Automated fibre placement (AFP) - A303

 

Automated fibre placement (AFP)
Document Type	Article
Document Identifier	303
Themes	Materials deposition and consolidation management Thermal management
Tags	Deposition Fibre-bed compaction Forming Thermal transformation Cure and crystallization
Prerequisites	[[A259|]] Materials deposition and consolidation management (MDCM) [[A201|]] Viscosity (resin) Prepreg

Overview

Automated Fiber Placement (AFP) is an advanced process for material deposition, primarily employed in the aerospace industry to manufacture high-quality structures. Leveraging advances in automation technology and computer numeric control (CNC), AFP can deposit narrow tapes of typically unidirectional prepreg under tightly controlled process conditions. AFP offers high deposition accuracy, repeatability, and reliability, enabling the lamination of parts with superior quality. Although this process can produce complex and high-performance parts, it requires a significant initial investment in equipment.

Automated fibre placement, similar to Automated tape layup (ATL), places multiple prepreg carbon fibre tows, typically between 1/8” to 1” wide, directly onto mould surface with a computer-guided robot. The main capability differences between the ATL and AFP is that AFP utilizes smaller width of prepreg and thus is capable of producing parts with more complex geometries and higher precision than ATL. Both ATL and AFP utilizes a method of applying heat, often an infrared system, to the prepreg during the laying process in order to heat up the resin to tack it into position on the mould. For AFP, the tows are always cut perpendicular to the length of the tow whereas ATL is capable of cutting the end to shape, which reduces scrap and post-processing trimming time. ATL and AFP are considered as the most prevalent automated composites manufacturing technologies since these technologies have widespread adoption in both government and commercial programs with equipment ranging from few square feet to hundreds of square feet in size. However, this widespread adoption exists only in industries such as aerospace where the companies are able to justify the large capital investment costs involved.

Significance

Automated Fiber Placement (AFP) enables the production of high-performance, large structures with moderately high complexity. Utilizing advanced automation technology, AFP ensures tightly controlled processing conditions, allowing for the fabrication of high-quality laminates. Compared to conventional material deposition methods, AFP systems provide enhanced reliability, repeatability, and a higher production rate.

Scope

This page provides an overview of the Automated Fiber Placement process. The process is explained from the MSTE perspective to include important variables and sub-systems of this process.

Process description

Broadly, the AFP process consists of an automated motion platform, such as a gantry or an articulated robotic system, that carries a fiber placement head. This motion platform produces the necessary movement to deposit narrow composite tapes along a predefined trajectory on the tooling surface. During the AFP process, multiple narrow composite tapes, referred to as ‘tows’, are aligned in the AFP head to form a ‘course.’ With each pass, an AFP head can deposit up to 32 tows simultaneously in a single course on a tool surface. Each tow is driven individually, and can be started, stopped, cut and restarted independently. Tows can be driven at different speeds enabling layup over complex geometries.

The material is supplied on spools, which are mounted on the creels of the AFP system. As these spools are unwound, the tow is fed into the process. At this stage, the material release film is removed, if present for the specific material system employed. Then, the tows are guided under controlled tow tension to the AFP compaction rollers. The AFP head incorporates a heat source positioned immediately in front of the roller to elevate the temperature of the underlying substrate prior to material deposition. Finally, the tow is placed on the substrate using pressure applied by the compaction roller. The compaction of the tows at elevated temperatures sufficiently consolidates the laminates, thereby eliminating the need for vacuum-assisted void removal that is commonly required in the hand layup process.

Deposition rate, layup temperature, compaction force, and tow tension are the primary process parameters controllable in an AFP system. These parameters must be tightly controlled during the process to successfully manage material deposition and consolidation, ensuring a high-quality and defect-free layup. The specific factors involved in managing these process parameters vary based on the type of material used. There are three main categories of materials, differentiated by their resin systems, used in the AFP process: thermoset prepreg tapes, thermoplastic prepreg tapes, and dry fiber tapes .

Schematics representation of the AFP process.

