Process and equipment for safeguarding the elasto-plastic characteristics of a stretchfoil during the packaging of products

Abstract: The film (101), which is cyclically extended in the station for wrapping the product (4), is acted on by a jet of hot air supplied from a thermo-electric generator (7) controlled by a control logic unit (11) which through suitable sensors (16-17) measures the ambient temperature and the temperature of the said film (101) and controls any other inputs, so that the said film is kept in optimal and substantially constant conditions to react without tearing to the pre-stretching and/or stretching to which it is normally subjected in the product packaging cycle.

Description: Most of the thermoplastic films used in the packaging of food products which must be kept in refrigerated environments, and particularly the stretchable films, undergo abrupt changes of their chemical and physical characteristics, particularly their elastic and plastic characteristics, at the low temperature which must be maintained even during the packaging of such products, and become extremely fragile, with a risk of tearing during the stages of pre-stretching and/or stretching to which such films are normally subjected when in use. The threshold of practical risk varies with the chemical composition of the film, for example according to whether it is made of polyethylene (PE) or polyvinyl chloride (PVC), and rises exponentially with a decrease in temperature. Prior art document EP-A-0 378 730 discloses a method for maintaining elastic and plastic characteristics of a film of stretchable thermoplastic material at optimal and substantially constant values during packaging of products, comprising the steps of: positioning a portion of the film in a packaging system; measuring temperature of surrounding environment in which the portion is used and determining whether said measured temperature is lower than an ideal temperature at which the film presents the best deformability characteristics for packaging; thermally conditioning said portion of film until the measured temperature of said environment reaches said ideal temperature by blowing hot air onto the portion of film positioned in the product packaging station. The method of present invention is defined by the steps according to claim 1. For the application of this method, the apparatus of claim 5 is provided. The control logic unit processes the collected data according to a suitable algorithm and cyclically activates the hot air generator, changing the operating temperature thereof and if necessary the speed of rotation of the fan, to ensure that the temperature of the portion of film cyclically extended in the packaging station and acted on by the flow of hot air is maintained at values such that the elastic and plastic characteristics of the said film remain at optimal and practically constant values. In the following description a preferred embodiment of the invention will be illustrated solely by way of example and without restriction, in the figures on the two attached sheets of drawings, in which Fig. 1 is a schematic side view of a packaging machine with the equipment according to the invention; Fig. 2 shows the hot air generator on a larger scale than that of the preceding diagram, and in transverse section; Fig. 3 shows the hot air generator in a plan view from above and in partial section. Figure 1 shows schematically the typical composition of a machine for packaging products with stretchable film, having at least one reel 1 from which the wrapping film is unwound cyclically by suitable means and is fed to a fixed distributor 2 from which the film is gripped at its end by a clamp 3 which moves horizontally with a travel proportional to the dimensions of the product 4 to be packaged, to extend a portion of film 101 of adequate length above the said product 4. During the transfer of the product 4 to the elevator 5 by a controlled-speed conveyor (not illustrated), known means 6-106 measure the dimensions of the product (width, length, height and flaring), to permit the said logical control of the clamp 3. According to the invention, a hot air generator 7 is provided next to the clamp 3 when the latter is in the position remote from the distributor 2, and this generator comprises: a set of electrical resistances 8; an electric fan 9 which sucks in ambient air and directs it onto the whole extension of the said set of resistances; a chamber 10 which guides the air leaving the electric fan against the said set of resistances 8 and directs the hot air in a uniformly distributed way under the portion of film 101 extended over the product 4 to be packaged and over the whole transverse extension of the said film. For this last purpose, the chamber 10 is provided with two or more adjacent flat discharge apertures 110 disposed under the clamp 3 and designed and orientated in such a way that the air discharged from them is made to contact the lower face of the portion of film 101 as uniformly as possible. The motor of the electric fan 9 may be of the fixed- or variable-speed type. The number 11 schematically indicates an electronic microprocessor-based programmable control unit, which through suitable interfaces and multiple-wire links 12-13 controls, respectively, the set of resistances 8 to vary the quantity of heat produced by it, and the motor of the electric fan 9, by varying its speed if necessary. Through the multiple-wire links 14-15, the unit 11 receives, respectively, the data relating to the ambient temperature in which the film feeding means operate, measured by at least one sensor 16 suitably remote from the generator 7 and disposed, for example, next to the reel 1, and the data on the temperature of the portion of film 101, measured by one or more sensors 17 located next to this portion of film. Operating with a predetermined algorithm, the unit 11 reads the electrical signal corresponding to the ambient temperature, and controls the parts 8-9 in a logical way, to ensure that the portion of film 101 is contacted by a flow of hot air which brings it into the best condition for pre-stretching. The electrical signal from the sensors 17 is used as a feedback signal and to avoid the exceeding of a maximum safety limit in the heating of the film 101. The hot air generator 7 is activated at least during the unwinding of the portion of film 101 from the reel 1, and possibly also during the pre-stretching of the said film, and is switched off automatically before the product 4 is raised against the said film 101 for the packaging of the said product. This timed activation of the generator 7 is provided by the unit 11 through its own timing means and with the multiple-wire link 18 to a sensor 19 which measures, for example, the position of the clamp 3 at the end of the active travel, for taking the end of the film from the distributor 2. The unit 11 may also modify the operation of the generator 7 according to the dimensions of the product 4 to be packaged and consequently according to the extent of pre-stretching of the film 101. For this purpose, the unit 11 may be provided with a multiple-wire link 20 to the means 6-106 already mentioned in the introduction to the present disclosure. The unit 11 may also modify the operation of the generator 7 according to the type of film used and/or other variables, and for this purpose the said unit 11 may be linked to a programming and interrogation keyboard 21 and to a visual display unit 22 which displays the various parameters. The control system 11-21-22 may be independent of that of the packaging machine, or may be integrated in the machine control system, with serial connections and predetermined communications protocols, the whole being done in a way that will be understood and easily applied by those skilled in the art. In the latter case, the sensor 19 is not used, since the starting of the hot air generator 7 is controlled by the computer of the packaging machine.

