So what do you do with all of those scraps of aluminum? Melt them down and make something of course! There are a few parts that it just makes sense to cast, like the Kort nozzle's for the thrusters, linkage ends for the control rods, and perhaps some nice control handles for the ballast controls. All of these parts could be built from aluminum sheet, aluminum pipe or molded from plastic, but casting 1300 degree aluminum looks like fun even if it is a little dangerous. Or perhaps because it is dangerous.
My foundry started from reading several of the very good web sites available on the topic. See the links at bottom. Foundries can be a simple hole in the ground fueled by coal or a walk in gas fired room. Mine is a modest foundry as home foundries go. I wanted to melt upwards to 15 pounds of aluminum so I started with a crucible (the actual melting pot) that is a little bigger than a standard #10 crucible. My crucible however is made from a 6 inch by 1/4 inch thick steel pipe with a 1/2 inch plate of steel welded onto the bottom. After a trip to the local junk dealer, the metal scrap yard, and a foundry supplier I had all of the parts. I made a simple burner using parts from the turkey fryer burner I picked up and used to melt my lead. A couple of of 30 gallon oil drums ($4 each) from the local junk dealer would form the body.
I had been working mainly with with aluminum up to this point. Cutting steel with my Dewalt cut-off saw wore out to $90 blade is short order. Since it cost $40 to sharpen I decided I had an excuse to buy an acetylene touch but I decided to try a cutoff blade in my grinder first. Harbor Freight sells a box of 25 - 7 inch metal cutting disk for $14, and a cheep angel grinder will cost $20 to $30. Unfortunately the cut-off blades worked great, so the torch will have to wait.
One of the 30 gallon drums was cut off at 18 inches to make the bottom of the furnace. These drums are 14 inches in diameter which is just right for my 6+ in wide crucible + 1 inch of space between the crucible and the refractory so that the refractory is about 2 1/2 inches thick.
The lid is 2 1/2 inches cut off of another barrel. These barrels have lids like paint cans, so instead of trying to weld on a lid I just used another $4 barrel. Others have used pressure storage tanks used by household water well systems and except that these are rounded on the top, I think they are a better solution; unless you are good at welding on the thin metal that the 30 gallon barrels are make from. I am not.
Refractory is the material you use to insolate your foundry. You can make your own refractory as described on some of the links, but I was having problems finding the materials. It seems the local schools purchased all local fire clay available in small quantities and they will not restock until next fall. So, I went the lazy route and purchased Mizzou Castable Refractory. As much as this stuff looks like concrete, it is not concrete, it also has a 30 day self life. Read the instructions! It is mainly made from alumina, which is aluminum oxide. Wear a respirator, gloves and wash up after using it. It took 4, 55 lb bags at $27 each to complete my furnace with enough left over to make several plinths (small blocks that support the crucible above the bottom of the furnace) and a flower pot furnace, which is great for small jobs. You only use 5 pints of water for each 55 lb bag so it looks like very dry concrete. The trick is to vibrate this stuff, and if you have an old table saw then you have the perfect vibrator. Just lower the blade, cover it with plastic, turn it on, and let it vibrate your refractory.
The lid was poured first. You can see the small steel bars welded in to reinforce the refractory . This is only done in the lid, so that the refractory is supported when it eventually cracks. A butane torch with a piece of cardboard wrapped around it sticks through a hole in the lid. This will form the site hole and vent in the top.
While the bottom was drying I made a crucible (melting pot) from a 6 inch steel pipe with 1/2 inch plate steel welded on the bottom. The crucible was then used to make the center form for the refractory. Wooded strips 1 inch wide were glued to the crucible with dabs of Liquid Nails and then those were wrapped with aluminum flashing. The flashing was cut and folded over to close off the top in order to make it easier to shovel in the refectory without have to try and miss the opening to the center form.
With the bottom dried, the center form was set in place along with the burner inlet pipe which is a 2 inch steel pipe shaped to fit the curve of the center form. It is held in place with Liquid Nails and a piece of metal that is inserted in the pipe and pierces the center form.
Curing will be many hours spent gradually heating up the furnace. The Mizzou Refactory came with a very specific set of instructions but I did it over multiple evenings, using heat lamps the first couple of nights, then the next night I started with the burner. About 5 or so hours with the burner pulled back out of the burner pipe, turned down low and only partially venting through the pipe. In order to achieve 250 degrees F, then higher temperatures on 4 consecutive nights until we were melting aluminum. After the first 3 hours above 250, steam was escaping from my porous welding job, so I would not recommend any faster curing than several days.
My burner is made from an aluminum 2 inch to 3/4 inch reducing bell turned on my lathe, but that is just because I could not find an 1 1/2 x 3/4 inch reducing bell at one of the local plumbing supply houses and I wanted to play on the lathe. The center is an 8 inch long 3/4 inch nipple, and the flare is a 1 inch by 3/4 inch reducer. I cut the threads out of the 1 inch side of this bell on the lathe, but I doubt that was necessary. I just borrowed the 5 PSI regulator hose and orifice from my bargain turkey fryer I picked up for melting lead. I welded a small cross piece in the large end of the aluminum bell, then drilled an tapped a hole in the center that would fit the gas hose. I also welded 6 small bits of flat steel onto the 8 inch nipple. This keeps the burner centered in the burner pipe which allows for additional air flow around the outside of the burner. One of the tabs of steel is notched to prevent the burner from sliding too far into the burner pipe. Make sure to keep the gas turned up enough to keep the flame burning only at the flared end of the burner. If too low the flame will burn back through the 3/4 inch pipe and start burning inside the intake bell. This will cause the gas hose fitting to over heat and it would soon melt of and catch fire. The propane tank would be next and after that you would not need to worry any more :)
I tested the foundry with a small melt and it worked great. I takes 30 minutes to melt a couple of pounds and over an hour for about 8 pounds. Larger burners will do the job in 15 minutes and that seems important to many people, but there is always something else to do other that watch the water boil. Also you really do not want the aluminum to get much hotter that it's melting point of 1300 degrees. I tried adding a blower taken from a discarded dryer, in order to force more air thought the burner and increase the efficiency, but just a small amount of additional air caused the burner to blow out.
I was very happy with the crucible tool. The design is a hybrid of a couple of others I saw.
