Thanks for correcting me, seems I had left my common sense and experience in bed this morning.
|Quoting Northwest727 (Reply 5):|
why is the Cirrus then cured at a lower temperature? Is there a higher cost involved with curing at higher temperatures?
Cost is almost certainly it. If you're curing much above normal room temperature you need an oven. You can make a curing room that will go to high temperatures when no one is inside but that would be inefficient as hell. Aircraft parts are often chunky, awkward shaped, or both so for production you generally need a big oven (especially to cure many parts in a batch). A 120ºC (248ºF) oven is about the hottest that can be made on the cheap (i.e. timber frame with plenty of insulation foam), but you also need heating elements, fans inside to move the air around, thermocouples to measure the surface temperatures of your parts in as many places as possible, and some sort of control system (as there is a maximum rate that you can ramp up or down the temperature without risking losing bond strength or warping the parts). In addition you need some form of vacuum pump as you bag your part and its mould, before removing the air so that the atmospheric pressure around it acts to consolidate the part. All this can be done for a few thousand dollars. This is the type of setup I use on a day to day basis making aircraft parts, albeit for a prototype.
More expensive ovens can go to higher temperatures, but where cost is less of a problem you may go for an autoclave, which is basically an oven combined with a pressure vessel. As you suck the air out of your vacuum bag in an autoclave, you can have the pressure of several atmospheres acting on your part improving the compression between layers of fibre as you set the resin thus guaranteeing a more consistently strong bond. They cost a lot to build, run, and maintain mind.
Electricity will be the big cost in serial production, you have to maintain the temperature for a long while (the lower the cure temperature the longer you need to hold it) to set the resin. The hotter you go the less effective your oven's insulation will be so you may see something approaching an exponential rise in electricity required to hold increasing temperatures. Therefore running the oven overnight for a 15 hours (an arbitrary example, this will change from one resin brand & type to another) at 70ºC (176ºF) may cost less than a 120ºC (248ºF) 90 minute cure. Also don't forget the limit in ramp up or down rates for your resin that must not be exceeded, it takes far less time to get from ambient to the low temperature cure and vice versa than it does to get to the high temperature, and this time must be added to the time at full temperature required to run the cure.
Basically it all comes down to cost, at some point they asked what value it would add to the aircraft to let people paint it any colour they wanted or to leave it outside in the hottest place on earth, and came to the conclusion that this did not sufficiently offset the cost of producing a structure that could take the associated maximum temperatures. Airlines place a big value on painting their aircraft distinctively to mark their bran and to fly anywhere any time, private owners and syndicates generally see it as an interesting benefit, but not worth massive additional cost.