Method and system of controlling a plant for dehumidifying and/or drying

Abstract

A method is disclosed of controlling a plant for dehumidifying and/or drying plastic material in granular and/or micro-granular and/or powder and/or flake or similar form, this plant including a process fluid generator and at least one dehumidifying/drying hopper for feeding a respective user machine associated with the plant and including a melting device for melting the plastic material and a moulding device for moulding, in particular by injection moulding and/or blow moulding and/or compression moulding, the plastic material. The method of controlling includes the steps of detecting in the user machine a pressure (P.sub.pwp) of the plastic material in melted state and modifying at least one process parameter (D.sub.p) of the plant on the basis of the detected value of this pressure (P.sub.pwp).

Claims

1. A method of controlling a plant for dehumidifying and/or drying plastic material in granular and/or micro-granular and/or powder and/or flake form, said plant comprising a process fluid generator and at least one dehumidifying/drying hopper intended for feeding a respective user machine associated with said plant, the user machine for treating and transforming a plastic material, the user machine including a melting device for melting said plastic material and a moulding device for moulding, by injection moulding and/or blow moulding and/or compression moulding said plastic material the system includes a control and management unit configured for: receiving a signal, detected by a measurer in said user machine, corresponding to a value of an injection pressure (P.sub.pwp) of said plastic material in melted state; verifying whether the detected value of said injection pressure (P.sub.pwp) is greater than a preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp); and sending command signals to said process fluid generator for adjusting, if said detected value of said injection pressure (P.sub.pwp) is less than or the same as said preset limit value (P.sub.lim), a process parameter of said plant; the method of controlling includes the steps of: detecting in said user machine the pressure (P.sub.pwp) of said plastic material in melted state; and adjusting at least one process parameter (D.sub.p) of said plant on the basis of the detected value of said pressure (P.sub.pwp) using the control and management unit.

2. The method according to claim 1, wherein said pressure is the injection pressure (P.sub.pwp) of said material in melted state in said moulding device.

3. The method according to claim 1, wherein said detecting includes detecting an injection pressure (P.sub.pwp) of said material in melted state in said moulding device and is performed in a feeding zone of said melting device through which said melting device feeds said plastic material in melted state to said moulding device; and/or wherein said melting device includes an extruding/plastifying cylinder in flow communication with said moulding device, an extruding/plastifying screw received inside said extruding/plastifying cylinder to transfer said plastic material in melted state from said extruding/plastifying cylinder to said moulding device, a hydraulic piston for axially driving said extruding/plastifying screw inside said extruding/plastifying cylinder, and a hydraulic circuit for driving said piston by a driving fluid, wherein said detecting includes detecting a hydraulic pressure exerted on said hydraulic piston and is performed in a feeding zone of said hydraulic circuit through which said hydraulic circuit feeds said driving fluid to said piston; and/or wherein said melting device includes an extruding/plastifying cylinder in flow communication with said moulding device, an extruding/plastifying screw received inside said extruding/plastifying cylinder to transfer said plastic material in melted state from said extruding/plastifying cylinder to said moulding device, a piston for driving axially said extruding screw inside said extruding/plastifying cylinder, and electric driving means for driving said piston, wherein said detecting includes detecting a force exerted by said piston on said extruding/plastifying screw; and/or wherein said melting device includes an injecting cylinder in flow communication with said moulding device, an injecting piston received inside said injecting cylinder to transfer said plastic material in melted state from said injecting cylinder to said moulding device, a hydraulic circuit for driving said injecting piston by a driving fluid, an extruding/plastifying cylinder in flow communication with said injecting cylinder, and an extruding/plastifying screw received inside said extruding/plastifying cylinder to transfer said plastic material in melted state from said extruding/plastifying cylinder to said injecting cylinder, wherein said detecting includes detecting a hydraulic pressure exerted on said injecting piston and is performed in a feeding zone of said hydraulic circuit through which said hydraulic circuit feeds said driving fluid to said injecting piston.

