Daniel Walczyk’s research while affiliated with Rensselaer Polytechnic Institute and other places

A novel co-extrusion system for continuous fiber reinforced thermoplastic composites in filament and narrow tape format was designed, fabricated, and tested. The new modified pultrusion process, called In Situ Impregnation, impregnates continuous dry fiber reinforcement tows in-situ with thermoplastic matrix for applications ranging from 3D printing using robotic manipulation to automated fiber placement. The technical goal of the system is to directly co-extrude and impregnate a reinforcement fiber tow (carbon) with thermoplastic matrix injected by an extruder fed with thermoplastic pellets. This approach uses inexpensive materials instead of ‘prepreg’ tow in order to streamline the additive manufacturing process, cut costs for advanced composites manufacturing, and deliver fully customizable fiber orientation. The purpose of this paper is to discuss analytical modeling of friction and fiber tensioning in the system which allows for the full impregnation of the fibers. Experiments were conducted on a working pultrusion system where load was adjusted through the tensioning system to better understand the amount of friction throughout the system, the magnitude of tension in the fiber tow, and to validate the models. The resulting friction model can be used by machine designers to estimate the tension in tows, ropes, fibers, etc. in similar tensioning devices, and estimate automated system specifications such as motor requirements. A brief description of the new manufacturing process is also provided. Future work includes commercialization of the technology, automation of the manufacturing system, and further modeling work to predict fiber spreading behavior based on geometric factors.

There is growing industrial and academic interest in manufacturing of biocomposite parts comprised of natural fibers in a thermoplastic matrix that begin as a commingled, unconsolidated preform. Unfortunately, little thermal property data exists in the literature for simulation/analysis of processes used to make parts (e.g., pultrusion, Automated Fiber Placement (AFP), and compression molding). In this paper, the authors explain how specific heat capacity and thermal conductivity values of both constituent materials and the biocomposite preform are measured in a direction transverse to the fiber length, and how the effect of entrained air is included. Thermal property values for hemp and flax fibers along with polypropylene and polyethylene filaments, measured both individually and combined into apparent values for the preforms, are compared with experimental values. Finally, determination of thermal properties for use in pultrusion simulation is explained as a case study.

This paper discusses a new method for decorticating bast fiber stalks through a mastication process without damaging the fiber for use in biocomposites. Conventional automated decortication methods provide high stalk processing throughput, but they significantly damage the bast fibers and adversely affect their performance in biocomposite applications. Initial experiments with industrial hemp using a matched set of tools indicate that indexing the stalk by, at most, half a tooling period for each mastication cycle maximizes both the crushed stalk flexing action and dehurding efficiency. Further process insight was gained through simple stalk crushing experiments (force vs. deflection) between matching teeth with no indexing, where force spikes correspond to initial collapse of the stalk cross section and initial hurd bending fracture along the stalk length. A more extensive experimental design with stiffer tooling reveals that adding spaces in the bottom die for hurd to fall through, and using the smallest practical indexing distance less than half a tooling period and also more teeth maximizes hurding efficiency. However, shorter indexing and more teeth also decreases throughput rate and complicates stalk handling. Future work for optimizing and commercializing the process are suggested.

This paper describes the application of a new manufacturing process for low-cost and rapid consolidation and curing of advanced thermoset composites that avoids the use of expensive prepreg, autoclaving, and thermally induced curing. The process, called VIPE, uses a novel tooling design that combines vacuum infusion (VI) of a dry preform with resin, a rigidly backed pressure focusing layer (P) made of an elastomer to consolidate the wet preform with uniform pressure, and high-energy electron beam curing (E). A VIPE tool is engineered and fabricated to manufacture 3D laminate bicycle seats composed of woven carbon fiber textile and an electron beam-curable epoxy acrylate. Details of the tooling design discussed include computational fluid dynamics (CFD) simulation of the vacuum infusion, iterative structural finite element analysis (FEA) to synthesize the pressure focusing layer (PFL), structural FEA to design the top mold made of a composite sandwich structure for electron beam transparency, and Monte Carlo electron absorption simulations to specify the e-beam energy level. Ten parts are fabricated using the matched tool (bottom aluminum mold covered with silicone layer and top mold with carbon/epoxy skins separated by foam core) after the dry textile preform contained within is infused with resin, the tool halves are clamped under load, and a 3.0 MeV e-beam machine bombards the tool for less than 1 min. Part thickness, part stiffness, surface roughness, and fiber and void volume fractions measurements show that aerospace quality parts with low cycle times are achievable, although there is high variability due to the small number of replicates and need for process optimization.

