| ... | ... | @@ -61,7 +61,7 @@ And you strongly mix it with the fork taking care of scratching the edges of the |
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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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The silicone is ready you do not need to degas it, though it can improve the quality of larger molds (100+ particle molds) to briefly degas under vacuum, then cure under pressure. Then, you slowly pour it in the backing mold, making sure that the particle-shaped posts are covered by at least 3 mm of silicone. If you have a small spill or drip down the sides of your backing mold, the silicone is easier to clean once cured:
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| ... | ... | @@ -72,55 +72,60 @@ After about 5 hours (over night if you want to be sure) the silicone is cured: |
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You can, now, safely pull the silicone mold away from the backing mold. It is very resistant so do not hesitate to pull it strongly:
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You can, now, safely pull the silicone mold away from the backing mold. It is very durable so do not hesitate to pull it strongly:
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To be sure that the mold is correctly cured, once demolded you can cure it few more hours at 60 C (this is not absolutely necessary but better if you want to keep your mold longer). Here is an example of mold you can get:
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To be sure that the mold is correctly cured, once removed from the backing mold you can cure it few more hours at 60 C (this is not absolutely necessary but better if you want to keep your mold longer). Here is an example of mold you can get:
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Once casted, this mold can be reused a lot of times (~100) until it gets a bit sticky. Also, remember that it can be easier to duplicate this casting step instead of duplicating the backing mold or making a bigger one.
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Once cast, this mold can be reused many times (~100) until it gets a bit sticky or accumulates damage. Molds for non-concave particles tend to last longer than those for particles with concave features. Also, remember that it can be easier to duplicate this casting step instead of duplicating the backing mold or making a bigger one - if the quality of your final particles starts to deteriorate, a helpful troubleshooting step is to create a new silicone mold.
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Picking the right urethane
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--------------------------
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Before casting urethane, you need first to choose the best for what you want to do. The photoelastic signal is a function of the strain: the larger the deformation, the more you will have fringes. But if fringes are too dense they will overlap and you will not be able to measure anything. This will happen if you load your sample too much or if it is too soft. On another hand if the sample is too stiff or if you do not load it enough the photoelastic signal will be too weak to see something or to measure accurately. Note that this also depends on the thickness of you sample. That is why, depending on your sample's shape and on how you load it, you need to pick the right urethane stiffness.
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Before casting urethane, you need first to choose the best for what you want to do. The photoelastic signal is a function of the strain: the larger the deformation, the more you will have fringes. But if fringes are too dense they will overlap and you will not be able to measure anything. This will happen if you load your sample too much or if it is too soft. On another hand if the sample is too stiff or if you do not load it enough the photoelastic signal will be too weak to see resolve the forces. Note that this also depends on the thickness of your sample. That is why, depending on your sample's shape and on how you load it, you need to pick the right urethane stiffness.
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The castable urethane we use here is a [Clear Flex](https://www.smooth-on.com/product-line/clear-flex/): 'a water white clear urethane liquid rubber compound designed for applications that require absolute clarity'. It is available in three different shore stiffness: 30, 50, 95. The one we use here is the [Clear Flex 50](https://www.smooth-on.com/products/clear-flex-50/). It is the most commonly used for centimetric samples that undergoes few Newton loadings. Also, note that playing (non linearly) on the quantity of cross-linker (part A) you add in the urethane, you can tune the material stiffness. But be careful because you also play on the curing time!
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The castable urethane we use here is a [Clear Flex](https://www.smooth-on.com/product-line/clear-flex/): 'a water white clear urethane liquid rubber compound designed for applications that require absolute clarity'. It is available in three different shore stiffness: 30, 50, 95. The one we use here is the [Clear Flex 50](https://www.smooth-on.com/products/clear-flex-50/). It is the most commonly used for centimetric samples that experience up to a few Newtons of load. Also, note that playing (non linearly) on the quantity of cross-linker (part A) you add in the urethane, you can tune the material stiffness. But be careful because changing ratios also changes the curing time, and can introduce more mechanical nonlinearity!