AFP of thermoset prepregs

Thermoset-resin prepreg tapes (specifically the slit-tape variation as discussed in the Materials Section) are the most common type of material used in AFP processing. In this case, tack, or adhesiveness, is the primary material property resisting the formation of layup defects. As-received B-stage prepreg materials are compliant and tacky above their glass transition temperature. Excess resin is removed and the resin undergoes a partial curing in B-stage prepreg. The process conditions are accurately controlled to ensure sufficient adhesion, or tack, between the newly placed tows and the existing substrate. Adequate tack between different plies is essential to maintain the intended fiber orientation, as any deviations or fiber misalignments are considered defects and require repair/correction.

With decreasing deposition rate, the tow has more time under pressure at the process nip point to develop a high-quality contact with the underlying substrate. However, a lower deposition rate is not desirable as it negatively impacts process productivity. Layup temperature significantly influences tack through the resin’s ability to flow (i.e., viscosity), and cohesive strength. An increase in temperature reduces resin viscosity which facilitates tack formation, but it also reduces resin strength (see also viscosity). For many epoxy-based resin systems peak tack is observed at about 30 to 40°C. Temperature should not exceed approximately 60°C to avoid triggering the cure reaction. While increasing compaction force can also enhance tack, its effectiveness reaches a plateau beyond a certain level.

Alongside tack, drape – or the material's ability to deform – is another crucial property pertinent to this process. The drapability of prepreg tapes is influenced by process parameters (primarily temperature) as well as their width, as wider tapes are more challenging to deform. The use of narrow tows in the AFP process enhances drapability, facilitating the lamination of more complex geometries. Additionally, the precise control over individual tows allows for accurate control of the layup boundaries, minimizing scrap and reducing the need for follow-up processes.

Improper layup design or management of process conditions can lead to development of various types of defects such as gaps, overlaps, improper boundary coverage, wrinkles, puckers, bridges, etc. in the layup. Some defects such as gaps and overlaps may be inevitable in producing complex parts since they are a natural consequence of producing a ply through depositing individual tows with a finite and constant width over a complex surface. Other types of defects such as wrinkles are unacceptable as they negatively impact mechanical properties of the part.

Laminates produced via AFP of thermoset prepreg materials, are typically bagged and cured in an autoclave process. This further ensures proper consolidation and cure of the part to achieve the high-performance properties that are required .

AFP of thermoplastic prepregs

Thermoplastic-resin based prepregs can also be processed using the AFP process. During this process, the temperature of the underlying substrate locally exceeds the melting temperature of the thermoplastic resin. The new tow is deposited under high compaction pressure to facilitate molecular interdiffusion at the interface and consolidate the material. Then, the tow and substrate are cooled in a controlled manner to ensure a high degree of crystallinity in the resin. Compared to the processing of thermoset prepregs, AFP of thermoplastic materials requires significantly higher temperatures (approximately 400°C) and greater pressure.

AFP processing with thermoplastic prepregs offers the potential for in-situ consolidation, wherein the laminate produced by the fiber placement process is fully consolidated and ready for service without any additional processing steps. Although this remains a significant area of research and development, most parts currently produced through this process undergo post-consolidation in an autoclave or a hot press. This additional step ensures sufficient time for complete interdiffusion of molecular chains at interfaces between plies under high temperature and pressure .

AFP of dry fiber tapes (Dry Fiber Placement)

Dry fiber tapes can be used in the AFP process (often referred to as Dry (Automated) Fiber Placement, DFP or DAFP) to produce reinforcement pre-forms. The pre-forms can be then used in a liquid composite molding process to infuse the reinforcements with resin, cure the resin system, and produce the final part.

Lacking resin, dry fiber tows are less compliant and more brittle compared to prepreg tows, which results in a different set of defects appearing during the process. Common issues include fuzz balls, fiber fluff, and loose fibers. Furthermore, the achievable deposition rates for dry fiber placement are typically lower due to the brittleness of the fibers, making the dry fiber tows prone to breaking.