The tool has 2 hooks that can reach down into the furnace and catch 3/4" studs welded onto the top outside edges of the crucible. The hooks are welded to a 7" piece of pipe that swivels on the tools handle. Once the crucible is lifted out of the furnace the handle is slowly rotated allowing the hooks to swivel and the tilting arm which is welded to the handle to rotate down and catch the bottom of the crucible. The crucible can now be tilted. The photos are only a demonstration, I'd be wearing gloves if this was real, and the spots on the crucible are just ash from the Liquid Nails that was used to hold the wood furring strips in place when the crucible was being use as part of the from to cast the furnace.
If I had to do this again I might build my furnace so that the lid included about 3 inches of the side of the furnace. This would increase the weigh of the lid, but the crucible tool would be much simpler. I could then cut holes in the side of your crucible and just slide end of the tool through the holes. You would want some notches in the tool to help keep the crucible from sliding off but you eliminate the need for the swivel, the hooks, and the the studs on the crucible.
Lost Foam Casting should be ideal for my needs so I when ahead and built a hot wire cutter that can be fitted to my old Craftsman table saw. It's build from 3/16" aluminum sheet only because that's what I am building my submarine from, but having it attached to the table saw is prefect because I can use the fence and push guide as well as tilt the wire if needed.
The saw blade is removed and the wire cutter is bolted onto the back of the spindle bracket where the safety guard normally goes.
I tested a battery charger as a power supply for heating up a cutting wire, but this was not successful as the charger apparently had a shutoff built in to keep it from overheating due to a complete short. So I found a regular transformer from American Science Surplus at www.sciplus.com that will output 7 amps at 24 volts AC. The transformer gets its power through a dimmer switch so the voltage on the wire can be reduced. One end of the power from the transformer actually is grounded most of the time because the wire lightly touches the blade guard, but the top end is isolated with a piece of plastic tubing that is glued onto the aluminum frame with epoxy. The wire is pinched between a couple of washers on a 1/8 inch bold. The small bolt is nice it can be bent if needed in order to get the wire at 90 degrees to the table.
You can buy Nichrome wire but steel guitar strings (not the wound kind) work great. Note that Nichrome is a brand name. What you are really looking for is "resistance wire". Others user fishing leader wire like the Stainless Steel Single Strand Leader Wire, 69lb. .018 Diameter made by Mason Tackle. Since my transformer outputs 7 amps at 24 volts I can also use .035 welding wire, but it is harder to control and much less accurate than the smaller wire. However, the welding wire can be bent into desired shapes, and when duct taped onto a piece of wood for a handle or mounted on a turn table or table saw it can be used like a router or shaper.
I since purchase a 1/4 pound spool of .01 diameter stainless steel wire from www.McMaster.com because I was going through too many $1.50 guitar strings. I also made a handy hand held wire cutter. The plastic clamp holds a steel rod which supports on end of the stainless steel wire as well as one side of the electrical connection from the transformer. A small bolt treaded through one jaw of the clamp holds provided a connection for the other wire from the transformer and the other end of the stainless steel wire. The wire slacks when it get hot, but that is not a problem when cutting out items using a pattern. A small block of wood can be inserted between the steel rod and wire in order to shorten the cutting area. In the photo the cutter has been used to core out 4 1/2 inches of foam between to parts of the jet pump in order to make an extension.
For slicing wide pieces of foam a plate of aluminum with an L shape bracket welded to the backside is clamped to the existing table saw fence.
Lost Foam is the perfect casting process for building a few odd pieces. I created the lower cabin hatch hinges from a solid block aluminum using a table saw, so I know the time required to shape a custom piece by removing what you don't want. Building the part out of foam will certainly be faster, then it is just a matter of pouring in molten aluminum and letting it burn the foam away leaving me with the perfect part. You have to admit that it sounds good anyway. In reality it's going to take several experiments before I get it right, but the learning is lots of fun too!
Here are my first cast, and frankly they look like crap, but that makes improving easier. I started with a chuck of Polystyrene from a local supplier, but you may be able find plenty in the trash bins behind appliance stores. The density of my test piece is 1 pound per cubic foot. They also have 2 pound per cubic foot, but that would produce more burn off and ash. After cutting it to shape I burned some 3/8" holes in it with a 1/4" bolt heated up with a torch. The holes would be for 3/8 inch bolts and I wanted some of the holes to be threaded so instead of tapping the threads after the part was cast, I burned a kerosene soaked rag held the bolts in the flame until they were well coated with soot. After they cooled the bolts were then positioned in the foam. Next the whole thing was coated with thinned down mixture of premixed sheet rock mud. About 1 part mud to 1 part water in the mixture. This was placed in front of a heater vent to dry over night. A hot water closet would be a good place too, just use your head about how close you want a flammable material to the heat.
First Test Pour
I also built a "Krush Head Pressure Tool" as recommended by Dave at www.buildyouridea.com. He has a very good Lost Foam Metal Casting Primer The design only varies slightly from Dave's. The KHPT tool is placed on top of the sand around and it allows the molten aluminum to pool above the foam parts in order to add pressure and a reserve of material to be drawn into the casting as the aluminum cools and contracts. The lock that holds the tool closed is a piece for 1 1/2" pipe that is flattened and cut lengthwise. It is shaped to slid up both handles when they are closed and the handles actually angle slightly inward toward each other as they come out from the 4" pipe so the lock gets tighter as it slides in toward the 4" pipe. The hinge is make from a piece of 5/8" bar and 4, 1 1/4 inch long pieces of 3/4" pipe. You simply weld every other piece to the other side of the 4" pipe.
Test Day II
Finally ready to make a some more test pours, the first step is to fire up the furnace a put on some aluminum scrap. I already learned that aluminum shavings from the table saw create a lot of unusable dross. Perhaps due to the amount of oxidation that forms on the abundant exposed surface?
While we are waiting for the aluminum to melt the parts are prepared. The mud coating over the foam has completely dried, and a pair of parts are set into a few inches of sand. The plan is to pour both at once and fill them until molten aluminum flows out of both risers. The risers are made from foam, just like the rest of the part but they will be cut away from the part after the pour. The aluminum will be poured into the two risers that are positioned closest together, technically these are called "sprues".
More sand is added to burry the parts and a good attempt is made to shake the tub in order to vibrate the sand into the crevasses around the parts. You never get too old to play in the sand box!
