THERMAL IMPULSE WELDING

Use of Vectra A950 liquid crystal polymer is described with reference to the production of barrier packaging for the containment of anaesthetic emulsions. The pouches were compression moulded and sealed by thermal impulse welding. Concentrations of lidocaine and prolocaine in the emulsion were studied for 14 weeks at two different temperatures, 40 degrees C and 60 degrees C, and at 100% relative humidity. Loss of substances from the emulsion was due mainly to adsorption onto the polar surface of the liquid crystalline polymer, it is reported. 21 refs. 

The heat is applied to the outer surfaces of the materials being welded, and in order to make the weld it must travel through the material to the interfaces. For many applications thermal sealing has been superseded by impulse sealing but the former process is in use still for welding thick sheets of flexible thermoplastics. The production equipment employs a heated wedge placed between the sheets, over which they are moved continuously. 

The essential difference between machines for thermal sealing and for impulse sealing is that, whereas the former employ electrode bars that are maintained at a constant temperature, the latter employ sealing bars which are heated only during the clamping and welding cycle. 

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