Figure: Single-Bore Ceramic Collars. The gray marks on the collars show where we have scraped silicone off the outer surfaces.
Once cured, we note that silicone has crept half-way up the outside of the collars. We scrape the silicone off with a scalpel. Cut cut the short ends to the same length, remove 2 mm of silicone to expose the wires, and cement into a tub of dental cement all together. We place in water and set up a motor-driven wire-flexer that pulls on the leads while they sit in a beaker of water. We clamp the leads 50 mm from the collars and we streth them by 10 mm once per second.
Figure: Wire Flexing. We are measuring isolation of the orange lead while flexing all four leads by 20% once per second.
After thirty seconds, we measure isolation resistance of each lead with respect to the water in which the cement fixture is immersed. For the blue, red, and purple leads, the isolation is only 1 MΩ, indicating the seal between the silicone and ceramic has broken. We are seeing the resistance of the microscopic film of water between the lead and the collar. The orange lead's isolation is >100 GΩ. We put the cement fixture in our ove at 60°C. After half an hour, we place in water again and isolation resistance is >100 GΩ for all four leads. We begin flexing. Within thirty seconds, isolation resistance is 1 MΩ for blue, red, and purple. The isolation of the orange lead remains >100 GΩ after 3600 s of flexing.
The fit between the lead and the collar in the previous four examples was tight only in the case of one of the smaller collars, although we did not make note of whether it was the orange or the purple. The only way to make the fit tight is to start with a lead of tapering thickness. We have eight such leads. We make four tight-fitting collars with silicone as adhesive and leave to cure in warm humidity. Another possible source of failure in the collars we glue in place with silicone is failure of the seal outside the collar because of silicone residue on the ceramic surface. We make another four and glue with Loctite Ultragel. We allow the glue to cure in warm humidity for one hour. We assemble into a dental cement head fixture and load into the wire flexer. We connect all four free ends of the leads together to our isolation meter. All four isolation resistances in parallel are 3 GΩ. After 600 s, combined isolation is 4 GΩ. After 5000 s, it is >100 GΩ. We leave the flexer running.
UPDATE: [16-MAR-16] After 18 hrs of flexing, leads 1-3 of our set of 4 have broken by fatigue where they pass over the lip of the beaker in our wire-flexing apparatus. Lead 4 is intact, with isolation resistance >100 GΩ. We remove silicone from the free ends of the other leads, and isolation resistance is >100 GΩ for all three. In one case, we have 65,000 flexes and the collar seal is still intact. In the other three cases, we have failure of the spring by fatigue before the collar seal fails by fatigue.
Figure: Four Surviving Collars. Leads 1-3 broke by fatigue some time between 5k and 65k flexes. Lead 4 survived 65k flexes.
UPDATE: [17-MAR-16] We left the four collars in water overnight. Their combined isolation resistance is >100 GΩ. We also have four collars cemented to leads with silicone dispersion. All four have a layer of silicone roughly 50 μm thick on the outer surface at the ends of the collar. We pull on the collars and find that the cement in three of them breaks easily, and in one of them a good tug gets it free. Given the success of the Loctite Ultragel cement, we abandon the silicone cement.
