Custom Lightweight Subwoofer Box - Plastic Fantastic - How-To

The mold for the enclosure begins with an MDF base. The basic shapes are cut out and glued in place with superglue. The vertical ribs require screws so the tool doesn't break in the forming process.

To create the bubbletop shape that mimics the roofline, the center of the front baffle board was trimmed using a flush-trim bit on a router.

With all the basic shapes taken care of, the sides were taped up for the 2-part expanding foam. The tape is overlapped each half-width, ensuring the foam doesn't leak.

We used Por-A-Foam from Freeman Manufacturing and Supply. This is higher-density foam at 8 pounds, compared to most other foams, which are 3 or 4 pounds. This foam will withstand more pressure when thermoforming.

The foam is poured into the cavity formed by the masking tape. It only takes a few minutes for the foam to fully expand and cure. The foam expands 10 to 14 times its original mass, so overfilling is a possibility.

The foam rises above the boundary of the enclosure. This excess gets trimmed off later.

With the tape removed, the excess foam is trimmed with a knife or coping saw.

The sides are sanded smooth using a DA sander. While a power sander isn't necessary, it makes things go faster than hand sanding. The final finish will be hand sanded to get exact contours.

The foam is strong enough to endure the forming process, but, for longevity, the entire mold is coated with Duraglas. Note how the top of the tool has been coated and smoothed.

Jason Carson lays down the final finish on the enclosure mold with a 120-grit sanding.

To ease the release of the mold from the hot plastic, a barrier coat of Slip Plate spray-on graphite is applied. This coat needs to cure for at least 24 hours before parts can be pulled from the tool.

To achieve proper vacuum, the surface of the mold is drilled with a 3/32" drill bit. The corners and recesses need to be drilled every 4" to 5".

The base of the mold is also drilled, but with larger bits. Notice the edges have a 1 1/2" wide strip, leaving the center recessed. This facilitates vacuum distribution, making for a uniform part.

The mold is bolted to the platen, a base with a solid bottom and perforated top: this lets the vacuum cover the entire base. The platen is clamped to the thermoforming machine, which is basically a giant oven. The rack above the mold slides into the oven and heats the plastic to 400F.

The plastic is 0.325" thick; this ensures a solid, uniform thickness and a sturdy enclosure.

The hot plastic is then draped over the mold and the vacuum sucks the air out, leaving the plastic tight against the mold. Once the plastic cools, the mold is removed.

The plastic enclosure needs to be trimmed from the large sheet. A bandsaw or jigsaw makes quick work of the plastic.

This is the dangerous part. A shaper table with a saw blade attachment is used to trim the tub. The plastic has a natural roll at the bottom that must be removed.

With the enclosure trimmed, the interior surface is layered with Scosche Accumat and the woofer holes are cut and reinforced with 1/2" MDF for holding power.

A 3/4" base was added to seal the enclosure and complete the build. The base was brad-nailed and securely sealed with silicone.

The enclosure and amprack are set in place in the trunk. The tub has been sanded smooth and is ready for paint.

Toby Ramsey painted the enclosure, while Trevor Meridith laid out the airbrush work.

With the enclosure back from paint, the three Kicker S8L7 8" woofers are mounted. The grilles were painted to match the rest of the install. The enclosure only weighs 10 pounds, not bad when compared to the mold, which weighed over 65 pounds.