Casting a Concrete Lamp Sculpture With a CNC Milled Styrofoam Mold

In this Instructable I'll show you a quick and simple way to make a concrete sculpture with a CNC router and a lost styrofoam mold.

• The CAD Model is just a quick sketch with a few arcs and lines to create a "flame"...
• I've used a 3D CAD and extruded the sketch to get a better idea how the final part will look like - but this is not really necessary for something that simple...
• Its size is about 500 x 420mm and you can download the "Flame.dxf" file here...
• Be aware: we're creating a negative mold - so the final part will be mirrored! In this case it doesn't matter - but if you e.g. add some text you'll have to mirror the drawing or the final part will be wrong!

• The mold is made from two 600 x 600 x 50mm pieces of fine extruded styrofoam...
• Extruded polystyrene foam offers much better surface quality and more stability than expanded polystyrene foam (the one with visible beads). It is also extremely easy to machine. I used "Styrodur" from BASF, I think a similar material in the US is called "Roofmate"...
• The final sculpture will be 90mm thick - but unfortunately my longest end mill is too short for this so I decided to cut the two 50mm pieces separately...
• You can see the CAM process using Estlcam and machining of the material in the video at the beginning...

• After the lower part has been finished the second 50mm sheet of styrofoam is just glued on top of the first using some spray glue...
• Make sure the glue you use is suitable for polystyrene - otherwise the mold may be destroyed because many solvents dissolve styrofoam rapidly...
• I've decided to glue before machining instead of later because this way the parts will automatically be aligned perfectly - there is no risk of displacements or distortions.
• CAM and machining is again shown in the video at the beginning...

• The lamp socket is a small E14 - a choice I regret somewhat because larger bulbs require E27 and may have looked better...
• The socket is just squeezed into the styrofoam with a little epoxy glue:
  • Just press it against the foam where you like it to be...
  • Then use a sharp knife to cut along the dent it created...
  • And finally add a few drops of glue and squeeze it into its final position...

• Mix your concrete and make sure it is pourable but not too liquid...
• After pouring I've knocked a few times against the molds sides pretty hard. This way most air bubbles will rise to the top. I should have done this a bit more thoroughly as there are still some bubbles visible in the final part but fortunately not too large and not too many...
• Be aware that most concrete varieties really need a few days to harden even if they appear solid after a few hours. If you are too quick the sculpture may break during demolding. I've used cement screed and left it 3 days to harden...
• The back of the sculpture is quite rough - if you want it to look nicer you'll need to scrub it after a few hours with some water and a felt pad...

• You'll not be able to remove the sculpture without destroying the mold anyways - so just take a box cutter and make a few relief cuts to make demolding easier...
• Add a few felt pads to the bottom to avoid scratching your floor or furniture...
• Then finally screw in a bulb and you're finished...