Filament Winding - The String and the Glue

The Process 

                What is filament winding?  Filament winding has been compared to "wrapping a whole bunch of string around a spool and then taking the spool out later.” That's a fairly simple analogy, but its close to the mark.  The ‘spool’ essentially is the internal part, referred to as the mandrel that forms the shape of the filament wound structure.  The ‘string’ is the reinforcing fiber that is systematically wound around the mandrel until it totally covers the surface area to the depth desired by the designer.  In order to keep the string in place, the fiber reinforcement is saturated with the ‘glue,’ or resin, which eventually cures and binds the fibers in place.

                 The filament winding process is very simple in concept, although the winding equipment and software control systems have changed significantly over the past 15 years.  Advances in winding software control systems have advanced considerably so that rather exotic parts such as 90° pipe elbows, ‘T’ pipe sections, paddles, and square cross-sections may now be fabricated.  In the early days, filament winding equipment concentrated on simple 2-axis machines ¾ a rotating part mandrel and a fiber delivery system that wound fiber as it traveled back and forth along the rotating axis.  This is comparable to a simple but effective lathe operation.  These simple 2-axis machines are still widely used to fabricate cylindrical tanks, pipe, pole, and tubular products.  Software and machine design advances added other axes of motion for more sophisticated part geometry. It’s not unusual to hear of 3-, 4-, 5-, 6-, and 7-axes machine capability.  Without going into an in-depth discussion, these additional axes typically add fiber delivery head motion to the radial direction (axis-3), vertical direction (axis-4), and localized fiber delivery ‘eye’ rotation (axes-5, -6, -7).

                The objective is the same, regardless of the available winding machine axes of motion: to be able to deliver resin-impregnated reinforcing fiber to the rotating mandrel shape in a controlled fashion the same way, part after part after part!  Filament winding utilizes continuous fibers so that structural performance is notably higher.  In fact, the process depends upon the fiber to be continuous because resin wet out, layer compaction, and void reduction all rely very heavily upon the ability to pull on fiber to create tension.  Chopped or discontinuous fibers do not allow this to happen.  If the string gets cut, or breaks, it must be spliced or tied together.

                 Because the filament winding process is automated, design and manufacturing efficiency are easily obtainable with this process.  Fiber lay-down rates, a measure of the processing speed, is dependent upon the part complexity and the market segment.  Simple parts such as tubular structures often are wound at fiber lay-down rates in the 300 to 3000 lbs/hr range.  Complex parts, those that require machine complexity over 4-axes of motion, and very high performance/high fiber volume aerospace products, often are fabricated in the 10 to 200 lbs/hr range.

 Materials of Fabrication

                 Filament winding requires continuous fiber reinforcement and a resin system to bind things together.  There are many types of materials that can be used in this process.  The choice is often a decision made jointly by the design engineer and the process, or manufacturing engineer.  Cost, structural performance requirements, environmental resistance, corrosion resistance, and regulatory issues all play an important part in this decision.

Fibers (The ‘String’)

                 The string, or continuous fiber reinforcement, provides the structural performance required of the final part.  The fiber is the primary contributor to the stiffness and strength of the composite.  The dominant commercially available fibers are: E-glass, S-glass, aramid (like DuPont's familiar KEVLAR^®), and carbon/graphite systems.  To summarize these systems:

• E-Glass is good strength (400-500 ksi), low modulus (10.5 msi), lowest cost fiber, available in many forms, widely used in commercial and industrial products, most-used in filament winding;

• S-Glass has improved strength (625-665 ksi), higher modulus (12.6 ksi), higher cost fiber, used in aerospace and high performance pressure vessel applications;

• Aramid is good strength (450-550 ksi), higher modulus (11.5-27.0 msi), higher cost fiber, very low density (one-half of glass fiber), excellent impact and damage tolerance properties, poor compression and shear strength; and

• Carbon/Graphite has a wide strength range (270-1050 ksi), highest modulus (33-120 msi), highest fiber cost, intermediate density (two-thirds of glass fiber), poor impact or damage tolerance, best tensile strength and stiffness properties;

Resins (The ‘Glue’)

                 The glue, or the resin matrix that holds everything together, provides the load transfer mechanism between the fibers that are wound onto the structure.  In addition to binding the composite structure together, the resin matrix serves to provide the corrosion resistance, protects the fibers from external damage, and contribute to the overall composite toughness from surface impacts, cuts, abrasion, and rough handling.  Resin systems come in a variety of chemical families, each designed to provide certain structural performance, cost, environmental, and/or environmental resistance.  (Note: Only the thermoset family is covered in this article.)  A few major resin matrix families of interest to filament winders are:

• General Purpose Polyester is classified as orthophthalic polyesters, lowest cost systems, widely used in FRP industry, moderate strength and corrosion resistance, room temperature cure;

• Improved Polyester is classified as isophthalic polyesters, slightly higher cost, good strength and corrosion resistance, widely used in FRP corrosion applications, room temperature cure;

• Vinyl Ester is a chemical combination of epoxy and polyester technology, excellent corrosion resistance, higher cost, excellent strength and toughness properties, widely used as corrosion liner in FRP products;

• Bisphenol-A Fumarate, Chlorendic are more exotic systems for improved corrosion resistance in harsh environments, higher cost resins, higher temperature capability, sees application in paper and pulp industry applications;

• Phenolic possesses excellent flammability properties (eg. flame retardance, low smoke emission), higher cost systems, lower elongation, moderate strength, applications involve fire resistant systems structures; and

• Epoxy has wide range of resins available, best strength properties, usually heat-cure required, good chemical resistance, higher viscosity systems, higher material cost, applications across broad market segment range.

