Aluminium Tooling: It Is Not Cheaper, It Just Survives

The tool is the cheapest part of the project and it decides the rest

In a thermoforming project the tool is usually 5 to 15 percent of the total cost of getting to a saleable part. It is also the item that decides the cycle time, the tolerance, the surface finish and the maximum volume, which means it decides everything that happens after it is paid for. Optimising the tool price is optimising the smallest number in the project.

The three materials in common use are epoxy, aluminium and steel. Epoxy is cast or machined from a tooling board, aluminium is machined from plate or billet, and steel is machined and often textured. The price ladder runs roughly 2,500, 11,000 and 55,000 for a comparable part, and the life ladder runs 5,000, 100,000 and over a million.

The decision looks like a simple division, and it is not, because the three materials do not produce the same part. They differ in heat transfer, in surface finish, in how they wear and in whether they can be repaired. A cost per part calculation that ignores those differences is comparing three prices rather than three tools.

Epoxy never loses on price, it loses on life

Cost per part is the tool cost divided by the volume plus a variable cost that covers wear, maintenance and the occasional repair. For epoxy that is 2,500 divided by the volume plus 0.03. For aluminium it is 11,000 divided by the volume plus 0.008.

At 2,000 parts epoxy is 1.28 per part and aluminium is 5.51. At 5,000 parts epoxy is 0.53 and aluminium is 2.21. At 20,000 parts epoxy is 0.155 and aluminium is 0.558. Epoxy is cheaper at every volume where it can be used, and the gap closes slowly: aluminium only crosses epoxy at around 390,000 parts, which is seventy-eight times epoxy's life.

So the honest statement is not that aluminium is cheaper at volume. It is that epoxy stops existing at about five thousand parts, and above that volume aluminium is the cheapest tool that still exists. The purchase is a life decision, and the price comparison is a distraction.

Life is what you are buying

An epoxy tool fails in a characteristic way. The resin softens as it heats, so the tool loses dimensional stability above about 80 C, which limits the sheet materials it can run. The seal edge, which is the thin land that crushes the sheet and makes the vacuum seal, wears and rounds off, and a rounded seal edge produces webbing and poor detail rather than a clean failure. And epoxy cannot be welded, so a damaged tool is a replaced tool.

Aluminium wears more slowly and can be repaired. A damaged seal edge can be welded and re-machined, a worn cavity can be re-polished, and a tool that has lost its surface can be resurfaced several times before it is scrap. That repairability is worth more than the wear resistance, because it turns a tool replacement into a tool repair.

Steel is the material for tools that have to survive a decade, hold a texture through millions of cycles, or run engineering materials at 200 C. It is also the material that takes the longest to make, which is why it is rarely the right answer for a product that is still being developed.

Conductivity is cycle time

A thermoforming tool has to remove heat from the sheet, and the rate at which it does that is set by its thermal conductivity. Epoxy runs about 0.25 W/mK, steel about 30 and aluminium about 170. The cooling time that follows from that, for a 1.2 mm sheet, is roughly 22 seconds on epoxy, 15 on steel and 8 on aluminium.

That difference is often worth more than the tool price difference. On a 45 second cycle, moving from 22 seconds of cooling to 8 saves 14 seconds, which is 31 percent more parts per hour from the same machine and the same labour. Over a hundred thousand parts, the aluminium tool pays for itself in machine time alone, before any of the life argument.

The conductivity also changes the part. A tool that removes heat quickly produces a part with lower moulded-in stress and better dimensional stability, because the sheet is held against the surface until it is below its softening range rather than being released while still soft. That is why aluminium tools hold tolerance better even when the geometry is identical.

Table 1 — The three tool materials on the properties that decide the project

Where epoxy is genuinely the right answer

Epoxy is not a compromise for every project. It is the correct choice when the volume is low, when the design is still moving, when the part is large enough that an aluminium tool would be very expensive, or when the sheet is thin and the cooling requirement is modest.

Table 2 — Choosing a tool material by volume and requirement

The failure mode to avoid is the middle. A project that buys an epoxy tool for 20,000 parts is buying two tools and a shutdown, and the second tool will be ordered in a hurry at a premium. The cheapest total cost is usually a tool whose life is at least twice the expected volume.

What to specify

A tooling specification should state what the tool has to do, not only what it is made of.

• The expected volume over the product life, not the first order.
• The sheet materials and the maximum forming temperature, because that sets the tool material.
• The tolerance the part must hold, which is a conductivity requirement as much as a machining one.
• The cooling arrangement: channels, baffles, and whether the core is cooled.
• Whether the tool must be repairable, and whether the seal edge can be replaced.

Then ask for the cooling time at the specified sheet thickness, in writing. A tool quote that does not state a cooling time is a quote for a shape, not for a process.