Background Art Drying plastic materials is generally necessary before these materials are used in subsequent operations, for example in extrusion or injection molding processes. The presence of humidity in the materials may in fact impair the quality and/or appearance of subsequent production. In some cases, drying may also be performed prior to a storage phase, although always in view of the subsequent need for material with a low degree of humidity.

In some particular cases, as well as drying, it may be necessary to heat the material to a predetermined temperature to obtain the desired molecular structural characteristics prior to use in subsequent processes.

For example, polyethylene terephtalate (PET) may also be subjected to pre-crystallization procedures before use.

In general, the most widely used heating/drying systems of the prior art are essentially based on storage of the plastic materials in containers of considerable size. In fact, containers are required with a volume ranging from 3 to 20 times the volume of the plastic material to be dried, generally processed in the form of flakes or granules, as a function of the apparent specific weight of the material, of the processing time and of the productivity required. The heating or drying process is performed by maintaining the materials inside these containers for a time depending on the diffusion speed of the water molecules through the polymer up to the surface of the granule or flake, and on the degree of humidity of the

material subjected to processing.

In discontinuous processes, or in batch processes, the material is heated by contact with the heated walls of the container. In some cases, vacuum pressure is produced inside the container to accelerate the drying process and evacuate the gases produced. The material is mixed continuously by stirrers to even out the temperature of the mass.

With specific regard to continuous processes, heating takes place by blowing heated air while, if necessary, the material can be maintained in constant movement by stirrers to prevent possible compacting. The volatile substances released from the material are thus removed from the container by means of the hot air current thus produced and then dispersed into the environment. To obtain particularly high degrees of dehumidification, that is up to a few tens of ppm of residual humidity in the processed material the process air is previously subjected to a drying phase using specific resins, until reaching condensation temperatures ranging from-40°C to-60°C, then heated and fed into the container.

The resins utilized for forced drying must in turn be regenerated, that is freed from the humidity absorbed. This is produced by air heated to high temperatures. The water released by regeneration of the resins is dispersed into the environment, with consequent loss of energy.

In more recent times it has been suggested to use microwave energy in heating and/or drying processes of plastic materials, for example, hygroscopic plastic materials, in which the humidity found in the same materials constitutes the means capable of absorbing this energy, or plastic materials that are in any case sensitive to this form of energy notwithstanding their capacity to absorb humidity.

An example of process and apparatus for continuously heating plastic materials with the use of microwave energy is disclosed in EP-Al-0535771.

The apparatus comprises a drying room with a plurality of drying zones

separated by partitions in the upper portion of the room in order to apply heating at different temperatures to the materials. A rotary shaft with agitating blades allows to stir the materials passing in the lower portion of the room and make them to advance in the drying room up to a material outlet.

During the continuous process, some particles and/or dust of the processed materials can settle on the internal walls of the drying room, mainly on the upper part of the drying room or on the partitions, without any possibility to remove them during the process. Particles and/or dust standing still on the drying room are subject to overheating with consequent deterioration and, when mixed to the other materials passing in the drying room, cause the deterioration of the processed materials. This applies not only to the apparatus of the cited patent application, but also to other examples of apparatuses of the prior art.

Indeed, overheating can also occur for materials advancing slowly in high temperatures zones, in particular when the internal shape of the process container can adversely affect the progress of the materials through the same, independently from the efficacy of the agitating means provided in the process container.

Summary of the invention This being stated, the task of the present invention is to propose a method and a device for heating, drying and/or crystallizing plastic materials in a continuous way which make it possible to overcome the drawbacks of prior art.

Within the scope of this task, a general object of the present invention is to propose a method and a device of the aforesaid type which make it possible to produce heating, drying and/or crystallizing of plastic materials in a particularly rapid and efficient way. A particular object of the present invention is to propose a method and a device of the

aforesaid type which make it possible to avoid overheating of the processed materials.

Another object of the present invention is to propose a method and a device of the aforesaid type which avoids deposition of materials inside the process container.

