Aluminum vs Steel Injection Molds: Cycle Time, Tool Life and What It Actually Costs

Aluminum wins on cycle time and loses on tool life. Which of those matters more is decided by the total piece count, not by the price of the mold. The most expensive mistake in this decision is quoting the mold in isolation. A mold is a capital item bought once; the cycle time is a cost paid on every shot for the life of the program, and on a long-running job it is the far larger number.

This article covers thermoplastic injection moulding. The figures are from a GLEICH technical study, and the tool life guidance at the end also covers polyurethane and elastomer work, where the answer is frequently that aluminum is the wrong material.

One note on the money. The study was costed in Germany and the figures below are converted at CAD 1.6161 per EUR, the Bank of Canada rate for 2026-08-06. The machine rate is a converted German rate, not a surveyed Ontario rate. Use your own. The conclusion does not depend on it, and the reason why is set out after the table.

The system cost, worked through

The example is a moulded ABS cover. The outer face carries a simple waffle texture, so the cavity has to be etched. The inner face has several retaining claws, which are undercuts, released by hydraulic slides so the finished part drops out under gravity and no ejectors are needed.

• Total quantity: 20,000,000 pieces
• Two-part mold with 10 cavities, so 20,000,000 parts is 2,000,000 shots
• Compared: steel St37 (S235JR) against G.AL® C330R cast aluminum plate

First, the plate. The aluminum mold is built slightly heavier in section, because the material is softer and the sections have to carry the same clamping load.

Now the part everyone stops at. The steel mold is rated for at least 2,000,000 cycles, so one mold covers the program. C330R is rated for 500,000, so the job needs four molds.

Each aluminum mold is 24% cheaper to build than the steel one, because it machines faster. But four of them cost roughly three times the single steel mold. At this point a lot of projects are decided, and aluminum is dismissed.

That is the mistake. The mold is not the cost of the job. It is a line item in the cost of the job.

The same two molds, costed as a system

Thermal conductivity is where the difference comes from. Steel St37 conducts at 24 to 29 BTU/h·ft·°F (42 to 50 W/m·K). C330R conducts at 72 to 90 BTU/h·ft·°F (125 to 155 W/m·K), roughly three times as much. Heat leaves the part faster, so the part is ready to eject sooner.

The aluminum route finished CAD 499,770 cheaper, which is 31% less per part, despite costing three times as much in tooling. The extra CAD 116,230 spent on molds bought back 5,867 machine hours.

Run this on your own job before you quote it. You need four numbers: piece count, cycle time in each material, your machine rate, and the mold cost in each material. If you can get the first two, we can help with the last two.

Where the 40% actually comes from

A 40% cycle reduction sounds like the kind of number that comes with conditions. It is worth seeing which part of the cycle changes, because only one part does.

Injection and hold pressure are identical. Cooling falls by 54%. Everything else is rounding.

That single row explains where aluminum earns its money and, just as usefully, where it does not. Cooling is 71% of the steel cycle. On a thick-walled part that holds a lot of heat, the saving is large. On a thin-walled part where cooling is already a small share of the cycle, aluminum saves proportionally less and the case gets weaker. Look at your own cooling fraction before you assume the 40% transfers.

Realistic tool life by polymer

Tool life is what people want a straight answer on, and there is not one. It depends on part geometry, surface texture, insert and overmoulded parts, mold construction, guide bushings and fit, cooling circuit design and coolant, maintenance, machine condition, resin moisture content and handling. GLEICH publishes the following as guide values from experience, and they are non-binding.

Read them as a starting point for a conversation, not a warranty.

The bands are: prototypes to 5,000 pieces, small series to 50,000, mid series to 500,000, large series to 2,000,000. For the three aluminum columns the entry is the highest band that material reaches. The steel column reads differently: steel will run any of these, so the entry is the point from which steel starts to earn its higher build cost.

Four things are worth pulling out of that table.

• PP, PE and polystyrene are where aluminum reaches furthest. Low melt temperature, moderate pressure. This is why so much packaging and automotive trim tooling is aluminum, and on PP and PS the 7000-series rolled grades reach large series outright.
• Polycarbonate is where aluminum effectively stops. At a 518 to 608 °F melt and up to 29,000 psi, C210R is not suitable at all and the higher-strength grades manage prototypes only.
• C210R outlasts the stronger grades at high mold temperature. On PEI, on elastomers and on high-temperature resin, C330R and 7075 are marked not suitable while C210R still reaches prototypes or small series. That is not a mistake. EN AW-5083 is non-heat-treatable, so it cannot lose strength by over-ageing; the 7000 series can, and does.
• RIM and R-RIM invert the usual order. C210R is not suitable, while C330R and 7075 manage prototypes. Reaction pressure and the shear from the heated cavity skin are what govern there, not temperature.

