Molding and casting 1David Carr


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concept and development of an assignment that involves digital and sculptural skills in product or prototype making

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Molding and casting 1David Carr

  1. 1. Step 0: Concept This week I decided to create a "political topography" map. The map shows the county by county results of the 2008 presidential election. Higher areas on the map correspond to areas that voted more democratic. The map data itself comes from a reprocessed version of the purple map. Step 1: Reprocessing In order to comvert the purple map into a mold I made a few modifications. To get the "democraticness", I extracted the blue channel from the image and converted it to greyscale. I then removed all of the more spindly map features that might break off (sorry, New Orleans) and added a thick black border around the image to serve as a perimeter wall on the negative mold. After processing, the image looked like this:
  2. 2. General Theory For any molding and casting the process almost always is one of casting a rigid (or soon to be rigid) material into a flexible material. This is so that the casting (the part you want) can be easily removed from the mold. In our case we add an extra step by cutting a rigid mold for the mold (kind-of confusing) out of wax. If the part you want to make is made from a flexible material (rubber or similar) you can simply cast into the wax mold. If, however, you want a rigid part, you will need to design your wax mold so that you can pour rubber into it to make the mold for your part. Think positive and negative space.For a great description of a possible workflow see David Carr's page. Step 1: Creating the Mold
  3. 3. There are several ways to create a mold, and your decision will mostly be based upon the scale of your desired final piece. Modella: This is most appropriate for table-top sized pieces, such as chess pieces or a gasket for an espresso maker. You will carve into a piece of hard wax, usually purple or green. The modella only really reads .png files if you're not creating something in, unless the machine has been hooked up to its actual software. *** Don't forget to design a wall onto your mold so that you have something to pour your final cast INTO! *** Make sure that the wax is stuck well in the Modella. If it moves in the middle of carving, it is a huge pain! Important things to know when setting it up: • dark is far, white is near • set height: xmin: 1 ymin: 1 --> do this to start at 1,1 the way we did for the pcb boards • xwidth: value (usually in inches) • yheight: value (usually in inches) • z min: set how deep you want the last cut to be. ie 1 = 1 inch. • z max: set to .005 so that the tool comes back up above the material between cuts • nz: number of steps between the top of the material and the deepest cut - definitely make it more than one if your cut has significant depth. 3-5 worked for me, but you should play around with this number. • tool diameter - usually 1/8" so .125 • xy speed = 10, z speed = 10
  4. 4. Sample work flows: • look into the program that you can get at pr960nSBP) • photoshop/autocad->3dmax->render->.png • photograph->.png image Shopbot: This is most appropriate for larger scale pieces. You can work with soft foam on the shopbot, which is very easy to work with but requires a lot of sealing. To seal it: 1. sand the piece with fine (120 -200 grit) sandpaper. 2. coat it with a thin layer of epoxy-- this is most easily done by brushing on a layer, waiting a few minuets until it has set up a bit and gotten sticky, and then brushing on another layer. 3. let cure then sand and repeat 4. do this until all the pores in the foam are well sealed. Step 2: Rubber Mold Here are some general notes for the rubber mold o wear gloves, it's messy! o shake the blue bottle well before using o mix the blue and yellow in a 1:1 ratio, but it's not an exact science so eyeballing is ok o mixing: avoid getting air into the mix. either pour back and forth between cups until you have a solid color (no lines) or stir in a figure eight (8) motion with the wooden end of an ear swab
  5. 5. o tap cup gently to get the final bubbles out o pour with a skinny neck to avoid bubbles o over fill mold, the extra parts make it easier to remove the mold later o cure at 150F for 1hr (convection oven on convection mode) or overnight at room temperature Step 3: Casting The Process: o mix your material according to the directions. Note how long the pot life (how long it stays liquid before it starts to cure) on the instructions to see how long you have to work with the material. o ** Don't Rush if you don't have to! ** o avoid air bubbles (see instructions in the rubber section above) o if you have a two part mold, make sure your mold has a pour spout and an air hole on the other end. o clamp the two parts of the mold together (often heavy duty rubber bands work for this) o pour into mold through one hole until material begins to come out air hole o Tap mold gently to get all the trapped air out o Leave to cure according to the material's instructions o ** Don't pour down sink, it will set & clog pipes o ** Don't eat, it will clog you!
  6. 6. Materials: For making the mold (after you've cut the wax): o Rubbers: there are two kinds of rubbers--Urethane and Silicone. Depending on the composition these rubbers can be very similar or very different. They can be strong or weak, need a variety of releases, or none at all etc. The rubbers that are stocked in the lab are: o Smooth On PMC121 This is a durable urethane rubber that takes 16 hrs at room temperature to cure or 1 hr at 150 degrees F in the oven. o Smooth On Oomoo 25 This is silicone rubber that is a bit weaker than the PMC121 but cures at room temperature in 75 min and is great for quick trys. For casting the part o There are a huge host of materials you can use to cast your part. Everything from plastics to metals to plaster to waxes. Theoretically anything you can get into liquid, or semi-liquid form can be cast into a mold. Materials that are stocked for this class are: o - thermoplastic, thermoset polymers - PDMS-- This is a clear, very high resolution strong rubber. - polyurethane - plaster, calcium sulfate - portland cement, calcium silicates - metal, ceramic powder - waxes - gels - Additives: change properties like density, conductivity, flexibility, color (eg. fibers, rebar, dye) In addition to these <Smooth On carries a wide range of materials and it's worth perusing their website. Or find some other great material of your own!