I have a chilling unit that was overly loud for the space in which it lives, so I decided I needed to make a sound deadening box. Since I was making a box, I wanted it to be a little more interesting than just a box. In order to reduce the alignment issues with making a box out of six sides, I decided to make four of the sides one piece, and have them fold-up into place with living hinges. Technically two of the hinges only get used once, during assembly, but the final one lives on in use.

There are two main concepts to this instructable: sound deadening enclosures, and living hinges.

Step 1: Sound Deadening: the Tortuous Path

Sound deadening requires forcing sound waves to pass through a tortuous
path from their source to the outside, such that they are damped away before making it to the outside. You could just build a fully sealed box so no sound could escape, but often times the equipment that makes noise also requires ventilation. A note: the walls of the box may still vibrate, sending out their own noise.The goal here is not to completely remove the sound, but to reduce it significantly. The tortuous path allows air flow, but kills off the annoying short frequency sound waves.

Step 2: Living Hinges

The living hinges were created by cutting HDPE sheet on a CNC router with a 90 degree chamfer tool. The thickness of material left behind was about 0.015". In order to achieve this thickness I faced down the bed of the cnc router. I took some test cuts (shown in the second image) to find a reasonable thickness of the hinge. You'll note that chamfer tool I used was of large diameter 1.25" and had some thickness at the point. This results in a small gap at the corners.

The goal of a living hinge is to make it as thin as possible while maintaining enough strength for the application. The reason for a thin hinge is that you want to minimize the stress during bending. Stress is due to strain (streching/compressing). When the hinge bends the inside compresses and the outside stretches, between the middle of them there is a point of zero strain, this is the neutral axis. The thicker the hinge, the further the outer edges are from this neutral axis, and the more the outer edges strain (stretch/compress). The larger the strain, the larger the stress, and the more likely the hinge is to crack or fail.

Step 3: CNC Routing the Box

After doing some sample cuts I cut the actual box. Here you can see it is folded up and holding shape decently.

Step 4: Machining the Self-aligning Slot.

The chiller had a hose coming out to utilize the chilled working fluid and I needed to make an exit that also sealed up around the hose. I utilized an angle cutter to cut a V-groove such that the closing piece (let's call it the seal door) would seal up nicely.

On the CNC router I first roughed out the slot with an endmill. I then programmed the angle cutter routine to cut midway through the thickness of the sheet material. This resulted in a nice v-groove visible in the first image.

On a vertical knee milling machine I took the seal door I had cut on the router and I then cut the male portion of the v-groove. I used the same angle cutter but came down and cut on either side of the midway line. Image two shows the cutting process.

Finally, testing the fit in figures three and four we can see that the v-groove male and female components managed to align nearly seamlessly.

Step 5: Assemble the Box.

Finally the box was assembled, the internal supports were installed, and the entire system had vibration damping foam glued into place. Initially I though a very high bond tape might work to adhering the damping foam to the HDPE, but that was not the case. Instead I used some standard hot-glue to stick the foam to the hdpe. Finally, the box is held closed with 90 degree latches.