Introduction: Build an Electric Foundry for Melting Aluminum (that Works on 120 VAC)
I've seen many projects out there for building a foundry for melting aluminum to make castings. Most are based on cumbustion furnaces (charcoal or propane). A few are electric which has many advantages. None that I have seen are as easy to build as the electric foundry furnace I will describe in this instructable and none that I have seen work effectively on 120 VAC (they require 240 VAC to reach high enough temperatures). Unfortunately 240 VAC is not available in many home workshops.
See the attached video for a very quick overview of the project. Then read the rest of this instructable or visit my website IWillTry.org for detailed instructions.
Step 1: Advantages of This Design
First off, this design is based on an electric furnace which offers several advantages over a combustion furnace:
- An electric furnace is much safer because there is no open flame and the furnace can be completely sealed. If insulated well enough the exterior temperature of the furnace need not be excessive.
- An electric furnace requires no ventilation which means you can use it indoors. This is a big benefit in winter.
- An electric furnace is completely silent.
- An electric furnace has no consumables (unless you consider electricity to be a consumable).
The primary disadvantage of an electric furnace is it cannot reach the same high temperatures that a combustion based furnace can, but either type of furnace is perfectly adequate for casting aluminum which melts at about 660 degrees C. An electric furnace also does not heat up quite as fast as a combustion based furnace, but the difference is fairly insignificant unless you are producing high volumes of cast aluminum parts.
This design also has several advantages over other electric furnace designs I have seen:
- It’s extremely easy to build. After collecting all the components, I was able to build it in about 90 minutes.
- It's lightweight, due to using ceramic fibre insulation rather than fire brick.
- It's outside temperature is quite low due to being very well insulated. It reaches only about 75 degrees C which is cool enough to touch (briefly) and not hot enough to ignite anything.
- It works even on 120 VAC. Most electric furnaces are not insulated well enough to reach adequate pouring temperatures if operated at 120 VAC. They require 240 VAC.
A disadvantage of my design, perhaps, is that it is a little less robust, but for the limited amount of metal casting I do it suits me fine and I think it is a good entry level furnace for anyone who wants to dabble in aluminum casting without investing a lot of effort up front.
Step 2: Safety
To say that working with electricity, high temperatures, and molten metals is dangerous is an understatement. Different people will have different “acceptable levels of risk” based on their knowledge, awareness and risk tolerance. Just because I’m comfortable doing something doesn’t mean you should be. Please do not blindly follow my lead or anyone else’s. Understand the risks yourself and choose what’s acceptable to you. At a minimum, have a fire extinguisher handy and be sure you know how to use it.
Step 3: Materials and Equipment
I already had all the materials and tools I needed to make this furnace. I don’t recommend buying parts new if you can salvage them but for those who wish, I have provided links to some suitable items from Amazon below (disclaimer… these are affiliate links which means I will get a small cut… thanks for your support).
- Discarded propane tank. If you don’t have a used propane tank kicking around, try searching Craigslist, or visit your city recycling depot as they are often discarded.
- 6″ Stove Element: http://amzn.to/2x3xTZC
- Ceramic Insulation: http://amzn.to/2x3xTZC
- 240 VAC power cord: http://amzn.to/2x3xTZC
(a 120 VAC cord will also work if you don't have a 240 VAC outlet)
- Solder: http://amzn.to/2vLfCfs
Solderable alligator clips: http://amzn.to/2vL1G55
- Safety glasses: http://amzn.to/2vL1Byx
- Heavy welding gloves: http://amzn.to/2x3xTZC
- Angle grinder: http://amzn.to/2xiKiJD
- Abrasive cutoff disc: http://amzn.to/2xjvL0i
- Reciprocating saw: http://amzn.to/2xirJ85
- Metal cutting blade: http://amzn.to/2xirDgJ
- Hack saw: http://amzn.to/2f9r1Q8
- Drill: http://amzn.to/2jbRtwH
- Step drill bit: http://amzn.to/2vKZYkb
- Soldering iron: http://amzn.to/2vLZhqI
- Welder: http://amzn.to/2vL8HDf
- Digital Thermometer: http://amzn.to/2x3xTZC
- Watt meter: http://amzn.to/2x3xTZC
Step 4: Cut Off the Top of the Propane Tank
Before cutting the propane tank, you must empty the tank, remove the valve, and ensure all propane is vented. Empty the tank by connecting it to a BBQ and cooking until the propane runs out. Then remove the propane tank valve. This can be a challenge. I was able to do it by using a large pipe wrench on the valve and a long bar through the handle of the tank for opposing leverage. There will probably be a bit of residual pressure in the tank that will be released as you loosen the valve so it is a good idea to do this in a well ventilated area.
