Resin flow - A263

 

Resin flow
Document Type	Article
Document Identifier	263
Themes	Materials deposition and consolidation management
Tags	Debulking Fluid flow Gas flow Resin flow

Introduction

Resin flow in composite materials plays an important role in the manufacturing process, directly affecting the quality and performance of the final product. Resin flow refers to the movement and distribution of resin within the preform, particularly during molding or infusion process. Achieving uniform resin flow through the reinforcement fibers is essential for achieving optimal fiber wetting and fiber volume fraction, and minimizing defects such as voids, dry spots, or uneven curing. Resin flow is influenced by a variety of factors, including resin viscosity, temperature, pressure, fiber architecture and permeability, as well as any obstructions within the mold.

In processes like vacuum infusion, resin transfer molding (RTM), wet layup, and compression molding, effective control of resin flow is essential to ensure consistent fiber impregnation and uniform mechanical properties. Insufficient resin flow can result in incomplete fiber wetting, leading to compromised mechanical performance and potential failure. Similarly, excessive resin flow or poor resin management may cause resin-rich areas, increased weight and porosity, all of which can negatively impact the material's properties.

The dynamics of resin flow are governed by the rheological properties of the resin, the interaction between the resin and fiber surfaces, and the specific processing conditions. Therefore, managing resin flow is a critical factor in the successful production of composite parts.

Significance

Resin flow plays an important role in determining the mechanical performance and structural integrity of composite materials. Improper resin flow can lead to incomplete fiber wetting, uneven resin distribution, and the formation of defects such as voids or dry spots. These defects can significantly weaken the composite, reducing its tensile strength, stiffness, and overall durability.

Scope

The dynamics of resin flow are influenced by various factors, including the type of resin, fiber architecture, processing conditions, and mold design. This page aims to explore the different aspects of resin flow, including types of resin flow, consumables for promoting resin flow, and modeling resin flow.

Level I

Level II

Common types of resin flow

Different flow strategies can be used to fill a part during an infusion process. The selection of the specific flow type is dependent on factors, such as part geometry/complexity, infusion distance, part layup, resin viscosity, and others. The most common types of resin flow are described below:

Center-out flow

Center-out flow is one of the simplest and widely used resin flow strategies, especially in vacuum bag infusion processes. In this approach, one or more resin ports are positioned along the perimeter of the part, and a vacuum is applied from the exterior. In its simplest form, a centre out infusion will consist of a single resin port at the center of a part and results in a circular flow pattern.

For larger or more complex parts, multiple resin ports are often necessary to ensure successful filling of the the preform. For large, simple parts, the ports may be spaced evenly throughout the part. For parts with intricate features, careful consideration must be given to the placement of the resin ports to ensure consistent flow across all areas, particularly near features that are more prone to incomplete filling.

The type of reinforcement used can significantly impact the flow pattern in a center-out infusion. Unidirectional reinforcements or bi-directional reinforcements with different permeability in different direction will often cause the resin to flow more easily in one direction, creating an oval-shaped resin front. This behavior must be taken into account when determining the optimal port placement to ensure uniform resin distribution throughout the part.

Centre-Out Flow	Centre-out flow with multiple resin ports	Centre-out flow showing non-uniform directional flow

Perimeter flow

Perimeter flow is the opposite of a centre-out infusion process. In this method, resin is introduced at the edges of the part, with the flow directed toward the center. Vacuum ports are placed at the center or other key locations within the mold to ensure the cavity is fully filled. Perimeter flow is commonly used in light RTM processes, where the resin flow channel is integrated into the B-side mould. The challenges in designing port placement for perimeter flow are similar to those encountered in center-out infusion. As shown in the figure below, the resin front typically moves uniformly outward from the perimeter ports. However, careful planning is essential to avoid areas of stagnation or poor flow.

Perimeter flow type

Linear flow

Linear flow is commonly used for composite parts with minimal geometric changes along their length. In this type of flow, a resin flow channel is placed along one side of the part, allowing the resin to move across the part uniformly. This method is particularly effective for flat or elongated components, such as sandwich panels, where the resin must flow over and under the core material to achieve full consolidation.

