FINAL MOLD DESIGN

Mold Iteration 3

Photographs of the Final mold iteration in different stages of assembly. Printed in Vero Clear.

The final mold design is a rigid design for a more robust mold that can be reused. It is split into 4 sections but this time, the rod in the middle that’s used for making the cavity inside the air retention device, is now in one part instead of two. Also, the black sections are made from a flexible material to allow for a tight fit at the interfaces. By using a razor blade, the PDMS could be cut to make the slit for the colonoscope to slip in instead of casting the slit into the part. During test casts, a phenomenon called cure inhibition was observed. Cure inhibition is when an incompatible material prevents the surface of the PDMS from curing. After consulting Dr. Monica Kim from UCI and Kim Quackenbush from Dow Corning, the cure inhibition was solved by preheating the mold to get rid of residual chemicals, and then painting the inside of the mold with Polyvinyl Alcohol (PVA) which is compatible with PDMS, and acts as a barrier between the mold and the curing PDMS silicone.

Mold Iteration 2

The mold was redesigned to cast the device in one part instead of two. It was made in four parts to prevent backdraft and locking the parts inside the mold. The mold splits were chosen at the flaps and flange so it would be easier to release the part instead of having to pry it out from a deep hole

Photographs of second iteration of mold. Components and features (Left), and Mold being used during PDMS casting (Right). Printed in Tangoblack.

All of the mold parts had keyways to maintain the correct position of the parts, and to create a seal to prevent PDMS from leaking out. A hole was cut at the bottom to allow PDMS to be poured inside. The problem encountered was that the Stratasys 3D printer used to print the parts had errors and printed masses of materials where it shouldn’t be, thus resulting in a poor part. PDMS was then mixed up, degassed, poured into the mold, degassed again, and then left to cure. The PDMS set and partially cured due to what was later determined to be cure inhibiting chemicals used in resin-based printers. This left a bad finish as the uncured PDMS stretched. The partially cured plug was then further cured by waving a heat gun in front of it. It was observed that the plug turned from a milky white to clear as it was heated up, which indicates that the plug was curing.

Mold Iteration 1

The mold design was done in parallel with refining the design of the plug. Mold Iteration 1 was a barebones mold designed to cast the device in two parts. The plug was made into a negative space in solidworks and was cut in half to get two open-faced molds. To produce the hole in the middle of the casted part, half of a rod was made. The 4 part mold was printed out using PLA from a MakerBot 3D printer. Some problems encountered was that the rods weren’t a perfect mate with the mold due to the cooling properties of the PLA and lack of compensation in the CAD, and thus the rods and the grooves where the rods sit in had to be sanded down which ruined the contours.

Casting in vacuum chamber. Note PDMS overflowing over top.

Then, the PDMS was poured the mold, but because the PDMS is viscous, it doesn’t go into every nook and cranny inside the mold, especially at vertices, and corners. Air would be entrained in those corners, and so the mold was placed into the vacuum chamber to be degassed again. However, the mold was cut exactly where the part ends, and so there wasn’t any extra space added. When the PDMS was degassed, it flowed over the mold, so after the pressure was released, the PDMS would be at a lower level and ruin the part. The resulting parts was in this case only a test, to decide on the mold design and to practice casting PDMS.

Photographs of the first iteration mold, meant to be casted in 2 parts and glued together after curing.

The mold didn’t have a great resolution and thus the surface of the product was rough and possessed ridges. Moreover, due to differences in heating and cooling, the mold didn’t have a flat bottom as there were some distortion, but fortunately the inner cavity of the mold was acceptable. Caleb, a graduate student in Dr. Tolley’s lab, helped cast the first prototype and was of tremendous help. At the lab, the PDMS (medical silicone) was mixed up and degassed by placing it into a vacuum chamber. When the PDMS was placed inside the chamber, it started to expand and bubble, and rise up on the container. The vacuum pressure had to be released so that the bubbles can pop and not just shrink back down into the PDMS. This was done multiple times to ensure that all the dissolved air was released.

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