Tow Impregnation Research Articles

Impact of additives on the quality of oxide/oxide tow-pregs obtained by continuous fibre impregnation

This paper focuses on the formulation of aqueous alumina slurries compatible with manufacturing alumina/alumina ceramic matrix composite (CMC) by a continuous tow impregnation process. Two hygroscopic organic plasticizers, sorbitol and glycerol, along with a gelling powder, boehmite, are evaluated for their impact on both slurry behaviour and the quality of impregnated tows. The interest of using glycerol instead of sorbitol is demonstrated, as glycerol exhibits smaller impact on slurry viscosity and stability, while being more hygroscopic and plasticizing, enabling a reduction in organic content. Combining glycerol with boehmite powder improves the surface and the tack of impregnated tows, although boehmite increases slurry viscosity and shear-thinning behaviour. Furthermore, thermal degradations of sorbitol and glycerol are studied through thermogravimetric analysis, to define an autoclave curing cycle for each plasticizer removing, to achieve optimal compaction of tow-pregs and hence reduce the porosity of the resulting CMCs.

Bulk factor characterization of heated debulked autoclave and out-of-autoclave carbon fibre prepregs

Tow impregnation during cure causes change in the laminate thickness, thus bulk factor, which causes variation in prepreg air permeability and large thickness deviations around corners. A noncontact eddy current thickness sensor monitored the laminate thickness throughout debulk and cure in an oven. Bulk factor values averaged to 1.1 for materials intended for autoclave use and 1.2 for out-of-autoclave, where a lower number is ideal for complex geometries. A heated debulk (HD) at around 48 °C for 2 h resulted in similar bulk factor values as a 16-h room temperature debulk (RTD), suggesting that this shorter cycle can be used as a replacement, resulting in a 26–71% cycle time reduction. For these thick sixteen ply laminates, up to six percent and five percent pore area can be expected of laminates processed RTD and HD, respectively. The highest short beam strength was achieved by a plain weave material intended for out-of-autoclave processing and lowest pore area was achieved by a 5-harness material intended for autoclave processing.

An investigation of in situ impregnation for additive manufacturing of thermoplastic composites

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.

The effect of processing on the microstructure of hoop-wound composite cylinders

Fibre-reinforced polymer composites are increasingly used to make pipes and pressure vessels. The relationship between wet-winding manufacturing, composite microstructure, and the mechanical performance is complex due to many process parameters and material properties involved. Efficient manufacturing aspirations however drive process innovations that include new, radically different tow impregnation methods. In this work, the process–property–performance relationship is experimentally construed for hoop-wound carbon fibre/epoxy composite cylinders. The difference between cylinders produced by a new tow impregnation system and cylinders from the reference impregnation system was investigated. Winding speed and cylinder wall thickness were considered as two additional variables. The results indicate that, within current scope, composite microstructure is relatively insensitive to the winding speed and to final cylinder thickness. Meanwhile, un-optimized changes for tow impregnation affect the void content, the size distribution of voids and the interlaminar failure mode in short beam shear.

Mechanisms of inter‐ply void formation during vacuum bag‐only cure of woven prepregs

AbstractThe removal of inter‐ply air is critical for limiting porosity in laminates. In this study, an in situ monitoring technique was employed to observe inter‐ply air evolution during vacuum bag‐only cure. Observations showed that reduced vacuum resulted in inefficient inter‐ply air evacuation, a more rapid bubble expansion rate, and formation of new air bubbles. A modified tow impregnation model showed that resin infiltration was impeded at reduced vacuum conditions due to the presence of intra‐tow air. However, the cured laminates showed that tows were fully impregnated in all cases, indicating that the entrapped intra‐tow air migrated to inter‐ply regions during cure. The interactions between intra‐tow and inter‐tow air at deficient vacuum conditions were revealed. Findings led to the conclusion that air remaining in intra‐tow regions contributed more to the increase of inter‐ply voids than the reduction in consolidation pressure difference associated with reduced vacuum.

