DESCRIPTION

Technical field

The present invention relates to a process for transforming a bulk polymer material by moulding or extrusion, having the characteristics outlined in the preamble of the principal claim, and an apparatus operating according to this process.

Background art

The extrusion method and the moulding method are known and widely used in the technical field of processes for transforming polymer materials, commonly referred to as "plastic materials" or more simply "plastics". The moulding method can be used in one of its various versions such as injection moulding, compression moulding, injection blow moulding, or extrusion blow moulding.

In the various versions of the moulding method, the polymer material is introduced in the molten or semi-molten state into a mould whose internal cavity reproduces the shape of the desired object, so that the polymer material, when brought back to the solid state, assumes the configuration thereof.

Similarly, in the extrusion method, the polymer material is pushed in the molten or semi-molten state through one or more through cavities formed in a suitable extrusion head, whose internal profile reproduces the shape of the desired object, so that the polymer material, when brought back to the solid state, assumes the configuration thereof. As a general rule, in these processes the polymer material to be transformed has to be loaded into one or more hoppers from which it is subsequently transferred, by gravity for example, into a moulding or extrusion apparatus which proceeds, respectively, to place it in the mould or to push it through the head in suitable conditions of temperature and pressure.

The cooling of the mould or the extrusion head, and the consequent solidification of the polymer material contained therein, is generally carried out by contact with a cooling liquid, for example water, conveyed by flowing through a suitable circuit. In this circuit, the cooling liquid comes into contact with an evaporation unit of a chiller assembly which brings it to a desired temperature which is sufficiently low to enable the mould or extrusion head to be cooled in the shortest possible time.

The polymer material loaded into the hopper and then fed to the moulding or extrusion apparatus is formed by a plurality of solid elements which are distinct and separate from one another, and are of suitable sizes and shapes, according to the working process to be performed and to the polymer material used.

In the present description and in the attached claims, this plurality of solid elements is indicated as a whole by the term "bulk material" (in accordance with the English term commonly used in this field), and comprises polymer material in the form of granules or powder or flakes, or alternatively in the form of semi-finished products as in the case of preforms supplied to a blowing apparatus for the production of plastic bottles. To promote the efficient and correct execution of the subsequent conversion step, the polymer material present in the hopper is properly treated, according to the type of subsequent processing and the specific properties of the polymer.

For example, it is known to heat the polymer material so as to reduce the time and energy required for the moulding or extrusion apparatus to bring the polymer material to a suitable thermal condition for its introduction into the mould or into the extrusion head. Alternatively or additionally, it is known to heat the polymer material so as to eliminate (or at least reduce to an acceptable level) the presence of any water that may be present on the surface of the solid elements forming the bulk material .

For some polymer materials with high hygroscopic properties, for example polyethylene terephthalate (PET), the treatment is not limited to heating but also includes a strong dehumidification of the polymer material, so as to bring its moisture content to a level below about 100 or 50 parts per million (ppm), depending on the field of application .

This strong dehumidification of the polymer material will be referred to in the rest of the description and the claims as "drying", and the resulting material will be referred to as "dried".

Actually, the presence of water inside or outside the polymer material supplied to the moulding or extrusion apparatus is entirely undesirable, as it may give rise to defects in the end product.

Transforming apparatuses of this type require a large energy supply for their operation, to the extent that, in many cases, the overall cost of the transforming process is largely determined by the amount spent on energy. In this field, therefore, it is considered to be highly desirable to find new solutions for reducing energy consumption as far as possible. Description of the invention

The problem underlying the present invention is that of providing a process and an apparatus for transforming a bulk polymer material by moulding or extrusion which is structurally and functionally designed to reduce the requisite energy supply.

Within the scope of this general problem, an object of the invention is to provide a solution that can be applied as widely as possible in these transforming processes, regardless of the characteristics of the polymer or the transforming method used.

This problem is resolved and this object is achieved by the present invention by means of a process and an apparatus realized in accordance with the appended claims.