Claims: A method for packaging products including steps for maintaining elastic and plastic characteristics of a film (1) of stretchable thermoplastic material at optimal and substantially constant values during packaging of products, said method comprising the steps of: positioning a portion (101) of the film horizontally at a product packaging station in a packaging system, in such a manner that this portion of film presents an upper face and a lower face; sensing the presence of said portion of film in the product packaging station; measuring temperature of surrounding environment in which the portion of film (101) is used and determining whether said measured temperature is lower than an ideal temperature at which the film presents the best deformability characteristics for packaging; thermally conditioning said portion of film (101) by blowing hot air at variable temperature onto the lower face of the said portion of film positioned in the product packaging station; measuring the temperature of the portion of film subjected to thermal conditioning; interrupting thermal conditioning; raising the product against the film after said step of interrupting; and packaging said product. A method according to claim 1, further comprising pre-stretching of the film during the thermal conditioning step. A method according to claim 1, in which the hot air for thermal conditioning is blown onto the lower face of the film in a longitudinal direction and in a direction opposed to that of unwinding of the film from a film supply reel (1). A method according to claim 1, in which the path of the said hot air is limited substantially to the portion of film positioned in the product packaging station by locating a suction system opposite an origin of said hot air. An apparatus for packaging products, having means for maintaining elastic and plastic characteristics of a film (1) of stretchable, thermoplastic material at a substantially constant and optimal value during packaging of products, said apparatus comprising: a product packaging station including means (3) for feeding a portion of said film (101) and positioning same horizontally at the packaging station in such a manner that this portion of film presents an upper face and a lower face, and means for at least one of pre-stretching and stretching said film; generator means (7) for generating at least one flow of hot air with a variable temperature to thermally condition the portion of said film, said generator means being arranged so as to direct the flow of hot air onto the lower face of the portion of film positioned in the product packaging station; at least one environment sensor (16) for measuring a temperature value of an environment in which the means for feeding the portion of said film operates, and for generating a proportional electric signal; at least one control logic unit (11) which receives as input the electrical signal generated by said environment sensor, and which controls said hot air generator means (7) so that the film (101) acted on by the hot air is stretchable by said pre-stretching and stretching means; at least one film sensor (17) for measuring film temperature of said film thermally conditioned by the hot air generator, wherein each of said film sensors sends a proportional electrical signal to said control logic unit for use as a feedback and safety signal to prevent exceeding critical temperatures, further comprising means (19) for detecting a presence of the film (101) in the product packaging station, for indicating said film presence to said control logic unit (11), for ensuring conditioning of the film by said hot air generator before pre-stretching or stretching, and for switching said hot air generator (7) off before raising a product up to said film to package the product; means (5) for raising a product up to the said film to package the product.

Inventors: DELLEDONNE JOSEPH, DELLEDONNE, JOSEPH
Applicants: AWAX PROGETTAZIONE, A.W.A.X. PROGETTAZIONE E RICERCA S.R.L.
Application year: 1995
Main classification: B65B61
IPC codes: B65B61, B65B11