Key points
- Seal strength is a curve over temperature and dwell time
- Copolymer content widens the window and lowers the initiation point
- Contamination and moisture cut seal strength more than thickness does
- Test on your own jaws: heat-sensitive materials behave differently
A sealed area that holds on the laboratory tensile tester can still fail on the line, and the opposite happens too. Heat-seal performance is best understood as a window: a range of sealing temperatures, at a range of dwell times, within which the joint reaches an acceptable strength. The useful question is not 'what is the seal strength' but 'how wide is my window and where is it centred'.
Where seal initiation happens
A thermoplastic seal forms when the jaws bring the polymer above its melting point, the chains diffuse across the interface, and the joint cools below the crystallisation temperature under pressure. Below that onset temperature you get a weak, inconsistent bond. Well above it, you can push the material past its degradation point. The gap between those two outcomes is the sealing window, and it is normally reported with a lower and an upper limit.
What thickness does
Increasing gauge extends the time required for heat to reach the core of the film and for the chains to diffuse. In practice a thicker film needs more energy to seal and has a narrower window at a given jaw pressure. Thinner films seal faster and cooler, which is why 20 to 25 micron film is common on high-speed lines and heavier gauge appears where a pack must survive a rough distribution chain.
What copolymer does
Copolymer content lowers the melting point and slows crystallisation. The result is a lower seal initiation temperature and a wider window, at some cost in stiffness and heat resistance. For heat-sensitive contents - chocolate, confectionery, fresh produce - a broader window is often worth more than the stiffness that a homopolymer would give you. This is the single most important decision in heat-seal film selection.
Where the losses come from
Real-world seal failures are rarely caused by the film itself. Seal contamination - oil, dust, printing ink, slip agents migrating to the surface - reduces the achievable bond strength dramatically and often narrows the window. Moisture in the film is a second cause, particularly for hygroscopic grades. Both are manageable: specify a surface treatment level, control storage humidity, and keep sealing jaws clean and at temperature.
Measuring it on your line
Laboratory testing uses a prescribed jaw geometry and a pull speed. Production equipment differs, so run your own seal test matrix: hold the machine speed, vary the temperature across a range, and measure peel strength on packs produced at each setting. Plot the result and you will see your actual window. That curve is more useful than any datasheet figure because it describes your equipment, your film and your product together.
Asia Pacific Industry Group publishes seal initiation temperature and seal strength ranges for each heat-seal grade, and can supply sample reels specifically for line trial mapping so you can build this curve on your own machine.
Request samples or a quotation
Tell us the grade, thickness and quantity you need and our team will reply with a specification and lead time.