Brief description of the drawings

The characteristics and advantages of the invention will be made clearer by the detailed description of some preferred exemplary embodiments thereof, illustrated, for the purposes of guidance and in a non-limiting way, with reference to the attached drawings, in which :

- Figure 1 is a schematic view of a first exemplary embodiment of an apparatus for transforming a bulk polymer material by moulding or extrusion, realized according to the present invention;

- Figure 2 is a schematic view of a second exemplary embodiment of an apparatus for transforming a bulk polymer material by moulding or extrusion, realized according to the present invention; and

- Figure 3 is a schematic view of a third exemplary embodiment of an apparatus for transforming a bulk polymer material by moulding or extrusion, realized according to the present invention. Preferred embodiments of the invention

With initial reference to Figure 1, a first exemplary embodiment of an apparatus for transforming a bulk polymer material by moulding or extrusion, operating according to the process of the present invention, is indicated as a whole with 1.

In particular, the apparatus 1 transforms the bulk polymer material by a moulding method.

The bulk polymer material treated in the apparatus 1, indicated by M, is any thermoplastic polymer material suitable for being formed by a moulding method, such as polypropylene, polyethylene, polyvinyl chloride, polystyrene, ABS, in the form of granules or flakes.

The apparatus 1 comprises, in general terms, a treatment unit 2, in which the bulk polymer material is subjected to one or more operations of preparation for moulding, and a subsequent moulding apparatus 3, designed to receive the bulk polymer material treated by the treatment unit 2 and to mould it into the shape of the desired product.

The treatment unit 2 comprises a hopper 4, designed to contain the bulk polymer material introduced into it via a loading line 4a, and a heating line 5, designed to convey a heating fluid to the hopper 4 in order to heat the bulk polymer material contained therein. In the preferred examples described herein, the heating fluid consists of air which is introduced into the hopper 4 via a diffuser 6 so as to come into direct contact with the polymer material contained in the hopper 4. In alternative embodiments, however, the heating fluid may be of a different type, for example another type of gas or a liquid such as water or oil, which, in the case of a liquid, is supplied to a coil or a jacket formed in the hopper.

In the preferred version shown in Figure 1, the heating fluid is conveyed along a circuit comprising a recovery line 7 running from the hopper 4 and a fan 8 for redirecting the heating fluid along the heating line 5. The circuit also preferably includes a top-up line 7a, for topping up the heating fluid in the circuit when necessary, a purification filter 7b and a heating device 9, for heating the heating fluid to the desired temperature, for example between 60°C and 190°C, before it enters the hopper 4. The moulding apparatus 3 comprises a mould 10 which is cooled by a cooling liquid, for example water, conveyed in a cooling circuit 11.

The moulding apparatus 3 further comprises a chamber 12 in which the polymer material from the hopper 4 is collected and brought to the molten or semi-molten state by means of a screw conveyor which pushes the material along the chamber 12 until it is injected into the mould 10. The cooling circuit 11 comprises a heat exchanger (or "chiller") 13, which cools the cooling liquid to a desired temperature, between about 7°C and 20°C for example, an inlet line 14, which carries the cooling liquid from the heat exchanger 13 until it comes into thermal contact with the mould 10, and an outlet line 15, which carries the cooling liquid flowing out of the mould 10 back to the heat exchanger 13.

A pump 16 causes the cooling liquid to flow through the cooling circuit 11.

According to a first aspect of the present invention, the apparatus 1 comprises a heat pump 20 including a compressor 21 for driving a process fluid through a closed process circuit 22 which includes a first heat exchange unit 23 and a second heat exchange unit 24. An expansion valve 26, through which the process fluid passes from a first pressure to a second pressure which is substantially lower than the first pressure, is provided on a line 25 of the process circuit 22 extending between the outlet of the first heat exchange unit 23 and the inlet of the second heat exchange unit 24.

Thus the process circuit 22 is divided into a high-pressure portion, extending from the compressor 21 to the expansion valve 26 via the first heat exchange unit 23, and a low-pressure portion, extending from the expansion valve 26 to the compressor 21 via the second heat exchange unit 24.

A line 27, extending between the second heat exchange unit 24 and the compressor 21, and a line 28, extending from the compressor 21 to the first heat exchange unit 23, complete the process circuit 22.

