Introduction: How to Lost Wax Cast an Aluminium Skull

Having learnt quite a list of skills off Youtube, Instructables and the internet in general I though it was time to give something back in the form of a photo guide of a recent aluminium casting project.

DISCLAIMER.

Listed here in this Instructable is not necessarily the best way of doing this, just the way I have found seems to work reasonably well.

Casting aluminium involves very high temperatures. SAFETY, SAFETY, SAFETY, make sure you use all of the appropriate protective equipment throughout. Aluminium in molten state can cause unmentionable burns, the fumes from a furnace can be quite nasty and your face and eyes are super important to protect.

Step 1: What You Will Need

For this project you will need.

  • Something to cast (made out of wax, ABS or PLA)
  • A candle (to use as a sprue)
  • Some plaster of paris
  • Some refractory material to re-enforce the plaster of paris (fine sand will do but I have used some calcined flint clay)
  • A tin can to hold the plaster mould
  • A tin can with both ends cut off to use as a 'header tank'

Step 2: Preparing the Mould

For this demonstration I am using a cheap skull candle as a mould. I have taken a 13mm drill bit and carefully drilled into the base then inserted a large taper candle to use as a 'sprue'. The sprue is the path that the molten aluminium will flow in.

You can apply similar methods to casting 3d printed parts in aluminium, instead I would recommend 'hot gluing' a polystyrene block to the base of the 3d printed part to use as a sprue.

Step 3: Setting the Part in Plaster

Mix up a thin slurry of plaster of paris. Dip the part in the plaster of paris immersing it up to almost the base of the sprue, making sure it is completely covered in plaster.

Immediately after it is covered in plaster drizzle refractory (fine sand or proper fine refractory), once again making sure that a little of the sprue base is left uncovered. Allow it to dry then repeat the process several times.

When I did this casting I went straight from plaster to sand then back to the plaster again, had I let the 'shell' dry after each layer of plaster and refractory the surface quality would have probably been better.

I coated my part in 3 layers of plaster and refractory before setting the part in plaster of paris and refractory mix.

The plaster of paris can struggle to cope with the heat involved in burnout and preheat so the refractory gives it more strength.

Alternatively you can avoid the shell method and go straight to casting the part in plaster and refractory mix.

I have done this before on an ABS 3d printed part with decent results.

Step 4: Burnout

After allowing the part some time to set ( I have done within a couple of hours of the plaster hardening) put the tin with you plaster and part in a fire (or ideally a burnout furnace if you have one ;) ) and melt and burn the wax or plastic out. Leave it quite some hours until you see no more burning and melting coming from the sprue hole.

Preferably go straight onto the next casting step while the mould is still hot. You will need pliers or similar to move your hot mould to adjacent to the furnace.

Step 5: Melt Your Metal and Pour

This step is pretty much self explanatory.

I wont go into too much detail here because it is the most dangerous step.

Do not embark on this step unless you are confident to do this safely

If you are not familiar with safely casting metal then go away and do some research and come back after. Myfordboy, Luckygen and some others do some excellent guides on doing this on Youtube, no doubt there are some excellent guides here also.

In the picture is myself tending my current home made furnace. It is constructed of a clay/ graphite crucible as a hot face and wrapped in approx 125mm of Kaowool then topped off with a mix of fireclay, calcium aluminate cement and perlite all inside a 2/3rd section of 200l drum.

It is fuelled by LPG and waste oil, (starting on LPG and then changing over to oil once it has been running for some time).

I can melt aluminium, copper or brass and cant be far off melting cast iron with this furnace.

Unfortunately my Wife who was taking photos of the process had to go out so there are some steps missing here.

On the top of the Mould apply a tin can with both ends cut out to use as a 'header tank' We have used play dough to set it to the mould but next time I will set this a little into the plaster itself.

There are some great pieces of info on aluminium casting on the internet that explain the reason why very well but basically the aluminium shrinks and the added volume and 'head pressure' seems to make the difference between a successful and an unsuccessful cast time and time again.

In the foreground you can see the galvanised bucket filled with refractory surrounding the plaster mould.

