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Urethane casted photoelastic samples
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Urethane cast photoelastic samples
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====================================
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Photoelastic samples can be casted! And this is, by far, the most convenient way to get it: more versatile (shape, mechanical properties, color...), cheaper and without absolutely any residual stresses. Different photoelastic materials are castable. In this section we will talk about urethane but you can also look [here](/molding-gel) to see how to make photoelastic samples from gels. Acrylics are another popular photoelastic material.
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Photoelastic samples can be cast! And this is, by far, the most convenient way to get it: more versatile (shape, mechanical properties, color...), cheaper, and without absolutely any residual stresses. Different photoelastic materials are castable. In this section, we will talk about urethane but you can also look [here](/molding-gel) to see how to make photoelastic samples from gels. Acrylics are another popular photoelastic material.
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This method is based on product you can find online at [Smooth On](https://www.smooth-on.com/). We emphasize here that the different authors of this wiki do not have any link of any sort with this company unless it is clearly specified in a paragraph.
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This method is based on products you can find online at [Smooth On](https://www.smooth-on.com/). We emphasize here that the different authors of this wiki do not have any link of any sort with this company unless it is clearly specified in a paragraph.
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The method described in details here have been first introduced by [Cox *et al.*](http://iopscience.iop.org/article/10.1209/0295-5075/115/64003/meta) and [Barés *et al.*](https://www.epj-conferences.org/articles/epjconf/abs/2017/09/epjconf162463/epjconf162463.html). In case you would like to refer to the method in a publication, please cite these papers.
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The method described in details here have been first introduced by [Cox *et al.*](http://iopscience.iop.org/article/10.1209/0295-5075/115/64003/meta) and [Barés *et al.*](https://www.epj-conferences.org/articles/epjconf/abs/2017/09/epjconf162463/epjconf162463.html). In case you would like to refer to the method in a publication, please cite these papers.
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The full method is separated in three different steps. First, you have to make a rigid backing mold to, then make a soft mold in which you can cast your sample. In the following we go step by step through the method.
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The full method is separated in three different steps. First, you have to make a rigid backing mold, then make a soft mold in which you can cast your sample. In the following, we go step by step through the method.
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Making a backing mold for the mold of the sample
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Making a backing mold for the mold of the sample
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------------------------------------------------
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------------------------------------------------
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The backing mold is a positive relief of your desired particle shape. You need it to cast the silicone mold that you will use to cast the urethane. Polyurethane is strongly adhesive to most mold materials. Silicone is an exception, so the backing mold is used to cast a silicone mold for the urethane particles rather than directly fabricating a mold for the polyurethane.
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The backing mold is a positive relief of your desired particle shape. You need it to cast the silicone mold that you will use to cast the urethane. Polyurethane is strongly adhesive to most mold materials. Silicone is an exception, so the backing mold is used to cast a silicone mold for the urethane particles rather than directly fabricating a mold for the polyurethane.
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Users have developed different techniques for fabrication of the backing mold. The simplest technique is to glue something the shape of the desired particles in the bottom of a cup. For more precise or reproducible results, you can 3D print or machine the mold. Here the material you will use must only be chemically compatible with uncured silicone (true of most materials).
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Users have developed different techniques for the fabrication of the backing mold. The simplest technique is to glue something in the shape of the desired particles in the bottom of a cup. For more precise or reproducible results, you can 3D print or machine the mold. Here the material you will use must only be chemically compatible with uncured silicone (true of most materials).
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* 3D printed small cylinders backing mold:
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* 3D printed small cylinders backing mold:
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| ... | @@ -22,7 +22,7 @@ Users have developed different techniques for fabrication of the backing mold. T |
... | @@ -22,7 +22,7 @@ Users have developed different techniques for fabrication of the backing mold. T |
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* Machined diabolo-shaped particle backing mold:
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* Machined diabolo-shaped particle backing mold:
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As in both of the examples pictured above, a mold that produces a silicone negative of uniform thickness and with regularly spaced particle wells will lead to the most reproducible and easy-to-produce castings. One innovation that has proven successful and improves the versatility of the backing mold is to machine a dish to precision flatness, then to drill an array of tapped holes into the dish. Any desired particle shape can then be machined or printed with a matching threaded post and screwed into the dish. Thus, a single backing mold can be used to create particles of any shape. Note that the [inverse analysis](/inverse-analysis) technique to back out the vector forces on urethane particles are only well-developed for circular particles.