A further object of the present invention is to propose a device and a method of the aforesaid type which make it possible to avoid slow progress of the materials in the process container.

Yet another object of the present invention is to propose a device of the aforesaid type which allows to be cleaned in a simple and easy way.

These objects are achieved by the present invention, which relates to a device for heating, drying and/or crystallizing plastic materials in a continuous way, including at least one process container, feeding means to continuously convey the materials inside the container, rotating means to stir the materials and/or make them to advance inside the container, drawing means to continuously remove the materials from the container and one or more microwave generators to heat the materials conveyed inside the container, characterized by recirculating means to connect the drawing means with the feeding means for the recirculation of at least part of the materials.

Recirculation of the materials allows to hold the process container substantially full of materials in all the operating conditions, without causing undesired slow progress or stops of the materials inside the container, mainly near the drawing means which draw the materials coming out from the device.

The recirculating means include preferably one or more motor driven augers receiving the materials to be recirculated through at least one outlet port provided in correspondence of the drawing means. The rotation of the motor driven augers is preferably regulated steplessly

between two different preset speeds in order to vary the fraction of the materials to be recirculated as a function of the operating conditions.

The process container preferably comprises at least two cylindrical chambers having a circular cross section and extending in the direction of the cylindrical axis, the chambers intersecting to each other along a direction parallel to the axis.

The rotating means include at least two stirrers positioned side by side in a respective chamber of the container and made to rotate about mutually parallel axes. Each of the stirrers includes a rotating shaft on which a plurality of radial blades are fixed. As well as contributing towards making the materials advance, the stirrers also exert a continuous mixing action on them.

The rotating elements can be produced in materials that reflect the waves of the electromagnetic field, so as to promote diffusion of the electromagnetic field in the mass of materials being conveyed in the container.

The microwave generators may be disposed so as to generate an electromagnetic field directed prevalently towards a direction which is offset with respect to the axis of rotation of the rotating element (s), preferably a direction shifted laterally towards the outside in respect of a perpendicular incident to the axes of rotation.

Therefore, in the process container kept full of materials, the stirrers allow to continuously advance the materials through the process container and avoid any deposition of materials along their path inside the container.

According to another feature of the present invention, the stirrers are removably housed inside the respective chambers and can be easily extracted thanks to a movable carriage supporting at least the gearmotor unit driving the stirrers. The carriage includes means for

supporting the shafts in the extracted condition and can be moved by a handling motor unit between an operative position of the stirrers within the chambers and an extracted position of the stirrers outside from the chambers. This allows to facilitate cleaning of the container and the stirrers, as for example when materials to be treated are different from those previously treated as regards colors or characteristics.

According to another aspect of the present invention, it is provided a method for heating, drying and/or crystallizing plastic materials in a continuous way, wherein the materials are fed continuously inside a process container, are made to advance inside the container by rotating means and are drawn continuously from the container, and wherein the materials are heated during their path inside the container by a microwave electromagnetic field, characterized by providing a recirculation of at least part of the materials from the drawing means to the feeding means.

The humidity released by the processed materials is preferably removed by maintaining a vacuum inside the container, or in any case by maintaining the pressure inside the container below the external atmospheric pressure.

Control of the degree of dehumidification of the materials subjected to processing is preferably performed by detecting the temperature of the materials exiting from the container. The power emitted by the various microwave generators disposed along the path along which the materials are conveyed inside the container is regulated as a function of the detected temperature at different points along the path of the materials, particularly in correspondence of each generator. One or more temperature sensors are thus provided, as well as means suitable to control the emission power of the microwave generators as a function of the detected temperatures.

