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Should I choose an aluminium or steel injection mould?

Choose aluminium for prototypes, bridge runs and lower volumes while the design may still change, and steel for long production runs, abrasive or high-temperature resins and fine polished finishes.

Most of the decision comes down to four things: how many parts you need over the tool's life, which resin you are moulding, what surface the part needs, and how settled the design is.

The trade-off in one table

Published aluminium life figures vary widely: Komaspec gives a maximum of 1,000 to 3,000 shots, MakerVerse 2,000 to 10,000, and Fictiv says a well-built aluminium tool run under the right conditions can reach 50,000 to 100,000 cycles, although Fictiv's tooling-life guide says aluminium usually supports 10,000 shots or fewer. Some of the higher figures come from suppliers who sell aluminium tooling, so ask your toolmaker to state the expected life for your part and resin.

Why aluminium is cheaper and quicker

Fictiv and Precision Expedited both put aluminium machining at roughly three to ten times faster than steel, and Fictiv says electrical discharge machining is usually four to five times faster too. Aluminium also conducts heat well, which Fictiv says allows simpler and cheaper cooling. Protolabs recommends aluminium when engineering changes are still expected.

Common aluminium tool alloys named by Fictiv are 7075 and QC-10, with 6061 used for some low-volume bridge tools.

Why steel earns its price

Steel is harder, so it resists wear from glass-filled resins and holds a finer polish. Precision Expedited names hardened H13 for fine polish and wear resistance, and pre-hardened P20 as a common production choice. Fictiv says aluminium is not suitable for abrasive plastics such as glass-filled nylon, and that corrosive grades such as PVC and POM can wear aluminium tools quickly. Protolabs recommends steel for volumes of a million or more and for high-temperature materials such as PEEK or Ultem.

Steel also suits more cavities. Protolabs offers aluminium tools with one, two, four or eight cavities, and steel above eight.

Steel is not always the dearer option. Fictiv notes that steel inserts in a standard interchangeable frame (often called a MUD frame) can be cost-competitive with a standalone aluminium mould.

The middle route

A soft-steel or bridge mould sits between the two. MakerVerse lists it at around €8,000 to €20,000, 4 to 8 weeks to build and up to 100,000 shots. Fictiv gives P20 a life of 50,000 to 100,000 shots. A bridge tool lets you start selling while a long-life steel tool is built, or covers a product whose demand is not yet proven.

How to decide

1. Estimate lifetime volume: annual quantity times expected product life, divided by cavity count, gives cycles.
2. Compare that number with the published life ranges above, using the conservative end for aluminium.
3. Check the resin. Glass-filled, abrasive, corrosive or high-temperature grades point to steel.
4. Check the surface. A high-polish or optical face points to hardened steel.
5. Check design stability. If the geometry may still change, the cheaper tool costs less to modify or replace.
6. Check timing. If parts are needed fast, aluminium or a bridge tool may get you to market while steel is made.
7. Ask for quotes on both, with tool life, cycle time and piece price at your volume, not just tool cost.

Formlabs' worked example shows the pattern: a machined aluminium mould is cheapest per part at 5,000 parts, and a machined steel mould at 100,000.

These are published planning figures from suppliers and design guides. Confirm tool life, finish and resin suitability with your toolmaker.

Questions buyers ask

Can I start in aluminium and move to steel later?

Yes, and many projects do. Plan for it: the steel tool is a new build, so budget for its own trial shots and inspection, and check parts from the two tools against each other.

Will aluminium parts look different from steel parts?

The part is the same resin, so function is the same, but finish can differ. Fictiv says harder steel surfaces support finer polish, so for a high-gloss cosmetic face, ask the toolmaker which finishes each tool can hold.

Does aluminium mean a slower cycle?

Not necessarily. Fictiv and Protolabs both say aluminium moves heat out faster than steel, which helps cooling. Cycle time still depends mainly on part wall thickness and resin.

Is an aluminium mould suitable for glass-filled nylon?

Generally not for long runs. Fictiv says aluminium is not suitable for abrasive plastics such as glass-filled nylon, and Protolabs says such resins wear aluminium faster. For a short trial, ask the toolmaker for the expected life with your exact grade.

Sources

• MakerVerse, Injection molding FAQ: https://www.makerverse.com/resources/injection-molding-faq/
• Formlabs, Injection molding cost: https://formlabs.com/blog/injection-molding-cost/
• Precision Expedited, Injection molding companies: questions to ask: https://precisionexpedited.com/injection-molding-companies-questions-to-ask/
• Protolabs, Aluminum vs. steel tooling: https://www.protolabs.com/resources/blog/aluminum-vs-steel-tooling/
• Protolabs, Soft vs. hard tooling: https://www.protolabs.com/resources/blog/soft-vs-hard-tooling/
• Fictiv, Steel vs. aluminum for injection molds: https://www.fictiv.com/articles/steel-vs-aluminum-for-injection-molds
• Fictiv, What to know about injection molding tooling life: https://www.fictiv.com/articles/what-to-know-about-injection-molding-tooling-life
• Komaspec, Injection mold tooling: https://www.komaspec.com/about-us/blog/injection-mold-tooling/