Lecture 7 Flashcards
what is photopolymerisation and what are the four key steps?
chemical process to solidify liquid resin into a solid object (light activated).
1) exposure:
resin consists of monomers, oligomers and photoinitiators. When exposed to light, the latter break apart generating free radicals.
2) initiation:
free radicals interact with monomers -> form a new reactive site at the end of molecule -> that will start the chain reaction
3) chain growth:
with each addition, the reactive site moves to the end of the chain -> this rapid growth transforms the liquid resin into a solid material
4) termination:
either combination, disproportionation (stable chain with no active site anymore), chain transfer
what is oxygen inhibition?
O2 can react with the free radicals, forming peroxy radicals -> these are far less effective in propagating polymerisation -> process slows down or stops completely
what are the steps in the core process of 3D printing with liquid resin?
1) layer by layer hardening:
light source (UV or visible) shines on liquid resin defining the geometry of the object. The resin solidifies in the desired pattern (80% polymerised so that the next layer can bond to it)
2) building the object: platform or light source moves to expose a new layer of liquid resin -> layers are stacked on top of each other
3) support structures: to hold complex geometries or overhangs (removed after printing)
4) post-curing: UV exposure to fully harden the object (improves strength and surface finish) + heat application to enhance toughness and thermal resistance
difference between top-down printing and bottom-up printing?
1) top-down: light source shines from above, build platform lowers for each layer -> large area and less force on the part, but high resin volume required and gravity can affect consistency
2) bottom-up (used more): light source shines from below, platoform lifts -> less resin used and thinner / high resolution layers, but strong adhesion required to counteract peeling forces
what are the 3 main resin printing technologies (VAT polymerisation) ?
SLA
digital light processing
Liquid crystal display
explai how SLA works, and + and -
STL model that is sliced, and a laser path is calculated needed to solidify each layer. The laser movement is controlled by galvanometer mirrors (rapidly). It is a layer by layer process.
+:
- smooth surface finish
- high resolution
- controlled heat dissipation
- consistent quality
- wide range of material compatibility
-:
- slower print speeds (infill areas)
- higher costs
- complex electromechanical system
- limited build volume
explain how DLP works and the + and -
digital projector that projects light onto an array of micromirros that can be moved to create a shape. An image will then be projected onto the resin.
+:
- speed (a whole layer at once)
- no light bleed
- low complexity
- wide range of material compatibility
- continuous layer manufacturing is possible
-:
- higher cost
- build size constraints
- heat generation
- pixalation
explain how LCP works and + and -
similar to DLP but uses a liquid crystal display instead of projector -> screen displays a layer as a static image which is exposed to UV to cure.
+:
- cheap
- no moving part
- very compact
- low maintenance
- high resolution
-:
- light bleed
- longer layer times
- exposure uniformity (light less intense on the side) + pixalation
- specialised resin needed
what is photopolymer jetting technology? (polyjet)
Small amounts of UV-curable material are sprayed onto a build platform (does not involve heat so parts don’t shrink) -> very precise.
Each nozzle contains a piezo valve -> material is dispensed as microdroplets. Piezo valves open exactly at the right moment according to a code. A roller smoothes the surfaces.
Parts in direct contact with the support material will exhibit a matte finish.
PolyJet can produce multicolored and multimaterial parts.
There is a water jet cleaning system.
what is the difference between thermoplastic and duroplasts (thermosets) ?
thermoplasts -> chain not chemically bonded to eachother, can be reshaped and remelted
duroplasts -> chains are chemically bonded during curing process (cannot be remelted)
why can we only use thermoplasts in 3D printing?
needed for layer by layer construction, layer that is currently printing needs to be partly melted to bond the new layer to it.
what happens when thermoplastic filaments or powders are heated?
thermal E increases kinetic E -> chains move more freely, the bonds are disrupted -> crystalline region starts to melt leading to a viscous state (chains can slide past eachother)
what is fused deposition modeling? What are the two types of extruders?
material used is a spool or filament.
1) direct drive: motors and gears are placed on the moving printhead -> more precision but slower
2) bowden tube: fixed to machine and connected to printhead with tube -> faster but flexible materials compress or get stuck
what are the different parts of the print head? (FDM)
- cold end with fan (prevents softening of material)
- heat break
- hot end with heater (to melt material)
- thermistor
- nozzle
what are the two things that the nozzle diameter impacts? (FDM)
- quality: smaller diameters produce finer details and smoother surfaces
- strength: larger diameters result in stronger prints (better layer bonding)
