Silicone-Cord Qualitative Composite Demonstration
Objective:
Qualitatively demonstrate orthotropic properties of composite materials and the effects of ply
orientation.
Description of Model and Procedure:
A clear elastomeric polymer was first obtained for use as the matrix material. The poly-dimethyl
siloxane known commonly as Sylgard 184 was chosen. The compound was chosen due to its clarity and
published values of 2.90 MPa for the elastic modulus. The material was also considered desirable due to
its initial fluidic state and ability to cure without application of heat. A small sample of the compound
was prepared to test whether a releasing agent, bond inhibitor, or special coating would be required on
the mold. The aluminum test mold and the material test sample are shown in Fig. 1.
Fig. 1: Test Mold and Test Material
The material test sample was easy removed from the mold. Use of a coating or bond inhibitor was
therefore deemed unnecessary. An aluminum mold was designed using SolidWorks solid-modeling
software and machined by means of computer-numerical-control milling. The material reservoir of the
mold was finished with sand blasting to remove surface features that resulted from the milling process.
Below are the both original design and the mold after completion.
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Fig. 2: Mold Design
Fig. 3 Completed Mold
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Holes were located such that the cord would be uniformly spaced with approximately .1 in. between
parallel cord segments. Fig. 4 shows the 3/32 in. diameter dowel pins that were finger-pressed into the
machined holes.
Fig. 4: Dowel Pin Close-up
Once the pins were placed, the free end of a braided fishing line was cemented to the mold edge using
cyanoacrillic glue. The line was laid as shown in Fig. 5.
Fig. 5: Demonstration of Cord-Laying
The pliers shown were used simply to guide the cord into position. Placing the tip of the pliers at the
base of a dowel pin allowed the cord to easily slide into place. Once the cord was laid, it was cemented
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to another edge of the mold to provide maintain a slight tension. This tension in the line helped prevent
the cord from sagging below the neutral axis. The inserts shown in Fig. 6 and Fig. 7 helped prevent the
cord from being pushed above the neutral axis by the addition of the silicone compound.
Fig. 6: 45-Degree Cord Placement Insert in SolidWorks
Fig. 7: Inserts and Cord With Mold
The first samples were prepared one with cord laid at 0 degrees from the longitudinal axis and the other
at 45 degrees. The polymer base and curing agent were mixed in a 10:1 ratio and poured into the mold.
Gas bubbles in the mixture were removed by pipette rather than troubling with vacuum degassing. The
first samples were allowed to cure at room temperature for approximately 54 hours. The 0-degree
sample was removed by pulling the insert away from the material, removing the dowel pins, and lifting
the material away from the mold. While removing the 45-degree sample, a small section of uncured
polymer material was found along the mold surface. This sample was allowed to set again for
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approximately 12 hours. The two samples were pressed firmly and a paper cutter was used to cut the
samples into strips. The response of the material to this method was somewhat unexpected and thus
these samples are not ideal strips. The mold was cleaned with acetone and a sample was prepared with
cord oriented 90 degrees from the longitudinal axis as well as a sample with no cord. These samples
were however cut into strips by pressing the material beneath sheet aluminum and carefully slicing into
the material with multiple long continuous strokes of a hobby knife.
Results:
Fig. 8 shows one of each of the prepared strips. To some extent the quality difference of the edges can
be observed.
The second method of cutting provided far better results than the first. The second method had
originally been considered as the preferred method, but the first method was attempted as a means of
experimentation.
Recommendations:
If additional strips are to be prepared, the second cutting method should be used. Additionally, the
strips of different orientations could be layered on atop the other and bonded in order to provide some
visual demonstration of overall composite lay-up.
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Relevant Files and Documents:
Results and Reports, End of Summer\Silicone-Cord Composite Strips\SolidWorks Files
Results and Reports, End of Summer\Silicone-Cord Composite Strips\Photos
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