Step 6: Finished Product

Let the cast part cool for 10 to 15 minutes then plunge in water. The 'heat shock' will often assist to break open some of the plaster and assist in its removal.

I had hoped for a better finish on the part but there were few things that conspired against me.

  • The mould was too cold; It was the next day after the burnout, I did preheat it a little in the furnace but not enough. The plaster may well have re adsorbed moisture.
  • I didnt leave the shell layers to set between layers. Pretty sure this caused some of the material to be a bit loose inside the mould causing some of the porosity and rubbish in some of the indented detail areas.
  • Didnt take enough photos; sorry folks this is my first instructable, hopefully the next one will be better.

Thanks for your attention folks, hopefully it was helpful or at least interesting to you.

Comments

author
WyckedStudios made it!(author)2016-03-03

I know I'm late to this party, but I may as well chime in;

The rough surface texture is a result of still having chemically-bonded water in your mold, and the aluminum being hot enough to cook it out, forming steam everywhere the metal meets the plaster.

PoP molds have both absorbed, and bonded water in them. Burn-out temperature is hot enough to drive off the absorbed water, but not the bonded. You need to get the mold to 600C (1100F) in order to break that chemical bond.

This is right at the upper limit of what PoP can tolerate. I'm very slow and precise with raising the temp in my kiln, and I still lose one out of every four or five molds.

author
Bruce_T_NZ made it!(author)2016-03-04

Hi, Wycked. It was a bit of an experiment and i'm pretty sure you are correct. I did a lost ABS 3d print and it turned out great even picking up the layer detail. I do not have a burnout oven so have done all such burnout in my wood burning fireplace in my house which can struggle to reach the appropriate heat on ocassions. Thanks for the pointer.
Bruce

author
XTL made it!(author)2015-11-01

A slightly improved mix is 50:50 casting plaster and silica sand(flour) at 100-200mesh. The silica flour imparts heat resistance while not interfering with the plaster's 'glue' like nature which holds it all together. You get a refractory that's good to about 850C. Subsequent shells can have sand mixed in. So first layer is 50:50 plaster/silica, then add 50% sand to the recipe. (Use sand or grog (calcined anything). Its better as it will expand less on heating.) Three layers total is fine. Let each layer dry a little (10 mins) before adding the next one. If the inner layer cracks the outer two layers will contain the spill. Embed the whole thing in sand as you describe before pouring.

author
Bruce_T_NZ made it!(author)2015-11-01

Wow, 50/50 didnt realise it was quite that high. Have tried POP without sand before and it failed miserably under the heat. I now have some superfine silica casting sand that I use sometimes and this seems to work well

author
Bruce_T_NZ made it!(author)2015-11-01

I have only done one lost 3d printing. Fully knowing the fume consequences I did a lost ABS, this allowed me to acetone vapour smooth it first for a better result.

I plan to try lost PLA but the 3d printer is on the fritz at the moment and still waiting for parts.

lost_abs.jpglost_abs_original.jpglost_abs_reveal.jpg
author
XTL made it!(author)2015-11-01

If 3D printing. use clear PLA. Less toxic when burning (no additives and not ABS). Keep to about 350C for cleanest burnout. To hot and mold may crack.

author
badideasrus made it!(author)2015-09-27

obviously this method works, but when making the sprue, effort should be made not to have a steep angle like the chin on the skull. when turned upside down to pour metal into, it'll act as a big air pocket. eitehr attach a small line from the chin to the surface to let air out, or angle the head to make it less likely to trap air.

author
Bruce_T_NZ made it!(author)2015-09-27

Thanks, I will keep it in mind. I was a bit constrained by the dimensions of the can or I would have placed it a bit differently.

author
Bruce_T_NZ made it!(author)2015-09-24

Thanks for the kind comments guys. To be honest the results are far from perfect but it does show great potential. With a bit of practice I'm sure I will be able to achieve near perfect results.

author
desertsniper made it!(author)2015-09-24

I have been wanting to try this! love the skull, thanks for the post.

author
BoboTheEpic made it!(author)2015-09-24

Nice casting work! Most lost wax/styrofoam casting I've seen has ended with less than desirable results, but you pulled it off very well.

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