4. The method according to claim 1, wherein said process parameter includes at least one between a value of the dew point (D.sub.P) of said process fluid and a value of the flow rate of said process fluid into said hopper.

5. The method according to claim 1, wherein said detecting and said adjusting are conducted continuously.

6. The method according to claim 1, including, before said detecting, verifying whether said plastic material consists of virgin resin, or of a mixture of virgin resin and recycled material.

7. The method according to claim 2, including, after said detecting, verifying whether the detected value of said injection pressure (P.sub.pwp) is greater than a preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp).

8. The method according to claim 7, wherein if the detected value of said injection pressure (P.sub.pwp) is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp), there is provided repeating in succession said detecting and said verifying in a subsequent instant of time; or wherein if the detected value of said injection pressure (P.sub.pwp) is less than or the same as said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp) said adjusting includes: letting a first preset time (TMP) elapse; and decreasing a current value of the dew point (D.sub.P) of said process fluid down to a minimum value (D.sub.pmin) of the dew point of said process fluid that is obtainable by said fluid generator.

9. The method according to claim 8, wherein after said decreasing said value of the dew point (D.sub.P) of said process fluid, said adjusting includes: letting a second preset time (T1) elapse; and verifying whether the detected value of said injection pressure (P.sub.pwp), optionally increased by a safety factor (DPS), is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp).

10. The method according to claim 9, wherein if the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp) said adjusting includes letting a third preset time (T.sub.stab1) elapse and increasing a current value of said dew point (D.sub.p) of said process fluid; or wherein if the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is less than or the same as said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp), said adjusting includes increasing a current value of the flow rate of said process fluid into said hopper.

11. The method according to claim 10, wherein after said increasing said current value of the flow rate of said fluid into said hopper, there is provided: letting a fourth preset time (T2) elapse; and still verifying whether the detected value of said injection pressure (P.sub.pwp) increased by said safety factor (DPS) is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp).

12. The method according to claim 11, wherein if the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is less than or the same as, said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp) there is provided emitting a warning signal and/or blocking said user machine; or wherein if the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp), said adjusting includes letting a fifth preset time(T.sub.stab2) elapse and increasing a current value of said dew point (D.sub.p) of said process fluid.

13. The method according to claim 12, wherein after said increasing a current value of said dew point (D.sub.p) of said process fluid there is provided letting a sixth preset time (T3, T.sub.stab3) elapse and still verifying whether the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp).

14. The method according to claim 13, wherein if the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is less than or the same as, said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp) there is provided emitting a warning signal and/or blocking said user machine; or wherein if the detected value of said injection pressure (P.sub.pwp), optionally increased by said safety factor (DPS), is greater than said preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp), said adjusting includes decreasing a current value of said flow rate of process fluid into said hopper.

15. The method according to claim 2, wherein said detecting includes detecting an injection pressure (P.sub.pwp) of said material in melted state in said moulding device and is performed in a feeding zone of said melting device through which said melting device feeds said plastic material in melted state to said moulding device; and/or wherein said melting device includes an extruding/plastifying cylinder in flow communication with said moulding device, an extruding/plastifying screw received inside said extruding/plastifying cylinder to transfer said plastic material in melted state from said extruding/plastifying cylinder to said moulding device, a hydraulic piston for axially driving said extruding/plastifying screw inside said extruding/plastifying cylinder, and a hydraulic circuit for driving said piston by a driving fluid, wherein said detecting includes detecting a hydraulic pressure exerted on said hydraulic piston and is performed in a feeding zone of said hydraulic circuit through which said hydraulic circuit feeds said driving fluid to said piston; and/or wherein said melting device includes an extruding/plastifying cylinder in flow communication with said moulding device, an extruding/plastifying screw received inside said extruding/plastifying cylinder to transfer said plastic material in melted state from said extruding/plastifying cylinder to said moulding device, a piston for driving axially said extruding screw inside said extruding/plastifying cylinder, and electric driving means for driving said piston, wherein said detecting includes detecting a force exerted by said piston on said extruding/plastifying screw; and/or wherein said melting device includes an injecting cylinder in flow communication with said moulding device, an injecting piston received inside said injecting cylinder to transfer said plastic material in melted state from said injecting cylinder to said moulding device, a hydraulic circuit for driving said injecting piston by a driving fluid, an extruding/plastifying cylinder in flow communication with said injecting cylinder, and an extruding/plastifying screw received inside said extruding/plastifying cylinder to transfer said plastic material in melted state from said extruding/plastifying cylinder to said injecting cylinder, wherein said detecting includes detecting a hydraulic pressure exerted on said injecting piston and is performed in a feeding zone of said hydraulic circuit through which said hydraulic circuit feeds said driving fluid to said injecting piston.

