a desired 3D part. Typically, stereolithography produces parts in discrete layers. Exposed areas are cured through the full layer height, whereupon the part is repositioned and recoated with resin before the next layer is exposed. Recently, continuous stereolithographic technologies have been developed which increase print speeds by eliminating the time-consuming repositioning and recoating steps. [19,20] Print speed in continuous stereolithography is dependent on the resin absorbance height, with low-absorbance resins allowing extremely high print speeds of up to 2000 mm h −1 at the cost of part fidelity. [20] In stereolithography, the penetration depth of light in the resin limits accuracy along the vertical axis: unaccounted-for light propagation can cause undesired curing, known as cure-through, overcure, [21,22] the back-side effect, [23] or printthrough error. [19,24] This phenomenon can also contribute to cross-linking heterogeneity, introducing internal stresses which can deform the part and further reduce fidelity. [25] The prevalent strategy to mitigate cure-through is to add nonreactive light absorbers to the resin formulation. [21,[25][26][27][28] Highly absorbing resins have been widely adopted despite the slower print speeds needed to ensure fully cured layers. Alternatively, cure-through can be mitigated without sacrificing speed by modifying the projected images, known as slices, based on modeling of the curing process. Optimization-based methods to eliminate cure-through by adjusting model dimensions have been developed for external surfaces and internal voids in traditional stereolithography. [22,24,29] Manual adjustments to account for cure-through have also been reported. [30] Nevertheless, slice correction has not been described for continuous stereolithography, where cure-through is a more significant and complex problem. Furthermore, existing models of continuous stereolithography are not tailored to this application. [31][32][33] Here, we present a curing model and a slice correction algorithm for continuous stereolithography. Previous noncontinuous approaches used iterative and heuristic processes to find optimal corrections and were restricted to black and white pixels; our correction method uses grayscale, which has previously only been used to improve lateral resolution, [34] along with an exact mathematical solution to precisely set the dose profile within a part. We also present experimental validation of our model and correction approach using a recently developed two-color continuous stereolithographic 3D printer. [20] These methods are Continuous stereolithography offers significant speed improvements over traditional layer-by-layer approaches but is more susceptible to cure-through, undesired curing along the axis of exposure. Typically, cure-through is mitigated at the cost of print speed by reducing penetration depth in the photopolymer resin via the addition of nonreactive light absorbers. Here, a mathematical approach is presented to model the dose profile in a part produced ...
Microfluidic devices are typically fabricated in an expensive, multistep process (e.g., photolithography, etching, and bonding). Additive manufacturing (AM) has emerged as a revolutionary technology for simple and inexpensive fabrication of monolithic structures—enabling microfluidic designs that are challenging, if not impossible, to make with existing fabrication techniques. Here, we introduce volumetric stereolithography (vSLA), an AM method in which polymerization is constrained to specific heights within a resin vat, allowing layer-by-layer fabrication without a moving platform. vSLA uses an existing dual-wavelength chemistry that polymerizes under blue light (λ = 458 nm) and inhibits polymerization under UV light (λ = 365 nm). We apply vSLA to fabricate microfluidic channels with different spatial and vertical geometries in less than 10 min. Channel heights ranged from 400 μm to 1 mm and could be controlled with an optical dose, which is a function of blue and UV light intensities and exposure time. Oxygen in the resin was found to significantly increase the amount of dose required for curing (i.e., polymerization to a gelled state), and we recommend that an inert vSLA system is used for rapid and reproducible microfluidic fabrication. Furthermore, we recommend polymerizing far beyond the gel point to form more rigid structures that are less susceptible to damage during post-processing, which can be done by simultaneously increasing the blue and UV light absorbance of the resin with light intensities. We believe that vSLA can simplify the fabrication of complex multilevel microfluidic devices, extending microfluidic innovation and availability to a broader community.
The processing and properties of a positive-tone, aqueous develop, epoxy crosslinked permanent dielectric based on a polynorbornene (PNB) backbone and bis(diazonaphthoquinone) (DNQ) photosensitive compound were investigated. The developing and cure properties of the films were studied as a function of cure temperature, epoxy crosslinker loading and DNQ loading. Reduced modulus measurements showed that crosslinking of the polymer film occurred via reaction of the polymer with DNQ. The final modulus of the DNQ-crosslinked film was 4.0 GPa. Swelling measurements for a UV exposed film showed material leaching from the film. Residual solvent from swelling measurements was analysed by gel permeation chromatography which showed the indene carboxylic acid form of DNQ leached out of the polymer film. The unexposed film did not exhibit material loss through leaching. When developed, films showed a decline in modulus to 2.6 GPa, likely due to the reaction of DNQ with the aqueous base developer forming nonreactive byproducts that did not contribute to crosslinking. An epoxy crosslinker was added to the formulation which helped crosslink the polymer film by inhibiting uptake of the aqueous base during developing. The epoxy inhibition of the base uptake was confirmed by quartz crystal microbalance, where an increase in epoxy loading led to a decrease in base uptake of the film during developing. Microelectronics packaging faces a continuing challenge to accommodate scaling of electronic components to smaller size and higher performance. A higher density of electronic components requires superior dielectrics, such as in the form of photo-definable dielectrics to insulate the components electrically and mechanically support them. 1 Polynorbornene (PNB) has shown promise for use as a dielectric because of its low dielectric constant and good mechanical properties. 2The photo-definability can be achieved with a negative tone or positive tone chemistry. Negative tone materials become less soluble in a developer when exposed to UV radiation, whereas positive tone materials become more soluble in a developer when exposed to UV irradiation. Negative tone PNB dielectrics have been well studied. [3][4][5][6][7] A positive tone chemistry is desirable for packaging applications, because the film is less sensitive to mask defects or particulates during exposure.Positive tone photo-definability has previously been demonstrated with a bis(diazonaphthoquinone) (DNQ) added to a polynorbornene polymer. 8 The DNQ additive in the PNB film inhibits dissolution by formation of a hydrogen bonded complex. Ultraviolet exposure causes the DNQ to undergo the Wolff rearrangement to form an indene carboxylic acid. Unlike DNQ, the indene carboxylic acid does not extensively hydrogen bond to the PNB, leading to the solubility switch of the PNB in aqueous base.9 DNQ-based photochemistry is compatible with an aqueous developer which is more environmentally friendly than solvent-based developers.A permanent dielectric can be achieved with an epoxy-based cross...
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