Canning and Fish Packaging Technology

Unit 11 - Heat Sealing

11.1.Heat sealing
Heat sealing is an effective way of closing packages and maintaining the protective qualities of the wrapper. Any material which is to be sealable by heat welding must either be fully thermoplastic (that is the plastic becomes fluid when heated and solidify again on cooling) or should have a layer of a thermoplastic. During heat sealing, two layers of thermoplastic films are held together under pressure, melt on heating, fuse into one another and seal together on cooling.

Three factors namely pressure, temperature and dwell time control the effectiveness of a heat seal. The dwell time is usually taken to mean the time during which the sealing jaws are closed. With hot bar sealing, this is the same as time for which the heat is applied but in the case of impulse sealing it also includes the cooling time. Each thermoplastic material has its own particular range of sealing pressure, temperature and time over which it will form a good heat seal.

Immediately after the heat sealing, the process joints are soft and without much mechanical strength. The original strength returns only after cooling (setting) to about room temperature and stresses should not be applied during this coating period.