Fiber-Resin Forms (Combining the Glue and String) 

                Combining the reinforcing fiber with the binding resin matrix essentially takes one of two paths before it’s placed onto the forming mandrel.  Thermoset resins are most often used in a liquid resin form termed “wet winding.”  In this process the dry fiber ‘rovings’ or `tows', as they are called, are passed through a resin tank just prior to encountering the mandrel.  The wet resin saturates the fiber bundle while in the resin tank.  As it leaves the resin tank, excess resin is squeegeed off to a predetermined resin/fiber content level specified by the manufacturer.

                 An alternate approach, gaining some degree of popularity with epoxy systems, is called prepreg winding, towpreg winding, or prepreg tow winding.  The resin supplier provides the roving or tow fiber in spools that already have a carefully controlled resin content saturated throughout the fiber.  The vendor does this by passing the dry fiber through a resin tank at the vendor's facility.  After wetting out the fiber roving or tow, the resin is partially cured (called B-staging) with heat and respooled for delivery to the filament winding manufacturer.

                 These two processes obviously have their distinct pros and cons.  The key differences in some of the economic, processing, and performance differences for these two methods are shown in Table 1.  The predominant method within the current filament winding industry is by wet winding.

 Table 1.  Comparison of Various Properties of Resin-Fiber Forms Available 

 Filament Winding Applications

          Filament winding has many commercial, industrial, and aerospace applications because of the ability to wind exotic shapes in a very carefully controlled and automated fashion.  There have always been two major rules in filament winding:

                 Rule Number 1: "If it’s round, it can be wound".  Consequently, many applications in the various market segments utilize filament winding to fabricate parts that are symmetrical and round in cross-section.  Table 2 shows a partial listing of typical applications and the resin families most often used to fabricate them.  Storage tanks, golf shafts, poles, shafts, pipe, and tubing lend themselves very nicely to the first rule because of their simple geometry.

                 Rule Number 2: "If it’s not round, it still can be wound".  This contradiction of Rule Number 1 simply says that, with a little innovation, a combination of filament winding with other composite processing techniques (like external compression molding or resin transfer molding) may accomplish a filament wound part regardless of shape complexity.  Some examples are kayak paddles, rectangular or square cross-section tubing or booms, and triangular parts.  Innovation in achieving the final resin-fiber compaction is they key to obtaining success with Rule Number 2.

                 It is fairly obvious in looking over Table 2 that most of the structures that are filament wound are typically round and derive their structural performance from the carefully oriented fiber directions that can be achieved with this process.  This, coupled with the ability to provide consistently high quality products in a fiber volume range from 40 to 65 percent.  For E-glass parts, this fiber volume range can be expressed in terms of a fiber weight percent that might be more meaningful to those working in the FRP industry.  For the same filament winding process, the weight percent of fiber in this situation ranges from about 60 to 85 weight percent.  Except for high performance resin transfer molding (RTM) and similar liquid molding (LM) processes, filament winding is the only process that can fabricate to such high fiber volume levels.

 Table 2.  General Comparison of Thermoset Filament Winding in Several Industries

                 The corrosion industry, the paper and pulp industry, and the oilfield industry, are the dominant users of products fabricated by filament winding.  The majority of applications in these industries utilize an E-glass fiber for the reinforcing string and either the polyester or epoxy resin family as the glue.  These industries represent over 90 percent of the market volume.

 Summary

                 The filament winding process represents a well-developed technology that is currently being used in many composite industry applications.  The process is capable of `laying down' structural, continuous fiber at rates of over 2000 lbs per hour in carefully controlled, automated processing.  We have briefly discussed in this article the typical fiber and resin families that are used in the filament winding process.  The choice of materials for a particular product depends more upon the economics, the environmental resistance, the weight limitations, and the strength performance requirements than limitations upon the filament winding process itself.  Filament winding, while somewhat sensitive to the materials selected, has been demonstrated with such non-conventional materials as cement, rubbery elastomers, optical fibers, steel wire, yarn, and a variety of other `string' and `glue' materials.  Innovation is the limit.

 Additional Reading and References: 

                Beckwith, Scott W. and Hyland, Craig R., Filament Winding Course: Principles, Methods, and Applications, Beckwith Technology Group Short Course, January 1997. 

                Beckwith, Scott W. et al, Filament Winding in Composites Manufacturing, Advanced Composites Seminars Clinic, January-February 1992-1996 (annual event). 

                Harper, Charles E., Handbook of Plastics, Elastomers, and Composites, 3rd Edition, Society of Plastics Engineers, 1997. 

                Lubin, George (editor), Handbook of Composites, Technomic Publishing Company, 1982. 

                Peters, Stan T. et al, Filament Winding Composite Structure Fabrication, Society for the Advancement of Materials and Process Engineering (SAMPE), 1991. 

                Schwartz, Mel M., Composite Materials, Volume II: Processing, Fabrication, and Applications, Prentice-Hall, Inc. 1997. 

                Strong, A. Brent, Fundamentals of Composites Manufacturing: Materials, Methods, and Applications, Society of Manufacturing Engineers (SME), 1991. 

Acknowledgments:

                 The author wishes to thank the Composites Machines Company, Delta Fiberglass, Engineering Technology Inc. (EnTec), McClean-Anderson, and Spencer Composites Corporation for their assistance in providing both information and illustrations for this article.