FAB14 Workshop: Digital Mechanical Material by Design, a physicist approach

Auxetic Mechanical Metamaterials

Materials
  • Acrylic sheets (thicknesses: 5 mm and 8 mm)
  • >50 dowel pins (6, 8 or 10 mm of diameter)
  • Scale
  • Silicone based polymer (fast curing)
  • Mixing cups
  • Mixing sticks
Machine
  • Laser cutter

Designing the mold

In this workshop unit, you will design and fabricate by molding and casting 2D auxetic mechanical metamaterials.

Using openSCAD, a free software, the first step is to design the mold. The mold is made of three laser cut acrylic plates (upper, middle, lower) and dowel pins (see photo below).

Here is the openscad code for parametrically designing the upper and lower plates:


              /*
              // FILE    : Auxetics Metamaterials with holes - Upper and lower plates
              // VERSION : 0.1 (draft)
              // LICENSE : M.I.T. (https://opensource.org/licenses/mit-license.php)
              // AUTHOR  : Denis Terwagne
              // TARGET  : OpenSCAD
              */

              kerf=0;

              //dowel pins diameter
              diam=8;
              //space in between holes
              space=diam/5;
              //number of holes horizontal and vertical
              nb_horiz=7;
              nb_vert=7;

              base=4;
              out_case=10;

            //Upper and lower plates

            difference(){
            square(size=[(nb_horiz+1)*diam+(nb_horiz+1)*space+out_case,nb_vert*diam+(nb_vert-1)*space+2*base+out_case],center=true);
                circle($fn = 200,d=(diam-kerf));

                for (i=[0:1:(nb_vert-1)]){
                    translate([0,(diam+space)*(i-(nb_vert-1)/2),0])
                    for (i=[0:1:(nb_horiz-1)]){
                        translate([(diam+space)*(i-(nb_horiz-1)/2),0,0])circle($fn = 200,d=(diam-kerf));
                        translate([-(diam+space)*(i-(nb_horiz-1)/2),0,0])circle($fn = 200,d=(diam-kerf));
                    }
                }

            }
            

Here is a SVG image example:

Here is the openscad code for the middle plate:


  /*
  // FILE    : Auxetics Metamaterials with holes - Middle plate
  // VERSION : 0.1 (draft)
  // LICENSE : M.I.T. (https://opensource.org/licenses/mit-license.php)
  // AUTHOR  : Denis Terwagne
  // TARGET  : OpenSCAD
  */

kerf=0;

//dowel pins diameter
diam=8;
//space in between holes
space=diam/5;
//number of holes horizontal and vertical
nb_horiz=7;
nb_vert=7;

base=4;
out_case=10;

//Middle plate
union(){
difference(){
    square(size=[(nb_horiz+1)*diam+(nb_horiz+1)*space+out_case,nb_vert*diam+(nb_vert-1)*space+2*base+out_case],center=true);
    square(size=[(nb_horiz+1)*diam+(nb_horiz+1)*space,nb_vert*diam+(nb_vert-1)*space+2*base],center=true);
    }

   for (i=[0:1:(nb_vert-1)]){
    translate([-(diam+space)*(nb_horiz+1)/2,(diam+space)*(i-(nb_vert-1)/2),0])circle($fn = 200,d=(diam+kerf));
    translate([(diam+space)*(nb_horiz+1)/2,(diam+space)*(i-(nb_vert-1)/2),0])circle($fn = 200,d=(diam+kerf));
   }
}

Here is a SVG image example:

Fabricating the mold

To make the mold, laser cut the 3 acrylic plates and prepare a number of dowel pins.

Disassembled mold
Assembled mold. The third plate with holes will be used later.

Molding the material

Now, it's time to cast our structure. To do this, follow these steps :

Use fast curing silicone rubber.
Pour an equal amount of BASE and CATALYST using a scale.
Stir and mix the 2 parts thoroughly.
Pour the liquid solution in the mold. It might be curing fast so do not loose time... Also if you can vacuum the solution using a vacuum pump and chamber prior pouring that will allow to remove air bubbles. If not, while pouring, make a very thin liquid column by levering up the container with the solution. This will also remove air bubbles.
Cover the mold with the 3rd acrylic plate. Be careful to avoid air bublles while covering.
Skeeze and hold the mold in place.
When it is completely cured. Remove the pins.
Demold gently the structure.
Clean your structure by removing excess material. Now, your structure is ready to be tested.

Testing the strutures

Hold the sample in between your two hands.
Compress the sample and observe how it shrinks. This is unusual and it is called an auxetic material. Usually when a material is compressed it expands laterally. Auxetic material behave the opposite.