JEC COMPOSITES MAGAZINE - Issue #36 - October 2007 - 63

Fig. 3: Quickstep QS20 machine showing tank arrangement material from the same batch, and mechanical properties equal or better to the baseline process, which had the benefit of many years of experience with this material. In a separate study with the same resin, using a unidirectional tape material, the glass transition temperature increased by 30ºC using Quickstep processing versus autoclave. While this would typically indicate a higher degree of cross-linking and, therefore, resin embrittlement, testing of the G1c fracture toughness by the double cantilever beam (DCB) method yielded significantly higher values for the Quickstep processed material [2]. A study undertaken by Kaiser et. al [3] shows that the panels made from Hexcel epoxy carbon-fibre prepreg material HexPly M18/1 with a resin content of 43% and a bidirectional [0°/90°], 4-harness satin-weave fabric G939 can be effectively cured using Quickstep technology. With this process, a time saving of 43% compared to the conventional autoclave cure cycle was observed. The monolithic panels manufactured in the autoclave and Quickstep processes provided similar values for the degree of cure and mechanical properties. Control of exothermic reactions Many of the resin systems have a danger of exothermic reaction due to accumulation of heat inside the laminate. This phenomenon is observed mostly in thicker laminates. The Quickstep process does not stop the resin from reacting exothermically; however, it does have a capacity to remove the heat generated by the reaction, by effectively transferring it into the HTF The HTF that moments . earlier had brought the resins to curing temperature, now acts as a massive heat sink with enormous thermal mass in comparison to the reaction heat generated, thus cooling the part to the temperature of the HTF This exotherm control is sometimes seen as a “blip” in . the temperature trace, where the probe inside the laminate records a rise in temperature above the HTF for a few minutes, then falls back to the HTF temperature as the heat of the reaction is absorbed (figure 5). As there is a layer of HTF 25-50 mm thick present across the part, this temperature can be held constant across the part, regardless of the levels of heat energy liberated from the exothermic reaction in thick or thin sections of the laminate. Fig. 4: Curing chamber for relatively flat components with vertical frame Unlike the consumables (e.g., pressurized nitrogen) in autoclave curing, heat energy can be accumulated efficiently over many hours, stored in the tanks, and used repeatedly for many cure cycles. The pumping and recycling of the HTF to the curing chambers can take place over minutes, thus allowing for rapid heat-up and cool-down of the part. With the high level of available heat energy, the crosslinking takes place rapidly and in a fraction of the time achievable within an autoclave. Industrial and performance advantages Due to the rapid heating and cooling characteristics and faster cure times, cycle times for the Quickstep process are notably shorter for current-generation prepreg materials. Savings of over 50% have been reported for a commonly used 175°C-curing aerospace prepreg, Hexcel 914 [1]. Significant savings are available to industry from reduced scrap rates for interrupted cure cycles, reduced tooling requirements, and increased flexibility in part manufacture. Since a part may be cured when lay-up is completed, the mould can be returned to the lay-up area sooner than is typical of batch processing, and lean manufacturing principles such as “flow processing” can be implemented. One study completed by a U.K. manufacturer on blocker-door production estimated a tooling saving of approximately 82%, due to a combination of reduced tool sets and lower costs of a Quickstep tool compared to an autoclave or oven tool [1]. As a result of such rapid curing, the properties of the final laminate can be somewhat different from those produced in traditional processes, and higher glass transition temperatures and material properties have been demonstrated for some materials. In the same UK study mentioned above, the test panels demonstrated a glass transition temperature 4% higher then the autoclave-cured Fig. 5: Control of exotherm during ramp-up of process temperature Tubular product manufacture For manufacturing tubular products, significantly shorter processing times and lower capital and tooling costs are essential. Current methods of tube manufacture include pultrusion, filament winding, and autoclave or oven-cured mandrel wrapping. The Quickstep process, with its characteristics for developing highquality, low-cost, and lightweight components using faster cure No36 October - November 2007 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #36 - October 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #36 - October 2007