When Aluminum Beats Plastic

An aluminum housing is chosen for four reasons, and it is worth knowing which one is driving your project because they lead to different construction methods.

Heat. Common wrought alloys conduct heat roughly 400 times better than ABS. An aluminum shell turns the entire enclosure into a heat sink and often removes a fan, a decision usually made alongside the rest of the thermal management strategy.

Shielding. A metal box is a Faraday cage. If your product has a switching supply, a motor drive, or a sensitive analog front end, an aluminum enclosure can be the difference between passing emissions testing and re-spinning a board.

Stiffness and abuse. Instruments, rugged field gear, and anything mounted to a machine benefit from a housing that does not creep, craze, or embrittle in UV.

Volume economics. Below a few thousand units a year, aluminum frequently costs less per unit than plastic, because plastic requires tooling that has to be amortized. This is the reason most low-volume industrial products are metal, and the general framing is in electronics enclosure design.

Four Ways to Build the Box

Extruded profile with machined end caps. The workhorse of instrument enclosures. A two-dimensional cross section is pushed through a die, cut to any length you like, and closed with two flat end plates. Internal slots for the board are part of the profile, so PCB retention costs nothing. A custom die runs roughly $600 to $2,500 with a three to five week lead time and a minimum order often quoted in hundreds of pounds of material. Standard off-the-shelf profile families let you skip the die entirely and only machine the ends, which is the fastest path to a professional-looking product at 50 units.

Bent sheet metal. Cheap, fast, and unbeatable for larger boxes and rack equipment. Weak on sealing, on tight tolerances across a bend, and on the premium feel a handheld product needs. Design it to the bend radii and relief rules in the sheet metal design guide or you will pay for it in fit-up.

CNC machined from billet. A unibody pocket milled out of solid stock gives the best tolerances, the best sealing surfaces, and shapes an extrusion cannot make. It costs $120 to $600 per housing in small quantities and does not get dramatically cheaper with volume, since the machine time is the cost. Right for 1 to 500 units, for premium products, and for anything that must hold a precise optical or mechanical relationship. Cut that bill substantially by following designing parts to cut CNC machining cost: fewer setups, larger internal corner radii, no deep narrow pockets.

Die casting. The volume answer. Tooling runs roughly $10,000 to $50,000 with a two to four month build, after which parts land in the single-digit dollars. Breakeven against machining usually sits somewhere between 3,000 and 10,000 units. It buys complex three-dimensional geometry, integral bosses, and thin structural ribs, at the cost of draft angles, looser tolerances, and porosity that limits both pressure sealing and cosmetic anodizing. The full economics are in aluminum die casting.

Choosing the Alloy

The alloy choice is mostly made for you by the process, but it has cosmetic consequences that surprise people at the finishing stage.

  • 6063 is the standard extrusion alloy. It flows into fine detail and anodizes to a clean, even, bright finish. Use it when the extrusion is a visible surface.
  • 6061 is the general machining alloy: stronger, tapping cleanly, but anodizing slightly duller and grayer than 6063. Mixing a 6061 end cap with a 6063 body on the same product and anodizing them together will produce a visible color mismatch. Machine the caps from 6063 if the color has to match.
  • 5052 is the sheet metal alloy, formable and corrosion resistant, with mediocre anodizing appearance.
  • A380 and other casting alloys contain enough silicon that they anodize dark, blotchy, and inconsistent. Cast housings get powder coat or wet paint, not decorative anodize.

Finish: Anodize and the Alternatives

Type II sulfuric anodize is the default: a 0.0002 to 0.001 in oxide layer that takes dye, resists corrosion, and gives the matte or satin look people associate with quality hardware. Type III hardcoat runs thicker and much harder for wear surfaces, but it darkens the part and grows dimensions enough to matter on a press fit. Chromate conversion coating is the choice where the surface has to stay electrically conductive, and it is also the right prep under paint. Cost drivers, color consistency across lots, and rack-mark placement are covered in anodizing aluminum.

Two traps catch first-time designers. Anodize is an electrical insulator, so every ground path, EMI gasket land, and bonding point must be masked or machined after anodizing, and that masking has to be called out on the drawing. And anodize is transparent: extrusion die lines, machining marks, and weld heat-affected zones all show through. Specify the pre-finish, typically a bead blast or a directional brush, as part of the spec rather than leaving it to the shop.

Design Rules That Keep It Manufacturable

  • Wall thickness. Hold extrusion walls at 0.060 in (1.5 mm) or more, and keep them reasonably uniform across the section, since thin regions cool faster and pull the profile out of shape. Die castings want 0.080 to 0.120 in (2 to 3 mm).
  • Section complexity. Extrusion die cost and scrap rate scale with the circumscribing circle and with deep narrow tongues in the profile. A hollow section costs more than an open one and holds looser tolerances.
  • Machining allowance. Extrusions are not precision parts. Where a mating surface or bore needs real accuracy, add stock and machine it after extrusion instead of tightening the profile tolerance.
  • Threads. Tapped holes in 6061 are fine for assembly, but any joint that will be opened repeatedly in service should get a threaded insert.
  • Sealing. An extruded body with a captured gasket groove in the end cap seals well and reaches a solid ingress rating; a die casting needs a machined sealing face because the as-cast surface will not hold an O-ring.
  • Dissimilar metals. Stainless fasteners in an aluminum housing outdoors will corrode the aluminum galvanically. Address it with coatings and isolation as described in corrosion protection for metal parts.

Get Your Enclosure Costed Properly

Projects House designs aluminum enclosures from concept through production drawings, including process selection at your real volume, alloy and finish specification, sealing and thermal design, and the DFM review that keeps a quote from tripling. Send your board outline, environment, and annual volume through the contact form.