I recently took an interest in conductive inks or pastes, in order to 3D print small electronic circuits. I was inspired by mickey77's instructable. The conductive pastes are made up of a conductive material, such as metal or graphite powder, mixed with a binder. I bought some graphite powder at a hardware store and 3 or 4 types of potential binder material to test. The problem was how to measure and mix small quantities of these materials with reasonable accuracy and without making a mess, and how to get a repeatable mix from batch to batch. The reference quantities, from mickey77's instructable, were 1.5cc of graphite to 1cc of binder (he uses contact adhesive).
So how do you measure and mix 1cc or 1.5cc of material? If I measure 1cc of gooey glue, even if I have a measure that will give me 1cc, I will have trouble transferring all of it into a mixing container. The solution I hit on was to 3D print a disposable item that combines a 1cc measure with a larger mixing volume, plus a separate 1.5cc scoop for the graphite.
Step 1: What You Will Need
- A tooth pick (really cheap). I prefer the round ones to the flat ones.
- A 3D printer (not so cheap)
- Whatever materials you are mixing
- A design. My stl files are provided below. If you design your own for different volumes, you'll need to do the math.
Step 2: The Binder Measure/mixer
My design for the mixing vessel consists of a hemispherical lower section with a volume of 1cc, plus a wider top section that gives room to add in the graphite powder and stir it all with a tooth pick.
Step 3: The Scoop
The scoop is a hemispherical bowl of 1.5cc and a handle
Step 4: Using It
- Fill the lower half of the mixer with binder. Gently squeeze it out of the tube until it reaches the rim of the 1cc section. It will probably dome up a bit, but this is not about scientific precision.
- Fill the scoop with graphite. My graphite came in a puffer bottle, sold as lock lubricant. I transferred it to an old jam jar so it can easily be scooped out. Tap the scoop on the inside of the jar, or scrape the excess material off the top with a box cutter, so you get a fairly accurate scoop full.
- Carefully tip the graphite on top of the binder. Be careful, and whatever you do do not sneeze while handling the powder. The stuff is virtually impossible to clean off surfaces!
- Using a tooth pick gently but thoroughly mix the two components. I find that twirling the tooth pick between my fingers is a good technique, as well as old fashioned stirring.
- Apply the product to whatever you are trying to make conductive.
Picture:Used mixer, good only for the garbage. The tooth pick is also unrecoverable.
Note this is about measuring and mixing, not about formulating conductive inks. That said, using graphite I get resistive material rather than very high conductivity. The binders that work are those that shrink as they dry/set, because (I surmise) they pull the carbon particles together so they can make contact with each other. So what works is contact cement, PVA glue and thick ABS slurry (scrap ABS dissolved in acetone). Araldite (two part expoxy) and silicone sealant do not work because they are deliberately formulated so they won't shrink.
Robert Murray-Smith has made a number of in-depth YouTube clips about conductive inks. Super Conductive Carbon Ink