3D printers print layer on top of layer, slowly building a three-dimensional object. A plethora of materials and methods are used to build these layers.
The Stratasys Dimension is a commercial 3D printer that uses ABS plastic. The ABS filament comes in a self-loading cartridge, and is fed into a heater block by two drive wheels. In the heater block, the ABS is heated to a semiliquid state and extruded through the tip, with layers as thin as 0.01″.
The open-source RepRap (reprap.org) project uses similar technology. The filament is a polymer, 3mm in diameter. This is fed into a chamber, held tightly against a drive screw. As the drive screw turns, it pushes the filament down into a heated barrel. The heat comes from a strip of nichrome wire that is wrapped around the barrel. At the tip is a nozzle with a diameter typically between 0.25mm and 1mm (smaller is more precise, but takes more time). A thermistor is embedded in the nozzle, so it’s possible to adjust the temperature for different polymers.
One of the easiest polymers to work with is Polymorph, which is marketed as Friendly Plastic or ShapeLock in the United States. Polymorph extrudes easily, is relatively strong, and melts at just 140°F. Unfortunately, it’s also expensive and deforms if regularly exposed to temperatures over 100°F.
RepRap enthusiasts have also experimented with HDPE, ABS, and PLA (polylactic acid). PLA is the most interesting, as it’s biodegradable and you can make it yourself. PLA is formed by heating lactic acid with stannous chloride; and lactic acid can be created by fermenting milk or starch. The process is sufficiently complex that it might not be terribly practical in small quantities, but it opens up some interesting possibilities for local production.
Another material the RepRap project has been experimenting with is EcoComp UV-curable resin, from Sustainable Composites Ltd. The resin is mixed with glass filler to make a paste, and hardens when exposed to ultraviolet light. A ring of UV LEDs mounted around the nozzle hardens the paste as it’s extruded. EcoComp resin is made from plant oil, and like PLA, will biodegrade if composted, making the material carbon-neutral. EcoComp resin, however, is very stable underground. If buried, it would actually reduce the amount of carbon in the atmosphere.
The Fab@Home (fabathome.org) project uses a much simpler extrusion method than the RepRap: the mechanism is simply a motor-controlled syringe.
A set of 30 syringe barrels and pistons costs $36, so it’s easy to experiment with different materials. This can lend itself to a lot of fun — peanut butter, icing, toothpaste — if you can squirt it out of a syringe, it’ll extrude. Making something sturdy out of peanut butter might be difficult, but the fun is in the experimentation. Betty Crocker Easy Squeeze Decorating Icing works well and could be great for 3D decorations.
Silicone also works well in the Fab@Home. It cures within 24 hours to a somewhat rubbery material. It’s also possible to get conductive silicone, which enables circuits to be embedded. For making solid models, FabEpoxy, from Kraftmark, looks like the most promising material. It’s a two-part epoxy that is very durable and machinable, and can be painted when cured.
All of these materials can be extruded at room temperature, which significantly reduces their complexity. Ironically, the Fab@Home syringe tool is very expensive to build, because of the $130 linear motor that plunges the piston. That’s a lot of money if you want to have numerous interchangeable extrusion heads.
Fortunately, there are lots of ways to hack around this. The first method that came to my mind is to adapt an old cookie press. These sell for $5 or $10 on eBay, and all you have to do is drill and tap a few holes to mount a motor. I chose a $6 Solarbotics GM3. My cookie extruder is pretty large, but a similar method could be used with a captured nut and a screw, on the standard Fab@Home syringe.
This method of fabricating that I’ve been describing is called fused deposition modeling (FDM). A similar method, called precision droplet-based net-form manufacturing (PDM) is being developed for use with metals by Melissa Orme, Qingbin Liu, and Robert Smith at the University of California, Irvine.
Their work has been with aluminum, but the process is expected to be suitable for a wide variety of metals. The molten aluminum is held in a graphite-lined canister with a vibrating plunger rod at its center. Perturbations from the plunger rod cause uniform droplets of aluminum to break away and exit the canister through a small hole in the bottom. In other words, it squirts out beads of molten aluminum. These beads cool quickly as they exit, and can be used to build objects in much the same manner as the plastic. This method is still being refined, but there are other additive methods in commercial use, such as selective laser sintering (SLS).
In the SLS process, a layer of powder is laid upon the bed. A laser then traces the shape of the desired object over the powder, fusing it together. Additional layers are laid down and then fused, building the object. This process can be performed with numerous materials, including plastics and metals.
A very similar technique is used in Arcam’s EBM (electron beam melting) machines, where an electron beam operating in a vacuum fuses titanium or chrome powder. While lasers and electron beams pose a relatively high barrier to entry, the same concepts can be applied to a simple heat gun. The CandyFab Project (candyfab.org) uses a $10, 500W air heating element to fuse together models made of pure sugar, with excellent results (see MAKE, Volume 12, page 38).
All of these 3D printing techniques share an important characteristic: they require a 3-axis Cartesian system to position the tool, whether that tool is a syringe or a focusing lens. Build one platform, and you can experiment with everything from cookie dough to laser sintering.
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