GB1604338A

GB1604338A - Screening device and method and apparatus for producing such a device

Info

Publication number: GB1604338A
Application number: GB23426/78A
Authority: GB
Original assignee: Hein Lehmann AG
Current assignee: Hein Lehmann AG
Priority date: 1977-06-18
Filing date: 1978-05-26
Publication date: 1981-12-09
Also published as: ES470763A1; DD137067A1; BE868175A; IT1104746B; AT366291B; ATA425378A; FR2394393A1; FR2394393B3; IT7849778A0; NL7806057A; DE2727427A1

Description

(54) A SCREENING DEVICE AND METHOD AND APPARATUS FOR PRODUCING SUCH A DEVICE (71) We, HEIN LEHMANN AKnENGEsELLscHAFT of Fichtenstrasse 75, D-4000 Dusseldorf, Germany (Fed.

Rep.); a German body corporate do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the follow ing statement:- The invention relates to a screening device and, in particular, to a screening device formed from metal wire coated with a high molecular weight thermoplastic plas tics material in the form of an elastomeric diisocyanate based polyester. Such a screen ing device is in the form of a fabric, mesh net of grid for drying, grading and/or clas sifying granular material which corrodes metal. The present invention further relates to a method and apparatus for producing such a screening device.

Wire screening devices in the form of fabrics, gauze, netting and lattices are widely used for dressing or preparation of minerals such as gravel, mineral ores and coal. The economy of such treatment is highly dependent upon a large screening area and the service life of the components or the screening device.

Known fabrics, gauze and lattices made of metal wires do, indeed have a large screen ing surface area. However, they frequently have only a short service life. In addition they must be inspected at regular, frequent intervals. Since however, the end of their service life is not readily noticeable, they are frequently replaced either too early, which is economically wasteful or too late, by which time the efficiency of the screening has been adversely affected.

To avoid the last-mentioned disadvan tages, it is known to provide the metal wires with a protective coating. The protective coating is extremely thin and is made from an organic thermoplastic resin. Meshes, fabrics and lattices made from such wire have not, however, proved successful for screening corrosive materials. In such screening devices, the metal wire must be able to absorb repetitive high tensile stresses and impulsive forces applied thereto by the material being screened. The coating only limits the abrasion of the wire by the material being screened for a short period of time.

Moreover, the accuracy and strength of the mesh produced therefrom often does not conform to the requirements of a user.

The present invention seeks to provide a screening device in the form of a fabric, mesh net or lattice having a highly accurate mesh size, and being relatively resistant to damage by the material being screened but which may be produced economically.

In accordance with the present invention, there is provided a screening device in the form of a fabric, mesh, net or lattice for draining, grading and/or classifying granular material, the service being formed from coated metal wire, the coating being in the form of a high molecular weight thermoplastic polymeric material which is an elastomeric diisocyanate-based polyester, the coating being firmly seated on the metal wire, wherein the thickness of the plastics material coating is between 0.3 and 5.0 times the diameter of the metal wire. The metal wire, in such a case, forms a support for the plastics material forming the coating or covering. This makes it possible for the plastics material forming the covering, when the coated wire is made into a screening device, to absorb the kinetic energy of the material being screened, so that metal wire absorbs only repetitive tensile stresses.

Furthermore, during the impact of the granules of material being screened against the covering, no high tension peaks occur, so that the abrasion resistence, and hence the life, of the screening device is favourably affected.

For screening particularly abrasive materials, such as mineral ores, coal or gravel, it has been found favourable for the thickness of the plastics material coating to be between 0.5 to 4.0 times the diameter of the metal wire.

The coating covering may be applied as a single coat, but may equally be applied in one or more layers as long as the coating is homogeneous.

It is obviously simpler to provide the coating in a single covering layer. However, irregularities in the circularity of the coated metal wire may be avoided if the plastics material is applied in two of more layers. It is particularly desirable to apply the coating in two or more layers if the ratio of the diameter of the coating to the diameter of the metal wire is greater than 3 and the diameter of the metal wire is less than 1 mm or if the above ratio is greater than 2 and the diameter of the metal wire is in excess of 1 mm.

It is particularly advantageous if the plastics material has a smooth exterior surface.