Brief description of the drawings Further characteristics and advantages of the present invention shall become more apparent from the description hereunder, provided purely as a non-limiting example with reference to the accompanying schematic drawings, in which: - Figure 1 is a simplified perspective view of a device according to a possible embodiment of the present invention; - Figure 2 is a front elevation view in section a device according to the present invention; - Figure 3 is a side view in section of a device according to the present invention in its operative condition; - Figure 4 is a simplified top plan view in section of a device according to the present invention; - Figure 5 is an enlarged view of a detail of the device shown in Figure 3; and - Figure 6 is a side view in section of a device according to the present invention with the stirrers extracted from the process container.

Modes for carrying out the invention Figure 1 schematically shows a possible embodiment of a device according to the present invention without some details, for sake of simplicity, as for example microwave generators and other elements of the device. The device is essentially constituted by a process container 10 made preferably in a reflecting material for the microwaves at the wavelength utilized.

The materials are fed continuously (arrow M) to an upper collecting member, such as a hopper 22 which is at atmospheric pressure, through known devices (not shown), for example augers, band or bucket elevators or the like. The hopper 22 is connected below to a compensation chamber 21 which is kept alternatively at atmospheric

pressure and at vacuum or low pressure condition in order to allow the materials to be transferred to a lower hopper 20 which is kept only at vacuum or low pressure condition.

When the materials are to be transferred from the hopper 22 to the camber 21, the latter is brought at atmospheric pressure and then a first commanded valve 23a is opened to allow the materials to fill by gravity the chamber 21. Next the valve 23a is closed, a vacuum or low pressure condition is restored in chamber 21 and then a second commanded valve 23b is opened to transfer materials by gravity from the chamber 21 to the lower hopper 20 connected directly to the process container 10.

This system allows to feed continuously the materials to be processed and constantly maintain at the same time a vacuum or low pressure condition inside the process container 10.

At the end of the process container 10 there are provided drawing means, including for example an auger 40 that brings the processed materials, indicated by the arrow N, towards a discharge outlet 45. The latter may be directly connected to the inlet of a subsequent processing machine, as for example an extruder or the like.

According to the invention, there are provided means to draw at least part of the materials and bring them to the hopper 20 in order to recirculate a possible excess of materials near the end of the process container 10 and near the drawing means 40.

The recirculating means include at least one motor driven auger 1 which is connected to an outlet port 2 through the drawing means 40 and a discharge duct 3, where the materials are fed by gravity. Means are preferably provided to steplessly regulate the rotation of the motor driven auger (s) 1 between two different preset speeds, in order to make it possible to vary the fraction of the materials to be recirculated as a function of the quantity of materials required at discharge outlet 45.

In the embodiment shown in Figure 1, the auger 1 is of the flexible type in order to bring the materials to be recirculated back to the hopper 20 through a feeding duct 4.

Inside the container 10, the hopper 20 and the recirculating means 1,3 and 4 are kept in a vacuum condition, or in any case at a pressure below the external atmospheric pressure in order to allow the removal of the humidity released by the materials. A pump (not shown) can be for example used to obtain the desired low pressure condition and extract humidity from the device.

As shown also in Figure 2, the process container 10 includes two cylindrical chambers 80 having a circular cross section and intersecting to each other along a direction parallel to the cylindrical axis. The container 10 includes preferably an external coating 15 made in heat- insulating material.

Inside each chamber 80 are rotating means, the function of which is essentially to stir the materials and/or make them advance. Two stirrers 90 are positioned side by side in a respective chamber 80 of the container 10 and are made to rotate about mutually parallel axes. Each stirrer comprises a rotating shaft 30 on which a plurality of radial blades 31 are fixed. Orientation of the blades 31 and the direction of rotation of the shafts 30 are chosen so as to make the materials advance and keep it moving towards a discharge zone of the container 10.

As shown schematically in the plan view of Figure 4, the radial blades 31 (of which only the fixing hubs are shown for simplicity) are preferably placed in alternate positions between the opposite shafts 90 (not shown in Fig. 4) in order to improve the mixing of the materials during rotation of the stirrers.

