Aluminium vs Polycarbonate Enclosures: A Cost-per-Unit Breakdown by Volume

Customers usually ask us "is aluminium or plastic cheaper?" and the honest answer is "tell me your annual volume and your finish spec and I'll tell you in two minutes." Material cost is rarely the deciding line. Tooling amortisation, post-processing, and the cost of a rejected finish dominate the per-unit number once you cross out of prototype quantities.

On a typical industrial box — call it 200×150×80mm, four PCB mounts, two cable glands, an EMI gasket channel — the per-unit cost stack we model looks like this:

• Raw material — 8–20% of unit cost. Aluminium 6063-T5 extrusion runs ₹260–320/kg; PC/ABS pellets sit at ₹220–280/kg.
• Process cost — 25–40%. CNC machining time on extrusion vs. injection cycle time on plastic.
• Tooling amortisation — 10–55%. The single line that drives every volume-tier decision.
• Post-processing — 15–35%. Anodise, powder coat, screen print, or in-mould labelling.
• Yield loss — 3–12%. Higher on anodised aluminium, lower on bare ABS.

"We have walked into customer reviews where the design team had already committed to die-cast aluminium for a 2,000-unit/year programme. The DfMA exercise moved them to extruded aluminium with CNC end-caps and cut unit cost by 41% before we touched the PCB." — Pioneer Horizon mechanical engineer

The rest of this article walks the actual numbers from prototype quantities through 50k/year, with the cross-over points we use as defaults when there is no project-specific reason to deviate.

At the prototype tier, tooling does not exist yet, so every unit cost is a process cost. The choice here is between subtractive manufacturing on aluminium, 3D-printed plastic (SLA or SLS), and sheet-metal bending.

CNC-machined aluminium 6061

Roughly ₹4,200–7,500 per unit on the box described above, depending on feature density. Two-day lead time at a regional shop. Surface finish out of the machine is suitable for engineering review but not customer-facing — budget another ₹600–900/unit for bead-blast plus clear anodise.

SLA / SLS printed plastic

₹1,800–3,200 per unit. Same-day to two-day lead time. SLS nylon (PA12) survives drop testing; SLA resin generally does not and will creep at 60°C. Useful for fit-check and EVT-level builds, not for customer demonstrators that need to live in a hot car.

Folded sheet-metal aluminium

₹2,400–4,000 per unit for a folded-and-riveted 1.6mm Al5052 box. Lead time three to five days. Tolerances on bends are ±0.3mm — adequate for most industrial work, marginal for sealed-gasket geometry.

Where each one breaks

• CNC is the safest choice for an EVT build that needs to survive vibration and thermal cycling.
• SLS is the right call for ergonomics, button feel, and customer-facing demos under 50 units.
• Sheet metal wins when you need EMI shielding and don't yet need IP-sealed.

One thing to avoid at this tier: do not commission injection-mould tooling to "save money" before EVT-2 sign-off. We have seen ₹18 lakh tools scrapped because a thermal redesign changed the standoff layout on the bottom plate. The cross-over to tooled plastic only makes financial sense once the mechanical interface is locked.

This is where the most expensive mistakes happen, because the volume looks just high enough to justify tooling but rarely is. Our default at this tier is extruded aluminium with machined end-caps, and below is the maths.

Extruded aluminium 6063-T5

Extrusion die cost is ₹40k–80k for a profile up to ~150mm wide. Amortised over 3,000 units, that is ₹15–27 per unit. Linear cost of extrusion runs ₹110–160 per metre of profile. End-caps are CNC-machined from 6mm plate at ₹180–240 each (two per unit). Anodising adds ₹40–70/unit in batches of 200+. Total: ₹620–870 per unit at 3k/year, sometimes lower.

Low-cavity injection moulded plastic

A single-cavity production tool in P20 steel for the same enclosure runs ₹6–9 lakh. Amortised over 3,000 units that is ₹200–300/unit on tooling alone, plus ~₹85–130/unit on the moulding cycle, plus post-processing. Total: ₹385–520 per unit at 3k/year — cheaper than aluminium on paper, but only if the finish spec is tolerant of weld lines and the colour is moulded-in. Add screen printing and the gap narrows fast.

