Thermoforming Mold Cost: Where the Money Actually Goes

The tool is mostly machining, not material

A single-cavity aluminium forming tool is usually quoted as one number, and the number hides a structure that matters when the project changes. Broken down, the aluminium block is about 34 percent of the cost, machining is about 38, surface texture is about 14, the vacuum system is about 8 and the trial run is about 6.

The machining share is the one that surprises people, because a forming tool is not a precision mould. It is a block with a cavity milled into it, drilled for vacuum, and finished by hand. What makes it expensive is not tolerance but size: a large tool needs a large machine, and large machine time is charged by the hour.

The texture share is the one that gets forgotten in a budget. A tool that produces a part with a grain, a matte finish or a logo needs that pattern cut, etched or applied, and it is a separate operation with a separate lead time. A project that budgets for a tool and then adds texture has under-budgeted.

The second cavity is the cheap one

Cost per part falls steeply with cavity count and then flattens. Taking a single-cavity tool as 100, a two-cavity tool gives 68 per part, a four-cavity 52 and an eight-cavity 46. The second cavity is where most of the gain is, because the block, the frame, the vacuum system and the setup are already paid for.

Past four cavities the curve is almost flat, because the extra cavities add machining and aluminium almost linearly while the shared costs are already amortised. An eight-cavity tool costs nearly twice a four-cavity tool and saves only about 12 percent per part.

This is why cavity count should be chosen from the annual volume and the changeover cost, not from a rule of thumb. If the volume is not there to keep eight cavities busy, the per-part saving never arrives and the tool is simply more expensive.

What actually moves the number

Depth is the first driver. A shallow tray in a 40 mm block and a deep draw in a 200 mm block are different tools with different machine time, and the difference is not linear in depth because deep cavities need draft, radii and often a multi-part tool.

Size is the second. Cost roughly follows the volume of aluminium removed and the footprint of the machine that can hold it. A tool that fits a 1 m machine and a tool that needs a 2.5 m machine are in different cost classes even if the part is the same shape.

Tolerance is the third, and it is the one most often over-specified. A forming tool does not need mould tolerance. Asking for a tool that holds 0.1 mm on a part that will be trimmed afterwards adds machining time for a tolerance that the process cannot deliver anyway.

Table 1 — The six things that decide a forming tool quote

What a cheap tool costs later

An epoxy tool on a wood backing costs a fraction of an aluminium block tool, and for a prototype or a short run it is the right answer. What it does not do is hold its shape. Epoxy creeps under heat and load, and a tool that was flat when it was made is not flat after a thousand cycles at 120 C.

The failure is slow and it looks like a process problem. Parts start to show a variation in depth across the run, then a variation between the start and the end of a shift. The operator adjusts the heater, which makes it worse, and the tool is the last thing anyone suspects.

Wood pattern tools are cheaper again and are for one-off parts. They are also dimensionally alive: wood moves with humidity, and a pattern that was correct in January is not correct in July. For a single part that is acceptable; for a production run it is not.

Table 2 — Tool materials, what they cost and what they last

The trial is six percent of the quote and most of the risk

The trial run is the smallest line on the quote at about 6 percent, and it is where a project either lands or does not. The tool is tried in the production sheet at the production temperature, the first parts are measured, and the draw, the radii and the venting are adjusted. That is one iteration if the tool was right and three if it was not.

The cost of a bad trial is not the trial. It is the tool modification, the material, the machine time and the week of schedule, and it is usually larger than the trial line itself. A tool quoted with one trial and no modification allowance is a tool that will produce an invoice for modification.

This is why the drawing package sent with the enquiry matters more than the price. A toolmaker who is given the formed part, the draw depth, the sheet and the trim method builds a tool that needs one trial. One who is given a shape and a hope builds a tool that needs three, and the difference is in the modification hours, not in the aluminium.

How to ask for a quote that can be compared

Two forming tool quotes are usually not comparable, because one includes texture and the other does not, or one assumes a trimmed part and the other assumes the tool produces the finished edge. The way to make them comparable is to send the same package to both: the 3D part, the draw depth, the sheet material and thickness, the texture, the tolerance on the formed part and the annual volume.

It also helps to say what happens after forming. If the part is trimmed on a CNC fixture, the tool does not need to hold the trim edge, and that is a real saving. If the part is trimmed by hand, the tool has to give the operator a line to follow.

And it helps to ask what the quote excludes. Freight, trial material, texture samples and modification after the first trial are the four things that most often turn a quote into an invoice.