Effects of debulk temperature on air evacuation during vacuum bag-only prepreg processing

Air removal prior to cure is critical for limiting porosity during vacuum bag-only (VBO) processing of prepregs. In this study, the effects of pre-cure dwell temperature (debulk) on air evacuation were investigated for both plain weave (PW) and unidirectional (UD) prepregs. In situ observations revealed that increasing dwell temperature promoted inter-ply air evacuation (by as much as 2x for UD prepregs). Through-thickness gas permeability increased with increasing temperature and decreased with increasing number of plies. The decrease in in-plane permeability during heated debulk was attributed to increased tow impregnation. The findings provide guidelines for cure cycle optimization. Heated debulk enhanced air evacuation in PW laminates, particularly as laminate width/thickness ratios exceed a threshold value. However, warm debulks were less effective, particularly for thicker laminates (>8 plies).

In situ monitoring and analysis of void evolution in unidirectional prepreg

In the layup of prepreg laminates, air is inevitably entrapped between adjacent prepreg plies, yet the removal of this inter-ply air under vacuum bag cure conditions is not well understood. In this study, an in situ visualization technique was developed to dynamically observe inter-ply air removal during the cure of an out-of-autoclave prepreg. The technique was used to investigate mechanisms of air removal and void evolution in unidirectional prepreg. Prepreg impregnation was also tracked by inspection of laminate cross-sections prepared at different times during the cure cycle. From these data, a three-stage air removal mechanism was documented based on the relationships between void content, resin properties, and tow impregnation as functions of time. Furthermore, a positive correlation was observed between the rate of evacuation and bubble elongation. Though discussed here in the specific context of voids in unidirectional laminates, the in situ observation technique developed for this work has broad potential to enhance understanding of processing phenomena associated with out-of-autoclave prepregs.

Characterisation of epoxy powders for processing thick-section composite structures

Epoxy powders were investigated as a processing route for fast, low-cost manufacturing of thick-section fibre reinforced polymer parts. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and parallel-plate rheometry were used to characterise the material for realistic processing conditions. The epoxy powders contained heat-activated curing agents and exhibited good thermal stability at and above typical processing temperatures (160–180°C). The exothermic heat produced during curing was found to be small when compared to some conventional epoxies. Similarly, it was shown that epoxy powders can be melted between 45 and 120°C to achieve low viscosities for fibre tow impregnation, without inducing significant cure. Semi-empirical cure kinetics and chemorheological models were presented, which can be used to predict the epoxy's behaviour during part consolidation and curing. Modifications were made to an existing cure kinetics model to better represent the behaviour of the epoxy at lower temperatures. The relationship between glass transition temperature and the degree-of-cure was described using the DiBenedetto equation and was implemented in an existing chemorheological model. The chemorheological model was applied to a standard process cycle to assess the accuracy of the model and the effectiveness of the process cycle.

A stochastic approach to model material variation determining tow impregnation in out-of-autoclave prepreg consolidation

Material variations are always present even though out-of-autoclave prepregs are machine-made. They strongly determine the consolidation and may eventually lead to voids within the final part, depending on applied process conditions. To capture any contingencies, stochastic differential equations are derived to describe various interacting phenomena in OoA consolidation. In a second step the probabilistic space is discretized using the Karhunen–Loève truncation and the Probabilistic Collocation method is applied in order to use deterministic solvers for flow and compaction problems. The initial degree of impregnation is represented by an Ornstein–Uhlenbeck process and calibrated with CT-images.

