http://www.theworkshop.ca/metcastmach/foamcasting/foamcasting.htm
Lost-Foam Metal Casting
This page was created in response to a few requests that I've had regarding Lost Foam Metal Casting method that I use. Unfortunately I can't compare Lost Foam to Cavity Molding or Green Sand Casting as I've never used green sand.
When I first built my Blast Furnace, I just wanted to try it out on something quick & dirty. I had read about Lost Foam, and thought it would be a quick way to get started. The results I had were so much better than what I expected that I've just continued on with it. Secondly, since I didn't have a Cope & Drag setup or the sand clay mixtures readily available I started to focus more on Foam.
The foam that I use is construction insulation. I buy mine in 2 ft by 8 ft by 2 inch sheets. This is great stuff to work with, as it cuts easily and can be shaped with common hand tools.
The tight cell structure is a key feature, I doubt the the larger bead type foam would be of much use. I first started using foam for molding fiberglass. During that experience I made a small hot wire foam cutter. Even though the cutter is fun to use, it does tend to distort the form as it cuts and requires quite a bit of hand finishing to clean-up plugs.
Generally I guesstimate the rough dimensions of a part and cut a chunk of foam with a hand saw at least 0.25" over-size. This generally leaves enough material to shape as closely as possible to finished dimension.
In this case I'm cutting out an octagon shape that will be turned down to be a face plate for my lathe. Notice the el' Cheapo drill press. I picked this up at a flea market for 5 or 6 bucks, and just love it, as it's bolted to a table on wheels, while my main press is bolted to a stationary 16ft bench.
Once the chunk is cut, I find the center, drill a hole to accept a length of 3/8ths threaded rod. A nut and large flat washer are located on either side of the foam block to help keep it flat or perpendicular to the threaded rod.
Generally I hand tighten the nuts to the point that the washers sink flat with the surface of the foam. This is adequate to keep the rod from spinning inside the foam as you shape it.
Obviously the threaded rod and foam block get chucked-up into an electric drill. It doesn't have to be a drill press, but for me it seems to be easier than having the drill flopping all over the bench.
I true-up the block into a cylinder with a wood rasp. At this point I'm using both hands and trying to steady off the base of the press. Even though it's just foam, there's enough velocity on the outer edges to give you nasty burn if you rub-up against a turning edge. I'm using my index finger on my right hand as a light sensor as the block spins.
After having shaped the block into a cylinder I use a woodworking chisel to turndown the cylinder into whatever shape I need.
It seems that the faster the foam spins the better finish I get. After having tried at slower speeds I noticed that there is a greater ripping action on the foam leaving a more mottled finish requiring more effort to produce a piece.
During the shaping process I tend to use the sharp corners of the chisel and the face as shown in this picture. Always angle the chisel point with the direction of rotation, or you may find that the chisel gets drawn into the foam and ruins the piece. Similarly you may loose your grip on the chisel and have it fly across the shop.
The last stage is to get a smooth finish without removing any more material than necessary. I like to start with a 200 grit sand paper to remove any little flakes and finish with a flat metal file.
These pictures are of a square shouldered faceplate, but pulleys are just as easy to make with the groove cut and finished.
The factory glaze or finish should be broken while on the press with the sand paper as well. The factory finish tends to cause a warp or curl on pieces once the opposite side is cut. This problem is more pronounced with flat square pieces.
Not that the specifics of this casting have much to do with Lost-Foam Casting but I thought this was a good illustration of how easy foam is to deal with.
I wanted 4 slots cut squarely on the face. Here I'm using my trusty (knock-off) dremel tool with a quarter inch routing bit.
Since it requires so little force to cut through the foam, it's easy to use a steel rule as a guide without damaging the bit.
The tub on wheels is full of used sand that I have yet to sift, the Steel bucket, wooden box, and old pot are what I use for pouring my castings into.
I will have to get more sand, but I have enough to pour off 3 castings at a time with this set-up.
Here my assistant carefully sifts the sand through screen stapled to a shallow box. By using a wisk any debris is easily swept off the screen onto the floor, the screened sand is poured off into a container.
I recently picked up 20lbs of the finest sand I've ever seen. Unfortunately it was on a beach in northern Quebec during a fly-in fishing trip. It took some convincing to get the pilot to let me bring it back, but it's in Ontario and about to be put to the test.
This stuff is so fine it feels soapy when you run your fingers through it.
Here is the faceplate plug with a 9/16th shaft through the bore and a sprue shaped and pinned into place.
This is the first time I'll be trying steel pins to hold 2 pieces together. Though it may seem paranoid to be fearful of any changes to process, it's always in the back of your mind that liquid metal could end-up flying in all directions whenever you encounter an unknown variable.
I opted to use my trusty soup pot, poured in about an inch and half of loose sand. Placed the faceplate plug roughly in the center and covered it with more loose, dry sand.
There is no clay, water, oil, petro-bond, just dry loose screened sand. This picture doesn't show it well, but I try and create a natural funnel with the sand to help direct the metal onto the foam sprue.
Ultimately, you want to pour the metal directly onto the top of the sprue as fast as it can take the metal. As I've said in other pages, there is a knack to pouring fast but carefully.
I always find that there is a split second when the metal pools ontop of the sprue and then there is a quite poof with a puff of black smoke. As this happens you are pouring and MUST KEEP pouring.
To make matters worse the sand does look like it is shifting slightly, this only adds to your anxiety that the pour didn't turn-out. Now all that is left to do is is clean-up your tools, pace around the mold pot, and generally wait. After about 10 minutes I'm almost crazed with anticipation.
I start by poking the top of the sprue to see how solid it feels. I can never tell much from the poke test but it does tend to kill at least 15 to 20 seconds.
This is it... The moment of truth, I grab a large set of pliers or channel locks and pull it out of the sand.
This time everything when well. Steel pins are OK, so is the pipe to place the bore hole. My slots seem to have come out fine as well.
The finish from the French sand also has noticeably improved this casting. Compared to previous pieces there is less pitting.
After a bit of clean-up with a wire brush and file, the sprues are cut-off. There is a slight concave property to the face plate that will be machined off. I think that this is from not taking off enough of the factory foam finish.
Hopefully this helps you explore the possibilities of Lost-Foam Casting.