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Note: The room-temperature vulcanizing silicone rubber used in moldmaking is expensive, and mistakes can be costly. In addition, many of the materials used in casting can be potentially hazardous. I take no responsibility for any losses you may incur by following these instructions, and please follow all safety precautions when molding and casting. When using casting resins, make sure to work in a properly ventilated area (or wear a respirator), and wear latex of vinyl gloves to avoid contact with the liquid components. Keep all molding and casting materials away from children.. About recasting: Obviously, the instructions below can be used to recast existing kits. I do not condone or advocate this activity. But the fact is, recasters will (and do) get the info somewhere. I offer this information to those who wish to make castings of their own prototype sculptures (or want to cast small parts to duplicate them for dioramas, etc). Part II -- Moldmaking: Pouring the Silicone First you will need to gather your materials for this next stage of moldmaking. The first step is to clay up your prototype and create mold boxes. For this you will need the following supplies: Silicone and
catalyst (RTV or room temperature vulcanizing) The most important material you need at this stage is room temperature vulcanizing (RTV) silicone rubber and catalyst. This two-part compound is what you will make your molds from. I buy my silicone from Silicones, Inc. in North Carolina. The brand I use is GI-1000, and they sell it in various kits (along with catalyst). for the Slimer I used a 9-lb. kit and a 2-lb. kit. You can contact Silicones at 336-886-5018.
Silicone kit from Silicones Inc. (www.silicones-inc.com) The other important piece of equipment you will need is a precision scale. Mixing silicone is complicated (a 10:1 ratio of catalyst is required) and so a precision metric scale is essential. I use a triple-beam Ohaus scale, which can be purchased at shops which sell, er, alternative smoking supplies (i.e. head shops). They're a bit expensive at around $100, but it will serve you well.
My trusty Ohaus scale Place the precision scale on a level, preferably newspaper-covered surface. Make sure the scale is "zeroed out" or that when all three sliders are on zero, the scale pointer rests on zero. If this is not the case you will need to adjust the scale. On the Ohaus triple bar scale, the adjustment knob is a silver cylinder just below the platform of the scale. NOTE: the following steps require careful attention to both measurements and calculations. Failing to mix in the proper ratios can adversely affect the cure time of silicone. Have your pencil and paper ready now as you will need to take notes. With all sliders set to zero, weigh the empty container. Work the sliders around until the indicator is centered. The weight is the TOTAL of all 3 sliders, probably under 35 grams. Write down this weight on your pad, though not at the top of the page, and write "container" next to it. Take the container off the scale and pour in the white part of the silicone into the container you just weighed. With smaller amounts use a plastic cup or even a plastic spoon to dip up some of the white silicone. Silicone is a thick, gooey substances with the consistency of white glue. When you want to stop pouring, you may not be able to stop. Use a spatula to cut off the flow and strings that inevitably result. Place the container back on the scale and weigh it. Write down this weight on your pad, just above your container weight. Label it "container & silicone." Subtract the weight of the container from the weight of the container with silicone. Label this as "silicone." Take the "silicone" weight and divide it by 10. This is easy, since it's metric. Move the decimal to the left one space. For example, if your silicone was 99.87 grams, moving the decimal gives you a result of 9.987 grams, which you can round up to the nearest half a gram, or 10 grams. Always round up to the nearest half a gram (i.e. a result of rounding off 8.124 grams would be 8.5 grams) Write this down and label it as your "catalyst" weight. Take the "catalyst" weight and add it to the "container and silicone" weight. This will give you the "total" weight. DO NOT ADD IT TO JUST THE "SILICONE" WEIGHT! YOU MUST ADD THE CONTAINER WEIGHT BACK IN AT THIS STAGE!!! Can you tell that I made this mistake once? I guess you can. With the container of white silicone still on the scale, change the slider settings to the "total" weight. Shake up the blue catalyst liquid. It sometimes settles a bit. Sometimes this catalyst is referred to as activator Pour a little of the catalyst into a paper 3 oz. Dixie cup. Crimp the rim of the cup so you have a pouring spout. Slowly add the blue catalyst to the white silicone, just drops at a time, until the scale indicator centers. If you go a