Die-cast aluminium

Die-cast tool cost is ₹12–22 lakh. At 3k/year that is ₹400–730/unit on tooling — almost always uneconomic at this tier. Reserve die-cast for the 10k+ tier where the tool amortises properly.

The honest cross-over: if your finish spec demands anodised aluminium feel (industrial instruments, lab equipment, anything where the customer touches the box every day), the extrusion route stays cheaper until ~7k/year. If the box lives inside a panel and nobody sees it, low-cavity plastic wins at ~2.5k/year. Our DfMA review walks both paths and we share the spreadsheet — see our contact page if you want us to run yours.

At 10k+/year, tooling amortisation drops below 10% of unit cost and the decision shifts to process and finish. Injection moulding is the default; the question is which polymer family and how many cavities.

Multi-cavity injection tooling

1. 2-cavity P20 tool — ₹9–14 lakh. Cycle time ~55 seconds for the 200×150×80 enclosure. Throughput ~120 units/hour.
2. 4-cavity hardened H13 tool — ₹18–26 lakh. Cycle ~65 seconds. Throughput ~220 units/hour. The right call above ~25k/year.
3. Family tool (top + bottom in one shot) — adds 15–25% to tool cost but cuts assembly handling and the lid-to-base colour-match problem in half.

At 25k/year on a 4-cavity H13 tool, the tooling line drops to ₹72–104/unit. Moulding cycle cost is ₹38–55/unit. Material at ₹240/kg on a 320g part is ₹77/unit. Post-processing (IML or pad-printing) is ₹18–35/unit. Total: ₹205–271 per unit. That is the number we benchmark every other route against.

Die-cast aluminium becomes viable

The 12–22 lakh die-cast tool amortises to ₹24–44/unit at 50k/year. Total cost lands at ₹420–580/unit including powder coat and machining of mating surfaces. We recommend die-cast at this tier only when the design needs the thermal mass for heat dissipation — see our companion piece on natural vs forced convection for when that calculation flips.

Finish quality at scale

Cosmetic yield at 25k+/year is a real line item. In-mould labelling (IML) gives near-zero rework but adds ₹4–7 lakh in label tooling. Pad-printing is cheaper per unit but rejects sit at 2–4% on dark-on-light schemes. Powder coat on die-cast holds yield around 1.5% rework with a competent finisher.

Below is the default we hand to product managers when there is no project-specific reason to deviate. Treat it as a starting point, not gospel — the right call depends on finish spec, ingress protection requirement, and how often the customer touches the box.

Default by volume

• 1–100 units (prototype): CNC-machined 6061 aluminium with bead-blast finish, no tooling commitment.
• 100–1,000 units (pre-production): Extruded aluminium + CNC end-caps, clear anodise. Tooling commitment under ₹1 lakh.
• 1k–5k units/year (low volume): Same extrusion route, with optional aluminium top cover for branding. Add powder coat if colour matters.
• 5k–25k units/year (mid volume): 2-cavity injection moulding in PC/ABS for indoor, PA66-GF for outdoor with UV exposure.
• 25k+ units/year (high volume): 4-cavity injection moulding, IML for branding, or die-cast aluminium if the thermal budget demands it.

Three honest caveats

1. IP rating overrides cost. If you need IP66+, the gasket geometry forces design choices that override the cost table. We treat the IP requirement first and then run the cost stack on whatever survives — see our notes on IP67 vs IP68 gasket geometry.
2. Flame rating overrides material choice. UL94-V0 is mandatory for many industrial deployments and not every polymer family hits V0 at every wall thickness. See our piece on UL94-V0 by material family.
3. Tooling lead time is its own line item. A 12-week tool delivery on a programme that needs to ship in 16 weeks is not a cost decision — it is a schedule decision.

If you would like us to model your own enclosure across these tiers — including a tooling-amortisation curve and a finish-quality risk register — share the mechanical spec and we will return a costed analysis within five working days.