The present invention relates to a controlled heat seal device for fastening wrappers in thermoplastic material, applicable in particular to machines for overwrapping packets of cigarettes and commodities of a similar nature.
More particularly, the invention refers to a device designed to seal the end folds of such wrappers by a controlled application of heat.
Controlled heat seal devices of the type currently in use comprise a pair of heated plates positioned on opposite sides of a conveyor belt along which the single commodities are carried, each enveloped in a relative wrapper. The plates are reciprocated through an operating stroke, which brings them into contact with the end folds of the wrappers, and a corresponding return stroke.
It is essential with such devices that the duration of contact between the plates and the ends of the wrappers can be varied, since the thermoplastic material from which the wrappers are fashioned must be heated up to but not beyond a given temperature, whatever the operating speed of the machine.
The general practice is in fact to associate the heat plates with a non-adjustable source of energy, for example one or more electrical resistances, and then to determine the exact duration of the contact between plates and wrapper employing a variety of means.
Given that no wrapping machine will ever operate at one constant speed, it follows that the duration of contact between the heat plates and the wrapper end folds needs to be matched to the different speeds of the machine.
In Italian Patent Application 3438A/74, by way of example, the duration of contact between the heat seal plates and the wrapper end folds is determined by means of a single cam type element which affords two or more profiles engaged by a following roller associated mechanically with the plates.
Whilst the heat seal device embodied according to this principle has proved capable of effective and precise operation over time, it nonetheless betrays a marked lack of flexibility inasmuch as a change in the operating speed of the machine dictates the replacement of the cam, with obvious drawbacks.
The object of the invention is to provide a heat seal device affording a high degree of flexibility and dependability over time.