Layup Strategies

A layup strategy describes how a ply is produced from depositing individual tows and it affects trajectory starting points, reference curves, and coverage across the surface. The first step in describing a layup strategy is the reference curve used to produce a course. There are several strategies available for creating reference curves, including fixed angle, geodesic, and variable angle. The fixed angle strategy generates a curve from a specified starting point that maintains a constant angle relative to a given axis across the entire surface. The geodesic curve method, which follows the natural path of the surface, is employed to avoid steering. On the other hand, variable angle guide curves alter the fiber orientation along the curve, enabling the creation of laminates with variable stiffness.

Reference curve strategies

A ply is constructed by populating the entire surface using the reference curves. Three primary coverage strategies exist for this purpose, namely, offset curves, shifted curves and independent curves.

The offset or parallel curves strategy is the most common one used for path planning. In this strategy adjacent curves are computed to be equidistant. The advantage of this approach is that the resulting ply will be free of gaps and overlaps between courses. However, over complex geometries, the offset curves may result in a fiber orientation that is different from the reference curve fiber orientation. Furthermore, if the initial reference curve has a curvature, the offset approach increases curvature leading to higher residual stresses and potentially defects in the tows.

The shifted curve strategy simply shifts the reference curve through a translation. While this method is very simple, it may lead to an increase in gaps and overlaps between the tows. Finally, it is also possible to populate the entire ply surface by independently designing reference curves. This approach can be tedious and time consuming but it may be necessary to ensure layup quality when laminating complex geometries .

Comparison between offset (left) and shifted (right) strategies

Material

Material Types

Fiber placement heads typically employ narrow tows with standard widths of 1/8”, 1/4”, or 1/2”, and deliver a standardized number of tows, 4, 8, 16, 24, or 32, in each course. The most commonly used material in this process is graphite/epoxy thermoset slit tape prepreg, which consistently delivers high-quality tows and achieves high-performance material properties. However, other types of materials, including thermoset or thermoplastic impregnated tows (towpreg) and dry fibers, are also used.

Thermoset tows

Thermoset tows consist of fiber reinforcements pre-impregnated with thermoset resin systems. High-performance graphite reinforcements paired with epoxy resin systems are the most commonly used materials in thermoset tows. Since the matrix is a B-stage resin, thermoset tows should be stored at -20°C to prevent the advancement of the degree of cure and material aging.

Thermoplastic tows

Thermoplastic tows consist of fiber reinforcements pre-impregnated with thermoplastic resin systems, with graphite reinforcements paired with polyether ether ketone (PEEK) being the most commonly used material system in this process. These tows provide higher damage tolerance, impact resistance, chemical and solvent resistance compared to thermosets, and are recyclable. Additionally, they have an unlimited shelf life and low storage costs, as they do not require frozen storage. They also offer the potential for rapid, out-of-autoclave (OOA) processing through in-situ consolidation .

Dry fiber tows

Dry fiber tows consist of dry fiber reinforcements produced in a tape format that are bound together using thermoplastic coating. This construction allows for the fibers to be handled and deposited to produce reinforcement pre-forms while maintaining alignment and tow integrity before final resin infusion. Graphite and glass fiber reinforcements are common in dry fiber tows.

The production of dry fiber tows does not involve a prepregging process, and their storage does not require freezing, nor do they have out-time limitations. As a result, they are less costly to manufacture and store. Typically, preforms created through DAFP are processed out of autoclave through a lower cost liquid composite molding process. However, the resulting parts are prone to quality challenges associated with resin infusion.

Material Manufacture Method

Slit-tape

Prepreg slit-tapes are produced by trimming broad sheets of prepreg materials to specific tow widths using rotary knives in a slitting process. Since these slit-tapes are manufactured by cutting prepregs, they achieve very high dimensional accuracy and further offer advantages with respect to productivity, reliability, and product quality . However, they are more costly as their production involves additional processing steps.

Towpreg

Pre-impregnated tows, or towpregs, are manufactured by pre-impregnating fiber tape pre-forms in their final width with resin .

Both slit-tapes and towpregs can be produced using thermoset and thermoplastic resin systems. Thermoset slit-tapes, which are tacky at ambient temperature, typically have one side protected with a plastic release film. This prevents the tows from adhering to one another and helps preserve the material's quality.

