The Creation of Fiberglass Tanks and Parts
for Autonomous Underwater Vehicle Constant Buoyancy Power Supply
MASSACHUSETTS N7Yfg
by
M F TECHNOLOGY
Jean H. Sack
'UN? "1,11
Submitted to the
Department of Mechanical Engineering
in Partial Fulfillment of the Requirements for the Degree of
Bachelor of Science in Mechanical Engineering
at the
Massachusetts Institute of Technology
June 2012
C 2012 Jean H. Sack. All rights reserved
The author hereby grants to MIT permission to reproduce and to distribute publicly paper and
electronic copies of this thesis document in whole or in part
in any medium now known or hereafter created.
Signature of Author
Department of Mechanical Engineering
May 18, 2012
Certified by
Douglas P. Hart
Professor of Mechanical Engineering
s Supervisor
Accepted by
'iJl
Samuel C. Collins
o 3Tn.ienhard V
of Mechanical Engineering
Undergraduate Officer
this page intentionallyleft blank
2
The Creation of Fiberglass Tanks and Parts
for Autonomous Underwater Vehicle Constant Buoyancy Power Supply
by
Jean H. Sack
Submitted to the Department of Mechanical Engineering
on May 18, 2012 in Partial Fulfillment of the
Requirements for the Degree of
Bachelor of Science in Mechanical Engineering
ABSTRACT
The purpose of this thesis was to construct and seal air and containment tanks and other
parts for a constant buoyancy power supply for an Autonomous Underwater Vehicle, or AUV.
While multiple materials and techniques were considered for construction, the final tanks were
made using lost foam molds and epoxy resin. This secondary purpose of this thesis was to
provide a detailed description of how to create fiberglass parts, especially custom designs with
challenging shapes. Molds were made using a variety of techniques, with best results achieved
using a hot wire. Most parts were finished by sanding and using a filler coat, though one of the
tanks was left unfilled for transparency. The tanks were machined, and the foam removed, and
valve, sensor and rail mounts were made and attached.
Thesis Supervisor: Douglas P. Hart
Title: Professor of Mechanical Engineering
3
Acknowledgments
The Author would like to thank many people for their invaluable assistance in the success of
this project. First, thank you to Ivan Sack, for numerous hours on the phone with advice on
materials, fiberglass layups, and lost foam molding. Second, thank you to Professor Doug Hart,
for additional instruction on materials and techniques, and especially for his attention to detail
and aesthetics. Thank you to Dave Dow and Patrick McAtamney in LMP for their incredible
patience, good humor and assistance in machining the tanks, and to MIT LMP, HML and
Lincoln Laboratory for providing lab space for the project, and the MIT Pappalardo Lab for use
of their hot wire for foam cutting. Thank you also to Professor Lagace for meeting with me to
discuss possible materials and helping identify important aspects of the project, to Benjamin
Harvatine and Brendan Rios for invaluable assistance in doing the layups, to Teresa Saxon-Fox
and Luke Mooney for their designs of the tanks and general assistance, and especially to Ms.
Saxon-Fox for her leadership, support and friendship, and to the rest of the members of 2.014 for
all of their efforts in making the overall project a success. Finally a heartfelt thank you to Karen
and Gary Sack, for their continuous love and advice, and for teaching me that there are many
ways to approach a problem, and with work and determination great things can be achieved.
4
CONTENTS
A b stra c t ..................................................................................................
.. 3
Acknowledgements....................................................................................4
T ab le of C ontents ......................................................................................
L ist o f F igu res ..........................................................................................
5
. 6
1. Introduction and motivation....................................................................
8
2. Requirements of tanks and selection of materials..............................................
9
2.1 Tank specifications....
............
...
..................................
2.2 Possible Materials.....................................................................................
9
10
3. Construction of Foam Molds......................................................................11
3.1 PreliminaryMolds............................................................................11
3.2 R efined Molds.................................................................................
4. F iberg lass Lay U p ..................................................................................
13
17
4.1 PreliminaryTanks using Fiberglassand PolyesterResin...............................17
4.2 Machining Tanks.............................................................................19
4.3 Refined Layup Using Epoxy Resin..........................................................22
4.4 Filler Coat......................................................................................23
4.5 R em oving Foam ................................................................................
24
5. Fin ishing T an ks ....................................................................................
26
5.1 Sanding........................................................................................
26
5.2 Leak Testing andPatching...................................................................27
5.3 FinalFillingand Sanding...................................................................28
5.4 Finishingthe Containment Tank without filler............................................29
5.5 Vacuum Testing and Sealing with Silicone................................................30
6. O utfitting Tan ks ...................................................................................
6.1 Attaching Fittings .............................................................................
6.2 Attaching Rail Mounts..........................................................................33
6.3 Valve B rackets...............................................................................
32
32
33
7. Creating Strangely Shaped Parts Using Fiberglass....................................34
7.1 Capabilitiesof Hot Wire Foam Cutting......................................................34
7.2FiberglassLayup ofDifficult Shapes........................................................35
8. Lessons Learned .................................................................................
39
5
LIST OF FIGURES
1.1:
Showing an annotated cad model (1 .1 a) and picture of the built system 1.1 b. This thesis
focuses on the creation of containment and air tanks, as well as the snorkel shroud, water
shroud and fins and nose cone, not shown.........................................................9
3.1:
Showing construction of the initial preliminary rough mold. 1 a) Shows the orientation in
which the foam segments were cut, lb) shows a roughed out shape and 1c) shows the
completed duct-tape covered mold............................................................12
3.2:
Showing construction of the first air tank mold 2a) and the mold covered in duct-tape 2b).
These pictures show the pockets on the tanks which provided challenges in fiberglass lay. . 13
up . ................................................................................................
3.3:
Shows the process of creating delrin templates to attach to the foam sheets for cutting
with the hot wire. 3.3a Shows setup for the water jet with lead weights on the delrin sheet
3.3b is cutting the third template with the first two cut out and 3.3c displays the finished
15
temp lates. ............................................................................................
3.4:
The final air tank (left) and containment tank (right) molds show considerably better
accuracy from using the hot wire to cut the sections. Effects of the molten foam from the
cutting is seen on the bottom section of the air tank and right most section of the
containment tank, and gaps from foam warpage are seen on the containment tank........16
4.1:
The first tank created out of fiberglass with polyester resin, wrapped in plastic wrap for
. . .. 18
curin g . .........................................................................................
4.2:
Air tank curing in the fume hood. The tape pattern was an attempt to press the foam
19
sections all the way into the pockets. .........................................................
4.3:
Unwrapped and partially machined preliminary air tank, showing the foam underneath.
This figure also shows two of the holes on the port hole face and the center marking....20
4.4:
Showing the sanded, partially machined final air tank with on the holes in the pockets.
On the other side, the hold is in the bottom of the pocket, instead of the back as shown
. . 21
h ere. ..............................................................................................
