A German BESS integrator ran the same enclosure on sheet metal for five straight years at a steady annual volume. The geometry had been frozen since year two; no engineering change after start of production. Every year of that run they paid a per-unit difference that injection molding would have removed, unseen, because it lived inside a unit price.
We see the mirror image in our quote log. A device team opened a tool on the strength of a unit price, then issued ECN number three: two bosses relocated, a snap fit re-cut. The revisions cost more than the per-unit saving the tool had been justified on.
Both teams asked the same question about a sheet metal vs injection molded enclosure, and both answered it from a material property chart. Properties do not decide it — two cost curves do, plus a formula with five terms you can fill in from documents you already hold. We build metal enclosures, and this guide says plainly when to take the drawings to a molding shop instead.
Key takeaways
- Two curves, not two materials. Sheet metal spends money on every piece; injection molding spends it up front in a tool. Where they cross is arithmetic.
- The crossover is a formula.
q* = (NRE_tool + Σ change costs + Δ finishing & assembly + tool carrying cost) ÷ (c_unit_metal − c_unit_molded)— five terms, each from a document you can ask for. - Published numbers disagree. aivon.com says 1,000–5,000 units, bsumwell.com publishes bands, newayprecision.com names seven variables.
- Revisions are asymmetric. A laser program or punch turret changes in hours; a tool change depends on whether you add material or remove it.
- Some enclosures rule the tool out first. Envelope, load path and feature integration come before any cost term.
- Metal is sometimes the wrong answer. Small, indoor, frozen, no EMI or thermal load, high repeat volume: take that pack to a molder.
Two cost curves: where sheet metal and injection molding spend their money
A fabricated enclosure spends its money on every piece — cutting time, bend count, welds, finishing, assembly labor and fitted hardware — and that per-unit cost falls only gently with volume, because the route carries no dedicated tooling. A molded enclosure moves the spend forward into a tool; after that, piece cost falls steeply with cavity count.
Two things set how steep each line is: the enclosure size, and how many features the process absorbs in one operation. Material sets neither.
| Cost item | Sheet metal | Molded | Dominates |
|---|---|---|---|
| Tooling and fixtures | Programs, standard tooling | The tool, plus trials | Before piece one |
| Unit labor | Cutting, bending, welding, assembly | Cycle time per shot, across cavities | Every piece |
| Unit material | Sheet, commodity level | Resin by grade and shot | Every piece |
| Finishing | Separate operation | Color in the process, or conductive coating after | Larger on metal |
| Assembly and hardware | Fasteners, inserts, seals | Bosses, snap fits, seal grooves | As volume grows |
| Change cost | A program edit, next piece | A tool modification, priced by direction | Every revision |
| Tool storage and refurbishment | Not applicable | Storage, maintenance, refurbishment | Late in program life |
Of those seven rows, this article uses one: the per-unit difference. The sheet metal enclosure cost page breaks a quote down line by line; here that difference is one term in an equation.
If the curves send you to metal, the next decision is a separate alloy one: which sheet metal alloy fits that service environment. That page assumes metal has already been chosen. This article is the step before that step.
Before the RFQ: get both routes to quote the same frozen drawing in the same finished condition, or the two curves describe two products.
The sheet metal vs injection molding crossover is a formula, not a number
The crossover quantity is the annual volume at which injection molding becomes cheaper than sheet metal for your enclosure. It is not a figure you look up: it is the point where the two routes' totals are equal, and it shifts every time one of five inputs moves. Published break-even numbers exist, and they disagree with each other, because each one solves a different enclosure.
q* = ( NRE_tool + Σ expected change costs + Δ finishing & assembly + tool carrying cost )
÷ ( c_unit_metal − c_unit_molded )Every term has a source, and none of them is an article:
- NRE_tool — the tool cost on the molder's quotation, for your geometry and cavity count.
- c_unit_metal − c_unit_molded — both suppliers quoting the same frozen drawing, in the same finished condition, at the same annual volume. The equation collapses if you compare an assembled, coated fabrication against a bare molding.
- Σ expected change costs — your ECN history: the geometry revisions that landed after tooling existed, in your last two programs.
- Δ finishing & assembly — what each route still does after forming: coating and hardware for metal; conductive coating, machining and inserts for molding.
- tool carrying cost — the molder's written position on who stores, maintains and refurbishes the tool.
