Aluminum or Steel Mold? Choosing for Your Stage

A lot of people treat an aluminum mold as a cheap stand-in for the real one, something to put up with until they can afford steel. I think that gets the decision wrong. Both metals make good parts. What decides between them is the stage your product is at.

The short answer: aluminum suits a part whose design may still change, and how long it lasts depends on the resin, the use and the care it gets. Steel suits a frozen design, a run that will go on for years, or a resin that would wear aluminum out. An aluminum mold used between prototype and production is often called bridge tooling, or a prototype mold. For most first products, my view is that it's the sensible place to start.

Aluminum probably suits you if:

• Your design is still moving
• You want real molded parts to test, show or sell before committing to production tooling
• Your resin isn't glass-filled or otherwise abrasive

What the injection mold decides

An injection mold is a block of metal with the shape of your part cut into it. Molten plastic is pushed into the cavity, cools, and comes out as a part. The mold is the large upfront cost. After the mold, each part costs mostly plastic and press time, which is why a part with a long cooling time isn't cheap just because the mold is paid for, and why the metal you choose shows up in the price of every part that follows.

How the metals differ

• Aluminum moves heat faster than steel, so parts often cool and cycle faster. It is easy to cut, which keeps the mold cost down. Glass-filled resins wear it much faster. Aluminum needs more care than steel, and it does not stand up to abuse.
• Pre-hardened P20 steel sits in between. It machines reasonably well and lasts longer than aluminum.
• Fully hardened tool steel is for the longest runs and the most abrasive materials. Hardened steel is slower to cut and slower to change, and fine features in it are often burned in with sinker EDM rather than cut.

The shop has built molds in both aluminum and P20 steel. Separately, its molding runs have gone from tens of parts to tens of thousands.

We design our molds and cut them in our own shop, typically in about two weeks.

When the design is still moving

The usual line is that aluminum is easier to modify. That's only half true. Taking metal out of aluminum is easy. Adding metal back by welding is harder than in tool steel, and some aluminum grades weld poorly.

Good mold makers plan for this by cutting steel-safe. They leave metal where a change is likely, so adjusting the part means removing metal rather than welding it back. If you expect a wall to get thicker or a boss to grow after the first parts are tested, say so before the mold is cut. It's one of the most useful things a client can tell a mold maker.

Where the mold sits while the design changes matters too. When the people who designed the mold work beside the machine that cut it, a change to your part is a conversation in one building. How design, mold making and molding work at one shop

When steel is the right call

If your design is frozen, your volume is large and steady, or your part is in a glass-filled resin, go to steel. Buying aluminum first would mean paying for a mold you already know you'll replace. And past a certain volume, a production molder running hardened multi-cavity molds will usually beat a short-run shop on cost per part. The signals for that move are in when to move from a short-run shop to a production molder.

If you're selling a 3D-printed product now and wondering when a mold pays for itself, my guide to going from 3D printed parts to a first molded run walks through the crossover and the questions to ask before you pay a deposit.

Pick the mold for the product you have now, and leave room in it for the one you'll have after your first customers use it.

Related services: Aluminum tooling, molding and machined parts Shop capabilities