Towpreg Proves Cost-competitive For Wound Pressure Vessels
Design group's new winding option streamlines new pressure vessel development.
By Barry Berenberg, Web Editor/Technical Writer | July 2003
Pressure vessels have long been manufactured by filament winding. Although they appear to be simple structures, pressure vessels are among the most difficult to design. Applications include breathing devices, such as self-contained breathing apparatuses (SCBAs) used by firefighters and other emergency personnel, SCUBA tanks for divers, oxygen cylinders for medical and aircraft uses, fuel storage for alternative fuel vehicles, fire extinguishers and aviation cylinders for emergency slide inflation, opening of doors or lowering of landing gear, etc. Composites are even used for paintball gas cylinders.
Source: HyPerComp
This fireman's SCBA, designed by HyPerComp and made by Draeger, features a 6061 aluminum liner fully overwrapped with pre-impregnated T700 carbon fiber tow and finished with a thin layer of E-glass for abrasion protection and clear gel coat for a smooth, easily cleaned surface.
Pressure vessel design requires a careful balance between multiple technical requirements and government specifications. Each application has different requirements for valves, bosses, support fittings, volume, stored material and operating conditions (temperature extremes and exposure to chemicals or other materials). Since the highly pressurized containers are often located near people, safety also is a key design factor. Standards established by various government agencies in the United States, Canada, Europe and Asia cover allowable materials, qualification testing and retesting requirements.
HyPerComp Engineering Inc. (HEI, Brigham City, Utah, U.S.A.) provides design, manufacturing and testing services for filament wrapped, high-pressure composite cylinders to manufacturers who are seeking to develop new, low-cost designs. In business since 1996, HEI has worked with both established manufacturers and helped with the start up of new filament winding factories in the U.S., England, Japan, China and South Korea.
Cylinders types
Cylinders are classified into one of four types: Type 1: all-metal construction; Type 2: metal lined, with fiberglass hoop wrap; Type 3: metal lined, with composite full wrap; and Type 4: plastic lined, with composite full wrap.
Type 1, 2 and 3 commercial cylinders are approved for life support, aviation, and the other applications mentioned earlier. Type 4 cylinders have only recently been developed and are not in widespread use. All-metal cylinders are the heaviest and have the longest history of use, but composite cylinders are generally chosen over all-metal cylinders when weight is of importance. Adding the fiberglass overwrap for the Type 2 cylinder reduces the weight but increases the cost. Type 2 cylinders are simpler and thus less expensive than Type 3 cylinders, but the full wrap is the lightest design. Type 3 cylinders can be wound with either carbon fiber or fiberglass overwraps (aramid and other specialty fiber overwraps also are produced). Carbon cylinders are more expensive than glass, but they provide higher performance, including lower weight, improved environmental resistance, and longer lifetime. Glass cylinders have a maximum lifetime of 15 years (by regulation), whereas carbon cylinders can be tested further to extend their life span up to 30 years.
Wet winding or towpreg?
Commercial composite cylinders have traditionally been made by wet winding. HEI designs for all processes, but has found that winding with towpreg offers some real advantages. Because the towpreg supplier controls resin content, the need for measuring and mixing is eliminated and the possibility for contamination is minimized — producing parts of consistent, high quality. Wet winding has a fairly high scrap rate, primarily related to these resin issues. Production throughput increases with towpreg because much less setup and cleanup time is required, and the winding speed is no longer limited by the fiber wetout requirements. The tacky prepreg resin protects the fibers as well, so winders can be run at higher speeds without the risk of fraying damage. Finally, curing is much simpler, because towpreg parts do not require a cosmetic gel coat, and they do not need to be rotated during cure to prevent sagging or running of excess resin that can occur with wet winding.
When comparing pot life and shelf life, however, the trade-off is less clear. Raw fibers and resins practically have an indefinite shelf life at room temperature, but once mixed, the pot life of a resin can be measured in hours. Most towpregs, on the other hand, have a limited shelf life and require cold storage, but can sit out for days before being used in the winding process. Materials like Alliant Techsystems Inc.'s (ATK, Edina, Minn., U.S.A.) TCR and Newport Adhesives and Composites Inc.'s (Irvine, Calif., U.S.A.) water-dispersed prepreg tow have an extended shelf life at room temperature, making their storage more competitive with wet systems. And whereas wet winding can use almost any fiber and resin combination, material selection is more limited for towpreg.
The biggest perceived difference is cost. Wet winding materials cost about $17.8/kg to $22.2/kg or $8/lb to $10/lb (resin accounts for 20 to 25 percent of the cost), and towpreg costs about $31.1/kg ($14/lb). Many manufacturers don't get past that initial cost difference. According to Thomas Hannum, director of operations for HEI, a full cost comparison takes into account the increased production throughput, lower scrap rates, and decreased setup and cleanup times. Related to the improved quality, HEI also has found that towpreg systems deliver 90 percent or more of fiber strand tensile strength, compared with 70 to 85 percent for wet wind systems, and the finished parts exhibit a smaller variation in properties. The net result is that less material can be used for towpreg vessels of equal performance.
Considering all of these factors, towpreg winding is cost-competitive with wet winding. "When starting with a new manufacturer facility, HEI has been 100 percent successful in making the case for towpreg," says Hannum. "For established manufacturers, however, the cost to requalify their existing wet-wound cylinders may be prohibitive." There also would be some cost to retrofit existing equipment, though it would be small in comparison to requalification costs. In HEI's experience, "No company has made a switch from wet winding to towpreg winding of pressure vessels," says Hannum.
