EP3755629B1

EP3755629B1 - Apparatus for filling containers with dosed quantities of powdered material

Info

Publication number: EP3755629B1
Application number: EP19712304.5A
Authority: EP
Inventors: Sandro Naldi
Original assignee: Romaco SRL
Current assignee: Romaco SRL
Priority date: 2018-02-23
Filing date: 2019-02-22
Publication date: 2022-01-12
Anticipated expiration: 2039-02-22
Also published as: CN111770882B; EP3755629A1; IT201800003007A1; ES2907110T3; WO2019162980A1; CN111770882A

Description

Technical Field

The present invention relates to an apparatus for filling containers with dosed quantities of powdered material.

Prior art

The need to automatically fill specific containers such as bottles and the like with a predetermined quantity of powdered material has known for some time.
Therefore, apparatuses have been developed to allow the introduction of dosed quantities of powdered material into relative containers conveyed in succession along a supply line.
According to a known type, these apparatuses comprise a dosing wheel rotatable around a substantially horizontal axis of rotation. The dosing wheel is located below a hopper containing the powdered material to be introduced into the containers. The dosing wheel has a series of channels formed in a peripheral portion of the wheel itself; in each channels a piston suitable for delimiting a collection chamber for the powdered material is inserted. Each chamber is associated with suction means suitable for creating therein a vacuum condition so as to suck a predetermined quantity of powdered material from the hopper inside the chamber, when the chamber occupies a working position near the hopper. In each chamber a fluid, such as compressed air, is introduced by conduction means to unload the quantity of powdered material from the chamber to a respective container.
The patent US 3,656,518 discloses an apparatus for filling containers with a powdered material comprising a dosing wheel, rotatably mounted on a rotation shaft carried by a supporting frame. A hopper containing the powdered material is placed above the dosing wheel. The dosing wheel has a plurality of openings extending along radial directions from the periphery towards the centre of the wheel itself. In each opening, there is inserted a sleeve in which a piston comprising a stem and a head is slidably arranged; the piston head is made of a material being porous to gases but impervious to powdered materials. The head of the piston delimits in the sleeve a dosing chamber suitable for collecting a predefined quantity of powdered material. The stem of each piston defines at one end a cam follower placed in contact with cam means suitable for driving the piston in translation inside the sleeve. Each dosing chamber is provided with a lateral opening communicating with a supporting element associated with the rear of the wheel. The supporting element forms an arched channel extending for the circumference arc comprised between the loading position of the material and the unloading position of the material and performs the function of allowing the holding of the powdered material into the chamber. The lateral opening and the channel of the supporting element are connected to a vacuum pump suitable for creating a vacuum condition in each chamber, as well as in the channel, in order to suck the powdered material from the hopper when the chamber is placed at the same level as the hopper and to allow the material to be held in the same chamber during the rotation of the dosing wheel up to the unloading position, respectively.
A further apparatus of this type is shown in the patent EP 202 868 .
An apparatus for filling containers with a powdered material according to preamble of claim 1 is also described in patent US 4,567,714 . The apparatus comprises at least one device configured for dispensing dosed quantities of a material, and arranged below a hopper containing the material to be dispensed. Each device comprises a stationary inner element and an outer element, arranged concentrically with respect to the inner element. The outer element forms a rotatable ring comprising a plurality of radially extending openings, suitable for housing relative pistons sliding inside the openings. One end of each piston is coupled with a channel which drives the movement of the piston in a radial direction, within the relative opening, during rotation of the outer ring with respect to the central element, at least for a portion of the circumference of the ring.
A specific sector of use of these apparatuses is the pharmaceutical sector. In this sector, the need is felt to automatically fill small containers with dosed quantities of a pharmaceutical powdered product ensuring a high precision and repeatability to the product dosage operation.
In particular, in the aforementioned sector of powdered pharmaceutical products, such as powders for injectable solutions, in order to better control the dosage of the active ingredient, it has been adopted the solution of filling the containers only with the active agent, excluding the excipients.
This solution allows, on the one hand, to carry out a better control on the quantity of material corresponding to the unit dose of product to be inserted in the respective container but on the other hand it determines the problem of managing very small quantities of powdered product, lower than the treated quantities of product in the case of mixtures including the active ingredient and the excipients. Known apparatuses for filling containers have dimensional and structural limitations which make it difficult to manage very small quantities of powdered product.
A further problem to be considered is the need of always meeting the precision and repeatability requirements of the dosage operation, which are particularly stringent in the pharmaceutical sector.

