busybee fluffy rambles Notes

Tools and materials

• drill mixer attachment
• hot glue gun
• plaster of paris ($2.20/lb at Amazon). only use this for molds which don’t need high reuse
• pottery plaster is more suitable for molds that need reuse. A typical mold needs 1-2kg (2-5 lb).
  • Amazon has it for $27.50/10 pounds ($2.75/lb) or $100/50 lbs ($2/lb)
  • Blick currently has it for $14.34/8 lb ($1.79/lb) or $22.68/25 lb ($.90/lb), after shipping. Obviously Blick is the better deal this time. This makes Blick’s pottery plaster actually significantly cheaper than plaster of paris, too. But it’s also persistently out of stock right now.

Object design

Making a two-part slipcast mold (using an injection-molded part as the template but the principle applies to your own items too):

Making one-part slipcast molds with a slip well:

Note to self: remember to design a foot into the object.

Mold making

A good technique for making a fully 3D-printed mold + outer form:

For my purposes I’m not going to build the “collar,” I’ll just caulk+hotglue everything down to a melamine board or metal tray or similar. In later videos he switches from a printed collar to an over-engineered flange that he attaches with silicone and threaded inserts and screws (although it does ensure proper centering, at least, but that could also be achieved by drawing some centering lines on the board).

Some OpenSCAD code for generating the outer form:

module plug() {
    // plug shape goes here; include the slip well
}

module mold() {
    // generates the interior of the outer mold with an appropriate thickness for slip casting
    minkowski() {
        plug();
        sphere(25);
    }
}

module volume() {
    // generates a mesh that you can provide to admesh to determine the plaster volume
    difference() {
        mold();
        plug();
    }
}

module form() {
    // generates the outer form
    intersection() {
        difference() {
            minkowski() {
                mold();
                sphere(3);
            }
            mold();

            cylinder(r=30,h=500); // adjust radius for pour spout
        }
        translate([-1000,-1000,y_offset]) cube(2000);
    }
}

When assembling the mold:

1. Center the inner plug onto the smooth surface
2. Attach it with silicone caulk and a bead of hot glue
3. Spray the surface with a misting of Windex or other surfactant (to prevent bubbles from clinging to the surface)
4. Center the outer shell and attach to the smooth surface (caulk + hot glue)
5. Give the silicone time to cure
6. Mix and pour the plaster

Slip casting plaster ratios; tl;dr: for 1 liter of mold plaster, mix .72kg of water with 1.04kg of plaster of paris

Inner shell can be demolded by pushing in compressed air between the plaster and the shell. Outer shell can be more easily demolded by cutting it off (can then tape it up if needed to make a new mold).

The mold needs to be at least 2.5cm thick, so when making a two-part mold it’s probably best to make the outer shell a cylinder (or similar) that has at least that much thickness at the thinnest point. Also remember to have enough room for your keys.

Slip making

How to slip cast ceramics gives some helpful information about making the slip for casting. Salient points:

• Slake the clay body into water and strain out all large clumps
• The density should be around 1.8g/mL
• This is about 75% clay (bone dry) to 25% water by mass

When actually slipcasting, it’s good to have a wide pan with a draining grid on it.

1. Pour the slip into the mold
2. Wait 15-30 minutes (depending on the thickness desired)
3. Pour the remaining slip back out
4. Place the mold upside-down and at an angle on the draining rack for the excess slip to drain off
5. Allow to dry for a while (a day or two probably?)
6. Use a metal kidney to remove the clay from the slip well
7. Palm the piece out of the mold and place onto a wareboard to finish drying