Modeling the resin flow and numerical simulation of the filling stage for vacuum-assisted resin infusion process

The preform is covered by a flexible vacuum bag in vacuum-assisted resin infusion process, and the preform compaction state is time and space dependent during the mould-filling stage, there is no practical tool for the filling simulation. In this paper, the resin flow governing equations for vacuum-assisted resin infusion are built based on the analysis of the preform compaction behaviour and the fibre tow impregnation. For obtaining the numerical solution conveniently and accurately, an alternative approach is proposed in which the primary flow is computed from the normal Darcy’s law, two-slave elements are added to each master element, named sink slave element and deformation slave element, to calculate the resin mass variation caused by fibre tow infiltration and preform deformation. Three examples are conducted to verify the precision and broad applicability of the approach. The influence of the preform deformation and the unsaturated effect on the mould filling is analysed.

Effect of room-temperature out-time on tow impregnation in an out-of-autoclave prepreg

Tow impregnation as a function of material out-time was investigated for an out-of-autoclave carbon fiber–epoxy prepreg. Prepreg was aged at ambient temperature for 56days. Every 7days, laminates were laid up and cured using vacuum bag only processing. Void content was calculated through image analysis of polished sections. Experimental results were used to validate an analytical model for tow impregnation. Model predictions were based on flow kinetics during processing conditions, taking into account increasing degree of cure and evolution of resin viscosity as a function of ambient aging time. The study found that no significant tow porosity occurred within the material’s stated out-life, that tow porosity increased once this out-life was exceeded and eventually stabilized due to the room-temperature vitrification of the resin. The model’s predicted trends were consistent with experimental results, suggesting that an increase in resin viscosity is indeed the main cause of out-time induced tow porosity and providing a means of predicting laminate quality as a function of room temperature aging time.

Modelling the effect of material properties and process parameters on tow impregnation in out-of-autoclave prepregs

Out-of-autoclave prepregs based on woven fabrics initially consist of dry tows and resin-rich areas. The tows allow air evacuation in the initial stages of processing and are subsequently infiltrated by surrounding matrix. The following study analyzes the relationship between material properties, process parameters and tow impregnation for three OOA prepregs. First, a representative model for tow impregnation is developed. Then, the model parameters are determined and the model predictions are correlated to impregnation data measured by X-ray microtomography. Finally, the model is used in a parametric study to investigate the effect of fibre architecture, cure cycle temperature and resin initial degree of cure on tow impregnation rate and to predict the possible occurrence of flow-induced micro-voids.

Multiscale modeling of unsaturated flow in dual-scale fiber preforms of liquid composite molding III: reactive flows

The woven, stitched or braided fabrics used in liquid composite molding (LCM) display partial saturation behind moving flow-front in an LCM mold which is caused by delayed impregnation of fiber tows. In this part 3 of the present series of three papers, a novel multiscale approach proposed in parts 1 and 2 [1,2] is adapted for modeling the unsaturated flow observed in the dual-scale fabrics of LCM under non-isothermal, reactive conditions. The volume-averaged species or resin cure equation, in conjunction with volume-averaged mass, momentum and energy (temperature) equations, is employed to model the reactive resin flow in the inter-tow (gap) and intra-tow (tow) regions with coupling expressed through several sink and source terms in the governing equations. A coarse global-mesh is used to solve the global (gap) flow over the entire domain, and a fine local mesh in form of the unit-cell of periodic fabrics is employed to solve the local (tow) flows. The multiscale algorithm based on the hierarchical computational grids is then extended to solve the dual-scale flow under reactive conditions. The simulation is compared with a two-color experiment and a previously published two-layer model. Significant differences between the temperatures and cures of the gap and tow regions of the dual-scale porous medium are observed. The ratio of pore volumes in the tow and gap regions, the effective thermal conductivity in the tows, and the reaction rate are identified as the important parameters for temperature and cure distributions in the gap and tow regions.

