Making a Plastic Enclosure for a Lightweight System with Heavy Sound

Innovation is a tricky thing; there are not many ideas that haven’t been tried. Even so, creative shops have to try new things to differentiate themselves in a crowd of everyday shops. AVC in Stillwater, Oklahoma is one of those innovators. AVC’s talented crew has over 50 years of combined experience, three having spent many years at Kicker and Q-logic.

When a local automotive enthusiast went to AVC looking to have a complete system installed in his 2007 Corvette Z06 he had only one limitation: the entire system, speakers, amps, subs and enclosure had to be under 110 lbs. Since the Z06 is a high-speed vehicle, the owner didn’t want it slowed down by a heavy system. His first Z06, a 2006 model, was built by Kicker, and it weighed in at over 200 pounds, that equals a loss of over 30 horsepower (the Z06 has an average of 1 horsepower per 6.5 pounds), which is significant for a speed freak. So this system required a serious diet weighing half as much, while using the same equipment. Since the equipment weight would remain unchanged, the weight had to be taken from the enclosure and amp rack. This is where a truly unique and seldom seen process takes place.

In order to build a lightweight yet strong enclosure, the decision was made to build it from plastic. They used a process called thermoforming. That’s when a plastic sheet is heated to over 400 degrees and draped over a mold built from MDF, 2-part foam and Duraglass; then vacuum is applied to pull the plastic tight over the mold, creating a solid, hollow plastic tub, which is made into the enclosure.

While the process is quite simple, there is an art to the science. Things such as draft, negative returns and plastic thinning must be taken into consideration. In order to get box that wouldn’t buzz and flex, the plastic used in this enclosure is .325”, that’s over 5\16” thick. Along with using thick plastic, the design itself must have strengthening characteristics such as ribs, opposing angles and rounded corners, as well as few flat areas as possible.

The building of the mold poses a few problems as well. The overall dimensions of the finished enclosure will be over 1/4” larger all around, so the tool must be smaller than the actual part. Along with that, the plastic has a natural rollover on the corners, so the design must incorporate that detail.

Since the enclosure was too large for the thermoforming machine at AVC, Kicker offered the use of their machine. Special thanks to Joe Gross and John Cronin for their assistance in this project.

While the plastic has a nice grain pattern that could be left as is, AVC chose to have the entire enclosure sanded smooth and painted, then finished with some custom airbrushing to complete the installation. Once all was said and done, the system weighed in at a svelte 106 pounds. Not bad considering the amps alone weigh 22 pounds each!

1. The mold for the enclosure begins with an MDF base. The basic shapes are cut out and glue in place with CA (Super Glue) glue. The vertical ribs require screws so the tool doesn’t break in the forming process.

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

3.  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.

4.  The foam we used for this project is 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 means the foam will withstand more pressure, which is important when thermoforming. The foam is mixed in a 1:1 ratio, then whipped with a stir stick until it changes to a caramel brown color.

5.  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 10-14 times its original mass, overfilling is a possibility.

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

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

8.  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.

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

10.  The final finish on the enclosure mold with 120-grit sanding.

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

12.  To achieve proper vacuum, the surface of the tool is drilled with a 3\32” drill bit. The corners and recesses need to be drilled every 4-5 inches.

13.  The base of the tool is also drilled, but with larger bits. Notice the edges have a 1.5” wide strip leaving the center recessed. This facilitates vacuum distribution, making for a uniform part.

14.  The tool is bolted to the platen. The platen is a base with solid bottom and a 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 tool slides into the oven and heats the plastic to 400 degrees.

15.  The plastic is .325” thick; this ensures a solid, uniform thickness and a sturdy enclosure.

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

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

18.  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.

19.  With the emclosure trimmed, the interior surface is layered a Scosche Accumat and the woofer holes cut and reinforced with 1/2” MDF for holding power.

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

21.  The enclosure and amp rack are set in place in the trunk. The tub has been sanded smooth and ready for paint.

22.  The enclosure was painted at Toby Ramsey, while Trevor Meridith laid out the airbrush work.

23.  With the enclosure back from paint, the 3 8” L7 woofers are mounted in the enclosure. The grilles were painted to match the rest of the install. The enclosure only weighs 10 pounds, not compared to tool which weighed over 65 pounds.