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As in both of the examples pictured above, a mold that produces a silicone negative of uniform thickness and with regularly spaced particle wells will lead to the most reproducible and easy-to-produce castings. One innovation that has proven successful and improves the versatility of the backing mold is to machine a dish to precision flatness, then drill an array of tapped holes into the dish. Any desired particle shape can then be machined or printed with a matching threaded post and screwed into the dish. Thus, a single backing mold can be used to create particles of any shape. Note that the [inverse analysis](/inverse-analysis) technique to back out the vector forces on urethane particles is only well-developed for circular particles.
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* A modular dish with cylindrical posts of two sizes:
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* A modular dish with cylindrical posts of two sizes:
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| ... | @@ -32,8 +32,8 @@ As in both of the examples pictured above, a mold that produces a silicone negat |
... | @@ -32,8 +32,8 @@ As in both of the examples pictured above, a mold that produces a silicone negat |
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Making a mold of the sample
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Making a mold of the sample
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Once we have got the backing mold, we can cast the mold. This one is made of [Mold Star silicones](https://www.smooth-on.com/product-line/mold-star/): 'cure to soft, strong rubbers which are tear resistant and exhibit very low long term shrinkage'. They are different sorts depending on what you are casting and how fast you want to go. In the following example we use
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Once we have got the backing mold, we can cast the mold. This one is made of [Mold Star silicones](https://www.smooth-on.com/product-line/mold-star/): 'cure to soft, strong rubbers which are tear resistant and exhibit very low long term shrinkage'. They are different sorts depending on what you are casting and how fast you want to go. In the following example, we use
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[Mold Star 15 Slow](https://www.smooth-on.com/products/mold-star-15-slow/) which fits for most of the cases.
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[Mold Star 15 Slow](https://www.smooth-on.com/products/mold-star-15-slow/) which fits most of the cases.
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Everything you need at this step is:
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Everything you need at this step is:
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* A and B parts of the Mold Star silicone.
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* A and B parts of the Mold Star silicone.
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| ... | @@ -46,7 +46,7 @@ You can find all the safety and processing recommendations for Mold Star 15 Slow |
... | @@ -46,7 +46,7 @@ You can find all the safety and processing recommendations for Mold Star 15 Slow |
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You need first to mix parts A and B in equal ratio to make the silicone. 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 ratios to make the silicone. Recommendations are to mix it in an equal ratio by volume. But it also works quite well in an equal ratio by mass and since it is easier, that is the last way we will use here.
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You pour a certain mass of part B:
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You pour a certain mass of part B:
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| ... | @@ -86,17 +86,17 @@ Once cast, this mold can be reused many times (~100) until it gets a bit sticky |
... | @@ -86,17 +86,17 @@ Once cast, this mold can be reused many times (~100) until it gets a bit sticky |
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Picking the right urethane
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Picking the right urethane
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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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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 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 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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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 by 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 color the sample for imaging purpose, you should use [So Strong](https://www.smooth-on.com/products/so-strong/) dye. In the following walkthrough of the casting method, we lightly tint our sample green because that coloration usually optimizes contrast in a polariscope.
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If you want to color the sample for imaging purposes, you should use [So Strong](https://www.smooth-on.com/products/so-strong/) dye. In the following walkthrough of the casting method, we lightly tint our sample green because that coloration usually optimizes contrast in a polariscope.
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More aggressive addition of dye can simulate the effect of using an optical filter or illuminating with monochromatic light. In the image below, we show sets of particles that are (left to right) darkly dyed, lightly tinted, and clear.
