By Captain J.R. Watson
Tubes are used on boats for hard tops, T-tops, Biminis, dodgers, bows, bow and stern pulpits, rails, canoe and kayak paddle shafts, boat hooks, and so on. Composite tubes built with epoxy and reinforcing fibers
offer advantages over metal in terms of light weight, custom shapes and sizes, and corrosion resistance. Composite tubes can be faired and painted to produce a seamless appearance to match the boat, or left to show
the carbon fiber.
I've been experimenting with approaches to building a variety of
composite tubes. Following are some things I've tried (some that worked and some that did not) that you may find of value if you want to produce composite tubes yourself.
For all of these composite tube projects, I chose WEST SYSTEM 105 Resin and 207 Special Coating Hardener. The hardener choice was driven primarily by the desire for plenty of working time, fast wet out of the fiber reinforcement (as I was often unsure how long it would take to accomplish what I had in mind), and the option of a clear finish.
The real challenge in building composite tubes is in creating the mold or mandrel (the core around which material can be cast, molded, or otherwise shaped) and releasing the composite successfully from the mold.
Fiberglass over a metal tube is a typical approach; however, I also experimented with using foam pipe insulation as a molding surface.
To form the composite tube, braid is slipped over the mandrel, wet out with epoxy, and then reinforced with additional fiber/ epoxy laminates to the desired thickness. The composite tube is then released from the
mandrel, and the sections are joined together if needed.
Braided fibers are a good choice for composite tubes. Braids are available in fiberglass, aramid, carbon fiber, and other materials. A
sleeve or tube of braided material behaves like a Chinese finger trap. Because the fibers are woven together at 45 degrees, you can simply pull the braid to lengthen it. This also reduces the fiber orientation to about 35 degrees. The material to lengthen is "stolen" from that formerly used to keep it wide. Total surface area remains constant, so the more you
lengthen it, the more the diameter shrinks (i.e., tightens).
|Figure 1 - Typical braided tube of carbon fiber. The length-to-diameter ratio can be altered like a Chinese finger trap.
The reverse effect is that if the ends are pushed, the diameter increases with a likewise increase in the fiber orientation to about 55 degrees. This, of course, is how the finger trap is removed; because the diameter is now larger, it slides off your fingers. This expansion and compression of the braided tubes can be utilized in the construction process. In these projects, I used carbon fiber braids.
Specifically, I chose 3K (3,000 fibers per bundle) .06 oz/ft carbon fiber braid purchased from CST, The Composites Store and A & P Technologies. These companies also carry braids in tape form, rather than sleeves.
Tapes behave like biaxial but are much more controllable as the braid yields a straight edge that does not unravel. Tapes go around a corner nicely and can be manipulated like the sleeves by pulling or pushing to alter fiber angle.
Molds for straight tubes: Fiberglass over a metal pipe or tube
My simplest and most successful approach to creating molds for composite tubes is to laminate a thin layer of fiberglass over a smooth metal pipe or tube (could be aluminum, chrome, steel, etc.) that serves as a mandrel.
Wax the tube. It must be very smooth to facilitate removal. Apply a thin layer of fiberglass with epoxy. After this thin layer has cured, cut the laminate along the tube's length with a utility knife and pop it loose
but still on the mandrel. Rotate it a little to make sure it is free.
Then, continue with the desired laminate schedule. The fiberglass has already been released, so the composite tube will slide easily off the mandrel.
Rope and Foam
A second approach uses rope and foam pipe insulation (with a self-sealing slit) that looks like a doughnut in section. The foam has a
slippery surface to which the epoxy does not bond very well, facilitating removal when used as a molding surface.
I strung a length of low stretch rope (about 25% greater than the desired tube length) between two points tensioned with tackle. Initially, I left the rope unattached on one end. Alongside the rope, I also strung the
same length of 50 lb monofilament fishing line. I covered the rope and monofilament
with 1-1/4" OD slit foam used for pipe insulation.
It comes in 4' lengths and is sized for
about a 5/8" diameter pipe.
I fed one layer of braided sleeve onto the
foam. I followed this with a second layer.
However, the second layer was gathered at
the "long end" where the 25% extra rope
was situated. Then I tensioned the rope to
reduce any curve or sag and worked the
braid toward the ends of the tube to tighten
it against the foam. Next, I wetted out the
braid with WEST SYSTEM® 105/207. I placed
layers of 703 Unidirectional Carbon Tape
over the braid and wet it out with more epoxy.
In this way, I could step-taper these layers
to achieve a buildup of thickness in the
center with less on the ends. Once I had
achieved the schedule I wanted, I slid the
outer braid up over the unidirectional laminations
using waxed twine so I could pull
and work one end of the braid toward the
opposing end and tie it off. Once
the braid was smooth and secure, I wet it out
and let the tube cure overnight.
|Figure 2 - Making a composite tube using
foam pipe insulation, rope, and monofilament fishing line.