6
4.5:
Sanded and machined refined containment tank showing the port hole (left) and the tank
interior to piping or bladder to piping attachment holes. ................................... 21
4.6:
The air tank (left) and containment tank (right) after being wrapped in plastic to cure.
The clamps on the air tank are holding pieces of cardboard against the tank pockets to
24
force them to cure in the desired shape. ......................................................
4.7:
Shows and completely cured air (left and containment (right) tanks after plastic wrap
was removed. It pictures the new shape for the air tank, as well as the poor surface
25
flatness created by using plastic wrap. .......................................................
5.1:
Shows the air tank after a second layer of filler was applied to the port hole face. This
also shows the other faces sanded down and ready for more filler coats....................29
5.2:
Fixing an air bubble in the containment tank.................................................30
5.3:
Picturing the vacuum test using a plastic bag duct taped to the outside of the tank. After
it was sealed, the water bladder was pumped dry and a pressure sensor measured change
in pressure. The bag was used to determine the location of the leaks as it moved if the
31
end it covered was not completely sealed. ....................................................
6.1:
The finished filled containment tank with port hole cover attached and the tubing used to
32
connect the water bladders on both tanks coiled around it. ................................
7.1:
Showing the stages of mold and layup done to make one of the fins. From left to right,
the roughly shaped fin as cut with the hot wire; the fin after sanding and applying mold
release wax; after the initial layup with rough filler; after sanding and another layer of
epoxy. Photo courtesy of B. D. Harvatine....................................................36
7.2:
Showing the stages of the mold for the nose cone in rough form on left, after it had been
smoothed into shape in center, and after initial layup on right................................37
7.3:
Shows the snorkel shroud, 7.3a; water shroud immediately after layup, 7.3b; and after
cured and cut off of the mold, 7.3c. In addition, 7.3b shows the spiral wrapping method
can be seen through the filler coat............................................................
38
7.4:
Showing many of the fiberglass parts that were made in their final state. Visible on the
containment tank is one of the rail attachments, and connected valves and sensors on their
respective mounts. The air tanks has port hole cover visible as well as the attached water
and air valves, and the snorkel and water shrouds in their final state are pictured.........39
7
1. Introduction
One of the limiting factors on current Autonomous Underwater Vehicles (AUV's) is their
limited deployment life. The purpose of this thesis was to create the customized fiberglass tanks
needed to implement the design created in the fall semester of MIT's 2.013 class, which
extended the autonomous life to 40 consecutive 12 hour missions.
This design features a
constant buoyancy hybrid internal combustion recharging system.
The power source is a Honda GXH50 engine, which is a small, 4- stroke gasoline engine.
The fuel is kept in bladders inside two sealed containment tanks that maintain constant mass and
buoyancy as the fuel is consumed. This is done by filling the evacuated space in the tanks with
water and air in a 70:30 ratio, and also eliminates moments on the system. Water goes into a
water bladder that fills as the fuel is consumed, and the remaining space is filled with air. Water
and air are provided by snorkel system, which takes in air and water to maintain positive
buoyancy and cool the engine while it is charging the batteries which power the AUV on its
missions. This system is composed of multiple redundant valves to ensure that the air line
remains clear of water, and that the air intake of the engine and containment tanks remains dry.
The air tank is pumped completely dry at the surface to allow for maximum buoyancy, and fills
with water during missions to allow for diving. A CAD model of the system is provided in
figure 1.1 a, and a picture of the system is provided in 1. 1b.
This thesis focused on making all of the fiberglass tanks, and assisting with the fabrication
of all of the other fiberglass parts, namely a shroud for the snorkel and water intake, and fins and
a nose cone for a mockup of the entire system. A large part of this project involved learning
methods of lost foam molds fiberglass layups. There do not appear to be any easily located
fiberglass resources on the MIT campus, so much of the tank creation was done through trial and
error.
8
1.1a
fuel bladder
water
bladder
cotai
ent tanks
Air Tank
-tainenk
1.1b
Figure 1.1: Showing an annotated cad model, 1.la and picture of the built system 1.1b.
This thesis focuses on the creation of containment and air tanks, as well as the snorkel
and water shrouds, and fins and nose cone, not shown.
2. Requirements of tanks and selection of materials
2.1 Tank specifications
The initial design on the containment tanks was to use the vacuum created from draining the
fuel tank to help siphon in water to maintain constant buoyancy. This required a rigid, air tight
tank that would not yield to the induced vacuum, or allow air to come in, thus equalizing the
pressure difference. The tank also had to be machine able to allow valves to attach the bladders
inside the tank and piping outside the tank, also while maintaining an air tight seal. A port hole
9
was needed to insert the bladders and connect them to the piping. It was critical that the outside
diameter of the tank was small enough to accommodate an attachment rail system to hold all the
components together and still fit within the shell of the REMUS. The interior diameter was also
critical as it had to fit custom fuel and water bladders that were custom ordered for the project,
and extremely expensive with a two month minimum lead time. Since the design required them
to be attached with Velcro to the inside of the tank to allow them to correctly fold as the contents
were emptied, too large of an internal diameter would make it difficult to attach, and too small of
a diameter would prevent the tanks from being entirely filled or correctly placed. Either of these
would not use the system to its fullest capacity, and would limit the run time of the engine and
therefore the autonomous life of the REMUS.
The air tank also needed to be water and air tight to control buoyancy of the system and
prevent leakage on the other components of the power supply, especially electronics. In order to
save room in the power supply to minimize overall length of the tube, pockets were designed
into the tank to allow the air and water valves to be recessed into the tank instead of outside on
both sides.
2.2 Possible Materials
Multiple materials were considered for the tanks. One option was to make them out of
sections of aluminum sheet welded together. This would have had the advantage of minimum
tolerance capabilities, but required access to a welding lab and proficiency in welding. It was
also not ideal because of the weight of the raw material and potential expense. Another option
that was considered was pre-impregnated fiberglass cloth: woven glass fibers already saturated
with resin. This material was not selected because of cost and also the need to bake the layup to
cure the resin. Since this would require a large oven and expose it to potentially toxic materials,
this was also not ideal. Another briefly considered material was Acrylic, which can be formed
through heating and bonded together using superglue.
This may have been a very elegant
solution but little was known about material properties, and it was not selected due to the
accessibility and availability of the final material, being fiberglass. This ultimately used material
was used with two different resins: polyester and epoxy resin. Polyester resin was used initially
due to easy availability, price and limited toxicity. All of the materials needed to make the
10
preliminary two tanks were acquired easily at a large hardware or auto repair store.
Epoxy
resin, ordered online through Aero Marine was used in the final tanks because of the better
quality final product, improved workability and elimination of very potent and lingering
polyester resin smell. Fiberglass was also ultimately selected because it was the easiest of the
materials to work with and gain proficiency in with limited instruction and time.
