Precision Tooling for High-Volume Production
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In high-volume production the tool holds the tolerance, not the operator. A mold, die or fixture that's slightly off makes every part slightly off, for hundreds of thousands of cycles. These are the things that decide whether it stays right.
Give the tool a small slice of the tolerance
A common rule of thumb for gauges is to make them to 10 percent of the part tolerance. Production tools don't need that everywhere, but critical features should use only a small part of the tolerance band. The rest is needed for things the toolmaker can't control: material variation, shrink, press and machine variation, and wear. A tool built at the edge of tolerance on day one has nothing left for month six.
Pick the steel for the volume and the material
- Pre-hardened P20, around 30 HRC, is a common choice for molds at moderate volumes. It machines easily and is quick to modify.
- Hardened H13, typically around 48 to 52 HRC, lasts much longer and stands up to glass-filled and other abrasive resins.
- Stainless mold steels such as 420 resist corrosion from resins like PVC and from humid storage, and polish well for clear parts.
- D2 and similar cold work steels, around 58 to 62 HRC, are typical for cutting and forming dies.
Aluminum molds have their place too, for prototypes and low volumes. We compare them in aluminum vs steel injection molds.
Plan for wear
Every high-volume tool wears, so put the wear where it's cheap to fix. Use replaceable inserts at gates, cutting edges and other high-wear areas, standard components for ejector pins, punches and guide pins, and keep a spare kit for the parts that wear first. A worn insert gets swapped between runs. A worn cavity block can mean weeks of welding and recutting while the line waits.
Prove it with numbers
Before a tool is released, the parts it makes should be measured, not eyeballed. First article inspection checks every dimension once. Capability studies check that the process holds them run after run. For automotive PPAP submissions, an initial study usually needs a Ppk of at least 1.67 on key characteristics, and 1.33 is a common minimum for ongoing production.
The measurement has to be trustworthy too. A gauge R&R under 10 percent is generally acceptable, 10 to 30 percent may be acceptable depending on the application, and anything over 30 percent isn't.
Keep it in spec
Count cycles or strokes and service the tool on a schedule, not when parts go bad. For molds that means cleaning vents and checking parting lines, ejectors and water lines. For dies it means sharpening punches and checking clearances. Keep a record for each tool: cycles, repairs and measurements of the last good parts. When a dimension starts to drift, the record tells you whether it's wear, a process change or a material change.
Fixtures and gauges are tooling too
Machining fixtures, welding jigs and checking fixtures follow the same rules: locate the part the same way every time and build in replaceable wear points. Our jig vs fixture guide covers the basics.
We design and build molds, dies and fixtures for manufacturers, including Ontario automotive suppliers. See tooling, and what drives the cost of a mold.