Continuum Dual-scale Modeling of Liquid Composite Molding Processes

In this study, a comprehensive continuum dual-scale model has been developed for LCM processes for woven fiber reinforcements. This model incorporates the main flow mechanisms involved, including mass exchange between dual-scale porosities, micro-diffusion within fiber tows between neighboring repeating unit cells (RUCs), and the capillary effect within fiber tows. Macro-and micro-flows are simulated at the same numerical scale. A sink and a source term are introduced into the two different scaled flows to couple them together. An unsaturated flow model is adopted to describe micro-diffusion within fiber tows between neighboring RUCs. The physics of the sink/ source term is modelled according to the impregnation of tows within a RUC. Two models developed in soil physics have been adopted to evaluate the capillary effect and the partially saturated permeability within fiber tows. A finite element method is employed to solve the governing equations, and a simple implicit time integration method is suggested. A 1D RTM example has been modeled. The modeling results indicate that all of the three main mechanisms have significant influences on the LCM processes, but the mass exchange between the dual porosities plays a dominate role, particularly under high injection rates.

Modelling microscopic flow in woven fabric reinforcements and its application in dual-scale resin infusion modelling

Abstract A physical unit cell impregnation model is proposed for the micro-scale flow in plain woven reinforcements. The modelling results show a characteristic relationship between tow impregnation speed, the surrounding local macro-scale resin pressure and the tow saturation within the unit cell. This relationship has been formulated into a mathematical algorithm which can be directly incorporated into a continuum dual-scale model to predict the ‘sink’ term. The results using the dual-scale model show a sharp resin front in inter-tow-pore spaces and a partially saturated front region in intra-tow-pore spaces. This demonstrates that the impregnation of fibre tows lags behind the resin front in the macro pore spaces. The modelling results are in agreement with two reported experimental observations. It has been shown that the unsaturated region at the flow front could increase or have a fixed length under different circumstances. These differences are due to the variation in tow impregnation speed (or the time required for the tow to become fully impregnated), the weave architecture and the nesting and packing of plies. The modelling results have also demonstrated the drooping of the inlet pressure when flow is carried out under constant injection rates. The implementation of the algorithm into a dual-scale model shows coherence with a single-scale unsaturated model, but demonstrates an advantage in flexibility, precision and convenience in application.

Numerical prediction and experimental characterisation of meso-scale-voids in liquid composite moulding

Pressure gradients that drive the resin flow during liquid composite moulding (LCM) processes can be very low while manufacturing large composite parts. Capillary pressure becomes the predominant force for tow impregnation and thus meso-scale-voids can be generated, reducing the part quality. In contrast, micro-voids are created at high resin pressure gradients. In this work, a numerical method is presented to predict the creation of meso-scale-voids and their evolution. Experimental validation is conducted by measuring void content of produced composite parts with micro-computed tomography (μ-CT). Additionally, the void content as a function of the modified capillary number Ca ∗ is determined and the influence of the fibre volume content in the bundles on the meso-scale- and micro-void content is studied.

Analytic characterization of the permeability of dual-scale fibrous porous media

It has been known for some time now that fabrics that contain fiber tows, which are bundles of thousands of aligned fiber filaments, can be represented as dual-scale porous media; these tows are woven or stitched together and the flow through the resulting fabrics can be modeled to account for the delayed impregnation of tows, which draw fluid away from the advancing flow front. From such flow simulations, one can predict not only the macroflow front, but also the saturation of the fiber tows, as well as determine the partially-saturated region. The simulations require one to input two material parameters to quantify the two distinct length scales of the porous media: the bulk permeability, which characterizes the overall resistance to the flow, and the tow permeability, which is the resistance to the flow within the tow and is related to the saturation of the fiber tows. Techniques to characterize the bulk permeability are well established and can be performed in one, two, and three dimensions; however, methods to measure tow permeability are sparse. Hence, an analytical model is developed to characterize both permeability values: the bulk permeability ( K) and the fiber tow permeability ( K t) from a one-dimensional constant flow rate experiment in which the inlet pressure profile during the filling is recorded. These two permeability values determine the time scales of resin impregnation in between the fiber tows and within the fiber tows thus playing an important role in the decision process of how long to leave the resin injection gate open to ensure filling of the fiber tows. For dual-scale preforms, the inlet pressure profile for a constant rate injection is not only dependant on the structure of the preform and fluid viscosity, but also on the mold length. If the mold is sufficiently long, it can be mathematically proven that the partially-saturated region is constant. Examination of the solution also reveals that the partially-saturated length increases as the bulk permeability increases and as the tow permeability decreases. The analytical methodology is demonstrated by characterizing four different fabrics.