The Krush Head Pressure Tool is set in place around one sprue from each part. The sprue rises about 1/2 inch above the sand. When the aluminum is poured it will cool on contact with the sand and then fill the bottom of the KHPT until is starts to burn the foam out and flow down into the part.
Once the aluminum starts to melt in the furnace it goes fairly quickly. You can add long pieces of aluminum thought the site hole in the top of the furnace and watch as they slowly slip into the melt. The scrap I am using is 3/16 inch 5082 and 5086 aluminum alloy sheeting from the submarine boat I am building. It produced very little dross, but I skimmed off the worst of it anyway. My skimmer is just a piece of bent steel pipe welded onto a handle.
Once comfortable that plenty of aluminum in the crucible, we were all set to pour. The first step is to hook the crucible seen in the photo below and left. Maybe next time I will remember to put the face shield down and not just wear it on my head. Things are not really as hot as I expected but I still like the fact that this tool allows me to stay far away from the crucible and off to the side.
The jack stands make a nice place to rest the tool in order to rotate the handle and bring the swing arm into contact with the crucible. ...and look who remembered the face shield!
You can add another level of safety by putting your furnace and casting area over a bed of dry sand. If the crucible breaks, or if aluminum is spilled onto concrete, the aluminum will vaporize the water in the concrete in as little as 2 or 3 seconds, causing it to explode or "spall" and fling chips of concrete and hot aluminum through the air. Just setting a newly cast piece on the concrete to cool will cause spalling.
Clink here to see a movie of the pour ...well at least part of the pour. It ends with the foam gasses bursting into flames and it was anti-climatic after the flames anyway. The first lesson is to warn your camera person; my wife :) that flames are involved. What you miss is the aluminum burning up through one of the risers and spreading out across the top of the sand. The problem is that once the molten aluminum burned thought one of the vents it quickly relieved the pressure built up by the head tool and aluminum poured out of the one open vent. So the second lesson is to make the vents the same height as the head tool or open them up so they all vent at the same time. In this case the second piece did completely form, it just never filled the second riser sufficiently to reach the surface.
After cooling for several minutes the parts are pulled from the sand. You better be wearing good gloves! The mud is easily removed from the part, but you can see a problem between the letters "A" and "Y". I don't think I got enough sand in the crevice there to support the mud and the aluminum broke through. Facing this side up is part of the solution, as well as taking more care to vibrate the sand into all the recesses, or reinforce the mud with some plaster, or all of the above. I learn latter that the mud works best if there are several coats applied with a day between each in order for it to dry. Subsequent coats should also be un-thinned.
By the way, my wife and lovely assistant for the past 24 years is "KAY". The chips of left over mud are sifted from the sand and we are ready to go again.
The photo to the right shows a couple more parts having been poured. The first part has two risers that are connected and the second with the KHPT still connected as separate second riser outside of the KHPT that delivered that pool of aluminum over the surface of the sand.
Again the mud was a bit too weak for the pressure. In the photo below on the left ("Pressure blowout") you can see where the mud on the top of the part split and was forced upward by the aluminum. Two coats of mud should hold (I since of learned that 8 to 12 coats are best, until there is at least 1/4 inch of very dry mud), and perhaps a little slower on the pour so the aluminum has time to burn thought the part and not build up so much pressure in the KHPT. More sand on top of the part, perhaps 4 or 5 inches in stead of 3. And finally I think the exit riser should be burned out in advance with a hot piece of metal in order to provide a vent for the gas and smoke. (This was the right path. I later started using acetone to melt the foam) Without an open passage, this gas would gather in the vent riser insolating the foam from the molten aluminum, there by delaying the burn through and allowing too much pressure to build inside the part.
I again tested casting threads, and again the treads are good, but not not great. It's important to get the bolt completely covered with soot as a couple of the threads stuck to the bolt.
Having tried to cut a uniform notch out of a block of foam I decided to try cutting the foam into slabs and then gluing them together to form a channel. I tested Polyurethane Adhesive and Hot Glue for bonding the slabs, and the Hot Glue definitely worked the best. Good thing too because it dries faster and is much easier to work with.
I also experimented with sanding the foam with 400 grit sand paper before coating it with the mud, and while you can still see the pattern from the foam beads, the sanded surface was much smother. A little grinding with an aluminum grinding disc and the surface looked great.
More Trial and Error
I tried casting a 14 inch ring that is about 1 3/4 inch wide and 1/2 inch thick. It had 5 vents and the vents were first opened up by pouring a little acetone into each one. This allowed the pressure to be relieved before the aluminum reached the top of the vent, and the part did not blow out like some of my test pieces It almost worked but as you can see in the photo on the left, it did not complete the last 5 inches of the ring, leaving a gap between the farthest two vents. I poured this piece quickly, knowing the aluminum had a long way to flow, so I'm not sure what the answer is yet. After getting some advice from the from the hobbicast forum the second attempt employed 4 spokes from a center riser along with 8 soda straws on the ring in order to vent the burn off gasses. It worked much better, but it needed more vents. In the photo you can see a black spot on the left where a void exist between two of the straws. The mold also broke again, but the large protrusion was easily cut away. The foam for this part is a low density 3/4 construction styrene insulation foam board coated with sheet rock mud thinned by 50% with water. I am going to add more vents to the second ring and try again. We'll hope that more vents is the answer. One vent every 4 inches should do the trick, but sheet rock mud, cut with 50% water is not holding its mustard. So I am going to try again with additional coatings of mud that has not been thinned. If that does not work, then I am going to try castable refractory, which is an idea supplied by Ken Weaver . (See sidebar).
Test Day III
I only had time for a couple of test this weekend, but they were both educational. First off - Sheet rock mud works fine as long as you keep adding coats until there is 1/4 thick layer of mud. The first coat can be and needs to be thin if there is detail in the piece. Plaster works well too but its harder to work with due to its quick drying time, it cost more than sheet rock mud and you'll have a lot more waste. Plaster is good for making molds from original parts. I have had some success making a cast of a sprocket but I still need to improve the gate and vents. Both plaster and sheetrock mud need to be completely dried before pouring. I put mine in a toaster over set on warm for 4 days. I am not going to try the ring again, in the mean time I found another solution and no longer need a ring. But brackets and a sprocket are on the list.