16. The method according to claim 2, wherein said process parameter includes at least one between a value of the dew point (D.sub.P) of said process fluid and a value of the flow rate of said process fluid into said hopper.

17. The method according to claim 2, wherein said detecting and said adjusting are conducted continuously.

18. The method according to claim 2, including, before said detecting, verifying whether said plastic material consists of virgin resin, or of a mixture of virgin resin and recycled material.

19. A control system of a plant for dehumidifying and/or drying plastic material in granular and/or micro-granular and/or powder and/or flake or similar form, said plant including a process fluid generator and at least one dehumidifying/drying hopper intended for feeding a respective user machine associated with said plant, the user machine for treating and transforming a plastic material, the user machine for extruding and subsequently for injection moulding and/or blow moulding and/or compression moulding said plastic material; the system includes a control and management unit configured for: receiving a signal, detected by a measurer in said user machine, corresponding to a value of an injection pressure (P.sub.pwp) of said plastic material in melted state; verifying whether the detected value of said injection pressure (P.sub.pwp) is greater than a preset limit value (P.sub.lim) of said injection pressure (P.sub.pwp); and sending command signals to said process fluid generator for adjusting, if said detected value of said injection pressure (P.sub.pwp) is less than or the same as said preset limit value (P.sub.lim), a process parameter of said plant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention can be better understood and implemented with reference to the attached drawings that illustrate some embodiments thereof by way of non-limiting example, in which:

(2) FIG. 1 is a partially sectioned schematic view of a system of controlling a plant for dehumidifying and/or drying plastic materials according to a first embodiment of the invention;

(3) FIG. 2 is a partially sectioned schematic view, of a system of controlling a plant for dehumidifying and/or drying of plastic materials according to a second embodiment of the invention;

(4) FIG. 3 is a partially sectioned schematic view, of a system of controlling a plant for dehumidifying and/or drying plastic materials according to a third embodiment of the invention;

(5) FIG. 4 is a partially sectioned schematic view of a system of controlling a plant for dehumidifying and/or drying plastic materials according to a fourth embodiment of the invention;

(6) FIG. 5 is a partially sectioned schematic view, of a system of controlling a plant for dehumidifying and/or drying plastic materials according to a fifth embodiment of the invention; and

(7) FIG. 6 is a block diagram of a method of controlling a plant for dehumidifying and/or drying plastic materials according to the invention.

DETAILED DESCRIPTION

(8) The present invention is advantageously applied to a method and to a system 1 of controlling a plant 2 for dehumidifying and/or drying plastic material in granular and/or micro-granular and/or powder and/or flake or similar form, for example polyethylene terephthalate (PET), intended for supplying at least one user machine 3 associated with such a plant, in particular a machine for treating and transforming a plastic material.

(9) The following disclosure will refer explicitly to this advantageous application without thereby losing generality.

(10) With reference to FIG. 1 there is illustrated a first embodiment of the control system 1 according to the invention.

