DESCRIPTION

Technical field

The present invention relates to a gravimetric metering device having the features set out in the preamble of the main claim.

In particular, the invention is used in a preferable though non-exclusive manner in the field of processing plastics materials.

Technological background

In the technical sector set out above, it is widely known to produce objects from polymer material from admixtures of different materials, which are prepared in accordance with recipes which define the precise relationship between the different materials.

These admixtures are configured in order to confer on the final product chemical/physical or mechanical characteristics which are adequate for the intended purposes for the use thereof and can provide for the combination of a consistent number of materials, including one or more polymer materials and one or more additives which may be both of an organic nature and of an inorganic nature.

In general terms, these materials can differ from each other as a result of any of their characteristics, such as, for example, the chemical composition or the form thereof or the degree of purity thereof or also the origin thereof.

By way of example, the materials to be mixed may be one or more polymers in granule form, a mineral filler and one or more additives, such as a stabilizing compound, an antioxidant compound or a colouring compound.

Furthermore, the polymers to be mixed can be of the same type, for example, PET or EVA, but may have different origins, such as, for example, virgin material or recycled material.

In order to prepare these admixtures, gravimetric metering devices are commonly used in the sector.

In general terms, these gravimetric metering devices comprise a plurality of hoppers which are all connected to a weighing unit which is positioned downstream thereof.

Each hopper contains a component of the admixture to be prepared and, in accordance with the type and the quantity of this component, may have a shape and dimensions different from the other hoppers.

The desired admixture is prepared by successively adding suitable quantities of the materials which come from the different hoppers in accordance with what is set out by the specific recipe.

In particular, the quantities of material from the single hoppers are measured by the weighing unit as a result, for example, of the provision of load cells, on which a receiving container, which is positioned under the discharging opening of the different hoppers, is supported.

In operational terms, once the recipe for the admixture to be prepared has been set and therefore the quantities of each single material which composes it have been calculated, a first hopper starts to charge the material contained therein into the weighing unit until the quantity of this material, measured with precision by the weighing unit, reaches the value predefined by the recipe. At this point, the connection between the first hopper and the weighing unit is closed and the connection between the weighing unit and a second hopper is opened so as to receive and weigh the desired quantity of the second material, and so on until the admixture is complete.

In this context, the Applicant has observed that, in known gravimetric metering devices, the material contained in the different hoppers is discharged into the weighing unit in accordance with two different methods.

In the first method, there is provision for the hopper to be connected to the weighing unit underneath by means of a sliding-gate valve.

In this manner, when the valve is opened, the material can pass rapidly from the hopper to the weighing unit, being able to complete the transfer of the quantity of material within relatively rapid times.

However, the Applicant has verified that this first discharge system does not allow the acquisition of high levels of precision in terms of the quantities of material discharged.

The second method provides for the hopper to be connected to the weighing unit underneath by means of a screw feeder.

In this case, the material from the hopper is transported by a screw as far as the inlet to the weighing unit. In this manner, by suitably sizing the screw and the rotation speed thereof, it is possible to control the quantity of material introduced into the weighing unit in a very precise and constant manner, and to respond rapidly to the instructions to end discharge.

However, the Applicant has verified that this second discharge system is very slow, which may lead to serious negative effects on the processing times.

In light of the particular characteristics of the discharge systems described briefly above, the Applicant has observed that the hoppers with a discharge system with a sliding-gate valve are used mainly to meter the materials at the highest proportions in the admixture to be prepared, wherein a lesser metering precision affects as a percentage to a lesser extent the total quantities of the discharged material, while the hoppers which are provided with a discharge system with a screw feeder are mainly used to meter the materials having the lowest proportions in the admixture to be prepared, wherein a high level of metering precision is instead required and wherein the penalization in terms of metering time becomes more acceptable.

However, the Applicant has noted that, during the production process of a number of types of articles, there are also required metering precision levels which are very high for materials which constitute a relevant proportion of the admixture to be prepared.

An example of such production processes is constituted by the sector of production of footwear, where together with a main polymer material there can be added a fraction of up to 30% or 40% of a polymer material which is recycled and which during the step of processing the polymer admixture gives rise to a considerable volumetric expansion and which consequently requires high levels of precision during the formation step of the admixture.

On the basis of what has been set out above, in this production application the recycled material would have to be metered by means of a hopper which is provided with screw feeders in order to comply with the strict margins of error allowed. However, this would unacceptably increase the production times.