I does not fit under Lost Foam but I need 8 sprockets on shafts that don't need to support much weight, so I am going to attempt to cast them using the one steel sprocket I have. It's too complex of a part to try making foam blanks so I am going to try a split plaster mold. Each tooth has a vent. The only real problem I had was getting the plaster to release form the sprocket without breaking. For release agents I tried Petroleum Jelly (Vasoline) which did a very poor job. Axel grease was much better because it did not dissolve in the wet plaster. Soot was alright but it would do better with parts that were smoother to start with. The best agent and turned out to be a thin coating of wax with a axel grease over coat. Just melt some candle wax and dip the part. The hotter the wax the thinner the coat, and if it is not right then melt it off and try it again. This adds a bit to the size of the part but not much when you consider that the aluminum will shrink some as it cools. I did a sloppy job pouring the aluminum into the mold, but the teeth of the sprocket did a surprisingly good job of forming even if they did have some air pockets in them. The mold unfortunately was destroyed by the heat, even though it was dried for days in a oven set on low. My thoughts are to try this again once I have acquired some sand casting capabilities.
...I now can cast sprockets! See: Lost Foam in Petro-Bond
More Lessons about Lost Foam and Mud
I now know that moisture is the enemy when pouring 1300 degree aluminum into a foam part coated with mud. So I placed the parts in a oven to dry and set the oven on low, which is just under 200 degrees in this case. Most of the parts did fine, but one of them split open like an over rip melon. It seams that the foam can swell when heated. It turn out with subsequent testing that 175 is too hot too, at least for the parts closest to the heating element. So if your not using your wife's new convection oven then 125 is my recommendation. This part had a coating of about 1/8 to 3/16 of an inch. The other important lesson is to assure that the mud is completely dry. I had several parts arranged with 1/4 inch separation between large 2 inch wide flat sections. After a few coats, the mud completely filled the space between the parts, but with no external surface it did not dry even after days under heat lamps. I washed out the foam with acetone and it quickly dissolved the foam and then the mud on the inside of the group of parts. When I turned the part over to pour out the acetone, a large glob of mud also poured out onto the floor. That brings me to my third lesson, which is using acetone to melt the foam before casting the part. Once the mud is 3/16 inch or better I can dissolve most if not all of the foam in the part with acetone. This does not really remove much of the foam from the part, instead it converts it into a goop that sticks to the insides of the casting with the consistency of used chewing gum. You may be able to pull some of it out of the casting, but the real advantage I see as it allows for you to make sure that each vent is clear, and with the vents already open, then as soon as you start to pour in the aluminum the burn off gasses and steam can easily begin to vent. Also since it is important to make sure the part is dry, dissolving the foam allows for air to flow though the part. I first blow air through using an old dryer blower, then and only when I am sure that all of the acetone has been evaporated, I place the part back into a warm oven for a final drying. Be sure to keep in mind that acetone is extremely volatile and if placed in an over it could easily explode. Finally, I you're going to dissolve the foam with acetone and you are gluing the parts up with hot glue, then only use a minimum about of glue so as not to coat the entire joining surfaces. Since acetone will not dissolve the hot glue, a completely covered joint will prevent the acetone from flowing through the joint. This is especially true for small joints such as where a soda straw is glued to a foam part in order to provide a vent.
SUCCESS ...well some anyway.
I started with the melt a hotter than what I had been using. This batch is just starting to glow orange. I am also using flux from www.budgetcastingsupply.com since this some of this aluminum has now been cast once or twice, perhaps more. My flux tool is a small section of pipe connected by a tee. I pour the powder flux down the handle and then submerge it into the melt. I also built a better skimmer which is basically a 4 inch round, 1/16 inch flat piece of steel with drain holes drilled thought it, and welded onto a steel pipe for a handle. I can't tell that the flux did much good, but it is comforting.
The parts have a 3/16 inch coating from several dips in of sheet rock mud. They were dried for days, then the foam was dissolved with acetone and air blown through the part to dry up the acetone that did not pour back out. Then another day in a 200 degree oven to make sure the mud was dry on the inside too. Finally the parts where set in sand and inspected by Sam (the dog). She looks worried. Perhaps she knew that I stood little chance of pouring a 10 lb pot of molten aluminum on the end of a five foot pipe and hitting the 2 inch opening of the of the riser. I originally had cone shaped funnels on the top of the risers, but these broke when I was melting the foam out. In any case next time I will have a bigger target.
Sam may have also realized that the foam was really still in the mold, it was just reduced to a gum like substance and ready to burst into flame when hit with the aluminum, which would happed quickly because the vents would really work well. Anyhow she was not around during the pour and so she missed the 5 foot high flames that shot from the vents, removed hair from my right arm and obscured my vision so that the 2 inch target became impossible to see. I will have to get a movie next time. Minutes latter the molds were still burning. Unfortunately, having spilled so much on the surface, I did not have enough to fill the last mold. I should have known that and waited, but the flames were too entertaining to resist.
The next day another batch is poured. I added a blower to keep the flames pushed back. My blower is salvaged from a cloths dryer, cost $0, and it did a great job. One part still has its mud cast funnel and the other had the vents far enough apart that I could use the head tool. It's much nicer having a bigger target and the smoke and flame blowing the other way.
It's also nice to have more melt on hand than needed. One year and 45 pounds latter on a low carb diet and I found that the old muffin tin works great for casting ingots from the left over melt. But don't set it on concrete or the heat will cause spalling. Of course 2x4s will burn, but that is manageable.
Tips and Notes:
More: "Secrets of the Lost Foam"
On the second attempt I connected the sprues to the bracket's flange and only added a small vent to the top of the part. In the photo beside, the first attempt is next to the hammer and the second and usable part is in the fore ground. You can see one small void on the part. The trick to here was using lots of melt. Once aluminum was flowing from the vent I noticed the gas pockets were still venting as well, so I just kept pouring until I did not see any more pockets of gas venting. In the end there was much more aluminum laying in the sand than what was actually in the part, but the part was great. If you click to enlarge the photo, you can also see a small ridge near the flange. After the foam was dissolved with acetone I accidentally broke the mold, but I was able to put it back together by coating the broken area with generous amounts of Plaster of Paris. The ridge is where the aluminum seeped into the crack between the broken parts.
One of the ticks to successfully dissolving the foam in a part with acetone is not completely coating the joint between two parts with hot glue. Doing so will prevent the acetone from completely flowing through the part. Glue on small vents by only applying hot glue around the edges.
