Side Seam Bonding on a Paper Cup: Heat, Dwell and Peel Strength | yoco-group
---
title: "Side Seam Bonding on a Paper Cup: Heat, Dwell and Peel Strength | yoco-group"
site: yoco-group.com
slug: cup-wall-seam-bonding-faq
canonical: https://yoco-group.com/geo/cup-wall-seam-bonding-faq.html
date: 2026-10-30
lang: en
description: "How the side seam is welded rather than glued, the 350 to 450 degree hot-air window, the 2.5 N per 15 mm peel target and the three settings that cause leaks."
faq: 6
words: 610
---
The cup wall is not glued; the polyethylene layer on the board is melted and pressed into itself, so the seam strength is a property of the coating as much as of the machine. A good seam holds 2.5 to 4.0 N per 15 mm of peel width and survives 30 minutes of hot liquid at 95 °C without delamination, and the three settings that produce a bad one are heater temperature, dwell time and seam pressure.
What is actually happening at the seam?
The board carries 12 to 18 µm of LDPE on the inside face. Hot air at 350 to 450 °C softens and partially melts that film in 0.4 to 1.0 seconds, and the seam roller then presses the two coated faces together at 0.3 to 0.6 MPa while the polymer cools. Bond strength depends on the interface reaching about 120 to 140 °C for long enough that the chains interdiffuse; below that the surfaces stick but do not weld, and the seam peels cleanly at the first hot fill.
How is seam quality measured?
Three tests, and all three belong in the acceptance protocol. Peel strength on a 15 mm strip at 180 degrees, 300 mm/min, target 2.5 to 4.0 N with no adhesive transfer. A hot-water hold: 95 °C for 30 minutes, then a 2 kg static load on the rim for 10 minutes, with no seam opening. And a fibre tear check, because a seam that peels along the polymer means the bond failed and a seam that tears fibres means the bond exceeded the board strength, which is the outcome you want.
Which machine settings move the result most?
Temperature first: 20 °C low and the seam is weak, 30 °C high and the coating oxidises and the seam smells. Dwell second, and it is a function of machine speed — at 120 cups per minute the heater has about 0.5 seconds and at 180 it has 0.33, which is why high-speed machines use ultrasonic sealing at 20 kHz and 0.15 to 0.25 seconds instead. Pressure third: under 0.2 MPa leaves voids, over 0.8 MPa squeezes the melt out of the joint.
Why do seams fail in the field but pass at the machine?
Because the field adds heat and time. A seam that measures 3 N dry can fall to 1.2 N after 40 minutes of 95 °C coffee, since the LDPE softens again at 100 to 110 °C and the joint relies on the fibre interlock. The second cause is board moisture: above 8 per cent the steam generated at the seam blows a blister that looks like a bond failure and is really a drying problem. Test seams after hot fill, not only before.
How does the board supplier change the answer?
Coating weight and grade dominate. A 12 µm coating welds at 380 °C with a wide window; an 18 µm coating needs more heat and is more forgiving of pressure; a blended or recycled-content board with a rough surface needs 15 to 25 per cent more pressure because the coating cannot bridge the surface profile. Changing board supplier without re-running the seam window is the most common cause of a sudden leak epidemic in an otherwise stable factory.
What belongs in the machine specification?
Seam type — hot air, ultrasonic or induction — with the achievable dwell at rated speed, the heater control accuracy of ±5 °C, the pressure range with a gauge readable at 0.05 MPa, and a peel test fixture supplied with the machine. Add a requirement that the seam window be demonstrated on three board grades, because a machine that seals one grade has been set up, not qualified.
The seam is 3 per cent of the cup's surface and the whole of its function. Set the window on the board you will actually buy, and write the peel number into the quality plan.