Which G.AL® grade for which job

Two alloy families, four products. The alloy decides the strength and the temperature behaviour; the surface condition decides how much machining you do before you start cutting the cavity.

Beyond the G.AL® range, the study also covers materials we do not supply, and it is worth being straight about that. Rolled EN AW-7075 or 7022 reaches further than C330R on several polymers. Rolled 6082 or 6061 is the right call for higher-density PUR foam and for any cavity that has to take an even etched texture. And where the resin is filled with glass, carbon or talc, the answer is usually steel.

Where aluminum is the wrong answer

This is the part of the study most likely to save you money, because it prevents a mold from being built at all.

Filled resins

Glass fibre, carbon fibre and talc are severely abrasive. In a filled-resin application, aluminum makes sense only for one-offs or very small quantities. Surface treatment does not rescue it: hard anodising or electroless nickel slows the wear but does not stop it, and aluminum remains a soft substrate underneath.

R-RIM

Reinforced reaction injection moulding is the clearest case. The fillers work the cavity surface away fastest around the gate. If the surface is etched, the difference between the gate area and the rest of the cavity is visible on the first part off the tool, and by about the fifth part the etch is only fragmentary. Dimensional accuracy goes the same way. For production parts, steel is the only sensible material. For a prototype or pre-series run, an aluminum tool needs hard coating with genuinely good sealing, or hard enamel. Hard anodising is not enough.

RIM

Unreinforced RIM is injected at 1,450 to 2,900 psi (100 to 200 bar), and the reaction generates at least 158 °F (70 °C), rising to about 194 °F (90 °C) in sections thicker than 0.4 in. (10 mm). That heat penetrates only about 0.06 in. (1.5 mm) into the metal, so the cavity skin tries to expand while the bulk of the mold does not, and the resulting shear in the metal is severe. Only high-copper alloys, EN AW-2017A and EN AW-2219, hold up. Rolled 7075 works at low quantities. Cast plate is not recommended for RIM at all, including ours.

Elastomers

Rubber processing is dominated by steel molds, for two separate reasons. Cast 5083 plate such as C210R handles the temperature without difficulty, but its strength falls far enough at working temperature that the mold deforms after only a few releases and thin features tear away, so only low-quantity prototypes are realistic. The 7000 series is ruled out on thermal stability. The 2000 series would be the obvious answer, except that several rubber compounds react catalytically with the copper in the alloy, and what comes out of the cavity is an adhesive mess that has to be dug out.

When a cavity gets damaged

The first instinct is usually to weld it. That is almost always the wrong move on an aluminum mold, for three reasons.

• Mold plate is usually a higher-strength aluminum. Welding causes local, total and irreversible softening, which can lead to outright failure of the repaired area.
• Most high-strength aluminum alloys, including C330R, are not weldable in the first place.
• The heat-affected zone changes the microstructure and the weld itself has a different structure again. Both change the local thermal conductivity. On some polymers that shows up on the finished part as a shadow or a colour change, sometimes called a phantom shadow, and it can appear even when the weld itself is invisible.

The better repair is an inlay: machine out the damaged area and shrink-fit a plug of new material. It costs more than a weld and it does not produce shadowing.

Two constraints people find out about late

Food contact

Not every aluminum alloy is approved for food contact. DIN EN 602 governs it. The 5000 series and most of the 6000 series are approved. The 2000 and 7000 series are not. In G.AL® terms that means C210R and C250 are approved and C330R and C330 are not. If a 7000-series mold is used for something like yoghurt pots, it needs a food-safe surface treatment such as electroless nickel plating.

Etched textures

Where the cavity needs an etched grain, rolled plate in 5083, 6082 or 6061 gives a more even etch than any cast plate, ours included. This is a genuine limitation of cast plate and it is worth knowing before the tool is designed rather than after the first etch trial. If the texture is critical, ask us and we will say so.

The short version

• Cost the program, not the mold. On the study job, four aluminum molds at three times the tooling cost still came out 31% cheaper per piece.
• The saving is cooling time. Injection and hold do not change. Check what share of your cycle is cooling before assuming 40%.
• Unfilled resin, high volume: aluminum is usually right. Filled resin, R-RIM, elastomers, or PC: usually not.
• C210R is the default and covers most Canadian mold work. C330R when you need the strength or the extra tool life.
• Do not weld a damaged cavity. Use a same-material inlay.