Presentation of the invention

The aim of the present invention is to solve the above-mentioned problems cited by devising an apparatus which allows to automatically fill the containers with dosed quantities of powdered material even in very small doses.
Another object of the invention is to provide an apparatus, which allows to perform the dosage and filling operations of the containers with high precision and repeatability.
A further object of the invention is to provide an apparatus which is structurally robust.
A further object of the invention is to provide an apparatus for filling containers with powdered material of simple constructional and functional conception, absolutely reliable in functioning, versatile in use, as well as relatively cheap.
The aforementioned objects are achieved, according to the present invention, by the apparatus for filling containers with dosed quantities of powdered material according to claim 1.
The apparatus for filling containers with dosed quantities of powdered material comprises a dosing wheel rotatable around a substantially horizontal axis of rotation, said dosing wheel comprising a plurality of channels extending along respective radial directions from a peripheral edge of said dosing wheel towards the centre of said dosing wheel; a hopper, arranged above said dosing wheel, and suitable for containing said powdered material; a plurality of pistons slidably inserted respectively inside said channels, each piston comprising a stem and a head integral with one end of said stem, each piston defining in said channel a calibration chamber arranged to collect a dosed quantity of said powdered material, said calibration chamber being delimited by said head of said relative piston and by an inner wall of said respective channel; an adjustment assembly, associated with said dosing wheel, suitable for allowing the adjustment of the position of each piston inside said respective channel; conduction means communicating with said channels and suitable for being connected alternatively to a negative pressure source and to a positive pressure source, so as to create a vacuum condition in each said calibration chamber and so as to introduce a flow of a fluid inside said calibration chamber, respectively, said head of each said piston presenting a cross-section having a smaller diameter than the diameter of the cross-section of an end portion of said stem, said end portion being opposite to said head.
It is observed that the provision, for each channel, of a piston with a head having a lower cross-section than the end portion of the stem, allows to obtain base surfaces of the calibration chamber having a reduced extension and therefore, a range of volume values of chamber which are lower than those obtainable with the known apparatuses of the same type.
The dosing wheel comprises a discoidal body inside which said channels are formed, each said channel having at said peripheral edge an opening that puts it in communication with the external environment.
Each said channel has an increasing cross-section from said peripheral edge towards the centre of said discoidal body. Therefore, the assembly of the pistons in the channel is facilitated thanks to the configuration of the channels themselves, which allow each piston to be inserted inside the relative channel from the centre of the discoidal body.
Preferably, the diameter of the cross-section of the head of each said piston has a value in the range of about 3.5 mm to about 4 mm.
Preferably, said diameter of the cross-section of each said head has a value of about 4 mm.
Preferably, said head comprises a filtering element provided with a plurality of holes suitable for guaranteeing the permeability of said head to the gaseous media and the impermeability of said head to said powdered material.
Preferably, the diameter of the end portion of the stem has a value of about 5 mm.
Preferably, said channels are regularly distributed on said discoidal body.
Preferably, said channels are arranged at the same angular distance with respect to each other.
Advantageously, each said channel comprises a segment near a central part of said discoidal body which is configured to allow the insertion of said relative piston during assembly.
Preferably, said segment near the central part of the discoidal body has a greater cross-section with respect to the section of the end portion of said piston.
Preferably, each said channel comprises an end segment on the side of said peripheral edge having a diameter of the cross-section substantially corresponding, unless the presence of a play, to the diameter of the cross-section of said head of said relative piston.
Preferably, said adjustment assembly comprises a plurality of actuator elements connected to respective pistons.
Preferably, said actuator elements are made by pins.
Preferably, each pin is inserted into a corresponding hole made in the end portion of the relative piston.
Preferably, said actuator elements are constrained to an adjustment ring and to a relative sliding guide.
Preferably, said adjustment ring is associated frontally with the discoidal body and occupies a substantially central portion of the discoidal body. Preferably, said adjustment ring comprises a series of grooves defining a series of spirals developing from the centre towards the periphery. Preferably, said spirals are Archimedean spirals.
Preferably, said adjustment assembly comprises a series of said sliding guides, one for each actuator element, extending along respective radial directions of the discoidal body.
Advantageously, said adjustment ring can be driven in rotation about said substantially horizontal axis of rotation to move the actuator elements simultaneously along a radial direction, inside said relative sliding guides, so as to translate the corresponding pistons along the relative channels.
Preferably, said conduction means comprise a plurality of conduits, one for each said channel.
Preferably, said conduits are angled with respect to the respective channels of a predetermined inclination angle having a value lower than 90°.
Preferably, said inclination angle has a value of about 50°. Experimentally, it has been observed that the arrangement of each conduit with an inclination angle presenting the aforementioned value is advantageous in terms of fluid-dynamic performance. In particular, from the experimental results it is observed a decrease of turbulence phenomena and this helps to make the apparatus more precise and provided with high repeatability.
Preferably, said inclination angle refers to the angle comprised between the median longitudinal axis of each conduit and the median longitudinal axis of the relative channel.
Preferably, said conduits extend from the respective channels up to a rear surface of the dosing wheel.
Preferably, a distributor element is associated with the rear surface of said discoidal body and occupies a fixed position during the rotation of the disk, said distributor element being suitable to be connected to said positive pressure source or to said negative pressure source.
Preferably, said conduits are put in communication, in certain operating steps of the dosing wheel, with said distributor element.

