An environmentally friendly alternative to the usual Starbucks cardboard.
This is made primarily through 3D printing and moulding silicone rubber.

Credits: Made with and instructions written by Dr.Roberts.
RC Roberts - Additive Microfabrication Laboratory - The University of Hong Kong

Step 1: Things You'll Need

What you will need:

five 3D printed four parts to form the mold and one as the handle
(stl files of the parts can be found here: http://www.thingiverse.com/thing:1030210)
10 M4-20 bolts with washers and nuts. You can substitute 5/16" by 3/4 long bolts if you aren't around metric hardware.
Hand drill with M4 or 5/16" drill bitallen key (M3) and/or screw driver for your bolts along with a pair of pliers to hold the nut.
paint
scraper
A tube RTV silicone. This is used for fish tanks or to seal windows and can be found at hardware stores.|
2 part silicone rubber (Dow Corning Sylgard 184 used here as it was laying around.
Smooth-On offers more economical options that be tested when the order arrives)
Container and stir stick to mix the rubber
Sandpaper to smooth the mold.

Optional (fancy lab equipment):

Scale for measuring rubber compound
Vacuum chamber for degassing the rubber (not needed for other rubbers)
Oven able to hold 80C (176F) - PDMS will cure at room temperature, it just takes ~24 hours without the over.

Step 2: Sticking Pieces Together Pt 1

A. Print out the 5 components.

B. Sand the surfaces that bolt together on a flat surface to minimize gaps if your printer does not print flat.

C. Use the drill and drill bit to size the holes to your bolt size.

D. Bolt everything together without any silicone to test the fit of everything and get familiar. Then unbolt everything.

E. Apply a bead of RTV silicone rubber to the surface between the two outside mold shells and tighten it together.

Step 3: Sticking Piece Together Pt 2

F. Apply a bead of RTV silicone rubber to the bottom surface of the outside shells and bolt it to the bottom plate.

G. Wipe the inside of the mold to remove any rubber that has squeezed out.

H. Apply a bead of RTV silicone to the center mold piece, farther from the edge.

Step 4: Sticking the Handle on the Mould

I. Test fit the handle and then apply a bead of RTV silicone to the bottom and sides before sliding it in place.

J. Apply RTV silicone to any seams on the mold to make it water tight.

K. Wait 24 hours (or follow instructions on the package) for the RTV silicone to dry. Fill with water and look for leaks. If there are leaks, drain the water and apply additional RTV silicone and let it dry. Once the mold is water tight, proceed.

Step 5: Moulding

L. Follow the instructions of your two part rubber. Use water to estimate the volume needed and let the mold dry before adding rubber.

M. If using Sylgard 184, mix the rubber in a 10:1 weight ratio on the scale and then degas for 30-60 minutes in a bell jar. Pour the PDMS rubber into the mold and then carefully degas the mold if needed. Please the filled mold in an 80C oven to allow it to cure, or leave overnight.

Step 6: Removing the Final Product From the Mould

N. Once your soft rubber has cured, start by unbolting the bottom plate. Use the scraper to gently pry off the bottom.

O. Peel off the RTV silicone around the handle and unbolt the outside shell. Carefully remove both sides of the mold. P. Use an allen key or tweezers to gently separate the inner mold from the rubber. Q. Push out the center mold. R. Trim any excess rubber on your casting. S. Enjoy your favorite hot beverage with a reusable mug handle! Stop wasting the paper insulators and show off your making expertise.