By Lauri Turunen
Exel Composites
In straight tubes, any force applied to the cylindrical wall causes hoop stress. Tool handles — a mop handle, for example — experience hoop stress when being used actively or roughly handled. Telescopic pole applications, where you must lock tubes by squeezing the surface with locks, creating hoop stress to the tube, are one example of applications that require high hoop strength. To ensure composite tubes can meet these challenges, the profiles must be manufactured with cross-aligned fibers, providing extra stiffness and high mechanical strength in the transverse direction.
In this article, we compare different manufacturing techniques and explore how the continuous manufacturing technology of pull winding can achieve high hoop strength.
The orientation of fibers in a composite profile has a significant impact on the profile’s mechanical properties. A typical alignment is unidirectional, where reinforcements are aligned in the longitudinal direction of the profile and provide good axial strength. Cross-wound reinforcements improve transverse stiffness and strength. Fabrics also are used and typically provide alignments at 45- or 90-degree angles. Popular manufacturing techniques like filament winding and using pre-impregnated (prepreg) materials can facilitate this.
Prepreg and filament winding
Both processes allow manufacturers to precisely control the fiber-to-resin ratio and ply thickness, but often require longer, energy-intensive curing processes that involve heating or cooling steps. This, combined with the high number of manual steps involved in the filament winding process, means filament winding is better suited for manufacturing thick tubes with large diameters.
In addition to the technical considerations, expense also is a factor. Because both filament winding and prepreg rolling involve move manual processes than continuous manufacturing techniques, such as pultrusion and pull winding, they often have high labor and resource costs and can take longer to produce the equivalent composite profiles.
This is leading many industries to turn to other, more cost-effective processes for manufacturing composite profiles with high hoop strength.
Considering pull-winding for high hoop strength
Like pultrusion, pull winding involves pulling fibers through a resin bath and heated die to be cured into the desired profile. The main difference, however, is that some of the fibers are helically wound around a mandrel in the transverse direction before being pulled through the heated die. While not dissimilar from filament winding, pull winding allows for greater control over fiber placement and tension, resulting in more uniform and predictable hoop strength.
Pull winding is particularly useful for manufacturing thin-walled composites with high hoop strength because it allows for precise, thin transverse fiber-layer control. This is achieved by combining the unidirectional alignment of fibers with helical winding, resulting in the ability to control fibers from 0 to almost 90 degrees. Furthermore, pull winding is highly automated and repeatable, ensuring high-volume, cost-competitive production across a single, in-line manufacturing process.
Lauri Turunen is product business owner for tubes, tool handles and telescoping poles and pull-winding expert at Exel Composites.
Exel Composites, headquartered in Finland, specializes in continuous manufacturing techniques, like pull winding, and combines this expertise with a forward-thinking, collaborative approach. Exel will discuss options and possibilities with customers throughout the development process, ensuring product design and development are optimized for each given customer’s specific requirements. This allows the company to work with you to develop the optimal high hoop strength composite tube for your application, giving you a solution that is lightweight and cost-effective, without sacrificing mechanical performance.
Find out more about Exel Composites’ pull winding process on the website at https://exelcomposites.com/guide-to-composites/our-manufacturing-processes/.
