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Thermoplastics 101

Understanding molded enclosures for commercial loudspeakers.

Thermoplastics 101

Jan 1, 2006 12:00 PM,
By Bruce Borgerson

Understanding molded enclosures for commercial loudspeakers.

As molded cabinet loudspeakers proliferate throughout the marketplace, the lexicon of the AV systems integrator has expanded—perhaps reluctantly—ever deeper into the vocabulary of chemical engineering. Roughly a dozen different terms describing molded enclosures can be found on loudspeaker cut sheets, although unfortunately not all of these descriptions are particularly informative. This article will attempt to cast some much-needed light on both the processes and the materials employed in the manufacture of molded enclosures for commercial loudspeakers.

Generic Terminology

A few manufacturers have labeled their loudspeaker cabinets as made from “polymer” or “thermoplastic.” The second is somewhat more specific than the first, though neither is much more elucidating than the generic term, “plastic.” Essentially, all molded loudspeaker cabinets use polymers of one kind or another, and all except some types of composites use one of the many types of thermoplastics. What differentiates the various enclosures are the exact type of thermoplastic material used and the process used to form the material into its final shape. These two factors determine the key attributes of weight, rigidity, and ductility (ability to form detail), as well as resistance to impact and environmental conditions such as heat, sunlight, and corrosive chemicals. They also determine manufacturing cost, which is reflected in the end cost to the user.

Injection Molding

The overwhelming majority of small and mid-sized molded loudspeaker cabinets are formed by injection molding. In this process, a thermoplastic material is heated to a liquid state and then forced under high pressure into a closed mold. After the mold is quickly cooled, it is opened at a seam and the finished part is removed.

Because injection molding allows formation of both interior and exterior surfaces simultaneously (rotomolding and hand lamination do not), the molded parts can incorporate internal structural features such as stiffening ribs for greater rigidity. Injection molding also accommodates a high degree of automation, and short cycle times allow production of pieces in high volumes at relatively low cost. However, initial costs for mold tooling can be quite high, and the machinery is bulky. Mackie, for example, puts a price tag of $700,000 on the 32-ton mold used to make cabinets for the SRM450.
A number of different thermoplastic materials are used for injection molding of loudspeaker cabinets, including:

  • Polypropylene—This is, by far, the most popular material for all but the smallest cabinets. It offers high impact resistance, good acoustical properties, and excellent overall ruggedness. Other uses for polypropylene include food packaging, laboratory equipment, and automotive parts. Popular loudspeakers made from this material include the JBL Eon Series, Mackie SRM350, Electro-Voice Zx5, and Crest Performance LQ Series, among many others.
  • High-impact polystyrene (HIPS)—This material is better suited to installed applications, as opposed to portable use, since impact resistance is relatively low. However, polystyrene offers very good stiffness and damping, excellent moisture resistance, and material cost is low. The JBL Control Contractor Series and QSC AcousticDesign surface-mount cabinets are among those molded from HIPS.
  • Acrylonitrile butadiene styrene (ABS)—ABS is a common thermoplastic used to make lightweight, rigid products such as automotive parts, golf club heads, and toys such as the familiar Lego bricks. Loudspeaker cabinets made from this material include the Yorkville C Series and the Gemini GX-350. ABS is classified as a copolymer, so it is likely that loudspeaker other cabinets labeled with this term (Electro-Voice SxA360, for example) are made either from this material or one chemically similar.
  • Polycarbonate—This is a very strong, durable, hard thermoplastic used to make, among other things, eyeglass lenses, DVDs, and Nalgene bottles. Lexan is a trade name for a polycarbonate. Loudspeakers using this material include the Electro-Voice Plasma P1 and the Alesis ProVenue.

Rotation Molding

Rotation molding, also know as rotomolding, is a manufacturing technique first developed in the 1940s. The process employs a large and rather ungainly-looking machine that rotates the mold on two axes at high speed, forcing pre-heated thermoplastic material uniformly into a single-sided mold. Rotation molding allows formation of much thicker walls and stronger corners than can be accommodated with injection molding, which makes it the preferred process for cabinets that must be extremely durable and resistant to environmental effects. Because the mold is one-sided and open, there is virtually no restriction on part size. Tooling costs are low because the molds can be formed from aluminum or sheet steel.

Nearly all rotation molded loudspeakers are made from polyethylene, primarily because this material’s relatively low melting point works well with the logistics of the process. Polyethylene is also relatively inexpensive and extremely durable. However, because polyethylene is somewhat less rigid than most other thermoplastics, some type of bracing or ribbing must be incorporated into the design to add stiffness in order to control cabinet resonance.

Loudspeakers cabinets made with the process include the entire Technomad series, as well as Community’s R.25-X, R.5-X, and R.5-Sub. Other rotomolded products of similar ruggedness include kayaks, highway barriers, outdoor play structures, and ATA-approved road cases.

Molded Composites

Composites, as the name implies, are engineering materials made from two or more components. One component is usually a strong fiber for tensile strength, such as fiberglass, Kevlar, or carbon fiber. The other component is a matrix substance used to form the material, usually a polyester or epoxy resin.

The most familiar composite is laminated fiberglass. This familiar material has a very high strength-to-weight ratio, and hand lamination allows complete control over thickness, glass type and core materials. Also, there is virtually no limitation on part size, as Community has ably demonstrated with its famous Leviathan series horns. However, the fiberglass has found limited use in loudspeaker enclosures because the material doesn’t allow the molding detail accommodated by the injection process. Also, costs are relatively high: The process requires a considerable amount of highly skilled labor, and it is subject to intensive air management restrictions to ensure worker safety.

Despite higher costs and molding limitations, fiberglass holds a steady share of the outdoor loudspeaker market. Nothing else can claim to be as durable, lightweight, and virtually impervious to extreme environmental conditions. Best known in this category are Community’s R1-X, R2-X, and WET models.

Taking their place alongside the familiar fiberglass are some newer, more exotic composite enclosure materials. QSC Audio new offers several products housed in a cabinet made from Composilite, a new ‘hybrid’ material that uses multiple skins of carbon fiber layered over a Nomex honeycomb core to form a rigid, seamless structure. JBL’s Vertec line arrays take the same concept a step further by encapsulating honeycomb structures, aluminum bracing and steel components into a single composite shell. Although the process is highly labor intensive and therefore costly, the end result according to JBL is a housing that is lightweight, rigid, durable, and free from acoustical anomalies. Though currently limited to high-end professional systems, this type of space-age loudspeaker cabinet likely points the way to a wider range of new molded products that will emerge over the decade to come.

The author would like to extend special thanks to Ted Leamy and Bernie Warner of JBL Professional and Bruce Howze and Steve Goodwin of Community for their indispensable contributions to the content of this article.

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