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Theller Engineering                         707-762-3820

Designers of the world's most advanced hot tack machines.
DEFINITIONS of TECHNICAL TERMS

hot tack -- strength of a hot seal measured at a specified time interval after completion of the sealing cycle but prior to the temperature of the seal reaching ambient.

dwell time -- the time interval during the heatsealing cycle when the sealing jaws are in contact with, and exerting pressure on, the material being sealed.

cooling curve -- the graphical depiction of the increase in strength of the seal with time, as it cools during the period immediately following conclusion of the sealing cycle.
  • The cooling curve is a plot of hot seal strength vs cooling time. The portion of the cooling curve of greatest practical significance is the first 1000 milliseconds following opening of the heatseal jaws.
cooling time -- the time interval from when the heatseal jaws open at conclusion of the sealing cycle, to the point at which the hot-tack force is determined.

delay time -- the time interval from when the heatseal jaws open at conclusion of the sealing cycle, to the point at which withdrawal of the sample from between the jaws is initiated.
  • Delay time in the Theller machine is effectively zero. Withdrawal of the strip is initiated in less than 10 ms after the jaws open.
withdrawal time -- the time interval from when withdrawal is initiated, to the point in time when all slack has been removed from the test strip between the seal and the grips, so that measurement of the strength of the seal can commence.

failure mode -- a visual determination of the manner in which the test strip fails during grip separation.

adhesive failure -- a failure mode in which the seal fails at the original interface between the surfaces being sealed.

cohesive failure -- a failure mode where either or both of the sealed webs fails by splitting or breaking, and the seal itself remains intact.
  • The term may be defined somewhat differently when applied to sealing systems involving an adhesive material as a separate component.
hot-tack curve -- a plot of hot-tack strength vs sealing temperature.
  • This is the basic curve used for comparing materials for their hot tack performance. It shows not only the maximum hot seal strength achievable by each material and the sealing temperature required, but also the breadth of the sealing temperature range at any specified level of hot tack.
sealing temperature -- maximum temperature reached at the interface between the two web surfaces being sealed, during the dwell time of the sealing cycle.
  • Sealing temperature will equal jaw temperature (both jaws at same temperature) if the dwell time is long enough for the interface to reach equilibrium with the jaws. When seal strength no longer rises with increasing dwell time, the interface has reached equilibrium with jaw temperature.
Seal-initiation temperature - minimum sealing temperature required to form a seal of significant strength.
  • The level of strength that must be reached to be deemed "significant" varies with the application. As a default value, 0.50 N (ca. 50 gm force) is commonly used.
burnthrough -- a state or condition of a heatseal characterized by melted holes and thermal distortion.
  • Burnthrough indicates that the sealing conditions (time and/or temperature) were too high for an acceptable seal.


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