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Acrylic Enclosure Using Laser Cutter and Strip Heater

Picture of Acrylic Enclosure Using Laser Cutter and Strip Heater
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Acrylic (Plexiglas(tm)) is great material for creating enclosures for electronic projects.  In this Instructables, I'll illustrate the steps in creating a 2-piece enclosure using laser-cut acrylic and a strip heater to make the bends.  In this case, the enclosure is for a cute vacuum-tube headphone amplifier (based on the NP-100V12 by Rogers Gomez--see http://diyaudioprojects.com/Solid/12AU7-IRF510-LM317-Headamp/).  But any time you want a nice, simple enclosure (optionally, transparent), this technique can be useful.

What you'll need:

Acrylic:  Polycarbonate ("Lexan") won't cut on the laser.  1/8" thick (often 3mm, around 0.118") works well.  Thicker will be more difficult to bend.  Fluorescent or smoke colors can add a nice touch--I got this fluorescent blue from Inventables.com:  https://www.inventables.com/technologies/fluorescent-blue-acrylic-sheet
Precise measurements of locations and diameters of knobs, jacks, ventilation holes, circuit board mounting holes, etc.
Form for bending (more later)
Laser cutter
Strip Heater

(If you live near Ann Arbor, MI, you'll find these tools and more at Maker Works, a makerspace.  Many makerspaces will have a strip heater if they have a laser cutter.)
 
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Step 1: Create your laser file

Picture of Create your laser file
You'll create the laser file based on what the acrylic pieces look like flattened.  Some things to consider:

Plan on a reasonable radius on bends.  I used 3/8" here for appearance.

Consider using slots instead of holes for mounting things to ease assembly.

Give yourself clearance around components that stick through the acrylic, especially threaded shafts (e.g., volume controls, jacks).

If you'll be using a form, you may wish to add an indexing hole or holes if you don't already have one naturally.  In the graphic above, this is the small hole in the center.

Remember to factor in the radius of the bends when planning the overall length of the flat stock.  I used a 3/8" bend radius, so the material is shorter than if you had a perfect 90-degree bend.

Measure your actual material thickness.

If you are using a graphics program, remember that for the Epilog laser drivers, cut lines must be thin ("hairline").

Add labels, part numbers, logos, or other engraved content.  If a piece is slightly non-symmetric (this one is--the PCB mounting holes are not the same distance from each edge), you might want to put a subtle indication on it.  I placed some labels and logos on the top of my enclosure, but didn't on the bottom.

Step 2: Cut on your laser

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As usual, leave the backing paper on the bottom of the acrylic when laser cutting to eliminate the smoke damage on the bottom you can get otherwise.

Once laser cut, remove any paper backing.  (Apparently you can leave plastic protective film in place for the bending operation, but I have never tried that.)

Step 3: Creating a Form

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In many cases you'll be able to get away without a form.  If eye-balling the bend radius and angle is fine, or you have a bending jig, then skip this step.

For a two-piece enclosure like this, getting the angles, bend radius, and spacing right was important--lots of parts need to stick through the front and back, and coordinate with the bottom piece.  Using forms means the spacing should be dead-on.

I made these forms out of Ash, expecting that I'd be using them quite a bit.  You could use MDF or softwood (e.g., 2x4) if you wanted.  The radii were created using a corner-rounding router bit on a router table for precision, but you could free-hand the radius on a sander.  Then some surface sanding, and a couple of holes for indexing the parts.  (You do have some holes you can use for indexing, right?)

For small holes, you can use a transfer punch or the shank of twist drills as an index pin.  For larger holes, try out your sockets from a socket set.  I found a 3/4" 12-point socket fit just right in the 1" index hole for the top piece.

Note that you'll want to make the forms wider and thicker than the acrylic, so the acrylic isn't hanging off an edge or protruding when you bend.

You may also want some clamps and/or weights.  (More on technique later.)

(Barely visible on the left-hand form you'll see I marked "front" and "back"--the top piece is not symmetric around the index hole (in this case, the hole for the vacuum tube).  If you have a choice, it is worth making things symmetric!

Step 4: Making the Bends

Picture of Making the Bends
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Acrylic is a thermoplastic--once heated, it becomes, well, plastic.  We'll use a strip heater to selectively heat a strip of the plastic to the point where it can be bent (or formed on our form).

To form acrylic, it needs to be in the area of 290-350 degrees F (depends on exact type).  It is important to get it hot enough because acrylic does not like stress--it will develop cracks and break.  Too much heat, though, and you can get bubbling (this is due to absorbed water), other surface degradation, too much pliability, and even combustion if you get it too hot.

You can find TAP's instructions on making your own strip heater elsewhere on Instructables, as well as their directions for using it.  Another great source of information are manufacturers themselves--for example, this document on forming Plexiglas:  http://www.plexiglas.com/literature/pdf/135.pdf  And of course the manufacturer of the strip heater will have suggestions for use.

In general, you'll place the acrylic piece on top of the heater (it is held above the heating element--acrylic should never be in contact with the raw heating element, though there are some inexpensive low-temperature heating strips that are placed in contact), heating up the line of acrylic to be bent.  For thinnish acrylic (1/8" and below), you can often just heat from the side that is the outside of the bend.  For thicker, you may need to flip the acrylic over from front to back periodically to heat the whole thickness up enough for the bend.

Periodically test the flexibility of the bending area until it is pliable enough to be easily bent or formed.  Just for fun I'll take the surface temperature, but I mainly go by how the plastic is reacting to bending.  The plastic will be hot (about 300 degrees F), so beware of burns!  Transfer quickly to the place where you'll bend or form the piece.

If bending by eye, bend to the desired angle (perhaps slightly past) and hold in place.  If you do not hold it in place until cool enough, it will tend to flatten.

(From what I've read, it sounds like it is a bad idea to actively cool the bend, for example with a wet sponge or something.  This would introduce stresses.)

If using a form, place the plastic on the form, insert any index pin(s) to get the positioning just right, fold the plastic around the bend, and hold or clamp in place until cool enough.  In the photo, the current bend is the one on the bottom of the form--the plastic I'm holding on the top was bent earlier and iis cool.  As with anything, if you need to create many of these bends accurately, you'll likely find that time spent creating forms, jigs, and clamps, is well worth it.

Note that you can use a combination of both an inside and outside clamp if needed.  For really tight bends it sounds like they'll be useful.

If you have multiple pieces to bend, do not place all of the pieces on the strip heater at the same time (even if they'll fit).  Put on one, then wait a minute or two before adding each additional piece.  This will give you time to take the piece off, make the bend, and let it cool before the next piece is ready.  Once you get going, you don't want to let pieces get overheated.

That's it--you have a precision-cut, acrylic enclosure!
Very informative Ible. The tube amp board looks great housed in the blue tinted acrylic enclosure.