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If you want to slightly color the sample for imaging purpose, you should use [So Strong](https://www.smooth-on.com/products/so-strong/) dye. In the following we color our sample in green because it is usually the optimum color for polarisers.
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If you want to color the sample for imaging purpose, you should use [So Strong](https://www.smooth-on.com/products/so-strong/) dye. In the following, we color our sample in green because it is usually the color which optimizes contrast in a polariscope. Coloring your sample can also simulate the effect of using an optical filter or illuminating with monochromatic light - in this case it is helpful to keep in mind that the stress-optic coefficient is a function of wavelength: coloration, in addition to stiffness and material thickness, controls the resolution and dynamic range of your photoelastic measurements. Longer wavelength (redder) light produces fewer fringes for the same strain than shorter wavelength (bluer) light.
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Casting method
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Once you have chosen the good stiffness, the next step consists in making urethane rubber and pouring it in the silicone mold. This one is the tricky part...
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The next step consists in making urethane rubber and pouring it in the silicone mold. This is the tricky part...
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Everything you need for this step is:
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* A and B parts of the Clair Flex.
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* green So Strong dye.
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* a cup and a fork to mix the urethane. In this case it is highly suggested to use disposable tools because it cannot be cleaned at all.
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For this step you will need:
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* Parts A and B of your chosen `Clear Flex' product.
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* green `So Strong' dye.
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* a cup and a tool to mix the urethane. It is highly suggested you use disposable cups because cleaning the clear flex residue is very difficult.
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* your silicone mold.
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* a syringe.
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* a syringe (or micropipette).
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* a scale. You can also use anything to measure volumes.
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* protection gloves.
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* a vacuum chamber coupled with a vacuum pump going as low as 0.98 bar (9 inches of mercury).
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* a piece of fishing wire or any very thin wire.
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Here again, you can find all the safety and processing recommendations for Clear Flex [here](https://www.smooth-on.com/tb/files/CLEAR_FLEX_30_TB.pdf). Remember to check this first.
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Here again, you can find all the safety and processing recommendations for Clear Flex [here](https://www.smooth-on.com/tb/files/CLEAR_FLEX_30_TB.pdf). Remember to check this first.
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Some general notes:
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- Because of the fast curing time, it is good to work in small batches when possible.
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- For larger batches, it is essential to mix the components thoroughly. A spiral arm paint mixer running at approximately 600 RPM is an effective approach.
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- One of the greatest challenges is depositing the mixed resin into the mold in uniform volumes. The high viscosity and non-linear rheology of the uncured resin make this a difficult task. Metering approaches that use positive displacement work best. Here we recommend the use of a syringe or pipette. If available, a chemical metering pump can provide improved results, particularly for larger batch sizes.
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You need first to mix parts A and B in equal ratio to make the urethane. Recommendations are to mix it in equal ratio by volume. But it also works quite well in equal ratio by mass and since it is easier, that is this last way we will use here.
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You need first to mix parts A and B in equal ratio to make the urethane. The manufacturer recommends mixing it in equal parts by volume. We have found it also works well in equal parts by mass, and since it is easier to dose by mass, that is the procedure we will use here.
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You pour a certain mass $`M`$ of part B:
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If you want to color the urethane do it now! First remember that the dye is called 'So Strong'... So if you want to keep the transparency of your sample just add very few.
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If you want to color the urethane do it now! First, remember that the dye is called 'So Strong!' A little bit goes a long way. If you want to your grains to remain transparent to the eye, just add the smallest amount (1 drop is generally enough). If using color to simulate a color filter or monochromatic light, we recommend carefully measuring the volume of dye you use, and determining the correct dose by trial and error. 1 microliter per 16g of `Part B' will produce nearly transparent particles. 40 microliters of dye will ensure only very monochromatic light will be transmitted, but the particles will be very dark, nearly opaque to the eye. The "dark particles" depicted below were cast with these higher volumes of dye.
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| ... | ... | @@ -220,4 +225,3 @@ Or if you trap a bubble: |
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