DIY planar using aluminized mylar

[Gustava]

Rob McKinley at ER Audio has this big roll of 12 micron aluminized mylar that he recommends for tweeters. Rob was kind enough to put a meter across the roll for me and measured 8 ohms across a one meter distance. So I ordered a bunch. I'm anxious to cut a piece 2cm wide and measure the resistance for possible use in a DIY ribbon.

But the real reason I ordered the mylar is that I've designed up a low/mid-range planar panel that I intend to put in a line array along side the ribbon.

Does anyone have experience etching aluminized mylar? There is a fair amount of info on the web about etching aluminum but nothing about aluminized mylar.

The problem areas I see are applying the initial photo-resist (spray on, I assume), UV exposure, development, doing the etching (how long?, which etchant? Will the mylar be weakened?) and then getting rid of the photo resist (what solvent?) without damaging the mylar or affecting the remaining aluminum which is very, very thin to begin with.

Any experience that can be shared would be appreciated.

--Steve

[MJ Dijkstra]

Hi,

You can etch with strong acids or bases, both will do the job as aluminium is amphoteric. Just be aware of the safety-issues when dealing with this kind of substances.
I did this a long time ago when metalised Mylar was the only source of Mylar I could get. I could not make any sharp boundaries as the acid did creep over the edge, but as sharp boundaries were not an issue anyway I didn't put any effort in improving on that. So you might do a better job than I did.
Theoretically these very strong acids/bases might attack the ester-bond in Mylar. (hydrolysis). Wether this weakens the Mylar in a significant degree I don't know for sure. I couldn't detect any failures but used the etched mylar for only a short time.

[Few]

I'd suggest you start with fairly dilute etchants. Even if the acid or base doesn't chemically bother the mylar, the high temperatures resulting from a fast exothermic reaction could leave your diaphragm badly crumpled. Back in "my youth" I tried to etch some aluminum that was glued to some mylar in order to fabricate a quasi-ribbon diaphragm. My first naive attempts sure etched the aluminum! The result was a shrunken and crumpled mass of plastic. If you slow the reaction you might not run into any problems.

Few

[Allen Wright]

If it's a tweeter panel, why do you want to etch it?

Apogee speakers use a single strip for the tweeters, as do Magnaplanar.

One long strip will have enough DC resistance to match amps fairly well I would think.

Regards, Allen

[Gustava]

Allen, I am planning on a single strip mylar for the 1.2m ribbon. The etching bit is for the planar mid-range. I will meander the circuit between six rows of neodymium magnets to keep 1m of circuit within a 0.3 Tesla field --a bit like the BG Neo8 but with more air movement to achieve a lower f0. I plan to stack six of these panels along side the ribbon in a dipole frame. I am hoping to get the planar response down to about 100Hz. My MathCad models look like I can get there but now comes the experimenting.

[Allen Wright]

Now it makes sense. I wish you success - I just use Apogee Scintillas at home, so I know what a good planer ribbon can sound like.

Regards, Allen

[DavidJanszen]

Not to be a pain, but 8 Ohms point-to-point across 1m on a continuous roll seems to me like it might be kind of high. Just guessing, but seems to me that the metalization might not be thick enough to carry the current. I would make a surface resistivity measurement and use that to figure whether you have what you need. Sheet resistance - Wikipedia, the free encyclopedia

[Spasticteapot]

If you're etching aluminum, a mixture of dilute ferric chloride and citric acid is supposed to be the ticket.

I'm very interested as to the results. Several companies in the mid-80s had great success with magneplanar headphones, and I've been wondering if it might be possible to reproduce them.

[Gustava]

Originally Posted by DavidJanszen

Not to be a pain, but 8 Ohms point-to-point across 1m on a continuous roll seems to me like it might be kind of high. Just guessing, but seems to me that the metalization might not be thick enough to carry the current. I would make a surface resistivity measurement and use that to figure whether you have what you need. Sheet resistance - Wikipedia, the free encyclopedia

David, thanks for the link on sheet resistance. I'll add the equations to my model and see how it turns out.

I am planning to try something a bit novel with the mid-range planars --an isobaric planar design where I have to membranes on each side of a center spacer and a sealed space between. That will give me twice the current handling even as it improves the sensitivity by placing each membrane into the higher flux area near the magnets. I don't know of anyone that's tried this before.

We'll see.

I found some really great information on etching aluminum on a website intended for the printing industry. NontoxicPrint.com | Nontoxic Printmaking
I intend to use spray-on photo-resist and standard PCB development techniques to prepare the membrane for etching. I still have to think through how I am going to keep a tension on the membrane through the photo, development and etching processes...

--Steve

[Steerpike]

Some photoresists use sodium hydroxide as a developer.
This will remove the aluminium - and quite fast too. (I've done it to get clear film from coloured gift-wrap)