(11) The dehumidifying and/or drying plant 2, controlled by the control system 1, comprises a process fluid generator 4, in particular air, the main components of which are bounded by a dotted line, and at least one dehumidifying/drying hopper 5.

(12) The generator 4, also known as a dryer, dehumidifies, heats, and delivers, by a first conduit 6, a defined flow rate of process fluid into the hopper 5, and, by a second conduit 7, receives the used process fluid into the hopper 5 to dehumidify and/or dry the plastic material.

(13) In particular, the process fluid generator 4 comprises a pair of towers 8, 9 for regenerating the process fluid that are connected together by conduits 10, between the regenerating towers 8, 9 valves 11 being provided that are intended to activate selectively one of the two regenerating towers 8, 9.

(14) The process fluid generator 4 further comprises a filter/cooler 12 for filtering/cooling the process fluid, and a pump 13, that works in a closed circuit, to pump the process fluid into the hopper 5.

(15) Also, the generator 4 comprises a heating chamber 14 to heat the process fluid to be sent into the hopper 5. The heating chamber 14 can, for example, be switched on or off by a PID (proportional-integral-derivative) control.

(16) The system 1 comprises, along the first conduit 6, a measurer 15 of the value of the dew point D.sub.p, of the process fluid, and a measurer 16 of the flow rate of the process fluid.

(17) The aforesaid hopper 5 is fed, with plastic material, by a receiver 17 of plastic material, the latter being supplied to the receiver 17 by a feeding conduit 18.

(18) The hopper 5 is intended for feeding the plastic material to the user machine 3, for example a machine for treating and transforming plastic material, such as a machine for extruding, and subsequently moulding by injection moulding and/or blow moulding and/or compression moulding, the plastic material.

(19) The user machine 3, for example an injection moulding machine, comprises a melting device 19, or extruder, for melting the plastic material, which includes, for example, an electrically heatable extruding/plastifying cylinder 20 and an extruding/plastifying endless screw 21, for example with a variable profile and section, which is rotatable in the cylinder 20 by a motor M, and a moulding device 22, in flow communication with the cylinder 20, for moulding, for example by injection moulding and/or blow moulding and/or compression moulding, the plastic material in melted state, in which the extruding/plastifying screw 21 transfers the plastic material in melted state from the extruding cylinder 20 to the moulding device 22.

(20) Also, the user machine 3 comprises a control and management unit 23 that controls the user machine 3.

(21) The system 1 comprises, between the melting device 19 and the moulding device 22, a pressure measurer 24 for measuring directly an injection pressure P.sub.pwp of the plastic material in melted state entering the moulding device 22.

(22) In the present context, injection pressure P.sub.pwp is the pressure at which the plastic material in melted state exits the melting device 19.

(23) The aforesaid pressure measurer 24 is positioned substantially at a feeding zone 25 of the melting device 19 through which the melting device 19 feeds the plastic material in melted state to the moulding device 22.

(24) Also, the system 1 comprises a control and management unit 26 connected, by respective communication lines 27 (illustrated by a dashed and dotted line), to the measurers 15, 16, 24 and to the control and management unit 23 of the user machine 3.

(25) In use, as will be explained better in the following description of the operating method, the control and management unit 26 receives a signal, detected by the pressure measurer 24, corresponding to a detected injection pressure P.sub.pwp value of the plastic material in melted state, verifies whether the detected injection pressure P.sub.pwp value is greater than a preset limit value P.sub.lim of the injection pressure P.sub.pwp and sends suitable command signals to the process fluid generator 4 to modify, if the detected the injection pressure P.sub.pwp value is less than the or the same as the preset limit value P.sub.lim, at least one process parameter of the plant 2.

(26) The method according to the invention is disclosed below with reference to the block diagram illustrated in FIG. 6.

(27) Initially, the method verifies whether the plastic material present in the hopper consists of virgin resin, or of a mixture of virgin resin and recycled material.