A variation of linear flow places the resin flow channel along the part's centerline. In this configuration, resin flows in both directions from the central port, often resulting in a shorter infusion time compared to unidirectional flow. However, this approach is less commonly used for sandwich panels, as it can be challenging to ensure the core’s bottom layer is adequately filled.

Linear edge-to-edge infusion	Linear centre-out infusion

Materials to Promote Resin Flow

External Flow Media

External flow media are used in vacuum bag infusion processes to improve resin flow. These materials, often paired with peel ply, help remove excess resin and flow media after the part is demolded. However, external flow media are not suitable for closed mold processes, as the peel ply becomes fully encapsulated within the resin, making it nearly impossible to remove the extra material. In closed mold processes, flow paths may be incorporated into the tooling design to achieve a similar effect to external flow media used in vacuum bag infusions.

The most common external flow media are polyethylene meshes (either LDPE or HDPE), which are placed on the bag-side surface of a laminate stack. The choice of material typically depends on the process and the materials used. LDPE flow media, often referred to as "red mesh," is used in lower temperature applications, as it has a maximum temperature limit of 150°F. HDPE flow media, often called “green mesh”, is designed for higher temperature applications, with a maximum temperature limit of 200°F. Resin typically flows faster through the green mesh than the red mesh.

In addition to the standard red and green meshes, alternative materials are also available. For example, EnkaFusion Nylon Flow Media is a nylon-based material that can withstand much higher temperatures than polyethylene meshes. Other alternatives, such as polypropylene meshes, are also available, providing further options depending on specific application needs.

Red mesh flow media from Fiber Glast	Green mesh flow media from Fiber Glast	Enkafusion external flow media from Fiber Glast

External flow media is often used to promote a specific flow strategy. When using mesh materials, additional mesh can be placed around the resin port to encourage resin flow from the port into the part. Alternatively, meshes can be arranged in a strip along the center of the part to facilitate a more linear, center-out infusion pattern.

In production, various methods are employed to achieve specific flow patterns. Materials such as spiral tubing or Omega flow channels can be used to promote even resin distribution and support a linear, center-out infusion process. These products are connected to the resin feed tube, and their geometry allows resin to flow through their length while evenly distributing the resin throughout the open sections.

Spiral tubing from Fiber Glast	Omega flow channel from Fiber Glast

Internal Flow Media

Various internal flow promotion methods are employed in Light RTM and other infusion processes to enhance the consolidation of composite components. These materials are integrated into the laminate stack, creating a high-permeability layer that facilitates resin flow. This significantly reduces filling times, especially for thick laminates or laminates with low-permeability reinforcements.

Several material options are available, each offering distinct advantages depending on the application. Below are examples of common types of internal flow media:

Some materials are offered as standalone "core" or "flow" plies, typically made from non-stitched synthetic materials. These flow plies provide excellent permeability and are ideal for improving resin distribution. Examples of such materials include Lantor Soric and EnkaFusion nylon flow media.

In addition to basic flow plies, there are structural reinforcements designed to provide both flow promotion and additional strength. These plies are usually constructed from a loose weave of fiberglass reinforcement, making them closer in thickness to standard reinforcement plies. They offer an alternative to a laminate stack composed solely of reinforcing plies, while significantly reducing fill times. Examples of such materials include Chomorat G-FLOW and SAERTEX SAERflow.

For RTM Lite, it is common to use reinforcements that combine both flow plies and additional structural reinforcement. These materials consist of a chopped strand mat (CSM) stitched to the core ply material. The CSM adds strength and allows these reinforcements to be used as the sole plies in the laminate, as it can also provide the required surface finish. Chomorat Rovicore and SAERTEX SAERcore are examples of such materials.

G-FLOW structural flow media from CHOMARAT	SAERflow structural flow media from SAERTEX	Lantor Soric internal flow media from Fiber Glast

Modeling Resin Flow

Modeling the flow within a composite structure is a complex process, influenced by various factors including the materials used in the layup stack, the chemical properties of the resin being infused, and the design of the tooling and process. These factors can vary significantly depending on the scale of analysis. Composite flow is typically evaluated at three distinct scales: microscale, mesoscale, and macroscale. Each scale considers different aspects of the flow dynamics, as illustrated in Figure below.