Three named sources, three different answers — which is the point. aivon.com puts the break-even "usually between 1,000 and 5,000 units". bsumwell.com publishes volume bands, with metal at the low end and molding taking over as volumes climb. newayprecision.com refuses a number and lists seven variables instead: tool cost, cavity count, resin grade, cycle time, enclosure size, cosmetic class and inspection plan. Any figure you read online solves somebody else's enclosure.
| Input | Which way it moves the crossover | Where you get it |
|---|---|---|
| Tool cost for your geometry | Right, towards metal | The molder's quotation |
| Cavity count in the quoted tool | Left, towards molding | Ask how many cavities |
| Envelope size | Right: a larger part needs a larger tool | Your model, the molder's limit |
| Expected revisions before SOP | Strongly right | Your ECN history |
| Volume as steady repeats | Left | Your demand plan |
| Finishing work after forming | Right, if molding needs conductive treatment | Both suppliers, finished condition |
| Program life and spares duty | Right | The contract, before you sign |
Three inputs get forgotten more often than the rest. The first is the shape of the demand rather than its total: steady monthly repeats justify a harder, multi-cavity tool, while one bulk order followed by silence does not. The second is how long the tool has to stay serviceable — a numerator cost, not a footnote. The third is the step from single-cavity to multi-cavity, which is a second purchase, not a deferred upgrade.
Investing in tooling and fixtures inside the sheet metal route has its own payback arithmetic. This article asks which of the two routes this enclosure should take. Both end in a payback calculation; they are not the same one.
Your move: fill in the one term only you can supply — the number of geometry revisions you still expect before SOP.
Before the cost maths: does the part even fit the process?
Envelope. Clamp tonnage and the tool envelope put a ceiling on a single molding. Past it, the molded answer becomes several moldings joined together, and the joint has to be sealed and inspected. A Texas coastal EV charger OEM came to us holding two molding quotations for the same housing; both came back as an assembly with a sealed joint, because the longest side would not fit inside a tool envelope anybody would quote. Enclosures whose size puts them outside the molding envelope are the ones we build. One supplier publishes the same position: at extra-large sizes, injection tools get extremely expensive.
Load path. A molded box carries point loads through bosses; a fabricated box carries them through flanges and continuous members. Transformers, drives and busbars bolted inside are sensitive to that difference, because the load does not stay where the boss is.
Feature integration. Where bosses, snap fits, light pipes and a molded seal groove let one part replace five, molding wins on assembly labor as well. newayprecision.com's feature list is the one to hold against your BOM.
Decide now: send the envelope dimensions to a molder before the RFQ, and ask one question — is this one shot, or an assembly?
The five axes that usually decide it
Heat
Metal moves heat out of a sealed box. Polymer holds it in. In a design with no forced air, aivon.com describes cases where the temperature rise requirement alone forced a change from plastic to metal. How much heat has to move, and what the venting and heat-spreading options cost, belongs with the thermal load the enclosure has to move.
Ask either supplier: what is the internal dissipation, and is the enclosure sealed? If it is, make the polymer route price its thermal solution before you compare unit prices.
EMI and RF: sheet metal vs plastic shielding
Metal gives you a conductive shell before anyone has designed anything. A plastic enclosure has to earn it — conductive coating, conductive filler or a metallic layer, each carrying adhesion, wear and inspection exposure. aivon.com reports molded enclosures failing radiated emissions because the conductive layer was incomplete in a corner or worn off during assembly.
The honest version is the one topcabinet.com states plainly: material alone does not guarantee EMI performance. Seams, door joints and every cable penetration decide what you measure, and a poorly gasketed steel door loses to a well-executed coated molding. If the product has to radiate RF from inside the box, a metal shell traps it — which is the motivation for the third route below.
Ask either supplier: which seams, doors and penetrations sit in your EMI path, and who owns continuity across each — the engineering sits in EMI continuity across seams, doors and penetrations.
Fire and smoke
Metal does not burn; a polymer enclosure depends on the resin and its flame-retardant package. That is all this article says on the subject: flame ratings start from their definition and verification, and that page owns them on this site. Here, fire behavior is one of five axes in a process decision and nothing more.
Ask your own compliance owner: which flammability requirement does the application carry, and who signs the position?
Impact, UV and outdoor aging
The impact case that catches buyers out is not the first drop, it is the last one: aivon.com notes a plastic enclosure passing a one-meter drop at room temperature can shatter in the same test after thermal cycling or UV exposure.
Ask either supplier: what is the impact energy at end of service rather than at first article? The conventions sit in impact resistance ratings for enclosures.
Finish and color
Molding puts color and texture into the process and removes a downstream operation. The counter-argument is repair: a coated metal panel can be touched up on site, while a damaged molded cosmetic part is usually a replacement.