I had been thinking about burning the foam out of the part before the pour and had tried it using the burner with little success. Having read about removing wax from lost wax molds on the hobbycast I decided to experiment with a couple of mud covered foam parts I ended up not needing.
The parts where made from high density foam and repeatedly coated with thinned sheet rock mud and allowed to dry until a 1/4 inch layer of mud encased the foam. Before burning out the foam, it must first be dissolved with acetone. This reduces the foam to a gum like residue on the walls of the part. The foam expands when heated, just like wax, so melting it with acetone prevents it from splitting mud apart. I then allowed the part to dry for 24 hours in a 120F degree oven. This makes sure that the volatile acetone is evaporated and allows the part to dry from the inside. Then place the part into the furnace with the vents facing down. As the part heats up the foam residue will melt and flow out of the vents and then burn away. After only 15 minutes or so the smoke will clear and the heat can be turned off. I do not recommend breathing any of the smoke as some polyethylene foams will give off hydrogen cyanide gas which is deadly! I am sure it contains all sorts of carcinogens too but who cares. Just remember the "dead" thing, okay?
Leave the part to cool slowly in the furnace. A few hours latter you can remove the part, which will now is essentially fired pottery and any remaining ash can be blown out. These are delicate parts, much more so than real pottery, after all this is sheet rock mud, and not clay.
It worked so well on the test parts I subjected a real part set and it survived as well. To pour the part I still buried it in sand for safety and to provide a little extra support, but it poured with no bubbling, no smoke, no flames, and so the normal flowing of massive amounts of molten aluminum through the part in order to carry off all of the gas bubbles was not needed. There is a movie above from pouring a set of parts were the foam was not not pre-burned. If you saw that movie then you'll understand just how boring this pour was. But the result is spectacular, even the 1/8 inch thick vents look great. The surface is very smooth with every detail of the original foam part. You can see the fine lines where the foam was glued together but also the lines in the sooth that remained on the mud after the foam was burned out. These parts are receivers for hatch dogs if you were wondering and the thin side of each of the 4 receivers is only 1/8 inch thick. The only blemish among the 4 parts in the set is show in the photo. I looks like an air pocked formed inside the receiver as the mud as the layers were being built up and this pocked filled with aluminum. You can also see thin ribbons of aluminum on the some of the outside corners where the mud must have cracked during drying.
Freedom Plaster/Silica Investment
The investment is whatever you use to coat the foam, and so far I have mostly used sheetrock mud after having thinned it down with water so it was easy to brush on or dip the part in. I have also used plaster in combination with the mud where I needed to fix a broken vent or and some structural stability to the part. The good thing about sheet rock mud is that it is already in a liquid and it does not harden in 10 minutes or so like plaster. The bad thing is that is takes days of dipping, and drying, and dipping again, and again in order to build up a 1/4 inch shell. It is not always necessary to build up a shell that thick. If you and set the part in sand and get the sand in contact with all of the surfaces so that it can support the think mud shell, the a thin coating of mud will work. However I have found that a few more days and a thicker shell means a much higher change of success on the first attempt. I have not tried it yet, but if you are not overly interested in a smooth surface, you can also place foam directly into Petro-Bond or even regular sand and allow the metal to burn it out.
I should have left good-enough alone and stuck with the sheetrock mud, but for reasons unknown to me I decided to try a mixture of 1 part fine white silica sand or silica flour and 1 part molding plaster which is also known of as "Plaster of Paris" or is that "Freedom Plaster"? I read about using silica in the mix because it allowed for better thermal expansion of the mold. I unfortunately tested this mixture on two exhaust manifolds that I spent many hours cutting and gluing together. After letting the part dry for a few days I placed it in an kiln and over a few hours raised the temperature to somewhere around 1000 F. Unlike sheet rock mud the plaster and silica mix actually weakened and cracked badly. I have one more part that uses the same investment mixture so I dry it to at 150F for a few days and then try it in the kiln. Perhaps my problem was just too much moisture? No, I dried the next piece to a crisp and tried it again but got the same results. Plaster/Silica flour does not take heat!
More Mud Slinging -- after all, it's an election year.
Needing to recover from the disastrous use of plaster/silica investment. I've have turned again to the proven ready-mix sheet rock mud and water method. It may be slow, but considering the many hours required to create the foam part, a reliable method is more important than a fast method.
Once the two halves of the gear box were ready, then were joined together with small dabs of hot glue and then dipped a few times in order to insure that the two halves were square to each other. Unfortunately I used too many dabs and did a poor job cutting then apart which seriously damaged the edge. To make a repair I built the edge back up with wax. Notice that I also added a small brace to the vent, which did a nice job of preventing it from breaking off during the week of dipping and drying. No more fishing broken vents out of the mud vat and gluing them back on.
The parts are allowed to dry between each dip in the mud. I have a 25 gallon plastic storage container filled to the brim with one part ready mix mud and one part water. My dryers are an electric meat smoker which adds a nice hickory sent to the shop and a stack of fire bricks that get rearranged to fit the size of the part as needed. This is heated with a heat lamp or an $10 electric coil originally intended to light charcoal briquettes.
Once there is a 3/8 inch layer or more of mud the pouring spout and vent are cut open using a hack saw, hand saw, or reciprocating saw. The reciprocating saw makes quick work of it but be sure the mud is thick and dried hard so that it can take the stress.
Next a couple of cups of acetone will melt the foam into a chewing gum like residue inside the part. Actually the polystyrene is completely dissolved by the acetone, but the foam contains other ingredients like plastic and contaminants that are not dissolved. With the foam out of the way, the part can now dry from the inside and it needs to be dried thoroughly. Acetone is very flammable and the fumes are explosive so do not put the part directly into a gas oven!
With the foam gone there is no danger of the foam expanding and breaking the mud, and the parts can be placed into the furnace in order to burn out the foam residue. This took about 2 hours for the gear box parts and about 900 degrees. Not all of the residue was removed but what remains should not cause too much burn off during the pour.
Each half was poured separately. There is about 10 pounds in the larger part. Sections of the part are more than 1 inch thick. There was some cracking of the mold, but the sand surrounding the mold did a fairly good job of containing the aluminum. The mold was in perfect shape before the pour but the part was cold and I think the cracking could be minimized if I were able burn out the foam residue and the pour the mold while it is still hot. I'll give that a try when I cast my wet manifolds.
