AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT WIPING EXCESS COATING FROM HOT DIP METAL COATED MESHES The following statement is a full description of this invention, including the best method of performing it known to me: WIPING EXCESS COATING FROM HOT DIP METAL COATED MESHES INTRODUCTION This invention relates to a novel method and an apparatus for wiping and controlling excess metallic coating layers and coat weights on long products or substrates in the form of mesh or net 5 or fabric (these product names are used interchangeably in this specification) in continuous hot dip metal coating operations like galvanising. The invention is effective and useful in coating operations of meshes in various sizes and shapes with metals and alloys, such as tin, aluminium, zinc, lead, copper and alloys thereof. A protective metal-coated finish is required on the product for reasons like protection against corrosion in normal use. This invention is characterised by 10 mechanical wiping forces of electromagnetic interactions generated within the liquid metallic coating layers on the running mesh outside the hot dip bath through the process of electromagnetic induction applied in a particular manner. The practical workability of this invention is due to the good electrical conductivities of various coating metals at the operating conditions as required for adequate coupling in the induction process. 15 Electromagnetic wiping, or wiping with mechanical forces of electromagnetic interactions, as stated above, of long hot dip metal-coated substrates of constant or uniform cross sections, which are essentially cylindrical, such as wires and strips, after their exit from the hot dip bath is well known and well established in the continuous hot dip galvanising practice. This invention is 20 particularly useful in electromagnetic wiping of long hot dip metal coated substrates of non constant or non-uniform cross sections, such as mesh or net or fabric, which are non-cylindrical or of varying geometries in cross sections, after their exit from the hot dip bath. This invention is useful for hot dip metal coating of "wire net" or "wire mesh" and the like. Such 25 nets and meshes are generally prefabricated with uncoated wires through processes like crimping, weaving, linking, knitting, twisting, stranding and spot welding to make a long fabric. Some meshes are also prefabricated from sheets or strips by perforating them all over the surface. Yet another such product is generally known as "Expanded Wire Mesh" or "Expanded Sheet Mesh" or simply "Expanded Mesh" and is generally prefabricated by partially and intermittently 30 shearing or slitting long uncoated sheets or strips along their widths and lengths and stretching 2 and expanding the slits in lateral directions. This invention and its guiding principles described herein are applicable and useful for all kinds of long and substantially flat nets, meshes and fabrics in all sizes and geometries and of all materials coated with all kinds of coating metals and alloys thereof in the molten or liquid state. However, for brevity, this invention is substantially 5 described herein for the most commonly manufactured galvanised steel nets consisting of wires or wirelike links. The use of zinc as the coating metal is only illustrative. PRIOR ART IN ELECTROMAGNETIC WIPING 10 Prior patents known to the applicant in the field of Electromagnetic Wiping are listed below. 1. Indian Patent # 156939 dated 24/12/1983 2. Indian Patent # 186942 dated 12/06/1997 3. United States Patent # 4,228,200 dated 14/10/1980 15 4. United States Patent # 4,273,800 dated 16/06/1981 5. United States Patent # 3,518,109 dated 30/06/1970 6. United States Patent # 4,033,398 dated 05/071977 7. German Patent # 2202764 dated 11/01/1972 8. German Patent # 3008207 dated 11/09/1980 20 9. Belgian Patent # 739,130 dated 19/09/1969 All the patents listed above, except No. 3, claim and describe suitability for cylindrical products like wires and strips only. No. 3, i.e. US patent # 4,228,200, claims its suitability for nets or meshes along with wires and strips but, as per the claims therein, the electromagnetic wiping 25 device is essentially and at least partly immersed in the molten zinc bath, thereby essentially including part of the bath in the electromagnetic induction and interaction phenomena used for wiping and coat weight control. The present invention differs from this in that the wiping device is located entirely outside and well away from the molten zinc bath, thereby excluding the bath from the electromagnetic induction and interaction phenomena occurring on the travelling coated 30 mesh for wiping and coat weight control. Apart from this, the present invention differs from the prior art in many ways, the main distinctive difference being that the wiping devices and systems in the present invention are specially designed to suit various types of meshes