Ask either supplier: how is a scratched cover handled two years into service? Matching color across batches belongs in the finishing stream the molding route either removes or adds.
The five axes in one table
The first two columns are the comparison everyone publishes. The column that changes a purchase is the third.
| Axis | Usually won by | The question for the supplier |
|---|---|---|
| Heat dissipation | Sheet metal | Internal dissipation, sealed or vented, plus the polymer thermal cost |
| EMI / RF | Sheet metal, seams designed | Which seams, doors and penetrations are in the EMI path |
| Fire and smoke | Sheet metal | Which flammability requirement the application carries |
| Impact and UV | Sheet metal at end of service | Impact energy at end of service, not at first article |
| Color and finish | Molding, on first cost | How a scratched cover gets handled two years in |
Put those five questions to both suppliers before you compare their numbers.
What a revision costs: sheet metal vs injection molding changes
A revision is not the same event in the two routes. On the sheet metal side it is a laser program edit and a new bend sequence, effective on the next piece. aivon.com puts it in the right units: changing a laser program or a punch turret is measured in hours.
On the molding side, the first question is directional. Are you adding material? Then a weld-up and a re-machined insert may carry it. Are you removing material? Then you are usually looking at a new insert or a new core, and every trial shot after that costs another cycle.
One instrument program in our quote log had already committed to a molding tool when it issued a third ECN on the enclosure geometry: two bosses moved, a wall thickened. The tool paid for itself on the unit price, right up to the point where that revision arrived.
The two routes do not carry change the same way, and that difference is the rule this article is built on: if you expect more than two geometry revisions before start of production, the crossover moves right, sometimes past the point where the tool was ever going to pay back. It moves right because every one of those revisions belongs in the numerator, and none of them appeared in the unit price that sold you the tool.
So the sequencing rule comes first: freeze the drawing, then commit to tooling on whichever side you choose. Where a program cannot freeze, the molding industry has a concept for the gap — prototype or bridge tooling, built to a lower duty for early builds, as newayprecision.com discusses. Raise it with your molder as a concept; it is not a service this article offers.
For keeping the revision count known rather than guessed: going from prototype to repeat production without a tool covers the no-tool route to volume, and drawing revision control and ECN handling covers the discipline that produces it.
Open your last two programs and count the geometry revisions that landed after the tooling existed, on either route.
> Freeze the geometry before you commit to either route. Send the model for a free CAD design and DFM review.
The third answer: a hybrid metal frame with plastic panels
Hybrid construction exists because the two routes fail on opposite things. The structure worth keeping is the one eabel.com describes: a metal skeleton carrying the load path, the earth continuity and the sealing frame, with polymer skins carrying corrosion resistance, RF transparency, appearance and weight. Its motivation, per eabel.com, is the IIoT problem — a device that has to hold a Faraday cage and pass RF cannot do both with one continuous shell.
The engineering is where most descriptions stop, and where projects get hurt. Metal and polymer expand at different rates, so their joint cannot be a continuous bonded line: it has to be controlled local anchors that let the two materials move. The skeleton and the tool must also share a datum system; get that wrong and polymer flashes onto a visible face, or the tooling is damaged on closing. That reasoning comes from sunmyhardware.com.
Where we sit in it: a hybrid enclosure means we build the metal skeleton, and the polymer skins come from a molding supplier you appoint.
Hybrid is also the wrong answer in identifiable cases: whole-enclosure high-grade EMI, high fire load, high vandal resistance, extreme UV combined with heavy impact. Each time the short side takes over, and you carry two processes' costs to arrive at one weakness.
Who owns the datum system between the skeleton and the tool, and who signs off the anchor pattern?
Program life: the costs no sheet metal or molding quote includes
Three costs sit outside every quotation and land inside every long program.
Start with the tool's own life: who owns it, who stores it, who maintains it, who pays to refurbish it. Ask the molder in writing what happens once the program outlives the first tool. Supply continuity comes next — a resin grade can be discontinued, and its substitute is a qualification exercise, while sheet is bought at commodity level and its substitute is a drawing note.
The cost that decides more outdoor programs than either of those is spares. Fabricated spares mean the drawing exists and one more can be made; a damaged panel is a repair. Molded spares mean a minimum order quantity, dead stock, or reopening a tool.
Before you compare the two quotations, write down what a spare cover costs you once the program has been running for years.
When sheet metal is the wrong answer
Take the molding route — and not narrowly — when every one of these is true:
- A small envelope, comfortably inside a tool's practical limit.
