Accumulation extruder head with output element of a mechatronic assembly integral with the expulsion piston

Abstract

An accumulation extruder head (1) for a container blow-moulding machine comprises an internal accumulation chamber (13a) with a central vertical axis (z), which is filled with a thermoplastic resin load which is expelled in the form of a tubular parison through a bottom outlet mouth (11) by means of a piston (18), coaxially inserted inside the accumulation chamber (13a) and movable in a reciprocating manner inside the accumulation chamber along the central vertical axis (z) in an expulsion direction, from a raised position into a lowered expulsion position, and, in an upward direction, from the expulsion position into a raised position. The head also comprises a mechatronic assembly (30) comprising an output element (31), displaceable in a linear manner along the central vertical axis (z) between a raised position and a lowered position and operated by a plurality of electric actuators controlled by a control unit. The output element (31) is rigidly connected to the piston for translation therewith and, during a step for filling the accumulation chamber, the output element (31) is raised so as to move the piston (18) in the upward direction so that the volume of the accumulation chamber increases in proportion to the volume of thermoplastic resin entering into the accumulation chamber.

Description

[0001] ACCUMULATION EXTRUDER HEAD WITH OUTPUT ELEMENT OF A MECHATRONIC ASSEMBLY INTEGRAL WITH THE EXPULSION PISTON

[0002] DESCRIPTION

[0003] The present invention relates to an accumulation extruder head for producing a thermoplastic resin parison in a machine for blow-moulding plastic containers and to an associated accumulation extrusion method.

[0004] It is known in the technical sector relating to machines designed to produce blow-moulded plastic articles that said articles are produced by dispensing a load, called a parison, by means of an extrusion head with an outlet mouth. The parison is in the form of a tube of thermoplastic resin with a given length, diameter and thickness which, once dispensed through the outlet mouth of the extrusion head, remains suspended underneath the extrusion head while waiting for a blowing mould to close around it and “capture” it internally. Once the mould is closed with the parison inside it, the article is formed by blowing compressed air inside the mould.

[0005] The thickness profile of the parison may be varied by means of a male element, suitably configured and coaxially inserted inside the extrusion head and movable inside it so as to vary the opening of the parison outlet mouth during the expulsion of material. The assembly with expulsion mouth configured in this way is also referred to by the term “die”.

[0006] The most common machines for blow-moulding plastic articles use a system for continuous extrusion of the parisons, in which an extrusion head continuously dispenses the thermoplastic material through an outlet mouth so as to supply successive capture and blowing cycles in the mould.

[0007] It is also known that the thermoplastic resins are characterized by a certain viscosity, which is variable depending on the type of resin. The viscosity determines the capacity of the melted mass to remain suspended below the extrusion head pending preparation of the mould for capture of the load. Since the weight of the said load tends to lengthen the parison, stretching it and altering its geometric characteristics, when the stretching is excessive for the quality requirements of the finished product, an accumulation (or intermittent) extrusion technique is used so as to reduce significantly the suspension time of the parison. An accumulation extruder head, also known as an intermittent extrusion head, is an apparatus designed for use in machines able to produce blow-moulded plastic articles with a large size compared to that which can be obtained with the more common continuous extrusion system suitable for smaller size articles.

[0008] The accumulation head is provided with a vertical-axis accumulation chamber inside which a quantity of extruded plastic resin is accumulated, said resin being supplied by a supply system connected to the chamber and generally comprising an extruder screw which continuously feeds the extruded material to a distribution system which distributes it inside the chamber. Once accumulated, the quantity of resin is subsequently expelled in the form of a tubular parison through a bottom outlet mouth in a single short dispensing step, pushed by a piston movable inside the accumulation chamber between a raised position and a lowered expulsion position.

[0009] For the expulsion stroke, conventionally the piston is operated from a raised position into the lowered expulsion position by means of a high-pressure hydraulic cylinder, the movement of which is controlled by a flow-regulating valve.

[0010] The system involves the possibility of adjusting the speed and the expulsion stroke in order to determine the quantity of expelled resin and the process parameters.

[0011] US2018 / 104878, US201 1 / 052746 and US5851579 describe accumulation extruder heads for a machine for blow-moulding containers according to the characteristic features in the preamble of Claim 1 .

[0012] The most important aspects in the market of machinery for the production of blow-moulded plastic articles are energy savings and production efficiency. These aspects help reduce the production costs and emissions resulting from energy consumption.

[0013] In this context it is known that one of the problems of the known apparatus with an accumulation extruder head is that the quantity of energy used for hydraulic operation of the piston represents a very high percentage of the overall energy used for the movements of the machine, generally more than 50% of the total in a machine moved entirely by a hydraulic system.

[0014] A further important aspect in the production of blow-moulded plastic articles is the quality of the finished product and repeatability of the production cycle. It is known in fact that thermoplastic resins are compressible and have a viscoelastic behaviour which may be influenced by parameters such as the speed of the flows, the pressure and the temperature. In the blow-moulding process, an important aspect for maintaining the repeatability of the process is that of keeping as constant as possible the behaviour of the loads, i.e. the reactions thereof to mechanical stresses. In order to reduce such stresses it is important that a pressure is kept as low as possible in the accumulation chamber, limiting the variations in volume.

[0015] For this purpose, the hydraulic accumulation extruder heads are configured to allow the resin to behave as naturally as possible during the accumulation step, eliminating as far as possible elements which may alter the reactions thereof. In particular, the expulsion piston is left free to move upwards during the accumulation step, pushed by the resin which enters into the accumulation chamber. Furthermore, the bottom surface of the piston has a thrust area sufficient to overcome the friction due to the material flow.

