GRAVIMETRIC METERING DEVICE AND CONTROL METHOD THEREOF

Abstract

A gravimetric metering device which is provided to add an additive material to a polymer material, includes: a hopper to contain the additive material, a supply member to transport the additive material discharged from the hopper towards the polymer material, and first and a second weighing elements to detect the weight of the additive material discharged from the gravimetric metering device.

A control unit determines, on the basis of the weight detected by the first weighing element, the flow rate of additive material discharged from the gravimetric metering device by regulating the supply member and controls the operation of the first weighing element by means of the second weighing element by comparing, at the start and at the end of a cycle for charging the hopper with the additive material, the weight detected by the first weighing element with the weight detected by the second weighing element.

Claims

1. A gravimetric metering device (1), which is provided to add an additive material to a polymer material, the gravimetric metering device (1) comprising: a hopper (3) configured to contain the additive material, a supply member (5) which is provided downstream of the hopper (3) in order to receive the additive material which is discharged from the hopper (3) and to transport the additive material towards a discharge opening (9) which is connected to the polymer material, a first weighing element (16) which is associated at least with the hopper (3) and which is provided to detect the weight of the additive material discharged from the gravimetric metering device (1), a second weighing element (17) which is associated at least with the hopper (3) analogously to the first weighing element (16) and which is provided to detect the weight of the additive material which is discharged from the gravimetric metering device (1), and a control unit (11) which is provided to determine, on the basis of the weight detected only by the first weighing element (16), a flow rate of the additive material which is discharged from the gravimetric metering device (1) and to regulate the supply member (5) in accordance with the flow rate, wherein the control unit (11) is configured to control the correct operation of the first weighing element (16) by comparing, at the start and at the end of a cycle for charging the hopper (3) with the additive material, the weight detected by the first weighing element (16) with the weight detected by the second weighing element (17).

2. The gravimetric metering device according to claim 1, wherein the supply member (5) comprises a screw which extends between an outlet (4) of the hopper (3) and the discharge opening (9) and a motor (10) for driving the screw.

3. The gravimetric metering device according to claim 1, wherein the first weighing element (16) detects the weight of the supply member (5) and of the hopper (3).

4. The gravimetric metering device according to claim 1, wherein the first and second weighing elements (16, 17) are positioned one adjacent to the other and are charged by the weight of the same components of the gravimetric metering device (1).

5. The gravimetric metering device according to claim 1, wherein the first and second weighing elements (16, 17) are identical to each other.

6. A method for controlling a gravimetric metering device (1) which is provided to add an additive material to a polymer material, the gravimetric metering device comprising: a hopper (3) configured to contain the additive material, a supply member (5) which is provided downstream of the hopper (3) in order to receive the additive material which is discharged from the hopper and to transport it towards a discharge opening (9) which is connected to the polymer material, a first weighing element (16) and a second weighing element (17) which are associated at least with the hopper (3), and a control unit (11) which is associated with the supply member (5) and the first and second weighing elements (16, 17), wherein the method comprises: detecting, by means of the first weighing element (16), the weight of at least the hopper (3), determining, by means of the control unit (11), the flow rate of additive material which is discharged from the gravimetric metering device (1) within a predefined time period from the weight measurements detected only by the first weighing element (16), regulating the supply member (5) in accordance with the flow rate of additive material which is discharged from the gravimetric metering device (1) within the predefined time period, detecting at the same time the weight of at least the hopper (3) by means of the first weighing element (16) and by means of the second weighing element (17) at the start and at the end of a cycle for charging the hopper (3) with the additive material, comparing the weight values detected by the first and second weighing elements (16, 17) and, if the weight values differ from each other by a quantity or a fraction greater than a predefined threshold, emitting an alarm signal.

7. The method according to claim 6, wherein the comparing comprises: calculating the weight of the additive material which is charged in the hopper (3) on the basis of the weight measurements of the first weighing element (16), calculating the weight of the additive material which is charged in the hopper (3) on the basis of the weight measurements of the second weighing element (17), and comparing the weights of the additive material charged in the hopper (3) as calculated from the weight measurements detected by the first and second weighing elements (16, 17).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The features and advantages of the invention will be better understood from the detailed description of a preferred embodiment thereof, which is illustrated by way of non-limiting example with reference to the appended drawings, in which:

[0067] FIG. 1 is a schematic perspective view of a gravimetric metering device realised according to the present invention;

[0068] FIG. 2 is a schematic side view of the gravimetric metering device of FIG. 1;

[0069] FIG. 3 is a schematic perspective view of a component of the gravimetric metering device of FIG. 1.

PREFERRED EMBODIMENT OF THE INVENTION

[0070] With reference to the appended Figures, there is generally designated 1 a gravimetric metering device which is realised according to the present invention.

[0071] The gravimetric metering device 1 is provided to add an additive material to a polymer material being supplied to a transforming machine, such as, for example, a pressing mould or an extruder, in which the polymer material is melted and moulded in order to form a desired article.

