Material test: aluminized mylar

I'm going to try some small scale mock-ups of paraboloid mirrors over the next few weeks.  First up: aluminized mylar, a thin plastic sheet with an aluminum coating.

I knew that there were some pretty serious disadvantages to using this material, and those turned out to be even more serious than I'd thought.  The only real advantage is cost.  I was able to make my handheld concave mirror for less than $5, and this video by Dan Rojas demonstrates that it's possible to build a concave mirror more than a meter in diameter for less than $25. His method involves stretching the mylar across a frame then applying suction to make it concave.

Caution: do not try this at home.  Fire hazard, blinding hazard.  This isn't just boilerplate. The blinding hazard turned out to be quite a bit worse than I'd expected, and I had thought I was already being a bit paranoid.  I would strongly recommend against copying the techniques I've used here; what I was doing was quick and crude, intended only to get a sense of how the material behaves.
Aluminized mylar is easy to obtain.  It's what camping emergency blankets are made out of.  For this exercise I just wanted a small, handheld mirror.  I used a microwave food cover as the frame.

I cut the mylar to shape, then stretched is slightly with pieces of tape on opposite sides.  I made it semi-airtight in a quick and dirty way by running tape around the rim.  That gave me a flat mirror.  I made the mirror concave by applying suction to one of the holes on the back of the cover and plugging it.  The initial result looked rather impressive: a smooth concave surface from rim to rim.  Not bad for a few minutes work.I regret that I don't have a picture of the completed mirror from the day of the test.  I hadn't expected it to degrade as quickly as it did.

Mirror shape was a problem.  It was not perfectly paraboloid, hence the focus was not particularly sharp.  I get a sense that it would be impossible to get the curvature as precise as I'd like.

Nevertheless, the focus was sharp enough to be a bit unsettling.  It took me a second to clue in to what I should have known already: staring at an object sitting at the mirror's focus is about as safe as staring directly at the sun.  Sunglasses are not adequate protection, not by half.  I have some proper eye protection on order, more on that when it arrives.  I'm putting off any mirror tests until then.

Durability was very poor, even worse than I expected.  These pictures taken just a couple of days later show a lot of sag when relaxed and a lot of crinkling around the edges when under suction.  That's partly due to the tape losing its grip (I'd been a bit slapdash) and partly due to the mylar itself stretching.  Repeatedly stretching and relaxing the mylar by changing the amount of suction was not good for its health.

I conclude that aluminized mylar is great if you want to build a great big Archimedean death ray and don't mind that it will likely be non-functional by next weekend.  If you need pinpoint focus and the ability to replicate your results aluminized mylar is not a great choice.

Next up: bespoke 3D printed mirror frames.  Given my unusually busy schedule and parts orders that have yet to come in design work on this project will be on hold for a few weeks.

EDIT Circa July 19

I did a quick retest of the mylar mirror just to see how effective my welding mask would be.  (answer: very.  light at the focal point was extremely bright to the naked eye, quite dim seen through the mask)

New pictures:

Flat, with a reflected image of a painting on my wall:

Concave under suction:

The right way to do a stretched mylar mirror (described here) is to stretch the plastic over something like a rubber O-ring before applying suction.  That way you don't get stretch marks seen around the perimeter of the mirror due to nonuniform anchoring.  I don't think it's worth my bothering with an improved O-ring version of this technique.  I'm going with a 3-D printed plastic substrate in subsequent tests.