(28) Subsequently, for each instant of time t, the measurer 24 detects, in the user machine 3, an injection pressure P.sub.pwp value of the melted plastic material. The injection pressure P.sub.pwp of the melted plastic material, measured continuously before the material enters the moulding device 22, as disclosed better below, enables maintenance operations to be anticipated on the dehumidifying and/or drying plant 2 and, at the same time, possible downtime of the user machine 3 and product rejects from the latter to be reduced.

(29) Also subsequently, the detected injection pressure P.sub.pwp value is compared, by the control and management unit 26, with a preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the detected injection pressure P.sub.pwp value is greater than the limit value P.sub.lim.

(30) The limit value P.sub.lim is a settable value that varies according to the type of user machine 3.

(31) If the detected injection pressure P.sub.pwp value is greater than the preset limit value P.sub.lim, this means that the dehumidification and/or drying process is under control, i.e. that the plant 2 is supplying to the user machine 3 a suitably dehumidified and/or dried plastic material that is such that the finished product can meet the required quality standards. Further, the fact that the detected injection pressure P.sub.pwp value is greater than the preset limit value P.sub.lim enables it to be affirmed that the dehumidifying and/or drying plant 2 does not require, in the immediate future, extraordinary maintenance interventions. After which, the method again detects the injection pressure P.sub.pwp value at a subsequent instant of time t+t and again compares the injection pressure P.sub.pwp value with the preset limit value P.sub.lim.

(32) Vice versa, if the detected injection pressure P.sub.pwp value is less than or the same as the preset limit value P.sub.lim of the injection pressure P.sub.pwp, i.e. the injection pressure P.sub.pwp is decreasing, this may mean that the dehumidifying and/or drying process is losing effectiveness, or that one or more of the components of the dehumidifying and/or drying plant 2 needs to undergo maintenance.

(33) At this point, the method of controlling according to the invention allows a preset first time TMP, for example 4 hours, to elapse, during which amount of time TMP the detected injection pressure P.sub.pwp value is compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp. This initial time TMP is allowed to elapse to avoid interventions if the decrease in injection pressure P.sub.pwp is due to an instantaneous problem, due for example to a loss of power of the user machine 3, or to the fact that the cylinder 20 has not filled correctly, etc.

(34) After the first time TMP has elapsed, if the detected injection pressure P.sub.pwp value is still less than or the same as the preset limit value P.sub.lim of the injection pressure P.sub.pwp, i.e. the injection pressure P.sub.pwp still tends to decrease, the method modifies at least one process parameter of the dehumidifying and/or drying plant 2.

(35) In particular, the method increases, by the control and management unit 26, a current value of the dew point D.sub.P of the process fluid, in particular increases the current value of the dew point D.sub.P of the process fluid to a maximum value D.sub.pmax of the dew point of the process fluid obtainable from the process fluid generator 4.

(36) In other words, in this step the method disables, by the control and management unit 26, a stabilising function with which the control system 1 can be provided, this stabilising function having the task of maintaining stable, at a preset value, the value of the dew point D.sub.P of the process fluid supplied by the process fluid generator 4.

(37) In particular, in order to rapidly increase the value of the dew point D.sub.P of the process fluid, the control and management unit 26 forces, by the valves 11, the change between the tower 8, 9 that is currently in use and the tower 8, 9 that is regenerated and waiting to be used, the latter being able to make the process fluid reach the maximum value of the dew point D.sub.pmax.

(38) This enables the plastic material present in the hopper 5 to be dehumidified, the viscosity of the plastic material to be increased, and thus the injection pressure P.sub.pwp to be increased.

(39) After this, the method of controlling according to the invention allows a preset second time T1 to elapse, for example 3 hours, during which the detected injection pressure P.sub.pwp value is compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the injection pressure P.sub.pwp is tending to decrease, has stabilised or is tending to increase.