- Indoor service, with no UV or weather exposure.
- Geometry frozen, or expected to freeze before any tooling is cut.
- No EMI requirement and no significant thermal load to shed.
- Weight-sensitive, with no heavy internals bolted to the walls.
- A high and stable annual repeat.
- An interior full of features molding can absorb into one part — bosses, snap fits, light pipes, an integrally molded seal groove.
Under those conditions we would rather tell you before you send the RFQ than after.
What to take to the molding shop is the same data pack the next section sets out: annual volume and its pattern, program life, substrate requirements, cosmetic class, change policy.
If you ticked every line in that list, close this article and put that pack in front of a molder.
The RFQ pack that makes the two quotes comparable
The commonest way to get this wrong is comparing two different products: one prices a fabricated, coated, assembled enclosure, the other a bare molding. What looks like a process difference is usually scope.
Send the identical pack to both routes:
ENCLOSURE PROCESS RFQ — [PROJECT / PART NUMBER]
1. Annual volume, and its pattern: steady monthly repeats, project spikes, or one bulk order
2. Program life, and the spares / service period required
3. Geometry status: frozen, or the number of revisions expected before SOP
4. Cosmetic class: declared in the terms used on this site, not invented for this RFQ
5. Finish specification: identical for both routes
6. Change policy: who pays for a tool modification or a program change
7. Both routes quote the finished condition — metal including coating and assembly;
molding including secondary operations and any conductive treatment
8. Tool ownership, storage and refurbishment terms, stated by the molding routeItem 3 moves the answer and is the one buyers leave blank. Item 7 makes the two numbers comparable. The commercial fields around that block are set out in what to put in an enclosure RFQ. Send both routes the same eight items and the two quotations that come back will at least describe the same product — which is more than most comparison exercises achieve.
Put items 1, 2 and 3 in the email before you attach the file.
Where the decision sits
The decision rests on numbers only you hold: annual volume and its pattern, program life, and the geometry revisions you still expect. Everything else comes from the quotations.
What we contribute is the metal side, including the skeleton in a hybrid build. Free CAD design and DFM review is available before you freeze the geometry; minimum order quantity is 1, with no tooling charge on the sheet metal route; standard parts ship in 7–15 working days, custom parts in 20–30 days. Where the evidence points at injection molding, we say so.
> We do not issue certifications or compliance declarations. The EMI, flammability and ingress positions are yours to hold, or your test house's.
Send the model and those three numbers, and we will return the metal side within 24 hours: talk to us about the enclosure program.
Frequently asked questions
At what quantity does injection molding become cheaper than sheet metal?
There is no universal figure: the crossover is the volume where the two routes' totals are equal, and it moves with tool cost, cavity count, enclosure size, expected revisions and program life. Three named sources give three different answers — aivon.com puts the break-even "usually between 1,000 and 5,000 units", bsumwell.com publishes volume bands, and newayprecision.com refuses a number and lists seven variables instead. Substitute your own tool quotation, per-unit difference and revision count into the formula in this article.
Can a molded enclosure reach IP65 or IP66?
Yes. Molding can make sealing easier, because a continuous molded face and an integrally molded O-ring groove are cheap to produce. The real difference is how the interface behaves in service: gasket compression set, cover stiffness, and whether a damaged sealing face can be repaired or replaced. The ratings are defined under IEC 60529, with NEMA classifications alongside. Ask what the interface does after years in the weather.
Is plastic always lighter?
No. The shell is usually lighter, and that is real where the shell is most of the assembly. The advantage is eaten by metal inserts, thicker walls needed to carry a load, or stiffening that the polymer section has to add back.
When should I switch from a plastic enclosure to a metal one?
When more than one of these appears at once: the enclosure has to shed real heat; it is going outdoors long term; it carries an EMI requirement; and someone in the field has to repair a damaged cover. One of those can usually be engineered around on the polymer side. Four together usually cannot.
Can I start on sheet metal and open a tool later?
Yes, and it is usually the right order, because it buys a frozen geometry before you spend on a tool. The condition is DFM compatibility: do not let a feature adopted for fabrication — an undercut that only works as a folded tab, a wall thickness chosen for a bend radius — block the molding design later.
Will switching to plastic break EMI compliance?
Treat it as a new test rather than an equivalent substitution. The reason is the one topcabinet.com states: material alone does not guarantee EMI performance. What you measured was a system of walls, seams, gaskets, doors and penetrations, and changing the wall material changes that system.