R&D: Frostruder MK2
One of our goals when we started MakerBot and designed the CupCake CNC was to automate one of the most tedious things of all time: frosting cupcakes. This incredibly difficult task has plagued mankind for centuries, but at long last we have found a reliable way to automate the process.
Our original design for a frosting extruder followed in the footsteps of Fab@Home’s paste extruder. They have a fairly elegant solution that uses linear actuators. Unfortunately, the linear actuators themselves cost more than our target price for the entire kit, so we attempted to come up with an emulated design that uses standard motors, lasercut gears, and some threaded rod to create a motorized plunger.
Well, this worked up until a point, but it was a pretty error prone and bulky solution. The gears were difficult to attach. The threaded rod based plunger required a design at least 2x the height of our desired syringe (60cc) and basically was heavy and didn’t work well. It was certainly an interesting design challenge for Bre, Will, and I that saw about 7-8 iterations and a failed appearance on TV, but ultimately it was fruitless. There’s a reason we never released the MK1 for sale.
Not only that, but the motorized piston based solution is an inherently flawed approach: The extrusion of a material is based primarily on the pressure, viscosity, and nozzle diameter. There’s not much you can do about viscosity and nozzle diameter, so we’re mucking with pressure. In the motorized piston approach, the pressure builds up gradually as you push the plunger down, and releases as the material either leaves the syringe or you back off the plunger. The result is that you either have extremely slow start/stop times or you have to deal with massive ooze problems.
Which brings us to the MK2. I was musing over the design failures one day when I had the idea that instead of trying to create the pressure in the syringe mechanically using a plunger, what if we directly applied pressure using air. I reasoned that we could use a commonly available air compressor and electrically controlled solenoid valves to push frosting out of a syringe tip. I had this idea for about 6 months, but it had to sit on the backburner for a while until I had a chance to work on it.
My first experiment was with some thick, chocolate frosting that you can find in nearly any grocery store. I simply wanted to see if it was possible to use air pressure to extrude frosting, so I wired up a solenoid to a switch and used that solenoid to turn the air pressure on and off to the syringe. I was using a 21GA (0.53mm) needle and a standard 60cc syringe. I hooked it up to the air pressure and opened the valve. Nothing happened right away, but I gradually turned the pressure up until about 50-60 PSI I started getting a frosting extrusion. I kept turning up the pressure to about 80 PSI where I got a really nice, very fast frosting extrusion that was about 0.5mm wide. Success!!!
Well, it wasn’t a total success: when I closed the valve, the syringe was still pressurized and continued to extrude. Obviously that is a problem, so I went back to the drawing board. I came up with the idea of adding a 2nd valve that would act as a relief valve and release the built-up pressure to the outside world. I grabbed a second valve and went back to the garage to experiment. The result was phenomenal! I was able to start and stop the extrusion at will, with zero oozing problems. This was excellent news. I soon had an Extruder controller wired up to the solenoids and a tester gcode script that would cycle the valves every 10 seconds. It was amazing to see a stream of frosting coming out and stopping every 10 seconds. I ran to get Bre and Adam to celebrate and we danced a frosting dance.
So: fast forward a few weeks and we’re gearing up for the Yahoo Hack Day in Times Square. We really didn’t have a solid game plan for what to do, but we knew we wanted to have fun and stay up all night hacking. I was really into the new frostruder design, so I brought it with us along with a portable air tank and a bike pump. We spent all night hacking on the frostruder and trying a variety of edible materials (frosting, peanut butter, and jelly). We ended up winning the Best Hardware Hack category with our New York Toast entry. It was a fun, fun hacking adventure.
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