Conventional wet installation of fasteners is both a labor intensive and time-consuming process. The long-term structural integrity of an aircraft depends on the quality and consistency of the installation of its fasteners, and manual wet installation brings into play a number of operator- induced variables. Both the volume of sealant applied and area covered can vary widely from fastener to fastener. As under-application of sealant threatens the strength of the bond, sealant is often over-applied. This necessitates time-consuming cleanup and waste disposal, and adds excess weight. Additionally, application of sealant to keep-out areas (such as fastener threads) can result in installation failures and costly rework. This issue affects the assembly of all sizes of aircraft, from small military helicopters to the largest wide body passenger jets. Air-framer facilities that produce military transports such as Boeing’s C-17 and wide-body jets like the Airbus A380 and Boeing 747 are equipped with robotic automated fastening systems; the leading cause of downtime in such systems is fouling caused by wet sealant contamination. QwikSeal® pre-sealed fastener technology is aimed primarily at original manufacture, but can be used at depot level repair facilities for high volume fastener installation
QwikSeal pre-sealed fastener technology is approaching full technical maturity, and the platform must be qualified to an appropriate standard before widespread use in aerospace applications can be realized. The main goals of this work were to develop a specification against which QwikSeal fasteners could be qualified, ensure that QwikSeal fasteners reliably met the performance criteria defined in the specification, and transition the technology to the aerospace manufacturing community. All tests were performed on PS 870 and PR 2001 as well as the subject sealant, and the data compared.
QwikSeal is a pre-sealed fastener technology developed by Systems & Materials Research Corporation (SMRC) along with their commercialization partner, PPG Aerospace – PRC-DeSoto International (PPG). QwikSeal consists of a fastener pre-coated with a one-part moisture curable sealant, and encapsulated with a rapidly curing moisture barrier. The sealant and moisture barrier are applied to the fasteners in a metered volume using an automated application machine. QwikSeal sealant is prepared by modification of a commercial two-part polythioether aerospace sealant with a moisture-sensitive cure-blocking agent. The barrier coat protects the sealant and allows easy handling for technicians. Upon installation, the barrier coat ruptures, and the sealant is exposed to ambient moisture and fully cures. Implementation of QwikSeal will result in tremendous savings in terms of labor hours as fastener installation time is expected to decrease dramatically in comparison to conventional wet installation. Additionally, QwikSeal significantly reduces the amount of waste generated in the form of unused sealant material and excess sealant waste removed after wet installation.
SMRC prepared 550 corrosion fasteners with QwikSeal sealant and both references sealants (PR 2001 & PS 870) divided evenly, as well as 150 puffer box fasteners and 120 SD-40 wing box fasteners using only QwikSeal sealant. All fasteners were barrier coated for comparison testing.
Corrosion panel installation went well aside from complications seen in only the PS 870 set of fasteners where the barrier coat cracked in the majority of samples and caused incomplete extrusion of the coating upon installation. It was discovered that PS 870 fasteners prepared earlier in the day saw less cracking than those prepared later. It was hypothesized that the differing chemistry seen in polysulfide sealants reacted with the chemistry of the barrier coat. The QwikSeal-coated fasteners saw no cracking in the barrier coat and installed as expected with 360° squeeze-out seen in the majority of fasteners. Sealant was allowed to cure for 30 days before being sent back to the Naval Air Systems Command for testing.
Puffer Box installations were performed manually using a speed/torque wrench combination. Cracking in the barrier coat was again seen in about 30% of this set of fasteners, although it did not affect the slower installation method. All fasteners installed as expected with 360° squeeze- out seen. The sealant was allowed to cure for 30 days before being sent back to the Air Force Research Laboratory for testing. The puffer box was filled with fuel, inverted, and pressured up to nine pounds per square inch (psi) over one week with no leaks observed in that time.
SD-40 Wing Box installations were performed using manual installation methods described in the Joint Test Protocol (JTP). Unfortunately, all of the fasteners showed cracking in the barrier coat to a more extensive degree than seen before. This is likely due to having to store the fasteners in -80 °C freezers for extended periods of time after preparation (1+ month), in addition to pressure changes during shipment from California. The fasteners were installed by a Lockheed mechanic who tested various installation methods in addition to the normal speed wrench technique. These methods included differing speeds of installation and tool type (wrench vs drill). Due to the extensive barrier coat cracking, installations took much longer than previously seen and pieces of the barrier coating had to be removed from the hole before installation could be completed. Even with these problems, installation of QwikSeal fasteners was faster than currently used methods and 360° squeeze-out was achieved. The sealant was allowed to cure for 30 days before fuel testing began. The wing box was filled with fuel (JP-5), inverted, and pressurized to 3.5 psi for four hours. After 20 minutes only one QwikSeal fastener was leaking and no change was seen over the full four hours. After pressure was removed the fastener was no longer leaking. The box was left inverted with fuel inside (no pressure) for one week with no change seen.
The cracking seen in the barrier coat of various fastener sets has never been encountered to this extent before the current testing. The degree of cracking seen in each set has led SMRC and PPG to believe that it was not caused by the same factors collectively. It is thought that the chemistry of the PS 870 sealant is a factor in the original cracking seen in the corrosion fasteners, but that temperature and pressure changes are the more likely cause in all other fastener sets. This is clear from the complete lack of cracking in all other corrosion fasteners that were not prepared using the PS 870 sealant with all other variables held constant.
SMRC and PPG are working with potential customers of QwikSeal to develop a business model compatible with the aerospace industry. Materials, equipment, and methodology exist to support high-volume production of QwikSeal fasteners.