Brief Description of the Drawings

The details of the invention will become more evident from the detailed description of a preferred embodiment of the apparatus for filling containers with dosed quantities of powdered material according to the invention, shown by way of example in the accompanying drawings, wherein:

figure 1 shows a front view of the apparatus for filling containers with dosed quantities of powdered material according to the invention;
figure 2 shows a front view of a detail of the present apparatus;
figure 3 shows an enlarged sectional view of the above-mentioned detail according to plane of trace III-III of figure 2;
figure 4 shows a front view of a further detail of the apparatus according to the invention;
figures 5 and 6 respectively show a rear front view and a sectional view according to the plane of trace VI-VI of the detail illustrated in figure 4;
figure 7 shows an enlarged front view of the detail illustrated in figure 4;
figure 8 shows a front view of a further detail of the apparatus;
figures 9 and 10 show a front view and a top view of a further detail of the apparatus.

Embodiments of the invention

With particular reference to these figures, the apparatus for filling containers with dosed quantities of powdered material according to the present invention has been generally indicated with 1.
It is specified that, in the following disclosure, the term "about" refers to a range of values around the indicated value, taking into account the tolerances foreseen in the technical sector.
The apparatus 1 comprises a dosing wheel 2 arranged to dose predefined quantities of powdered material and to fill a series of containers with such dosed quantities of powdered material, for example a pharmaceutical product.
Preferably, the containers are specific bottles.
The powdered material is inserted inside a hopper 3 arranged above a dosing wheel 2. The hopper 3 is fed by a specific tank, not shown, provided with the powdered material therein. The hopper 3 can include mixing means able to guarantee a continuous mixing of the material.
The containers to be filled are arranged in succession along a supply line which can include a conveyor belt, suitable for defining an operative surface on which the containers are resting.
The conveyor belt brings the containers to the dosing wheel 2. In particular, the conveyor belt comprises at least one segment extending below the dosing wheel 2 so that the filling operation is carried out.
If the powdered material is a pharmaceutical product, the apparatus 1 is installed inside a sterile chamber characterized by environmental parameters such as temperature, pressure, humidity and the composition of the air having values falling within pre-set intervals. The sterile chamber provides a high-efficiency filtering system, for example a HEPA filter suitable for guaranteeing the maintenance of an adequate level of air cleanliness in the chamber.
The dosing wheel 2 is rotatably supported by a shaft associated with a supporting frame, not shown in the figures. More precisely, the dosing wheel 2 is driven in rotation around a substantially horizontal axis of rotation A by a motor element which is not shown. Preferably, the dosing wheel 2 is driven in rotation with intermittent motion.
The dosing wheel 2 comprises a discoidal body 4 provided with a plurality of channels 5 extending along respective radial directions from a peripheral edge 6 towards the centre of the discoidal body 4. The channels 5 are regularly distributed on the discoidal body 4 and are arranged at the same angular distance with respect to each other.
Each channel 5 is formed inside the discoidal body 4 and has, at its end corresponding to the peripheral edge 6, an opening 7 which puts it in communication with the external environment.
The diameter of the cross-section of each channel 5 varies along the radial direction increasing from the peripheral edge 6 towards the centre. In particular, each channel 5 comprises an end segment 8 on the side of the peripheral edge 6 presenting a diameter of the cross-section d0 substantially corresponding, unless the presence of a play, to the diameter d4 of the cross-section of the head of a piston inserted therein, as will be explained better below.
The segment of each channel 5 arranged near a central part C of the discoidal body 4 is instead configured so as to allow the insertion of a relative piston during the assembly. More specifically, the segment near the central part C of the discoidal body 4 has a greater cross-section with respect to the section of the end portion of the piston.
A piston 9 is slidably inserted inside each channel 5, being suitable for defining a calibration chamber 10 arranged to collect a predetermined quantity of powdered material.
Each piston 9 includes a stem 11 and a head 12 integral with an end of the stem 11. In the mounting configuration, the stem 11 extends for a portion of the channel 5 and the head 12 faces the peripheral edge 6.
The head 12 preferably has a hollow cylinder shape and comprises a filtering element 13 mounted on a base surface.
The filtering element 13 comprises a plurality of holes 14 having a predetermined size so as to guarantee a permeability to the gaseous media and an impermeability to the powdered material.
The stem 11 has a decreasing cross-section from one end opposite to the head 12 to the end integral with the head 12. More precisely, the stem 11 forms a first end portion 15 opposite to the end where the head 12 is arranged, and a second intermediate portion 16, adjacent to the end portion 15, which has a first narrowing of the cross-section. The diameter d2 of the intermediate portion 16 is smaller than the diameter d1 of the end portion 15.