Shape

As a result of the narrow tapes used in AFP, the material has high drapeability and therefore complex geometries can be manufactured using this process. Single or double curved complex surfaces can be manufactured using AFP. It should be noted that the specific equipment used, especially the size of the fiber placement head, limits feasible geometries.

Tool

A typical setup involves a one-sided tool with a flexible vacuum bag. AFP of thermoset tapes commonly involves autoclave curing and therefore, the tool material should be able to withstand temperature and pressure of the material cure cycle. AFP of thermoplastic tapes involves application of high local temperatures and pressures during processing.

Equipment

Automated motion platform

Different motion platforms have been implemented to produce the motion required to deposit tows over desired trajectories. Typically, 6 degrees of freedom, 3 translational and 3 rotational are required for the AFP head to maintain a perpendicular orientation with respect to an arbitrary layup surface and trajectory. Cartesian gantries and articulated robotic arms are two general architectures available for this purpose.

In gantry platforms, the gantry provides three translational degrees of freedom (x, y, z). The AFP head is mounted on the gantry and is equipped with three additional mechanical rotational axes. Cartesian gantries are particularly suitable for the layup of long and wide structures that demand high precision and productivity. Their design is highly flexible and of high payloads can be supported, allowing for installation of the machinery required to deposit of a greater number of tows, and layup larger structures. Various gantry configurations have been implemented before. Horizontal and vertical (or column) gantries are among common configurations.

An articulated robot is a robot with arms that are connected to each other via rotary joints in a chain. The robot uses all its revolute joints to access its workspace. AFP heads can be used as the robot’s end effector to perform fiber placement operations. Compared to gantries, robots have a more limited payload and workspace. To increase the workspace additional axes can be incorporated in the system. For instance, the robot can be installed on a linear rail or suspended from a horizontal gantry.

Schematics representations of a horizontal gantry laminating a spar	Schematics representations of a column or vertical gantry laminating a fuselage section	Schematics representations of an articulated robotic arm with an AFP head as end effector laminating a concave tool

Material storage

Two common strategies for storing material spools in the AFP process include storing them directly on the AFP head or in a cabinet creel. Storing the spools on the AFP head minimizes the travel distance of each tow from the spool to the process nip point (compaction roller), which can enhance process efficiency. However, this approach significantly increases the weight and footprint of the AFP head. An important implication for system design is the need for a more rigid motion platform to support the additional payload, which can increase costs. On the other hand, storing material spools in a creel cabinet results in a smaller AFP head, which is particularly advantageous when laminating small concave tools. However, this setup can pose challenges in preventing resin buildup in the tow delivery mechanism.

Delivery system

Material spools are individually driven using a servo system. A separate spool removes the backing paper and rewinds it before feeding the tow into the system. The delivery system guides the tows from the storage module (either a creel cabinet or directly from the AFP head) to the compaction roller. Guide pulleys, rollers, and tubular guides are employed to continuously feed the tows and continuously guide them through the system. Active or passive tension control mechanisms are used to ensure accurate and stable tow tension control throughout deposition. Typically, the delivery system is cooled down to about 5°C to minimize prepreg tack, which helps limit the buildup of resin residue, maintains a clean delivery system and prevents blockage.

Compaction unit

In AFP processing of thermoset prepreg tows, the material is applied to the substrate using a deformable compaction roller. Ideally, when subjected to compaction force, the roller deforms against the tool surface, adapting its shape to apply a uniform compaction pressure to the tow. Solid rollers, constructed in one piece, can be used to deposit tows. However, as the number of tows increases, wider rollers should be used, which can lead to nonuniform pressure distribution across the roller width, especially pronounced in more complex geometries. To address this issue, segmented rollers have been developed. In segmented rollers, each segment is responsible for the application of a single tow, and all segments can move independently relative to one another. This design enables the roller to deform more effectively against the tool surface and produce a much more uniform pressure distribution. Polyurethane is the most common material choice for manufacturing compaction rollers.