3. Construction of Foam Molds
3.1 Preliminary Molds
The first containment tank mold was constructed by cutting pieces of standard 2 inch pink
insulation foam into sections 12 inches long, and screwing the segments together as shown in
Figure 3.1 a. A plywood template created from the drawing of the end of the tank was used to
make an end section, and then both the template and the end piece were used as guides to shape
the corners into a rounded shape. This method, while crude, allowed the quick construction of a
preliminary mold with only a hacksaw and screws. It resulted in very poor accuracy and surface
finish, however, as pictured in figure 3. lb. Once the tank was shaped and all the corners were
rounded, it was covered in duct tape to prevent the foam from dissolving when fiberglass was
laid up, as shown in Figure 3. 1c.
The air tanks shape suggested a different approach, which ended up being much more
effective. For this mold, the pieces of foam were cut directly from the plywood template, and
then stacked up as shown in Fig. 3.2. Cutting the pieces slightly oversized and sanding the edges
proved particularly effective for creating a nice surface finish, and creating the most continuous
mold possible out of the sections. Finally, screws inserted from the sides, instead of all the way
through each layer into the next, allowed for the use of shorter screws and easier construction.
11
1a)
-b)
1c)
Figure 3.1: Showing construction of the initial preliminary rough mold. la) Shows the
orientation in which the foam segments were cut, lb) shows a roughed out shape and ic)
shows the completed duct-tape covered mold.
12
3.2a)
3.2b)
Figure 3.2: Showing construction of the first air tank mold 2a) and the mold covered in
duct-tape 2b). These pictures show the pockets on the tanks which provided challenges
in fiberglass lay-up.
3.2 Refined Molds
Since using a hacksaw off a plywood template produced poor accuracy and edge
smoothness, another method of manufacturing the molds was created.
A few options were
13
considered, such as CNC milling or water-jetting each piece and stacking them up as done with
the air tank, but both were vetoed by the shop manager. For CNC, the concerns were two fold;
first that no appropriate cutting tools or clamping devices existed to securely and accuarely cut
foam, and second the extent of the mess that would be created from cutting semi-low density
foam with a cutting tool. The water jet was not used mainly because of concerns with water
seeping into the foam and deforming or warping the sections.
Therefore it was decided that templates would be cut out of delrin sheet with the waterjet in
LMP, with these templates being attached to the foam for cutting with a hot wire. After the
shapes of the outermost dimensions on the tanks were put into mastercam, they were transferred
to the water jet computer. One template was made for the containment tank, and two were made
for the air tank, with one for the segments with pockets, and one for the middle section of the
mold. Fig. 3.3 shows the water jet cutting out templates. The containment tank design was
changed slightly from 11.75 inches long to 12.2 inches to allow the 2 inch sections to be used.
Once the templates were cut, they were attached to the pieces of foam with double sided
tape. The large foam sheets were first cut into more manageable sections using the hot wire in
the Pappalardo Lab, and then the templates were traced. Cutting using the hot wire is not
immediately intuitive, as it is important to find the correct speed. If the cut is done too quickly,
the wire bends which causes undercuts on a curved path and results in tapered parts. If the cut is
done too slowly, on the other hand, the foam will begin to melt, resulting in deformed edges and
a rough and hardened surface finish. At the correct speed, before a residue of melted foam
builds up on the wire cutting surface, the foam melts away and the part can be cut with almost no
resistance.
The best finish was produced by adding relief cuts into the radiuses so that pieces
could be removed as the cut was made, which allowed for a continuous cut while removing this
buildup on the wire. Without these relief cuts, the melted foam built up on the wire forming long
molten strands which melted into the mold section but then fell off, leaving indents on the edges
as seen on the rightmost segment of the containment tank shown in fig. 3.4. These areas then
had to be sanded down to allow for a smooth layer of fiberglass, and resulted in difficulties
matching up the pieces.
The tape which attached the template to the foam was considerably stronger than needed,
and damaged the surface of the foam to which it was applied when the template was removed. It
worked better for the subsequent segments, using the same tape, since it adhered less after
14
multiple uses. For future attempts, only a few, small sections of tape are needed to sufficiently
hold the template in place.
3.3b
3.3a
3.3c
Figure 3.3: Shows the process of creating delrin templates to attach to the foam sheets
for cutting with the hot wire. 3.3a Shows setup for the water jet with lead weights on the
delrin sheet 3.3b is cutting the third template with the first two cut out and 3.3c displays
the finished templates.
15
Attaching the molds segments to each other was challenging. The foam segments were
slightly warped, likely from storing the sheet on edge in a humid environment, which prevented
the segments from going together flat. This significantly reduced surface area between the two
segments, which required a particularly strong bond where they were in contact.
The first
attempt was to use wood glue, but after 24 hours, only the glue that dripped out the edges had
dried. It was then discovered that 51b strength double stick tape, while not strong enough to
correct for the warpage, did successfully hold the sections together, when applied to clean foam
with the glue residue removed. The edges were then sanded down, though there ended up being
some gaps and variations in surface flatness where the hot wire undercut the foam pieces. The
surface could not be entirely fixed by sanding, as excess sanding on the foam distresses the
structure and can result in large chunks falling off. All sharp edges were rounded, but since
epoxy resin does not dissolve foam, the molds were not covered with tape. Unfortunately it was
discovered that epoxy resin affixes itself with great determination to foam, and as a result, the
foam was almost impossible to remove after machining the tanks. Therefore, the final foam
mold was covered completely with mold release wax which helped significantly with foam
removal, detailed in section 4.5. The final molds are shown in Fig. 3.4
Figure 3.4: The final air tank (left) and containment tank (right) molds show
considerably better accuracy from using the hot wire to cut the sections. Effects of the
molten foam from the cutting is seen on the bottom section of the air tank and right most
section of the containment tank, and gaps from foam warpage are seen on the
containment tank.
16
4. Fiberglass Lay Up
4.1 Preliminary Tanks using Fiberglass and Polyester Resin
The first tank made was the containment tank. The lay-up was done as crudely as the mold,
but again produced a usable tank with only three hours of labor and very limited materials. The
most problematic aspect of using polyester resin is the smell, which while being non-toxic, is
extremely odorous. This required working outdoors, which then became weather dependent as a
reasonably warm, dry day was needed. The ends were laid up with a layer of cloth followed by a
layer of mat, finished with a layer of cloth, and the middle section was simply wrapped with
cloth. Between each layer, resin was mixed with hardener in 6oz amounts and the cloth or mat
layer was soaked through via a combination of brushing on resin and pouring and spreading with
the brush. Since it was mixed in small amounts with slightly less hardener than recommended
and used quickly, there were no issues with working time.
It was still extremely messy,
however, especially when cutting had to be done on the ends to allow the cloth or mat to fold flat
around the corners instead of wrinkling. Each of the flat sides on the end was cut so it could be
directly laid down, and each rounded segment was cut into 3 sections for a total of 12 cuts on
each end. These segments were then overlapped, and the cloth wrapped around the middle was
laid over the top of them to hold all sections down. Because of frequent use of both resin and
scissors, the gloves and scissors quickly became covered in resin and difficult to work with. This
was especially true when loose glass fibers from the cut sections started sticking to everything.