(40) After the second time T1 has elapsed, the method verifies, by the control and management unit 26, whether the detected injection pressure P.sub.pwp value, optionally increased by a DPS safety factor (so as to take account of possible swings of the injection pressure P.sub.pwp), is greater than the preset limit value P.sub.lim of the injection pressure P.sub.pwp.

(41) If the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is greater than the preset limit value P.sub.lim of the injection pressure P.sub.pwp, the method of controlling, by the control and management unit 26, allows a preset third time T.sub.stab1 to elapse, during which the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the injection pressure P.sub.pwp has stabilised.

(42) After the third time T.sub.stab1 has elapsed, the method decreases a current value of the dew point D.sub.p of the process fluid, i.e. restores the stabilising function of the control system 1. This means that the action of increasing the value of the dew point D.sub.p has had the effect of returning the injection pressure P.sub.pwp to a value above the limit value P.sub.lim.

(43) On the other hand, if the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is less than or the same as the preset limit value P.sub.lim of the injection pressure P.sub.pwp, the control method, by the control and management unit 26, drives the generator 4 so as to increase a current value of the flow rate of process fluid into the hopper 5, in particular to increase a ratio between the flow rate of the process fluid and the hourly production of the hopper 5, this ratio being defined, in the field, as the K factor, up to a preset value K.sub.AIPC that is a function of the plastic material and of the user machine 3.

(44) In other words, in this step the method disables, by the control and management unit 26, an optimisation function with which the control system 1 can be provided, this optimisation function optimising the energy consumption of the plant 2.

(45) In particular, in order to rapidly increase the value of the flow rate of process fluid into the hopper 5, i.e. the K factor, the control and management unit 26 increases the rotation speed of the pump 13 until the preset value K.sub.AIPC is reached.

(46) This surplus of energy enables the plastic material present in the hopper 5 to be dehumidified further, the viscosity of the plastic material to be increased, and thus the injection pressure P.sub.pwp to be increased.

(47) After this, the method of controlling according to the invention allows a preset fourth time T2, for example of 2-3 hours, to elapse during which the detected injection pressure P.sub.pwp value is compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the injection pressure P.sub.pwp is tending to decrease, has stabilised, or is tending to increase.

(48) After the fourth time T2 has elapsed, the method verifies, by the control and management unit 26, whether the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is greater than the preset limit value P.sub.lim of the injection pressure P.sub.pwp.

(49) If the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is less than or the same as the preset limit value P.sub.lim of the injection pressure P.sub.pwp, the method of controlling emits a warning signal and/or stops the user machine 3.

(50) Vice versa, if the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is greater than the preset limit value P.sub.lim of the injection pressure P.sub.pwp, the control method, by the control and management unit 26, allows a preset fifth time T.sub.stab2 to elapse, during which the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the injection pressure P.sub.pwp has stabilised.

(51) After the fifth time T.sub.stab2, has elapsed, the method decreases a current value of the dew point D.sub.p of the process fluid, i.e. restores the stabilising function of the process control system 1. This means that the actions of increasing the value of the dew point D.sub.p and the flow rate of the process fluid, i.e. of the K factor, have had the effect of returning the injection pressure P.sub.pwp to a value above the limit value P.sub.lim.

(52) After this, the method of controlling according to the invention allows a preset sixth time T3 to elapse, for example of 2-3 hours, during which the detected injection pressure P.sub.pwp value is compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the injection pressure P.sub.pwp is tending to decrease, has stabilised, or is tending to increase.

(53) After the sixth time T3 has elapsed, the method allows a preset further sixth time T.sub.stab3, for example of 2-3 hours, to elapse, during which the detected injection pressure P.sub.pwp value is still compared continuously with the preset limit value P.sub.lim of the injection pressure P.sub.pwp to verify whether the injection pressure P.sub.pwp is tending to decrease, has stabilised, or is tending to increase.

(54) These time windows enable the tendency to be verified over time of the injection pressure P.sub.pwp to decrease, stabilise or increase.