Implementation of renewable materials and building systems research into practice has proven challenging. Technologies such as mass timber, engineered mycelium, or hempcrete have understandably garnered attention, but their widespread adoption has been hampered by specific, yet unsatisfied performance metrics. This paper proposes an interscalar approach for the early stage validation of such technologies and for the development of a practical framework to guide further research. This approach begins by analyzing the design problem and identifying relevant and quantifiable performance metrics, before organizing them under “performance scales” that reflect a particular research discipline. The resulting framework provides identification of insurmountable obstacles, identification of required research expertise, and organization of the research effort into manageable tasks. This paper presents a case study utilizing this approach, specifically regarding non-corroding natural fiber composite reinforcing, for cementitious materials.Concrete is an indispensable component of infrastructure systems, but most of America’s infrastructure was built with little concern for long term durability. Many of these structures are nearing the end of their service life and trillions of dollars must be spent to repair and maintain their operation. Only 2% of reinforced concrete is reinforcing steel by volume, but corrosion of that 2% leaves the remaining 98% of concrete at risk of failure. There are commercially available anti-corrosion rebar technologies available, but none are perfect solutions: some are easily damaged, others are incompatible with all concrete, and some are prohibitively expensive.This paper aims to address these concerns by utilizing an interscalar approach to validate the viability of a non-corroding composite rebar made from natural fibers and thermoplastics. This approach found that at the structural scale, natural fiber composites could achieve the same strength as steel and the same elasticity of GFRP with a fiber ratio of 44%-50%. At the processing scale, preliminary experiments indicate that “jacketing” the natural fibers with thermoplastic during the commingling stage resulted in the best fiber saturation at the consolidation stage of production. Finally, at the environmental scale, preliminary calculations indicate that natural fiber composites can be produced with 30%-50% less embodied energy than other non-corroding rebar technologies. These results not only demonstrate the viability of natural fiber composite rebar, but also the benefits of using an interscalar approach for early stage technology validation.

Interest in biocomposites is growing worldwide as companies that manufacture high-performance products seek out more sustainable material options. Although there is significant research on biocomposite material options and processing found in the literature from at least the last two decades, there are few experimentally based case studies published to help guide product designers and engineers when considering these materials. This paper discusses the use of biocomposites in the seat of an electric bus. Although it is clear that biocomposite material options are quite limited, the authors eventually settled on three natural reinforcements (cellulose, hemp, flax), two epoxies (one low and the other high viscosity) with high biobased carbon content, and one flax precoated with bioepoxy for consideration. Laminate plates with a 4mm nominal thickness are manufactured using VARTM (low viscosity epoxy only), hand layup as a surrogate for prepregging (high viscosity epoxy only), compression molding, and an out-of-autoclave process called the Pressure Focusing Layer (PFL) method. Permeability of the three reinforcements infused with the high viscosity epoxy and fiber volume fractions are determined experimentally to provide insight into VARTM processing and mechanical performance. The tensile modulus, maximum tensile stress, flexural modulus, and maximum flexural stress are measured for all combinations of reinforcement, resin, and processing using tension testing and three-point bending based on ASTM standards. Basic conclusions are drawn about the specific application and more generally about the process of using biocomposites in commercial products.

Components made from thermoplastic composites (TPC) by additive manufacturing (AM) and automated fiber placement (AFP) are gaining popularity because of the material system’s inherent advantages over thermosets, but high material costs and limited production flexibility are hindering more widespread use. This paper discusses prototyping, fluid flow modeling, and testing of a novel in situ preimpregnator (prepregger) for lower-cost TPC feedstock used in AM and AFP. Prior attempts by researchers to address this technical need have generally resulted in low-quality prepreg with low fiber volume fraction (Vf) and high void content, which can be explained by Darcy’s law applied to the highly viscous polymer melts. Uniformly spreading the fiber tow thin before impregnation is the key to rapid and complete impregnation. Based on initial experiments with a flexible experimental setup to quickly test concepts, a prototype prepregger consisting of dry fiber payout reel with slight torsional resistance, fixed pin fiber spreader, double slot die coater fed with polymer melt from an microextruder, consolidator with pultrusion exit die, and prepreg downstream drive mechanism was designed and built. Predictions from a steady-state flow model of the coating die/microextruder system assuming creeping flow and Newtonian fluid behavior agreed well with experimental flow measurements for a variety of polymer melts and operating conditions. Once ideal input parameters were determined, prepreg tape specimens exhibited complete impregnation but low Vf and excessive encapsulation due to insufficient consolidation and removal of excess resin. Experiments where pultrusion dies were used for filament manufacturing showed significantly improved Vf and prepreg quality. Future work is suggested to facilitate commercialization of this promising process technology.