The cloth was pulled as tight as possible, and then the entire tank was wrapped in plastic wrap to
help maintain its shape while curing. During the early stages of the curing process, the tank
became warm but not hot enough to melt the plastic wrap. Even after being left outside for 5
hours to cure, and wrapped in further layers of plastic, however, the smell from the cured resin
made rooms unpleasant to inhabit, and resulting in headaches to the occupants.
Somehow,
likely due to errors in the mold, the tank cured with a significantly slanted end. A picture of the
first tank is provided in Fig. 4.1.
17
Figure 4.1: The first tank, created out of fiberglass with polyester resin, wrapped in plastic
wrap for curing.
The first air tank was created using a fume hood, in an attempt to contain the noxious odor
of the polyester resin. Created with assistance, less of a mess was made as one person was able
to only cut while the other smoothed on the cloth and resin limiting the spread of resin to tools.
The air tank was, however, a much more complicated shape, as it had pockets in both sides as
seen in Fig. 3.2 and 3.4. Cutting the cloth to not fit the pockets and ensure complete coverage of
all of the corners proved to be extremely difficult, especially as the resin seemed to harden much
more quickly in the fume hood. As a result, a significant amount of resin was wasted as it set
before it could be used. For this tank, a layer of mat was used over the entire tank, instead of just
the ends, which absorbed much more resin and was extremely difficult to work with. Not only is
mat more reluctant to bend than cloth, but the loose glass fibers adhere to everything and create
large sticky hairballs which are difficult to control. Once the mat layer was applied, it seemed as
though the lay-up attempt had failed, and was going to need to be completely removed and
redone, as nothing was retaining shape.
Fortunately the final cloth layer was able to hold
everything together and smooth out the mat layer. The pockets were difficult to hold while
curing; sections of foam were wrapped in plastic wrap, inserted into the pockets and then the
entire tank was plastic wrapped and taped as shown in figure 4.2.
18
Figure 4.2: Preliminary air tank curing in the fume hood. The tape pattern was an attempt
to press the foam sections all the way into the pockets.
After the tanks were fully cured, the plastic wrap was removed, though at times it was difficult as
it was glued to the outside of the tank. The unwrapped preliminary air tank is shown in fig. 4.3.
4.2 Machining Tanks
In order to machine ports and valve holes into all of the tanks, the Solidworks drawings for
the parts were imported into Mastercam to create CAD files to be run on the CNC Bridgeport
mill in the LMP machine shop. Since the tanks are a large and awkward shape, it was not
possible to use a standard vice to hold them securely enough for machining. Instead, the vice
faces were reversed to allow a flat metal L to be clamped in place. The flat sides of the tanks
were then clamped with bar clamps between the metal flat and a plywood section to spread out
the forces and avoid cracking the tanks. The tanks were also checked with a level to ensure that
the cuts were straight, especially with the first containment tank which needed to be shimmed to
19
account for the slanted end. The containment tank was made with metal screw heads in the ends
of the foam mold, so these screws needed to be located with magnets to avoid damaging the
cutting tool by running into them. It was possible to cut through the entire thickness of fiberglass
using a
1/4
inch end mill with one pass, going reasonably slowly. Cutting was done with a
vacuum directly following the cutting path to prevent glass dust and fibers from spreading. The
cut air tank is shown in figure 4.3.
Figure 4.3: Unwrapped and partially machined preliminary air tank, showing the foam
underneath. This figure also shows two of the holes on the port hole face and the center
marking.
For the final air tank, holes were also added in the pockets. This was difficult, as holes
needed to be exactly centered in the pockets which were not entirely square because of the
difficulties in layup described in sections 4.1 and 4.3. One of the holes in the pockets was in to
the back of the pocket, and was reasonably simple once the position was marked.
More
challenging was the hole on the other side located in the bottom of the pocket. For this hole, the
cutting tool was held by only the very end to be long enough to reach all the way to the bottom
of the pocket, and the cut was made a reduced speed.
The port hole was cut using a hole saw,
though this created less accurate placement, and the cut itself was more difficult, though it
eliminated the need for the elaborate setup for using the CNC mill. One of the pocket holes is
20
shown in figure 4.4. The containment tank was simpler and had only a port hole centered on one
side with six center drilled holes for the screws, and three % inch holes placed on the back for
fittings to the air piping, and water and fuel bladders and piping, as shown in fig. 4.5
Figure 4.4: Showing the sanded, partially machined final air tank with on the holes in the
pockets. On the other side, the hold is in the bottom of the pocket, instead of the back as
shown here.
Figure 4.5: Sanded and machined refined containment tank showing the port hole (left) and
the tank interior to piping or bladder to piping attachment holes.
21
4.3 Refined Layup Using Epoxy Resin
The final tanks were created using #300 epoxy resin with #21 Non Blushing
Cycloaliphatic Hardener, ordered from aero marine. Since epoxy resin has only a faint odor, it
was possible to work with inside, in a reasonably well ventilated room. First, the molds were
covered with a thin layer of resin, and then a layer of cloth was adhered to the mold. As done
previously, the ends were covered first, followed by a wrap of cloth around the middle of the
tank. This time the end sections were not as long, which caused overlapping to be less effective.
The epoxy resin was very nice to work with, though it had a much shorter working time.
The first cup made was a large volume and was allowed to sit for too long in an attempt
to allow the hardener to diffuse completely into the resin.
Since epoxy resin cures in an
exothermic process, a large amount of heat is generated, and the cup that was holding the resin
began to smoke, and then deform as the plastic melted. The tank with the first layer of epoxy on
it also melted through the light weight drop cloth used, adhering plastic to the first layer of glass
cloth.
Subsequent batches of resin were mixed in much smaller quantities and used more
quickly. As soon as heat could be felt, only about 5 minutes of good working time remained,
and any resin not used within that time became gummy and unusable.
In small quantities,
however, the cups did not become unpleasantly warm or deform.
As with the previous lay-ups, the mat cloth proved to be difficult to work with, though
the faster set helped when the pieces were properly and precisely placed, cut, and overlapped. It
became problematic, however, whenever cloth or mat was accidentally shifted, especially under
other layers, as it was not always possible to rearrange. Also as before, after the mat layer was
applied, everything started falling apart and only held together after the final cloth layer was
applied.
This was the first containment tank using mat over the entire surface, and some
wrinkles that surfaced in the mat when the cloth was stretched tight, prevented a completely
smooth finish layer. The epoxy was worked into the layers by sticking the new fibers to the
previous layer, pouring a small amount of epoxy on a surface, and spreading it with rubber
spreaders. The final layer of cloth was simply placed on top of the saturated mat layer and
worked with a metal roller until it was also saturated with excess resin from the matt layer. These
methods proved to be faster and more effective at conserving resin than brushing on coats.
