Cup Side-Seam Overlap: The Dimension Nobody Measures
The side seam overlap is the length of the leak path, and in most cup factories it is set by the sheet width and the forming die and never measured again. Specify the overlap in millimetres with a tolerance, verify it on a section rather than on the flat blank, test the lower limit with a hot fill, and measure one cup per lot plus one per shift. A cup that leaks at the top of the seam usually had an overlap that was adequate for a cold drink and marginal for a hot one, and no dimensional check of the rim will ever show it.
Scenario
A converter produces 8-ounce hot cups for a coffee chain, at a running rate of about 200 cups a minute on a single machine. Six months into the program the chain reports that a share of cups weep at the side seam, always within about 15 millimetres of the rim, and always with a hot drink. The complaint is accompanied by a photograph of a bead of coffee on the outside of the seam.
Incoming inspection measures the rim diameter, the height, the bottom diameter and the grammage of the board, all within specification, and the leak test used at the factory is a static water fill at room temperature held for ten minutes. The cups pass. When a cup from the returned carton is cut through the seam and laid flat, the overlap measures 3.9 millimetres against a drawing value of 5.5. Nothing on the drawing had a tolerance, and no routine had ever measured the overlap, so the change had been invisible since whichever tool service had produced it.
Why Overlap Is the Leak Path
A heat-sealed side seam is two coated surfaces pressed together at temperature. On a real cup, the bond is never perfect across the full overlap: there are micro-channels at the edges of the seal, at the start and end of the jaw cycle, and wherever a coating is thin. A liquid inside the cup reaches those channels, and what decides whether it reaches the outside is the distance it has to travel before it runs out of seal.
That distance is the overlap. With a wide overlap, a micro-channel at the inner edge is surrounded by bonded material, and the liquid stops. With a narrow overlap, the same channel reaches the outer edge and the cup weeps. The defect in the seal is identical in both cases; only the geometry decides whether the customer ever sees it. This is why a material saving that narrows the overlap by a millimetre changes the field behaviour without changing a single inspection result.
The Second Reason: Heat Changes the Coating
Hot fill raises the temperature of the seam, softens the polyethylene coating and increases the vapour pressure inside the cup. A seal that holds at twenty degrees holds for a different reason at ninety-five: the coating flows more, the bond can creep under load, and a marginal channel opens as the polymer softens.
The effect is concentrated at the top of the seam, for three reasons that compound. The top of the seam is where the cup leaves the sealing jaw and where the jaw is most likely to be slightly cooler or slightly less loaded. The top holds the liquid when the cup is full. And the top is where the cup is squeezed when it is lifted, which is a mechanical load added to a thermal one. A leak that begins as a bead within 20 millimetres of the rim is showing you all three at once.
What to Specify
The overlap in millimetres with a tolerance, the method, the seal parameters with ranges, and the tests that verify the lower limit.
| Parameter | Typical value | Why it matters |
|---|---|---|
| Seam overlap | 5.0 to 6.5 mm, tolerance plus or minus 0.5 | Length of the leak path |
| Seal temperature | 180 to 220 Celsius, per coating grade | Bond without burning the coating |
| Jaw pressure | 0.3 to 0.5 MPa, uniform across the seam | Contact without thinning the board |
| Dwell time | 0.6 to 1.2 seconds | Time for the coating to flow |
| Coating weight | 15 to 25 gsm inner, matched outer | Enough polymer to bridge the fibers |
| Hot fill test | 95 Celsius, 30 minutes | Reproduces the field condition |
| Squeeze test | 10 N at grip height, then inspect | Reproduces the lifting gesture |
The tolerance is not decoration. Without one, a tool service that removes a shim or replaces a wear plate can move the overlap by half a millimetre and the drawing will still be satisfied, because a nominal value with no tolerance describes an intention rather than a limit. With one, the first article after the service is either inside or outside, and the decision is a comparison.
How to Measure It
Cut a cup through the seam with a sharp blade, lay the section flat and measure the overlap with a steel rule or under a magnifier with a scale. Measure at three heights: 20 millimetres below the rim, the middle, and 20 millimetres above the bottom. The top reading matters most, because that is where the field leaks appear.
One cup per shift and one cup per lot is a reasonable frequency for a stable process, recorded with the value and the height. Increase to one per hour after a tool service, a sheet lot change or a coating lot change, and run that increased frequency for the first eight hours of the new condition. A process that has just changed is the process most likely to be outside a tolerance that nobody has checked in a year.
