Making The Core Box and Cores
You need a mold to make the sand cores. This is called a core box. I made one out of two 6 inch 2"x4" pieces. These were clamped together and two 1/4" holes drilled through each corner. A 1/4" dowel was glued in each side. This acts as a guide to align the to halves later. With the two halves clamped together again, a 1-3/8" hole was drilled 4-1/2" deep into the end exactly down the middle. An additional smaller hole, about 1/4" or less, was drilled on through to the bottom of the core box. I used masking tape to mask off the wood except for the large hole down the middle. This was then painted (blue in this case) to make a smooth surface to help make the sand release easier later.
Very fine silica sand is mixed with corn flour, boiled lindseed oil and sodium silicate. I use 4 cups of sand, 3 tbl corn flour, 3 tbl lindseed oil, and 1/4 cup sodium silicate. This must be thoroughly mixed. You need to wear rubber gloves as the sodium silicate is a strong alkali. Sodium Silicate is available from Budget Casting.
Ram the sand mixture into the mold with a piece of wood. A round dowel would probably be better to use than the square piece shown in the photo. Once firmly rammed, use a straight edge to trim the top smooth.
Mext I invert the mold and push a hole completely through the sand core via the small hole that was drilled in the bottom of the box. I used a sharpened wire to do this. The hole will allow CO2 gas (next step) to permeate the sand.
Mext I inject CO2 into the core through the small hole that was drilled in the bottom of the core box. CO2 causes the sodium silicate to harden so the sand mix becomes solid. CO2 is available from welding supply houses and brewery/wine kit stores. I injected the gas at around 4 pounds of pressure for about 30 seconds.
Generally, just firmly rapping the mold will allow the finished core to drop out so you don't have to pull the mold completely apart as shown in the second photo to remove it. You will want to gently remove the flashing along the sides with a file or sand paper.You will need one of these cores for each fish you plan to make.
You can use the cores as is but I prefer to cook them dry. This will make them stronger and they will last longer in storage. I store mine in plastic zip bags. My core furnace is a propane smoker I bought at a local hardware store. It works quite well. I cook the cores at 400-500 degrees F for about an hour. They will turn a nice brown color as you will see later. The lindseed oil cooking has a disagreeable odor so the smoker probably would not be suitable for later use on meat. The second photo shows cores for another project ready for cooking.
Now that the cores are complete, we move on to the pattern. We need to use the core box to make one more core shaped piece. This piece will be made of Bondo automotive putty rather than sand. Rub a bit of petroleum jelly on the core box to keep the Bondo from sticking to it. Fill the hole with Bondo and let it set. Once it is set it will be ready to join to the original fish casting to be use for the final pattern piece. I placed some modeling clay in the fish to help support the Bondo core.
The rod (called a core print) is placed in the mouth of the fish. You can see there is a space around it. This needs to be filled with a wax fillet to create a smooth joint here. I use bee's wax which I extruded into round lengths with a clay extruder from the hobby store.
I pressed several pieces around the joint as shown. Then using a torch I heat up a hand made tool. The tool is basically a thick wire that has been beaten into a spatula/spoon shape at one end.
Using the heated tool, I melt and smooth the wax around the core print. You are not filling the fish, just making a smooth surface between the metal pattern and the core print.
We are ready now to make the sand mold. Shown below is the fish pattern/core print on a molding board. The molding board in on top of the "cope" side of the flask. I used some small blocks of wood to hold the fish in place with the core print horizontal.
If molded as is, the sand mold would be a little too low in the drag side of the flask so I have added a temporary frame to hold the flask a little higher. This will have the effect of placing the parting line of the mold between the two flask halves. The drag half is attached as shown in the second photo. Notice the flask pins are pointing down, this is to allow the mold to be easily separated later.
Now we dust the surface with parting dust to keep the sand from sticking to the pattern. And now we add sand using at first a kitchen sieve to riddle very fine sand into the flask to cover the pattern. Once the pattern is covered you do not need to sieve (riddle) the sand.
Using a homemade bench rammer, I pound the sand firmly around the pattern. Once full, I use a straight edge to even the surface of the mold.
Next, the bottom board is placed on top and the whole thing is rolled over with the empty cope side on top.This is a handful. You may want to start this project without the drag in place. Just leave the cope off in the beginning and add it in place at the appropriate time in the following steps.
Now we remove the cope and pattern board. The underlying frame and support boards are revealed. I use a thin spatula to cut around the edge to facilitate removing the temporary frame. The support pieces are also now ready to be removed.
Using a the spatula, a soft paint brush and (carefully) compressed air, I remove the sand down to the halfway (parting) line. It is an important that this be done carefully, so take your time.
Here is the pattern with all of the sand cleared to the parting line. The pattern has been dusted with parting dust once again. Now place the cope side of the flask in place and riddle fine sand onto the mold just as you did on the drag side.
Ram the sand as before and scrape the surface smooth.Now you need to cut the sprue hole. I usually use a piece of sink drain to punch the hole. Clean the top of the hole and cut a funnel shape to help guide the metal into the sprue when the metal is poured. Clean all loose sand from around opening. Finally, I use a rubber mallet to tap the edges of the flask to help loosen the sand from the pattern.
Now we very carefully lift the cope off of the pattern. Hopefully it will lift cleanly. The loose pile of sand in the first photo is from cutting the sprue. Blow it out carefully with compressed air. The second photo has an odd appearance due to the shadows...it is the cope half of the mold we just lifted off.
Now we prepare to drag side of the mold. First, we dribble a little water around the pattern and core print to firm the sand. Too much water will cause problems. In taking the photo I spent a little too much time and got the mold too wet. The second photo shows tools used to cut a "gate" from the sprue to the pattern. This is where the metal will flow into the mold to create the casting. Use your fingers to smooth the gate and "cup" of the sprue and blow out any loose sand.
Using a small metal rod, tap on the pattern and core print to help loosen them from the sand. Very carefully lift the pattern from the sand. If you mess up here, you will likely have to start the whole process over again (voice of experience).
Time to put a sand core into the mold. The core is used to create the hollow space inside of the casting. First, since this will be a brass casting, I sprinkle flour on the surface of both the cope and drag. Usually, I keep the flour in a cloth sack and using the air compressor, blow air through the bag to carry the flour to the molds. The flour helps the metal flow in the mold. After this is done the sand core can be put in place. I used some 80g sand paper to shape the end of the core to round it off. This will make the back of the inside of the fish rounded as well. As you can see, the core is a little shorter than the print. This is not a problem. You can also see now that the extra length of the sore provides the support for the portion that is inside the fish.
The mold is nearly complete. We carefully put the cope half back on. Then the surface is scratched to provide an even surface for the weights that will be stacked on top of the mold. Be careful not to get any sand knocked down into the sprue. The mold is taken outside and weights added. The weights keep the mold together when the metal is poured. Without them (about 15 lbs in this case) the hot liquid metal would force the two halves of the mold apart and run out on the ground. I put a temporary cap over the sprue opening to keep out insects and debris until I am ready to pour.
No photos of the actual pour as I needed both hands to do that, but here is the just after photo. It's a mess! Remember I said that I got the sand too wet while wetting the edge of the pattern earlier? The water turned to steam and blew out much of the metal that was flowing in. The result was a disaster. You learn from mistakes and start over. The core did what it was supposed to do, you can see it sticking out of the fish's mouth in the second photo.
Eventually you will be rewarded. Here is a completed big mouth fish that has been polished and is ready to decorate some home.
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