A screening device in the form of a fabric, mesh, net or lattice having a high mesh accuracy and strength and a high degree of stability is produced from coated metal wire and it will be apparent that the plastics material forming the coating must have a high adhesion abrasion, high abrasion resistance and good flexibility at low temperature.

Due to its high degree of elasticity, which means that it can absorb impacts and that it has a high abrasion resistance, an elastomeric diisocyanate-based polyester is used as the plastics material since it retains its relevant properties in the temperature range of-40 C to 80"C.

The plastics material may have added thereto additives for providing it with resistance against the effects of ultra-violet radiation and hydrolysis, if the plastics material itself does not have these properties. This prevents premature embrittlement or swelling of the plastics material by light or water.

It is also desirable if the metal wire is centrally located in the plastics material.

n contrast to lattices made solely from metal wire by weaving, in which the wires forming the warp are pre-crimped to 30% and the weft wires are pre-crimped to 70% before being processed on a lattice loom, the coated metal wire used to form the screening device in accordance with the present invention may be pre-shaped in such a manner that the warp and weft forming wires produced therefrom are each precrimped by 50%. The pre-crimping is of course, effected after the coating has been applied to the wire.

If the coated metal wire is made into a screening device in the form of a fabric on a loom having a plug arm or similar device, a high mesh accuracy and mesh strength may be attained by slightly pre-crimping the weft-forming wires and by not crimping the warp wire.

If, on the other hand, the coated metal wire is made into a wire fabric or mesh on a loom having a picker, the warp and weft wires may be left uncrimped. It is thus possible to use a uncoated metal wire of relatively large overall diameter for interweaving.

In a further aspect, the present invention provided a method of producing such a screening device wherein metal wire is preheated two a temperature of 100" to 110 C and is then supplied to a transverse extrusion head incorporating a nozzle region, a thermoplastic plastics material in the form of an elastomeric diisocyanate based polyester being simultaneously supplied to the nozzle region under the influence of pressure and heat so as to be molten when applied to the wire, the thus-coated metal wire there being formed into a screening device. By carrying out such pre-heating, the plastics material does not become chilled on contact with the surface of the metal wire and air residues possibly drawn in therewith are no longer able to escape to the rear. Because of the elevated wire temperature, the adhesion of the plastics material on the surface of the wire is improved, since the pressure coating in the transverse extrusion head is carried out at a temperature at which the plastic material is fully molten. To achieve really good adhesion between the wire and the plastics material it is extremely desirable that the surface of the wire should be free from grease and is dry.

The adhesion between the coating and metal wire may be still further improved if the coated metal wire, directly after leaving the nozzle region, is thoroughly and rapidly cooled to substantially ambient temperature. Thus, the cooling agent used is so selected, and is supplied at such a temperature and speed, that the heat dissipated from the exposed surface of the coating, which is dependent upon the heat conductance of the plastics material, is continuously and speedily removed and is not allowed to build up.

It is therefore desirable to effect the cooling by spraying the coated wire with water.

A smooth exterior surface for the plastics material coating may be achieved if the nozzle region is maintained at a temperature at least 5%, preferably 10 to 15%, lower than the composition temperature of the plastics material during the coating stage.

It has been found that a homogenous and satisfactory coating may be achieved if the exact high extrusion pressure necessary for the pressure coating is maintained in the orifice region. This pressure, which must be maintained adjacent the discharge end of the nozzle, is more difficult to maintain with thicker coating thicknesses of the plastics material. This problem particularly arises when plastics materials of very low viscosity, that is to say, plastics material which are highly fluid at their melting or softening temperature, are being used. This particularly applies to polyesters. In order, nevertheless, to permit the coating to be applied in one operation, it has been proved particularly desirable, when carrying out such a process to suitably dimension the nozzle portion.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a sectional view through a coated metal wire from which a screening device in accordance with the present invention is made, and Fig. 2 is a transverse sectional view through an extrusion head for producing coated metal wire.

In Fig. 1 a metal wire 1 is shown embedded in a covering coating 2, the covering 2 having a thickness 3 which is between 0.3 and 5.0 times the diameter 4 of the metal wire, and is preferable 0.5 to 4.0 times such diameter. The diameter of the coated metal wire is of course, equal to the sum of the diameter 4 of the metal wire and twice the coating thickness 3. The surface 5 of the covering coating is smooth and has a surface roughness of between 5 and 10 . The metal wire 1 is preferably a round, bright-drawn spring steel wire having a diameter of from 0.3 to 5 mm. Such coated metal wire is then processed into a screening device such as a fabric, mesh net or lattice for draining or drying, grading and/or classifying granular material such as gravel, sand, coal and mineral ores. The material being screened, and thus, impacting the screening device, generally has a high kinetic energy and so tends to cause a high degree of abrasion and sets up considerable dynamic and static stresses.

