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1. Effect of Fiber Bed Architecture on Single Resin Droplet Spread for Prepreg Manufacturing

   Martinez, P.; Jin, B.; Nutt, S. (July 2020, SAMPE journal)

   Previous studies have shown how discontinuous resin formats can increase the robustness of Vacuum Bag Only (VBO) prepregs. Current formats of this discontinuous resin format, dubbed USCPreg, all rely on a discontinuous film being applied on a fiber bed using only pressure. However, efforts are currently being undertaken to apply the discontinuous resin to the fiber bed directly, without a separate filming step. These methods should allow broader and more diverse characteristics of the prepreg, and allow a reduction in bulk factor, customization of the resin distribution, and potentially enable the production of prepreg “on demand.” To understand how applying discontinuous resin to a dry fiber bed at temperatures suitable for resin deposition may affect the final distribution, small-scale experiments were conducted. A fluid with controlled viscosity, matching the viscosity of epoxy resin during hotmelt processing, was used to minimize variability. The experiments consisted of a sessile droplet of facsimile fluid being deposited on the surface of a single ply of reinforcement. The spread of the fluid was then recorded, using a goniometer as well as a standard camera. Post-processing of these recordings was performed to obtain the spreading of the fluid in three directions: in the plane directions and the out-of-plane direction. The fluid was constant, a 30Pa.s rheological standard, but the reinforcement was varied to determine how the fluid interacted with different reinforcements. Macro-scale changes, such as fabric weave and fabric areal weight, and micro-scale parameters, such as tow width and fiber size, were varied to observe their effects on fluid distribution. The experiments yielded maximum in-plane spread distance, time for the resin to fully impregnate into the fibers, and aspect ratio of spreading, particularly for non-symmetric weaves. The results can be used to guide how the resin is deposited on different reinforcements, in order to achieve a resin distribution that will consistently yield high-quality parts. In addition, it is possible these observations can be applied to resin flow in standard continuous film prepreg, such as predicting the final degree of impregnation. 

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2. Nano Z-threading Strategy Enhanced Multifunctional Carbon Fiber Composites

   Hsiao, Kuang-Ting (November 2021, Prof. Joseph Koo WebSymposium on Polymer Nanocomposites, Advanced Materials WebCongress 2021, 16-18 November 2021.)

   Carbon fiber reinforced polymer (CFRP) composites have been increasingly used to replace metal parts in many industries such as aerospace, marine, automotive, and sporting goods. The CFRP parts compared with their metallic counter parts have the advantages of lightweight, significantly higher tensile strength, stiffer, and corrosion resistant. On the other hand, compared with many metal parts, the CFRP parts have many well-known disadvantages including the lower toughness, lower through-thickness tensile and shear strengths, lower thermal conductivity, lower electrical conductivity, and lower operating temperature. These disadvantages have made the conversion from metal parts into CFRP parts challenging and costly to design, manufacture, and maintain. The use of nanoparticles in polymer has been studied in the recent two decades. Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been dispersed in various thermoset and thermoplastic polymers and improved the mechanical, electrical, and thermal properties; however, the properties were not comparable to CFRP. Later, researchers tried to infuse CNTs or CNFs into either carbon fiber preforms [1] or glass fiber preforms [2] for improving the mechanical properties. But the results were marginal and with great uncertainty due to the challenges of nanoparticle dispersion, filtering, and alignment while being infused through the fiber preform. The glass fiber preform experiments ended with relatively more consistent improvement than the carbon fiber preform experiments since that the glass fiber preform has significantly larger pores than the carbon fiber preform' s. The small pore size presented a great challenge for infusing millions of unaligned long CNTs or CNFs through the carbon fiber preform without being filtered. To infuse long CNFs or CNTs through the carbon fiber preform and achieve reliable improvements, especially for 55% or higher carbon fiber volume fraction with increasingly tighter pores, an innovative plan for the processing and nano-reinforcing strategy is necessary. The z-threading strategy [3, 4, 5] has been reported to have consistent experimental successes in achieving the statistically meaningful improvement in multifunctional properties. The manufacturing steps of the CNF z-threaded CFRP (ZT-CFRP) are: (1) disperse the CNFs in a resin, (2) use a strong electrical field to align the CNFs in either the B-stage epoxy film or a CNF/resin impregnated sponge layer, whereas the CNFs are aligned in the through-thickness direction of the film or sponge layer. (3) place the resin film or sponge layer on a preheated dry carbon fiber fabric and press the resin film into the hot carbon fabric and allow the resin flow to carry the well-aligned CNFs to thread through the pores in the carbon fabric. (4) cool down the resin saturated and CNF z-threaded carbon fiber fabric to obtain the ZT-CFRP prepreg. (5) use the ZT-CFRP prepreg to make the ZT-CFRP laminate. Compared with the traditional CFRP, the ZT-CFRP laminates were reported of having improvement in the Mode-I delamination toughness, interlaminar shear strength, longitudinal compressive strength, through-thickness electrical conductivity, through-thickness thermal conductivity, and can reach the carbon fiber volume fraction of 55-80%. It is an effective approach to achieve a multifunctional CFRP for potentially expanding CFRP's applications. 

