Filament winding - A299

Filament winding
Document Type	Article
Document Identifier	299

Overview

Filament winding is an automated composites manufacturing process where resin-impregnated filaments such as fibreglass or carbon fibre in tension are wound around a rotating mandrel to make a tank or pressure vessel. This automated process was developed in the 1970s in Europe for the manufacturing of fibreglass pipes and tanks . Filament winding is particularly known for its ability to create cylindrical or spherical shapes, though variations of the process can be adapted to produce more complex forms. There have been continuous developments & improvements made to this automated process throughout the years to improve capabilities from a 2-axis classic lathe-style to 6-axis bed-style or goal post-style machines which increases the number of degrees of freedom.

Common filament winding machines are 4 axis machines with computer control over multiple movements to ensure the accurate placement of fibers. These machines control the mandrel rotation, which determines the speed and direction at which the part rotates during winding. The carriage translation can be also controlled by computers, enabling the fiber feeder to move along the length of the mandrel and deposit fibers uniformly. The cross feed mechanism adjusts for variations in mandrel diameter and allows the machine to accommodate different part geometries. Finally, the guide rotation is linked to the winding angle, controlling the angle at which the fiber is laid down relative to the mandrel surface.

For longer tubular parts such as piping, a process called continuous filament winding is used. Continuous filament winding generally involves an stationary feed and translation of the mandrel in the axial direction. One of the key advantages of this process is that it eliminates the creation of end dwell, which is a common issue in other winding methods where the fiber may accumulate at the start or end of the winding cycle, resulting in an uneven distribution or weak points in the composite structure.

Significance

Filament winding is a process in the production of high-strength, lightweight composite parts used in various industries. This technique enables the manufacturing of high-performance tubular and cylindrical structures. By precisely controlling the winding pattern and fiber orientation, filament winding ensures optimal mechanical properties for the final part. As an automated process, it also offers enhanced repeatability and reliability, making it well-suited for high-volume production of composite components.

Scope

This page provides an overview of the Filament Winding (FW) process. The process is explained from the MSTE perspective to include important variables and sub-systems of this process.

Applications

Filament winding is used for pressure vessels and bottles due to it’s high specific strength and the lighter weight of filament wound parts compared to traditional steel manufacturing methods .

Tubular products such as composite piping can be made with the filament winding process, and benefit from corrosion and degradation resistance. Driveshafts and couplings can be made from filament winding have an advantage of being insulated, separating metallic components from each other. Composite driveshafts have also been used in motorsports for their lightweight properties . Utility poles also benefit from degradation resistance offered by filament winding, but they the filament winding process also provides ease of transportation since poles can be made in segments and then secured together on site. The high specific strength of composite winding is useful in applications like energy storage flywheels due to high specific strength .

Process description

Filament winding is an additive manufacturing process, where fibers are continuously wound onto a rotating mandrel to create strong, lightweight components. In Wet Winding, towpreg systems or continuous strands of fiber are passed through a carriage guide and onto a rotating mandrel. In this process, the fibers are impregnated with resin either before or during winding. One challenge in filament winding is the creation of "end dwell", which occurs when the carriage guide needs to change direction at the end of the mandrel. This transition can cause the accumulation of waste material, as the fibers may not be laid down in the most efficient way during this movement, resulting in an uneven or thicker buildup at the ends of the winding.

There are two types of resin baths used in wet filament winding: the dip type and the drum type systems. In the dip type system, fibers are submerged in a resin bath, and the resin is evenly distributed by a squeegee or similar mechanism that ensures proper metering. In a Drum type system, the bottom of the drum is immersed and fibers are drawn over the top of the drum to pick up resin. The drum system uses a doctor blade to meter the resin, providing more precise control and less friction on the fibers during the impregnation process. To facilitate optimal resin impregnation, resin baths are often heated to lower the viscosity of the resin and improve saturation of the fibers. An advantage of wet winding is that rollers are often not necessary since the resin acts as lubrication. This reduces the complexity of the system.

When the roving is wound around the mandrel, it forms a winding band. The winding band generally is not thick enough to cover the entirety of the mandrel in one pass. In the diagram below the part is wound from left to right (in green) and then the carriage travels in the reverse direction (shown in red). This process repeats until the desired part is constructed. The filament winding process will often leave a distinctive diamond pattern .

Schematic of fiber band interweaving during filament winding. Adapted from .

Definitions

• Creel: The creel is the component that holds the spools or rolls of roving (fiberglass or carbon fibers) used in the filament winding process. It can be designed in different ways to facilitate fiber feeding to the winding apparatus. There are two primary types of creels: center-pull and outside-pull. In a center-pull creel, the roving is unwound from the center of the spool, which helps to reduce the possibility of tangling and ensures a smoother fiber feed. Another advantage of center pull creels is that they can be daisy chained together to form a single long fiber strand. In an outside-pull creel, the fiber is unwound from the outer edge of the spool, which can be useful when the spool has a large diameter or when working with multiple spools.

• End Dwell: End dwell refers to the portion of the part that is affected by the need to reverse the direction of the machine at the end of the mandrel. As the carriage guide moves back and forth along the length of the mandrel, the part at the transition point can exhibit excess resin build-up or uneven fiber distribution. This results in waste, as this section is not part of the usable finished part and is typically discarded

• Mandrel: The mandrel is the central tool or form around which the fiber is wound. It can be made from various materials such as metal, aluminum, or plastic, depending on the application. The mandrel serves as the shape template for the winding process. In some cases, the mandrel may be a single, continuous piece, while in other instances, it could consist of multiple sections that are assembled or disassembled as needed.. The mandrel can also be left in the part after the winding process.