[0016] In this sense the hydraulic system is advantageous because it allows the piston to assist naturally the accumulation of the resin inside the accumulation chamber. However, this is in any case uninfluential since the pressure of the plastic inside the accumulation chamber must overcome a series of frictional forces due, for example, to the oil inside the cylinder and / or the weight of the piston, which act on the expulsion piston resulting in a pressure - albeit limited - inside the accumulation chamber.

[0017] This pressure may have collateral effects: during reloading, the configuration of the die, even though completely closed, does not ensure a perfect seal, a reason for which the pressure must be prevented from increasing excessively resulting in the loss of material from the expulsion mouth.

[0018] In order to deal with this problem, it is known that the hydraulic cylinder which equips the traditional systems may be a double-acting cylinder, i.e. a cylinder which is able to provide a thrust both in the downward expulsion direction and in the upward direction for accumulation reloading.

[0019] During the upward movement, i.e. during reloading, a pneumatic thrust may be applied on the bottom side by means of a valve and a pressure reducer with compressed air in the chamber opposite to that supplied for the expulsion thrust, thus resulting in an upward thrust able to offset these loads. The air pressure may be modulated so as to have the values necessary for obtaining the minimum necessary thrust. This function is referred to as “accumulation decompression”. The application of the thrust introduces, however, into the system a further variable element which reduces the precision of the system and therefore the repeatability of the cycle, since it varies the natural expansion of the thermoplastic material.

[0020] During reloading, especially with very viscous plastics, the opposite effect may also be obtained, namely the pressure inside the accumulation chamber will not be sufficient to prevent the air becoming trapped in the load. The movements of the die when the load is inactive may result in air being drawn in from outside the head through the die. In such a case, the pressure inside the accumulation chamber must be increased. During the upward movement of the accumulation piston and therefore the hydraulic piston, the oil must be discharged from the cylinder chamber. By modulating the oil flow being discharged it is possible to create a “counterpressure” which allows the material to be compacted and therefore prevents the entry of air, restoring the nominal pressure required inside the accumulation chamber.

[0021] EP 3,287,255 describes a method for extruding molten resin in an accumulation space of an extrusion head, which comprises a passage for separating a top plate from a bottom plate and retracting the top plate upwards by means of a rotational movement of an electric motor and then loading the molten resin into the loading space with a loading pressure sufficient to push the piston upwards against the weight of the bottom plate and the piston inside the loading space, the bottom plate being arranged on the accumulator and coupled to the piston from above, the top plate being engaged with the electric motor and arranged on the bottom plate such as to be able to enter superficially into contact with the bottom plate; and a passage for extruding the molten resin from the extrusion mouth by moving the top plate downwards, the top plate and the bottom plate coming into superficial contact with each other by means of a rotational movement of the electric motor and moving therefore the piston downwards into the loading space. In this solution, before performing loading into the accumulation chamber, the top plate is retracted upwards completely to a top end-of-travel stop. This results in the need for long idle travel during the downward stroke, before coming into contact with the bottom plate, creating a delay during the expulsion step with major consequences in terms of uncertainty in the process management, quality of the end product and other mechanical safety functions. With the known electrical system it is also not possible to control the movement of the piston during the upward movement, nor perform the accumulation decompression and counterpressure functions such as to guarantee the optimum pressure inside the accumulation chamber during reloading.

[0022] A further problem of the known extrusion head is that it is complex to manufacture and maintain and, during the head calibration, installation and maintenance steps, when there is no resin inside the accumulation chamber, it is not possible to intervene on the piston without at least partially disassembling the extrusion head, so that these operations are particularly complex and costly. The technical problem which is posed therefore is that of providing an accumulation extruder head for thermoplastic resin which solves or at least partly overcomes one or more of the said problems of the known heads.

[0023] A particular aim of the present invention is to provide an accumulation head which is more efficient from the point of view of energy consumption and repeatability of the production cycle.

[0024] A further desirable aspect is that the accumulation head should allow a greater quality of the finished product to be obtained.

[0025] A further desirable aspect is that the accumulation head should be more versatile when managing the production of products which are different and / or are made using different thermoplastic resins.

[0026] In connection with this problem, it is also required that the head should be simple, compact and easy and inexpensive to manufacture.

[0027] These results are obtained with an accumulation extruder head according to Claim 1 .

[0028] The accumulation extruder head according to the present invention for use in a blow-moulding machine for blow-moulding containers therefore comprises: a head body having: an internal cavity, inside which there is defined an accumulation chamber with a central vertical axis z designed to be filled with a thermoplastic resin load; and a bottom outlet mouth for expulsion of the thermoplastic resin load in the form of a tubular parison, the accumulation chamber communicating at the bottom with the outlet mouth; a piston, coaxially inserted inside the accumulation chamber, the piston being movable in a reciprocating manner inside the accumulation chamber along the central vertical axis z in an expulsion direction, from a raised position to a lowered expulsion position, and in an upward direction, from the expulsion position to the raised position; a mechatronic assembly comprising : an output element, linearly displaceable along the central vertical axis z between a raised position and a lowered position; and a plurality of electric actuators, each able to rotationally drive a respective rotating element; wherein the output element is rigidly coupled to the piston for translation therewith.

[0029] Each rotating element is coupled to the output element so that rotation of the rotating element displaceably drives the output element in the vertical direction and the electric actuators are controlled and configured to drive in a coordinated manner said rotating elements so as to jointly displace said output element and therefore the piston both in the expulsion direction and in the upward direction.

[0030] A control unit of the mechatronic assembly is configured to control operation of said electric actuators in a synchronized manner so as to displace said output element and therefore the piston.

[0031] According to a preferred aspect of the invention, in the filling step for filling the accumulation chamber, the output element of the mechatronic assembly is displaceably driven with a controlled upward movement so as to move the piston in the upward direction in such a way that the volume of the accumulation chamber increases in proportion to the volume of thermoplastic resin entering into the accumulation chamber.