Trimming the mating edges so they fit flush together started on the table saw. I clamped a 90 degree brace to hold the part vertical and a piece of aluminum between bottom edge of the part and the table saw's fence provided a straight pass. This was followed by lots of delicate grinding after a piece of paper was slipped between the two halves in order to find and mark high spots.
Next, 3 - 3/16 inch stainless steel pins were tapped into the halves to assure the halves always had the same alignment when placed together and then holes were drilled and tapped for 8 -3/8 inch stainless steel bolts that will hold the two halves to together.
Finally the recesses were cut for the bearings using a router and jig. A mill would be the the best tool for this task, but I don't have one, so I turned a circle on my lathe and then welded that to a plate that I could position and bolt to the box. This required ordering a couple of extra long straight router bits from www.toolbarn.com. The first bearing was allied with the drive shaft from the Berkeley Jet Pump. Then using a paper template printed from CAD a second jig was built to alien the other bearings from the first recess.
I made a couple of the recesses a bit too large, but metal epoxy works nicely to fill these in. After 24 hours, metal epoxy can be machined just like aluminum. It holds up nicely to 350 degree heat and 2500 PSI pressure but is susceptible to shock stress.
After destroying one router and bit when the router got away from me, I decided to slow the router speed down by powering it though a common dimmer switch. Actually this is the same dimmer switch I use with my hot wire cutter. The results were excellent! The dimmer switch allows the router to run at less that half its normal speed, making it much easier to do fine work. With the slower speed, I could use the router free hand in order to shave the recesses for the bearings until I eliminated any binding once the gears were assembled.
Round Two of trying to cast wet manifolds for the diesel engine. After many nights the cutting and gluing I once again have a set of foam manifolds to start coating with mud. After breaking off the vents a couple of times during the mud dipping process I decided to reinforce those parts with Plaster of Paris.
Having spent a couple of weeks getting the foam to the point where I though it was ready to cast, I decided to build a pre-heating box. The pre-heat box is built for thin steel sheet and designed to incase the mold in sand. A steel lid is cut and welded into place and sealed with plaster to hold the sand inside, while exposing the spure and vents. The box has pipes welded onto each end that allow the entire box to be rotated upside down. Once up side down the box was then enclosed inside a fire brick box whit a steel lid and the temperature with brought up to 700 degrees over 4 hours. The idea was to burnout the foam, then rotate the box upright and immediately pour the aluminum in order to reduce the thermal shock to the mold.
Unfortunately that was not what happen. After 4 hours it was apparent that the foam was not burning out because I did not get the smoke normally associated with this process. My guess is that the sand surrounding the mold was acting as insulation and preventing the inside of the mold from getting hot enought to burn out the foam. With a crucible now brim full of molten aluminum I decided to press forward with the pour. I removed some of the fire bricks and rotated the box into position for the pour. As soon as the molten aluminum hit the mold, smoke and steam shot from the two vents. The smoke was expected, but the steam was not. Even after emptying all 15 pounds of aluminum into the mold, it was still not full, which was another very bad sign. The end product was a horrible casting with large voids and aluminum in several places where it should not have been. In stead of trying to cast the second manifold I decided to dissect it. What I found was that the internal parts of the mold had dry surfaces but hid large amounts of very wet mud underneath. I had taken 2 weeks to dip the manifolds once a day and then allow them to dry in a 100 degree oven, but even that was far too fast.
I learned some good lessons with this attempt but having spend the better part of 2 months trying to cast manifolds I am ready to try a different approach to the problem. So for now I am setting such an ambitious cast aside and I will take a stab an cutting and welding a manifold together.
Click Here to see building a manifold form welded parts on the "Engine and Jet Drive" page.
Months ago, when I was building my large foundry I had some left over Mizzou Castable Refractory and so I made some plinths and a flower pot furnace. The flower pots were just normal clay pots borrowed from a pile the neighbor had beside his house. Ok, I stole them.
Well today I finally needed just a small amount of melt and so I finished making the Flower Pot Foundry which still needed a burner hole and a sight/vent hole. I used a standard 1/2 masonry drill bit and drilled several holes to make a crude hole about 1 1/2 inches wide for the burner and several more in the the top for a vent. The burner is only resting on steel and the flame directed through the burner hole. The crucible is made from a gas cylinder cap I found on the high way, and a couple of bits of steel have been welded in to close off the holes on it's sides. Another couple of bits of angle have been welded to the bottom to support it off the bottom of the furnace. Finally two new 1/2 inch holes have been drilled at the upper lip, which allow the crucible to be lifted with a custom crucible tool. The crucible tool is a jack handle with a 3/8 inch bolt welded onto the end and then the bolts head was cut down so that it became a slight hook in order to prevent the crucible from sliding off. An couple of pieces of flat bar are also welded to the handle so that when the tool is rotated they catch the bottom of the crucible in order to tilt it for pouring.
I didn't have to remove the clay flower pot from the castable refractory; just heating it up once for about 4 minutes to help cure the refractory did that for me. This refractory had been sitting in a dry spot for months so curing was not much of an issue, otherwise the refractory comes with very specific instructions for curing.
This is the same burner I use in my larger foundry with is converted from a turkey fryer. The details for the larger foundry and burner are described above. The large furnace is great for melting up to 15 pounds but takes almost 2 hours and unfortunately it also takes about 30 minutes to melt just a couple of pounds because it takes a while to heat up all that refractory. On the other hand the Flower Pot Foundry melts a couple of pounds in just 15 minutes. There are always plenty of things to do around the shop, so it's not that I am in a rush, but I am cheep and the quicker melt time saves on propane. Not only that but handling the small crucible is much, much easier so pouring funnels are not necessary.
The support legs on the submarine will be deployed and retracted using all-thread rod for lead screws and the lead screw nuts will be common 3/4 inch nuts, but I need to fix these inside the 1 1/2 inch pipe that will make the leg so one at a time the nuts were placed on a section of all-thread and placed inside a 2 inch section of 1 1/2 inch pipe. The pipe rest on an aluminum plate and the all-thread passes through that plate and rest on top of a second plate. Everything but the nut is covered with a generous layer of soot from burning diesel on a rag. The parts came out usable, but only hours latter I changed my mind and I am going to use square tubing and not round. Oh, well.