The new design for the air tank, as shown in the mold making in Figs 3.3 and 3.4, and the
final tank in 4.4 and 4.7 resulted in a noncircular tank, with cutouts on the sides to allow more
22
room within the REMUS tube for piping. This change in design eliminated the option to pull the
wrap layer around the outer surface tight. In leaving enough slack to accommodate the pockets, it
became very difficult to keep the cloth in place and prevent wrinkles while working on other
faces. This was particularly difficult as wet epoxy sticks to everything, and loose cloth is prone
to wrinkling and pulling away from other wet epoxy layers in favor of sticking to a dry plastic
dropcloth. Strips of cloth placed perpendicular to the wrap where the cutaways began were
somewhat successful in helping to maintain the shape, but sanding these areas flat as described in
section 5.1, resulted in holes which required patching as described in section 5.2
4.4 Filler coat
After all the layers of fiberglass were applied, phenolic micro balloon filler was mixed with
the resin to create a finishing layer. A large amount of filler mixes into the epoxy before it
become as paste, as several tablespoons were added to half a cup of resin before any notable
change in consistency took place. Several more tablespoons were added before the mix ceased
to be runny, and looked roughly like chocolate pudding in color and consistency. It was then
spread over the entire tank using a plastic spreader, and then the tank was wrapped in plastic
wrap. Since a significant amount of resin had been lost due to premature hardening, there was
insufficient resin left to create a full filler coat on the air tank, resulting in a mottled finish.
The plastic wrap was not only unnecessary for the containment tank; it was extremely
detrimental to the surface finish. Since it clung to the filler where it touched, even when an
attempt to pull it tight was made, it still ended up wrinkling and as a result the filler coat had a
very uneven surface. The plastic wrap was, however, potentially helpful for the air tank, as the
pockets were clamped with pieces of cardboard cut into the shape of the pocket, and plastic wrap
was useful to prevent the cardboard from adhering to the tank. Also, since there was insufficient
resin to make a thick filler coat, much of what was applied was better worked into the existing
fiberglass layers which were not as affected by wrapping in plastic. Figure 4.6a shows both
tanks wrapped in plastic wrap, and also shows the clamping on the air tank to preserve the shape
of the pockets. Though this was quite effective on preserving the desired shape, the clamps did
leave an indent on the side of the air tank from the gripping section.
After the tanks had
23
completely cured, the plastic wrap was removed, and a different angle of the tanks is provided in
figure 4.6b. In this figure, the mottling from lack of filler coat is particularly apparent, showing
the clear section where no filler was added
The final containment tank was made without filler or plastic wrap, as a clear tank was
desired to allow some visibility in what was happening inside the tank. The top layer of cloth
did not go on completely smoothly, and the finish was much rougher without the filler, but there
was some transparency with the final tank.
4.6a
4.6b
Figure 4.6: 4.6a Shows the air tank and containment tank after being wrapped in plastic to
cure. The clamps on the air tank are holding pieces of cardboard against the tank pockets to
force them to cure in the desired shape. 4.6b Shows and completely cured air and
containment tanks after plastic wrap was removed. It pictures the new shape for the air tank,
as well as the poor surface flatness created by using plastic wrap.
4.5 Removing foam
Removing the foam from the cured tanks was significantly easier for the first tanks that were
made with duct tape completely covering the foam mold. Though polyester resin was used with
those molds, it seems likely that epoxy resin would also adhere less to the closed surface of the
tape, instead of the open cell foam. With the first containment tank, the middle chunks of foam
were chiseled out using a combination of a very sharp, hooked carpet knife, screwdrivers, and
the chisel end on a tire iron. The port hole that was cut in the end was filed down and smoothed
to minimize imbedding glass shards in forearms while working inside the tank. This tank was
24
made with the foam sections stacked on top of each other as seen in fig. 3.1, with the 2.5 in long
screws holding the sections together perpendicular to the direction of foam removal, except on
the end piece. Therefore, it became challenging to remove the foam as the screws were sharp,
embedded in large sections of foam, and difficult to work around. Once a significant amount of
foam had been chiseled out of the tank, sections were gradually able to be peeled off of the sides
allowing larger pieces to come out. Once sufficient volume was removed, the remaining duct
tape and foam was easily pulled out in one piece. This left the inside completely clean, though
small, smooth ridges covered the entire inside of the tank from the surface finish of the tape.
Since the design for the air tank was changed after creation of the first air tank, foam was not
removed from that tank.
The process did not work nearly as nicely for the second air and containment tanks. Since
the duct tape layer was eliminated and nothing added in its place, the epoxy resin completely
bonded the fiberglass to the foam mold. Sections could no longer simply be pried off the walls;
it was now extremely difficult to extract any foam except for the sections directly under the port
hole which could be directly cut out. Therefore, the only way to remove overhung sections of the
containment tank and the majority of the air tank was to melt out the foam using acetone. When
the acetone was poured into the tank, the foam deformed instantly on contact from the rigid pink
structure, to extremely sticky and elastic dark purple substance. It was somewhat interesting to
be able to see variations in the foam density throughout the foam sections, as at times the acetone
would completely melt a section, and at other times it melted around parts leaving structures left
in the foam. The melted foam was pulled out and deposited on crumpled paper to allow the
acetone to dry so the remaining foam could be discarded. The acetone was considerably faster at
removing large amounts of foam than chiseling and cutting had been, but it was still a fairly
slow, unpleasant task. Even after most of the foam was removed, it was impossible to remove the
entire melted foam residue from the insides of the tanks. Once the acetone remaining in the tanks
had evaporated, the remaining foam residue adhered itself to the interior of the tank and
hardened into a second layer that was extremely resistant to removal. Some of it was able to be
chiseled out, and the rest was somewhat smoothed using the electric sander, but without
significant time and determination, complete removal is impossible.
The final tank was made using a considerable amount of mold release wax to finish the
mold. Therefore, emptying it out was considerably easier. Though the mold release was not as
25
effective as the duct tape had been at preventing adhesion, it was still possible with some amount
of melting, chiseling and prying to get a reasonably clean interior with considerably less effort.
Unfortunately, since it required considerable prying force especially to remove the foam directly
under the overhand, considerably damage was made to the fiberglass directly around the port
hole. Several small rips and tears were made where the tool had levered off of the surface, which
were easily repaired by applying more resin to the damaged areas, and allowing the edges to
bond as the resin cured.
5. Finishing tanks
5.1 Sanding
After the tanks cured, they were unwrapped and were shiny in appearance. Since the
wrinkles from the plastic wrap were cured into the tanks surface, significant sanding was needed
to even it out the surface. This was done with a small hand-held sander outside to reduce dust
inhalation. The sanding pads quickly filled up with dust from the filler, and had to be changed
out and cleared frequently to maintain effectiveness. It was reasonably easy to determine how
far it was possible to sand without damaging the integrity of the tank, as the cloth fibers became
visible when the filler coat was removed. In some cases it became necessary to sand through the
outer layer of cloth to eliminate a wrinkle, or in places where the outer layer was not fully
bonded to the mat layer underneath.
The most important part, however, besides removing
anything clearly raised, was simply roughing up the entire surface so that a second layer of filler
could be applied to make the surface more uniform. Later sanding was done by hand before the
filler was completely cured, as this proved to be easier. The sanded tanks are pictured in figure
4.4 and 4.5.