Due to the increasing popularity of additive manufacturing with continuous fibers thermoplastic composites (TPC), this paper investigates the process of in situ impregnation of dry fiber tow with molten thermoplastic resin as a means to reduce material cost. To better understand impregnation with high-viscosity polymer melts, a 1D version of Darcy’s Law is derived along with Gebart’s equations for estimating fiber tow impregnation and permeability. Rheology of the two polymer melts tested, nylon 6/12 and polycarbonate, were measured using a capillary viscometer. The experimental setup consisted of a heated and weighted plunger system to maintain polymer melt pressure in a temperature-controlled pultrusion chamber, through which a dry 3 K carbon fiber tow passes through at a constant velocity for in situ impregnation. The resulting TPC tape was collected on a reel and not used directly for AM. The setup allowed three process variables in Darcy’s equation, i.e. chamber pressure, polymer viscosity (via temperature), and exposure time, to be varied. The degree of impregnation of the thick tape specimens – quantified using digital microscopy and image analysis on polished cross-sections – matched theoretical predictions quite well (average errors of ∼10% for nylon and ∼15% for polycarbonate). However, despite pressures up to 1.2 MPa and exposure times (5.5−15 sec) consistent with current AM system feed rates, the extremely high melt viscosities (290–980 Pa⋅sec) make full impregnation of thick tapes nearly impossible in a practical setting. Improvements to the in situ impregnation process based on experimental results and impregnation models are suggested.

This technical brief discusses material degradation issues and potential remedies related to advanced thermoset composites manufacturing using a new out-of-autoclave consolidation and curing process called ‘Electron Beam processing of Specialized Elastomeric Tooling combined with Resin Infusion’ (EB-SETRI). The design process for EB-SETRI tooling based on finite element structural analysis and Monte Carlo simulations of EB attenuation within tooling materials is briefly described. Of particular interest in this paper is the elastomeric mask, since it exhibits significant changes in mechanical properties based on prior work. Samples of five different silicone blends (four different durometers and two different catalysts) and one urethane (elastomeric mask materials of choice) were irradiated by an EB source with 3.0 MeV maximum power to simulate the conditions experienced by EB-SETRI tooling during processing. Changes in surface hardness and compression modulus were measured using ASTM D575 and D2240 as a function of dosage. Urethane embrittles and becomes unusable even at low doses, whereas silicone generally hardens to a maximum level at higher doses, presumably due to increased crosslinking density, and modulus increases linearly. The embrittlement of silicone is shown to be a result of the EB irradiation and not due to a temperature increase from energy absorption. Changes in elastomer mechanical properties confound process performance as a result, and several concepts for dealing with these changes are suggested. Although the experimental focus is on EB-SETRI, results are applicable to any manufacturing process that combines the use of EB irradiation and elastomers.

Purpose This paper aims to summarize the up-to-date research performed on combinations of various biofibers and resin systems used in different three-dimensional (3D) printing technologies, including powder-based, material extrusion, solid-sheet and liquid-based systems. Detailed information about each process, including materials used and process design, are described, with the resultant products’ mechanical properties compared with those of 3D-printed parts produced from pure resin or different material combinations. In most processes introduced in this paper, biofibers are beneficial in improving the mechanical properties of 3D-printed parts and the biodegradability of the parts made using these green materials is also greatly improved. However, research on 3D printing of biofiber-reinforced composites is still far from complete, and there are still many further studies and research areas that could be explored in the future. Design/methodology/approach The paper starts with an overview of the current scenario of the composite manufacturing industry and then the problems of advanced composite materials are pointed out, followed by an introduction of biocomposites. The main body of the paper covers literature reviews of recently emerged 3D printing technologies that were applied to biofiber-reinforced composite materials. This part is classified into subsections based on the form of the starting materials used in the 3D printing process. A comprehensive conclusion is drawn at the end of the paper summarizing the findings by the authors. Findings Most of the biofiber-reinforced 3D-printed products exhibited improved mechanical properties than products printed using pure resin, indicating that biofibers are good replacements for synthetic ones. However, synthetic fibers are far from being completely replaced by biofibers due to several of their disadvantages including higher moisture absorbance, lower thermal stability and mechanical properties. Many studies are being performed to solve these problems, yet there are still some 3D printing technologies in which research concerning biofiber-reinforced composite parts is quite limited. This paper unveils potential research directions that would further develop 3D printing in a sustainable manner. Originality/value This paper is a summary of attempts to use biofibers as reinforcements together with different resin systems as the starting material for 3D printing processes, and most of the currently available 3D printing techniques are included herein. All of these attempts are solutions to some principal problems with current 3D printing processes such as the limit in the variety of materials and the poor mechanical performance of 3D printed parts. Various types of biofibers are involved in these studies. This paper unveils potential research directions that would further widen the use of biofibers in 3D printing in a sustainable manner.