| ... | ... | @@ -123,81 +123,85 @@ Pour this drop in the cup: |
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Depending on how strong you want the color to be, you can add more.
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You add $`M/2`$ of part A:
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Then you add $`M/2`$ of part A:
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Note that you can use any elongated tool to conveniently guide the liquid flow out of the bottle. Also note that it is very important to carefully clean the A part bottle before closing if you do not want the crosslinker to permanently seal the bottle. You will see what I am talking about the next time you will make a sample...
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Then you strongly mix part A and B with the fork taking care of scratching the edges of the cup. First the liquid is blurry:
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sticky use talk
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Then you strongly mix part A and B with the fork taking care of scratching the edges of the cup. First the liquid is blurry:
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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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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 farther).
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So to fill the mold taking all these points into account, here is a suggestion. First, use a syringe. This is the most convenient way to manipulate urethane without adding bubbles (still you need to be careful). Choose a syringe with a convenient volume: if small you will be accurate when pouring but if the sample is large it will take time to fill it (and you need to be fast); if big it will go faster but it will be less convenient (or even impossible) to add the right quantity. You can also use different syringes: a large one to fill in the mold rapidly and a smaller one to add the exact quantity. Also try different syringe mechanisms because some can have stick-slip behaviors when you push on them which make the accurate filling impossible to get.
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So begin by filling in the syringe with urethane without trapping bubbles:
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Then, fill rapidly the mold to almost the right quantity:
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Here you see that there is not enough:
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Then, go back and add the exact quantity plus a little bit more:
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A good way to control the volume you added is to play with the light reflexion instead of looking directly, since the urethane is mostly transparent. For example you see here (from left to right) that the first hole is filled with the exact mold volume which is not enough. On the contrary, there is too much urethane in the third one. The second and the forth ones are OK. How do I know that? I trained... That is the secret of the method it is a trial and error process.
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If you need to add just a little bit more in the mold (and if this is less than a drop) pull this quantity out of the syringe and deposit it at the top of the mold hole:
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Remember also that, if you put too much you can still suck it back. Same if you add a bubble you can suck it back in the syringe and then trash it.
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I said previously that you need to be fast. This is because the urethane gets more and more viscous very rapidly so that it is almost impossible to manipulate accurately after around 20min.
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First, when just mixed, it is liquid like water:
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After 20min it is more viscous:
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And after 30min you cannot manipulate it with a syringe at all:
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You let it at room temperature for about 16 hours depending on the sample size and then demold it:
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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 let 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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| ... | ... | |