in their existing 3 galvanising practices, taking into account their non-cylindrical shapes, which has not been done in prior art. WORKING OUTLINE OF PRESENT INVENTION 5 As per this invention, the net with the liquid metal coating layers thereon continuously emerges from the bath, and thereafter travels through a zone of an effectively localised externally applied alternating magnetic field having non-zero components of required strengths parallel to various wire links constituting the net. In a preferred embodiment of this invention, such conditions are 10 achieved by having the travelling net, after its emergence from the bath, run freely, i.e. without physical or electrical contact, through a loop of electrical conductors, such as copper bars or tubes, in which an alternating current of required frequency, waveform and magnitude is maintained with the help of a variable electrical or electronic power source. Under such conditions, alternating currents are induced in the liquid coating layers on the net while in the 15 vicinity of the conductor loop. The interactions between the existent alternating magnetic flux and the induced currents in the liquid coating layers generate net or effective mechanical forces acting in all directions away from the conductors of the loop and this effects rejection, through squeezing and wiping, of the entrained excess coat weight or coating mass on the net. The wiped away excess coating mass returns to the bath under the action of gravity. 20 In another preferred embodiment, the intensity of the wiping forces is further enhanced, through an increase of the magnetic flux density for the same magnitude of current in the said conductor loop, by substantially covering the conductors forming the loop with soft ferromagnetic cores on all outer sides except directly in between the conductors and the coated net or mesh. Examples of 25 such cores are 'Horse Shoe' or 'C' shaped Ferrites, Tape Wound and Cut Toroids, Stacked Laminations and Iron Powder Compacts. In order to achieve satisfactory wiping on all the links of the net and junctions thereof, the entire wiping device consisting of the assembly of conductors and cores is preferably located adequately 30 apart from the bath level in order not to subject the bath to an appreciable degree of magnetic induction, which would interfere with the electromagnetic wiping phenomena occurring in the coating layers on the running net while within the said device. Furthermore, the entire wiping 4 device is located and oriented above the bath level in such a position and manner that the coating layers entraining the travelling net remain at around the same temperature as that of the bath and that no wire links of the net are entirely parallel to the conductors of the loop while passing through the said device. 5 In another preferred embodiment, the net travelling between the bath level and the conductor loop is kept surrounded by an inert gaseous atmosphere that does not rapidly oxidise the coating layer and that is maintained at around the same temperature as the bath. Examples of suitable inert gases are oxygen-depleted air or nitrogen, argon and other gases that do not cause rapid oxidation 10 of the coating metal at the operating conditions. The cross-sectional geometry of the conductor loop and accompanying cores and their positioning relative to the running mesh have marked effects on the wiping intensity that can be achieved. For example, the gap between the opposing conductors, the conductor and core span heights or 15 widths facing the net, and the angles between the conductors and various links of the net all affect the extent of wiping action. Furthermore, the frequency, waveform shape and magnitude of the loop current also affect the wiping intensity that can be achieved. Increasing the current increases the intensity of wiping forces and thus results in decreasing the retained coat weights on the product. The loop current magnitude thus provides a stable and repeatable control variable for 20 controlling the obtainable coat weight. The wiped away liquid coating mass drains off backwards along the lengths of the running links of the net under the action of gravity and eventually returns to the bath. APPLICATION IN CONVENTIONAL MESH GALVANISING 25 In the contemporary continuous hot dip mesh galvanising process, a long mesh is continuously cleaned and longitudinally drawn through and out of a bath of molten zinc. The nascent surface of the mesh, freshly exposed due to cleaning, bonds with the zinc metal in the bath, forming a thin Iron/Zinc alloy layer. The bonded layer also exerts a viscous drag on the surrounding liquid 30 free zinc in the onward direction. Once outside the bath, part of the entrained liquid coating mass returns or drains back to the bath under gravity while the remaining mass is supported and dragged onward against gravity due to its viscosity. The drag force