages/Disadvantages of Composites===========
Page 4 The Matrix===================.==============
Page 5 Setting Up The Shop===========================
Page 7 The Core and Its Preparation.====================
Page 8 Hot Box Pictures and Dimensions.======.==========
163 Home Page====================================
Composites Corner No. 6 - Preparing to Layup and More Tools
This time I am going to start becoming more detailed in the description of the composite fabrication process.
Specifically I will describe the glassing or layup process and the considerations that one should take while
executing this task.
First make sure that you have available all of the materials and tools necessary to complete the job. For instance
you do not want to run out of resin part way through a layup of a composite part. You do not want to run out of
glass or other fiber for the same reason. It is unlikely that you could get more of either before your part becomes
hardened junk. Also not having the tool you need when you need it can cost you your part. In most cases you
will need to have your cloth cut to the proper shape and fiber orientation before you start the layup. Have them
layed out in the order in which they will be applied in advance. This especially true for large layups like wings
and fuselages. Sometimes it may be necessary to cut the cloth as you go but not often. Handling cloth with
gloved hands stickey with resin can become a subject of a Three Stooges movie. Have plenty of disposable
gloves available since you will find it necessary to have unstickey hands when positioning the cloth layers and
will have to change them often. Bare hands? No unless you like the feel of MEK or acetone attacking you skin
when you try to remove the resin. Remember the part about developing allergic reactions to the resins as well.
Remember that once the resin is mixed the clock starts running and you do not have forever to use it. Most
composite parts must have all of the fiber cloth ( usually fiberglass ) layed out and resin applied in one session.
This ensures that the part is the bonded single piece unit that is the hallmark of the composite process. You will
join individual parts together later on in the building process by sanding the surfaces to be joined and applying
the requiste number fiber cloth layers, often in the form of a fiber tape. As you can possibly imagine sanding
every inch of the surface is not something that you would want to do for every layer of fiber cloth used in a
composite part, especially the very large critical ones like wings and fuselages. Also some components like
spars for example are often required by the designer to be made in one session. As long as each resined
layer is tacky the next layer may be applied without sanding.
As the previous paragraph implies there are two main ways to bond fiber cloth layers together: The "wet layup"
where all of the fiber cloth layers are fused to each other while the resin is still wet ( or tacky ) and the bonding
of the layers after they are hard ( cured ) which requires thorough sanding for each layer. The latter is usually
reserved for bonding cured complete components to each other later in the building process such as the
fuselage sides and internal bulkheads.
Now a few words about sanding: A material called Peel Ply can be applied to fiber surfaces while they are still
wet and removed later after resin cure. This provides a roughened surface that is suitable for making a good
bond. Sanding and Peel Ply also clean the bonding area. Clean surfaces are absolutly essential to achieving a
good bond since oil or grease of any kind will prevent or weaken the adhesion of the resin to the cured joint.
( It's not good for wet bonds either ). So don't work on your Chevy engine or anything oily and greasy in the
vacinity of composite work! You will be surprised how far little droplets of this stuff can travel and they seem to
want to home in on anything about to be bonded or painted.
Avoid over-sanding the surfaces to be joined. This is a condition best learned by seeing it but the best
description I know is to sand until the fibers can be seen just at the surface of the resin surrounding them. They
will have a dull appearance in contrast to the shiny surface of the unsanded resin. You will note that there will
often be tiny pockets of unsanded resin surface in between the fibers of the weave. Sanding down to this level
will begin to sand through the high points of the fibers in the weave effectively cutting them part way through.
This is to be avoided in order to maintain the maximum strength of the fibers so don't sand down this far. The
tiny unsanded pockets are not generally considered to be a problem. The surface is truly over-sanded when
the next layer of fiber below the surface one appears. Peel Ply's advantage is that when it is applied to the wet
resin surface it pushes the fibers below the surface and provides the desired roughened surface texture when it
is later peeled off after cure. It cannot be used in all cases, must be applied correctly and it is another expense.
Most often 80 grit sandpaper is used in my experience but that information is usually provided by the plans and
kit designers. This course a grit can very quickly cut through the fibers and do a lot of damage that must then be
repaired but its very courseness can reach down into the aforementioned pockets with careful sanding technique.
