Aluminum or steel injection molds for a first production run: which fits, and when should you switch?

Aluminum injection molds cost less, make first parts sooner and are cheap to change, but they wear out sooner: usually a few thousand to tens of thousands of shots (one shot is one cycle of the mold), against about 50,000 to several hundred thousand for pre-hardened steel such as P20, and about 500,000 to a million or more for hardened steel. For a product that starts with a few hundred units and grows to a few thousand a year, the real choice is between two paths: aluminum for the first run and steel once the design settles, or steel from the start. If you start in aluminum, switch once the design is frozen, and order the steel mold while the aluminum one still has about two to three months of parts left, enough to cover building and approving the steel mold.

Aluminum first is a kind of insurance. It pays when the first run is likely to change the design, or when you'd rather not pay for steel before demand is proven, and the bigger the price gap between aluminum and steel where the molds are built, the cheaper that insurance is. If the design is already settled and demand is clear, a single-cavity steel mold (one part per shot) from the start usually costs less over the product's life, because the other path pays for two sets of molds and a second round of approval. Corrosive or high-temperature plastics, glass-filled plastics at high volume, a high gloss that has to hold over tens of thousands of parts, and parts with several slides (moving side pieces of the mold that form clips or holes on the side of a part) push toward steel from the start.

What do SPI mold classes 101 to 105 mean?

SPI mold classes describe how many cycles an injection mold is built for, on a scale from the US plastics industry association, the Plastics Industry Association, formerly the SPI: class 101 for a million cycles or more, 102 for under a million, 103 for under 500,000, 104 for under 100,000, and 105 for prototype molds of 500 shots or fewer. Classes 101 and 102 also require cavities and cores hardened to at least 48 HRC, so a P20 mold, at about 28 to 34 HRC, is a class 103 mold even if a quote calls it 102. An aluminum mold for a first run is usually a class 104 mold; a quote that says class 105 covers no more than 500 shots. Shops use the classes loosely, so ask which class a quote assumes and what it means in that shop.

Aluminum vs steel injection molds: how do they compare?

The table compares aluminum, pre-hardened steel and hardened steel injection molds for a small plastic housing: life, SPI class, cost, time to first parts, changes, cooling, finish and suitable plastics. Aluminum molds are often called soft, rapid or prototype tooling, and hardened steel molds hard or production tooling; the names vary, so ask what a quoted mold is actually made of.

Property	Aluminum (such as 7075 or QC-10)	Pre-hardened steel (such as P20 or NAK80)	Hardened steel (such as H13) or stainless (such as S136)
Typical life	A few thousand to tens of thousands of shots; shops' claims for the same grade range from 2,000 to a million or more	About 50,000 to 500,000 shots	500,000 to a million shots or more
Usual SPI class	104 (105 only for a prototype mold of 500 shots or fewer)	103	101 or 102
Cost	Lowest; much lower than steel at US rapid-tooling shops	Higher; in many Asian shops, only somewhat more than aluminum for a simple single-cavity mold	Highest
Time to first parts	About 1 to 4 weeks	About 2 to 6 weeks	About 6 to 12 weeks or more
Design changes	Cheap and quick to re-machine; adding metal usually means a new insert, since aluminum welds poorly	Slower, but the steel can be welded and re-cut	Slowest
Cooling	Conducts heat about four to five times better than P20, so cycles can be shorter when the mold has proper cooling	Slower	Slower
Finish	Takes polish and texture, but holds a high gloss poorly over a long run, and etched textures can look less crisp	Good; NAK80 for high polish	Best for mirror finishes and long runs
Plastics	Unfilled plastics such as ABS, PC/ABS and polypropylene	Most plastics, including glass-filled grades for a few thousand to tens of thousands of shots, often with hardened inserts at the gate	Glass-filled and abrasive plastics at high volume and high-temperature plastics; stainless for corrosive ones
Slides, many cavities, hot runners (heated channels that feed the plastic)	Limited; steel inserts are often added where aluminum wears	Yes	Yes

To compare a mold's life with your plan, count shots, not units. A single-cavity mold makes one part per shot, a two-cavity mold two, and a family mold (one that makes several different parts, such as both halves of a case) one set. So 10,000 sets of a two-part housing on single-cavity molds is about 10,000 shots on each mold, plus a few percent for setup and scrap.

Why do mold quotes vary so much in shot life and price?

