3D Printer Technology Basics

Picking a 3D Printer Technology

There are a number of good surveys of 3D printer technology, including commercial choices that are fridge-sized and cost from $15K-$1M, and hobby kits that cost $200.

Stereolithography (SLA) uses an optically sensitive material, typically a vat of UV-curable photopolymer, to selectively cure 2D slices into a 3D object.  Speed and accuracy can be excellent depending on the optics used, in particular with newer digital light processing (DLP) projector chips, but the cost of the photopolymer is fairly high (at least $65/liter or Kg), and most photopolymers result in objects that are not very impact-resistant.  SLA was one of the first 3D printing technologies, and still dominates for small-size high-precision work. The standard file format "STL" (STereoLithography) derives its name from this technique.

PolyJet printers use a similar photopolymer to SLA, but use inkjet-type technology to spray the photopolymer onto the object before curing.  This allows them to use a wide variety of materials, including rubbery materials, and to grade materials into one another.

Direct Metal Laser Sintering (DMLS) or the higher temperature Selective Laser Melting (SLM) uses a laser to fuse a flat bed of powder or a gas-carried stream of powder into a solid, depositing material layer by layer.  With an inert atmosphere and high temperature chamber, this can be made to work with metals, including steel and aerospace metals like titanium or inconel.  This also provides automatic support structure, since the powder is deposited at the same time as the part.  Electron Beam Melting (EBM) uses an electron beam for the same purpose, but requires a vacuum chamber. 

Fluid Deposition Modeling (FDM) (or the generic term Fused Filament Fabrication / FFF) deposits a semi-solid material onto a build platform to assemble the shape, and is the dominant technology for home 3D printers. Unlike SLA, it is inherently a 1D fabrication process, since the material emerges in a single line.  Most models work by feeding a filament into a hotend, but this is only suitable for materials that have a viscous intermediate state--this includes most plastics, chocolate, and even glass when using a sufficiently hot hotend, but the low viscosity of most molten metals makes them unsuitable for FDM/FFF directly.  However, FFF can make a mostly-metal preform for later sintering, as in the Metal X or Desktop Metal.  Another option is to extrude self-curing materials like epoxy or cement.  Advantages of FFF include inexpensive filament feedstock (as low as $20/Kg), and simple heating elements can be home fabricated, unlike lasers or optics.

Picking a 3D Printer Filament

The physical format of the plastic feedstock can be:

The chemical composition of the filament can be:

	Composition	Why use it?	Smell	Density	Tensile Strength	Elastic Modulus	Glass Temp Tg	Printing Range
PLA	Polylactic acid, plus plasticizers	Biodegradable	pancakes	1.25 g/cc	55 MPa	3.5 GPa	57C (very low!)	160-200C
ABS	Butadiene rubber plus acrylonitrile and styrene	Impact-resistant	headache	1.05 g/cc	40 MPa	1.4-3.1 GPa	80-125C	200-240C
HDPE	High-density polyethylene	Cleaning your hotend, chemical resistance	candlewax	0.95 g/cc	15 MPa	0.8 GPa (soft!)	-125C	220-230C?
Nylon	Polyamide 66	Strong & flexible	??	1.14 g/cc	70 MPa	2-4 GPa	50C	240+C
Lexan	Polycarbonate	Strong & clear	??	1.2 g/cc	50 MPa	2.6 GPa	150C	250-305C
PET	Polyethyelene terephthalate	Low shrinkage	slight chemical	1.38 g/cc	55 MPa	2.7 GPa	70-80C	240C
TPU	Thermoplastic Urethane	Flexible parts	None	1.2 g/cc	26 MPa	0.012 GPa	-35C	220C
Tin	w/ alloying copper, silver	Low melting point	toaster	7 g/cc	10-40 MPa	50 GPa	n/a	>230C
Aluminum	w/ alloying zinc, silicon	Low density	toaster	2.7 g/cc	110 MPa	69 GPa	n/a	>660C
Steel	w/ alloying carbon, chromium, nickel	High strength	fire	7.9 g/cc	400 MPa	200 GPa	n/a	>1370C
Glass	Silicon dioxide with boron or sodium flux	Transparent	none	2.5 g/cc	33 MPa	50-90 GPa	300C	1000C

Several filaments are designed primarily as dissolvable support material, such as high impact polystyrene (HIPS), which dissolves in limonene; or polyvinyl acetate (PVA), which dissolves in hot water.  Their properties are compared here.

Many filaments are actually alloys of different substances--in particular, ABS, HDPE, and polycarbonate will intermix well.  And there are a variety of "improved" filament chemistry options.

Filament makers can also add small particles of various filler materials to the plastic matrix:

• Iron particles result in heavy magnetic filament, for printing magnetically active parts like motor windings or inductor cores
• Copper or brass particles result in a heavy and metallic-looking print
  
• Wood particles result in a lightweight stainable print
• Graphite or graphene additions can result in slightly conductive filament, typically in the Kohm/cm, which is enough conductivity to fabricate some buttons and sensors, but not enough conductivity for most wiring (yet!).  Conductive filament can also be used for electroforming metals onto the surface of the print.
  

Fibers

• Mechanical Properties of Components Fabricated with Open-Source 3D Printers, 2014
  
• Hotend Design Theory
  

A key technology for designing toolpaths is calculating material flow rates:

• Why slicers get the dimensions wrong, by nophead (Chris Palmer)
• Extrusion Multiplier: Why your 3D Printer won't Print, by me