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Materials Science in Additive Manufacturing Ceramic vat photopolymerization

it, stable solid structures form. The threshold is further n is the medium’s refractive index; and NA corresponds to
influenced by the TPA cross-section (δ) of the photoinitiator, the objective’s NA.
which quantifies the probability of simultaneous photon
absorption per unit time (Equation IV): 2.4. Continuous liquid interface production
dn p NF 2 (IV) Traditional SL methods employ a layer-by-layer approach,
resulting in slow printing speeds unsuitable for mass
dt production. In addition, oxygen inhibition during
where N denotes the molecular number density photopolymerization often leads to incomplete curing by
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(absorbing molecules per unit volume), while F = I/hν either deactivating photoinitiators or forming peroxides.
represents the incident photon flux, where h is Planck’s To address these limitations, Tumbleston et al. introduced
constant and ν is the optical frequency. Since: continuous liquid interface production (CLIP), an advanced
SL technique. In CLIP, printing occurs above an oxygen-
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dW dn p h (V) permeable window, forming a thin “dead zone” (tens of
dt dt micrometers thick) where oxygen suppresses curing. This
uncured layer prevents adhesion between the window and the
Hence, δ can be expressed as:
printed part, enabling continuous fabrication. As illustrated
3
16 h 2 in Figure 4, CLIP operates by projecting UV images through
Im[ ] (VI)
3 ()
cn N a transparent, oxygen-permeable window into a liquid resin.
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Unlike conventional SL, which requires intermittent resin
where δ has a unit of 10 m s/photon. 14 renewal and layer repositioning, CLIP continuously draws the
−58
4
Upon TPA, a portion of the excited initiator undergoes cured part upward while fresh resin flows into the dead zone.
intersystem crossing to the triplet state, followed by bond This eliminates stepwise interruptions, significantly boosting
cleavage to generate reactive radicals or ionic species, printing speeds – up to 500 mm/h or higher – without
thereby initiating TPP. The efficiency of radical formation, compromising resolution. Moreover, print speed remains
termed radical quantum yield (φ), combined with the TPA independent of layer thickness, further enhancing efficiency.
cross-section (δ), determines the initiator’s performance. Key advantages of CLIP include: (i) elimination of oxygen
Optimizing these parameters enables efficient TPP at inhibition via controlled dead zone formation; (ii) continuous
reduced laser power and higher writing speeds. The lifetime printing without pauses for resin replenishment; (iii) high-
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of radicals and polymerization kinetics are influenced speed production while maintaining precision; and (iv)
by multiple factors, such as radical quenching (e.g., by flexibility in layer thickness without affecting speed. This
oxygen or other radicals), intramolecular recombination, innovation overcomes critical drawbacks of conventional SL,
and chain transfer reactions with surrounding molecules. making it viable for scalable, high-throughput manufacturing.
These dynamics vary depending on the photoinitiator, The CLIP process critically depends on establishing an
monomer, solvent, and environmental conditions (e.g., oxygen-inhibited dead zone, achieved through an amorphous
temperature, atmosphere), ultimately affecting achievable fluoropolymer window that combines high oxygen
resolution and processing parameters. Although the permeability with UV transparency and chemical resistance.
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theoretical resolution limit of TPP is extremely small – The dead zone’s thickness is determined through differential
determined by the voxel dimensions at the polymerization measurement techniques and varies with both oxygen
threshold – practical feature sizes are constrained by laser availability and the window’s permeability characteristics. Key
stability, material composition, and the numerical aperture observations include: (i) When using ambient air instead of
(NA) of the focusing objective. For high-NA objectives pure oxygen, the dead zone thickness approximately doubles.
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(NA > 0.7), the optical resolution can be estimated using (ii) Increasing photon flux reduces the dead zone thickness.
the following equations: 21 (iii) Nitrogen environments completely eliminate the dead
.
0 325 zone, preventing CLIP operation. For systems operating with
r (VII)
xy
2 NA 091. ambient air, the dead zone thickness can be calculated as:
PI 05.
C
0
0 532 1 Dead zone thickness D (ix)
.
r (VIII) c0
z
2 n n NA
2
2
where φ is the intensity of incident photons, α is the
PI
0
where r and r denote the lateral and axial resolution product of photoinitiator concentration and the wavelength-
z
xy
limits, respectively; λ represents the excitation wavelength; dependent absorptivity, D is the resin reactivity, and C is a
c0
Volume 4 Issue 3 (2025) 5 doi: 10.36922/MSAM025200031