You can improve some characteristics of 3-D printed FDM things by filling them with other stuff.
“Filling” with plastics usually means mixing the plastic with “filler”, originally just some random solid material that was cheaper than the plastic (thus the name), but often these days something that imparts other superior properties to the plastic: higher strength and lower gas permeability in the case of bentonite clay; higher strength and density, plus reflectiveness in the case of steel or aluminum (aluminum-filled nylon is what Shapeways calls “alumide”); color; conductivity, when graphite, silver, or gold is the filler; thermal conductivity, using e.g. iron oxide; surface texture, as when sawdust is used as a 30% filler in PLA filament for 3-D printing to produce “printable wood”; and so on.
That’s not what I’m talking about. I’m talking about using FDM to print a hollow shape, and then filling the hollow spaces with some other material, in order to provide different properties to the resulting object. In some cases, you might want to actually remove the FDM plastic entirely afterwards, sort of like lost-wax casting, but without the intermediate step of making the negative mold around your wax positive.
What kinds of properties might you want to get?
Mass is a big one. FDM is capable of producing impressively light and thin objects, but it’s a very slow process for massive objects. And for many applications, “light and thin” equals “cheap-feeling and fragile”. So simply filling a cavity with a cheap, heavy material can make a big difference.
Here are some materials you could reasonably use for adding mass:
| | g/cc | max temp | |
| paraffin wax | .9 | 46°–68° | <https://en.wikipedia.org/wiki/Paraffin_wax> |
| water | 1 | 0° | |
| sugar syrup | 1–1.6 | 0°–186° | <https://en.wikipedia.org/wiki/Sugar> |
| silicone caulk | 1.04 | >260° | <http://catalogue.airtech.lu/product.php?product_id=30&lang=EN> <https://en.wikipedia.org/wiki/Silicone_rubber> |
| ABS | 1.07 | 210° | <https://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrene> |
| epoxy resin | 1.1 | 78°–162° | <http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_004e/0901b8038004e7b6.pdf?filepath=epoxy/pdfs/noreg/296-00312.pdf&fromPage=GetDoc> |
| PLA | 1.3 | 60°-140° | <https://en.wikipedia.org/wiki/Polylactic_acid> |
| salt | 2.2 | 801° | <https://en.wikipedia.org/wiki/Salt#Chemistry> |
| portland concrete | 2.2 | 573° (quartz) | |
| gypsum (plaster) | 2.3 | | <https://en.wikipedia.org/wiki/Gypsum> |
| quartz | 2.6 | 573° (phase transition) | <http://www.mindat.org/min-3337.html> |
| whitewash/chalk/calcite | 2.7 | 600° | <https://en.wikipedia.org/wiki/Calcium_carbonate> |
| aluminum | 2.7 | 660° | <https://en.wikipedia.org/wiki/Aluminum#Physical> <https://en.wikipedia.org/wiki/Solder> |
| magnetite | 5.2 | | <https://en.wikipedia.org/wiki/Magnetite> |
| steel | 8 | | <https://en.wikipedia.org/wiki/Steel#Material_properties> |
| 37% tin, 63% lead solder | 9.8 | 183° | <https://en.wikipedia.org/wiki/Solder> <http://www.metallurgy.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf> |
| lead | 11.3 | 327° | <https://en.wikipedia.org/wiki/Solder> <http://www.metallurgy.nist.gov/solder/NIST_LeadfreeSolder_v4.pdf> |
| glass powder | 1.7 | 850° (Ts) | <http://www.expertsystemsolutions.com/biblio/HSM/sintering%20behavior%20of%20glassceramic%20frits.pdf> |
The cheapest filler material is water, but it also has the lowest density of any solid here. All the others listed here are currently pretty easy to buy in kilogram quantities, a criterion which excludes otherwise interesting materials like cyanoacrylate, gallium, Wood’s metal, gelatin, and albumen.
Salt, gypsum, concrete, and whitewash have the property that you can convert a powder into a solid mass at room temperature by adding water and, in the case of whitewash, carbon dioxide. Paraffin is similar in this, in that you can divide it into grains and then sinter them, but it also melts at a low enough temperature that you may be able to pour it into an FDM-printed mold, possibly carrying other filler materials with it. (Sugar can also be extruded directly, as Jordan Miller has done: http://www.thingiverse.com/thing:26343/#instructions.)
Epoxy resins and silicone caulks, similarly, will harden at more or less room temperature, starting in a liquid state.
Silicone is flexible and high-temperature tolerant.
You should be able to melt PLA or ABS off of silicone once it’s hardened; also of concrete, salt, plaster, chalk, or metals. You can also dissolve ABS with acetone or PLA with a mixture of alcohol (ideally propanol, although I’ve heard ethanol or even water works too) and sodium hydroxide. Weld-On #5 solvent also works to dissolve PLA, but it's nasty, nasty shit, made of methylene chloride, glacial acetic acid, and methyl methacrylate monomer. Miller-Stephenson MS-111 stripping agent can also dissolve PLA, and also epoxy (!!) https://groups.google.com/forum/#!topic/ultimaker/8s1bq_9LsRM. http://www.vinland.com/blog/?p=68. MS-111 is 50% methylene chloride, 20% phenol, 15% formic acid and will blister your skin.
Dichloroethane can supposedly also dissolve PLA.
Acetone, MEK, tetrahydrofuran, and TCE will plasticize PLA but not dissolve it. I think all three of them will dissolve ABS, and MEK and TCE may be more available here in Argentina than acetone.
Ethyl acetate can vapor-polish PLA, which suggests it can dissolve it: http://www.printedsolid.com/smoothpla/
My attempts to polish PLA with ethyl acetate, or dissolve it in a test tube, were unsuccessful.