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Hello everyone!
On a Cross Cockpit Collimanated Display System, what would happen to the Mylar Mirror if the vaccum is left un-operational for a long period of time say more than 1 Month, will it damage the Mirror?
Also does anyone have some kind of document which specifies the storage and the optimal operational conditions for MYLAR, such as temperature and humidity ?
Regards
Pretty sure you're overthinking this whole thing. What happens to your shoes if you don't wear them for a month? They get dusty, that's about it. As long as you don't leave it in direct sunlight in the middle of a desert, it should be fine. Not 100% sure but the useful life of the kinds of coatings that mylar mirrors have is generally 20 years or so at which point they lose their reflectivity (not sure whether its due to oxidation or because the material itself starts to break up, ruining the smoothness of the surface... either way it's oxidation that causes the problems). In terms of optimal conditions, you can probably find the specs for mylar on the internet i.e. what the operation temeperature is for the material. Other than that, obviously when it's hotter, you'll need less vacuum pressure than when it's cold etc etc.
The mylar won't be damaged by being left in the relaxed state.
I haven't noticed a significant difference in required vacuum between hot days and cold days, but then I haven't measured that directly. The main issue Gene and I have had with temperature is that the adhesive we used gets soft when it gets hot, letting the mylar move on the frame.
Lessons learned:
- Use the thinnest mylar you can get your hands on - it will require less vacuum to draw down, so will put less shear stress on the adhesive.
- give yourself plenty of edge margin. We've got 3" all around, and it's barely adequate. The pull-away happened in one spot where the edge is 2-3/4".
- Keep things cool. Hot adhesive is weak adhesive.
If it does get dusty, be very careful when cleaning it. the aluminum coating scratches very easily. Draw it down just far enough to take out the slack, then use an air duster to blow off the loose dust. if it's still dusty, use plain water in a sprayer, soak up the runoff as it runs to the bottom of the mirror, then blow it dry. DON'T wipe it.
Mylar itself is very tough stuff. Back in the day, mylar was used as a drafting film and for developing circuit board art work because of its durability and dimensional stability.
If chip packets are anything to go by, should be strong enough to easily withstand wiping, not sure though as I haven't ever touched the stuff but the manufacturing process is the same as far as I know.
A chip bag is probably about 2 mil or thicker. Unless you have a really strong vacuum and insanely strong adhesive, you'll need much thinner mylar. We're using 0.5 mil.
While the mylar itself is tough, the aluminum coating that makes it reflective is VERY easily scratched. Even a soft cotton cloth will leave visible marks if you aren't very gentle. Moreso, if the mylar is drawn down to proper shape, it is very close to its yield stress. At this point, it doesn't take much force to put a permanent 'dent' in the mirror.
Take my word for this; I speak from experience.
Point taken.
If you sneeze at 1mil wrong, you'll mar it. Even a dampened paper towel can leave scratches in it. The one thing I've found that works well is a Swiffer dry duster. It doesn't mar the Mylar, but you need to be damn careful that the plastic handle never comes in contact with the Mylar because that WILL mar it.
g.
I just had a thought about how to keep the noise down; use the shop vac to drop the pressure down to where you want it, then switch to one double-acting cylinder or two single-acting cylinders hooked up to one or two high-torque motor(s)/linear actuator(s) to maintain the vacuum. If you use a large cylinder, you won't need a lot of speed, just torque.
DOH! Why didn't it occur to me before? YOU DON'T NEED MYLAR! Alsa Corp. makes amazing chrome paints, Mirrachrome and Killerchrome. You could make a rough mirror form in sections, add an inch or so of plaster (or w/e), have a CNC rig mill each section down to the exact shape, then finish and paint the assembled sections.
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Excellent suggestions from Gene and Wayne ...... i always listed to the experienced ones !!
Thanks. I was searching the same information since many months. Thanks for your hard work.
Well the reason mylar is used is because it is a pretty much perfect surface when stretched tight. Paint, chrome or not, is only as perfect as the surface it is applied to and more often deteriorates in visual quality exponentially depending on the method used to put it on a surface. Not to mention it is more easier to create the correct spherical section by using mylar stretched into a concave shape than to "roughly" create said shape. Remember that the shape is very important and takes a boatload of calculations to determine otherwise you'll end up having to play pin the tail on the donkey (or move the mylar mirror around to find a location where pilots are not looking at the wall and ceiling behind the cockpit).
Trust me, we have three ginormous spherical sections with plaster surfaces in front of our FNPT device, currently acting as support for a rather large piece of mylar that was cut out of our Level D FFS during its yearly mirror change. The workers manage to drop it when moving it to the FNPT room so its essentially useless in the long term but it will be good practice for applying it to the mylar frame and showing the bosses that it can be done, thus freeing up funds to purchase a new roll of the stuff for the actual application.