Old Technology, New Enhancements

The process of manufacturing composites by winding fiber filaments over a rotating mandrel dates back to the 1940s, and after decades of upgrades and improvement the technique is still in wide use today. From golf club shafts and boat paddles to pressure vessels and aerospace structures, filament winding remains a preferred manufacturing method for hollow, cylindrical or oval-shaped composite products.

The material formulations in filament wound products like tanks and pipes make them strong and lightweight. “In a hand lay-up process, the resin reinforcement is about 65% resin and 35% reinforcement. But in filament winding it flips, so you wind up having a much higher degree of reinforcement versus resin,” says Kevin Lambrych, manager of Alta Performance Materials’ Corrosion Science Center (CSC) in Ohio and manager of corrosion and fire-retardant resins PD and TS. “That allows you to make a lighter, stronger laminate. You can lay up a lot of reinforcement in a short period of time for a very strong tank or pipe wall.”

Preventing Corrosive Breaches

At the CSC, Alta partners with fabricators, engineering firms and end users to ensure the products they need, like scrubbers, tanks and pipes, can withstand corrosive materials, such as acids, bases and other aggressive chemicals. “We also help companies optimize their fabrication techniques. With filament winding, that means achieving good, effective cures so they can lay up thicker windings and improve their production efficiency,” says Lambrych.

Creating a corrosion-resistant tank begins with the hand lay-up of a corrosion barrier on a mandrel. The barrier, which provides most of the corrosion protection, consists of surfacing veils (100 to 200 mils thick) with a backing of chopped strand glass and resin. The first 10 to 20 mils are 90% resin, while the balance is chopped strand glass with 65% resin. The discontinuous chopped strand glass prevents chemical attack from moving easily from fiber to fiber.

For each application, Alta recommends the veil type (C-glass, synthetic polyester or carbon), number of veils, type of glass and thickness of the chopped strand glass mat.

“We try to recommend both the materials with the best performance and best ease of processing for the fabricator,” Lambrych says.

The next step is the filament winding of the structure, which should be made with the same resin as the corrosion barrier in case there’s a breach. Alta produces Derakane and Derakane Signia epoxy vinyl ester systems, which are designed with high resistance to oxidizing acids, solvents and caustics.

For customers working with new chemical environments, Alta will test coupons of various material combinations in reactor kettles to determine the loss of strength, flex modulus and hardness that occur over time. The goal of testing is to identify the best material combination for the application’s environment.

One reason for the continued popularity of filament winding is flexibility of construction; filament wound storage tanks can be made on a worksite. “If you can get the mandrel to the location, and you ship the reinforcement and the resin on trucks, you can then essentially set up a fabrication shop right there,” says Lambrych. “Tanks up to 60 feet have been made onsite.”

One example is the El Boleo Project in Baja California, Mexico, which used onsite fabrication to make 40-foot diameter corrosion-resistant tanks used in the extraction of copper, cobalt, zinc and manganese from ore. Other tanks are manufactured offsite and shipped to the location, like the corrosion-resistant filament wound tanks being used in a project that is shortening the time it takes to extract lithium from the water in geothermal power plants.

“As we come up with new technologies like these, filament winding is going to continue to be a pretty important piece of the composites manufacturing industry,” says Lambrych.

Stronger Replacement Poles

As aging wood utility poles continue to fail during hurricanes, windstorms, fires and other natural disasters, some power and telecommunications companies are hardening their infrastructure by strategically replacing wood poles with stronger and more resilient FRP composite poles.

Composites manufacturers can use pultrusion or filament winding to build FRP poles. While both methods deliver strong, lightweight, resilient structures, each has its advantages. Because pultrusion is a continuous process – making poles of constant cross section with uniform strength – this manufacturing method is good for large-scale, economical production of standard profile poles.

Resilient Structures, which was founded in 1995, uses the filament winding process, which enables production of modular, tapered poles with continuous axial and circumferential reinforcement where necessary.

“A utility pole is structurally efficient when it is designed as a tapered column,” says Scott Holmes, Resilient Structures’ chief technical officer. “Although the top of a pole can be loaded with equipment and wires and gets buffeted by the wind, the highest stresses are going to be near the bottom of the pole.” Tapered poles offer better load distribution and aerodynamic efficiency.

