• Covalent bonds form between the molecules in the liquid, forming a solid material (this reaction is called photocrosslinking)
  • Absorption of photons as they pass through the liquid limits crosslinking to a small layer (typically tens of μm thick) at a time
• Parts are more isotropic (versus FDM), have almost matte finish
• Two machine design approaches:
  1. Right: Light shines up through a window; part gradually drawn up out of a tray of resin
     • Requires more liquid
  2. Projection Printing (below): Light shines down onto liquid surface; platform moves down into the vat as the part builds up
     • Printed object drawn upwards out of tray of resin
     • Separation from tray: peeling or O2-inhibited dead layer

• A newer, potentially faster, approach for processes that rely on photocrosslinking is to expose each 2D layer in one single exposure step, rather than scanning a beam of light. The digital micromirror devices that are an integral component of modern video projectors can be used to pattern the photocrosslinking wavelength of light. Even with this layer-by-layer approach, there is still a need to
• Whether 3D printing will always involve this slicing approach is an open question. Since slicing often imparts anisotropic mechanical properties to printed components and limits their layer-to-layer strength, there may be a strong case for developing full 3D control over the material deposition process, so that, for example, the deposition head would move in the x, y and z directions simultaneously. Path planning to avoid collision between the machine and the component would be a significant challenge slice a 3D object into multiple 2D images.

• Gelation just occurs at the curing depth, .$D_c$ where the resin receives the curing dose, .$E_c$: $$E_c = E_0 e^{-D_c/D_p} \Longrightarrow D_c = D_p \ln \bigg( \frac{E_0}{E_c} \bigg)$$

• Cured material may not be very strong at curing depth, so layers typically overlap in .$z$ direction to ensure material is all well-gelled

• Thermal and light curing are done to finish these surfaces

• Inkjet-dispensed polymer inks are built up layer by layer and photocured (crosslinked). Layers down to ~16.$\mu$ thick.
• Elastic modulus and color can be varied spatially by mixing inks
• A wide range of mechanical properties (including elastomers) are now possible
  • Rigid glassy polymers to soft rubber-like performance possible
  • Note that these are “simulated” materials: the printed inks are photocured whereas the final production material may be, e.g., thermoplastic
• Typical machines can carry about five different material cartridges at a given time, and can tune material properties by depositing finely interspersed patterns of droplets of these materials.
• Far more expensive than FDM or basic SLA tools.
• Electrically conductive materials are beginning to emerge

• Available in Jacobs Hall:
  • Objet260 Connex3
  • Objet350 Connex3
  • Interchangeable ink cartridges
• Soft, rubbery, tacky:
  • Tango range
  • ~ few MPa Young’s Modulus
• Rigid:
  • Vero (white, black, yellow, magenta, cyan, clear available)
  • Digital ABS
  • ~ few GPaYoung’s Modulus
• Soluble support:
  • FullCure705
  • Remove by hand, water or NaOH solution

• Can be applied to thermoplastic polymers (esp. Nylon), metals, and even ceramics
• A thin layer of the powder is rolled across the printing bed, and then a laser is scanned across the layer, delivering enough heat that the powder particles melt only at their surfaces.
• Atoms or molecules at the contacting interfaces between particles diffuse faster in the elevated temperature and cause adjacent particles to connect to each other.
  • Powder particles are heated enough that surfaces melt– atoms/molecules at their surfaces interdiffuse, bonding the particles.
• Because the powder is not fully melted, parts usually retain some porosity
  • Can weaken the component compared to continuous bulk material
  • The pores can serve as stress concentrators and promote crack propagation across the component
  • The surface finish can be rough — the roughness is comparable to the metal particle size

Selective Laser Melting (SLM) #

• Also called Direct Metal Laser or Directed Energy Deposition
• Laser (infrared light) actually melts the powder
• Removes porosity by melting the particles
  • Thermal gradients can be a problem
• Computer generators now account for thees gradients and deformations
  • Changes pattern order and energy distribution
• There has been a move away from metal SLS towards complete melting of the metal powder that is being fused.
  • Greater control of grain structure and lower porosity than with selective laser sintering
  • Fully dense structures achievable
• Scanning laser beam completely melts the powder during processing
  • No binders being employed but rather the structure of the component being built up directly from metal.
• These processes may be layer-by-layer or, increasingly commonly, via a direct spray
• Isn’t feasibly monetarily right now

• Application: pressure sensor housing in General Electric jet engine compressor
  • Made using SLM
  • Co-Cr alloy
  • 19 of them in the GE LEAP engine
  • Replaces a casting process
  • Development time up to a year faster than using casting

• Example of remanufacturing with SLM
  • Used when material is very expensive and not recyclable/reusable

• Like SLM, produces fully dense parts
• Operates in vacuum to avoid metal oxidation and so that the electrons can follow a straight path without colliding with air molecules.
  • In contrast, SLM uses an inert gas atmosphere
  • Vacuum allows higher temperatures to be used (> 800 K)
  • Reduces oxidation and porosity due to adsorbed gases
• Materials more limited than SLM
  • Examples: Ti grade 2, Ti6Al4V, Inconel 718, CoCrMo
• Features down to a few micrometers are possible although printing times are slow compared with SLS or even SLM

One approach to printing objects from powders (which may include metal, polymer or ceramic powders) is to selectively dispense a binder (a sprayable ‘glue’, usually a polymer) on to layers of dry powder, holding the powder together in specific locations. After printing, the part is removed from the surrounding unbound loose powder. For polymeric parts, the binder can be colored and constitutes part of the final object. For metal or ceramic printing, the binder is subsequently be driven off (vaporized) with high heat after the whole part has been printed, and the powder is also sintered together by the heat (see below for explanation of sintering).

• Polymer powder production
  • Ball milling: grind below glass transition
• Metal powder production
  • Solid-state reduction: crush ore, pass through furnace
  • Atomization:rapidly freeze molten spray
  • Electrolysis: deposition in powder form (e.g. Cu)
  • Chemical, e.g. precipitation from solution
• Size distributions typically Gaussian
  • SLM: typical average ~ 30 microns
  • The more spherical, the more easily it flows
  • Some powders more porous