Keep the section. A section photographed at the time of measurement is the record that answers a complaint six weeks later, and a section with the bond line visible shows where the seam will fail rather than only that it did. A leak test tells you that the cup leaked; a section tells you why.
Reading the Seal Itself
The section shows more than a width. A good bond shows the two layers merged into one, with the coating bridging the fibers and no line of separation along the overlap. A weak bond shows a visible boundary, a lighter band where the coating did not flow, or a section that separates when it is flexed.
Three conditions are worth naming on the inspection sheet. A bond line that stops short of the outer edge is the classic weep: the seal exists, and it does not reach the point where liquid would exit. A bond that is complete but thin, showing a bright band under the magnifier, is a temperature or pressure shortfall that will fail under hot fill. A bond with a crease or a wrinkle in the overlap is a forming problem, and it will leak at the crease regardless of the width.
Why the Change Was Invisible
Because the overlap is set by the tool and by the sheet, and every check in the factory was measuring something else. The rim diameter is set by the forming collar, the height by the blank, the bottom by the bottom die, and the grammage by the board supplier. None of those measurements moves when the overlap changes, and the visual check of a finished cup shows a seam that looks exactly as it did before.
The general lesson is worth stating plainly: when a property is set by the tool rather than by the operator, the inspection has to look at the tool's output in a way that reveals it. For a side seam, a section is the cheapest such look, and it costs a cup and two minutes.
Test Methods That Reproduce the Field
A static water fill at room temperature is the most common factory test and the least informative, because it reproduces neither the temperature nor the load that the field applies. Two additions close most of that gap.
The hot fill. Fill the cup with water at 95 degrees Celsius, stand it on a dry surface and inspect the seam at 5, 15 and 30 minutes. Look for a bead on the outside, for darkening of the board at the seam, and for softening that allows the seam to move when the cup is lifted. A cup that passes at 5 minutes and fails at 30 is showing you creep, which is a coating and pressure issue rather than a width issue.
The squeeze. After the hot fill, grip the cup at the height where a hand naturally grips, apply a defined load, hold for a few seconds and inspect. A seam that holds still can open under a squeeze, and the squeeze is the gesture that produces most field leaks. Without this step the factory is testing a cup nobody handles the way the test does.
A dye or a pressure test can be added for a more sensitive result, and it should be treated as an investigation tool rather than a routine, because it detects channels that a hot fill would not reach in a normal holding time. A routine that passes both a hot fill and a squeeze is a routine that covers the field condition.
Machine Settings and Their Interaction
Temperature, pressure and dwell are the three settings, and they interact in a way that makes single-variable adjustment dangerous. Raising temperature to fix a weak bond can burn the coating and produce a seam that looks bonded and fails later. Raising pressure to compensate for a low temperature can thin the board at the seam and reduce the mechanical strength. Extending dwell to fix either adds cycle time and heat input to the whole cup.
Log the three values hourly against the section width, and plot them. A drift in a jaw heater shows as a falling bond quality with an unchanged width, and a drift in the sheet shows as a changing width with unchanged settings. The two causes look alike in the finished cup and are distinguishable in the log.
What Triggers a Stop
A change of more than half a millimetre in the measured overlap, a visible unbonded line on a section, any moisture on the outside of the seam in the hot fill test, or a seam that opens under the squeeze. Each of these should be written on the shift sheet with its sample frequency, so the decision is a comparison with a number rather than a judgement about a cup.
Set the stop values with the customer's application in mind. A cold drink cup can carry a narrower overlap than a hot drink cup, and a cup for a vending machine can carry a different value again. A single specification across all products forces the plant to build every cup to the hardest case, which is safe and expensive, or to the easiest case, which is the story of the scenario above.
What Belongs in the Specification
The overlap with a tolerance and a measuring method, the three heights at which it is measured, the seal settings with ranges, the section frequency, the hot fill and squeeze procedure with pass criteria, the trigger values, a section photograph from the first article, and the requirement that the overlap appear on the drawing rather than only in the process sheet.
A dimension on the drawing is checked at first article and at every tool service. A note in the process sheet is checked when someone remembers. The difference between those two is exactly the difference between a specification and a hope.
Bottom Line
The overlap is the length of the leak path, it is set by the tool, and it is the dimension nobody measures. Put it on the drawing with a tolerance, cut a cup and look at the section once a shift, test the lower limit with a hot fill and a squeeze rather than with a room temperature fill, and keep the section photograph with the lot. A cup that weeps at the seam has not failed a leak test; it has failed a dimension that no test was written to find.