| ... | ... | @@ -12,15 +12,24 @@ The full method is separated in three different steps. First, you have to make a |
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Making a backing mold for the mold of the sample
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The backing mold is nothing but the positive of your sample. You need it to cast the silicone 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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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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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 or machine it. Here the material you will use does not really matter since the silicone we will cast does not sticks at all.
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* 3D printed small cylinders 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] technique to back out the vector forces on urethane particles are only well-developed for circular particles.
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* A modular dish with cylindrical posts of two sizes:
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* Examples of other post shapes, with threaded rods revealed:
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Making a mold of the sample
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| ... | ... | @@ -212,4 +221,3 @@ Or if you trap a bubble: |
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