Front view schematics representation of AFP compaction rollers

Using solid rollers can lead to nonuniform pressure distribution	Segmented rollers adapt to tool geometry more effectively producing a more uniform pressure distribution

Roller design considerations for AFP processing of thermoplastic prepreg tows are significantly different due to the higher pressure and temperature requirements of the process. Steel rollers, capable of withstanding the high temperatures employed during processing, are a common choice. However, they only produce a line contact, which significantly reduces the time under pressure available for molecular interdiffusion. To address this, shoe-like compaction systems are implemented to provide high compaction force over a larger area. Additionally, deformable rollers equipped with a silicone tire have been introduced to achieve a more uniform pressure distribution and greater contact area. These rollers are cooled to enhance their lifespan, yet they still degrade over time due to exposure to high temperature and should be replaced regularly.

Heating source

As discussed in the section on process description, application of heat is a critical step in the AFP processing. Three main methods have been used for heating up the substrate hot gas torch, infrared lamps and laser.

Hot gas torches have been utilized in AFP systems for over two decades, using nitrogen (N2) or carbon dioxide (CO2) as the heating medium. This method uses forced convection to heat the prepreg materials, allowing operators to control the flow rate and gas temperature. However, maintaining a consistent temperature with hot gas torches can be challenging, which might affect process consistency and quality.

Infrared heaters, mainly featuring quartz lamps, are the most commonly used heat source in AFP processes. These heaters operate via radiative heat transfer, where the input power to the lamp can be controlled. However, they can be inefficient and create non-uniform heating, due to the wide dispersion of heat they produce. Additionally, infrared heaters generally lack the energy required to effectively process thermoplastic materials, limiting their use to thermoset resins.

Laser heaters represent a more modern approach, primarily utilized for processing thermoplastics due to their high energy density and focused energy delivery. This technology operates through both radiation and conduction, offering precise control over input power and energy application. The strategy involves using a light wavelength that is absorbed by the fiber material, minimizing damage to the matrix. However, lasers are not suitable for all materials; for instance, they cannot be used with glass fibers as these do not absorb laser energy effectively. Additionally, the use of laser heaters requires strict safety measures, including laser shielding and specialized personal protective equipment (PPE) to protect against potential hazards.

On-the-fly cutting

Modern AFP systems are able to control deposition of individual tows in a course. One of the features of this capability is to cut the tows at precise locations during the deposition process. This capability, called on-the-fly cutting, allows the AFP machines to produces ply boundaries and cutouts in the laminate, accurately.

Advantages

Primary advantages of the Automated Fiber Placement process are higher quality, enhanced repeatability and quality consistency, and higher production rate. In addition, this process is more scalable, both in terms of size of structures and production capacity, less reliant on operator skillsets and can lead to lower scrap material.

Disadvantages

Automated Fiber Placement requires high capital investment in machinery. It furthermore requires expert knowledge to manage various aspects of the material deposition process to achieve high quality parts.

Application

The primary area of application for Automated Fiber Placement process is lamination of aerospace structures, such as fuselage sections, wings, spars, and cryogenic tanks.

Latest state-of-the-art technology

• Automated in-situ inspection systems integrated into AFP head to inspect Boeing 777X wing panel structures using a series of laser projectors, high resolution cameras, laser profilometers, and advanced computer software algorithms (Electroimpact & Aligned Vision).

• Automated inspection of dry material placement using laser profilometer sensors to map out the edges of the dry materials during layup (Danobat Composites).

• Fiber orientation and defect detection with the use of a camera system and advanced computer software algorithms. Combined with a 3D laser scanning system, this inspection technology is capable of superimposing the raw image data onto the surface of the 3D scanned geometry (Hexagon Manufacturing Intelligence – Apodius Vision System).

• Automated in-situ surface profile inspection system developed by National Research Council of Canada (NRCC) utilizes advanced profilometer at the end of the AFP head to provide faster and more accurate part inspection as compared to infrared interferometry. This process was reported to be capable of reducing processing time by up to 30 percent as compared to manual inspection (Fives and NRCC).

