The primary proposed design for
this project requires the use of CNC machining to fabricate the mold. In this
process, a SolidWorks design of the mold cavity will be created from the design
parameters given by the sponsor. The model will have five 0.05 mm (diameter)
cavities for the placement of the electrode spheres. A channel with a diameter
of 0.25 mm will be engraved into the mold for placement of the lead wires. The
SolidWorks model for the mold cavities are shown below
Negative Image of Engraved Channel with First
Design Approach using CNC machining (dimensions in mm).
Corresponding SolidWorks Model for Negative
Image of Engraved Channel with First Design Approach (dimensions in mm).
In order to use CNC machining to
fabricate the mold, the SolidWorks design must be converted into a MASTERCAM
model. This model will include a user programmed pathway that will yield the
required drilling/milling path the CNC machine must take to fabricate the mold.
In addition, a stepper must be used to reach higher drill speeds around 30,000
rpm. This stepper requires a housing component that would have to be
manufactured before it can be utilized. The mold will be fabricated out of
6061-T6 Aluminum, which was chosen for its strength per weight ratio,
workability, and resistance to moisture. Stainless Steel was the secondary
choice for the mold material that was quickly ruled out by the team due to its
extreme high cost.
The procedure for fabricating the electrode
ball contacts will not be changed from that already used by Dr. Clement’s
research staff. The process will still employ the use of 0.25 mm 90% Platinum
and 10% Iridium wire. A pre-measured segment of the wire will be exposed to a
flame which will melt the tip into a ball-shaped electrode. The mold that will
be fabricated by the CNC machine will have five cavities with 0.5 mm diameters
for the electrode balls to be placed in. These cavities will be coated with PVA
(Polyvinyl Alcohol) to prevent the electrode surfaces from being covered with
silicone. The team will run a series of impedance tests on the wire to
determine how much surface area the silicone needs to cover to get the required
impedance of 20 kΩ.
The mold itself will consist of two halves.
The wires and electrode spheres will be placed into the bottom half in their
respective channels/cavities. The top half of the mold will be a mirror image
of the bottom half without the electrode cavities. Pins will be set on the
outer diameter of the two molds to secure the piece together. Silicone will be
injected through holes situated axially to the mold cavity. A pumping system
will apply positive pressure to one end of the mold to inject silicone into the
channels. Negative pressure will be applied to the other side of the mold to
draw the silicone through the channels. The pumping system used in this design
will be two syringes, one applying positive pressure and one applying negative
pressure.
After the silicone is injected through the
system, the mold will then be placed on a hotplate at 65° C to cure the
silicone. The pins holding together the two mold plates will be removed and the
electrode array will be run under hot tap water to wash off the PVA. The final
array itself will be examined and tested for any imperfections or errors.