Once the valve is off, make sure all propane has been expelled from the tank. You can accomplish this by letting the tank sit outside for several days, or blowing compressed air into the tank for a few minutes, or filling the tank with water till it overflows, and then draining it.
Once the tank has been vented, use a felt pen held at a fixed height to mark a line around the tank. I chose a height that looked right to me, knowing that the top of the tank will become the lid of the furnace and will need room to pack in about 3 inches of ceramic insulation.
To start the cut use an angle grinder with abrasive cut-off disc. This is the point at which the tank may explode if you haven't properly vented it ;-). Once the cut is started you may wish to switch to a reciprocating saw with a metal cutting blade if you have one. This will be much faster than the abrasive cut-off disc. It should only take a couple minutes to saw around the full circumference of the tank.
Step 5: Prepare the Element
The terminals of the range element need some extensions added to reach through the wall of the tank. I used some threaded steel rod I had handy (difficult to tell from the pictures). Non-threaded rod like welding rod or coat hanger would suffice for quick experimentation but threaded rod will make it easier to properly insulate the terminals where they pass through the housing and attach an electrical cord safely (unfortunately I don't describe that process in this instructable since I haven't done it yet... more on that later). Bend a loop in the end and screw it to one of the terminals. Cut the threaded rod to length with a hack saw such that it will stick out about 1″ from the side of the furnace when the element is centered. Repeat this process for the other terminal.
Step 6: Drill Holes in the Base for the Terminals
Hold the element 3 inches above the inside bottom of the propane tank (the eventual height of the insulation) and mark the location of holes where the terminals will need to pass through the side of the tank. Drill two holes about 1/2″ in diameter using a step drill.
Step 7: Assemble the Base of the Furnace
Fill the bottom of the propane tank with random chunks of ceramic insulation until you’ve filled in the concave portion of the tank. Using the top of the tank as a template, cut a circle of insulation to cover all the random chunks and provide a nice flat surface on which to rest the range element. You may need to experiment with different amounts of insulation to get the element terminals to line up with the holes you drilled.
Wrap the element terminal extensions (the threaded rods) individually with plenty of ceramic insulation and pass them through the holes in the housing ensuring there is no possibility of the rods shorting out on the housing. If there is any contact it could cause a short circuit or present a risk of electric shock. This method is just a quick stop gap solution for testing purposes. For a more robust and safe design I would machine some insulators out of teflon (working temperature 260 degrees C) to sandwich the housing where the threaded rods pass through. I would secure these with a nut on the threaded rod on either side of the housing. If I take the design further I will add pictures and instructions explaining that process.
Once you’re happy with the position of the element, add a couple layers of ceramic insulation around the inside walls of the tank. Note the image shows only the first layer of insulation around the walls. Another layer is added next to the first bringing the insulation thickness to about 3".
Step 8: Assemble the Lid of the Furnace
Weld 4 screws inside the top of the propane tank in such a way that they will hold insulation in the lid. Let the screw heads protrude a bit past the lip of the lid so that when the lid is in place on the base of the furnace the screw heads will help align the lid with the base.
Using the lid as a template, cut a circle of ceramic insulation to fit. Fill the concave part of the lid with random chunks of ceramic insulation and the cover those with the circle of insulation, tucking the insulation under the screws to hold it in place.
Step 9: Make a Power Cord
The first power cord I tested was a salvaged 120 VAC cord that I soldered a couple alligator clips to (sorry... teaching how to solder is beyond the scope of this instructable). If 120 VAC is the only power you have available it will suffice, but performance will be greatly improved at 240 VAC. Lucky for me I have a 240 VAC outlet in my garage for a large air compressor. If you have access to 240 VAC, use a 240 VAC power cord with whatever plug type you need to connect to your power source. The attached image shows the 240 VAC power cord I ordered from Amazon to match my power outlet.
