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Matching Cup and Lid in One Mold Set: Why Seal Geometry Decides Leak Rate

Time: 2026-10-26 09:59:47.651

The leak is rarely the lid's fault

When a beverage chain reports leaking cups, the first instinct is to blame the lid supplier. In most cases the cup rim is the cause: a rim that is not flat, a rim radius that is too sharp, or a rim that ovalised after trimming. Because the lid is a snap fit, even 0.1 mm of rim distortion changes the interference and the seal. Shantou Longsheng Industry Co., Ltd. makes cup molds and lid molds as a matched set so that both sides of the joint are controlled by one drawing.

Three things the rim must deliver

  • Flatness — measured across the full circumference, not at four points
  • Rounded seal bead — a radius that lets the lid snap over rather than cut into the cup
  • Concentricity — rim and base must share an axis, otherwise the lid sits skewed

How the two molds stay matched

A cup mold and rim mold pair is built from the same shrink allowance. PP shrinks differently depending on wall thickness and mold temperature, so a cup molded at 0.55 mm wall and a lid molded at 0.9 mm will not share the same shrinkage factor unless both tools are cut for their own conditions. We model the shrink separately, then verify the interference by physical fit test across a batch of 50 assemblies before shipment.

Process settings that keep the seal

  • Mold temperature stability within plus or minus two degrees
  • Consistent trim blade sharpness; a dull blade drags the rim and leaves a burr that breaks the seal
  • Cooling time long enough that the rim does not relax after ejection
  • Stacking pressure during packing, which can ovalise a warm rim

Choosing the machine

FS-8556 full-servo cup machines are typically used for cups, with lids run on the same platform using a lid stack mold. HSS-8556 hydraulic servo covers heavier lid designs; multi-station thermoforming handles flat and dome lids in high volume.

About Shantou Longsheng

Founded 1998, Shantou, Guangdong Province, China. 30,000 m² factory, "Longsheng" trademark, 25 granted patents (1 invention, 24 utility models), high-tech enterprise. Sheet extruders, multi-station thermoformers, vacuum forming machines, cup making machines and precision molds for PP, PS, HIPS, PVC, PET, PLA, PE and biodegradable materials.

https://www.stlongsheng.com

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