Fundamentals of filament winding: systems, processes and design principles - A422

 

Fundamentals of filament winding: systems, processes and design principles
Document Type	Article
Document Identifier	422
Themes	Materials deposition and consolidation management Residual stress and dimensional control management Machining and assembly management Quality/inspection management
Tags	Deposition Process step Defects Quality

Introduction

Filament winding (FW) is a cost-effective and repeatable process for manufacturing high-performance composite tubes, pipes, and pressure vessels. This article provides an accessible overview of FW fundamentals to support engineers, technicians, and learners in understanding how materials, machine systems, and process parameters combine to produce reliable wound structures. It introduces the basic terminology, identifies the main equipment components, and explains how design considerations interact with process control.

Significance

The article first defines FW system elements: machines (2-axis, 4-axis, and multi-axis), fiber creels, delivery and tensioning devices, resin impregnation systems, and mandrels. It then outlines the winding sequence, emphasizing how rotation speed, fiber angle, and band placement determine laminate architecture and mechanical performance. Next, it explains resin and fiber compatibility, addressing the influence of viscosity, cure kinetics, and fiber sizing on product quality. The design section discusses liner selection, thickness control, and strategies for minimizing defects such as slippage or gaps. Schematics and step-by-step illustrations align with the MSTEP manufacturing framework, showing how FW fits within material preparation, structure design, tooling, and process execution.

Scope

Readers gain a clear understanding of the FW process workflow (from setup to curing) and how machine control and material selection affect end-use performance. The article provides the foundational vocabulary and system knowledge necessary for interpreting subsequent KPC entries on testing and advanced applications.

Definition and Role of Filament Winding System Elements

Filament winding (FW) is a highly integrated composite manufacturing process in which machine configuration, material delivery, and tooling collectively determine product quality and performance. FW systems are fundamentally composed of winding machines, fiber creels, delivery and tensioning devices, resin impregnation systems, and mandrels. Machines are commonly classified based on the number of controllable axes, including 2-axis systems (mandrel rotation and carriage translation), 4-axis systems (adding eye rotation and cross-feed), and multi-axis systems capable of complex, non-geodesic winding paths. Increasing the number of controlled axes expands the design freedom of fiber placement, enabling variable angle laminates and localized reinforcement.

Figure 1. Basic 2-axis filament winding system

Figure 2. A 4-axis filament winding system capable of controlling mandrel rotation, carriage translation, eye rotation, and cross-feed (left), and multi-axis systems capable of complex, non-geodesic winding paths (right)

In filament winding, the design and selection of fiber guides, such as the pay-out eye, are influenced by the processing method. In wet filament winding, the presence of resin provides inherent lubrication, allowing the use of static guides without excessive fiber friction. Alternatively, rollers or pulleys can be employed to further reduce friction. Due to resin buildup, cleaning and maintenance requirements are an important consideration, and careful selection of guides and their surface material is recommended, with common choices including stainless steel and polytetrafluoroethylene (PTFE), which offer smooth surfaces and good resistance to wear and chemical exposure.

Figure 3. Fiber guides (pay-out eyes) featuring static guides (left) and rollers (right)

Fiber creels serve as the supply system for continuous rovings, ensuring uninterrupted fiber delivery during winding operations. Dispensing fibers from creels is coupled with suitable tensioning devices, which maintain consistent fiber tension as fibers are guided from the creel to the resin impregnation unit and finally to the mandrel surface.

Figure 4. Fiber tensioning unit for outside-pull creels (left) and conversion of creels from center-pull to outside-pull via the cardboard and foam core installation (right)

Fiber material for FW is available as direct (single-end) roving and assembled roving, which differ in how the fibers are produced and combined. In the former case, individual filaments are spun and gathered directly into a single roving. In contrast, assembled roving is formed by combining multiple strands to create a heavier roving. While assembled rovings can increase production rates, care must be taken to avoid catenary effects, as small differences in strand length can lead to fiber misalignment and unequal fiber tensioning.

Fiber creels are available in two configurations: center-pull or outside-pull. In the former case, creels typically rest on shelves and tension is applied to the fiber strands via mechanical systems such as s-wrap tension bars. For outside-pull creels, cardboard cores facilitate mounting each creel to a computer-controlled drive that applies torque and thus tension to each strand using a feedback control system with an appropriate device for measuring fiber tension.

Tension control is emphasized as a critical variable because it promotes fiber straightness, material compaction and part consolidation, including the reduction of voids, and bonding between subsequent layers during placement. However, excessive fiber tension may lead to fiber damage and/or slippage of fibers along the mandrel for low winding angles.