The first heat exchange unit 23 is placed between the process circuit 22 and the heating line 5 in order to heat the heating fluid by means of the process fluid, while the second heat exchange unit 24 is placed between the process circuit 22 and the cooling circuit 11 to cool the cooling liquid leaving the mould 10 by means of the process fluid. In particular, the first heat exchange unit 23 is associated with the heating line 5 downstream of the fan 8 and upstream of the heating device 9.

Preferably, the first heat exchange unit 23 operates at a temperature of at least 60°C, and more preferably at a temperature between 90°C and 130°C.

The second heat exchange unit 24 is associated with the outlet line 15 of the cooling circuit 11. The cooling circuit 11 also includes a line 15a extending so as to short-circuit the second heat exchange unit 24, and a control valve 15b which regulates the flow rate of the cooling liquid which is conveyed towards the second heat exchange unit 24 and the flow rate of the liquid which short-circuits this unit by passing through the line 15a. Preferably, the heat pump 20 comprises a heat exchanger 29 placed between the outlet line 25 from the first heat exchange unit 23, upstream of the expansion valve 26, and the inlet line 27 of the compressor 21. Because of the provision of this additional heat exchanger, heat can be transferred from the process fluid leaving the first heat exchange unit 23 to the process fluid leaving the second heat exchanger 24, upstream of the compressor 21, thus making the heat pump 20 more energy efficient. Preferably, the process fluid flowing in the heat pump 20 is carbon dioxide. More preferably, the C0 ₂ operates in the heat pump 20 in transcritical conditions.

In particular, the first heat exchange unit 23 operates in temperature and pressure conditions above the critical point of the C0 ₂ (supercritical conditions), while the second heat exchange unit 24 operates in temperature and pressure conditions below the critical point of the C0 ₂ (subcritical conditions), such that the C0 ₂ is at least partially evaporated. Therefore, in the first heat exchange unit 23, the process fluid does not undergo a change of state, whereas in the second heat exchange unit 24 it changes from the liquid state to the gaseous state. Consequently, the first heat exchange unit 23 acts as a gas cooler, while the second heat exchange unit 24 acts as an evaporator.

The apparatus 1 operates as follows.

The bulk polymer material M is loaded into the hopper 4 via the loading line 4a, where it is heated to a suitable temperature, dependent on the characteristics of the polymer and the type of moulding to which it is to be subjected.

The bulk polymer material M is heated by introducing heating fluid, for example air, having a predetermined temperature and flow rate, into the hopper 4 for a predetermined time.

The process of heating various bulk polymer materials is known to those skilled in the art who are able to identify the appropriate temperature, residence time and flow rate for achieving the correct heating of the polymer material to be treated.

The heating fluid is introduced into the hopper via the diffuser 6 into which the heating line 5 opens, and is recovered at the top of the hopper 4 by the recovery line 7 which returns it to the fan 8.

At the end of the heating step, the bulk polymer material M is transferred from the hopper 4 to the moulding apparatus 3, by gravity for example, where, after being brought to a molten or semi-molten state in the chamber 12, it is introduced into the mould 10 so as to assume the shape of the latter, and is subsequently extracted, after cooling, from the mould 10 for the subsequent processing steps.

The mould 10 is cooled by means of the cooling liquid flowing in the cooling circuit 11.

The heat pump 20 is operated in such a way that the process fluid (C0 ₂ in this preferred example) is supplied to the first heat exchange unit 23 at a pressure approximately between 110 and 140 bar and at a temperature approximately between 110°C and 145°C. At this temperature, it provides heat to the heating fluid present in the heating line 5, which is consequently brought to a temperature approximately between 70°C and 120°C. If the temperature is insufficient for introduction into the hopper 4, the heating fluid is additionally heated in the heating device 9.

Having released some of its heat in the heat exchanger 29, the C0 ₂ is expanded by the expansion valve 26 to a pressure between about 40 and about 50 bar, in subcritical conditions, and introduced into the second heat exchange unit 24 where it evaporates, thus cooling the cooling liquid flowing in the outlet line 15.

