Lasercut Menger Sponge

One of our members, Jaye, has been up to a rather special project in the last couple of weeks, a 3D Menger Sponge from lasercut acrylic. Here’s her writeup:

Hey there coruscating hivebrain,

As there was some interest when I mentioned it previously, I figured I’d write up my first attempt to build a Menger sponge with the laser cutter, acrylic sheet, and water-thin solvent based acrylic adhesive.

To test the assembly technique, I went with iteration M2 of the sponge in 3mm transparent red acrylic. Obviously, just cutting out all the little bits for the internals of the sponge and trying to assemble them one at a time in the right position would be fiddly and insane, so instead of doing that, I cut patterns out that could be used to assemble a whole layer of the sponge at a time.

The attached Plan image shows the cut file I used (and the cut file itself is attached as an Inkscape SVG). Black lines indicate cutting (laser settings: 40% power, 6mm/s speed), and red lines indicate engraving (laser settings: 40% power, 15mm/s speed). The idea was to include sprues in the cut design that hold the overall structure of each layer together, but to do deep engraving across the ends of every sprue so that:

  • The water-thin acrylic adhesive that tends to wick between layers by capillary action would be stopped at the edges of the actual sponge elements, but not spread into the sprues.

  • Once a layer was bonded and cured, the sprues should be easy to snap out.

This more or less worked. :)

Things that I learnt during the assembly:

  • Make sure that you let the acrylic bonding agent have plenty of time to cure before trying to snap out the sprues.

  • A chisel is handy for cutting across the engraving points, but you’ve got to be careful not to force the sponge pieces out of position.

  • A thin syringe and needle is the best tool for applying water-thin acrylic adhesive. Tilting the layers to catch the light lets you very easily see how much adhesive is going where.

  • Do tests to work out the right engrave settings for the laser cutter. If you set things too hot, the layers will disintegrate remarkably easily. Often during removal from the laser cutter. :)
Things that I’ll be changing for the next revision:
  • 3mm acrylic, which, if you want the sponge to be a cube, means you’ve got a “cell” size of 3x3x3mm ends up being pretty fiddly during assembly. Being tiny, the resulting sponge shows up assembly inaccuracies to an undesirable degree. Thicker acrylic will be easier to make look more accurate.

  • Transparent acrylic probably isn’t so good if you’re interested in seeing the structure of the sponge. It just gets all overlaid and messy, and the layered assembly structure and slight angle on the laser-cut edges doesn’t help.

  • 3mm acrylic and 2mm wide sprues, and engraving each sprue-end twice at a separation of 0.5mm means that the structure of some layers does warp a little when getting cut: there just isn’t enough volume for the heat to dissipate into without making the acrylic a bit melty. This means the structure of the sponge gets a little compromised. I figure thicker acrylic, with perhaps 3 or 4mm sprues should prevent this.

  • Engraving only twice at the end of each sprue, at a separation of 0.5mm meant that occasionally little fragments of the actual sponge would come away at the sprue end. Messy. I figure engraving across the sprue ends 6 times at a separation of 0.25mm will make the breaking away of sprues neater.

  • Using 3mm acrylic, the breaking force required to twist out the sprues that connect to the small 1x1x1 cell elements of the sponge that are in the corners was often higher that the solvent bonded joint between layers could handle, and the 1x1x1 element came away with the sprue. I figure that thicker acrylic and the subsequent larger bonding area for a 1x1x1 cell, combined with more and closer sprue engraving will fix this.
Build process photos attached.

We now return you to your regularly scheduled programming. :)