Modeling the Effect of Fiber Diameter and Fiber Bundle Count on Tow Impregnation during Liquid Molding Processes

This paper explores the effect of fiber diameter and fiber bundle count on tow impregnation during liquid molding processes. Resin and fiber wetting properties play an important role on the resin infiltration behavior within a single fiber tow. Dynamic and equilibrium microflow models, based on Darcy’s Law and force equilibrium at the resin flow front, assume that the infiltration of the resin is driven through capillary pressure and hindered by the entrapped gas within the tow. The results of the dynamic model show that the fiber tow count and the fiber diameter have a significant effect on the tow infiltration time. The infiltration time is 10 times longer for a 60 K carbon fiber tow than for a 6 K one. The infiltration time is also inversely proportional to the fiber diameter. As the fiber diameter is decreased 100 times, from 3.5 mm to 35 nm, the infiltration time increases 100 times for the same diameter tow. It also shows that fiber volume fraction also has a significant effect on the tow infiltration time and any fiber volume variations along the fiber tow could lead to dry spots in the composite due to the large differences in infiltration rates. The model indicates that the entrapment of a critical amount of air within a tow will lead to higher than acceptable void contents.

Modeling the Unsaturated Flow in Liquid Composite Molding Processes: A Review and Some Thoughts

The liquid composite molding (LCM) processes for manufacturing polymer composites involve injecting a thermoset resin into a fiber-packed mold cavity. Very often, the fiber preform behind the resin front is partially saturated during the mold-filling process in LCM, giving rise to an unsaturated flow in that region. This paper first discusses the role of dual-scale porous media in inducing unsaturated flow in certain directional (woven, stitched, or braided) mats due to the delayed impregnation of tows. The approach of using a sink term in the mass balance equation is compared with the traditional approach of using unsaturated permeability for modeling unsaturated flow in dual-scale directional mats. Later, the relation of unsaturated flow with the phenomenon of bubble formation and migration is discussed. The lack of connection between the unsaturated flow research on the one hand, and the bubble creation and migration research on the other, is highlighted next. A few suggestions are offered on related topics, including the modeling of bubble motion, and the need for accurate experiments in unsaturated flow.

Governing equations for unsaturated flow through woven fiber mats. Part 2. Non-isothermal reactive flows

Correct modeling of resin flow in liquid composite molding (LCM) processes is important for accurate simulation of the mold-filling process. This paper seeks to develop an accurate physics for the unsaturated flow witnessed in woven, stitched or braided fiber mats during the LCM mold-filling. Such a fibrous porous medium is characterized as a dual-scale porous medium as the gaps between the fiber tows are much bigger than intra-tow pores found between the fibers. The delayed impregnation of tows, as the invading resin in large gaps quickly passes by the dense tows without fully wetting them, is the cause of the unsaturated flow in dual-scale porous media. The mathematically rigorous phase-averaging method is employed to develop a new set of gap-averaged temperature and cure equations in the dual-scale media. Like the sink term in the mass balance equation introduced in the first part of this two-paper series, the temperature and cure equations for the gap region are also shown to have new source and sink terms. Such terms are created due to the convection and diffusion of heat and cured resin into the late-impregnating fiber tows. Implications of other terms created in the two equations due to the averaging process are discussed. Simultaneous solving of the gap and the tow transport equations is presented as a strategy to model non-isothermal reactive flows in dual-scale porous media.