[0032] According to preferred aspects of the invention, the mechatronic assembly may in particular be configured so that, during the movement of the piston in the upward direction in the filling step for filling the accumulation chamber, the output element is displaced in the upward direction so as to maintain a pressure inside the accumulation chamber within a predefined pressure range.

[0033] Advantageously, a pressure detection sensor may be provided inside the accumulation chamber. The control unit of the mechatronic assembly may therefore adjust the displacement of the output element during the upward movement in the filling step depending on a detected pressure signal emitted by the detection sensor, so as to keep the pressure inside the accumulation chamber within the predefined range of pressure values and preferably in accordance with a predefined reference pressure value.

[0034] According to further preferred aspects of the invention, during the filling step for filling the accumulation chamber, a control unit of the mechatronic assembly adjusts the rotational operation of each electric actuator so as to maintain a predefined reference value of the torque output by the actuator.

[0035] The coupling between the output element of the mechatronic assembly and each rotating element is preferably a coupling of the irreversible type, in particular a male / female coupling.

[0036] The present invention relates furthermore to an accumulation extrusion method according to Claim 13.

[0037] Preferred embodiments are described in the dependent claims which are fully cited herein.

[0038] Further details may be obtained from the following description of a non-limiting example of embodiment of the subject of the present invention provided with reference to the attached drawings in which: iqure 1 : is a partially sectioned schematic view of a first example of embodiment of an accumulation extruder head according to the invention; iqure 2 is partially sectioned schematic view of a second example of embodiment of an accumulation extruder head according to the invention;

[0039] With reference to Figure 1 assuming solely for the sake of easier description and without a limiting meaning a set of three reference axes in a vertical direction Z-Z, parallel to a central vertical axis z of the accumulation extruder head and a direction of expulsion of a parison, transverse direction Y-Y perpendicular to the vertical direction and corresponding to the width of the head, and longitudinal direction X-X, corresponding to the depth of the head, a preferred example of embodiment of an accumulation extruder head 1 according to the present invention comprises a substantially cylindrical housing head body 10, extending in the vertical direction Z-Z between a bottom mouth 1 1 and a top end 12.

[0040] The head body 10 has a substantially cylindrical internal cavity 13 inside which there is defined an accumulation chamber 13a with a central vertical axis z suitable for being filled with a thermoplastic resin load. The communication chamber 13a communicates at the bottom with the bottom outlet mouth 1 1 for expulsion of the thermoplastic resin load in the form of a tubular parison designed to be blow-moulded in a blow-moulding unit (not shown) in order to obtain a plastic container.

[0041] The bottom end with the outlet mouth 1 1 of the head body 10 is conventional per se and not described in further detail.

[0042] In the preferred example shown, the head body 10 has in particular a cylindrical housing with a side wall 10a which defines the substantially cylindrical internal cavity 13 with a central vertical axis z.

[0043] The head body 10 furthermore comprises a mushroom-shaped head 16 coaxially inserted inside the cavity 13 and fixed to the top end of the side wall 10a of the cylindrical housing.

[0044] The mushroom-shaped head 16 has a central coaxial hole 16a and an inlet 16b with a longitudinal axis for supplying the extruded resin, connected using conventional technology to distribution ducts 15 and supply ducts 14, formed inside the mushroom-shaped head and designed to connect the inlet 16b for the resin to the accumulation chamber 13a defined inside the cavity 13 of the head body 10, so that the extruded thermoplastic resin supplied to the inlet 16b is distributed by the distribution ducts 15 and dispensed inside the accumulation chamber 13a by means of the supply ducts 14, so as to be accumulated there and then expelled in the form of a parison through the outlet mouth 1 1 .

[0045] The accumulation chamber 13a is in particular defined inside the volume of the cavity 13 between the side wall 10a of the cylindrical housing and the central vertical axis z, in a cylindrical volume radially comprised between the mushroom head 16 and the side wall 10a.

[0046] During use, the head body 10 will be fixed to a fixed frame of the container forming machine and, according to conventional technology, the inlet mouth 16b for the extruded resin will be connected to an extrusion unit for example comprising an extruder screw with a longitudinal-axis outlet duct connected to the inlet mouth 16b.

[0047] A top plate 4 is fixed to the head body 10 by means of uprights 4a, at a distance in the vertical direction 7-7 from the top end 12 of the head body 10.

[0048] According to conventional technology, the male element 20 is coaxially inserted inside the cavity 13 of the head body 10. The male element 20 is movable coaxially with respect to the housing body 10 so as to vary the opening of the bottom outlet mouth 1 1 for the resin so as to adjust a thickness profile of the parison which is expelled. The mouth 1 1 and the male element 20 therefore form a die assembly for dispensing the parison.

[0049] The male element 20 is moved by an actuating unit 21 with an electric actuator 22 mounted on the top plate 4 and a transmission chain 23, which are arranged coaxially with respect to the central vertical axis z. The transmission chain comprises in particular a rod 23 coaxially inserted inside the central hole 16a of the mushroom head 16, which connects an output of the electric actuator 22 to the male element 20.

[0050] A piston 18 is coaxially inserted inside the accumulation chamber 13a; in particular the piston 18 is housed inside a cylindrical space between the mushroom head 16 and the side wall 10a.

[0051] The piston 18 is movable in a reciprocating manner inside the accumulation chamber along the central vertical axis z:

[0052] - in an expulsion direction, from a raised position into a lowered expulsion position; and

[0053] - in an upward direction, from the expulsion position into a raised position.

[0054] According to an advantageous aspect of the invention, the accumulation extruder head 1 comprises a mechatronic assembly 30, mounted on the plate 4 fixed on top of the head body 10.

