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

Picture of 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.

Step 1: CAD Design

Picture of 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!

Step 2: CNC Machining the Lower Part of the Mold

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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...

Comments

NeilB13 (author)2015-09-03

Very cool idea and I really like the look of it.

Just some tips with concrete with a project like this:

1. Use reinforcement, I would have made a PVC skeleton to help with tensile strength and to bring the weight down, and it could also be used to rewire the lamp if need be.

2. Using an self consolidation admixture in the concrete with work like this would produce a higher quality concrete (no bubbles)

3. A general rule of thumb is 7 days to cure ( the first day must be in covered with a wet towel to prevent cracking)

BG_instructs (author)2015-09-03

nice, great idea to use this type of styrofoam, idea's pop up in mymind....

Carlos AlbertoP (author)2015-09-02

Now, as you make the CNC machine? lol.
Good work

You can also make it with a manual router and templates - it is just a little more work than using a CNC machine.

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