Warning!!! Bare terminals and alligator clips are super dangerous when working with 120 or 240 VAC. My primary motivation in this project was to determine if I could melt aluminum effectively with a simple range element. I wanted to determine that as quickly as possible so I took shortcuts with calculated risks. I do not suggest this is a safe or permanent solution. Duplicate at your own risk. If you do, take extreme care to avoid touching the bare terminals or tripping on the power cord or shorting the terminals to the housing. If I were to take this design further I would machine teflon insulators to secure the element terminals where they pass through the housing, and use ring terminals to attach the electrical cord to the element terminals outside the housing. I would also attach the housing to ground so any short circuit will trip the breaker, significantly reducing the risk of electric shock.
Step 10: Make a Crucible
There are many ways to make a crucible. The simplest I know of is to use an empty soup can. The metal of a soup can is quite thin, however, and it will quickly oxidize at high temperatures, likely lasting for only a few heatings. I made my crucible from a piece of 1/8″ wall steel pipe and a steel plate that I had in my scrap bin. After welding the plate to the end, fill the crucible with water to check for leaks. You do not want molten aluminum leaking out of your crucible while it’s in your furnace.
Step 11: First Heat
Place the furnace base in a safe location away from anything flammable. Connect the alligator clips of the power cord to the terminals. Fill your crucible with some pieces of aluminum and place it on the element. Place the thermocouple probe near the top of the crucible and put the lid on the furnace. Plug in the power cord and monitor power and temperature as the aluminum heats up and melts.
Power meters for 240 VAC can be expensive, but if you can test the power at 120 VAC, there is no real need to do so at 240 VAC. Power is proportional to V squared, so at 240 VAC you will get 4 times the power you get at 120 VAC. I measured about 350 to 360 Watts at 120 VAC, so I know that at 240 VAC I will get about 1400 Watts. Power is also equal to voltage times current. Thus current is equal to power divided by voltage. So at 120 VAC I am drawing about 3 Amps and at 240 VAC I am drawing about 6 Amps. Those figures are likely typical of most 6″ range elements.
Aluminum melts at around 660 degrees C but needs to be about 700-750 degrees C to ensure a good pour without any unwanted solidifying. The temperature indicated by your digital thermometer will be the air temperature above the crucible which will be initially hotter than the aluminum since the air will heat up faster than the aluminum. If you are operating at 120 VAC, I would wait till the temperature reads around 730 to 750 degrees C before pouring.
If you are operating at 240 VAC, the air inside the furnace will heat up much faster than the aluminum and the thermometer reading will be higher than the actual aluminum temperature. Therefore I recommend “soaking” for 5-10 minutes at temperature readings between 700 and 750 degrees C. Basically wait till the temperature reading hits 750 degrees C and then unplug the power cord. Wait till the temperature reading falls to 700 degrees C and plug the power cord back in. Repeat for 5 to 10 minutes and then pour.
If you plan to make a lot of castings, it may make sense to add a temperature controller, but for limited hobby use, you can be the temperature controller, monitoring the thermocouple readings and plugging/unplugging the power cord to maintain the desired temperature.
Step 12: Pouring the Molten Alulminum
Heavy leather welding gloves and a pair of vise grips is all it takes to handle the crucible of molten aluminum. Remove the furnace lid and set it aside. Wearing the gloves, grab the crucible with the vise grips. Carefully pour the molten aluminum into your mold, if you have one prepared (sorry... a discussion of pattern and mold making is beyond the scope of this instuctable). I was just testing the furnace so I did not have any molds ready and simply poured some ingots for later use. A muffin pan is the ingot mold of choice for the backyard foundryman. Whatever mold you use, make sure that the finished ingot will be small enough to fit in your crucible when you want to melt it again.
Step 13: Performance Evaluation
When connected to 120 VAC this furnace produces about 350 Watts and takes about 2 hours to melt some aluminum and bring the temperature up to adequate pouring temperature.
When connected to 240 VAC the furnace produces about 1400 Watts (theoretically... I didn't measure it) and takes only about 25 minutes to melt a similar volume of aluminum and bring the temperature up to adequate pouring temperature.
I'm uncertain how long the range element will last (it is certainly not designed for this type of application), but it has survived several heatings at 240 VAC so far and shows no sign of decay.
Considering how easy it was to build this furnace I'm very pleased with the results.
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