Hi YouTube, my name’s Geoff and I’m the Veg Oil Guy.
Lost foam casting… … let’s make an aluminium sprocket.
Hi guys. Today I thought I’d share my technique for lost foam casting in the hope that it
may be useful to someone out there. It’s nothing special, more a combination of common
practices really, but it has great results for me.
It begins with the choice of foam. Most folks use Expanded Polystyrene Foam (EPS) when lost
foam casting. That’s the sort of foam typically found in packaging.
But there’s also Extruded Polystyrene Foam (XPS) much favoured in model making. It has
many of the same properties of Expanded foam but it’s much denser.
Extruded foam can be tooled and worked without falling apart, unlike Expanded foam, so if
I’m looking for detail, that’s the foam I opt for and that’s what I’m using here.
You’ll find it easily enough on the internet or at your local model shop.
It cuts a treat on a hot wire foam cutter but this piece is two thin for my needs. But
I can easily double its thickness by gluing another piece to it. I use expanding foam
glue, the sort used in woodwork. Whilst not quite the same, it behaves very similarly
in the foundry and that’s all I care about. Apply the foam glue very thinly, you don’t
want much. Then apply weight and ideally leave it 24 hours.
If you need to reduce the thickness of your foam, the hot wire cutter can cope with it
or you could use ordinary sandpaper on Extruded foam if you have the patience. Here I’m
placing a straight cutting router bit in my drill press, though this one isn’t as sharp
as it could be. The router bit can cut away the bulk of the material though it’s not
perfect. Sanding is required. You saw me making these wooden sprockets in
an earlier video and they’re going to be our template.
I drilled some fine holes in the wooden sprocket to accept some panel pins. These can be hammered
through the foam holding it firmly in place. A flush trimming router bit can be used to
follow the shape of the wooden template and the results aren’t too bad. You couldn’t
do that with expanded foam. It is possible to be neater if you have the
patience and if it’s required. Here I’m using the hot wire cutter to remove
as much material as possible. Using an ordinary steel bolt, I’m going
to make a crude but effective flush sanding drum.
I place the bolt in my drill press, turn it on and go to work with the angle grinder.
The idea is to round the head of the bolt and reduce it in size until it’s just a
little wider than the shaft. Here I’ve cut some coarse sand paper and
it can be glued with good contact adhesive to the shaft of the bolt.
Used once again in the drill press, it’s very effective and gradually sanding away
the excess. A blast of air prevents the sand paper from clogging and it lasts for ages.
If there are any imperfections you can drip candlewax into them. Build this up as necessary
and shape it with a sharp blade or sand paper. I’ve used wax here to rebuild damage to
a tooth and the wax will vanish easily in the foundry.
Here I’d adding a couple of scrap pieces for the sprues. Only two are glue but two
more help to balance some weight whilst the glue goes off.
Using standard copper pipe I’m pushing some holes through some scraps. These will make
up the sprues as you’ll see in a moment. It’s nearly casting time and I make a mistake
here in using my small flask that will come back to haunt me later. You’ll notice the
bottom flask has a solid bottom even though I’m using green sand.
It’s important to realise that. I’m using green sand not dry sand for my lost foam casting.
Typically dry sand is used but I’ve had poor results. With green sand as the foam
evaporates the sand maintains its shape, allowing the molten metal to fill it. Dry sand can
drop into the void and spoil this. You’ll notice I’m removing all voids,
flattening the sand well and forming a nice flat surface.
The upper flask is added and it’s bolted in place.
The foam sprocket is inserted centrally to and sand is sifted onto it. This makes sure
the finest of the sand comes into contact with the foam.
I’m using my fingers to make sure the sand is well pressed in. There can’t be any voids
and I don’t want to damage the foam – so fingers really are the best tool for the job.
Now come the copper pipes and these gently tapped into the firm sand, making sure they
do NOT come into contact with the foam. More sand is applied and firmed to hold these pipes
in place. You can see the two sprue tabs just sticking
out from the sand and ideally at this point you want the sand to be fairly flat and pressed
down. I apply some more foam glue to these though
it’s not strictly necessary, then I slide the other section of the sprues down the pipes
to contact the glue. These will form the feed and the vent and they need to be well buried
under the sand. And that was my big mistake. By using my smaller flask I didn’t have
the height needed to cover these properly. You need a good couple of inches at least
on top of your highest foam and stupidly I thought I could get away with it here.
The empty food tins help extend the height of the feed and vent. I picked up this practice
from Myfordboy and the idea is to add more pressure to the molten metal as it is poured.
It works well. Everything is well packed down and ready and
the foundry is fired up and metal melted.
Just before the pour the copper pipes are removed.
The molten metal is poured and – arrraghhh – out it comes. All because I didn’t bury
the sprues deep enough. What typically happens and what I expected
to happen was this… the molten metal is poured into the void left by the copper pipe.
At the bottom of this, the foam virtually evaporates leaving voids in the sand. Gases
escape through the feed and vent and the metal fills the space left behind.
But that didn’t happen and so, in a bit of a strop I threw aside my flask and banged
out the content grumbling about my first failure. Then I spied a hopeful something…
Success. Even though I completely cocked it up, the process is forgiving enough to still
work. So far this technique has worked for me every time.
Once everything had cooled and back inside my shed, you can see how the feed worked.
You can also see how it failed… and for that reason the finish isn’t perfect, but
it’s good enough for my needs. With the sprues removed and everything cleaned
up a little, you can see the one side of the finished sprocket looks excellent. You can
clearly see the pattern of the Extrude foam and this could be easily sanded or filed away
if required. The other side, which was the top, is not
so could. These imperfection look like cracks but I think they’re surface only, caused
by the pour spilling out of the flash and there not being enough head pressure to push
the molten metal fully to the sand. I don’t believe there’s any structural weakness,
at least not enough for my purposes, and I could possible grind away these cracks or
maybe even fill them. So if you do as I say rather than as I did
folks, I’m hoping this lost foam method of mine may help some folks out there. And
that’s a finished video guys. If you’ve got any questions or comments,
as always feel free to send them in. If you’ve got any video requests, I’m happy to try
and help there too. Please do check out my YouTube channel and
of course my other videos. If you haven’t subscribed yet, please do.
So that’s it for now folks, and thanks very much for watching.