Description

There are disclosed herein a device and a process for sealing a film material, particularly a heat-shrink film, to the raised peripheral frame of a tray made of a polymeric material.

In the field of food packaging, polymeric trays, e.g. made of PE, PP, PET, PS materials or combinations thereof, are known to be used.

Such trays are usually manufactured by injection molding, although other technologies may be used, such as thermoforming.

Once the food product has been introduced into the tray, the latter is closed by heat-sealing a covering film to the peripheral frame of the tray.

The tray covering film, usually a clear film, may be a monolayer or multilayer film (e.g. a film made from a linear polyolefin material such as LDPE, LLDPE with appropriate butene, hexene, octane, etc. additives for providing the stretching effect) or a laminated film; various additives may be added to the film material, to change some of the physico chemical properties of the material.

For reliable sealing, the covering film material shall be compatible with the tray material.

According to a prior art technique, the tray is carried below the covering film, then a vertically movable heated mold is lowered to cause an annular portion of the film to contact the mold frame and perform sealing.

The shape and size of the heated mold depend on the shape and size of the tray (and possibly on the number of trays if the machine is designed to carry out simultaneous sealing of multiple trays).

The heated mold simultaneously provides:

heating of the film material to a medium temperature (depending on the physical properties of the packaging film, e.g. about 70°C) over a ring that substantially and/or generally corresponds to the film portion designed for bonding with the tray frame; heating to a low temperature (also depending on the physical properties of the packaging film, e.g. about 40°C) over the area of the film material that is inscribed in the medium temperature-heated ring.

Medium-temperature heating causes the film to be sealed to the tray frame.

Low-temperature heating is not necessary for sealing, but is required to cause shrinkage of the film, thereby removing bulges and flattening it; this improves the exterior appearance and facilitates later application of product marking labels.

Excess film is generally removed by a mechanical punching process, with a punch having a shape mating the shape of the tray frame.

This prior art technique has a few drawbacks.

For example, as the tray size is changed, the production line must be stopped to change the sealing molds, which involves production and delivery delays.

Furthermore, the trays to be sealed are required to be in line with the mold, such that sealing occurs exactly at the peripheral frame of the trays.

The means for aligning the trays with the mold (e.g. conveyor means formed with tray seats) have operational reliability problems.

The object of this invention is to provide a solution to prior art problems and particularly to the above mentioned problem.

This object may be fulfilled by a device as defined in claim 1 and a process as defined in claim 8.

Further advantages are achieved by the additional features of the dependent claims.

A possible embodiment of the device will be now described with reference to the accompanying drawings in which:

Figure 1 is a general schematic view of a tray sealing device;

Figure 2 is an enlarged view of a detail of the device of Figure 1 (LED matrix);

Figure 3 shows the temperature distribution on the LED matrix of Figure 2.

In the drawings, numeral 1 schematically designates a device 1 for sealing a plastic film material 2 to the raised frame 3 of a food-grade tray 4.

The device 1 comprises means 5 for feeding a film material 2.

In a possible embodiment, the means 5 for feeding a film material comprise a powered unwinding roller and a synchronized winding roller, which move the film 2 through a working area 20.

A sealing mold 6 is provided, which operates in the working area 20.

The mold 6 is vertically movable and can heat-seal a film material 2 to the frame 3 of at least one tray 4 which is oriented with the concave portion downwards.

The mold 6 is designed to create a first heating area 61, at an average temperature Tl , which extends over a first ring, substantially and/or generally conforming with the shape of the frame 3 of the tray 4.

The mold 6 is also designed to create a second heating area 62, at an average temperature T2 < Tl, which extends in the area inscribed in the first ring 61.

The temperatures of the first heating area 61 and the second heating area 62 depend on the physical properties of the film material 2.

By way of example, the temperature Tl may be of the order of about 70°C, whereas the temperature T2 may be of the order of about 40-50°C.