Shot life depends on who's selling. For the same aluminum grade, one shop's table says 2,000 to 5,000 shots, another says 10,000 to 25,000, and a third claims a million or more with the right plastic. Steel varies too, so compare the shot count each molder will commit to in writing.
Where the mold is built changes the gap. US rapid-tooling shops quote a simple aluminum mold in the low thousands of dollars, against $25,000 to $100,000 or more for a production steel mold, though that comparison often sets a simple single-cavity aluminum mold against a multi-cavity steel one. In many Asian tool shops the gap is much smaller: a simple single-cavity P20 mold can cost only somewhat more than aluminum, with first samples in a few weeks, so some teams there skip aluminum and start in steel.
The same words mean different molds. "Production mold" can mean a single-cavity P20 mold or a multi-cavity hardened mold with a hot runner. Compare the material, class, cavities and the life the molder commits to, not the label.

Should you start with an aluminum mold or a steel mold?

Aluminum first fits when:

the first run is likely to change the design, because changes on aluminum are cheap and the steel mold is then cut once, to the settled design;
you'd rather not pay for steel before demand is proven;
the molds are built where aluminum costs far less than steel, as in the US, which makes that insurance cheap;
the whole product's life might fit within what the aluminum mold can make, in which case you may never need steel at all.

Steel from the start fits when:

the design is settled: printed or cast parts have been tested and the design-for-manufacturing review is done;
the plastic is high-temperature or corrosive, such as some flame-retardant grades, or glass-filled at high volume;
the part needs a high gloss or fine texture that has to hold over tens of thousands of parts, or has several slides, lifters (similar moving pieces that form undercuts on the inside) or thin, tall ribs;
the molds are built where a single-cavity steel mold costs little more than aluminum;
the product will sell more than an aluminum mold can make.

If you start in steel, ask the toolmaker to cut fit-critical features steel-safe: leave extra metal where a dimension may still change, so the first adjustments only remove metal, which is quick and cheap.

Middle options:

An aluminum mold with steel inserts, often called a hybrid mold, at the gates (where plastic enters the part), the shutoffs (where two metal faces meet to form a hole) and the slides, where aluminum wears first.
A standard frame with interchangeable inserts, often called a master unit die (MUD). The molder owns the frame and you pay mainly for the inserts.
A family mold that makes both halves of a case in one shot. It saves a mold, but both parts share one plastic and color, run in equal numbers, and stop together when one needs a change.

Example: two paths for the same housing

This is an illustration, not a customer project. The product and all the numbers are made up.

A two-part housing needs 300 units first, then about 3,000 a year for three years, about 9,300 sets in all. Both paths assume the molds are built at the same shop.

Path A, aluminum then steel (made-up numbers): two aluminum molds at $8,000 each, with their first samples included, $16,000; later two single-cavity steel molds at $20,000 each, $40,000; and a second round of sampling and approval for the steel parts, $3,000. Total: $59,000. First parts come about three weeks sooner than on path B.
Path B, steel from the start (made-up numbers): two single-cavity steel molds at $20,000 each, $40,000, plus adjustment rounds after the first samples, $3,000. Total: $43,000.

In this made-up example, if the design is settled, path B saves $16,000. But if sales stop after the first 300, path A has spent $16,000 on molds and path B $43,000.

Now suppose the first 300 units show a change the steel can't absorb cheaply. Cutting metal out of a mold is easy; putting metal back means welding or a new insert. A thinner wall over a large area, or a reshaped outer surface, needs metal put back. (A small change, such as moving a screw post, can often be plugged or welded in P20 for much less.) On path B, that might mean new core or cavity inserts for both housing parts, here a made-up $15,000 and four weeks, bringing path B to $58,000. On path A, the same change means new aluminum inserts, about $4,000, and the steel is still cut once, to the settled design, bringing path A to $63,000. With these numbers, one such change still leaves path B $5,000 cheaper but four weeks later; two make path A cheaper, $67,000 against $73,000.

The general rule: aluminum first pays when the design changes you expect would cost more on steel than a second set of molds and a second round of approval, or when you want less money at risk before demand is proven. A third path can cost the least: if the molder commits in writing to aluminum molds that last all 9,300 sets and hold the finish, stay on aluminum and never buy steel.

Piece price can go either way. Aluminum cools faster, but some rapid-tooling shops run simple molds with hand-loaded inserts or basic cooling and price parts higher. Compare the quoted piece prices at your quantities: across about 19,000 parts, every 10 cents is about $1,900.

When should you switch from an aluminum mold to steel, and what has to be redone?