[0055] The mechatronic assembly 30 comprises an output element 31 , linearly displaceable along the central vertical axis z with respect to the fixed head body 10 between a raised position and a lowered position. According to the invention, the output element 31 is rigidly connected to the piston 18 for translation therewith.

[0056] The mechatronic assembly 30 further comprises a plurality of electric actuators 32,33, each able to rotationally drive a respective rotating element 32a, 33a; each rotating element rotates about a respective vertical axis which is parallel and axially offset with respect to the central vertical axis z and is coupled to the output element 31 by means of a coupling, for example a male / female coupling, so that the rotation of the rotating element 32a, 33a displaces the output element in the vertical direction. The electric actuators 32,33 are controlled and configured to operate in a coordinated manner said rotating elements 32a, 33a so as to jointly displace said output element 31 ;

[0057] In greater detail, the mechatronic assembly 30 of the preferred example shown comprises two electric linear actuators 32,33, each preferably comprising a vertical-axis electric gearmotor with servomotor, encoder and reduction gearing. Each actuator 32,33 rotationally drives a respective vertical-axis screw 32a, 33a which is axially offset with respect the vertical central axis z of the accumulation chamber and the outlet mouth. Therefore, the actuators 32,33 and the screws 32a, 33a are located laterally with respect to the axis z of the head body, preferably in diametrically opposite positions in the transverse direction Y-Y, this enabling both joint operation in a balanced manner of the output element 31 and coaxial housing of the actuating unit 21 for operation of the male element 20 which, in order to be able to operate correctly, must be coaxial with the accumulation chamber 13a and the outlet mouth 1 1 .

[0058] The output element of the preferred mechatronic assembly shown consists of a plate 31 extending in a longitudinal transverse plane, with a thickness in the vertical direction, and designed to move along the central vertical axis z in the expulsion direction, from a raised position into a lowered position, and in the upward direction, from a lowered position into a raised position.

[0059] The plate 31 has a central hole 31 b designed to allow insertion of the actuating unit 23 for actuating the male element 20 for adjusting the outlet mouth 11 .

[0060] The plate 31 is coupled via a male / female coupling to each of the rotating screws 32a, 33a of the linear actuators, so that the rotation of the screws displaces the plate 31 along the central axis z. In particular the plate has a pair of vertical-axis holes 31 a with a female thread arranged on opposite sides in the transverse direction Y-Y with respect to the central hole 31 b. A respective screw 32a, 33a of a respective electric actuator 32,33 is housed inside each of the holes 31 a.

[0061] Preferably, the screws 32a, 33a are irreversible and / or form with a respective gearmotor an irreversible mechanical transmission.

[0062] An electronic unit for controlling the movement of the mechatronic assembly 30 is able to control electronically the electric actuators and therefore the movement of the output element 31 and the piston 18 in the vertical direction Z- Z.

[0063] In greater detail, the electric gearmotors are synchronized electronically so as to rotationally drive in a coordinated manner the respective screws 32a, 33a and therefore displace the output plate 31 in a balanced manner. For example, the gearmotors 32,33 may be electronically controlled in a master-slave configuration. A master actuator 32 is actuated according to the desired movement configuration and one or more slave actuators 33 follow the movement of the master actuator. The electronic control system is therefore able to ensure simultaneous movement with a high degree of precision, for example to within one minute of an angular degree of the said motor which, referred to the reduction ratio of the reduction system, is able to maintain alignment of the plate to within one hundredth of a millimetre.

[0064] Therefore, the screws 32a, 33a rotate inside the female threads 31 a simultaneously causing the downward or upward movement of the plate 31 in accordance with the pushing and filling steps of the production cycle.

[0065] The synchronization system, alternatively, may for example be formed by means of any mechanical transmission (of the belt, chain, gear, etc., type)

[0066] The mechatronic assembly 30 may comprise more than two electric actuators, preferably arranged angularly equidistant along a ring around the vertical central axis z of the head 1 so as to maintain the mechanical equilibrium of the thrusts acting on the output element. This allows the thrust potential to be increased and / or the cost and dimensions of the mechatronic assembly to be optimized. In the case where there are more than two electric actuators, the output plate 31 will have a corresponding number of holes arranged angularly equidistant along a circumference.

[0067] A pusher assembly 40 for pushing the piston 18 is arranged between the output plate 31 and the piston 18 and is rigidly coupled to the piston 18 for translation therewith.

[0068] The pusher assembly 40 comprises preferably a top ring 41 which forms a movement input element designed to cooperate with the output plate 31 of the mechatronic assembly 30. The ring 41 is rigidly connected to the piston 18 for translation therewith, in the preferred example by means of a plurality of vertical columns 42 arranged in a cylindrical volume defined in the top part of the head body 10.

[0069] According to an advantageous aspect of the invention, the output element 31 of the mechatronic assembly 30 and the input element 41 of the pusher assembly 40 are rigidly coupled together so as to move integrally in the vertical direction Z-Z both so as to push the piston 18 inside the plastic accumulation chamber 13a in the expulsion direction during a pushing step for expulsion of the parison, and during the upward movement of the piston during filling of the accumulation chamber.

[0070] In the preferred example shown, the output plate 31 has, on its bottom side, an inset annular seat coaxial with the central vertical axis z and designed to house the input ring 41 of the pusher assembly.

[0071] Preferably, the input element 41 of the pusher assembly 40 is mechanically coupled to the output element 31 by coupling means, in particular a series of fixing elements, such as collar screws 31 c, which integrally fasten the ring 41 to the plate 31. This connection between plate 31 and ring 41 is particularly advantageous also during the head calibration, installation and maintenance steps, allowing the piston 18 to be raised by means of the plate 31 when the apparatus is not in operation.

