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How Synthetic Strings Are Made

By Greg Raven

Synthetic strings for racquet sports come in all shapes, sizes, configurations, and colors. String construction involves three basic steps:

1. Design;
2. Manufacturing; and
3. Packaging.

However, the manufacturing step itself can be broken down further into five key steps:

1. Selecting the raw materials and other ingredients;
2. Creating the filaments from which strings are made;
3. Multifilament construction;
4. Coating and sizing (in the case of multifilaments); and
5. Post-processing, if any.

Here’s a closer look at the basic five-step manufacturing phase.

Raw materials

All synthetic strings are made from polymers, which are essentially long chains of monomers. Monomers are small, single molecules. Bonding these single (mono-) molecules into long-chain (poly-) molecules is called polymerization.

For “nylon” strings, manufacturers typically use polymers comprised of amide functional groups such as aliphatic polyamides (including Nylon 6 and Nylon 66), and polymers comprised of ketone functional groups such as Zyex (AKA polyetheretherketone, or PEEK). The amide family of polymers can also be blended, mixing Nylon 6 with Nylon 66 to make copolymer Nylon 6/66, for example. “Synthetic gut” is another name for nylon strings, although it can be misleading when used as a generic term because no synthetic string is constructed using the same manufacturing techniques as natural gut, let alone the raw materials.

For “poly” strings, manufacturers typically use polyesters comprised of ester functional groups— such as polyethylene terephthalate (AKA PET polyester), co-PET, and thermoplastic polyester elastomers (TPE). There are also “poly” strings made from polyolefins, which are polymers such as polyethylenes and polypropylenes, comprised of alkene functional groups, and which have different properties (including molecular weight) compared to polyesters. As with the ingredients for “nylon” strings, these ingredients can also be blended, as would be the case when mixing PET polyester with thermoplastic polyester elastomers.

The words “nylon” and “poly” are in quotes because these are not so much descriptions of strings as they are broad classes of filaments. For example, the aramid used in bulletproof vests and in some extreme-duty strings is also in the polyamide family along with nylon, although it is an aromatic polyamide. If that’s not confusing enough, there are ultra-high molecular weight polyethylenes that have mechanical properties similar to aramid despite being in a different polymer family.

String manufacturers have two options when it comes to sourcing ingredients for their products: They can buy ready-made filaments from companies such as DuPont, or they can make their own filaments from raw materials. In either case, manufacturers choose ingredients (either ready-made filaments or raw materials) to meet design goals that include strength, elongation, flexibility, abrasion resistance, friction properties, stress relaxation (tension maintenance), elasticity, resiliency, and color.

Ready-made filaments arrive at the factory on five-pound bobbins. As filaments, they have already gone through the extrusion and drawing stage (described in the next section), and go directly to the construction stage if the end product is a multifilament. Ready-made filaments are available both as single filaments and as bundles of filaments called yarns. When making monofilaments, the ready-made filament can be packaged more or less as-is, or it can undergo a further drawing. Manufacturers use additional drawing stages to fine-tune the characteristics of the string in-house, and it allows them to create different final products from one “base” string.

Raw materials arrive at the factory as pellets or beads (AKA “chips”) in 100-pound bags and look something like grains of rice. These pellets go to the extrusion and drawing stage, along with additives in the form of powders and liquids used for coloring the string and giving it different properties.

Creation: Extrusion and Drawing

Making filaments from raw materials is a complex process using costly machinery, but it gives the manufacturer a lot of control over the end product. The machine that makes the filaments is called an extruder, and it consists of a hopper, a heated barrel, an extrusion screw, an extrusion die, and a water bath.

The pellets and additives are poured into a hopper. Depending on the process, the materials also can be modified before being added to the hopper, as Luxilon does with its Big Banger line of strings. The ingredients are blended together in the hopper, and then fed at a controlled rate into the barrel section that typically lies beneath the hopper. The extrusion screw moves the raw materials through various zones inside the barrel. At the hopper end is the feed zone, where the extrusion screw has deep channels to pull in the mixture. The channels in the extrusion screw gradually become smaller along its length, so that they are shallower where they carry the mixture through the heating or “melt” zone. Melting the pellets alters the characteristics of the polymer so it flows more easily, although the temperature must be controlled to prevent irreversible degradation of the basic polymer structure. In the final section, the metering zone, the channels become shallower still, which pressurizes the mixture, allowing for a controlled feed of the material through the extrusion die.

