Cup Mould Maintenance: The 120 000 Cycle Interval and What Wears First
A cup forming tool set is not one mould but five: the mandrel, the bottom die, the curling ring, the knurl ring and the blanking die. They wear at different rates, and the one that fails first is almost never the one maintenance watches. Setting inspection intervals by cycle count rather than by calendar, and knowing which part wears at 120 000 cycles, is the difference between planned and unplanned downtime.
The Mandrel and Its Wear Mode
The mandrel is the male form the cup is wrapped around. It wears at the shoulder radius and along the side where the blank slides. At 120 000 cycles a hardened steel mandrel shows 0.02 to 0.04 mm of wear at the shoulder, which is enough to change the cup's top diameter by 0.3 mm and make lid fit inconsistent.
Measure the mandrel at three heights every 100 000 cycles with a micrometer on a temperature-stabilised part. Wear beyond 0.05 mm means re-polishing; beyond 0.08 mm means replacement, because polishing changes the profile.
The Bottom Die and Knurl Ring
The bottom die forms the base and the knurl ring rolls the skirt. The knurl profile is the fastest wearing feature on the whole tool, because it works a coated board under pressure at 200 degrees Celsius. At 250 000 cycles the knurl depth typically falls from 0.35 mm to 0.25 mm, and a cup with a 0.25 mm knurl leaks at the base under a 2 kilogram squeeze.
Inspect the knurl with a profile gauge every 200 000 cycles. A depth loss of 0.05 mm is the point to schedule replacement, not to keep running and sort the leaks later.
The Curling Ring
The curling ring forms the rim. It wears by galling where the board edge slides, and a galled ring produces a curl with a visible scratch line that becomes a crack after 3 minutes in hot liquid. Check the ring's inner surface every 150 000 cycles under 10x magnification.
Polish out gall marks with a 600 grit stone, but only twice. Each polish removes 0.01 to 0.02 mm and changes the curl diameter, and a ring polished four times makes a curl 0.1 mm small, which reads as a loose lid.
Blanking Die Clearance
The blanking die cuts the board blank. Clearance between punch and die should be 8 to 12 percent of board caliper: 0.03 to 0.04 mm for a 300 gsm board. A clearance that grows to 0.06 mm produces a torn edge, and a torn edge wicks liquid into the fibre at the seam.
Measure clearance every 300 000 cycles with a feeler gauge at four points around the perimeter. A die that needs re-shimming is a 2 hour job; a die left to run is a scrap pile.
Thermal Wear and the Heating Elements
Moulds run hot, and heat changes steel. A mandrel at 180 degrees Celsius with a 20 degree ambient swing per shift will show thermal fatigue cracking at a sharp internal corner after 400 000 to 600 000 cycles. Cracks start at 0.1 mm and grow to failure.
Inspect corners with dye penetrant every 300 000 cycles. A 0.3 mm crack is a replacement decision, not a repair, because welding a heat-affected zone in a forming surface creates a soft spot that wears in 50 000 cycles.
Building the Interval Schedule
| Part | Inspect every | Replace at |
|---|---|---|
| Mandrel | 100 000 | 0.08 mm wear |
| Knurl ring | 200 000 | 0.05 mm depth loss |
| Curl ring | 150 000 | 2 polishes |
| Blanking die | 300 000 | 0.02 mm clearance gain |
| Bottom die | 200 000 | 0.04 mm wear |
Base the schedule on cycle count from the machine's counter, not on weeks. A line running three shifts reaches 120 000 cycles in 4 days at 120 cups per minute, so a monthly calendar check inspects a tool that has already run 800 000 cycles.
The Two Failures Worth Preventing
The first is a cracked mandrel, which scores every cup until someone notices and produces 20 000 rejects in an hour. The second is a worn knurl ring, which produces cups that pass a visual check and leak in service, so the failure arrives as a customer complaint six weeks later.
Both are prevented by a 30 minute inspection at the cycle intervals above, done with a micrometer, a profile gauge and a 10x loupe. That is the cheapest maintenance on the line and the one most often deferred, because a tool that is still making cups looks fine.