AFP Developers

• FIives
• Electroimpact
• MTorres
• Coriolis Composites
• Trelleborg

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents	How to measure gel time - M101 How to measure reinforcement content (and corresponding matrix content) - M109
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Practice for Developing a Deposition Step - P157
Case Studies	Cost Comparison Study of a GFRP Leisure Boat Hull Manufacturing Method; Spray Up vs Resin Infusion - C115 Optimizing Cost vs Weight for Low Production Volume Parts - C112
Perspectives Articles	Composite materials engineering webinar session 5 - Manufacturing processes - Introduction - A124 Composite materials engineering webinar session 6 - Manufacturing processes - Prepreg processing - A125 Composite materials engineering webinar session 7 - Manufacturing processes - Liquid composite moulding - A126 Fabric Forming: how it affects design and processing, and how simulation can address this - A310 Fibre Architecture: Availability, pros and cons, and selection for my application - A309 Pultrusion of Thermoplastic Composites - A334

References

About	Help

Pre-impregnated (prepreg) material refers to fibre that is already combined with resin. It is the most common material form used in aerospace.

During prepreg production, (e.g. fibres are run through a resin bath), prepreg is heated and partially cured to B Stage (< 5 % degree of cure). Thermoset prepregs (e.g. epoxy prepreg) have to be kept in a freezer at around -20 °C. At room temperature, the epoxy starts to cure.

Carbon fibres are composed of large aromatic sheets similar to those in graphite. These graphitic layers form the basic structural units in the shape of ribbons. The structure of carbon fibre ribbon is believed to be a columnar arrangement of disoriented graphite crystallites parallel to the ribbon length. The idealized tetragonal crystallites are stacked above one another, with slight disorientation between the crystals in the direction of fibre axis, trapping sharp needle like voids, where the boundaries between the stacks represent the disordered regions.

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.

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

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.

The act of combining reinforcement fibre, resin to shape on a mould (tool).

There are three main methods to combine resin and fibre to deposit onto a tool (i.e. material deposition):

• Wet Processes: Mixing liquid resin and dry fibre directly in the mould, e.g. Wet Compression Moulding process for parts in BMW 7 series
• Liquid Composite Moulding (LCM) processes: Infusing dry fabric with resin using a pressure gradient, e.g. High Pressure Resin Transfer Moulding process (HP RTM) for parts in BMW i3
• Prepreg Processes: Laying up using prepreg where resin and fibre are already combined, e.g. Automated Tape Laying (ATL) process for some of the parts in Boeing 787.

Thermosets are a class of polymer that undergo polymerization and crosslinking during curing with the aid of a hardening agent and heating or promoter. Initially they behave like a viscous fluid. During curing, they change from viscous fluid to rubbery gel (viscoelastic material) and finally glassy solid.

If heated after curing, initially they become soft and rubbery at high temperatures. If further heated, they do not melt but decompose (burn)

Comes in two parts: part A (resin) and B (hardener). When mixed, curing reaction starts and is not reversible.

Examples include epoxy or polyester.

A class of polymer, some common examples include polypropylene and polyethylene.

They soften and melt upon heating (i.e. potentially recyclable), high viscosity when melted, therefore difficult to saturate fibres. Usually needs a lot of pressure and heat to process.

The glass transition temperature (T_g) is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. It is one of the most important properties of any amorphous polymer.

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.

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability

Degree of cure (DOC) is an indication of how far the chemical curing reaction (crosslinking process) has advanced in a thermoset resin.

DOC is defined with a number between 0 and 1 (or 0% and 100%) where 100% is a fully cured resin. It does not have to fully reach 100% for the resin to become solid or the part to be used. In some aerospace applications, resins are only cured to about 90%. Higher the degree of cure, higher the mechanical properties.

Drapeability refers to how well a material conforms to a complex shape (non developable surface).

Trade off: typically the more drapeable a fabric, the less dense the weave (lower V_f), and less stable during processing.

A single unit of fibre reinforcement that cannot be separated further without breaking it. It often has a circular cross section that is formed by the dies used to create it or its precursor material. Many filaments are manufactured in parallel to produce a strand or tow. The number of filaments in a strand or tow is often references by how many thousands of filaments comprise the tow. e.g. an 18k tow is made up of ~18000 filaments.