[0072] Preferably, the polymer material is in granular form and comes from a drying hopper (which is conventional per se and is not illustrated in the appended Figures), where it is heated and dried before being supplied to the transforming machine. In particular, the gravimetric metering device 1 introduces the additive material inside a pipe 2 which is depicted with broken lines in FIG. 2 and which connects the drying hopper to the transforming machine.

[0073] The polymer material is preferably PET and the additive material, which is also in granular form, is, for example, a stabilizing compound or a colouring compound.

[0074] The gravimetric metering device 1 comprises a hopper 3 which is connected by means of an outlet 4, which is formed on the base of the hopper 3, to a supply member 5. The supply member 5 comprises a screw which is received inside a cylinder 6 which extends in a longitudinal direction X between a first end 7 and a second end 8 where a discharge opening 9, from which the additive material is introduced into the pipe 2, is defined.

[0075] The screw of the supply member 5 is rotated by a motor 10 which is mounted at the first end 7 on the cylinder 6. In turn, the motor 10 is controlled by a control unit 11 so as to regulate the rotation speed of the screw and, consequently, the quantity of additive material per unit of time (in other words, the flow rate) which is present in the supply member 5 and transported towards the discharge opening 9.

[0076] The hopper 3 and the entire supply member 5 (cylinder, screw and motor) are mounted on a plate 12 which is L-shaped and which is in turn supported on a weighing unit 15.

[0077] Preferably, the plate 12 is removably fixed, for example, with screws 13, to a corresponding plate 14 of the weighing unit 15.

[0078] The weighing unit 15 further comprises a first weighing element 16 and a second weighing element 17 which are formed, for example, by two respective load cells which are mutually identical and which are positioned one beside the other and which are configured to detect the weight of the plate 14 and of everything supported thereon.

[0079] The first and the second weighing elements 16, 17 are further independently connected to the control unit 11 which receives and processes the weight signals detected thereby with the main objective of establishing the quantity of additive material per unit of time discharged from the gravimetric metering device 1 and consequently to regulate the supply member 5 so as to comply with a predefined target flow rate value.

[0080] The gravimetric metering device 1 operates as follows.

[0081] The additive material to be metered inside the polymer material is charged in the hopper 3 in accordance with a predetermined percentage which is imposed (manually or automatically) in the control unit 11.

[0082] As a function of the flow rate of polymer material, the control unit 11 establishes the flow rate of additive material to be introduced into the pipe 2 during the passage of the polymer material and, consequently, regulates the rotation speed of the screw which transports the additive material.

[0083] When the additive material inside the hopper 3 ends, or reaches a minimum level, the hopper 3 is charged with new additive material to be introduced into the pipe 2.

[0084] The weight of the hopper 3, the supply member 5 and the plates 12 and 14 is detected in a substantially continuous manner by both the weighing elements 16, 17, but the regulation of the supply member 5 carried out by the control unit 11 is based only on the weight value detected by the first weighing element 16, while the weight value detected by the second weighing element 17 is used by the control unit 11 only to control the correct operation of the first weighing element 16.

[0085] In particular, the control unit 11 determines the flow rate of additive material which is discharged from the gravimetric metering device 1 simply by calculating the weight decrease detected over time by the first weighing element 16. In fact, a weight variation detected by the weighing element may be attributed only to the variation of additive material present in the gravimetric metering device 1.

[0086] The flow rate value of additive material which is calculated is compared with the target value and, if necessary, the rotation speed of the screw is possibly adjusted.

[0087] At some times of the process, the control unit 11 provides for a comparison of the weight value detected by the first and second weighing elements 16, 17 in order to verify the congruence thereof.

[0088] If the values detected by the two weighing elements are congruent or at least differ by an acceptable and predefined tolerance range, then the metering process of the additive material continues in the preceding manners while instead, if the values detected by the two weighing elements differ by a value or a fraction greater than the tolerance range, then the control unit 11 emits an alarm signal, such as, for example, an indication on the monitor where applicable accompanied by an acoustic signal.

[0089] By way of example, the tolerance range may be defined as 5% of the weight value detected by the first or second weighing element.

[0090] Following the alarm signal, the operator or the control unit 11 itself may continue to interrupt the process for supplying the polymer material and metering the additive material and to start the maintenance of the weighing elements which can, for example, provide for a recalibration or replacement thereof.

[0091] Still by way of example, the control unit 11 can be programmed to carry out the comparison between the weight values detected by the two weighing elements 16, 17 on the occasion of the periodic charging cycles of the hopper 3, for example, by measuring the weight values before and after it is charged.

[0092] In particular, the weight measured by the first and second weighing elements 16, 17 is detected at the start of charging and at the end of charging so that the weight of additive material charged in the hopper is calculated, as a result of the difference, from the weight values measured both by the first and by the second weighing elements 16, 17.

[0093] The weights calculated in this manner are therefore compared with each other and, if they deviate by a quantity greater than a specific threshold, for example, equal to 5%, then an alarm signal is generated.

[0094] The person skilled in the art will know how to apply suitable variants to the present invention in accordance with any specific requirements, all being included within the scope of protection defined by the appended claims.

[0095] For example, the metering device may have any number of hoppers.