This is how to build a 1 part bonded sand mold for casting metal. I made this at Techshop. www.techshop.ws
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Step 1: Step 1: Make a Pattern
First step in creating a bonded sand mold is to create a pattern to be cast. In this case I decided to experiment with party balloons. I chose this material because the patter needs to be made from something that will be completely evaporated by the metal being poured into the mold. In most cases, polystyrene is used because molten metal will burn the pattern away and the cavity will be replaced with metal. This is real nasty for the environment and your health. So I encourage creative use of less harmful materials for your pattern. You do not need to think about undercuts but holes can be tricky to pack.
Step 2: Step 2: Acquiring a Flask
Step two: After you have created a pattern to cast, you will need a flask. This is a box or some sort of receptacle that your pattern can fit inside with at least 2 or 3 inches of spacing between the pattern and the top, bottom and sides of your flask. I used scrap plywood that I glued and nailed together. You can use just about anything that has rigidity or wont flex or bend as you pack sand into it. Try concrete cylinder forms or a metal pale in place of building a wood box.
Step 3: Step 3: Tools
Step three: Now it is time to get ready to begin collecting tools needed in the sand casting process. All you need are some basic packing tools and a tub to mix the sand and epoxy in. I recommend a large mixing tub that can be found at any contractor supply or mega hardware store. You will need two mixing containers one for binder (24oz +) and one for catalysts (3oz +). As far as packing tools, you can use 2 x 4 cut offs or any solid blunt object with a flat surface or get real high speed and make something a bit more accommodating like the ramming tools in the photo. I like to keep a steel brush and a scraping tool on hand to clean sand build up off of the mixing tub and ramming tools. This prevents solidified bonded sand from mixing in with uncured sand during packing.
Step 4: Step 4: Materials
Step four: Now that you have all of your tools on hand its time to get your materials ready. I like to use fine silica sand. Commercial grade Quikrete fine, kiln dried and screened can be found at most contractor supply and mega hardware stores. I use the 80 lb bags because this is all I can physically manage to mix by hand. This weight also works well with the ratio of epoxy that is recommended for this tutorial. Before ripping into the bag of sand, make sure you ware a respirator mask to avoid inhaling the fine silica dust that will be all over you during this process. If you have sensitive skin I also recommend wearing gloves and long sleeves to avoid contact with the chemicals during mixing.
Step 5: Step 5: Materials 2
Step five: Once you have dumped the sand into the mixing tub, it is time to measure out the binder and catalysts. You will be mixing the binder into the sand first so measure it first to keep track. Pour 24oz in to your cup; this is approx. 3 cups. Then fill the next measuring cup with 3oz of catalysts; this is less than a half-cup. To acquire these materials simply find a local mold making supply dealer and ask for bonded sand mold making chemicals. These materials can very in brand and chemical consistency so be aware that you may be using a different epoxy than what I am using. Be sure to consult the direction of use for the proper ratios of mixtures before mixing.
Step 6: Step 6: Placing Pattern in Flask
Step six: Place your pattern in the flask and determine the best position for it to allow the metal to flow through the form. Often a slight angle helps the metal to flow in and the air to be pushed out. Consider high points where the metal will have to travel up ward and make sure those areas are still below the highest point of the pattern in the mold. Once you have decided this position mark the side of the flask with a marker to indicate where you will drill your holes in the mold for the main gate and vents. You want to mark the highest point of the mold for your vent and the lowest point for a gate to pour the metal in. If you have other high points that seem like they may be too high for the metal to fill mark those points as well, so secondary gates or vents can be drilled into the mold to supply metal to those areas.
Step 7: Step 7: Mixing Sand and Epoxy
Step seven: Now that you have determined the position you will place the pattern in the flask it is time to get mixing. You will add the binder to the sand first. The key to getting an even mixture and a solid strong bond is to gradually add the binder into the sand as you mix the sand. This is done by pouring a light drizzle over the moving sand; this is a two hand action. Keep and eye out for beading, this is bad. The binder needs to coat the sand it you see little balls this means the sand is coating the binder and isn’t mixing properly. Also be sure to pull dry unsaturated sand from the corners of you container and be sure to roll the top to the bottom and vise versa. Once the binder is fully mixed, you should be able to pack a ball in your hand that cracks not crumbles when squeezed. Now it is time to add the catalysts. This is the tricky part you only have about 10 minutes or so before the two parts dry and harden. At this point you will not be able to use or pack the mixture of epoxy and sand. So just like above you will need to gradually add the catalysts while continuing to mix the sand with your other hand. Remember to be careful of beading. This needs to be done fast. You should be finished mixing the entire 80 lbs of sand in about 3 minutes to be able to have time to pack the mold.
Step 8: Step 8: Packing the Mold
Step eight: Quickly lay a 1 to 2 inch base on the bottom of your flask. Do this by throwing sand into the flask and then use your packing tool to pack it in tightly. Next place your pattern in the flask and begin packing sand around it. Begin by creating a slight grade to incline your pattern, make sure during this process you are keeping an eye on the placement of the pattern in relation to the registration marks drawn on the sides of the flask. Pack the sand into all the little nooks and crannies by pushing your fingers into these spots. Use your hands to press the sand up under the pattern. Keep gradually adding sand in layers and patting it down as you go. Be sure to pack the sides around the walls of your flask very tightly using a tool. Once you have covered the entire pattern with sand be sure to pack the sand with a tool to compact any areas you may have missed while using your hands. Now that you have a your pattern locked in a good solid base, fill the rest of the flask packing it tightly until the sand has reached the top of the flask. Continue packing until you hear a change in sound. The packing sound will go from a hallow sound to more like a thud. At this point the sand should be getting dry and will no longer be workable. Take you scraping tool or any flat surface and skim the top of the flask to level out the top surface.
Step 9: Step 9: Preparing Gates and Vents
Step 9: I generally let the sand cure over night but that’s probably over kill. If you are sure the sand is completely bonded its time to drill your gate and vent holes. I used a 10in long 1 in diameter masonry bit. Drill straight down into your pattern. I do this by making multiple plunges with the drill checking to make sure I drilled far enough down to hit the pattern but not too far. Depending on your specific form being cast, gate and vent risers may be helpful in the pouring process. These help provide additional metal to be pulled into the mold during cooling and shrinking. If you decide to do so, pack some of the access sand during the packing process into some party cups then after they have cured drill some holes in them. Place these over the gates and vents. These are not always needed if you decided not to do this make sure to widen the top of the gate hole to allow the metal to be poured easier. Once this is complete try to blow out any loose sand that my have dropped into the holes. At this point the mold is complete and you are ready to pour molten metal into your mold.
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