(55) After the further sixth time T.sub.stab3 has elapsed, the method verifies, by the control and management unit 26, whether the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is greater than the preset limit value P.sub.lim of the injection pressure P.sub.pwp.

(56) If the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is less than or the same as the preset limit value P.sub.lim of the injection pressure P.sub.pwp, the method of controlling emits a warning signal and/or stops the user machine 3.

(57) Vice versa, if the detected injection pressure P.sub.pwp value, optionally increased by the DPS safety factor, is greater than the preset limit value P.sub.lim of the injection pressure P.sub.pwp, the control method, by the control and management unit 26, decreases the air flow rate of the process fluid, i.e. decreases the K factor with respect to the preset value K.sub.AIPC, i.e. restores the optimisation function of the control system 1. This means that the actions of increasing the value of the dew point D.sub.p and the flow rate of the process fluid, i.e. of the K factor have the effect of returning the injection pressure P.sub.pwp to a value above the limit value P.sub.lim.

(58) With reference to FIG. 2, a second embodiment is shown of the control system 1. This second embodiment differs from the embodiment illustrated in FIG. 1 by the fact that the user machine 3 is provided with a own measurer 24 of the injection pressure. In this embodiment, the pressure measurer 24, shown by a dashed line, is thus optional.

(59) With reference to FIG. 3, a third embodiment is shown of the control system 1. In this third embodiment the melting device 19 comprises, in addition to the extruding/plastifying cylinder 20 in flow communication with the moulding device 22 and to the extruding/plastifying screw 21 received in the extruding/plastifying cylinder 20 to transfer the plastic material in melted state from the extruding/plastifying cylinder 20 to the moulding device 22, a hydraulic piston 28 to axially drive the extruding/plastifying screw 21 in the extruding/plastifying cylinder 20, and a hydraulic circuit 29 for driving the piston 28 by a driving fluid, for example hydraulic oil. It should be noted that in the third embodiment the extruding screw 21 is rotated around its axis by a motor M, and is driven axially by the piston 28. Also, in the third embodiment, the user machine 3 comprises a measurer 30 of pressure positioned substantially at a feeding zone 31 of the hydraulic circuit 29 through which the hydraulic circuit 29 feeds the driving fluid to the piston 28. The measurer 30 measures the hydraulic pressure exerted on the piston 28 and sends a corresponding signal to the control and management unit 26. The latter, by suitable conversion factors, converts the hydraulic pressure detected by the measurer 30 into an injection pressure P.sub.pwp value of the plastic material in melted state entering the moulding device 22. In other words, in the third embodiment, the injection pressure P.sub.pwp of the plastic material in melted state is detected indirectly by converting, by the control and management unit 26, the hydraulic pressure detected by the measurer 30. Lastly, in the third embodiment, the pressure measurer 24, shown by a dashed line, is optional/alternative to the pressure measurer 30.

(60) With reference to FIG. 4, a fourth embodiment is shown of the control system 1. In this fourth embodiment the melting device 19 comprises, in addition to the extruding/plastifying cylinder 20 in flow communication with the moulding device 22 and to the extruding/plastifying screw 21 received in the extruding/plastifying cylinder 20 to transfer the plastic material in melted state from the extruding/plastifying cylinder 20 to the moulding device 22, a piston 32 for driving axially the extruding/plastifying screw 21 in the extruding/plastifying cylinder 20, and electric driving means, not shown, for driving the piston 32. It should be noted that in the fourth embodiment the extruding/plastifying screw 21 is rotated around its axis by a motor M, and is driven axially by the piston 32. Also, in the fourth embodiment, the user machine 3 comprises a measurer 34, for example a load cell, positioned in such a manner as to measure a force, measurable in N (Newton), exerted by the piston 32 on the screw 21. In particular, the measurer 34 measures the thrust exerted by the piston 32 on the screw 21 and sends a corresponding signal to the control and management unit 26. The latter, by suitable conversion factors, converts the thrust detected by the measurer 34 into an injection pressure P.sub.pwp value of the plastic material in melted state entering the moulding device 22. In other words, in the fourth embodiment, the injection pressure P.sub.pwp of the plastic material in melted state is detected indirectly by converting, by the control and management unit 26, the thrust detected by the measurer 34. Lastly, in the fourth embodiment, the pressure measurer 24, shown by a dashed line, is optional/alternative to the measurer 34.