26
5.2 Leak testing and patching
After the surface was sanded reasonably smooth, the tanks were checked to see if they
were air and water tight. The containment tank was a reasonably simple construction, and since
it lacked areas where three faces joined at one point as on the air tank, the tank was completely
sealed except for the area around the port hole on the clear containment tank, where the
fiberglass had been ripped in the process of prying against it to get the foam out.
This was
easily patched by simply applying a small amount of resin to the damaged faces, bonding the
fiberglass back together. The tanks were then taken to the sink, and a few cups of water were
poured into them. The freshly sanded surface on the outside conveniently changed color when
wet, but no water leakage was detected on the bottom of the containment tank.
Such was not the case with the air tank, which had edges with a considerably tighter
radius, and corners both at the outer top and bottom back corners of the pockets. The bottom
edge had also not bonded correctly, and after sanding leaked significantly. The final areas that
provided issues were the edges and corners of the cutaways, which was a result of the probelms
detailed in section 4.3. Circles were drawn with a marker around any areas that had shown
leakage, and the tank was allowed to dry out before patching was attempted.
In order to patch the tank, all areas around holes were sanded down to cloth, and then the
patches were applied in the same way as the initial layup was done. A thin layer of epoxy was
applied directly to the area, and depending on the severity of the leak and amount of structure
remaining, layers of cloth and matt were applied as needed. For many of the minor patches, one
layer of cloth was sufficient, while others needed a layer of matt and multiple layers of cloth.
Patching is not difficult but does require sufficient time, as it is necessary to wait for the initial
layup to cure before sanding can be done and layers added.
In many cases, however, patching
may be a better alternative to spending significant time and energy trying to ensure that the initial
layup is perfect. If filler is being used and enough time is taken to sand a fill correctly, the patch
will be undetectable in the final product.
27
5.3 Final fillingand sanding
Through cycles of filling and sanding, it is possible to make the surface of fiberglass
projects as smooth and flat as desired. For this project, functionality was chosen over aesthetics
to allow the rest of the team members on the project as much time as possible to work with the
tanks and make the subsystems functional. Therefore, while an attempt was made to smooth out
overall roughness, the most care was taken on the sides where the rail attachments were
mounted. It was critical that these sides be as flat and smooth as possible to allow the greatest
bonding surface area between the Aluminum bar stock and the tank. In order to do this, the top
coat of filler was roughed up again with sand paper, and the phenolic micro balloon filler was
mixed up to a very thick consistency. It was difficult to determine the correct thickness for
optimal coverage and surface finish; too runny and it dripped off while curing, too thick and it
did not adhere to the tank, and would flake off. Had more time been available, this step would
have been done over the course of a few days with multiple layers. Instead, it was done in one
attempt with mixed results.
The bars attached to the containment tanks were most of the length of the tank itself, and
smoothing a layer over that large an area was extremely difficult. There was variation with the
tank due to the tapered edges from cutting on the hot wire as described in section 3.2, which
made smoothing a large area spanning more than two or three foam segments much more
difficult. One side came out well, but the filler applied to the other side was too thick of a layer
and not mixed quite thick enough and resulted in drips while curing. The air tank, being much
narrower, was considerably easier, and was done even while others were working on different
aspects on the same tank. The filler was mixed thicker, went on smoothly, and cured well.
A
layer of filler on the air tank is shown in figure 5.1. The filler was easiest to sand about 20 hours
after the parts had been laid up, as that allowed for the epoxy to be mostly but not entirely cured.
Also, the more filler that was added in the filler coat, and therefore the thicker the paste that went
on was, the softer the overall finish. This helped with sanding, though it did not provide as
resilient of a finish. To improve the hardness and resistance to scratches and dents, a final layer
of epoxy could be applied, which would result in a very smooth, glossy finish.
28
Figure 5.1: Shows the air tank after a second layer of filler was applied to the port hole
face. This also shows the other faces sanded down and ready for more filler coats.
5.4 Finishing the Containment Tank without filler
Since a clear tank was desired, the final containment tank was made without adding a filler coat.
The epoxy resin used was non-blushing and dried mostly clear, as shown in Fig. 5.2. This
eliminated the option to go back and fix flaws after the tank had cured as patches and flaws could
not be covered over and held together by a filler coat. It was difficult to make the edges of the
cloth lay flat, as it all had to be done without the filler to bury loose glass fibers and hold edges
down and in place.
These edges were both aesthetically undesirable and also somewhat
hazardous, as ends of glass fibers sticking up once epoxy has cured become rigid and extremely
sharp. After this containment tank was allowed to dry, it became apparent that the final layer of
fiberglass cloth did not seal completely to the mat layer over the entire tank. Instead, wrinkles
and air bubbles were present in a few areas, likely as a result of imperfect molds. Fortunately,
most of the wrinkles were present in the center of one end from the stand that was used, so that
end was chosen to be milled for the port hole cover. The larger air bubble on the side of the
tank, however, was not as convenient to eliminate. A somewhat crude fix was made by slightly
sanding the surface to open up the texture of the cloth so that resin could be worked into the
bubble, and then the top layer was pushed down onto the tank using the sharp corner of the metal
roller handle, taped down as shown in figure 5.2. This fix was surprisingly successful at bonding
29
the layers together, though it resulted in a potentially more brittle section on the tank from a
section of hardened epoxy not saturating cloth or mat.
Figure 5.2: Fixing an air bubble in the containment tank. This picture also shows the clarity
of the cured epoxy, as the lines running parallel to the electrical tape are the gaps between
sections of the foam mold, and are visible through two layers of cloth and a layer of mat.
5.5 Vacuum Tests and Sealing with Silicon
The original design of the tanks used the vacuum from the evacuated fuel to provide
sufficient suction to fill the water bladders at the desired rate. This required completely air tight
tanks, so a test was created to determine if and where leaks existed. The port hole cover was put
on, the water bladed was filled and attached to a pump, the fuel valve remained closed and the
open air valve was attached to a pressure sensor. Then a plastic bag was duct taped over the end
of the tank with the port hole cover, with care made to completely seal the edge of the bag to the
tank as shown in figure 5.3. The bag trapped air between the end of the tank and the bottom of
the bag, and since the plastic is extremely light, any change in pressure or air movement would
result in the bag changing shape. For the expected result of an incorrectly sealed tank, the bag
would gradually collapse, as air from the bag would be sucked into the tank to equalize the
pressure difference.
The water bladder was pumped dry, the pressure gauge was monitored to see the
changing pressure in milivolts, and the plastic bag was monitored for movement to determine if
the end was sealed. In the first test, there was very little change in pressure detected from the
30
Digital Multimeter (DMM) attached to the pressure sensor, and the bag collapsed, as expected.
