Mold for PDMS Frame

Initial prototypes of the mold were created using the Formlabs Form 2 SLA printer. The first iteration was created as a proof of concept using Durable resin (Figure 22). The first prototype of the mold was used to show design feasibility and to finalize PDMS frame dimensions. It contained the inverse of six frames with a slot for Thermanox®. Three of the wells had grooves for a glue fixturing system, and three were for a slit-based system. Prior to use, the wells of the mold were washed with 70% isopropyl alcohol to remove any uncured residual resin left behind from the fabrication process. PDMS was poured into the mold, vacuumed, and cured following standard procedure. The PDMS frames successfully released from the mold using a razor blade to separate the PDMS from the sides. This iteration of the mold was used to support the decision to use slits as a fixturing method instead of glue.

Figure 22. Durable resin mold

The second iteration of the mold was printed using Formlabs Biocompatible resin (Figure 23). This mold had six wells configured for the slit fixation method. Well dimensions were updated, and the overall mold dimensions were optimized to decrease material usage and manufacturing costs. Biocompatible resin was chosen to minimize the risk of any potentially cytotoxic resin components leaching into the PDMS. Prior to filling the wells with PDMS, the mold was soaked in 70% isopropyl alcohol for five minutes and thoroughly flushed with water to remove any uncured resin. However, after baking the PDMS for 2 hours at 60°C and letting it sit at room temperature for an extended period of time, the PDMS did not fully cure and would not cleanly release from the mold. The mold was given subsequent five-minute soaks in 70% isopropyl in an attempt to remove any residue that was interfering with the curing process that was not removed during the first wash. New batches of PDMS were also made to verify that the curing issue was not caused by improper mixing. These corrective measures failed to resolve the issue, and surfaces of the PDMS frame that were in contact with the biocompatible resin

continued not to cure properly. Once the frames were removed from the mold and allowed to sit for an extended period of time, full curing occurred, however the surface features were distorted with low resolution. This preliminary testing showed that biocompatible resin was not a feasible material choice for printing. In addition to preventing the PDMS from curing, printing with biocompatible resin is more than double the cost of printing with other resins.

Figure 23. Biocompatible resin mold

The third iteration of the mold was created to decrease PDMS frame removal time. Instead of a 1-piece design, this mold had separate side and bottom components (Figure 24). A rubber gasket was placed between the side and bottom pieces to prevent PDMS from leaking out of the wells. Due to the project timeline and material availability, the two-piece mold was printed using Formlabs Tough resin. The Tough resin was found to be unsuitable for this application, as it was flexible and warped before heating. The team also anticipated having issues with PDMS curing, similar to what was experienced with the biocompatible resin. The complexity of this

design also created more potential for user error and variability. These factors caused the team to designate the two-piece design unfeasible and not proceed with further testing.

Figure 24. Tough resin two-piece mold. The sides of the mold press fit onto the bottom of the mold. When PDMS is cured, the frames can easily be released from the mold by removing the side piece.

Additive manufacturing using the Formlabs Form 2 SLA printer was chosen as the preliminary prototyping technique due to cost effectiveness, time, and the availability of a biocompatible material. Additionally, some features on the original prototype were too small to be machined accurately. After testing three mold prototypes created with SLA printing it was determined that the Form 2 resin post-print curing process was not consistent enough to be used reliably with PDMS. Each resin cured differently, and the amount of uncured resin left after post-processing varied between materials and print iterations. PDMS would not cure consistently and it was difficult to ensure that all uncured resin was removed from the part.

To avoid the issues associated with Form 2 mold prototypes, a 1-piece 6-well mold was machined from Delrin™ acetal plastic using a 3/32”end mill bit. A 3-degree draft angle was

added to the sides of the mold to facilitate easy removal of the frames. PDMS poured in the Delrin™ mold cured fully after one hour at 60°C and the frames lifted out of the mold cleanly. Figure 26 shows a frame made in the Delrin™ mold. This iteration of the mold was chosen as the team’s final design and material because acetal plastic allowed the PDMS to fully cure and release from the mold. Additionally, the inclusion of a draft angle and the rounding of the corners facilitated easy frame removal. Acetal plastic has a heat deflection temperature higher than the PDMS curing temperature of 60 °C and does not leach cytotoxic elements into the PDMS. The feasibility of the Delrin™ mold was verified using testing described in the following sections and it was selected as the team’s final design. Three more identical molds were

machined to increase design validation data throughput.

Figure 25. Machined Delrin™ acetal plastic mold Figure 26. PDMS frame made in Delrin™ mold