These stresses are absorbed by the coating or covering. The covering thus substantially absorbs the impact forces and, due to its abrasion resistance, substantially reduces wear of the screening device.

Before being made into a screening device, coated metal wire has, hitherto, usually been straightened. To obtain a high degree of mesh accuracy and strength, the coated metal wires are usually pre-crimped to some extent. Thus, if the mesh is to be made on a lattice loom, it has been found favourable for the wires which will form the warp and the weft to each be pre-crimped by 50%. If a loom having a plug arm or similar device is used, it has been found that, utilising the above-described coated wires present invention, the wires which will form the warp should not be crimped and those which will form the weft should only be slightly crimped. In such a case, both the warp and wett wires are crimped by the tensile torces set up during inter-weaving. During such pre-crimping, it should be ensured that, for a predetermined mesh width, the pitch is reduced by from 5 to 30% and the effective crimping depth is increased by from 30 to 90%, depending upon the thickness of the covering. This applies similarly to wire lattices. If the coated metal wire is to be made into a screening device by interweaving on a loom with a shuttle, neither the warp nor the weft wires need be pre-crimped. Coated metal wires having a comparatilvely large overall diameter may be processed on such looms.

The coated metal wire may be processed within wide limits. Its possibilities of use in screening devices are manifold due to its high bearing capacity and high abrasion resistance. The screening devices, which are in the form of wire meshes, fabrics and lattices may advantageously be provided with stressing or tensioning devices. The warp and weft wires in such screening devices are in contact with one another without clearance. At the intersections they are in firm and immovable abutment with one another, which improves the bearing capacity and wear resistance of the screening device. The composite effect is to provide highly elastic, considerably torsion resistant structure in the temperature range in which such a device is likely to be used, that is to say, in the temperature range from 40 to 80 C.

Such screening devices are thus highly suited for use in the open air. They have a high alternating bending strength, with satisfactory adhesion between the metal wire and its covering. They have a high abrasion resistance and a high notch toughness relative to fissuration due to deformations.

The low specific gravity of the plastics material provides a considerable saving in weight in comparison with screens of identical mesh size made solely from steel. The elasticity of the plastics material, especially if it is an elastomeric polyester, provides the screening device with a recoiling action which improves the acceleration and the conveyability of the material being screened.

Fig. 2 shows a transverse section through an extrusion head, for producing coated metal wire. The extrusion head comprises a body portion 6. An adaptor 7 is used for supplying the plastics material, such as an elastomeric polyester 7. In the body portion 6, the metal wire to be coated is guided in a spindle sleeve 9. The spindle sleeve 9 leads into a nozzle device 10 which includes a nozzle region 11. The nozzle region 11 has a generally conical portion 12 adjacent the spindle sleeve 9, which merges into a cylindrical nozzle region 13. The length of the cylindrical nozzle region 13 is from 10 to 40 times, preferably 20 to 30 times, the thickness 14 of the covering coating 15. The length (12 + 13) of the nozzle region 11 is selected in dependence upon the flow prop erties of the plastics material, the extrusion speed, the desired thickness of the coating or covering and the desired diameter of the completed coated metal wire. In the extrusion head, there may be provided temperature gauging devices for measuring the composition temperature of the plastics material. The nozzle device 10 may include a cooling and/or heating device, not shown for regulating its operating temperature.

The arrow shown in the drawings indicates the direction of travel of the metal wire 8.

The extrusion head coats the wire by pressure coating.