The stem 11 forms a third portion 17 adjacent to the intermediate portion 16 which has a further narrowing of the cross-section with a diameter d3 being smaller than the diameter d2. The head 12 of the piston 9 is integrally associated with the third portion of the stem 11.
The diameter d4 of the cross-section of the head 12 is smaller than the diameter d1 of the cross-section of the end portion 15 of the stem 11. Preferably, the diameter d4 of the cross-section of the head 12 has a value in the range of about 3.5 mm to 4 mm. Even more preferably, the diameter d4 of the cross-section of the head 12 has a value of about 4 mm.
Preferably, the diameter d1 of the end portion 15 of the stem 11 has a value of about 5 mm.
Each calibration chamber 10 is delimited by the inner wall of the respective channel 5, in particular by the end segment 8 of the channel 5, and by the base surface of the head 12 of the piston 9.
Experimental tests have shown that, by providing a piston having a head 12 diameter of about 4 mm and considering a material having a specific weight of 0.55 g / cm³, the quantity of powdered material that the calibration chamber 10 can contain is in the range of values between a minimum of 20 mg and a maximum of 60 mg.
The range of values of quantity of material contained in the chamber corresponds to the range within which it is possible to vary the volume of the calibration chamber 10.
The adjustment of the volume of the chamber 10 is carried out by adjusting the position taken by the piston 9 inside the channel 5. The sliding of the pistons 9 in the respective channels 5 is operated by an adjustment assembly 18 associated with the dosing wheel 2.
The adjustment assembly 18 comprises a plurality of actuator elements 19 connected to respective pistons 9.
Each piston 9 forms a hole 20 in the end portion 15 through which it is inserted the respective actuator element 19. Preferably, the actuator elements are constituted by pins 19. The pins 19 are constrained to an adjustment ring 21 and to a relative guide 22.
The adjustment ring 21 is associated frontally with the discoidal body 4 and occupies a substantially central portion of the discoidal body 4. The adjustment ring 21 comprises a series of grooves 23 defining a series of spirals developing from the centre towards the periphery. The spirals 23 are Archimedean spirals.
The adjustment assembly 18 provides a plurality of sliding guides 22, one for each actuator pin 19, extending along respective radial directions of the discoidal body 4. The length of each guide 22 is equal to the stroke of the piston 9.
The adjustment ring 21 can be driven in rotation around the substantially horizontal axis A to move the pins 19 simultaneously along a radial direction, inside the relative guides 22, and then to translate the corresponding pistons 9 along the relative channels 5.
Each channel 5 is in communication with the conduction means 24, suitable for being connected to a negative pressure source so as to create a vacuum condition in each calibration chamber 10, in order to suck a predetermined quantity of powdered material from the hopper 3. Said conduction means 24 can also be connected to a positive pressure source to introduce into the channel 5 a flow of a fluid, for example compressed air, suitable for allowing the unload of the powdered material contained in the calibration chamber 10 towards a relative container to be filled. The negative pressure source is for example a vacuum pump. The positive pressure source is for example a compressor.
The conduction means 24 comprise a plurality of conduits, one for each channel 5, which are inclined with respect to the respective channels 5 of a predetermined angle a. The angle α refers to the angle comprised between the median longitudinal axis of each conduit 24 and the median longitudinal axis of the channel 5. The angle α has a value which is lower than 90°. Preferably, the angle α has a value of about 50°.
Experimentally, it has been observed that the arrangement of the conduit 24 with an inclination angle presenting the aforementioned values is advantageous in terms of fluid-dynamic performance. In particular, from the experimental results it is observed a decrease of turbulence phenomena and this helps to make the apparatus more precise and provided with high repeatability.
The conduits 24 extend from the respective channels 5 to a rear surface of the dosing wheel 2 and are put in communication, during certain steps of operation of the dosing wheel 2, with a distributor element 25.
The distributor element 25 is associated with the rear surface of the dosing wheel 2 and occupies a fixed position during the rotation of the dosing wheel 2.
This distributor element 25 is formed by a ring which forms a first suction channel 26 in the shape of a circumference arc extending in a loading area which is oriented towards the hopper 3 in a mounting configuration.
The first channel 26 is provided with at least one suction hole 27 suitable for allowing the connection of the channel 26 with the negative pressure source to create a vacuum condition in the first channel 26.
The distributor element 25 forms a second channel 28 in the shape of a circumference arc which is consecutive to the first channel 26 and extends between the first channel 26 and an unloading area opposite to the first channel 26 which is placed, in the mounting configuration, towards the containers to be filled.
