Paper Cup Bottom Seam Failure: Why the Base Leaks Before the Rim
A bottom leak starts at the seam where the wall meets the base, and it has four causes: insufficient sealing pressure, a base disc that is too dry or too damp, a wall that is not round at the bottom, and a glue or heat profile that does not match the coating. Diagnose by cutting failed cups: the failure looks different for each cause, and the difference decides whether the fix is in the tool, the paper or the machine.
Scenario
A chain reports that a small share of cups leak at the base within a few minutes of filling. The cups hold water in a factory test for an hour. The converter inspects a returned cup and finds the seam looks closed. A second cup, from the same case, is cut and the section shows a thin line of unsealed coating at one point of the circumference. The machine had been running slightly fast, and the sealing pressure had drifted as the sealing jaw heated through the shift.
Pain Point
The bottom seam is a circular joint that is formed in a fraction of a second, and it is inspected by eye at speed. A seam that looks closed can have a gap that is a fraction of a millimetre wide and only a few millimetres long, and that gap is enough for a hot liquid under a head of pressure. The process variables that create the gap are invisible in the finished cup: jaw temperature, dwell time, pressure, paper moisture and wall roundness all interact, and a change in one can be masked by the others until a shift boundary. The factory leak test runs at low pressure with cold water and it often passes a cup that fails in service because the test does not reproduce the temperature and the head.
Solution
Control four variables and inspect with a section.
Sealing pressure and dwell: record them per shift and check the gauge against a reference at the start of the shift. A jaw that heats through the shift changes the effective dwell.
Jaw temperature: measure with a contact probe at the start, middle and end of a run, because a jaw that is too cool gives a weak bond and one that is too hot damages the coating.
Paper moisture: measure the base disc and the wall separately, since a dry disc and a damp wall seal differently.
Wall roundness at the bottom: measure the diameter on two axes and check for out-of-round from the forming mandrel.
Inspection: cut one cup per hour and inspect the seam section under magnification, and run a hot water hold at the serving temperature for the serving time.
| Symptom | Likely cause | Evidence in the section |
|---|---|---|
| Thin unsealed line | Low pressure or short dwell | Gap with intact coating edges |
| Seam opens when hot | Coating not fused | Smooth separated surfaces |
| Base disc cracks | Dry or brittle disc | Radial crack from the centre |
| Leak at one point only | Out-of-round wall | Uneven seam width around the circle |
| Delamination of the seam | Jaw too hot | Bubbles or discolouration at the bond |
| Leak after minutes, not at once | Slow wicking path | Dark fiber along the seam |
ASTM International publishes test methods for paperboard and for packaging, including conditioning and seal strength procedures that can be applied to a cup bottom seam under defined conditions.
Result
A converter that added a section cut per hour found the gap appeared on one side of the cup, which pointed to an out-of-round wall rather than to pressure. The forming mandrel was worn, and the wall was slightly oval at the bottom. With the mandrel replaced and the pressure logging added, the leak rate fell to a level the chain accepted. The important change was diagnostic: the section cut told them where to look, and the leak test alone would have led to a pressure increase that would have created a different failure.
Why the Hot Test Matters More Than the Cold Test
A cold water test is a rapid check for gross defects and it is useful at the machine. It does not reproduce the condition in service, where the liquid is hot, the cup is held, and the seam is under a head of liquid. Heat softens the coating, raises the vapour pressure inside the wall and increases the permeability of the fiber. A seam with a marginal bond passes cold and fails hot, and the interval can be minutes rather than seconds. Run a hot hold for the serving time on a sample per lot, and include a squeeze and a tilt, because the customer does that too.
The Base Disc and the Paper Specification
The base disc is a separate blank, and it can come from a different reel than the wall. If the disc is drier than the wall, the seal is uneven; if it is damper, the seam can steam and blister. Specify the moisture for both, and store the reels in the production area long enough to equalise before running. A converter that changes base disc supplier to save cost without telling the brand can move the leak rate without changing any machine setting, which is why the notification clause belongs in the purchase order rather than in a general terms document.
Machine Speed and the Shift Effect
A machine that runs faster than the sealing window allows will produce a seam that looks closed and is not. The effect grows through a shift as the jaw heats, because the dwell in milliseconds stays the same while the temperature rises, and the operator may compensate by increasing pressure until the seam distorts. Log speed, pressure and temperature per hour, and plot them against the leak rate. A pattern that follows the shift rather than the lot is a machine problem, and a pattern that follows the lot is a paper problem. That distinction is the fastest route to a fix.
What to Ask the Converter
Ask for the sealing parameters with tolerances, the paper moisture at the wall and the base disc, the mandrel inspection interval, the section cut frequency and the hot hold test method. Ask for a retained sample per lot and for notification of any change of paper or base disc supplier. A converter that can answer all of that has a controlled process; one that answers with a leak test result has a test and not a process.
Preventive Maintenance on the Sealing Jaw
A sealing jaw drifts before it fails. The jaw surface wears, the coating on the jaw picks up paper dust, and the heater cartridge loses contact with the body, so the surface temperature falls while the controller reports the set value. Add three checks to the maintenance routine: clean the jaw surface at every shift change, verify the surface temperature with a contact probe against the controller reading once a week, and inspect the jaw profile for wear against a reference block once a month. A jaw that is worn across part of its width produces a seam that is sealed on one side of the cup and marginal on the other, which is exactly the pattern that a section cut reveals and that a leak test at the machine misses.
Bottom Line
Bottom leaks come from a seam that is closed but not bonded. Cut a cup, look at the section, and identify whether the gap is pressure, temperature, moisture or roundness. Then run a hot hold at the serving temperature and log the machine per hour, because the failure follows the shift more often than it follows the paper.