After everything had cured and I removed
rope, I used the 50-lb monofilament to cut
the foam into segments so it could be removed.
The monofilament is like dental
floss. Using a sawing motion it cuts right
thru the water pipe foam insulation almost as
if it were butter. I was eventually able to get
the foam out, but not without some difficulty
as it was stuck to the inner carbon braid.
This approach works on sections up to 10',
but may not for longer lengths. Considering
how little weight the foam represents, it may
not be worth the effort to remove the foam.
Polyethylene Terephthalate (PET)
Another approach I tried was to cover foam
pipe insulation with PET plastic braid, commonly
used to cover cables or electrical wiring,
with the intent to force the foam to become
more round. Tests where I tried to
bond to the PET resulted in what I
wanted the epoxy didn't bond to it, which
seemed to allow easier removal of the foam,
although the PET remained inside the composite
Fluorescent light tubes
An approach I tried that didn't work was to
laminate over a fluorescent light tube. I liked
the idea as the inside surface would be perfectly
smooth. I applied mold release to the
glass tube and laminated the carbon fiber
braid and unidirectional fiber as described
above. The plan was to break the bulb (they
implode) and vacuum the particles out.
However, I was disappointed when I broke
the end and the bulb did not implode but
stayed attached to the carbon fiber. Evidently,
there was too much surface area to allow
removal. So I have a carbon fiber tube
that is glass lined.
Making curved and tapered tubes
Based on this lesson, I tried a different approach.
I bonded a light layer of fiberglass
fabric over the florescent tube with mold release
on the surface. When cured, the single
layer of fiberglass is flexible enough that you
can break the bulb and remove the broken
glass. I then laminate enough fabric to the
outside to yield the required stiffness. Now I
have a composite tube that is perfectly
smooth and round inside.
|Figure 3 - A 6' tube built over a mold of foam pipe insulation. The PET plastic braid that remained is visible inside the carbon fiber tube.
|Figure 4 - PET is used on the outside of a carbon sleeve to serve as "handles" to help pull the sleeve taunt.
To make curved tubes, I bent 1/2" diameter
copper tubing to about an 8" constant radius.
Then I placed pipe insulation foam over the
copper tubing and proceeded with the laminating.
When everything was cured, the copper
tubing slid out. Again, I used monofilament
to cut the foam to allow removal in
|Figure 5 - A tapered tube section joined to a
To make a tapered tube, I placed pipe insulation
foam over a metal tube that was greater
than the size the pipe insulation was intended
for, which opened the slit. Then I attached
eyebolts to each end of the tube and stretched
two 50 lb monofilament lines. I aligned one
end of each with the edge of the open slit of
the pipe insulation and the other end on the
foam away from the edge to form a desired
taper. Using the line as a guide, I then cut along
the lines and remove the long thin wedges of
foam along the open slit. I placed the trimmed
foam over a taut rope as described earlier.
When I sealed the slit shut, I had a tapered
molding surface to laminate over.
Joining tube sections
To create one continuous tube, sections "straight or curved" can be joined to
other sections. I wanted matching diameters
within a 1
16" so I could easily blend the sections
to become visually "joint-less."
I've used two approaches for joining tubes
and both involve fabricating a sleeve that fits
in the ends of the tubes. The sleeve length
should be about four times the diameter of
the tubes you are joining half of the sleeve
length goes into each adjoining tube.
|Figure 6 - A short sleeve used to join two sections has
a smaller diameter than the sections.
The first approach to create a sleeve is to follow
the procedure described above for laminating
one layer of fiberglass over a polished
metal tube and then cutting the laminate to
free it. After that, add a couple more layers
to get the outside diameter needed to match
the inside diameter of the tubes to be joined.
The other approach I used was to laminate a
sleeve over a thin wall (1
16" or less) aluminum
tube mold. The outside diameter of the aluminum
tube represents the inside diameter of
the tubes to be joined less the thickness of
the sleeve. After sanding and polishing the
aluminum tube, I wrapped it in fiberglass
tape saturated with epoxy. Then I heated the
aluminum and the laminate to accelerate the
cure and also expand the aluminum.
After all was cured, I placed the laminate in a
snow bank (a freezer will work) to shrink the
aluminum. A 70F temperature drop is usually
sufficient to allow the cured laminate to
slide off the aluminum tube. I sanded the
sleeve to adjust the fit and then bonded it to
the inside of the tubes to be joined with
Composite tubes offer lots of possibilities. I
hope you find some of these approaches
Epoxyworks 26 / Spring 2008
Copyright © 2002, Gougeon Brothers, Inc. All rights reserved.
Last Modified on 10/28/02.
Reproduction in any form, in whole or in part, is expressly forbidden without the consent of the publisher. EPOXYWORKS, Gougeon Brothers, WEST SYSTEM, Episize, Scarffer and Microlight as used throughout this publication, are trademarks of Gougeon Brothers, Inc., Bay City, Michigan, USA.