The extrusion die is a plate located at the end of the barrel that has at least one hole through which the extrusion screw forces the molten materials. Of course, there can be several holes to allow the manufacture of multiple strands at the same time. The holes themselves can be cylindrical as would be the case for standard strings, or they can have a polygonal, oval, or other cross-section to influence the final shape of the extruded filament. This is where shaped strings — such as the octagonal Babolat Pro Hurricane Tour and RPM Blast, and the 16-pointed Poly Star Turbo — begin, but even the flat ribbons used in Head’s RIP strings are extruded as flat sheets and then cut to width. The holes in the die are larger than the finished size of the filaments that are being made. Ultra-fine filaments are called microfilaments.

Filaments can also be co-extruded, where the extrusion machine has two or more barrels and extrusion screws delivering different molten raw materials to different ports within the same filament position in the extrusion die. By extruding different materials together within the same filament, unique performance characteristics can be achieved.

In the case of Gamma TNT2 Tour nylon string, the center core filament is a co-extrusion that surrounds a high elasticity nylon in six central sections of the filament with a high modulus nylon material, to provide a combination of elasticity and resiliency that can not be achieved by blending the two materials together or by another construction method.

In the case of the Gamma Zo Tour polyester string, a softer, more elastic polyester is co-extruded within a stiffer, harder polyester to create a polyester monofilament string that is elastic and still has good abrasion resistance and tension maintenance.

As the extruded filaments exit the die, they immediately enter a water bath for cooling, which solidifies them enough to make them easy to handle. It would be nice if the long polymer chains were all stretched out and aligned in a neat laminar flow throughout the string, but what is more likely is that they are kinked, coiled, and folded every which way.

At this point, the filaments have most of their shape but only a rough size. To complete the process, the filaments move to the drawing machine. The drawing machine can either be in-line with the extruder as shown in the illustration, or the raw filaments can be taken up on spools for later drawing or for transport to a drawing machine at another location.

The drawing machine is a series of sets of rollers. There can be any number of sets of rollers, but typically there are three to five sets. The rollers control the movement and temperature of the filaments. Between sets of rollers is the drawing zone, so if there are four sets of rollers, there are three drawing zones. Each successive set of rollers in the drawing machine rotates slightly faster than the previous one. The speed differential between the sets of rollers progressively stretches and thins the filaments. The more differential there is between any two sets of rollers, the higher the “draw ratio,” and the more the filaments will be stretched and thinned.

The repeated cycles of heating, stretching, and cooling in the drawing stage align and compact the polymer chains, making the string harder (more abrasion-resistant) and stiffer, but also increasing its strength and ability to hold tension.

Even so, the string is still pliable at this stage, which allows the manufacturer to apply a lasting twist, or to run the string between forming “gears” to give it latitudinal texture (as seen in Luxilon Alu Power Rough, for example). Twisting the strings can be done either to give the string a different final shape, or to introduce additional springiness into the filament.

As important as the initial raw materials selection is, the drawing stage is just as important, as the draw ratio, heating, and cooling of the filaments each have tremendous influence on the final product, as does the time over which the filaments are in the drawing stage.

At the end of the drawing machine, the filaments will have their shape, size, and twist. The finished filaments come off the final set of rollers onto large spools. Monofilament strings are ready for printing, cutting, packaging, and shipping at this point, unless the manufacturer opts to apply an outer coating or lubrication to the string.

Filaments that were created to be part of a multifilament string are transferred from the spools to smaller bobbins that can be mounted into the machines used to combine the filaments. The tension on the filaments is closely calibrated as differences in tension when winding the bobbins can produce different results in the finished string.