In the present description and in the appended claims, the term "granular material" is intended to be understood to be a plurality of solid elements which are different and separate from each other and which have suitable dimensions and formation in accordance with the processing to be carried out and the material used, including the polymer material in powdered or flaked form.

The admixture is based on polymer material when at least 40% of the material of the admixture is formed by one or more polymer materials, preferably at least 60%.

The polymer material may be any suitable material which is normally used during processing operations for plastics materials. Preferably, it is a thermoplastic material, such as, for example, polyolefins, polyesters, polyamides and the copolymers thereof. Preferred examples of polymer materials are polyethylene terephthalate (PET) and the copolymer ethylenevinyl acetate (EVA).

Disclosure of the invention

The problem addressed by the present invention is to provide a gravimetric metering device and a method for forming an admixture based on polymer material which is structurally and functionally configured to at least partially overcome one or more of the disadvantages set out above with reference to the cited prior art.

This problem is solved by the present invention by means of a gravimetric metering device and a method for forming an admixture based on polymer material according to the appended claims.

Brief description of the drawings

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

Figure 1 is a schematic front view of a gravimetric metering device constructed according to the present invention;

- Figure 2 is a schematic side view of the gravimetric metering device of Figure 1;

- Figure 3 is a schematic rear view of the gravimetric metering device of Figure 1; and

- Figure 4 is a schematic plan view from above of the gravimetric metering device of Figure 1.

Preferred embodiment of the invention

With reference to the appended Figures, there is generally designated 1 a gravimetric metering device constructed according to the present invention.

The gravimetric metering device 1 is provided to form an admixture based on polymer material to be used in the context of processing plastics materials, preferably in the sector of producing footwear soles.

The gravimetric metering device 1 which is described herein as a preferred embodiment of the present invention forms part of a processing plant for plastics materials, for example, a production plant for footwear soles.

In particular, the gravimetric metering device 1 is preferably positioned upstream of a machine for converting the prepared polymer admixture, for example, a press mould, in which the polymer material is melted and moulded in order to form a desired article, such as, for example, a sole of a footwear item.

The gravimetric metering device 1 comprises a weighing unit 2 above which a first hopper 3, a second hopper 4, a third hopper 5 and a fourth hopper 6 are positioned, all being connected to the weighing unit 2 so as to be able to discharge a portion of the material contained therein into the weighing unit 2 when instructed.

In order to promote the descent of the material contained therein, each hopper 3-6 may be provided with a vibrating member, for example, of the pneumatic type.

The weighing unit 2 is conventional per se and comprises therein a container which is intended to receive the material from the hoppers located above. This container is advantageously supported on load cells so as to allow the precise measuring of the quantity of material introduced into the weighing unit 2. Furthermore, in the lower portion thereof, the weighing unit 2 may comprise a mixing member which is rotated by a motor 2a and which is intended to mix the components of the admixture, making it homogeneous.

The first hopper 3 is connected to the weighing unit 2 by means of a first valve 7 which is preferably of the sliding gate type and which is positioned at the opening of the discharge of the first hopper 3 inside the weighing unit 2. Furthermore, according to an aspect of the present invention, the first hopper 3 is further connected to the weighing unit 2 by means of a first screw feeder 8.

In particular, the first screw feeder 8 comprises a screw which extends inside a cylindrical pipe 9 and which is inclined upwards from the inlet of the screw up to the outlet thereof, and a motor 10 which is provided to rotate the screw. The inlet of the screw is connected to the first hopper 3 by means of a supplying pipe 11 which extends downwards and which is open in the first hopper 3 at a location upstream of the first valve 7.

The outlet of the screw is connected to a supplying pipe 12 which is open in the weighing unit 2 at a location which is different from the discharge of the first hopper 3 and which is controlled by the first valve 7 so as to be able to introduce material into the weighing unit independently of the opening or closing state of the first valve 7.

The second hopper 4 is connected to the weighing unit 2 by means of a second valve 13, which is preferably of the sliding gate type and which is positioned at the opening of the discharge of the second hopper 4, which is different from the discharge of the first hopper 3, inside the weighing unit 2.

The third hopper and the fourth hopper 5 and 6 are both connected to the weighing unit 2 by means of respective screw feeders 14 and 15 which in turn are both connected in terms of discharge to the supplying pipe 12 in order to supply the individual material inside the weighing unit 2.