The port hole cover was removed and more silicone caulk was added between the tank and the
port hole cover attachment. After the caulk had dried, a second test was done with electrical tape
covering the connection between the port hold cover and attachment. This time, a noticeably
higher voltage, 14 mV, was recorded from the pressure sensor, and the bag did not move. More
tests showed that without the port hole cover taped there was leakage, though it was less than
that found in the first test.
Figure 5.3: Picturing the vacuum test using a plastic bag duct taped to the outside of the
tank. After it was sealed, the water bladder was pumped dry and a pressure sensor
measured change in pressure. The bag was used to determine the location of the leaks as
it moved if the end it covered was not completely sealed.
These experiments showed several things; first was that there existed significant leakage
in the pipes and valves coming out of the tank, and second that nowhere near the required
vacuum was being reached to produce a siphon effect.
Therefore it was decided that an
additional water pump would be used to fill the water bladders as the fuel was consumed to
maintain constant mass and buoyancy.
31
6. Ouffitting Tanks
6.1 Attaching Fittings
Once the tanks were finished and sanded, fittings and port hole attachments and covers
were screwed on. Two of the three fittings on the back of the containment tank were somewhat
difficult to attach, because the plastic nuts that were designed to screw on to form a seal from the
outside of the tank, were located too close to each other to screw both together. Therefore, it was
easiest to put on one of the fittings by screwing it into the plastic nut from the inside, and then
rotating it such that the most room possible was provided to the other fitting. The second nut had
the corners grinded off, and was held with a pair of needle nose pliers while the inside fitting was
screwed into it with a wrench, again from the inside of the tank. The holes were small enough
that the fittings needed to be screwed through the fiberglass, which resulted in a water tight seal,
even before silicone caulk was applied to the threads.
Figure 6.1: The finished filled containment tank with port hole cover attached and the
tubing used to connect the water bladders on both tanks coiled around it.
Some amount of filing had to be done on the pockets of the air tank to allow the valve
that needed to be placed in the pockets to fit, but through a combination of filing and wedging,
32
the valves fit snugly. A benefit to only having three layers of glass fibers on the tanks was that,
while maintaining structural rigidity, they were still able to slightly elastically deform. The tank
with the port hole cover attached is pictured in figure 6.1
6.2 Attaching Rail Mounts
Other team members designed and built a rail system to hold all of the components of the
containment tank together and in place, and to provide power for all elements of the system such
as valves, pumps and sensors.
Attachments were therefore needed to connect all of the
components to the rail system. For the tanks, these bars were made out of aluminum angle stock,
with the wider flat side glued to the side of the tank using super glue. Tests were run to
determine the best adhesive for metal to fiberglass or epoxy and phenolic micro balloon filler,
and it was determined that Gorilla Glue Super Glue created the strongest bond.
To attach the rails to the tank, the surface of the tank was roughed up with sand paper,
and lines were carefully drawn with a level at precisely the correct height. Measuring this was
somewhat difficult, as the bottom of the tanks was rounded. It was done by shimming up the
tanks with a level on top of a flat surface on a fuel valve for the containment tanks, or a water
valve for the air tank. Then the midpoint was located and marked on the tanks, and the line was
drawn half an inch higher to account for the thickness of the rails. It was critical that the rails be
parallel with each other so that the tanks were not able to rock back and forth.
This was
necessary to avoid applying torques to the adhesion of the rail attachments to the tank as would
have been the case had the attachments been tightened to rails not attached parallel to each other.
Once this had been done, glue was generously applied to the rail attachments which were
immediately pressed against the tank wall. They were moved around slightly to spread the glue
to maximize bonding surface, and then held in place for a few minutes with pressure applied
from the inside of the tank outwards, again to increase bonding surface area as much as possible.
The rail mounts are pictured in figure 7.4
6.3 Valve brackets
In order to fit all of the piping into a small space and avoid damaging the sensors,
attachments were made out of G10 Garolite which was cut into an L shape on a band saw. The
33
fastening holes on the valves were marked on the brackets, and clearance holes were drilled out
on a drill press. As with the rails, the section of tank that the brackets were attached to was
roughed up with sand paper, superglue was applied to the bracket, and it was placed on the tank
with care to make sure it was straight. The first few attempts were done without marked lines,
and ended up being significantly crooked, due to the difficulty of making the tanks level. In
these cases, the glue dried before the placement could be fixed, and the brackets had to be pried
off using a knife blade and flat head screw driver. Then all super glue residue had to be removed
from both the attachment and the tank before it could be reattached. The fuel valve was attached
to the mount using standard M3 screws, and the air and water valves were attached using M2. A
final mount was made for the water pump out of the same material. The mounts are pictured in
figure 7.4
7. Creating Strangely Shaped Parts Using Fiberglass
While the tanks were the most functionally important fiberglass component for the bench
top model, other parts were needed for the overall mockup. Fins and a nose cone were needed to
outfit the tube for eventual water testing, and a snorkel and exhaust sheath were needed to
protect exposed external piping components and help keep unwanted water out of the system.
The snorkel and exhaust sheath turned out well using fiberglass, as for the most part they were
symmetrical and therefore not difficult. Fiberglass was not ultimately used for the fins and nose
cone, but a considerable amount was learned about fiber glassing capabilities through attempts to
make them work.
7.1 Capabilities of Hot Wire Foam Cutting
The easiest way to make to make molds, especially for small, thin parts proved to be
using the Pappalardo Lab's hot wire again, as described in section 3.2. The fins presented a
unique challenge, as they were to be less than an inch thick, and the only foam available was 2
inches thick. The base shape was cut out using a composite board template, and was reasonably
simple due to straight edges. Then, a one inch thick ruler shaped composite board was taped to
34
each side of the fin shaped foam section and the foam section was now cut in half horizontally
producing two fins, slightly thinner than one inch due to shrinkage from melting. These cuts
were surprisingly easy to do, and produced very nice, consistent results.
The molds were then
shaped and coated generously with mold release wax to smooth the surface and allow for the
possibility of foam removal. Various stages of the fins construction is shown in figure 7.1.
Since four separate circles were needed to make the nose cone, instead of making
templates, a marker attached to a string centered with a pin in the center of the mold traced out
the circle to be cut.
Then the parts were cut out free hand with the hot wire.
Since a
considerable amount of foam removal by hand cutting and sanding was needed to create a
perfectly smooth taper, the sections did not need to be precise and this was the fastest and easiest
way to get roughly the right shape. As with the fins these were then shaped by hand as shown in
figure 7.2. Since there was no reason for the mold to be extracted from this part however, no
mold release wax was added, with the intent being to allow the epoxy to permanently adhere the
mold to the layup. This was done for structural soundness as well as potential buoyancy to
counter the weight of the fiberglass.