Three (3) scales for evaluation of flow - depicted from

The macroscale is often the simplest and quickest model to consider, as it examines the flow through the entire laminate stack. At this scale, flow properties can be averaged across the stack, providing a broad overview. At the mesoscale, the focus shifts to the individual plies and their architecture. Factors such as tow size, weave type, ply compaction, and other physical properties are taken into account to develop a more detailed flow model. Mesoscale modeling is more complex and demands greater computational power than macroscale modeling. The microscale, on the other hand, is used to analyze resin flow around individual fibers within the tows and their interactions with neighboring fibers. This level of modeling is the most intricate and computationally intensive, typically used for small, localized areas to assess specific material properties, rather than for evaluating the flow across the overall part geometry.

Effect of Ply Drape on Flow Modelling

The geometry of the plies within a composite part has a significant effect on the flow behavior. Key characteristics of the reinforcing material, such as tow size, weave type (if applicable), yarn compaction, fiber type, and yarn angle, all impact the flow dynamics. Part geometries can cause deformation of the plies during mold placement, which may alter flow characteristics. For simple parts or cases where precise flow results are less critical, these effects can be overlooked. However, when employing more detailed modeling techniques, particularly at the mesoscale and microscale, it is essential to account for the draping effects, as they can significantly affect resin flow and material consolidation.

Darcy’s Law

As the resin flows from the inlet(s) towards regions under lower pressure, the resin velocity decreases according to Darcy's law. Darcy’s law governs flow through porous medium\[Q = -\frac{KA}{\mu}\frac{\Delta P}{x}\]

Where,

\(Q = \) Volumetric flow rate

\(K = \) Preform permeability

\(A = \) Preform cross-sectional area

\(\mu = \) Resin viscosity

\(\Delta P = \) Pressure differential across preform

\(x = \) In-plane flow distance of pressure differential

The time taken for resin to flow a certain distance from the inlet can be derived from Darcy's law. Please see How to assess the resin filling time in LCM processes for more detail. When manufacturing large components, multiple resin inlets and vacuum outlets at different locations are typically used. The inlets and outlets can be controlled to open either simultaneously or sequentially. In practice, it is common to open and close the inlets and outlets to manipulate flow fronts in order to fully impregnate the preform. The sequence however, is largely empirically and can even vary from part to part.

Numerical Simulation

Resin flow is typically modeled using generalized forms of the Navier-Stokes equations and Darcy’s law. To simplify the modeling process, many flow models assume uniform properties across the ply stack, neglecting through-thickness effects . Several strategies have been explored over time to develop accurate flow simulation models. The two most commonly used methods are variations of the Control Volume / Finite Element (CVFE) method and the Volume-of-Fluid (VOF) method . These approaches offer distinct advantages in simulating resin flow through composite structures, each suited to different aspects of the process.

Control Volume / Finite Element Flow Simulation Method

The Control Volume Finite Element (CVFE) method is widely used due to its efficiency and accuracy across various scenarios. This approach, however, does not account for the presence of air within the laminate stack. It works by solving a series of small, steady-state flow problems sequentially.

In the CVFE method, the area is subdivided into distinct elements and nodes, with each node corresponding to its own control volume. The pressure gradient between the inlet and the advancing flow front is calculated using Darcy's law, alongside the mass continuity equation for incompressible Newtonian fluids. From this pressure gradient, a velocity field for the flow front is derived, allowing for the determination of the time required to fill the next element at each location. The first element to be fully filled dictates the next time step increment. The remaining cells are partially filled according to the results, and a new flow front is established. An example of this process is illustrated in Figure 18. This cycle is repeated for each successive flow front until the entire control volume is completely filled.

Flow advancement in the CVFE method - depicted from

The CVFE is an efficient method for determining flow progression though can be very time consuming for solutions with a large number of control volumes.

Volume-of-Fluid Methods

The Volume of Fluid (VOF) method is particularly well-suited for simulating flows involving two or more immiscible fluid phases that cannot coexist in the same space . In the context of resin flow modeling, the two phases typically considered are the resin and the air within the system. The domain is divided into discrete control volumes, where the volume fraction of all phases sums to one, and a set of equations is applied to describe the behavior of each phase.

