Wednesday, February 20, 2013
Another spectacular failure
Last night I mixed and added the second half to my second attempt at making a mold. Up until then, everything had gone according to plan, and I had managed to avoid the mistakes of my first attempt. Unfortunately the mold failed anyway. I haven't taken any pictures of the light tank model recently, so the above image shows it without many of the details which have been added since. The second mold was an attempt to replicate the track assembly, and very similar to the first attempt, except in that I used blu-tack instead of plasticene and the perspex box was smaller and closer in dimensions to those of the finished mold.
In the next two images the blu-tack can clearly be seen below the first half of the silicon mold. Cure time was 6 hours, but I left the mold over night and didn't remove the blu-tack until 18 hours had passed. As can be seen in the image below, the blu-tack came away with relative ease and did very little damage to the model. At this point, things were looking good. I let the mold sit for another 10 hours then mixed the silicone and hardeing agent for the second half. I applied a generous spray of release agent as shown by the various tutorials I have read and seen and poured the mix last night.
I examined the result today and found a silicone brick. The release agent had done nothing to prevent the two halves from welding together so completely that even after I ripped the whole thing apart and examined it closely, I found no indication of anything like a seam.
In the image below; 1. shows the only damage sustained by the model. A few tracks came away during extraction showing how easy pvc plastic can be released from the silicone. 2. shows seepage points where the silicone had found minute gaps in the model. If the mold had been successful this 'flash' would have had to be removed with a scalpel before resin casting. 3. shows the location of the keys. Its noticable that the silicone was toughest at these two locations.
Having spoken to my brother since, I have decided to conduct tests using lego bricks until I perfect the alchemy involved. From my research, I hadn't anticipated this much difficulty. The process as described online, and shown on You Tube seemed pretty straight forward.
My first test will use talcum as a release agent (as advocated by an online tutorial I read), another will use hair spray (suggested by my friend Claus) and the third will use PTFE - Teflon/graphite locksmithing oil (suggested by my brother Peter).