Resin impregnation systems are responsible for wetting the fibers prior to deposition. In thermoset-based FW, this is achieved using resin baths or metered impregnation units, where fibers pass through the resin before winding. In a dip-type resin bath, the fibers are fully immersed in a resin-filled tank, with resin content controlled downstream using a squeegee or similar device. In contrast, a drum-type resin bath minimizes friction on the fibers by wetting them with a thin resin film formed on the surface of a rotating drum. In thermoplastic FW systems, impregnation may occur upstream during tape or tow fabrication, with consolidation occurring during winding through applied heat and pressure .

Figure 5. Resin baths for (liquid) thermoset resins: dip-type resin bath (left) and drum-type resin bath (right)

The mandrel functions as both a forming tool and a load-bearing element during winding, defining the internal geometry of the structure and providing support during curing or consolidation. Mandrel material, surface finish, and release characteristics are therefore integral to successful part fabrication. Mandrels can either be removed after curing or remain as an integral part of the fabricated component, as is the case for certain pressure vessel designs. In more complex geometries, multi-part mandrels can be employed to accommodate undercuts and enable removal after curing. Mandrels can also be actively heated to maintain a controlled temperature during winding, and in advanced configurations may incorporate heat-pipe technology enabling effective thermal management and, in some cases, supporting in-situ curing of the composite structure.

The final fabrication step for thermoset composite materials is typically curing at elevated temperatures. Temperature-controlled ovens are commonly employed to match the thermal conditions to the cure kinetics of the material. Wet winding requires mandrel rotation during curing to avoid resin drip-off and promote part roundness. Prior to curing, parts may be wrapped with peel ply, resin-absorbing cloth, and shrink tape to promote consolidation, remove excess resin, and create a uniform textured surface.

Winding Sequence and Kinematic Control

The winding sequence governs how fibers are deposited on the mandrel and directly controls the laminate architecture of the final composite structure. The interaction between mandrel rotational speed, carriage translation speed, and fiber angle determines the fiber trajectory and band placement. These parameters define whether fibers follow hoop, helical, or polar winding paths, which are combined to achieve desired structural performance. The fiber angle relative to the mandrel axis is a function of the ratio between these two motions and is identified as one of the most influential design variables in FW structures. Small changes in winding angle can lead to significant differences in stiffness, strength, and load distribution, particularly in vessels and pipes subjected to internal pressure. Band placement strategies, including the width of the fiber band and the degree of overlap between adjacent passes, are also highlighted as important considerations. Uniform band placement promotes consistent thickness and minimizes stress concentrations, whereas misalignment or irregular overlap can introduce local defects.

Figure 6. Schematic helical winding sequence illustrating fiber band deposition and interweaving for six winding circuits completely covering the mandrel surface

Relationship Between Laminate Architecture and Mechanical Performance

The mechanical performance of filament-wound structures is linked to laminate architecture, which is itself controlled by winding parameters. Fiber orientation determines how loads are transferred through the composite, with hoop-dominated laminates providing high resistance to circumferential stresses and helical layers contributing to axial load-bearing capacity. Design studies on filament-wound pressure vessels demonstrate that fiber angles must be selected to match the anticipated stress state. Improper angle selection can result in inefficient material usage or premature failure. Maintaining consistent laminate thickness is therefore essential for predictable mechanical response.

Mechanical performance is not just dictated by fiber orientation but also by placement accuracy and repeatability. Deviations from intended winding paths can introduce fiber waviness or resin-rich zones, which degrade load transfer efficiency. As a result, precise control of machine kinematics and real-time monitoring of winding parameters are necessary to translate design intent into manufactured performance.

In practice, laminate architecture is further influenced by process parameters and their control during winding. Commercial filament winding suppliers offer sophisticated design and simulation software that enables detailed path planning, parameter optimization, and prediction of laminate buildup. Despite these capabilities, designing winding patterns for complex geometries, such as pressure vessels with domed end caps or components with variable diameters, remains challenging, often requiring an iterative approach combining simulation, experimental validation, and incremental refinement of winding parameters.