A feature of the mold of the device is that it includes a matrix of heating elements which cover the pressing surface of the mold 6 in a grid pattern.

In the preferred embodiment, the mold 6 comprises a plurality n of LEDs, e.g. infrared LEDs 7, 7, ... 7, in such an arrangement as to cover in substantially and/or generally uniform fashion a surface A defined by a perimeter P.

In a possible embodiment, the plurality of infrared LEDs 7, 7, ... 7 cover a square or rectangular surface A.

Means 8 (typically an electronic regulator) are also provided for selectively controlling the power of each LED 7.

The power regulating means 8 are actuated as a processor 14 processes the image of the edge 3 of the tray 4 as taken by a camera system 11, and by a laser barrier 12 or camera 13 device for measuring the tray height.

Preferably, the LEDs have a cross section of less than 2,0 mm ² , thereby allowing sufficiently accurate definition of the edges of the first and second heating areas 61, 62.

Preferably, the LEDs have an electric power of at least 10 mW such that, with the LEDs in side-by-side arrangement, a power of at least 0.25 watt/cm ² is emitted.

With these characteristics, trays of different shapes and sizes may be sealed without replacing the sealing mold, by simply selecting the LEDs to be actuated to heat the first and second areas 61, 62 of the film 2.

The LEDs 7, 7,... 7 define a generally flat working surface, which is adapted to be pressed against the film 2.

Each LED of the mold 6 may be protected by a layer of a material that is transparent to the radiation emitted by the LED (such as alumina oxides, crystalline AI ₂ O ₃ , CaF ₂ , etc.), preventing the softening polymeric material from sticking to the surface of the mold 6.

As an alternative to the above solution, in which each LED is covered, the entire LED matrix may be covered by a single plate of a material transparent to the radiation emitted by the LEDs.

The use of a covering material that is transparent to the LED-emitted radiation is preferable when the LED emission spectrum substantially corresponds the absorption spectrum of the material that forms the covering film 2.

As an alternative to the above solution, each LED of the mold 6 (the whole LED covered area) may be protected by a layer of a material that is opaque to the radiation emitted by the LED (such as silicon oxides, rare earths, or an aluminum sheet as thick as a few tens of microns, etc.), also preventing the softening polymeric material from sticking to the working surface of the mold 6.

This solution is preferable when the LED emission spectrum is far from the absorption bands of the material that forms the film, or when the device operates with covering films 2 having a variable chemical nature.

In one embodiment, the infrared LEDs 7, 7, ... 7 have an emission peak for a length in a range of about 900 nm < λ < 1000 nm.

The use of a matrix of LEDs 7, 7, ... 7 provides instantaneous emission of light and/or heat radiation, which is not allowed with resistive or induction heaters, where thermal time constants are higher.

In the illustrated embodiment, the punch 6 is designed to create a third heating area 63, at an average temperature T3 > Tl, which extends over a second ring, external to the first heating ring.

Typically, the temperature T3 is 90 °C or more.

With this characteristic, the film 2 is cut and excess film is removed by melting.

In a first embodiment of the device 1 , the operator may manually select (from a set of default values), the shape and size of the trays 3 which reach the working area 20 of the mold 6, whereby the power of each LED 7 is controlled according to preset rules.

The device 1 may have means 10 for successively moving a plurality of trays 2 into the working area 20 of the movable mold 6.

In a possible embodiment, the device 1 comprises an imaging system 11 , which is located above a conveyor belt 10, to detect the shape, size and orientation of the frame 3 of each tray 4 to be sealed.

Additionally, the possible embodiment as described above comprises a laser barrier 12 or camera 13 measuring system, for measuring the height of the trays that come into the packaging line.

With this characteristic, no arrangement is required to impart a particular orientation to the trays, and the power of each LED 7 is controlled according to the shape, size and orientation of each tray.

It shall be particularly noted that the provision of the imaging system allows the use of simple conveyor belts to move the trays.

The imaging system may include a line-scan or an area-scan camera.