[0072] Preferably, the mechatronic assembly is configured so that, during the accumulation step, the output plate 31 is operated upwards so as to move the piston 18 in the upward direction so that the volume of the accumulation chamber increases in proportion to the volume of the material entering into the accumulation chamber.

[0073] In greater detail, at the start of the accumulation step for filling the chamber, the piston is in the lowered position which produces a minimum volume of the accumulation chamber and the die 1 1 is generally closed, so that the material supplied does not pass out through its opening owing to the pressure inside the chamber.

[0074] During the accumulation step, the material is fed into the chamber and exerts a pressure inside the chamber 13a such that, by means of the front area of the piston 18, it performs the upwards pushing of the entire integral kinematic chain consisting of piston 18, pusher assembly and plate 31 . However, because of the high reduction ratio, the system does not have sufficient reversibility to allow the thrust exerted by the material on the piston to be able to perform the upward movement of the entire assembly.

[0075] The control unit of the mechatronic assembly 30 must therefore drive all the electrical actuators rotationally in the opposite direction so as to recall the plate 31 and therefore the piston 18 upwards, increasing the volume of the accumulation chamber so as to allow the plastic material to accumulate.

[0076] In a preferred embodiment, shown in Fig. 1 , a sensor 35 is arranged inserted inside the accumulation chamber, preferably close to the outlet mouth, and is able to detect the pressure inside the accumulation chamber 13a. The sensor 35 may be for example a detection sensor of the diaphragm or other type suitable for use in thermoplastic resin extrusion apparatus.

[0077] The control unit includes means for setting a predefined pressure value or range to be maintained inside the accumulation chamber. For example, the unit may include a user interface by means of which an operator may set a predefined pressure value which is stored in the control unit. In addition or alternatively, one or more predefined value or ranges may be stored in the control unit, for example divided up according to the type of material, product and / or other process parameters, so as to selectable at the start of the cycle.

[0078] The control unit therefore modulates the upward movement speed of the plate 31 , for example by modulating the speed of rotation of the electric actuators, so that the pressure inside the chamber is kept within the predefined range and in particular as close as possible to the predefined value stored.

[0079] If the pressure detected is higher than the predefined value or upper limit of the predefined range, the motors of the electric actuators accelerate the rotation and consequently increase the speed of upward movement of the piston 18, whereas if the pressure is lower, the motors decrease their speed.

[0080] The mechatronic assembly thus designed is therefore able to adapt automatically the upward movement of the piston to the speed of entry of the material into the accumulation chamber.

[0081] If the entry speed is high, the tendency is for there to be an increase in the pressure and therefore the actuators are controlled so as to increase the speed of upward movement of the piston 18, causing a more rapid increase in the volume of the accumulation chamber. Vice versa, if the entry speed is low, the pressure detected decreases and the actuators are controlled so as to slow down the speed of upward movement of the piston 18, causing a slower increase in the volume of the accumulation chamber. The configuration therefore determines a control mode such that the volume of the accumulation chamber increases in proportion to the volume of incoming material. The value of the reference pressure must be included within a minimum, which prevents the formation of voids inside the accumulation chamber, due to air being drawn in from outside (excessively fast upward movement of the piston), and a maximum (excessively slow upward movement) such that the material is damaged by the excessive pressure and / or leaks from the die.

[0082] It is within the competence of the person skilled in the art to determine a suitable reference value or range for the pressure inside the chamber, for example empirically, taking into account also the fact that the die is generally unable to ensure that the high pressure values are maintained inside the accumulation chamber.

[0083] Generally speaking, the electric actuators of the mechatronic assembly 30 (for example of the type with a brushless servomotor) may be controlled with a vectorial management system, i.e. so as to carry out a predefined scheme in terms of speed and distance, modulating the torque output by each actuator so as to follow perfectly said scheme. This method of controlling the actuators is generally used during the expulsion step.

[0084] In some embodiments of the present invention, during accumulation, the servomotors of the electric motors are instead controlled with management of the output torque, with variation of the speed of rotation depending on a predefined output torque value of the actuator being maintained or reached.

[0085] In a preferred embodiment, shown in Fig. 2, the mechatronic assembly is configured and controlled to modulate the speed of rotation of the screws and therefore the upward movement of the plate 31 and the piston 18 so as to maintain a predefined torque value output by each actuator.

[0086] This means that any thrusting force in the upward direction resulting from the piston 18 (due to an increased in pressure inside the chamber for more rapid entry of resin) is added to the torque output by each motor: if the thrust increases, the motor control system “senses” a lightening (namely a reduction in the resistance torque of the plate 31 ) and reacts by increasing the speed of rotation of the respective screw and therefore the upward movement of the plate 31 ; on the other hand, if the thrust produced by the piston / plate assembly decreases, the control system senses an increase in load due to the smaller thrust of the resin entering the accumulation chamber and reduces the speed of rotation of the servomotor so as to slow down the upward movement of the plate and therefore of the piston 18.

[0087] This mechanism allows the system to sense a variation in pressure inside the accumulation chamber due to a variation in the supply of the material entering the head: the greater the speed of entry of the resin into the accumulation chamber, the greater will be the pressure and each motor will increase proportionally its speed. The mechatronic assembly reacts in the opposite manner when the entry speed of the material into the head decreases. If the extruder is stopped, the pressure inside the accumulation chamber tends towards zero and the motor stops because the predefined reference torque alone produced by the motors is unable to raise the entire kinematic chain.

[0088] The reference value of the torque which the motors must supply to the screws and therefore to the plate 31 may be determined by the operator in an empirical manner, i.e. by observing the effects of the setting. If the reference torque set is too high, the system could be raised without pushing the material; on the other hand, if it is too low, there are leakages of material from the die despite the fact that it is closed.