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3. DESIGN TOOL FOR DISCONTINUOUS RESIN PATTERNS IN VACUUM BAG-ONLY PREPREGS

   Schechter, Sarah G.; Centea, T.; Nutt, S. (September 2019, Proceedings of the Composites and Advanced Materials Expo (CAMX))

   Discontinuous resin distributions facilitate transverse air removal in vacuum bag-only prepregs during out-of-autoclave processing, and enable robust manufacturing. Methods to create discontinuous resin distributions with various pattern types and feature sizes have been demonstrated in recent reports. However, this new capability has expanded the design space for prepreg manufacturing, and optimum pattern characteristics have not been identified. In this work, a geometric model was developed to simulate prepregs and laminates with discontinuous resin distributions of various pattern type, feature size, stacking orientation, and ply count. Key metrics were employed to explore the capacity for air evacuation at room temperature. In particular, the projected surface area exposed was calculated to examine the fraction of uninhibited transverse air evacuation pathways. Secondly, sealed interfaces corresponding to the percentage of closed interlaminar regions within laminates were estimated. Finally, the tortuosity (the ratio of actual average gas transport path to straight-line path) of the dry pore network was calculated. A full factorial design, analyzed by n-way ANOVA and multi-comparison tests, was conducted to reveal the aspects of prepreg designs having the greatest influence on these metrics. Finally, these insights were used to fabricate prototype prepregs and experimentally measure their transverse permeability. Results revealed that a large number of sealed interfaces and high tortuosity were associated with lower permeability, indicating that these metrics can be used to screen resin patterns using the developed model. Broadly, the results validated a methodology to differentiate between discontinuous resin patterns with regards to air evacuation of the prepreg at room temperature, and therefore reduce the design space. Ultimately, this work can be used to guide prepreg design and to support manufacturing of high-quality composites by out-of- autoclave methods. 

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4. Effects of sample shape and adhesive type on rheology of unidirectional carbon fiber prepregs

   Das, A; Chan, KJ; Dillard, DA; De_Focatiis, DSA; Bortner, MJ (June 2022, Annual Technical Conference - ANTEC, Conference Proceedings)

   The viscoelastic properties of carbon fiber reinforced thermoset composites are of utmost importance during processing such materials using composite forming. The quality of the manufactured parts is largely dependent on intelligent process parameter selection based on the viscoelastic and flow properties of the polymer resin. Viscoelastic properties such as the complex viscosity (η*), storage modulus (G'), loss modulus (G''), and loss tangent (tanδ) are used to determine the critical transition events (such as gelation) during curing. An understanding of the changes in viscoelastic properties as a function of processing temperature and degree of cure provides insight to establish a suitable processing range for compression forming of prepreg systems. However, tracking viscoelastic properties as a function of cure during the forming process is a challenging task. In this current work, we have investigated the effect of sample size and adhesive type on the rheological properties of a commercially available carbon fiber prepreg material. Specifically, determining the linear viscoelastic region (LVE) as a function of sample configuration and different adhesive chemistries were explored. The results suggest that the square-shaped sample geometries coupled with cyanoacrylate based adhesive are optimum for conducting rheological characterization on the carbon fiber prepreg system. 

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5. Powder bed non-uniformity investigations via discrete element modelling for binder jetting technology

   https://doi.org/10.1177/00325899251361213  

   Grippi, Thomas; Maximenko, Andrii; Rios, Alberto_Cabo; Jiang, Runjian; Torresani, Elisa; Olevsky, Eugene (July 2025, Powder Metallurgy)

   In the binder jetting (BJ) process, as in most of powder bed additive manufacturing technologies, the powder is periodically recoated onto the substrate layer-by-layer. The elements of the current deposited layer corresponding to the part being manufactured are bonded together using a polymeric binder. In all cases that require a thermal process for sintering, the internal structure of the finished part is defined by the internal structure of the powder bed. This article focuses on the discrete element modelling (DEM) of various powder spreading methods during recoating and their impact on the powder bed structure particularly applied to binder jetting technology. The article demonstrates that despite the thinness of the deposited layers, they typically exhibit porosity and particle and pore size non-uniformities along the build-up direction. These irregularities contribute to the anisotropic sintering shrinkage observed in green BJ bodies during experiments. However, the experiments presented confirmed by modelling show that without binder deposition, the powder bed – except for a narrow surface layer – remains relatively uniform, regardless of the recoating method used. It is the binder injection into the porous structure of the powder bed that disrupts this homogeneity, locks in large surface pores, and exacerbates the effects of powder segregation during spreading. Finally, several strategies, explored via simulation, are proposed to reduce porosity variations during BJ: using a combined roller-wide blade method for powder spreading and a two-hopper approach, where each layer consists of small particles deposited over larger ones. 

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