Finer grits will not do this and even this grit cannot be certain to get the pockets completely so sand the surface
as described earlier and continue on. Also remember to lighten the sanding pressure when sanding curved
surfaces especially when transitioning from a flat area to curved one or you will find that you will have cut through
fiber cloth layes at the apex of the curve ( like the leading edge of your wing for instance). The force necessary
to sand a flat surface is much more than that required sand a curved one. I like to hand sand in these areas
because one can control the force applied by feel.
I will discuss the manufacture of the core over which the cloth is layed in more detail later. For now I will say that
the smoother the core is the smoother and lighter will be the finished part. Dips and other irregularities in the core
will require more filler material to smooth out the finished surface and this will increase the weight of the part.
Before I describe the layup process I would like to tell you about a device that will be of considerable aid in
maintaining the proper warmth for the resin. You will most likely be using a resin dispenser that will automatically
meter out the proper proportions of resin and hardener ( in the case of the epoxys ) and even if you are weighing
them out yourself or using the catalyst type resins you will need to keep these materials at the proper temperature
to ensure good flow and mixing. The device is called a "hot box" and can be made for around $25.00 from a
4 X 8 ft. sheet of 1/2 in. urathane foam house insulation. Make a box from this sheet big enough to enclose your
resins and a source of mild heat like a 40 watt light bulb. Don't let the bulb get too close to or contact the
containers since that might overheat the resins and scorch their containers and might cause a fire. If the
containers get hot to the touch then the bulb is too close. The higher the wattage of the bulb the further away it
must be to be safe. Experience has shown that 40 watts is the best choice. Make the box so that you have
good access to the dispenser or containers. The urathane sheet is easily cut with a knife and can be either put
together with long nails ( allowing easy disassembly ) or better yet, a hot glue gun. A small vent hole or more
at the top of the box will prevent the accumulation of resin vapors that may arise from the heating and allows
excess heated air to escape. They may be plugged as necessary to maintain the proper temperature.
I put a thermometer inside next to the containers on my dispenser. The proper temperature varies somewhat with
the resin type but is generally around 85 to 95 F. This can be controlled by use of a timer and trial and error or
careful selection of the bulb wattage and cutting vents into the top of the box until the correct temperature range is
achieved. All of this is cumbersome to do and eventually works well but the addition of a $10.00 thermostat greatly
simplifies the whole process. You must use trial and error to get the set point of the thermostat but once achieved
the thermostat will turn the bulb on and off as necessary to hold the correct temperature.
If all is in readiness as described then it is time to do the layup. If it is a large one get you friends there to help
you move and postion the cloth. If you are laying up over a foam core then usually one must apply a filler to fill in
the rough surface and help bond the first layer of cloth to it. The filler is often a mixture of the resin and an
expansion agent like microballons mixed to the consistency of thick cream. It can made of other materials but this
is the type that is most common to my knowledge. Follow your plans in any case. Remember that once you start
this step the you must continue to the end. The clock is now running. Don't let the filler harden without the cloth
being applied since sanding the resultant fragile surface is next to impossible without doing it great damage.
The core material alone is very weak.
The cloth should be marked with lines showing the orientation of the fiber threads within it using a Sharpie marker
or whatever may be recommended in the plans. These lines will be a great aid in ensuring that the cloth threads
are properly oriented again as per your plans. This is usually done when the cloth is cut.
From now on the surface will be very sticky and once the cloth contacts it it will be difficult to remove and reposition
without tearing and distorting the threads in the cloth so take great care to position the cloth properly before letting
it contact the surface. This is why you will need another pair of hands on large layups like your wings. The threads ideally must be straight with no kinks or sharp curves. No waves should be allowed to remain in the threads
especially in structures like the spars. Remember that fibers bear loads in tension and that means straight threads.
Waves are stress points.