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A more aggressive addition of dye can simulate the effect of using an optical filter or illuminating with monochromatic light. In the image below, we show sets of particles that are (left to right) darkly dyed, lightly tinted, and clear.
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While the darkest particles appear opaque to the eye, placing them on a backlit grid shows that all three colors of particle transmit light:
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While the darkest particles appear opaque to the eye, placing them on a backlit grid shows that all three colors of particles transmit light:
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Indeed, in the blue channel of the image, the particles are nearly indistinguishable:
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Indeed, in the blue channel of the image, the particles are nearly indistinguishable:
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| ... | @@ -111,7 +111,7 @@ Thus, darkly colored particles can be quite useful in application: a single phot |
... | @@ -111,7 +111,7 @@ Thus, darkly colored particles can be quite useful in application: a single phot |
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Casting method
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Casting method
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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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The next step consists in making urethane rubber and pouring it into the silicone mold. This is the tricky part...
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For this step you will need:
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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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* Parts A and B of your chosen `Clear Flex' product.
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| ... | @@ -133,21 +133,21 @@ Some general notes: |
... | @@ -133,21 +133,21 @@ Some general notes: |
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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 need first to mix parts A and B in equal ratios 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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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!' 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. Here are three sets of particles:
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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 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. Here are three sets of particles:
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The dark particles on the left were made with 40 microliters of blue dye per 16g of Part B. These transmit nearly monochromatic light, but the particles will be very dark, nearly opaque to the eye. The particles in the middle were made with 1 microliter of dye per 16g of Part B, and are nearly as transparent as particles without dye, like those on the right, but with a light tint.
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The dark particles on the left were made with 40 microliters of blue dye per 16g of Part B. These transmit nearly monochromatic light, but the particles will be very dark, nearly opaque to the eye. The particles in the middle were made with 1 microliter of dye per 16g of Part B, and are nearly as transparent as particles without dye, like those on the right, but with a light tint.
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Plunge few millimetres of the fishing wire in the dye to get a small drop:
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Plunge a few millimeters of the fishing wire in the dye to get a small drop:
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Pour this drop in the cup:
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Pour this drop into the cup:
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Depending on how strong you want the color to be, you can add more.
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Depending on how strong you want the color to be, you can add more.
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| ... | @@ -156,11 +156,11 @@ Depending on how strong you want the color to be, you can add more. |
... | @@ -156,11 +156,11 @@ Depending on how strong you want the color to be, you can add more. |
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Then 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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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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Then you strongly mix parts 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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Mix again, until the liquid is perfectly clear:
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| ... | @@ -178,12 +178,12 @@ When degassing you will see more and more bubbles, then less and less: |
... | @@ -178,12 +178,12 @@ 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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After 2-3 minutes when just a 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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It is now time to pour the urethane into the mold. This is the most sensitive part. You need first not 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 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 to use anything to carefully add the right volume (especially for small quantities), it will not be accurate and, most likely, you will add bubbles to 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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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 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 makes 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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So begin by filling in the syringe with urethane without trapping bubbles:
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| ... | @@ -197,14 +197,14 @@ Here you see that there is not enough: |
... | @@ -197,14 +197,14 @@ 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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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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A good way to control the volume you added is to play with the light reflection 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 fourth 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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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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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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I said previously that you need to be fast. This is because the urethane gets more and more viscous very rapidly so 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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First, when just mixed, it is liquid like water:
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| ... | @@ -219,11 +219,11 @@ You let it at room temperature for about 16 hours depending on the sample size a |
... | @@ -219,11 +219,11 @@ 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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You get a very sticky material. To finish the curing, put it in an oven at 100C for 5 hours or let it at room temperature for 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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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 lose the transparency but it will not be sticky anymore.
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Finally here is an example of what you can get if you pour too much urethane:
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Finally here is an example of what you can get if you pour too much urethane:
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| ... | | ... | |
