Heat sealing of semicrystalline polymer films. I. Calculation and measurement of interfacial temperatures: Effect of process variables on seal properties

A finite element analysis (FEA) modeling technique was used to predict the interfacial temperature as a function of time during the sealing of semicrystalline polymer films. An experimental technique using micro-thermocouples to measure rapidly changing interfacial temperatures during sealing was also developed. Agreement between predicted interfacial temperature profiles and measured values for polyethylene films was good except at temperatures substantially above the final melting point of the polymer. This deviation is caused by film-thickness changes occurring during sealing that are not taken into account in the calculations. The effect of heat-sealing process variables (seal bar temperature, dwell time, and pressure) on seal properties (seal strength, seal elongation, and seal energy) of polyethylene films has also been quantitatively determined. Seal properties are determined primarily by the maximum temperature achieved at the interface during heat sealing. Dwell time must be sufficiently long to bring the interfacial temperature to a desired level, but longer times at a given interfacial temperature do not improve seal properties at the conditions of our experiments. A slight pressure is helpful in bringing two microscopically uneven film surfaces into intimate contact, but higher pressure has no beneficial influence on seal properties. However, increased pressures and dwell times at temperatures above the final melting point of the polymer are detrimental to seal appearance due to material deformation in the sealing area. © 1994 John Wiley & Sons, Inc.