Only at the opposite ends of the stirrers there are provided blades 32 and 33 faced to each other on the opposite shafts, namely two facing

blades 32 in correspondence of the inlet zone of the materials feed by the hopper 20 and two facing blades 33 in correspondence of the discharging zone near the discharging outlet 2. These blades 32 and 33 avoid deposition of materials on front and rear internal walls of the container 10.

With reference again to Figure 2, the materials advancing along the container 10 are heated by microwave electromagnetic field generators 50 disposed along the path of the materials in the container 10 and positioned so as to generate inside both the chambers 80 an electromagnetic field directed prevalently towards the materials being conveyed.

Each microwave generator 50 is mounted on the container 10 through a manifold 55, with interposition of a partition 56 made of a material transparent to microwaves (e. g. glass, Teflon or the like) between each manifold 55 and the container 10.

The manifolds 55 have one or more holes to keep the inside of the manifold 55 at atmospheric pressure. In particular, the manifolds 55 can have for example perforated walls, with hole having shape and dimensions that avoid the diffusion of microwaves at the outside.

These provisions are made to insulate the generators 50 from the low pressure environment inside the container 10 because a sudden decrease of pressure, due for example to a break in a transparent partition 56, could damage the respective microwave generator 50.

Referring also to Figure 4, where the positions of the microwave generators 50 is indicated for simplicity only by the assembly flanges 51 of the same, it should be noted that microwave generators 50 are disposed to act in both the chambers 80 and directed prevalently towards the external space in respect of the axes of rotation of the respective rotating shafts 90. In other words, the electromagnetic flux

from the generators 50 is directed towards a direction which is offset with respect to the axis of rotation of the stirrers. Furthermore, the electromagnetic flux from the generators 50 is oriented towards areas free of rotating blades 31 on the respective chamber, while a rotating blade 31 is mounted on the shaft in the other chamber. In this way, the materials are continuously moved from one chamber and overheating of the same is so avoided.

The device according to the present invention may utilize generators 50 commonly available on the market. For example, generators with a power between 2 and 10 kW can be used, designed to emit microwaves at a frequency of around 2450 MHz. In the embodiment herein disclosed, up to ten generators can be mounted, each having a power of 6kW, in order to reach a productivity of about 600 kg/h (according to an estimate of about 100 W/kg per hour).

The number, layout and power of the generators 50 may obviously vary as a function of the length of the container 10, and as a function of the conditions and characteristics of the materials that are prevalently processed in the device and the flow rates of materials that the device must be capable of sustaining.

It must nonetheless be born in mind that the powers of the most widely used generators on the market for industrial applications in general do not exceed a power of 20 kW, and it should be also considered that too many generators of low power could be somewhat disadvantageous from the viewpoint of cost.

With reference to Figure 3, rotation of the stirrer shafts 30 is imparted by a gearmotor unit 35 which is regulated by a control unit to allow speed to be varied as a function of the conditions and characteristics of the processed materials. The gearmotor unit 35 is provided for rotating the stirrers in opposite directions, to improve the mixing action on the

materials. The rotation in opposite direction may be imparted by the gearmotor unit 35 at the same speed or, alternatively, the gearmotor unit may be designed to impart rotation at different speeds in order to obtain a random action of the blades 31 on the materials.

At the end of the container 10, the auger 40 of the drawing means is driven by a gearmotor unit 44 which can be subject to a control unit that determines the rotation speed as a function of the output rate to be obtained. This is particularly useful when a device according to the present invention is directly connected to the feed section of an extruder, or of any other similar machine, positioned immediately downstream of the device.

As far as the adjustment of the emission power of the microwave generators 50 is concerned, a control unit (not shown) is preferably provided to allow the emission power of each of the generators 50 to be varied as a function of the characteristics, the conditions of humidity of the processed materials, and the degree of humidity to be obtained in the materials delivered.

Control is preferably produced as a function of the temperature of the materials detected in different points of the container. Temperature sensors 60 are thereby provided to produce a signal representing the temperature in each zone of the container 10 in which they are installed.