The second channel 28 is also provided with a respective suction hole 27 suitable to allow the connection of the channel 28 with the negative pressure source to maintain the vacuum condition in the channel.
A third suction hole 27 is made at the discharge area so as to allow the connection of the conduits 24 occupying this position with the positive pressure source.
In the circular sector between the third suction hole 27 and the first channel 26, a third channel 29 is provided having at least one suction hole 27 for the connection with the positive pressure source to operate an optimum cleaning of the pistons 9 before a following filling of the chambers 10.
The operation of the apparatus for filling containers is easily comprehensible given the foregoing description.
Initially, the volume of the calibration chambers 10 is set based on the quantity of powdered material to be inserted in the containers. The adjustment ring 21 is then driven in rotation to move the pins 19 simultaneously along the respective radial directions with a consequent translation of the relative pistons 9 along the channels 5.
The dosing wheel 2 is then driven in rotation regularly spaced around the axis of rotation A to perform the sequential filling of the calibration chambers 10 with predefined quantities of powdered material and the subsequent emptying of the chambers 10.
In particular, in order to disclose an operating cycle of the apparatus, consider a calibration chamber 10 which initially occupies a position in the area below the hopper 3. In this position the chamber 10 communicates via the conduits 24 with the negative pressure source which creates the vacuum in the first channel 26 and in the chamber 10. The vacuum created in the calibration chamber 10 allows to suck the powdered material and fill the chamber 10 with an accurate quantity of powdered material.
The chamber 10 is brought by the dosing wheel 2 through the area corresponding to the second channel 28 wherein the vacuum is maintained so that the powdered material contained in the chamber 10 remains inside the chamber 10 itself and no material losses occur due to the forces to which the chamber 10 is subjected during the rotation.
The chamber 10 is then brought into the unloading area of powdered material and the powdered material is expelled into the underlying container thanks to the flow of compressed air introduced into the chamber 10.
Finally, the dosing wheel 2 brings the chamber 10 through the area corresponding to the third suction channel 29 wherein the delivery of a further flow of compressed air into the chamber 10 allows to clean efficiently the piston 9 and to make the calibration chamber 10 ready for a new operating cycle.
The apparatus according to the invention allows the dosing operation of small quantities of powdered material to be carried out optimally, thanks to the configuration of the pistons, as well as allowing an easy assembly, thanks to the configuration of the channels. In particular, the provision for each channel of a piston with a head having a lower cross-section than the end portion of the stem allows to obtain base surfaces of the calibration chamber having a reduced extension and therefore, a range of volume values of chamber which are lower than those obtainable with the known apparatuses of the same type.
It is to be considered that a piston with a head having a diameter of about 4 mm, which substantially corresponds to the diameter of the base surface of the calibration chamber, allows to dose quantities of material up to 20 mg, considering a specific weight of the material of 0,55 g / cm³.
It should be noted that the end portion of each piston having a cross-section which is greater than the section of the head guarantees the maintenance of an adequate stability and structural strength of the adjustment assembly.
It should also be considered that the configuration of the channels according to the present invention, wherein each channel has a cross-section which increases starting from the peripheral edge towards the centre of the discoidal body, allows to overcome the difficulties of mounting the pistons introduced by the reduction of the section of the end segment of each channel, as a consequence of the provision of the head of each piston with a lower cross-section. This configuration allows to insert the pistons inside the respective channels, from the centre of the discoidal body.
In this regard, it is noted that the segment of each channel near the central part of the discoidal body is configured so as to allow, during the assembly, the insertion of the relative piston.
A further prerogative of the present invention consists in the fact that the conduction of a flow of compressed air or the creation of a depression in the calibration chamber by means of an inclined conduit of an angle lower than 90°, preferably of an angle of 50°, with respect to the median axis of the calibration chamber allows the reduction of turbulence phenomena. In this way the load losses are reduced, and this makes the loading and unloading operations of the chambers more reliable and accurate.
Experimentally, it has also been observed that the improved fluid-dynamic behaviour of the fluid in the calibration chamber allows to obtain a high repeatability of the machine.
The apparatus, disclosed by a way of example, is susceptible of several modifications and variations depending on the different requirements.
In the practical embodiment of the invention, the materials used, as well as shape and dimensions, may be any according to requirements.