At the outlet of the second heat exchange unit 24, the C0 ₂ in vapour phase is pre-heated in the heat exchanger 29 and then returned to the initial temperature and pressure conditions by the compressor 21.

When operating as described above, the heat pump 20 has a coefficient of performance (COP) of about 3.5 calculated on the hot side (that is to say, at the first heat exchange unit 23), and about 2.5 on the cold side (that is to say, at the second heat exchange unit 24).

Because of the provision of the heat pump 20, the energy requirement of the apparatus 1 is substantially lower than that of a similar conventional apparatus without a heat pump. For example, in the case of transforming PET (described below) by moulding, the estimated energy saving is about 20% in the step of heating the heating fluid and about 10% in the step of cooling the mould cooling liquid.

The apparatus 1 has been described in relation to the treatment and moulding of bulk polymer material in granule or flake form, but it can also be designed for the blow moulding of bulk material in the form of semi-finished products, such as the preforms used for manufacturing bottles.

The apparatus may also be similarly designed to treat and mould bulk polymer material having highly hygroscopic properties, such as polyethylene terephthalate (PET). In this case, a dehumidification device, of the molecular sieve type for example, is provided in the circuit of the heating fluid associated with the hopper 4, when this fluid is a gas such as air, for drying the heating fluid before it is re-heated it and introduced into the hopper 4. By way of example, the heating fluid is dehumidified until it has a dew point of below -20°C.

Thus the bulk polymer material present in the hopper 4 is not only heated but also suitably dried before being supplied to the moulding apparatus 3. For example, the bulk polymer material is dried to a moisture content of less than 50 - 100 ppm .

Figure 2 shows schematically a second exemplary embodiment of an apparatus, indicated as a whole by 100, operating according to the process of the present invention.

Components of the apparatus 100 similar to components of the apparatus 1 described above are indicated by the same reference numerals.

The apparatus 100 differs from the apparatus 1 in that it comprises a second hopper 101 in which the bulk polymer material M, pre-heated in the hopper 4, is additionally treated before being supplied to the moulding apparatus 3.

In this case, the hopper 4 only pre-heats the bulk polymer material to a first temperature, and the heating fluid present in the heating line 5 is heated only by the heat pump 20.

The step of heating the bulk polymer material to a final temperature, higher than the first temperature, and, where necessary, the step of drying the bulk polymer material M, takes place in the second hopper 101, because of the provision of a second heating line 102 having a heating device 103, a fan 104 and, if required, a dehumidification device (not shown in the figure).

The advantages associated with this type of process (and the corresponding apparatus) are the elimination of excess moisture and better control of the process.

Figure 3 shows schematically a third exemplary embodiment of an apparatus, indicated as a whole by 200, operating according to the process of the present invention.

Components of the apparatus 200 similar to components of the apparatus 1 described previously are indicated by the same reference numerals.

The apparatus 200 is designed for treating and moulding bulk polymer material with highly hygroscopic properties, and particularly for drying the polymer material by a vacuum stripping step.

The apparatus 200 therefore differs from the apparatus 1 in that it comprises at least a second hopper 201a, positioned downstream of the hopper 4 and upstream of the moulding apparatus 3, and a depressurization line 202 connected to the second hopper 201a to generate a sufficient degree of vacuum in the second hopper 201a to reduce the moisture content of the polymer material to the desired level. The degree of vacuum generated in the second hopper 201a by the depressurization line 202 is at least 800 mbar and is preferably between 900 and 1000 mbar.

The depressurization line 202 includes a vacuum pump 203 for creating the requisite degree of vacuum in the second hopper 201, a filter 204, and a shut-off valve 205a for putting the vacuum pump 203 into communication with the second hopper 201a.

Preferably, a heat exchanger 206 is provided upstream of the vacuum pump 205, to put the depressurization line 202 into contact with a branch 207 of the cooling circuit 11 of the mould 10. Thus the moisture extracted from the bulk polymer material M present in the second hopper 201 is condensed and separated from the flow of air passing out of the second hopper 201.

The branch 207 is preferably detached from the inlet line 14 downstream of the heat exchanger 13 and, after it has passed through the heat exchanger 206, is re-attached to the outlet line 15 upstream of the heat exchanger 13.