Multifilament construction

Multifilament strings use two or more filaments. During the construction process, these filaments are wound, twisted, braided, woven, or wrapped together, and bonded to each other.

Bonding is accomplished either by using solvent to partially dissolve the outer surfaces of the filaments so they will stick to each other and fuse as the solvent evaporates, or by bathing the filaments in a two-part resin that holds the filaments in place after it cures. Microfilaments are so thin that solvent would cause their deterioration, so they are bonded — typically with polyurethane (as popularized by Tecnifibre), although other materials are sometimes used (as in Wilson Sensation).

As twisting the filaments increases springiness, manufacturers can change the flexibility and elasticity of the string by varying the angle at which they wind the filaments together. Higher angles produce tighter twists, which creates more flexible and elastic string, while lower angles produce looser twists and firmer, stiffer string.

As you might guess, there are few limits on how filaments can be combined, so that filaments of different materials can be configured in the same string. The manufacturer can also combine various sizes of filament and microfilaments, both in the core and in the wraps. Thicker filaments in the wraps can even give the finished string a stronger surface texture. One limit is the speed at which the string is made: Some machines make as few as three 40-foot sets of string per hour. This means that manufacturers must have multiples of any given machine to keep up with demand.

Coating and sizing

Multifilament strings can be coated in an extruder that is similar to the extruder that makes the filaments in the first place, but for coating, the raw string runs through the center of an extrusion die, which applies the melted coating material around it. The coated string then runs through a funnel-type device to ensure uniformity of the coating, and to set the final size (or gauge) of the string.

In this stage, the manufacturer can choose the coating material based on the characteristics it desires in the final string, whether harder for better abrasion resistance and durability, or softer to increase the friction between the strings.

Post-processing

For many strings, post-processing involves little more than printing identifying information to the string with a special ink-jet printer and applying a lubricant (of which Tecnifibre’s SPL is one example) to the string to ease handling during installation.

Perhaps the most involved post-processing occurs with Gamma strings, which undergo bombardment by gamma radiation to split the long chain polymer molecules so they connect to one another at the molecular level through covalent bonds. According to Gamma, the shorter molecular chains are able to move more freely relative to one another, making the material more elastic, while the additional bonds between the molecules aide in returning the molecules to their original positions after the material is stretched and released. However, as the alignment of the molecular chains and compaction of the molecular chains are unaffected by this process, the strength and hardness of the material remain the same, with no reduction in durability.

There is one additional step that could be considered post-processing: Testing. Manufacturers continuously test filaments and strings during production. Various visual examinations, tensile tests, dynamic tests, and tests for tension maintenance are done with samples taken out of the ongoing production. When multiple machines are involved, samples must be tested from each machine. On finished strings, dynamic testing starts off in the lab with the hammer, cannon, and repetitive impact tests, and some manufacturers use high-speed cameras to check spin and stringbed deformation during impact. Static tests employ a dynamometer to check linear, loop, and knot resistance. At this point, the finish string is ready for play testing.

After post-processing, the finished strings are ready for cutting, packaging, and shipping.

When you consider all the ingredients and combinations of ingredients, and all the processing and variations in processing, you can see how different one model of string can be from another. If you love your current string, you might have a deeper appreciation for all the factors that had to be just right for that string to be perfect for you. If you are looking to change strings, you can take hope that no matter what power, spin, control, stiffness, comfort, or durability you need from your string, there is quite possibly a string out there for you.

Of course, whether you are looking for a new string for yourself or for a customer, USRSA tools such as the String Selector on-line, String Specifications on-line, and String Selector Maps in RSI magazine can help.

Thanks to Ashaway, Babolat, Gamma, Head, Luxilon, Pacific, Poly Star, and Wilson for supplying background information for this article.

See all articles by Greg Raven

About the Author

Greg Raven  is an associate editor for Tennis Industry magazine and technical writer. He is certified as a Master Racquet Technician by the U.S. Racquet Stringers Association. He can be reached via e-mail at greg@usrsa.com, or through Facebook, LinkedIn, and Twitter. He plays tennis three to five days a week, and is turning into an avid cyclist.

From the March 2011 issue
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