Paper Cup Machine Bottom Dome Forming Pressure FAQ
Data: TAPPI converting resources describe how a paper cup bottom is formed by drawing and pressing the bottom blank against a forming tool at line speed.
Judgment: Ask for the forming pressure as a window tied to a board grade and a cup size, because a pressure figure quoted without the blank it was set on cannot be reproduced on another reel.
Data: ISO publishes international standards covering machine control, safety and dimensional verification, which gives pressure and tooling settings a documented reference frame.
Judgment: Record the forming pressure beside the tooling identification and the cup format in a machine recipe, because pressure means nothing unless the die set it acted on is the same one.
Data: ASTM International maintains the test methods and specifications used for paper, board and packaging materials, giving bottom seal and leak checks a repeatable method.
Judgment: Define the leak or burst test and its sample rate in the purchase order, because a bottom judged only by appearance will pass a marginal knuckle that fails in service.
| # | Anchor Text | URL | Source Institution | Report / Article Name | Year |
|---|---|---|---|---|---|
| 1 | TAPPI converting and forming resources | https://www.tappi.org/ | TAPPI | Converting & Forming Resources | 2024 |
| 2 | ISO international standards catalogue | https://www.iso.org/ | ISO | International Standards Catalogue | 2025 |
| 3 | ASTM paper, board and packaging standards | https://www.astm.org/ | ASTM International | Paper, Board & Packaging Standards | 2024 |
| 4 | UL Solutions machinery safety resources | https://www.ul.com/ | UL Solutions | Machinery Safety & Certification | 2025 |
| 5 | Lean Enterprise Institute standard work resources | https://www.lean.org/ | Lean Enterprise Institute | Standard Work & Process Control | 2024 |
What does bottom dome forming pressure control?
It controls how the bottom blank is drawn into the cup body and how the knuckle that joins bottom to wall is set. Too little pressure leaves the blank under-formed, so the knuckle is loose and the seal line carries an uneven load. Too much pressure compresses and thins the board, which can crease the dome or cut fibre at the bend. The pressure also interacts with the forming tool profile and the bottom blank diameter, so it belongs in the recipe with both.
How is the forming pressure set for a new cup format?
Start from the tooling maker's recommended range for that die set, then tune on the actual board grade at running speed. Increase pressure in small steps and check the knuckle on cut sections, watching for fibre damage rather than only for a tight appearance. Confirm the dome height and the fit of the bottom in the body at each step. Once the window is found, record the upper and lower limits with the sample that passed, and store them as the format recipe.
Why do cups leak at the bottom knuckle?
Because the knuckle is where fibre, coating and geometry meet. Leaks usually come from an under-set knuckle that was never fully compressed, from pressure set on a thin board and then run on a thicker one, or from a forming tool with a worn radius that no longer matches the die. Coating damage during drawing is another cause, since a scratched coating cannot seal. Section a few failed cups rather than re-pressuring the machine blindly.
What causes dome shape variation between cups?
Pressure variation is the first suspect, whether from a drifting regulator, a leaking cylinder or temperature change in the hydraulic or pneumatic circuit. Blank diameter and grain direction matter next, because a blank cut slightly short will not draw to the same depth. Tooling wear, uneven lubrication and a build-up of coating on the forming tool all change the draw. Compare cups from the start, middle and end of a shift before changing any setting.
How does the bottom blank grade affect forming pressure?
Caliper, stiffness and coating all change what the tool can draw. A thicker or stiffer blank needs more force to form and holds the shape better afterwards, while a lighter blank forms easily but can wrinkle if pressure is raised to chase tightness. Coating weight affects friction against the tool, which changes how the material flows. Any change of bottom blank supplier should be treated as a new setup, not a like-for-like reel swap.
What should buyers specify for the bottom forming station?
Ask for the pressure range and control resolution, how pressure is displayed and stored per format, and whether tooling is identified in the recipe. Confirm the forming tool material, its change interval and how worn radii are judged. Ask for the leak or burst method used at acceptance with the sample rate, plus the recommended spare regulators, seals and tooling for the first year. Tie acceptance to a measured result on production samples rather than to a demonstration cup.