Manufacturers of filament wound poles look for ways to improve the efficiency of their fabrication processes. “If you’re building a very large structure layer by layer it’s really critical that you get a lot of material down quickly. You’ve got to minimize the time that you spend winding as much as possible,” Holmes says.

Increased optimization and control of the winding process has helped. “We can program the machine to put fibers at a particular angle and to optimize the number of layers or circuits to cover the entire part,” he says. “But even with computer control, and all the latest software improvements, you still need an operator there because you’re dealing with a live material, taking fiberglass and impregnating it with resin. The operators know what they’re looking at and can make adjustments as needed to ensure a consistent process.”

Resilient Structures uses a proprietary polyurethane resin for its poles. The formulation is environmentally friendly – without the use of solvents – and has excellent structural properties and built-in UV resistance.

Holmes says changes are on the horizon for manufacturing of filament wound poles. Today most companies use structural testing to validate how they perform. But Holmes says more companies are beginning to use finite element analysis (FEA) of winding patterns to optimize the laminates.

“FEA enables a stress analysis on a complex composite structure, because even a tapered pole is not a simple structure to analyze. Using these programs, you can say, ‘I want a winding pattern to look like this, and analyze it to come up with a more optimized laminate design,’” he adds. In the future, such analyses will likely use a digital twin of a pole and the associated manufacturing process to reduce structural testing and further optimize the pole.

A Hybrid Model

Filament winding is a fast, cost-effective way to manufacture many products, but it is limited in the reinforcement capabilities, materials and the shapes it can produce. Addcomposites, based in Finland, has developed a hybrid automated fiber placement/filament winding system that could overcome some of those restrictions.

In 2019, a year after its founding, the company introduced its compact AFP-XS system, which is designed to convert existing robotic arms into automated fiber placement (AFP) systems. “As we evolved and understood industrial robotics a bit more, we realized that the AFP system platform we built can be used to do filament winding as well,” says Pravin Luthada, company co-founder and CEO.

The AFP-XS system features interchangeable compaction mechanisms that can support AFP’s localized pressure application or filament winding’s continuous tension control. Dual-mode also supports the thermal management modules to enable in-situ consolidation of thermoplastics or controlled thermoset winding. Changeover from one process to another is fast; the system can be programmed to stop at any point to make the switch.

Luthada says there are several benefits to this hybrid system. Up to now, filament winding has primarily been done with thermoset resins, in part because traditional filament winders are not built to handle in-situ consolidation for thermoplastics. The head on the AFP-XS, however, can be fitted with a heat source that would allow that curing. Having the ability to build composite components with recyclable thermoplastics is an environmental advantage in today’s market.

Combining AFP capabilities with filament winding also enables better placement of reinforcements at critical places within a structure. “With filament winding, you have to follow a geodesic path, but with AFP you have no such limitation. You can follow any path on the surface as long as it’s allowed by the fiber, and can place reinforcing patches where you need them,” says Luthada. That reduces the use of materials in areas that don’t need reinforcement, ultimately cutting the weight of the composite part.

The hybrid system makes it possible to produce more complex shapes in a single manufacturing process as well. Filament winding can be used only for convex shapes, while AFP can handle both convex and concave surfaces.

One application for this hybrid process is missile structures and carriers for them. “These are slightly complex windings because there are dips, and there are certain sections where extra fibers are needed so that you can attach a handle or some sort of holding mechanism,” says Luthada. “You could make them with this system, and they would be extremely light.”

The AFP-XS system was deployed by a European defense manufacturer for the construction of a rectangular pipe. In another demonstration project, a manufacturer produced a next-generation hydrogen tank with a 41% weight reduction. The tank included a 15-layer filament wound CFRP shell, localized AFP reinforcement and an integrated thermoplastic liner.

With technological advances like these, decades-old filament winding production is likely to be around for many more years.

Mary Lou Jay is a freelance writer based in Timonium, Md. Email comments to mljay@comcast.net.

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