The gravimetric metering device 1 further comprises a control unit (not illustrated) which is suitably programmed to receive instructions concerning the admixtures to be composed, in order to process the weight measurements which are received from the weighing device 2 and which is provided to control the operation of the first and second valves 7, 13 as for the first screw feeder 8 and the respective screw feeders 14 and 15.

The control unit may be specifically dedicated to the gravimetric metering device 1 or may be integrated in the control unit of the processing machine downstream thereof so as to allow the operator to control with a single instrument and a single interface both the processing machine (for example, the press) and the gravimetric metering device.

The gravimetric metering device 1 operates in the following manners.

There are charged in each of the hoppers 3-6 respective materials which are intended to form the desired admixture at the suitable ratios. Preferably, the admixture is based on polymer and comprises a first polymer material which is charged in the first hopper 3, a second polymer material which is charged in the second hopper 4, a third material which is charged in the third hopper 5 and a fourth material which is charged in the fourth hopper 6.

In a preferred embodiment, the first and second polymer materials are recycled EVA and virgin EVA, respectively, while the third and fourth materials are additives which are commonly used in the sector for the same applications. Once the recipe for the admixture to be prepared has been set, the control unit calculates (or receives as direct input from the operator) the respective quantities of the materials which compose it and, therefore, the quantity of the first, second, third and fourth materials which have to be discharged from the respective hoppers in order to correctly form the admixture. These quantities can be defined as target quantities of the individual materials to be discharged.

Purely by way of example, the following target quantities can be fixed: 25 g of the first polymer material, 72 g of the second polymer material, 2 g of the third material (first additive), 1 gramme of the fourth material (second additive).

The first polymer material has the characteristic of having a high expansion coefficient in the subsequent processing operations for the admixture, for which it is necessary for it to be metered with a high level of precision, for example, with a margin of error less than 0.2 g (or 0.2% with respect to the main material).

The control unit initially controls the discharge of the hopper which contains the material which has a greater proportion in the admixture, therefore in this case the second hopper 4. The discharge is carried out by controlling the opening of the second valve 13 and by waiting for the weight of the second material, which falls as a result of gravitational force into the weighing unit 2 and which is measured by the load cells, to reach the target quantity of 72 g. Once the target quantity of 72 g has been reached, the second valve 13 is closed in order to end the discharge of the second material.

The quantity of second material introduced into the weighing unit 2 is verified and, if it is different from the target quantity (an event which is possible given the very low precision of the discharge system with a sliding-gate valve), the target quantities of the remaining materials are recalculated so as to maintain the exact ratio required by the recipe.

At this point, the first polymer material which is contained in the first hopper 3 is metered.

In order to carry out this operation, an intermediate quantity which is less than the target quantity of 25 grammes is initially established by the control unit, for example, of approximately from 10 to 15%, therefore in this case approximately 22 g.

The first valve 7 is therefore controlled so as to open by the control unit in order to discharge a first portion of first polymer material equal to this intermediate quantity of 22 g.

This first portion of the first polymer material is therefore discharged rapidly by the first hopper 3 into the weighing unit which, when a weight equal to the intermediate quantity is measured, brings about the closure of the first valve 7. The real weight of first polymer material which is discharged from the first hopper 3 and which is measured by the measurement of the weighing unit is then verified and, on this basis, the quantity of first material which is missing in order to reach the target quantity is calculated precisely.

Once this missing quantity is defined, the first screw feeder 8 is actuated so as to introduce the first polymer material into the weighing unit 2 in a precise and controlled manner until the target quantity is reached.

The operation for forming the admixture is therefore completed by the subsequent metering of the third and fourth materials which are introduced into the weighing unit by means of the respective screw feeders 14 and 15.

As a result of the gravimetric metering device 1, therefore, it is possible to meter components of an admixture based on polymer in an extremely precise and generally rapid manner, increasing the efficiency and the productivity of the production plant and substantially reducing the quantity of processing waste.

The person skilled in the art will know how to apply suitable variants to the present invention as a function of any specific requirements, all being included within the scope of protection defined by the appended claims.

For example, the metering device may have any number of hoppers and any number of them may be provided both with a rapid discharge with a sliding- gate valve and with a discharge controlled by means of a screw feeder.