PERSONAL FAB

BY TOM OWAD

Three-Dimensional Printing Methods

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

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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

Build one platform, and you can experiment with everything from cookie dough to laser sintering.

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