These signals are sent to the control unit to adjust almost instantly the power emitted by each generator 50.

According to a possible embodiment of the present invention, one or more sensors may be provided to detect the level of the materials in one or more zones of said container. For example, at least one sensor 66 can be provided in the upper portion of the discharging area inside the container 10-the first area that can be partially emptied-in order to send signals representative of this detected condition to a suitable

control unit. The latter may be programmed to continuously vary the amount of materials coming out from the process container 10, i. e. the sum of the flow rate of materials drawn by the auger 40 and the flow rate of materials recirculated through the auger 1. The control unit therefore operates on the gearmotor units driving auger 1 and 40 in order to restore the required filling condition inside the process container 10.

According to another feature of the present invention, electrical conductive elements are placed between the container and the rotating shafts of the stirrers to keep the same in electrical contact to each other. This is due to the fact that electromagnetic fields can induce electrical current on metal elements. It must therefore be assured in any case-also in presence of lubricating materials-the electrical continuity between the rotating shafts 30 and the container 10 in order to avoid electrical discharges mainly inside the low pressure or vacuum environment of the container 10.

A possible embodiment of this feature is shown in the enlarged view of Figure 5, where a bush 70 made of an electrical conductive and hardwearing material is pushed against the respective shaft 30 by spring means 71. Moreover, rotating sealing means 73 are provided for preventing the passage of processed materials towards the conductive bush 70. Sealing means 73 are also made of a conductive material suitable to operate at high temperatures in presence of variable electromagnetic fields.

During steady operation, the materials to be processed are fed continuously to the container 10 and expelled through the discharge port 45.

The flow rate of the materials coming out through the outlet port 2 determines the speed of advance of the materials inside the process

container 10. A preset flow rate through the outlet port 2 can avoid accumulation of the materials at the end of the container.

When the flow rate of materials drawn by the auger 40 is changed, for example due to the change of demand by an extruder connected downstream to the device, the speed of the recirculating auger 1 is changed in inversely proportional mode in order to keep constant the flow rate at the outlet port 2.

In order to assure the filling of the container 10 without exceeding in the compression of the materials inside the process container 10, the rotation speed of the stirrers 90 is automatically varied within a preset range in order to keep a preset torque on the gearmotor unit 35 driving the same stirrers. The transport capacity of the stirrers is determined by setting a pre-established inclination of some blades 31 as a function of the physical properties of the materials to be processed.

The signal detected by the level sensor 66 (Figure 3) is used to determine the flow rate at the outlet port 2 for keeping the required filling level and therefore assuring the desired cleaning action on the internal surfaces of the process container 10.

In the start-up phase of the process, in which the temperatures of materials conveyed in the various zones of the container have not yet reached the pre-established values, materials can be fully recirculated from the discharge port 2 towards the feed hopper 20. In this case, the auger 1 is driven to its maximum speed for the time required to bring the temperatures of the materials to the pre-established values.

In any case, the time required to reach the steady operating temperatures is particularly fast and in any case much less than the times required to reach the steady phases in common drying systems using heated air currents. The device is thus particularly suitable and inexpensive even in the case of small quantities of materials to be

processed.

According to another feature of the present invention, the stirrers 90 are removably housed inside the respective chambers 80 of the process container 10.

Figure 6 shows the device according to the invention with the stirrers 90 in extracted position. The gearmotor unit 35 is mounted on a carriage 200 which is movable with respect to the supporting structure 100 of the container 10. Means 201 are provided on the movable carriage 200 for supporting the stirrers 90 when they are extracted from the container 10.

Advantageously, a handling motor unit (not shown) can be provided for moving the carriage 200 between the operative position (Figure 3) and the extracted position of the stirrers 90 (Figure 6).

The stirrers 90 are designed to be easily removed to facilitate cleaning operations, especially when materials with different characteristics must be processed in succession, and also maintenance operations on the blades 31 and the shafts 30.