(61) With reference to FIG. 5, a fifth embodiment of the control system 1 is shown. In this fifth embodiment, the melting device 19 comprises an extruding/plastifying cylinder 38, an injection cylinder 35 in flow communication with the moulding device 22, an extruding piston 36 received and axially movable in the injection cylinder 35 to transfer the plastic material in melted state from the injection cylinder 35 to the moulding device 22, a hydraulic circuit 37 for driving the extruding piston 36 by a driving fluid, for example hydraulic oil, and an extruding/plastifying screw 21 received in the extruding/plastifying cylinder 38 to transfer the plastic material in melted state from the extruding/plastifying cylinder 38 to the injection cylinder 35, in which the extruding/plastifying cylinder 38 is operationally positioned above, and in flow communication with, the injection cylinder 35. Also, in the fifth embodiment, the user machine 3 comprises a measurer 39 positioned substantially at a hydraulic circuit 37 feeding zone 40 through which the hydraulic circuit 37 feeds the driving fluid to the extruding piston 36. The measurer 39 measures the hydraulic pressure exerted on the extruding piston 36 and sends a corresponding signal to the control and management unit 26. The latter, by suitable conversion factors, converts the hydraulic pressure detected by the measurer 39 into an injection pressure P.sub.pwp value of the plastic material in melted state entering the moulding device 22. In other words, in the fifth embodiment, the injection pressure P.sub.pwp of the plastic material in melted state is detected indirectly by converting, by the control and management unit 26, the hydraulic pressure detected by the measurer 39. Lastly, in the fifth embodiment, the pressure measurer 24, shown by a dashed line, is optional/alternative to the pressure measurer 39.

(62) It should be noted how the method and the system 1 according to the invention enable downtime of the user machines 3 associated with the dehumidifying and/or drying plant 2 to be reduced or even eliminated.

(63) In fact, in the method according to the invention it is possible to counteract early, compared with known methods, injection pressure P.sub.pwp, variation phenomena by acting on the dehumidifying and/or drying plant 2 and not, as is taught in the prior art, on the user machine/s 3.

(64) Also, it should be noted how the actions of disabling the stabilisation and/or optimisation functions enable the correct injection pressure P.sub.pwp to be restored rapidly, greatly reducing, compared with the prior art, machine downtime and the production of rejects.

(65) Also, it should be noted how continuous monitoring, i.e. monitoring in real time, of the injection pressure P.sub.pwp enables the dehumidification/drying process trend to be interpreted and possible maintenance on the dehumidifying and/or drying plant 2 to be anticipated compared with scheduled times. In fact, a tendency of the injection pressure P.sub.pwp to fall below the limit pressure P.sub.lim, may mean a deterioration in the operating efficacy of the dehumidifying and/or drying plant 2 due to a deterioration and/or malfunction and/or breakage of one or more of the components thereof.

(66) Also, it should be noted how the (if necessary) continuous modification of the process parameters of the dehumidifying and/or drying plant 2, carried out upstream of the user machine 3, and thus upstream of the finished product, i.e. on the melted plastic material, enables timely intervening to avoid variations in the quality of the finished product, with a consequent great reduction in the number of products to be rejected and thus sample checks of the finished product to be avoided, with consequent reduction of costs linked to personnel or devices intended for such checks.

(67) In one version, the method according to the invention continuously monitors whether the injection pressure P.sub.pwp value is comprised between an upper limit value P.sub.limsup and a lower limit value P.sub.liminf, i.e. between a band of values.