Invaribly you will find that this is often not achieved. The fix is to very carefully identify the threads that have the
curves in them and to pull on those threads only until the curves are removed. Pulling on straight threads will
induce waves. This pulling is done on both the threads in the cloth's weave alternating between the two until the
waves are removed from both sets of threads. Ideally the threads of the cloth weave will be straight and 90
degrees to each other. Again follow the guides in your plans where they exist. If you have no experience with
layups doing a training project not involving you aircraft is suggested so that you get the idea before you do
Make sure that your work area is at the proper working temperature or that you have an effective way of warming
the surface and resin as it is applied to ensure good penetration ( wet out ) of the weave. I have discussed this
before. Now lay the first layer of cloth over your core using the care mentioned above. Go to your resin
dispenser and meter out the resin and hardener being sure to mix them completely. It is advised usually to mix
small amounts that can be applied quickly so as to avoid wasting the resin if the quantity used cannot be used
before it starts to jell. Mix thoroughly. The more material in one cup the sooner it will jell. When this happens
the resin will no longer properly wet out the cloth and must be discarded.
Take you brush in hand and paint the resin over the cloth covering the whole surface with a solid coat. Do this
as fast as possible - the clock is running - but not in a panic. Just work quickly and steadily. Now is when you
will discover if the temperature is warm enough. If it is the resin will begin within minutes to soak the cloth fibers
making them almost the same color as the resin. If it is too cold then the resin will sit on top of the cloth and the
fibers will show through it with a shimmery fish scale like appearance. In this case penetration is not being
achieved and you must warm the area to get it. A hair dryer works but now extra work and time will be required
to get the proper wet out and you must take care not to burn the resin with the dryer. All in all it is best to have
the work area heated properly but sometimes ideal conditions cannot be had. A hair dryer or heat gun can be
used for spot problems as well along with a brush technique called stippling.
After the first coat is applied and the wet out has begun use your brush in a vertical jabbing motion to force the
resin into the fibers until the whole cloth layer has the wet out appearance described in the previous paragraph.
This is called stippling. A caution here is that stippling should not be done with too much force especially on
the first layer of cloth. If done too vigerously here the filler material will be forced up to the surface of the cloth
and this will make its bond with the next layer weaker than it should be. ( A note of caution here is that fillers
don't have much strength and therefore should not be bonded to in structural areas ). Also too much and too
vigerous stippling will foam up the resin drawing air into it. This is not too big a problem and will be mostly
removed by the next step.
Once the surface is properly wetted out then one must remove any excess resin if necessary. How do you tell
if you have too much or too little resin in the cloth? Mostly by experience but I will try to describe both conditions
here. It is too dry if the resin has penetrated the fibers but the color is much lighter than the resin by itself as seen
in the mixing cup and is also lighter than the areas around it that are known to be thoroughly wetted out. It may
have a fuzzy look as well. Add more resin with the brush. It is too wet when the weave appears as if it has a
deep clear coat over it and especially if the resin appears to have pooled over the weave. A proper wet out
has been achieved when the threads of the weave can be clearly seen above the resin and the color is almost
the same as the pure resin in the cup. That color will be slightly lighter in appearance than the pure resin if the
cloth is light in color. ( Dark cloths like carbon-graphite are another matter ). The procedure here is to draw a
squeege lightly over the surface removing the excess resin ( into a waste cup usually). Any foamed up resin
due to stippling will usually be removed by this process. Also on the first layer of cloth too much pressure while
squeeging can draw the underlying filler material up the surface. Not good as stated earlier in reference to stippling.
The clock is running.
One other step is to see if there are any air bubbles under the cloth. They are usually obvious but the small ones
can have a fish scale appearance and can be seen to be like bubbles under ice when looked at up close. They
are removed by stippling the area somtimes with more resin on the brush. Once the bubbles are gone the area
is squeeged again to remove the excess resin. Note that too much force while squeeging can draw air into the
weave and create bubbles. Just watch the area being worked for this and try again. Remember the Burtism,
"Take care of the ounces and the pounds will take care of themselves". The clock is running.
Check the weave for proper alignment of the threads as discussed removing any waves and kinks. The thread
( fiber ) orientation had best be correct at this point because pulling off the cloth and repositioning it without
destroying it is not an option. If you make this mistake you had best be very fast at cutting a new cloth piece.
( Neither a beer nor the majic words will fix the problem but they might make you feel better for a while ).
The clock is running.
All the above having been done on layer one it is now time to do the same thing again for each layer of cloth
applied. For wings and other large surfaces this can make for a long day. Speaking of which it is time to go to bed!
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