When to order the steel mold. Once the design is frozen, and while the aluminum mold still has enough life left to cover the steel mold's build and sampling, often two to three months of demand. Keep the aluminum mold until the steel parts are approved, and build some stock before you retire it.
New first samples. Expect new first shots from the steel mold, a check of the critical dimensions, and one to three rounds of adjustments: often two to six weeks of sampling on top of building the mold.
Gate, parting line and ejector pins. Ask the steel mold's maker to keep the approved gate location, parting line and ejector-pin layout. If they move, gate marks, knit lines and pin marks move with them, which matters on cosmetic faces, and more so when a different shop builds the steel mold.
Shrink and settings. Steel and aluminum move heat differently, so the molder's settings and the part's shrink shift. If the steel mold uses less draft (the slight taper that lets the part come out of the mold) than the aluminum one, the part changes and needs approval again.
Fit with everything else. Recheck snap fits, screw posts, button feel, light pipes and gasket compression with the boards and mating parts; a shift of 0.1 mm can change how a button feels.
Color, gloss and texture. The same texture specification can look different on a new steel surface. Check new parts side by side with the approved sample, then sign a new golden sample, the approved part the factory checks production against.
Certifications. FCC testing depends on the electronics, so a housing change with the same boards, the same antenna position, the same plastic and no new metal normally doesn't need new testing; confirm with your test lab. For safety listings, the certification file (for UL, the follow-up procedure the factory is inspected against) usually names the resin's maker and grade, its flame rating and the minimum wall thickness. The same grade and color at the same or greater thickness usually keeps it; a substitute "equivalent" grade, a different colorant or a thinner wall needs the certification body's review. Write the exact grade into the purchase order, especially when the steel mold is built in another country.

What should you ask a molder before ordering an aluminum or steel mold?

The mold material, the SPI class the quote assumes, and the shot count the molder will stand behind in writing. An aluminum quote that says class 105 covers no more than 500 shots.
The grade of metal for the cavities and cores, written on the mold drawing, with the material maker's certificate.
What happens if the mold wears out early: who repairs or rebuilds it, and at whose cost.
The number of cavities, whether it's a family mold, and where steel inserts are used.
A design-for-manufacturing and mold-flow report, showing the gate, parting line, ejector pins and draft, to approve before any metal is cut.
The time to first samples, and how many rounds of samples are included.
What a design change costs, and how long it takes, on this material.
Whether your plastic suits the mold material, especially glass-filled or flame-retardant grades.
Which finish and texture the mold can hold, and for how many parts.
Who owns the mold, or the inserts if it's a standard frame, where it's kept, and whether another shop could run it.
If you start in aluminum: whether the same shop will build the steel mold, and whether any of the aluminum mold's cost is credited toward it.

For the design side of molding, see Moving from a 3D-printed prototype to injection molding; for the processes below a few hundred units, 100 to 1,000 units: bridge tooling, urethane casting or machining?; and for what a mold change costs later, Where does unit cost drop on a second production run.

Where we come in

If you're deciding how to tool your first run, this is one of the situations Anamor takes on. We start from your target unit cost, your quantities and how settled the design is. If the first run is likely to change the housing, we may suggest aluminum molds first and steel once the design settles; if the design is settled and steel pays off at your volume, steel from the start. If neither pays at your quantity, we suggest a bridge process such as urethane casting or machining, or tell you plainly if we're not the right fit. Our quotes list tooling separately from unit prices, at the quantities you're planning, and the sample production review on How it works prices one plan both ways.

Anamor takes US hardware teams with a working prototype into their first production run: we redesign the product for production, line up partner factories and suppliers in the US and overseas, price it at your quantities, and run the build through to shipping.

Send what you have: photos of the prototype, CAD if you have it, the plastic you have in mind, and how many you plan to make, now and over the next few years. Need an NDA? We'll send our mutual NDA, or review yours, before you send files. Within two business days, a real person on our team will reply with a free first look at your production run and a few times to talk.

Frequently asked questions

How many shots does an aluminum injection mold last?

An aluminum injection mold usually lasts a few thousand to tens of thousands of shots, or mold cycles, against about 50,000 to 500,000 for pre-hardened steel such as P20. Glass-filled plastics wear aluminum faster, and shops' claims for the same grade range from 2,000 to a million or more, so ask the molder for the shot count it will commit to in writing.

When should you switch from an aluminum mold to a steel mold?

Switch once the design is frozen and demand will outgrow the shot count the aluminum mold is rated for. Order the steel mold while the aluminum one still has enough life to cover building and sampling the steel mold, often two to three months of demand, and keep the aluminum mold until the steel parts are approved.

Does switching from an aluminum to a steel mold require new FCC or UL testing?

Usually not for FCC: emissions depend on the electronics, so a new housing with the same boards, antenna position and plastic, and no new metal, normally needs no new test. A safety listing's certification file names the resin grade, flame rating and minimum wall thickness; keeping all three, and the same color, usually keeps the listing, but the certification body decides.