I have had another opportunity to use aluminum as a mold and have learned something that you do not want to do. The ballast sled photo to the right shows the preparation for casting the lead screw nuts, and 3 mistakes were made. See if you can guess. The 1 inch diameter lead screw has sections of 1/8 inch wall 1 1/4 inch diameter aluminum pipe covering it. Aluminum cut from soda cans was used to shim the pipe so that the lead screw was centered in the pipe. The pipe has also been roughened in order the provide better grip for the molten aluminum. The lead screw also passes thought 2, 1 inch nuts. Half of the mold was formed from by the sides of the sled, and the other half is made from the same 3/16 inch aluminum sheet spot welded together with a pouring funnel and vent at one end and holes for the lead screw to pass through on both ends. This half of the mold was spot welded in place. The nuts rest against the side of the sled so most of the material in the cast is on the outer side but there is a no less that a 1/4 inch gap between the pipe and the sled. Before pouring I pre-heated the mold so that the aluminum would be less likely to choke as it flowed around the pipe and nuts. Did you find the mistakes? Here they are: #1 Don't use unprotected aluminum as a pouring funnel, the constant high temperature on the molten aluminum hitting one spot on the funnel is enough to rather quickly burn through the 3/16 inch material. #2 Don't pre-heat the mold. The outside mold was only spot welded together where the edges of the two sheets came together and they were already hot. Without enough mass or contact surface to conduct the heat way from the joint, the molten aluminum burned thought the joint between the sheets of the outside mold. Slowing down the rate of the pour allowed the leak to choke off so I kept pouring. I worked around the first two mistakes but the third one would have destroyed the part even anyway. #3 Aluminum always contracts about 2% as it cools , but with more material on the outside of the pipe than on the inside the force was much greater on the thick side and so the entire piece was bowed. The force did not bend the threaded rod, but it did bend the pipe enough to force the treads to cut into the aluminum shims, and it forced the two nuts at enough of an angle that they were locked onto the threaded rod. Once I had cut the part in half, between the two nuts, and without cutting through the threaded rod; both ends could easily be unthreaded.
"If at first you don't succeed..." I poured into the center so the distance would not be so great and added a lot of heat sinks to the form to keep it from burning through. I also did not preheat the form this time. And the results? Well it did not burn through the form, but it choked off quickly. So I when from too hot to too cold. Maybe the last bowl of porridge would be just right but I decided instead it was time to change the plan. The new plan (photos left) enclosed the nut and a section of the threaded rod in a temporary box. That was the molded into sand, and the temporary box was removed before the nut was cast. That one worked. I did used one nut with this rig but I had changed my mind on that along the way too. Using two nuts together always runs the risk of them acting like jam nuts when the casting expands and contracts with temperature changes.
If you already have a part and the part can draw a parting line that will allow you to separate the part into an upper and lower section then sand casting is a good option. Lots of people use green sand, but I have heard very positive comments about Petro-Bond sand so I choose skip green sand and go straight to Petro-Bond. The only problem with Petro-Bond is the price. With green sand I could have picked up the sand out of the Arkansas river that winds through Tulsa, Oklahoma and then added in some fire clay but instead I bit the bullet and paid $100 for 100 pounds pre mixed Petro-Bond. The price included shipping from a near by supplier. If you order Petro-Bond, note that you want the pre mix and not Petro-Bond II which is just the additive that is added to the sand. Mixing it by hand is also not recommended because it really requires a muller mixer to do right. If you want to go with green sand, there are good detailed instructions at www.backyardmetalcasting.com.
Start by building some casting flask that will hold the sand. Casting is far to macho to just call them sand boxes. Each flask has 4 parts. My top or "molding board" and the "bottom board" are 1/2 inch plywood. I added runners to the plywood to stiffen them and make the easy to pickup. The frames are pine secured by 2 screws at each corner. Nearly any board would work. You can use brackets to strengthen the corners if needed. The inside surface should be rough so the sand will stick to it, or you can add a thin strip of wood or cut a shallow groove to help hold the sand in place. You must be able to accurately align a pair of frame together after that have been taken apart, so I added a few dowel rods to each flask for this purpose. Others use interlocking blocks on the outside of the frames. A pair of old door hinges would work well too, and would have the added advantage of allowing you to insert the pin and lock the frames together. Locking the frames together is optional but it will prevent the molten metal from ever lifting the top frame. The only thing to keep in mind here is that one of the frames in a pair must not have anything that would prevent the the bottom molding or bottom board from resting flush against the frames top or bottom. Notice that the dowels on my frames are all fixed to one side and the latches I used can fold back so the do not extend above the edge of the frame.
If you'd like more details on sand casting and gating, I highly recommend Steve Hoerner's "Foundry 101" pages at www.host33.com/casting
Need a quick part or one that will not release easily from the sand? This is one more method that came to me from Master Ken, my casting mentor. I needed a small box that I could make into a waterproof switch enclosure, so I cut and glued some low density foam together then packed it into a flask of Petro-Bond and poured it in less that an hour. The result is what you would expect form low density foam but plenty good for what I needed. You can do this with any sand, but Petro-Bond works very well in holding its shape, preventing blow outs, and producing a smooth surface. Used and burned Petro-Bond works too.
I got to wondering if the higher density insulation foam; the pink stuff, would work? As it turn out, I was back to needing some aluminum chain sprockets for the fairly low tork job of moving the tiller and reverse gate on the Jet Pump. I used the one steel sprocket I had for a pattern and cut some copies out of 1 inch thick foam board. I hot glued a few of pieces together packed them into a flask of Petro-Bond, adding a 1/2 in diameter sprue through which to pour the aluminum. When poured it burst into flames as expected but much there was no violent build up of gas pressure that would spit the aluminum back out the sprue. Without a hard plaster or mud shell the pressure was obviously defusing into the sand.
The results were fantastic! This is the kind of reliable fast part production I had been looking for. Once I had the bar stock, it is just a matter of slicing off the individual sprockets.
Obvious next step was to see if it would work with the large sprocket. If I were really talented I could cast the part by just using the Petro-Bond, but as it were I could never remove the part from the sand without messing up several of the teeth. This way I just made a foam sprocket, packed it into the sand and left it there.