[0089] In this embodiment also, the mechatronic assembly is able to adapt automatically the upward movement of the piston to the speed of entry of the material inside the accumulation chamber. The configuration therefore determines a control mode such that the volume of the accumulation chamber increases in proportion to the volume of incoming material. It is also not necessary to measure the pressure inside the accumulation chamber 13a.

[0090] According to a preferred example of a method for accumulation extrusion of a parison of thermoplastic resin which uses the head 1 of the present invention, a resin load is accumulated inside the chamber 13a, said resin being fed by an extruder screw connected to the inlet 16b of the head body 10, distributed by the distribution ducts 15 and supplied to the accumulation chamber 13a by means of the supply ducts 14. The accumulation of resin inside the chamber 13a pushes the piston 18, causing an increase in pressure inside the chamber 13a and a variation in the thrust of the piston 18 against the plate 31 in the upward direction towards the raised position; during this phase, the piston 18 is operated so as to move upwards by means of the electric actuators 32,33 which operate the screws and therefore the plate 31 in the upward direction, so as to increase gradually the volume of the accumulation chamber 13a in proportion to the speed of entry of the material therein.

[0091] At the end of the accumulation step, the chamber is filled with the predefined load of plastic resin and the expulsion step starts, so that the control unit operates the electric actuators 32, 33 and the screws 32a, 33a in the opposite direction of rotation, so as to cause the downward movement of the plate 31 and therefore the downward movement of the piston pushed by the pusher assembly and the expulsion of the plastic resin accumulated inside the accumulation chamber 13a via the outlet mouth 11 .

[0092] In greater detail, during the expulsion step, the screws 32a, 33a operated by the respective electric actuators 32,33 move the plate 31 downwards in the vertical direction 7-7. The plate is integral with the ring 41 of the pusher assembly, so that the downward travel of the plate 31 pushes the input ring 41 , the transmission columns 42 and the piston 18 so that they are displaced integrally along the vertical axis z as far as the lowered position corresponding to complete expulsion of the parison by the piston 18.

[0093] During expulsion, the male element 20 of the die is operated so as to vary the opening of the outlet mouth 11 in accordance with a programmed thickness profile defined for the parison being expelled.

[0094] Advantageously, as a result of the mechatronic assembly 30, the downward stroke movement of the plate 31 and piston 18 may be controlled both in terms of speed and in terms of length of the stroke. In a preferred configuration, the electronic control unit for controlling the movement of the mechatronic assembly may in particular perform a feedback control of the actuators 32,33, for example by means of the encoder with which they are provided. The movement may therefore be easily programmed by the operator who may, for example, set precisely various speeds at the various points of the downward stroke during expulsion, with reference, for example, to a predefined graph of the speed during the downward stroke (vectorial control).

[0095] Once expulsion of the parison has been completed, the die is closed by operating the male element 20 and the accumulation chamber is supplied with a new thermoplastic load; the plate 31 is then displaced towards the raised position by the electric actuators 32,33 which reverse the direction of rotation of the screws 32a, 33a, causing the upward movement of the piston into the raised position, pushed by the new thermoplastic resin load supplied to the accumulation chamber 13a.

[0096] In greater detail, the control unit of the mechatronic assembly detects immediately the pressure inside the chamber by reading the signal emitted by the pressure sensor, or the torque output by each actuator, and intervenes, operating the plate 31 and therefore the piston 18 so as to maintain the predefined pressure or output torque values for the entire upwards stroke of the piston 18.

[0097] Advantageously, in a preferred configuration, for the upward stroke of the output plate 31 during the accumulation step, the electronic unit for controlling the movement of the mechatronic assembly 30 may perform feedback control of the electric actuators 32,33, using a feedback control configuration different from that for expulsion, where reference is made to the set pressure or the set reference torque.

[0098] In this way, the plate 31 moved by the actuators and the piston 18 assists the natural reactions of the piston to accumulation of the thermoplastic resin inside the chamber 13a, in a manner proportional to the entry of the resin into the accumulation chamber, reproducing the natural upward behaviour of the piston of the hydraulic systems.

[0099] Since the output plate 31 is displaceably integral with the ring 41 and therefore rigidly connected to the piston 18, it is possible to set the reversal of the movement for the next expulsion step, during any stage of the cycle or for any load amount. With this option it is possible to keep substantially unvaried the other systems for correction of the machine cycle, such as, for example, adjustment of the speed of the extrusion screw and management of the other parameters, normally performed by means of hydraulic expulsion.

[0100] The integral connection, virtually without any play, between the output element of the mechatronic assembly and the expulsion piston 18 therefore offers the advantage of providing a system where there is no delay in response due to recovery of the play or the distances between the elements of the kinematic chain.

[0101] As a result of management of the upward movement of the piston during accumulation by means of an electronic control unit which controls the operation of the electric actuators in a proportional manner as described above, it is also possible perform in a substantially automatic manner all the counterpressure and accumulation decompression functions which, in conventional hydraulic heads, require additional devices and settings and which are instead not possible in the known electric actuating heads in which the piston is detached from the electric drive system during the upward movement. Therefore the quality of the product is not negatively affected by sudden changes in pressure during the accumulation step.

[0102] The accumulation extrusion head with mechatronic assembly according to the present invention is able to reduce significantly the energy consumption, while ensuring a greater precision of the movements as regards control of the both the speed and positioning curves. In addition there is a significant improvement in the repeatability of the upward and downward movements of the piston, so that the sequential expulsion and reloading cycles are very similar to each other. The mechatronic actuating assembly is a “power on demand” system which uses only the amount of energy needed. The gearmotors react instantaneously so as to keep the movement vector in accordance with the programmed cycle, with direct modulation of the torque and therefore the power.

