Anelok: the art of window-making

Werner Almesberger werner at
Sat Jan 31 13:59:03 UTC 2015

I promised more details on keeping the paint out of the OLED window.
This is the journey so far:

The masking piece (not masterpiece)

Exhilarated with how smoothly the CAD-to-CNC process worked in the end
for cases, I decided to also try this to solve another problem: to
keep paint from the display area. For this, I made a little piece of
acrylic the size of the OLED panel, complete with a raised base for
extra strength, and a nice little knob to handle it.

The piece snugly fit into the bezel, so it would be exactly where the
OLED will later be. Here it is, already with paint applied:

The next picture, with the piece removed, illustrates the two things
that were wrong with this idea:

First, some pain merrily crept under it. But worst, the resulting
window is the wrong size ! There's a lot of panel around the OLED's
active area, and it doesn't look pretty enough for us to want to
put it on display:

Ewww. What was I thinking ?

The whole idea relied of course on the ability to use the bezel as
a mechanical guide, so I couldn't just make the piece a bit smaller.
Back to the drawing board.

Masking the masking liquid (or trying to)

So I sought the help of an old ally, the masking fluid. If I could
make a mask for that, it would in turn keep paint from obscuring the

One issue with this is that the mask now has to have an opening. A
mill can cut perfectly pointy corners if there's more "outside" than
"inside", but if this isn't the case, the corner will be rounded due
to the mill having a non-zero diameter.

So I couldn't use the sturdy 1/8" (3.175 mm) endmill I chop wood and
acrylic with, but had to use something small, e.g., the 35 mil / 0.89
mm bit I cut PCBs with. Alas, this one is also quite short and it
would take forever to grind down a block of acrylic with it. But it
does cut PCBs really well, so this is what I used.

Here we have the case top from the previous experiment, now about to
receive the paint that's been missing:

Fits nicely. On with the masking liquid:

A deep, reassuring blue. Looks as if it's gonna work ! After letting
it dry for a while, I took off the mask. This is what I found:

Even though the masking fluid is quite viscous, it clearly had no
difficulty at all getting under the mask.

With the help of a straight piece of PCB and a knife I then manually
trimmed the masking fluid. This didn't look too bad:

And the result was quite acceptable:

Note that a little paint still crept into the window in the upper
right corner.

Okay, that did not go as smoothly as planned but it worked in the end.
I had a second case to make, and I decided to try to do better this
time, e.g., by trying to apply more pressure on the mask.

Masking the masking liquid (trying again)

The previous case looked a bit austere. So I thought of also making a
window for the slider, e.g., to paint over later in a different color.
I designed another masking piece, milled it, held down the masks
firmly with a toothpick-like stick, and applied the liquid:

A deep, reassuring blue. This time it certainly must have worked.
Let's see what it looks like from the other side:

Oh no, not again ! Ah well, more cutting to to then. But let's be
smarter this time and cut the masking fluid with the mask still in
place, so that the PCB can act as guide.

Well, to make a long story short, the cutting was so ineffective that
the entire layer of fluid merrily detached from the case - in both
cases !

All I had to show for my troubles were the ugly knife marks.

Going dry

Having had it with liquids, I decided to return to good honest masking
tape. Since the area is quite small and not easily accessible, I didn't
use painter's tape from the hardware store but something specifically
made for small stuff.

To have an indication of where to place the tape, I drew the outline of
the case and the position of the windows, and placed this under the

After painting, the inside looked good:

Now, let's check the outside:

Argh ! Not the creepiness again !

This is what the case looked like after removing the tape:

I then removed the extra paint with the help of thinner. This is not
very precise and dulls the plastic a little, but the overall result
is tolerable:

There are of course still the ugly knife marks, but there isn't much
I can do about them. They also don't stick out in real life as much
as they do on this picture. (And neither does the OLED ever look so
blue. This is from the photo where the supply voltage was too low
for proper operation.)


While I prevailed in the end, the process requires way too much
improvising for my taste.

Maybe another possibility would be to make an acrylic piece the size
of the display's visible area, plus spacers to maintain the correct
distance from the bezel. To facilitate removal, the spacers would
have to be split. I suppose I could use a small vise / clamp to put
more pressure on the window mask as well.

By the way, what's the general opinion on having a window in the
slider area as well ? I was thinking of painting it in some light

- Werner

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