7.2 Fiberglass Layup of DifficultShapes
The fins created unique challenges in fiber glassing, as they were almost entirely
composed of tight corners. The small cutaway sections at the base proved to be impossible to
cover as it required cutting and folding from too many directions. This resulted in too much
material adding up in order to get all the edges covered, and eventually the section was ignored
and all material cut flush to the bottom edge of the widest part of the fin. The large flat surfaces
glassed reasonably well, though the top edge was also difficult. Different approaches were made
to try to cover the surface as smoothly as possible, and involved a combination of one piece of
cloth or mat covering the entire surface, and the use of multiple sections to cover the area. In the
later case, a wrap were made around the fin with all material on the top edge cut slightly below
the top of the mold. A second strip, the width of the top section was then laid on perpendicular
to the first layer. Through multiple layers of cloth going both directions, this helped to hold
edges down, and prevented the cloth and mat sections from trying to flatten back out instead of
following the radius of the mold.
Finally, the entire layup was completely covered in a thick
layer of filler, as shown in figure 7.1. While the filler helped maintain shape, the top corners
35
were still difficult to seal, and as shown in the most finished fin in figure 7.1, the top corner was
not quite covered. Since it was determined that the fiberglassed fins were too thick and did not
come down to a sharp enough point on the back edge, the parts were eventually 3-D printed and
painted with lovely results. It was still very useful, however, to discover how tight of a radius
could be forced without vacuum bagging, and the fin on the far right of figure 7.1 had the nicest
surface finish of any fiberglass parts made for the project.
MIND-
Figure 7.1: Showing the stages of mold and layup done to make one of the fins. From
left to right, the roughly shaped fin as cut with the hot wire; the fin after sanding and
applying mold release wax; after the initial layup with rough filler; after sanding and
another layer of epoxy. Photo courtesy of B. D. Harvatine.
The other part that was not ultimately used was the nose cone. This part also proved to
be extremely difficult to make, due to the extremely tight tolerances needed to make a snug fit to
the tube. The rounded section was difficult, as it curved outwards as part of the taper. This
meant that when many slits were cut into the circular sections of cloth, they failed to overlap
with each other, causing gaps to be present between each strip. The solution to this was to rotate
each section of cloth or mat such that the gaps were covered by other layers, but at the most
crucial edge, there ended up being far too many ends to make a clean corner at the edge, as seen
on the upside-down picture on the far right of figure 7.2. As was found with the air tank layup
detailed in section 4.3, even with the mat layer cut clean to the edge, this sharp corner prevented
the fiberglass cloth from being pulled tight, and as the mold had to be turned upside-down to
glass this section, the carefully placed strips on the top started falling off the mold. In an attempt
36
to create something workable, the bottom was covered in one layer of cloth and then a layer of
mat, and then a filler coat was applied.
While a filler coat directly on the mat worked
surprisingly well, without a way to preserve the shape, the top side of the nose cone did not cure
flat. The strips were not immediately cut at the edge in the hope of later re-attaching them to the
base, but enough epoxy had gotten on them that after all had cured, the cloth was no longer
workable and the ends had to be cut. Though it may have been possible to salvage the attempt, it
was determined that the part would more accurately be made purely out of foam and epoxy resin.
Figure 7.2: Showing the stages of the mold for the nose cone in rough form on left, after
it had been smoothed into shape in center, and after initial layup on right.
For a part like this to be attempted in the future, it may be more successful if the base was
glassed first and allowed to cure. Then the first layer of cloth and mat could be put on the top
and allowed to cure, and finally after sanding the entire part, a final layer of cloth could be
applied. Without doing the layup in multiple attempts, it seems unlikely that the layers would go
on smoothly.
The final two parts that were created were the shroud for the snorkel and water manifold.
The first of these was relatively simple, as it was simply an elliptical tube. While it was still
difficult to make the first layer of cloth and mat to stay smooth and flat while the top layer was
pulled tight, once the filler layer was added, the part cured nicely. The water manifold, on the
other hand, was more challenging due to the rounded ends. Though the bottom of the manifold
was to be open, it was easiest to make the cloth lay flat by wrapping around the entire mold, and
later cutting off the back side. In this case, the cloth was cut into a narrower long strip and spiral
wrapped around the mold to help cover the ends. Then additional, small strips were placed
37
around the ends to hold all of the cut and overlapped ends together.
The finished manifold is
pictured in figure 7.4.
7.3b
7.3a
Figure 7.3: Shows the snorkel shroud, 7.3a; water shroud immediately after layup, 7.3b; and
after cured and cut off of the mold, In addition, 7.3b shows the spiral wrapping method can
be seen through the filler coat.
For all of these parts, they were placed on upside-down plastic cups to cure, to minimize
adherence of the filler coat to other surfaces.
Though this left indentations at the point of
contact, these areas were much more easily detached and sanded than would have been possible
had the parts adhered to the drop cloth, or when they were covered with plastic wrap. A final
picture of the shrouds is shown in figure 7.4
38
Figure 7.4: Showing many of the fiberglass parts that were made in their final state.
Visible on the containment tank is one of the rail attachments, and connected valves and
sensors on their respective mounts. The air tanks has port hole cover visible as well as the
attached water and air valves, and the snorkel and water shrouds in their final state are
pictured.
8. Lessons Learned
Through this project it was discovered hat perfectly suitable fiberglass tanks could be
constructed with no previous experience. It was also discovered that the most important stage of
the tanks was making the molds perfect. A combination of more experience with the hot wire
and new, flat foam would have resulted in much more uniform tanks. The more mold release
39
agent used, the easier foam extraction was, and for future construction, removing most of the
centers of the mold segments not on the end might be tried to further aid removal.
The largest problems that were encountered were messes in the middle stages of fiberglass
layup, including most of the stages that involved fiberglass mat or corners. When sufficient time
exists and a filler coat is added, it seems that patching is easier than trying to get everything right
in the initial layup. The hardest part of the project was anything involving recessed pockets or
areas that did not allow for the fiberglass cloth to be pulled tight, as on the air tank.
If the tanks were redone, changes would be made to the molds themselves to simplify the
foam removal process after the tanks had cured around them. Since little structural support is
needed for the molds, a considerable amount of the center of each part could be cut away before
assembly. A better stand would be used to allow the tanks to cure without contact, and plastic
wrap would not be used except where absolutely needed to reduce the amount of sanding needed.
For a finished project, another few rounds of filling and sanding would have been done on the
tanks, and ultimately they would have been painted.
The project was a success, however, as functional tanks were created in only a few weeks, at
very low cost compared to the rest of the system. The tanks were airtight, and it was determined
to be possible to completely seal any holes into the tank. Mounts and rail mounts were easily
attached to the sanded filler coat using superglue, and were strong enough to maintain position of
valves, pumps and sensors, as well as hold the entire weight of the tanks on the rails.
As for the other fiberglass parts, it became evident that while some parts come out
beautifully using fiberglass, if the required radiuses are too small, such as on the fins and air
tanks, either the design must be reworked, or a different material should be used.
The most useful lesson on this project was the merit of quickly and easily creating a rough
prototype. This allows for the anxiety of trying to get it exactly right to be alleviated, and
produces a product that could be used if necessity required them to be. The only way to learn to
fiberglass is to simply do it, though hopefully this document can be used to educate readers on
things to watch out for, what is easily done, what is done with difficulty, and which things to
avoid.
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