I first rigged a tall fence to my hot wire cutter which is really my table saw with some minor conversions. You can see my hot wire cutter here: Hot Wire Foam Cutter. With this I can slice a 10 inch wide sheet of foam so that it is 1/4 inch thick. I then use a few dabs of hot glue to secure my steel sprocket to the piece of foam I carefully used it as a pattern for cutting the foam is the hot wire. It is important to keep the wire as cool and possible and not to pause while making a cut against the pattern. This keeps the cutting line as thin and smooth as possible.
Once the foam is cut out, it is freed from the steel pattern and a 1 inch spur is glued on. A layer of Petro-Bond is then placed in the bottom of a flask and the steel sprocket is hammer down into the sand to ensure that sand is level. Baking Soda dusted onto the steel will ensure that the sand does not stick to it. Once the sand is ready the steel sprocket is removed and the foam sprocket takes its place. More Petro-Bond fills the flask but only 1/2 of a flask is used, so it is only 3 1/2 inches high. And any sturdy heat resistant container would work. There is no need to use a flask.
Using the flower pot furnace to melt the aluminum I was able to cast five sprockets in an afternoon, and four of them were good. One failed to completely form likely because the aluminum was not hot enough. I used both fresh Petro-Bond and the old burned stuff. Both worked equally well. When using Petro-Bond, I separate the burned sand out an keep in a separate bucket. Before using it again it only needs to be riddled to break up the clumps. A riddle is nothing more than a piece of metal window screen or a sifter from the kitchen. Burned Petro-Bond is fine for this type of casting since it does not have to maintain its shape, while a part is removed.
There is always something else to try. I needed to make an marine exhaust manifold for my diesel engine and after looking at other manifolds with all of their curved shapes, it seamed like wax was the way to go. Having read on the hobbycast group I learned that normal paraffin wax available from the hobby store was not suited for casting because it was not hard enough to easily cut or machine. I knew that was true, having placed a chunk in my lathe one day just to see what could be done.
The answer to soft spongy wax is to purchase Machinable Wax from a supplier like www.freemansupply.com or mix your own by adding polyethylene plastic to the mix. Since cheep is my middle name I came home from Home Depot with a role of plastic.
I am certain it will depend on the actual type of polyethylene plastic you use, but a mixture of 4 parts wax and 1 part plastic by weight was enough to raise the melting point by 80 degrees and make a much harder material from the wax. Perhaps not true "Machinable Wax" but better named "Plastic-Wax"
To make the wax pipe for the manifold I leveled a sheet of aluminum and clamped some scraps of aluminum to form 3/8 inches walls. A large cookie sheet or baking pan would have done equally as well. I then greased it up with either dish soap or petroleum jelly. Both of these worked well as release agents. You need to grease the PCV pipe that will be used to form the wax pipe too.
Be careful not to get the mixture too hot. I finally got use my fire extinguisher when the mixture burst into flames as I was pouring it out. Wish I had that photo! Once on the aluminum it quickly cools enough to be handled with gloves. The edges can be cut free with a knife and the sheet pulled free. With help from my lovely assistant we wrapped it around a PVC pipe, trimmed it to fix, and wrapped it up with masking tape so it could cool. Once it has cooled the seam down the length of the pipe can be fused together with a hot knife blade.
Well that is as far as it goes for now, because shaping wax pipe to make turns is a royal pain involving cutting apart a PVC pipe, making a plaster cast, casting the "Plastic-Wax" and then fusing the part together. That was taking way too long and I could not get the wax very uniform. Some places were 3/16 of an inch and others were 1/4 inch. So instead I abounded the lost wax approach and redesigned my manifold to use square channels, which are easy to make from foam. You can see the progress down this path on the Engine and Jet Drive page under Exhaust Manifold. Maybe I'll come back to lost wax casting when I need to make something nice for the wife.
While I was playing with my batch of "Plastic-Wax" I cast a 4 inch cylinder in my KHPT (see above). Now I am turning it down to make patterns for sand casting and also to try some lost wax casting. The "Plastic-Wax" turns beautifully. It acts just like aluminum, except the tool never needs sharpening. The wax can also be cut freehand with something a simple as a knife. In the photo to the right I have just finished a double pulley. This will go on my engines alternator so it and the water pump can be driven by dual belts. Care must be taken to only use light pressure as the chuck only has a tenuous grip on the part. Wrapping the cylinder with duct tape helps give the jaws more gripping power.
I tried forming the wax pulleys in sand but I could not get the Petro-bond packed in tight enough. Mainly due to fear of breaking the wax part. The next idea was to cast them in a split plaster mold. I read somewhere that Pam or other non-stick cooking spray makes a good release agent so I gave it a try. It's not true! The plaster stuck to wax like glue. So plan "C" was wax lost, so I poured in the rest of plaster.
Once the plaster had dried a couple of days at about 100 degrees, I turned the heat up to about 200. That was enough to melt away the paraffin wax but not the "Plastic-Wax". However it also dried the plaster enough that the plaster split apart cleanly along the joint line leaving a nice split mold so I proceeded to fire up a batch of aluminum.
I thought it odd that there was no wax in the pan I had placed under the plaster mold when I put it in the oven. It turn out that the paraffin wax that formed the sprues and runners was actually absorbed into the plaster. When it was hit with molten aluminum it acted much like foam, but with a lot more smoke. Needless to say the pour did not work. Without vents to allow the smoke and gases to vent form the mold the gasses backed up the sprues, blowing out aluminum and formed a pocket on the top of the pulleys. The heat also cracked the plaster mold so I took some pleasure dropping it on concrete in order to remove the part.
This would have worked if I had made the sprues and runners from "Plastic-Wax" as well or if I had carefully heated the plaster enough to burn off the absorbed wax prior to pouring.
I'm sure I'll apply these lessons, but wanting to move forward, I went ahead an cut a block from an aluminum bar and spend the extra time turning in into the 2 pulleys I needed.
Turns out this "Plastic-Wax" stuff is great for decorating pumpkins too. We make a plaster cast of Randi's face while she bit into an orange half. Then make a plastic wax casting from the plaster, glued it into a pumpkin and painted it orange. The high melting point of the "Plastic-Wax" even kept the candle from melting the casting.
What if you could take that 3D CAD image and print it directly to a 3D plaster mold? Well, you can! At $25,900 it's a bit pricy for the home foundry, but how totally cool! It just kills me that the example happens to be a manifold and a prop! See more details at the www.zcorp.com
http://manufacturing.stanford.edu "How Everyday Things Are Made" (Covers Casting)