Terminator Graveyard; How To, and How Not to Lost Foam Cast Aluminium

Intro: Terminator Graveyard; How To, and How Not to Lost Foam Cast Aluminium

My recent project in 'lost foam' and a few pointers to things that can go well or go wrong.

STEP 1: How 'Terminator Graveyard' Started Out.

Having previously had some very effective results using the 'Lost Foam Casting' method I looked for some time on interesting polystyrene (Styrofoam) objects that I might be able to cast up.

For those not familiar with the method an object of polystyrene has a 'sprue' attached which allows the molten aluminium a path to flow into the object to be cast.

The object to be cast is buried in dry sand with the sand level immersing the object and most of the way up the sprue. (I have been using something special called fine calcined flint clay because I have it available but most people use sand)

Aluminium is then melted and poured atop the sprue and it melts the sprue and the object perfectly filling the void.

It is an awesome method because it doesn't require particularly specialist equipment and can give some great results.

Now the part I should not have to say but will anyway; Foundry Furnaces, Molten Aluminium and Casting are incredibly hazardous. This is not a safety guide, do not do such a project without excellent safety equipment and a good understanding of the risks involved and how to mitigate those risks. If you do choose to do so, it is at your own risk. I have included a video of the furnace running to show you just how intense it is.

STEP 2: The Polystyrene Prep

When browsing Trademe (New Zealand's answer to eBay) I stumbled upon a polystyrene mannequin head.

The facial details on the face and the head were stunning and the opportunity just could not be passed up.

When the mannequin arrived it was not small by any means and largely filled up a 20 litre bucket.

Firstly I fashioned sprues out of some 25mm polystyrene sheet and hot glued them to the base of the mannequin.

Next I used some medium fine (180grit) sandpaper to sand off the raised edges of the moulding defects on the polystyrene.

STEP 3: Getting the Mould Ready

The next step is to bury the polystyrene mould and part of the sprue in sand.

In the second photo you can see my 'pouring cup' this provides a clear defined reservoir for aluminium to prevent defects from shrinkage and head pressure to ensure the mould fills completely. In most smaller cases this would be a tin can with both ends cut off but because this was larger than most I used a short section of 100mm pipe.

For my projects thus far I have used calcined flint clay (which is fire clay that has been fired, crushed fine and graded) instead of sand, it has worked well for me but most people use silica sand.

The general rule of thumb for casting is the finer the sand then the finer the surface detail you will see (my calcined flint clay is graded to 0.05mm)

Some people do things like coat the polystyrene in plaster of paris or tape at this point to get good surface quality but I have not found it necessary myself. If you did give it such a coating it may prevent sand from collapsing in if you do not do the pour properly.

For this step I have never yet rammed the sand in, it has always flowed in loose like a liquid and filled any voids/ surface details.

STEP 4: Melt Some Metal

Now that the mould is prepared it is time to melt some metal.

As mentioned earlier, at the start of this project I did not even have a suitably sized crucible so I made one out of steel cut from an old air compressor pressure tank.

I cut it apart with a grinder and then welded a steel base on.

The final crucible was approximately 200mm in diameter and 300mm high with a volume of 8ish litres of aluminium.

Since I had never worked with such a big 'pot' of aluminium before I made a loop on either side so myself and a helper could put a bar through and lift it together.

STEP 5: Pour Some Metal

This is around about the place where we go from how to do lost foam casting to how not to do lost foam casting.

I had fashioned a jig to hold the crucible of aluminium above the mould.

We pulled the crucible of aluminium out, skimmed the dross then set it up on the jig above the mould. Unfortunately I had only guessed heights and things started to go 'bendy'.

The hot crucible of metal touched the side of the bucket (holding/ forming the mould) and set the bucket alight.

Hurriedly I poured the metal down the sprue and then the curve of the pouring on my jig no longer lined up with the sprue interrupting the pour.

I saw the 'sand' slump and new immediately what that meant, I carried on pouring but knew full well that this meant at the very least a partial fail.

Very quickly the mould would accept no more aluminium so the remainder was poured in an ingot tray.

The secret of lost foam casting is a continuous pour. No matter whether fire and smoke comes out (which it will) you must keep pouring. It seems the hydrostatic pressure of the incoming aluminium is sufficient to prevent the sand from caving in. Every time if the pour has been interrupted the results have been poor, if however the pour has been continuous the finish has exceeded expectations with no cave ins.