• Towpreg: Towpreg refers to prepreg (pre-impregnated) fiber that is used in a filament winding process. Unlike traditional wet winding, where the fiber is impregnated with resin during the winding process, towpreg is a cleaner, more controlled system where the fibers are pre-impregnated with resin before being wound onto the mandrel.

• Catenary effects: this refer to the forces and curvature experienced by the roving during filament winding, caused by the unequal lengths of fiber as they are drawn from the creel and laid onto the mandrel. These effects occur due to the tension differences that develop between the fibers as they are wound in the process, potentially leading to issues such as fiber misalignment, variations in the fiber's tension, or even fiber breakage if not properly controlled.

• Denier:Denier is a unit of measurement used to describe the mass of the fiber or roving. Specifically, it is defined as the mass in grams for 9,000 meters of fiber. Higher denier values indicating thicker or heavier fibers.
• TEX: TEX is another unit of measurement used to describe the mass of roving, but it differs from denier in that it is based on 1,000 meters of fiber rather than 9,000 meters. Heavier rovings speed up production (higher rate of material deposition) but they may reduce part quality and performance.
• Yield: the length of roving (in yards) per pound of fiber. It is a measure of how much fiber can be obtained from a given weight of material.

Materials forms

The most common material forms used in filament winding include direct roving, assembled roving, prepregs, and yarns.

Direct Roving vs. Assembled Roving

Direct Roving, also known as single-end roving, is a manufacturing technique where continuous filaments are spun into a single roving strand. All the fibers in direct roving are continuous throughout the length of the roving, ensuring a uniform structure. This form of roving is widely used for applications requiring fiber continuity.

Assembled Roving, on the other hand, is formed by combining multiple direct rovings to create a thicker, stronger roving strand. While this increases the overall strength and weight, small variations in the length of the individual rovings can introduce catenary effects—slight length discrepancies that may cause uneven tension distribution. This misalignment can lead to unequal load-bearing between the individual strands, affecting the mechanical properties of the final composite.

Rovings, whether direct or assembled, are typically classified based on either filament count (common in carbon fiber materials) or linear mass density (e.g., glass fibers). Filament count refers to the number of individual filaments in a strand, while linear mass density measures the mass of fiber per unit length, providing an indicator of the roving's thickness and weight.

Prepregs

Prepregs (pre-impregnated fibers) are fibers that have already been impregnated with a resin matrix before being wound onto a mandrel. These materials are typically used for high-performance applications, such as aerospace and motorsport, because they offer superior control over resin content and fiber orientation. Prepregs can be made from a variety of fibers, including carbon, glass, or aramid, and are often preferred for their ability to provide uniform fiber wet-out.

Yarn

Yarn is a continuous strand of fibers twisted or braided together, often used in filament winding for applications that require more flexibility and maneuverability. Yarns are typically less rigid than rovings and can be easily manipulated for complex winding patterns. While they may not provide the same strength as high-density rovings or prepregs, they can be ideal for applications requiring lighter materials or intricate shapes.

Tows and Bundles

Tows and bundles are similar to rovings, but they typically contain fewer filaments. A tow is a collection of continuous filaments bundled together without twisting, making it suitable for processes where flexibility and easy manipulation are important. These forms are commonly used for filament winding for applications that require thinner layers and finer detail.

Tow Pregs and Resin-Impregnated Tows

Tow Pregs are a variation of prepregs. These are tows impregnated with resin, which allows for the precise control of resin content during the winding process.

Latest state-of-the-art technology

• High-speed 3D filament winding machine developed in the UK to produce complex layups that have varying shapes and cross-sections. Cygnet Texkimp

• Integration of filament winding with automated fibre placement (AFP) and other processes such as 3D printing was developed in Macedonia and Germany. MIKROSAM and CIKONI

• Adaptation of filament winding process developed in Germany that wet winds carbon fibre/epoxy around aluminum pegs inserted into a tooling plate. This process is referred to as FibreTEC3D and it utilizes two collaborative robots, one to maneuver the tooling and the other that wet winds the carbon fibre/epoxy. This creates a lightweight structure that is tailored to loads and transitioning away from typical rotational structures of traditional filament winding. FibreTec3D on Youtube

• Multi-filament winding capable of 48-180 tows/fiber inputs to reduce cycle time as compared to conventional single-tow/tape winding. This was developed in Japan in 2015 and the first units are working to characterize the process and begin prototyping. Murata Machinery Ltd.

• Integration of 3D printed cores with automated 3D filament winding to reduce tooling costs for low-volume, one-of-a-kind products. This was developed in Germany as a solution to manufacture robot grippers because each gripper is very specialized and different, but tooling costs to make these are very high due to the low volume. The system 3D prints a mandrel with pins and then 3D wet wind carbon fibre/epoxy around these pins and cure at room-temperature. [CIKONI https://cikoni.com/en/engineering-for-additive-manufacturing-and-bionic-design]

References

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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.

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.

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.

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.

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.

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.

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.

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

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