Epoxy Tooling: It Moves Before It Wears

What epoxy actually buys

An epoxy tool is cast or machined from a tooling board, and it is ready in about five working days. An aluminium tool takes fifteen and a steel tool forty. That lead time is the product. A product that is still being designed cannot wait six weeks for steel, and a bridge tool that will be thrown away when the design settles should not cost as much as the final tool.

The second thing it buys is size. Epoxy is cast, so a very large tool does not require a very large block of aluminium. For a part over about a square metre, the cost advantage of epoxy grows faster than the size, because the alternative is machining away most of an expensive billet.

The third is repairability of the concept rather than the tool. An epoxy tool can be patched with more epoxy, which is not a precision repair but is often enough to get through a trial. That is a legitimate use and it should be planned for rather than discovered.

The cavity moves with temperature before anything wears

Epoxy has a coefficient of thermal expansion around 30 to 45 parts per million per degree, which is three to five times that of aluminium and ten times that of steel. A tool that is 400 mm across and 60 degrees warmer than the temperature at which it was machined grows by 0.7 to 1.0 mm across that dimension.

Cavity growth measured against tool surface temperature: nothing at 20 C, 0.02 mm at 40, 0.06 at 60, 0.15 at 80 and 0.40 at 100. Against a part tolerance of 0.10 mm, the tool is out of tolerance past about 70 C. That is below the temperature a thermoforming tool reaches when it is running polypropylene at a normal cycle.

So the practical limit on an epoxy tool is not the number of parts. It is the sheet material and the forming temperature. An epoxy tool running PVC at 60 C is fine for thousands of parts. The same tool running polypropylene at 145 C sheet temperature will not hold tolerance on the first shot, and the parts will be oversize in a way that looks like a material shrinkage problem.

The seal edge is the part that wears

A thermoforming tool has a land around the cavity that crushes the sheet and seals it against the vacuum. On an epoxy tool that land is the weakest feature, because epoxy is brittle and the land is thin. It rounds off with use, and a rounded land does not fail suddenly: it produces webbing, then poor detail, then parts that will not draw at all.

The failure is progressive and it is easy to miss because the parts still come out. A tool that has run 3,000 parts has a land with a radius of maybe 0.3 mm instead of a sharp edge, and the operator compensates by raising the vacuum and the temperature, which accelerates the wear. By the time the parts are visibly bad, the tool is past saving.

Table 1 — Where an epoxy tool degrades, and how it shows on the part

Where epoxy is the right tool and where it is a trap

Epoxy is right for prototypes, for bridge tooling before a production tool, for large parts at low volume, and for any project where the design is likely to change within a few months. In each of those cases the tool is a means to a decision rather than an asset, and the cheapest tool that answers the question is the correct one.

It is a trap when the volume is quoted as "about five thousand a year for three years" and nobody multiplies. Fifteen thousand parts is three epoxy tools, three tool changes, three qualification runs and three interruptions, and the total cost is higher than the aluminium tool that would have done all of it.

Table 2 — Where an epoxy tool is the right answer, and where it is the expensive one

Getting the most out of a tool you know is temporary

If the tool is going to be replaced, the goal is to get the volume out of it before the land goes. Three things help and they cost almost nothing.

• Run the coolest tool temperature the part will tolerate, because thermal growth is linear in temperature.
• Keep the seal land sharp by touching it up on a schedule, before webbing appears.
• Record the cavity dimensions at the start of the run, so that a later dimensional complaint can be attributed to the tool rather than to the material.

The dimensional record is the one nobody keeps, and it is the one that ends arguments. A part that measures 0.2 mm oversize in month three is either a tool that has grown or a material lot that has changed, and a baseline measurement from day one tells you which within a minute.

What to specify

An epoxy tool specification has to state the temperature it will run at, because that is what sets its dimensional life.

• The maximum tool surface temperature, which follows from the sheet material.
• The cavity dimensions at 20 C, and the dimensions required at running temperature.
• The seal land geometry, and whether it is a replaceable insert.
• The expected volume, stated as a total rather than as a rate.
• Whether the tool is a bridge, and what it is a bridge to.

Then measure the cavity when the tool is hot, not when it is cold. A cold measurement of an epoxy tool tells you what was machined. A hot measurement tells you what the parts will be.