Paper Cup Bottom Seam FAQ: Diagnosis, Hot Tests and Machine Drift

A bottom leak is a seam problem that looks like a paper problem. These questions cover how to read a section cut, why a hot test finds what a cold test misses, and how to tell a machine drift from a paper change.

How do you find the cause of a bottom leak?

Cut the cup through the base and inspect the seam section under magnification, then compare it with a known good sample. Four patterns cover most cases: a thin unsealed line points to low pressure or short dwell; a smooth separated surface points to a coating that never fused; a radial crack in the base disc points to dry or brittle paper; and an uneven seam width points to an out-of-round wall. Each pattern leads to a different fix, and the section is the only inspection that separates them. A leak test tells you there is a hole and nothing about where or why.

Why does the leak appear only with hot liquid?

Because heat softens the coating, raises the vapour pressure inside the wall and increases the permeability of the fiber. A seam with a marginal bond holds cold water and fails in a hot drink, often within minutes, because the liquid is also under a head and the customer squeezes the cup. Run a hot hold at the serving temperature for the serving time, with a squeeze and a tilt, on a sample per lot. A cold test remains useful as a quick check at the machine, and it is not a release criterion for a hot-fill product.

What machine variables cause the seam to drift?

Jaw temperature, pressure, dwell time, machine speed and paper moisture, and they interact. A jaw that heats through the shift changes the effective dwell, and an operator may compensate with pressure until the seam distorts. Log the variables per hour and plot them against the leak rate: a pattern that follows the shift is a machine problem, a pattern that follows the lot is a paper problem. That single distinction saves days of argument, and it is only possible if the log exists before the complaint arrives.

Why does the base disc matter separately?

Because it can come from a different reel than the wall. A disc that is drier than the wall seals unevenly, and a disc that is damper can steam and blister at the seam. Specify the moisture for both, store the reels in the production area long enough to equalise, and measure the two separately at the start of a run. A change of base disc supplier without notification can move the leak rate without changing a single machine setting, which is why the change clause belongs in the purchase order.

How often should a section be cut?

One cup per hour and one from every lot, plus one after any change of reel, jaw or speed. The cut takes a minute with a sharp blade and a jig, and the photograph takes another. Keep the photographs by lot so a later complaint can be compared with the condition at the time of production. A converter that cuts a section only when a complaint arrives is diagnosing history rather than controlling a process, and the cost of the habit is a few minutes a shift.

What does a good seam look like?

A continuous bond with no gap, a coating that has flowed into the fiber on both sides and a seam width that is even around the circumference. The fiber should look compressed rather than torn, and there should be no bubbles or discolouration that indicates overheating. A reference photograph of a good seam and three bad ones should hang at the machine, because a verbal description of a bond is interpreted differently by each operator.

What should be asked of the converter?

The sealing parameters with tolerances, the paper moisture for wall and disc, the mandrel inspection interval, the section frequency, the hot hold method and a retained sample per lot. Add a clause that any change of paper or disc supplier is notified before shipment. A converter who can answer all of that has a controlled process; one who answers with a leak test result has a test and not a process.

Does cup size change the bottom seam risk?

It does, because a larger cup holds more liquid and the head pressure at the base is higher, and the wall may be thinner for the same paper grade. A small cup that passes a hot hold can fail in a large size with the same seam parameters. Test each size separately rather than transferring an approval, and check the base disc diameter against the wall roundness for each size.

How does printing affect the seam?

A printed wall is compressed in the printing nip and can be slightly different in thickness at the seam, and an ink that sits in the sealing area can prevent the coating from fusing. Keep the print away from the seam allowance, and check the register at the seam on a production cup. A print that looks correct can still be the cause of a leak if it reaches the sealing band.

What is a reasonable reject criterion for a hot hold test?

Any visible moisture on the outside of the cup after the stated time at the stated temperature, under the stated stack and squeeze. A damp patch counts, not only a drip. Define it in the inspection plan, because two inspectors will otherwise disagree about what a leak is, and the disagreement will surface in a customer complaint.

How should the machine be restarted after a stop?

Run a few cups to waste, cut a section, then resume. A jaw that cooled during a stop produces a run of weak seams until it returns to temperature, and those cups are usually the ones that reach the customer. A short scrap run costs a handful of cups and removes the most common cluster of leaks in a shipment.

How should a new cup size be qualified?

As a new product rather than a scaled version. Run the section cuts, the hot hold and the squeeze test on the new size at the new machine speed, and check the base disc diameter against the wall roundness. A size change moves the head pressure, the paper area and often the wall grade, so an approval from the smaller size does not transfer. Keep the first article and five production cups from the first run, and compare them with the sample after a month of production, because the tool wears and the seam changes with it.

What should a chain specify to its cup supplier?

The hot hold condition, the section frequency, the sealing parameters with tolerances, the moisture of wall and disc, and a notification clause for any change of paper or disc supplier. Add a retained sample per lot kept for the shelf life of the cups. A chain that specifies only the print and the size has specified the product it sees, and the product the customer experiences is the seam, so the specification should be written around the seam.