The use of fluids to activate and control chemical reactions is not new. It has been applied in the production of chemicals and controlling chemical reactions in industry for generations. This is because fluids contain thousands of times the heat energy per volume of a gas and therefore the effective heat transfer rate is so much higher.
QUICKSTEP HEAT TRANSFER FLUID (HTF)
With the Quickstep Process the HTF is stored in three separate tanks, a hot tank, medium tank and cold tank. Heat energy can be efficiently accumulated and stored in the tanks and used over and over again for many cure cycles unlike gases, such as nitrogen, which is a costly consumable in autoclave curing. Heat is “topped up” to maintain a constant temperature in the two heated tanks, as heat is transferred into the curing of parts.
The pumping and recycling of the HTF to the curing chambers can take place over minutes thus allowing for rapid heat and cool down of the part being cured.
During the curing cycle , the HTF is pumped through the curing chambers to maintain the temperature required for the curing process. Once the part is cured, and the reaction is complete, the HTF is returned to the appropriate storage tank. In this way, the bulk of the heat energy is not lost as it is contained in the HTF, which is returned to the tanks.
RAPID SETTLEMENT
Vibration through the HTF appears to improve performance in many areas. It provides improved fibre compaction, enhanced mechanical performance by aligning the fibres and imbedding them more effectively within the resin matrix. The vibration of the HTF disperses any air bubbles present through the laminate. The use of vibration also appears to have an annealing effect on the laminate contributing to minimizing the residual stresses left in the part. This may also in part explain the lack of 'spring-back' noted to date. The mechanical data acquisition and development program is continuing and further work on the type of vibration and its impact on the process is being undertaken.
RAPID CURING
As noted above the Quickstep Process uses the fluids to transfer heat rapidly from fluid storage tanks to the curing chamber and back to the tanks with the part "floating" in the mould. This approach results in a self-limiting/controlling and rapid curing/cooling system whereby the part can be heated rapidly without fear of uncontrolled exotherm regardless of the rate of ramp up.² The part is then held at the optimum curing temperature for the cure to be completed before being rapidly cooled to room temperature.
The curing process can progress rapidly as there is substantial energy available to fluidize the resin and excite the molecules for them to cross-link and cure. As there is excess energy available for curing, curing takes place very rapidly in a tightly controlled time frame, regardless of whether the laminate is thick or thin.
This is considered to be a result of the rapid oscillation, agitation, and vibration of the resin molecules as a consequence of the high available heat energy. With the high level of available heat energy the cross-linking takes place rapidly and in less time than that normally associated with autoclave curing processes. The final cure result can be markedly different from those of an autoclave and higher glass transition temperatures have been demonstrated on a number of aerospace resins.
SUPERIOR PERFORMANCE
Based on testing to date, the rapid curing combined with vibration can result in superior performance in terms of peel strength, glass transition and mechanical toughness.
Since liquefaction of the resin is rapid and vibration compacts the parts, the pressure applied to the part need only be 1 to 4 psi. Vacuum is required in almost all applications. The resin in the entire laminate is melted within 1-5 minutes, dependent upon the pre-preg selected and size of part. It is believed full air release between all plies occurs before the resin in the outside plies reaches gelation which in turn has resulted in low void content in the parts produced to date. Multiple step curing cycles can be more precisely executed due to the exceptional thermal control offered by Quickstep.
With complex, multiphase toughened resin systems and pre-pregs, the activation of the reaction process can be controlled precisely. This allows the reactive sites to be placed evenly throughout the resin matrix to produce small, fine molecular structures with even dispersal of the toughening agents.
- 1. Water for example contains 2,500 times the energy of air per volume and in practice results in 25 times the heat transfer rate.
- 2. This is subject to the thickness of the laminate and the type of material and resin in the laminate. 1" thick glass has already been trialed without exotherm.