| ... | ... | @@ -45,11 +45,11 @@ You pour a certain mass of part B: |
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You add the same mass of part A:
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And you strongly mix it with the fork taking care of scratching the edges of the cup:
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The silicone is ready you do not need to degas it. Then, you slowly pour it in the backing mold, making sure that every edge of the mold is reached. You do not need to take care about bubble they will go away naturally before curing. You can also pour it in access without cleaning, it is easier to clean once cured:
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| ... | ... | @@ -138,7 +138,7 @@ Mix again, until the liquid is perfectly clear: |
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By mixing we trap bubbles in the urethane. We need to get rid of them to have a perfectly clear sample. To do so we will degas the urethane in a vacuum chamber:
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When degassing you will see more and more bubbles, then less and less:
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| ... | ... | @@ -150,7 +150,7 @@ When degassing you will see more and more bubbles, then less and less: |
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After 2-3 minutes when just few small bubbles are remaining you can pull the cup out of the vacuum chamber. You should get something like this:
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It is now time to pour the urethane in the mold. This is the most sensitive part. You need first not to add bubbles by pouring or by sucking up into the syringe. And you also need to pour the exact quantity to fill the mold... plus a little bit more! Indeed you want your geometry to be the exact one but the Clear Flex will retract when curing. So imagine that the top of your sample corresponds with the top of the mold, if you add exactly the right volume of liquid, you will get a flat surface before curing and something convex after. Which is definitely not what you want for an accurate photoelasticity. Instead if you add a little bit more than the mold volume, you will get something flat after curing. Because the liquid is viscous you can forget using anything to carefully add the right volume (especially for small quantities), it will not be accurate and, most likely, you will add bubbles in the liquid. Last problem... you need to be fast because the liquid will be more en more viscous very rapidly (we come back on this point a bit further).
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| ... | ... | @@ -191,15 +191,17 @@ You let it at room temperature for about 16 hours depending on the sample size a |
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You get a very sticky material. To finish the curing, put it in an oven at 100C during 5 hours or let it at room temperature during one week:
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The sample will be less sticky. If you want something even less sticky, you can slightly add a little bit of talk on top of it. You will somehow loose the transparency but it will not be sticky any more.
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Finally here is an example of what you can get if you pour too much urethane:
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Or if you trap a bubble:
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| ... | ... | |