The second hopper 201a is also associated with a heating circuit 210 designed to bring the bulk polymer material, after drying, to the correct temperature, or to keep it at the correct temperature, for supply to the moulding apparatus 3.

The heating circuit 210 preferably introduces hot dry air into the second hopper 201a and comprises a fan 211, a heater 212, a filter 213 and a shut-off valve 214a.

The apparatus 200 preferably comprises a pair of second hoppers 201a and 201b, substantially identical and operating in parallel and alternatively to one another, both connected to the depressurization line 202 and to the heating circuit 210, and both connected to the hopper 4 (positioned upstream) and the moulding apparatus 3 (positioned downstream). In particular, the second hopper 201b is connected to the depressurization line 202 and to the heating circuit 210 via respective shut-off valves 203b and 214b.

In this case, the apparatus 200 causes the bulk polymer material M, after it has been heated to a suitable temperature in the hopper 4, to be supplied to one of the two second hoppers 201a, 201b, for example the second hopper 201a, so that it can be dried.

The connection of the second hopper 201a to the depressurization line 202 via the shut-off valve 203a is opened, and the polymer material is subjected to a stripping step for the predetermined time required to reduce the moisture content to the desired level. At the end of this step, the shut-off valve 203a is closed and the bulk polymer material, having been heated and dried, is supplied to the moulding apparatus 3.

The discharge speed of the second hopper 201a is dependent on the demand for material of the moulding apparatus 3, and, in order to keep the temperature of the bulk polymer material remaining in the second hopper 201a above a limit value, the heating circuit 210 may be actuated and may be put into communication with the second hopper 201a via the shut-off valve 214a.

Simultaneously with the execution of the vacuum drying step in the second hopper 201a, a second amount of bulk polymer material M is heated in the hopper 4, this second amount being discharged, after the heating step, into the second hopper 201b to be dried by the opening of the shut-off valve 203b.

Because of this apparatus configuration, the hopper 4 can operate continuously, without waiting for the material newly dried by the second hopper 201a to be fully discharged into the moulding apparatus 3.

Thus, while the step of drying the polymer material is taking place in one of the two second hoppers 201a, 201b, which therefore uses the depressurization line 202, the other of the two second hoppers 201 is in the course of supplying the moulding apparatus 3, and uses the heating circuit 210 if necessary.

In a further embodiment of the present invention, not shown in the attached figures, the apparatus 200 does not have a pair of second hoppers 201a, 201b operated alternately for vacuum drying the polymer material and supplying the dried polymer material to the moulding apparatus 3, but comprises a single second hopper 201, designed to dry the polymer material, and a third hopper, having a heating circuit and located downstream of the second hopper, the dried polymer material waiting to be supplied to the moulding apparatus 3 being discharged into this third hopper.

Thus the apparatus has three hoppers in series, one for heating the polymer material, one for vacuum drying it, and one for maintaining its temperature while it waits to be supplied to the moulding apparatus.

The configuration of the apparatus 200 and its variant with three hoppers in series is advantageous by comparison with conventional systems with a single hopper in that it gives rise to smaller losses of time and production due to changes in production, and therefore offers greater flexibility of management.

Additionally, since the polymer material is dried at a pressure below atmospheric pressure, the temperature to which the polymer material has to be heated when contained in the hopper 4 is lower than that required in a process of drying at atmospheric pressure, and therefore much of the energy required for heating the heating fluid is supplied by the heat pump 20.

Furthermore, the residence time required for a given drying temperature is less than in a conventional apparatus.

In the preferred examples described above, the transformation of the bulk polymer material takes place by moulding, but it should be noted that all the characteristics of the present invention can refer in an entirely similar way to a process and an apparatus in which the transformation of the polymer material takes place by means of an extrusion method, in which the moulding apparatus is replaced by an extrusion apparatus and the mould is replaced by an extrusion head.

5 The present invention therefore enables considerable savings to be made in terms of energy, while offering numerous other advantages, including the fact of being usable in many different transforming processes by extrusion or moulding of polymer materials, regardless of the shape and size of the elements forming the initial polymer material and the specific i n characteristics of this material .