Resin and Fiber Compatibility

Resin-fiber compatibility is identified as a foundational requirement for achieving high-quality filament-wound composites. The rheological properties of the resin, particularly viscosity, directly influence fiber wetting and impregnation quality. Low-viscosity resins promote effective fiber wet-out but may increase the risk of resin drainage or uneven distribution, whereas high-viscosity resins may hinder impregnation and lead to void formation. Cure kinetics in thermoset systems are also critical, as premature gelation can restrict fiber movement and compromise consolidation. Fiber surface treatments and sizing are noted as additional factors that influence interfacial adhesion between fibers and the matrix. In thermoplastic FW systems, resin compatibility is framed in terms of melt behavior and consolidation efficiency. Processing temperature, applied pressure, and residence time govern the degree of intimate contact between fibers and matrix.

Structural Design Considerations and Liner Selection

Structural design in filament winding extends beyond fiber orientation to include liner selection, thickness control, and geometric constraints. Liners can serve multiple roles, including providing a gas or fluid barrier, defining internal geometry, and supporting the composite during winding and curing. Liner stiffness and thickness influence how loads are shared between the liner and the composite layers. A liner that is too compliant may lead to excessive deformation, whereas an overly stiff liner may carry excessive loads, reducing the efficiency of the composite overwrap. Liner properties must therefore be considered concurrently with composite design rather than treated as an independent component. Thickness control is another key design consideration, as variations in thickness can introduce stress concentrations and reduce structural reliability. Design strategies therefore emphasize uniform coverage, controlled overlap, and smooth transitions between winding patterns to minimize abrupt changes in thickness.

Defect Formation and Mitigation Strategies

Manufacturing defects are an inherent risk in filament winding and are closely linked to process control. Common defects include fiber slippage, gaps between adjacent bands, overlaps, resin-rich areas, and voids. These defects originate from improper tension control, inaccurate machine kinematics, or mismatches between material properties and processing conditions. Fiber slippage occurs when friction is insufficient to maintain contact with the mandrel surface or preceding layers. Gaps and overlaps result from errors in band placement or changes in winding speed. Resin-rich regions may form due to excessive resin pickup or insufficient drainage during winding, while voids are often associated with poor wetting or incomplete consolidation.

Process optimization approaches are highlighted as effective means of defect mitigation. By systematically varying key parameters such as winding speed, tension, and temperature, optimal processing windows can be identified that reduce defect prevalence. In addition, tooling such as pin rings can be introduced to mitigate fiber slippage in layups that include low winding angles.

Integration Within the MSTEP Manufacturing Framework

FW is positioned as a manufacturing process that naturally aligns with the MSTEP framework, encompassing material selection, structure design, tooling, execution, and process control. Material preparation involves selecting compatible fibers and resins and conditioning them for consistent delivery. Structural design defines fiber angles, stacking sequences, and thickness distributions based on performance requirements. Tooling, primarily in the form of mandrels and liners, provides geometric definition and mechanical support throughout the process. Execution encompasses the coordinated control of machine kinematics, fiber tensioning, resin impregnation, and curing. These stages are interconnected rather than sequential, as decisions made during material selection influence feasible winding speeds and impregnation strategies, while tooling design constrains achievable fiber paths. Filament winding is therefore characterized as a system-level process in which materials, design, and manufacturing must be considered simultaneously.

Relevance to Modern Composite Manufacturing

FW remains a technically mature yet evolving manufacturing method with continued relevance to modern composite applications. Its ability to produce high fiber volume fraction and repeatable axisymmetric structures makes it suitable for pressure vessels, pipes, and other load-bearing components. Advances in machine control, material systems, and process optimization further expand its applicability to both thermoset and thermoplastic composites.

References

References

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.

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.

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.

Sizing or fibre sizing refers to a coating that is applied to fibre during manufacturing. Highly proprietary (formulation and process).

Sizing serves two functions:

• Protects and aids fibre during processing
• Aids in the bonding of fibre and matrix

Material and process (M), Shape (S), Tooling and consumables (T) & Equipment (E) - all have an interlinked effect on the Process step (P). (See MSTE factory ontology)

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.

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

A central processing theme in the manufacturing cycle. This theme is concerned with managing the thermal response of materials during storage and handling or parts/tools when they are subsequently heated.

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.

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, simulation refers to either:

• Process simulation (predicting manufacturing outcomes, or the inverse problem of predicting manufacturing parameters to achieve a desired outcome), or
• Performance simulation (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 "Modelling")

With regards to modelling & simulation, validation demonstrates the accuracy of a computational model — in the context of its intended use — to check that it represents the physics of the problem and the ‘reality of interest’ (needs are satisfied).

With regards to manufacturing, risk is the combination of the probability and consequences of undesirable manufacturing outcomes. Manufacturing risk can lead to technical issues, program/schedule delays, and cost overruns.

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

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