Introduction: Casting a Concrete Lamp Sculpture With a CNC Milled Styrofoam Mold

By Estlcam.com

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.

Step 1: CAD Design

  • 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!

Attachments

Step 2: CNC Machining the Lower Part of the Mold

  • 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...

Step 3: CNC Machining the Upper Part of the Mold

  • 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...

Step 4: Inserting the Lamp Socket

  • 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...

Step 5: Mixing and Pouring the Concrete

  • 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...

Step 6: Final Steps

  • 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...
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