STEP 6: The Finished Product

As mentioned earlier I knew there would be problems with the final cast from the interrupted pour but I carried on anyway.

Another tip is that you can never leave the cast too long before disturbing it. I am always impatient and sometimes this has meant that there is still molten aluminium inside.

Leave the cast for several hours and while it will likely still be hot, it should no longer be molten.

No surprises that there were voids but in some ways I was very happy with it.

A post apocalyptic piece of art maybe; I call it Terminator Graveyard (and just in time for Halloween too ;) )

Better luck next time...

Comments

[Riffifi]

if the back looks a bit torn, at least the face is complete, and the defects are interesting. a perfect cast may have been boring :)

[Bruce_T_NZ]

Thanks heaps, while I was a little disapointed I kind of agree.

[MichiganDave]

I think it looks great. No disappoinmtent here at all. Thaks for sharing.

[Bruce_T_NZ]

Thanks for the kind comm

[Wolfbane221]

even though it was a failed pour it looks pretty cool. I'm quite interested in your waste oil burner there, actually this weekend I picked up some perlite and sand in preparation to make one. I already had the Portland on hand, but need to order the high temp clay. think you could post some pictures or share details on your burner? I basically plan on making a steel pipe with a taper, and using an air break with a blower. Being my first one I'm curious about measurements of the pipe.. does it matter or will I be fine as long as I mix the oil and air correctly.

[Bruce_T_NZ]

Hi Wolfbane. I wont post my burner pics for now as my setup is pretty complex for a first setup.

In mine I use a commercial heating furnace nozzle (I have a few; in the video clip it was probably running 2.75GPH) running off a proper heating furnace high pressure pump coupled to a motor via a bell housing I made to adapt the 2 together. I then use a dimmer style speed control to control the speed of a spa blower with the whole burner assembly having and OD of 2.5". My furnace burns diesel too which will perform a little different to waste oil.

In theory it is the air to fuel ratio that matters so it is possible to achieve a good mix by having either the air fixed and the fuel varying but both adjustable is preferable.

Depending on your blower style your available options tend to be a damper on the suction (for a squirrel cage blower, bouncy castle blower or leaf blower) or a dimmer speed control and maybe a ball valve ( on a spa blower or shop vac etc).

For varying your fuel flow the most common options are to raise your reservoir of oil to increase the head pressure and thus the flow and/or to install a needle valve to precisely control the flow.

Another option is to use a sealed reservoir and a regulator with a supply of air to pressurize the oil.

I have chosen to go with a 'proper' fuel misting nozzle because from my experience it gives the cleanest start-up and shut off with the least smoke.

Be aware that 'normal' cement works due to some chemistry with water and at high temperatures it will become no longer effective and can turn to dust. If you can get Calcium Aluminate cement it does perform better at these elevated temperatures. (Just starting out you might be OK but don't be surprised if it doesn't last particularly long)

Better still if you can afford professional refractory I do suggest you do so.

Your idea does sound workable but I would suggest a propane or lpg pre heater. If you use propane or lpg to get the furnace hot first then slowly start adding your waste oil you can get away with a much simpler fuel delivery system (simply dripping/ squiring the oil in through a 1mm or so nozzle) than mine and with acceptable amounts of smoke.

For best burner performance (least smoke and no unburnt fuel dripping back out of the burner) I suggest that you make sure that the nozzle exits direct into the furnace itself rather than back in the blower pipe, this does make a significant difference, even with a misting nozzle.

Another thing that I have learnt the hard way and have not found mentioned a great deal on the internet is regarding lid design. To keep smoke to a minimum the lid should seal as much as possible. My rule of thumb (others may well disagree) is that the size of the exhaust size should be approximately double the diameter of the burner/ air inlet size.

Hopefully this answers some of your questions and gives you a few pointers.

[Wolfbane221]

Bruce, you helped out a bit with some stuff others don't go into detail with. I'll probably start with a simple fuel delivery and look at more complicated ones for the future, Yours sounds ideal. Nobody else has ever discussed their exhaust so that may help me out in the design. Off to fusion to see what I can draw up before cutting into anything. Thanks for the help!

[Bruce_T_NZ]

No problem at all. Good luck.

One other thing that I just remembered is Kaowool. If you go with your homebrew refractory mix the perlite will be insulating so you may not need extra insulation. If you go with commercial refractory this tends to perform much better on the hotface but is a poor insulator. Most people will have 2 or three layers of kaowool between the drum or whatever contains their furnace and the refractoty hot face.