Seal Initiation Temperature and How to Read the Seal Curve

ASTM F2029 describes a heat seal curve as a plot of measured seal strength against sealing temperature, at a fixed dwell time and sealing pressure. The seal initiation temperature is the point on that curve where strength first trends upward from zero.

Other definitions are in use. A published study of polyethylene films defined SIT as the temperature at which a specific, low level of seal strength is reached. Check which definition a data sheet or specification uses before you compare two SIT values.

The temperature in F2029 is the set point of the sealing jaw. The standard notes that the set point is often higher than the surface temperature it produces. An SIT is therefore tied to the sealer, the dwell time and the pressure used to measure it.

F2029 covers making the laboratory seals and treating the strength data, but not measuring the strength. Each seal is pulled to ASTM F88, which measures the force needed to separate a strip containing the seal and identifies how the specimen failed.

A typical series keeps dwell time and pressure constant and changes only the jaw temperature. F2029 asks for an adequate or agreed number of replicates at each temperature, so that a mean seal strength can be plotted at every point.

Let the seals cool before you pull them. F2029 separates hot tack, measured while the seal is still hot, from ultimate seal strength, reached once the seal has cooled to ambient and stabilized. Hot tack has its own method, ASTM F1921.

Our ASTM F88/F88M-23 standard overview covers specimen width, pull rate and report items for the strength test.

Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].

Studies of polyethylene sealants report the same general shape. Strength stays near zero at low temperature, rises steeply above SIT and then levels off. Researchers mark three points on that shape.

• Seal initiation temperature, where strength begins to increase rapidly with temperature.
• Plateau initiation temperature, where strength stops changing much as temperature goes up.
• Final plateau temperature, where strength starts to fall quickly and the seal distorts badly.

The plateau has a physical meaning. Work on polyethylene films linked the start of the plateau to the final melting point of the sealant. The height of the plateau depended on the yield stress of the film.

The span from SIT to the final plateau temperature is the sealing range. A broad range gives room for variation in jaw temperature and other plant conditions, such as humidity or food contamination in the seal area.

A gradient sealer maps these points in fewer runs. Our STH-5 gradient heat seal tester has five 40 × 10 mm jaws, each set to its own temperature up to 250 °C, with the same pressure and dwell on all five. It prepares seals for ASTM F2029 work, and the strength is then measured on a tensile tester to F88.

Temperature is the main control. In a bar sealing study of laminated films with LDPE and LLDPE sealants, platen temperature was the primary factor in seal strength. Dwell time had to be long enough to bring the interface up to temperature.

Once the target strength was reached in that study, more dwell time added nothing. Pressure had little effect above the level needed to flatten the films into good contact. Laminates that conducted heat poorly needed a higher platen temperature to seal.

The film structure moves the curve as well. F2029 notes that the overall construction and thickness affect sealing properties, not only the seal layer. A thick or heat flow resistant structure slows the transfer of heat from the jaw to the sealing surface.

For these reasons, compare seal curves only when they were made on the same sealer with the same dwell time and pressure.

Treat a laboratory seal curve as a starting point. F2029 warns that a lab sealer and production equipment can differ in scale, sealing area and speed. The curve gives starting conditions for full scale equipment, which still need their own validation.

Record the failure mode as well as the number. In the laminate study, seal strength and failure mode were closely related. F88 identifies the mode of failure for every specimen, so note it at each temperature on the curve.

We sell direct from our factory, so you work directly with our factory team. Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].