[0103] The blowing-moulding process is such that relatively important parameters are subject to variations, so that by managing to reduce the greatest number possible of variables and the amount of said variations it is possible to reduce the tolerances of the parameters and increase the efficiency of the process. The head according to the present invention is therefore able to reduce the amount of plastic used in the articles produced, reduce the cycle time of the blowmoulding machine, improve the productivity and reduce also the production waste, which is notoriously high in the blow-moulding process, increasing the efficiency of the machine from 93 / 95% (efficiency of a process regarded as being stable with a hydraulic machine) to values in the region of 96 / 98%. Furthermore, the production efficiency remains more or less constant at any load and speed.

[0104] For example, a head according to the present invention which uses a pair of gearmotors able to develop a combined power of 28 Kw in the most demanding conditions is able to produce a maximum pressure of 200 bar in the accumulation chamber and a movement speed of 90 mm per second, with a minimum energy saving of 57% compared to a comparable oil-hydraulic system. It is also clear how with the accumulation head according to the present invention it is possible to maintain a more or less constant viscoelastic behaviour of the thermoplastic resin load during accumulation, by raising the piston upwards with a movement proportional to the entry of the new resin inside the accumulation chamber 13a. The mechanical stresses acting on the load being accumulated are therefore kept to a minimum, as are the variations in volume. The resin thus behaves in a natural manner.

[0105] The preferred control unit of the mechatronic assembly is able to overcome the intrinsic drawbacks of mechatronic systems which, using mechanical reduction gears which are irreversible or difficult to reverse, would require a major effort on the user side in order to reverse the movement of the system and allow the natural upwards movement of the piston.

[0106] By controlling the reversal of the mechatronic system during accumulation on the basis of a reference pressure or torque it is possible to perform in a substantially automatic manner also the overpressure and accumulation decompression functions.

[0107] With the accumulation head according to the present invention, in fact, advantageously it is possible to prevent the piston from moving up more rapidly than the feed rate of the load being fed by the extrusion screw, thus preventing the accumulation in the accumulation chamber of air bubbles which would alter the volume of load, resulting moreover in the risk of auto-combustion of the plastic owing to the presence of oxygen. Furthermore, the piston does not move up too slowly, thereby avoiding inside the accumulation chamber the generation of an excessive pressure which alters the actual volume of the load and causes the leakage of plastic from the die, with negative consequences for the process. The mechatronic assembly according to the present invention is moreover able to assist the expansion of the resin which occurs during reversal from the expulsion step to the loading step, thus avoiding further disturbance of the process. Said expansion, due to a drastic drop in pressure inside the chamber owing to cessation of the pushing force, occurs during an initial upward movement, in which the residual material expands rapidly and then restabilizes its volume when the normal loading pressure is reached.

[0108] Furthermore, it is clear how the head according to the present invention does not require the use of pressurised oil, eliminating the risks of breakage of the components of the pressurized plant and therefore the associated safety systems which are greatly simplified with the mechatronic system. The machines for machining the thermoplastic materials have resistances which heat the components of the extrusion system to as high as 300 degrees C. The accidental leakage of pressurised oil from a hydraulic system atomizes the oil to the point that contact thereof with the very hot resistances may result in the risk of fire. Furthermore, the hydraulic system comprises a storage tank from where the oil is drawn by the pump and sent to the circuit. Then the oil returns into the tank where it is cooled and filtered. The tank must have dimensions so as to keep the variation in the oil level within certain limits and, owing to the return oil flows, which are sometimes very voluminous, it must have air vent valves which are provided with filters, but do not manage in any case to retain all the atomized oil, so that part thereof is dispersed in the air. Where articles for the medical or food sector are produced, it must be ensured that the air inside the production plant has a level of contamination which is kept within certain limits, something which mechatronic systems are able to be ensure much more effectively.

