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Urethane casted photoelastic samples
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====================================
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[Smooth On](https://www.smooth-on.com/)
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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)
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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 stress.
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This method is based on product you can find online at [Smooth On](https://www.smooth-on.com/). We specify here that the different authors of this wiki do not have any link of any sort of link 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 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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Making a backing mold for the mold of the sample
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---------------------------
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The backing mold is nothing but the positive of your sample. You need it to cast the silicon mold that you will use to cast the urethane. Unfortunately, the urethane strongly sticks to almost everything (but the silicone we will use) so you definitely need this silicone mold. This means you need its backing mold. You cannot directly machine the urethane mold.
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You have different way to make this backing mold. The simplest is to glue something that have the same shape as your final sample in the bottom of a cup but just like in the examples below, you can also 3D print it of machine it:
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Making a mold of the sample
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---------------------------
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[Mold Star silicone](https://www.smooth-on.com/product-line/mold-star/)
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[this one](https://www.smooth-on.com/products/mold-star-15-slow/) fits for most of the cases
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| ... | ... | |