This transverse extrusion head is particularly suitable for use during the coating of a metal wire by an extrusion process. During the production of the coated wire, the metal wire is supplied at high speed, that is to say, at a speed which is of the order of 200 metres per minute, to the extrusion head. At the same time, an extrusion screw presses the plastics material, such as an elastomeric polyester through the adaptor 7 into the nozzle region 11 of the device 10. For an elastomeric polyester, the pressure is between 250 and 350 bar. The length of the cylindrical portion of the nozzle enables the necessary backwash pressure to be exerted on the extrusion screw. The backwash pressure is of considerable importance when dealing with highly fluid plastics material at its melting or softening temperature. The metal wire is pre-heated to 1000 to 11 00C and, directly after leaving the orifice 10, is rapidly but thoroughly, cooled back to substantially environment temperature by means of, for example, water. The rapid cooling of the plastics material both inwardly and outwardly though prevents after-annealin of the plastics material as, for example, is effected when producing electric cables. However, stresses are produced in the plastics material and "frozen in" by such cool prig. This, when using high molecular weight plastics material, causes strong adhesion between the covering coating and the metal wire.

Strong adhesion is achieved when there are substantially no capilliary passages or any other type of cavity between the metal wire and the coating and is an essential condition for a long service life. The completed coated metal wire may be wound, in a conventional manner, onto drums or into rolls.

The stability, mesh size and mesh strength of the fabrics, meshes and lattices produced from such coated metal wire may be further improved if the external surface of the coating is smooth and has a good abrasion adhesion. It is particularly desirable to accurately control the orifice temperature, because the quality of the plastics material coating is affected thereby. A high gloss on the external surface is generally obtained by afterheating the coating in the orifice. Extensive tests, however, have shown that excessively high temperature after-heating adversely affects the plastics material. An explanation for this is that excessive local heating destroys the molecular structure of the plastics material. This causes the plastics material to lose its abrasion resistance properties. This may be avoided if the nozzle region 10 is kept at a temperature which is between 10 and 15% below the composition temperature during the coating stage. If an elastomeric polyester is used as the plastics material, the optimal composition temperature during the coating stage is about 235"C and the temperature of the nozzle region is about 220"C. During coating of the metal wire, it is particularly important to maintain a constant optimum composition temperature. The optimum composition temperature lies in the region in which a substantially homogenous coating of the wire is achieved without causing over-heating of the plastics material, which can lead to a destruction of the molecular structure.

Careful monitoring of the composition temperature and the temperature in the nozzle region is therefore of substantial importance for producing a coating having the desired properties.

WHAT WE CLAIM IS: 1. A screening device in the form of a fabric, mesh, net or lattice for draining, grading and/or classifying granular material, the device being formed from coated metal wire, the coating being in the form of a high molecular weight thermoplastic polymeric material which is an elastomeric diisocyanate based polyester, the coating being firmly seated on the metal wire, wherein the thickness of the plastics material coating is between 0.3 and 5.0 times the diameter of the metal wire.

2. A screening device as claimed in claim 1, wherein the thickness of the plastics material coating is between 0.5 and 4.0 times the diameter of the metal wire.

3. A screening device as claimed in claim 1 or 2 wherein the coating is applied in a single layer.

4. A screening device as claimed in claim 1 or 2 wherein the coating is applied in a plurality of layers, the coating being homogenous.

5. A screening device as claimed in any one of claims 1 to 4 wherein the plastics material coating has a smooth exterior surface.

6. A screening device as claimed in any preceding claim wherein the plastics materlal is resistant to decomposltion by ultraviolet radiation or has an additive admixed therewith for making the material resistant to decomposition by ultra-violet radiation.

7. A screening device as claimed in any preceding claim wherein the plastics material is resistant to hydrolysis or has an additive admixed therewith to make the material