This method is very effective at modelling flow progression and is able to account for open channels and other areas that the CVFE is unable to solve for. The success of VOF methods is highly dependent on the establishment of appropriate mesh and time discretization; otherwise the solutions may not converge.

Other Considerations

Several additional factors can impact the accuracy of flow simulations and must be carefully considered. Variability in the properties of reinforcement materials, arising from differences in handling and cutting of the plies, can significantly affect flow behavior. Factors such as wrinkles, ply distortion, and variations in material properties can alter flow paths, potentially leading to race-tracking. If not properly controlled, race-tracking can cause significant deviations in flow behavior. To address this, probabilistic methods have been developed to account for such variability in flow models.

Design considerations can further complicate the modeling process. Many composite designs include multiple vacuum ports, which are closed off as the resin reaches them. Additionally, flow media may be applied only in specific areas of the part, with the optimal placement of these media typically determined through infusion trials. These trials, while effective, are time-consuming and resource-intensive. To streamline this process, various studies have explored methods to improve flow control modeling. One such approach involves using centroidal Voronoi diagrams, which have been shown to significantly reduce the number of iterations required to determine optimal placement. However, introducing these complexities into the simulation model should be carefully evaluated, as doing so can significantly increase the computational complexity and make the system more challenging to solve.

The methods discussed so far primarily focus on in-plane flow properties within a laminate, assuming a rigid tool structure. These assumptions are generally valid for thin laminates, where the in-plane flow dominates. However, as laminate thickness increases, through-thickness properties and tool compaction effects become more influential in the resin flow process. Tool compaction is especially critical when using flexible tools that may deform under pressure changes during the infusion process, impacting the final flow behavior.

Explore this area further

• Resin flow - A263
  • Darcy's law - A362

References

About	Help

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

‘Preform’ is the term for the fibre reinforcement. This is the stage between the raw material form after it is processed into an architecture (fabric, mat, etc.) and becoming a composite.

Volume fraction of either matrix or fibres with respect to total composite volume (matrix + fibre).

In composites processing, viscosity is an indicator of how easily the resin matrix will mix with the reinforcement and how well it will stay in place during processing. The lower the viscosity, the more easily resin flows. Resin viscosity ranges considerably across chemistries and formulations.

By scientific definition, viscosity is a measure of a material’s resistance to deformation. For liquids, it is in response to imposed shear stresses.

Permeability refers to the resistance to fluid flow through a porous material.

• Resin flow through fibre
• Gas flow through prepreg

Resin transfer moulding (RTM) involves loading a preform into a two (or more) piece, matched tool, closing it, and injecting resin under pressure (~15-100 psi, or ~1-7 bar).

Well suited to small to medium sized parts, limited to large sizes due to injection pressure loads and tool cost.

Chopped strand mat - A reinforcement material made of short, randomly oriented glass fibers bonded with a resin.

A tow is a bundle or yarn of individual fibres. The tow size is inherent to the fibre manufacturing process (i.e. a tow is manufactured in one process, rather than each fibre individually then bundled together after).

Typically, smaller tows are better because they result in a more homogeneous material.

The larger the tow:

• The faster it is to deposit material
• The easier is it for resin to flow between tows
• Harder for resin to saturate

Typical tow sizes:

• 1k (thousand)
• 3k
• 6k
• 12k
• 24k
• 50k

Any manufacturing and/or decision making activity that occurs during any stage of the development design cycle (e.g. conceptual design to production).

In the context of Knowledge in Practice, practice refers to the systematic use of science based knowledge to reduce composites manufacturing risk, cost, and development time.

Flow front refers to the leading edge of the resin as it is flowing into and through the fibre preform.

The use of multiphysics models to predict the outcome of real-world scenarios. May be analytical (closed form, "hand calculations") or computational (implementation on computers is required due to the large number of calculations involved. e.g. finite element method, finite difference method)

Most often in composite materials engineering, modelling refers to either:

• Process modelling (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance modelling (predicting the stiffness/strength/suitability of a structure, or the inverse problem of the material properties and dimensional requirements to achieve a desired stiffness/strength/suitability of a structure)

(same as "Simulation")