Claims

CLAIMS1) Accumulation extruder head (1 ) for a blow-moulding machine for blowmoulding containers, comprising: a head body (10), having: an internal accumulation chamber (13a) with a central vertical axis (z), designed to be filled with a thermoplastic resin load; a bottom outlet mouth (11 ) for expulsion of the thermoplastic resin load in the form of a tubular parison, the accumulation chamber (13a) communicating at the bottom with the outlet mouth (11 ); a piston (18), coaxially inserted inside the accumulation chamber (13a), the piston (18) being movable in a reciprocating manner inside the accumulation chamber along the central vertical axis (z);- in an expulsion direction, from a raised position to a lowered expulsion position; and- in an upward direction, from the expulsion position to a raised position; a mechatronic assembly (30) comprising: an output element (31 ), linearly displaceable along the central vertical axis (z) between a raised position and a lowered position; a plurality of electric actuators (32,33), each designed to rotationally drive a respective rotating element (32a, 33a); a control unit of the mechatronic assembly configured to control operation of said electric actuators; wherein each rotating element is coupled to the output element (31 ) such that rotation of the rotating element displaceably drives the output element in the vertical direction; and the electric actuators are controlled and configured to drive said rotating elements in a coordinated manner so as to jointly displace said output element (31 ); wherein said output element (31 ) is rigidly connected to the piston (18) for translation therewith; and the control unit is configured such that, during a filling step for filling the accumulation chamber (13a), the electric actuators are controlled to displaceably drive the output element (31 ) from the lowered position to a raised position thereby moving the piston (18) in an upward direction, in a manner suchthat the volume of the accumulation chamber increases proportionally in relation to the volume of thermoplastic resin entering the accumulation chamber characterized in that, during the upward movement in the filling step, the control unit of the mechatronic assembly adjusts the displacement of the output element (31 ) depending on a signal indicating the pressure detected inside the accumulation chamber and emitted by a detection sensor (35) arranged inside the chamber so as to keep the pressure inside the accumulation chamber within a predefined reference range and preferably substantially equal to a predefined reference value.2) Accumulation extruder head according to Claim 1 , characterized in that the control unit of the mechatronic assembly adjusts the displacement of the output element (31 ) during the upward movement in the filling step by controlling the speed of rotation of each electric actuator so as to maintain a predefined torque output by each electric actuator.3) Accumulation extruder head according to Claim 1 , further comprising a pusher assembly (40) arranged between the output element (31 ) and the piston (18) and comprising an input element (41 ) rigidly coupled to the piston (18), wherein the output element (31 ) of the mechatronic assembly (30) and the input element (41 ) of the pusher assembly (40) are rigidly coupled together so as to be displaced integrally in the vertical direction (Z-Z) in order to push the piston (18) in the expulsion direction inside the thermoplastic resin accumulation chamber (13a) during expulsion of the load in the form of a parison and to pull the piston in the upward direction during filling of the accumulation chamber.4) Accumulation extruder head according to one of the preceding claims, characterized in that each rotating element rotates about a vertical axis parallel and axially offset with respect to the central vertical axis (z).5) Accumulation extruder head according to one of the preceding claims, characterized in that it comprises two electric actuators and respective rotating elements arranged in diametrically opposite positions with respect to the vertical central axis (z) or more than two electric actuators and respective rotating elements arranged angularly equidistant along a circumference centred on the vertical central axis (z).6) Accumulation extruder head according to one of the preceding claims, characterized in that a male element (20) is coaxially inserted in the cavity (13)of the head body (10) and is movable coaxially with respect to the head body (10) so as to vary the opening of the bottom outlet mouth (1 1 ) for expelling the resin in order to adjust a thickness profile of the expelled parison, wherein the male element (20) is moved by an actuating unit (21 ) with an electric actuator (22) and a transmission chain (23), which are preferably arranged coaxially with the central vertical axis (z).7) Accumulation extruder head according to the preceding claim, characterized in that it is configured to drive said male element so as to close the output mouth opening during the step of filling the accumulation chamber.8) Accumulation extruder head according to one of the preceding claims, characterized in that the output element of the mechatronic assembly is formed by a plate (31 ) coupled by means of a male / female coupling with each rotating element (32a;33a), said plate (31 ) having preferably a plurality of female-thread holes and each rotating element being inserted inside a respective one of said holes and having an external thread mating with the female thread of the hole.9) Accumulation extruder head according to one of the preceding claims, characterized in that said rotating elements of mechatronic assembly are each in the form of a respective screw with an external thread and / or wherein the coupling of the rotating elements together with the output element provides an irreversible transmission.10) Accumulation extruder head according to one of the preceding claims, characterized in that the electric actuators are linear electric actuators (32,33), each comprising a vertical-axis electric gearmotor with encoder.11) Accumulation extruder head according to one of the preceding claims, characterized in that the control unit is configured to control said electric actuators with different control modes during the resin expulsion step and during the step for filling the accumulation chamber.12) Accumulation extruder head according to the preceding claim, characterized in that during the expulsion step the control unit performs vectorial control of the electric actuators.13) Method for accumulation extrusion of a thermoplastic resin load in the form of a tubular parison, comprising: feeding an extruded thermoplastic resin to a head body (10) of anaccumulation extruder head, the load being fed into an accumulation chamber (13a) with a central vertical axis (z) defined inside an internal cavity (13) of the head body, until the accumulation chamber is filled with a predefined load of thermoplastic resin; expelling the accumulated thermoplastic resin load in the form of a tubular parison through a bottom output mouth (1 1 ) communicating with the accumulation chamber (13a); wherein a piston (18) is coaxially inserted inside the accumulation chamber (13a) and moves in a reciprocating manner inside the accumulation chamber along the central vertical axis (z);- in an upward direction during the filling step, from a lowered expulsion position into a raised position;- in an expulsion direction during the expulsion step, from a raised position into a lowered expulsion position; wherein the head comprises a mechatronic assembly (30) comprising an output element (31 ), linearly displaceable along the central vertical axis (z) between a raised position and a lowered position, and a plurality of electric actuators, each of which rotationally drives a respective rotating element which is coupled to the output element (31 ) by means of a coupling such that the rotation of the rotating element displaceably drives the output element in the vertical direction; wherein the electric actuators are controlled and configured to drive in a coordinated manner said rotating elements so as to jointly displace said output element (31 ); wherein the output element (31 ) of the mechatronic assembly (30) is rigidly coupled to the piston (18) for translation therewith and, during the expulsion step, the output element is driven in the expulsion direction from a raised position to the lowered position so as to push the piston (18) in the expulsion direction towards the lowered position; wherein, in the filling step for filling of the accumulation chamber, the output element (31 ) of the mechatronic assembly (30) is displaceably driven with a controlled upward movement so as to move the piston (18) in the upward direction in such a way that the volume of the accumulation chamber increasesin proportion to the volume of thermoplastic resin entering into the accumulation chamber; characterized in that during the upward movement of the piston in the filling step for filling the accumulation chamber, a detection sensor measures the pressure inside the accumulation chamber (13a) and the output element (31 ) is displaced in the upward direction so as to keep the pressure inside the accumulation chamber within a predefined reference range and preferably substantially equal to a predefined reference value. 14) Method according to the preceding claim, wherein each of said rotating elements rotates about a vertical axis parallel and axially offset with respect to the central vertical axis.15) Method according to one of Claims 13-14, wherein, during the filling step for filling the accumulation chamber, a control unit of the mechatronic assembly adjusts the rotational operation of each electric actuator so as to maintain a predefined reference value of the torque output by the actuator.

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