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. or covering and the desired diameter of the completed coated metal wire. In the extrusion head, there may be provided temperature gauging devices for measuring the composition temperature of the plastics material. The nozzle device 10 may include a cooling and/or heating device, not shown for regulating its operating temperature. The arrow shown in the drawings indicates the direction of travel of the metal wire 8. The extrusion head coats the wire by pressure coating. This transverse extrusion head is particularly suitable for use during the coating of a metal wire by an extrusion process. During the production of the coated wire, the metal wire is supplied at high speed, that is to say, at a speed which is of the order of 200 metres per minute, to the extrusion head. At the same time, an extrusion screw presses the plastics material, such as an elastomeric polyester through the adaptor 7 into the nozzle region 11 of the device 10. For an elastomeric polyester, the pressure is between 250 and 350 bar. The length of the cylindrical portion of the nozzle enables the necessary backwash pressure to be exerted on the extrusion screw. The backwash pressure is of considerable importance when dealing with highly fluid plastics material at its melting or softening temperature. The metal wire is pre-heated to 1000 to 11 00C and, directly after leaving the orifice 10, is rapidly but thoroughly, cooled back to substantially environment temperature by means of, for example, water. The rapid cooling of the plastics material both inwardly and outwardly though prevents after-annealin of the plastics material as, for example, is effected when producing electric cables. However, stresses are produced in the plastics material and "frozen in" by such cool prig. This, when using high molecular weight plastics material, causes strong adhesion between the covering coating and the metal wire. Strong adhesion is achieved when there are substantially no capilliary passages or any other type of cavity between the metal wire and the coating and is an essential condition for a long service life. The completed coated metal wire may be wound, in a conventional manner, onto drums or into rolls. The stability, mesh size and mesh strength of the fabrics, meshes and lattices produced from such coated metal wire may be further improved if the external surface of the coating is smooth and has a good abrasion adhesion. It is particularly desirable to accurately control the orifice temperature, because the quality of the plastics material coating is affected thereby. A high gloss on the external surface is generally obtained by afterheating the coating in the orifice. Extensive tests, however, have shown that excessively high temperature after-heating adversely affects the plastics material. An explanation for this is that excessive local heating destroys the molecular structure of the plastics material. This causes the plastics material to lose its abrasion resistance properties. This may be avoided if the nozzle region 10 is kept at a temperature which is between 10 and 15% below the composition temperature during the coating stage. If an elastomeric polyester is used as the plastics material, the optimal composition temperature during the coating stage is about 235"C and the temperature of the nozzle region is about 220"C. During coating of the metal wire, it is particularly important to maintain a constant optimum composition temperature. The optimum composition temperature lies in the region in which a substantially homogenous coating of the wire is achieved without causing over-heating of the plastics material, which can lead to a destruction of the molecular structure. Careful monitoring of the composition temperature and the temperature in the nozzle region is therefore of substantial importance for producing a coating having the desired properties. WHAT WE CLAIM IS:

1. A screening device in the form of a fabric, mesh, net or lattice for draining, grading and/or classifying granular material, the device being formed from coated metal wire, the coating being in the form of a high molecular weight thermoplastic polymeric material which is an elastomeric diisocyanate based polyester, the coating being firmly seated on the metal wire, wherein the thickness of the plastics material coating is between 0.3 and 5.0 times the diameter of the metal wire.

resistant to hydrolysis.

8. A screening device as claimed in any preceding claim wherein the metal wire is centrally located in the plastics material.

9. A screening device as claimed in claim 1 substantially as hereinbefore described.

10. A screening device as claimed in any preceding claim wherein the device is produced by weaving coated metal wires, the coated metal wire constituting the warp and the coated metal wires constituting the weft each being pre crimped by 50% prior to weaving.

11. A screening device as claimed in any one of claims 1 to 9 wherein the device is produced by weaving coated metal wires, the coated metal wires constituting the weft being slightly pre-crimped and the coated metal wire constituting the warp being uncrimped.

12. A screening device as claimed in any one of claims 1 to 9 wherein the device is produced by weaving coated metal wires, the coated metal wire constituting the warp and the coated metal wires constituting the weft each being uncrimped.

13. A method of producing a screening device as claimed in claim 1 wherein metal wire is preheated to 100" to 1 100C and is supplied to a transverse extrusion head incorporating a nozzle region, a thermoplastic plastics material in the form of an elastomeric diisocyanate-based polyester being simultaneously supplied to the nozzle region under the influence of pressure and heat so as to be molten when applied to the wire, the thus coated metal wire then being formed into a screening device.

14. A method as claimed in claim 13 wherein the metal wire is supplied in a grease-free and dry state.

15. A method as claimed in claim 13 or 14 wherein the coated metal wire is thoroughly and rapidly cooled to at least substantially ambient temperature immediately upon leaving the nozzle region.

16. A method as claimed in any one of claims 13 to 15 wherein the nozzle region is maintained at a temperature at least 5% lower than the composition temperature of the plastics material during the application of the coating.

17. A method as claimed in claim 16 wherein the temperature in the nozzle region is between 10 and 15% lower than the composition temperature of the plastics material during the application of the coating.

18. A method of producing a screening device substantially as hereinbefore described.

19. A screening device whenever produced by a method as claimed in any one of claims 13 to 18.