Claims

Apparatus for filling containers with dosed quantities of powdered material, comprising a dosing wheel (2) rotatable around a substantially horizontal axis of rotation (A), said dosing wheel (2) comprising a discoidal body (4) inside which it is formed a plurality of channels (5) extending along respective radial directions from a peripheral edge (6) of said discoidal body (4) towards the centre of said discoidal body (4), each said channel (5) having an opening (7) at said peripheral edge (6) which puts it in communication with the external environment;
a hopper (3) arranged above said dosing wheel (2) and suitable for containing said powdered material;

a plurality of pistons (9) respectively slidably inserted inside said channels (5), each piston (9) comprising a stem (11) and a head (12) integral with one end of said stem (11), each piston (9) defining in said channel (5) a calibration chamber (10) arranged to collect a dosing quantity of said powdered material, said calibration chamber (10) being delimited by said head (12) of said relative piston (9) and by an inner wall of said respective channel (5);

an adjustment assembly (18), associated with said dosing wheel (2), suitable for adjusting the position of each piston (9) inside said respective channel (5); characterized in that the apparatus further comprises conducting means (24) communicating with said channels (5) and suitable for being alternately connected to a negative pressure source and to a positive pressure source, so as to create a vacuum condition in each said calibration chamber (10) and so as to introduce a flow of a fluid inside said calibration chamber (10), respectively,

wherein said head (12) of each said piston (9) has a cross-section having a diameter (d4) which is lower than the diameter (d1) of the cross-section of an end portion (15) of said stem (11), said end portion (15) being opposite to said head (12), and in that each said channel (5) has a cross-section which increases starting from said peripheral edge (6) towards the centre of said discoidal body (4).
Apparatus according to claim 1, characterized in that said diameter (d4) of the cross-section of said head (12) has a value in the range of about 3.5 mm to about 4 mm.
Apparatus according to claim 1, characterized in that said diameter (d4) of the cross-section of said head (12) has a value of about 4 mm.
Apparatus according to claim 1, 2 or 3, characterized in that said channels (5) are regularly distributed on said discoidal body (4), said channels (5) being arranged at the same angular distance with respect to each other.
Apparatus according to one of the preceding claims, characterized in that each said channel (5) comprises a segment near a central part (C) of said discoidal body (4) which is configured to allow the insertion of said relative piston (9) during assembly.
Apparatus according to claim 5, characterized in that said segment near said central part (C) of said discoidal body (4) has a cross-section being greater than the section of said end portion (15) of said relative piston (9).
Apparatus according to one of the preceding claims, characterized in that each said channel (5) comprises an end segment (8) on the side of said peripheral edge (6) having a diameter of the cross-section (d0) substantially corresponding, unless the presence of a play, to the diameter (d4) of the cross-section of said head (12) of said relative piston (9).
Apparatus according to one of the preceding claims, characterized in that said conduction means (24) comprise a plurality of conduits, one for each said channel (5), said conduits (24) being inclined with respect to the respective channels (5) of a predetermined inclination angle (a) having a value lower than 90°.
Apparatus according to claim 8, characterized in that said inclination angle (a) has a value of about 50°.
Apparatus according to one of the preceding claims, characterized in that said head (12) comprises a filtering element (13) provided with a plurality of holes (14) able to guarantee the permeability of said head (12) to the gaseous media and the impermeability of said head (12) to said powdered material.