little over, that's better than being under. Being over has never caused me a problem, but being under has been a very bad (and very messy) mistake. In fact, if I pour precisely correct, I still add a few extra drops of catalyst, just in case. Here is the measurement sequence broken down into steps: Step one: Container weight = C Step two: Container + Silicone = CS Step three: CS - C = Silicone (S) Step four: S divided by 10 = catalyst or Activator (A) Step five: A + CS from step 2 = Total Weight (set scale sliders to this weight) Now you're ready to stir it up. Stir until the powder blue color (see illustrations below) is consistent, with no lighter or darker streaks. This is the hardest work of the process. Use a spatula to scrape the edges and makes sure that everthing things gets in the mix. Use the spatula to bring up the stuff from the bottom center periodically, too. After it's mixed, transfer it to a fresh container and mix again for absolute homogenization of the mixture. This is critical to ensure a good cure with no gummy spots in the mold. Mark a line on the interior of your mold box that is ½ to ¾ of an inch above the highest point of your master If there are any areas you want really good coverage on, dip a soft detail brush in the silicone and "paint" the silicone on the master, making sure no air bubbles are trapped against the surface of the master. Hold the silicone at least a foot, maybe more, above your mold. You want to pour a thin thread into a corner of your mold, and allow that to pile up and flow over you original. Use a spatula to cut off the flow when your pour hits the mark on the interior of the mold box.
Starting to pour (note powder blue color of silicone)
Continuing to pour Note that I am holding the container high up from the mold box. This helps burst bubbles in the silicone as it pours. Note: did you know you can recycle old silicone molds? This is because silicone bonds with itself.
Recycle your old silicone molds! Cut old silicone molds into small pieces and push them beneath the surface of the poured silicone (make sure you don't mess up the clay surface or push any of the pieces up against the prototype). This is a good way to save on unmixed silicone. Just be careful. Tap the mold box against the table top a few times, and walk away. Don't mess with it beyond that. It will take close to 24 hours to fully set. This allows bubbles to rise away from your original, and you'll see them rising and popping in the silicone pour surface. Your mixing tub and spatula should just be allowed to set overnight, too. You'll be able to peel off and discard the cured siilcone, making them both completely resusable.
Silicone has cured for at least 18 hours, and mold box is cut open After the RTV cures for at least 18 hours, carefully remove the mold box and invert the RTV/clay setup so that the clay bed is now on top.
Removing the clay bed from the main body mold Here is where building it up in blobs becomes useful. The clay comes off fairly easily. Note the air vent embedded in the silicone. Try not to remove the clay from pour spouts. If you do, you will need to rebuild them in clay. As you can see, the key dimples are now bumps. When the second pour is done, dimples will be formed again, and the bumps and dimples will lock together when the mold halves are together.
Finishing the pour spout and air vent At this stage you want to finish up the air vent and the pour spout. Build the pour spout up above the surface of the silicone and make sure they are both airtight against the prototype.
Cleaning the excess clay up Remove the clay that has stuck to your prottotype at this point. Use a tool, and follow up with a brush and isopropyl alcohol. Now it's time to prepare for pouring the second half of the mold.
Lubricate the surface of the mold with Vaseline Using soft brushes, lubricate the entire surface of the mold with Vaseline. This is extremely important, because silicone will bond with itself if not prevented by the layer of lubricant. If you omit this step, your prototype will end up locked into a solid block of silicone. Once the surface is well-lubricated, rebuild your mold box around the mold.
Remember this guy? With the clay removed you can see the complex contours of the mold. The bottom will eventually be the "top" of the mold (note the placement of the pour spout and air vents). Once your mold box is rebuilt, follow the steps above to mix the silicone and pour the second mold half. After curing, break down the mold box, and the two halves of the mold can gently be separated and the sculpture can be removed. You can use an Exacto knife to enlarge the sprues and air vents if need be. Your mold is complete and ready for casting! That's it for this installment! In Part III, we will mix resin and make our castings. Go to Moldmaking & Casting Part III |
kay,
we've made our clay beds and mold boxes. Let's pour some silicone!