is generated at the iron/zinc 5 interface due to chemical bonding and also at the outer surface of the liquid coating layer due to its surface tension. In due course, the liquid zinc layer, which is normally much thicker than the chemically bonded layer, freezes onto the mesh. The total mass of zinc retained per unit surface area of the ungalvanised product is generally known as the 'coat weight' or the 'zinc carry' or the 5 'zinc pick-up' or simply the 'pick-up' in the particular galvanising process. The coat weight is also sometimes expressed as the average thickness of the coating layer on the product in solid state. After its exit from the bath, the mesh runs in a substantially planar and ascending manner either 10 vertically upwards or at an acute angle to the horizontal, mainly in order to facilitate draining off of the excessively entrained liquid coating mass back to the bath under the action of gravity. Nets and meshes consist of different patterns of wires or wirelike links linked or joined together at various angles. For example, square or rectangular meshes contain wires that are parallel to the long side of the mesh as well as wires that are perpendicular or transverse to the long side. Excess 15 zinc can drain back to the bath easily along the parallel wires under gravity. However, liquid zinc cannot drain away easily from the transverse links, when they are horizontal, owing to the high tension on its outer free surface, which prevents drop formation required for drainage. For this reason, on some galvanising plants or lines, a square or rectangular mesh is made to exit the bath in a direction oblique to its normal straight course of travel on the line, thus orienting the 20 longitudinal as well as the transverse links at acute angles to the vertical, in order to facilitate draining off of the excessive coating mass from all the links under gravity - refer to FIG. 9. In modern manufacturing practice, attempts are constantly made to increase the speeds of wire nets through the hot dip bath for reasons of higher productivity and lower costs of production. It 25 is also known that the zinc coat weight on any constituent wire link in the hot dip process increases as the speed of the wire net increases; and at speeds desirable and otherwise attainable in the manufacturing practice, the obtainable coat weights far exceed the minimum specification. It becomes desirable to wipe off or squeeze off the excess coating metal from the running wire net and recover it to the hot dip bath to save material and costs. Also, for enhanced product 30 quality and marketability, uniform and even distribution of the coating mass, giving a smooth surface, is desirable. This invention aims at achieving both of these requirements (coat weight control and uniform distribution) in the manufacturing practice through suitably designed wiping 6 systems and devices. Furthermore, this invention can be suitably applied to all the existing modes of exit of the wire net from the zinc bath without having to modify any of the existing set up except replacing the current wiping devices by the wiping devices and systems as per this invention. 5 Until now, some wiping methods involving physical and chemical means have been developed and enjoyed limited success. These include: * Providing additional external heating to the net exiting the bath e Maintaining inert or oxygen free atmosphere around the exiting net 10 * Maintaining a bed of crushed charcoal around the exiting net e Blowing air or nitrogen on the exiting net * Providing brushing or squeezing rollers on the exiting net e Treating the exiting net with surface active chemical fluxes 15 These methods have been mainly known to 'break' the external surface film of the liquid coating layer outside the bath and thus reduce the amount of coating mass that can be supported. However, these methods have very little effect or influence on the liquid coating layer supported by the constituent wire link's bonded surface. This invention effectively breaks the external surface film of the liquid coating layer outside the bath and also effectively reduces the coating 20 mass supported by both the surfaces (refer to FIG. 2) to any desired extent and at any desired speed of the mesh running through the bath. This invention is further described and explained with the help of some schematic figures, listed below, relating to some preferred embodiments. However, the drawings herein in no way limit 25 the scope of this invention. Moreover, in the said drawings, same numerals and letters are used to denote the same parts and entities. LIST OF FIGURES ATTACHED 30 FIG. I depicts a sectional elevation of the hot dip bath and the galvanised mesh. FIG. 2 shows a sectional view of a travelling wire link and its liquid coating layer. FIG. 3 depicts an isometric view of the basic wiping system as per this invention.
7 FIG. 4 depicts in section electromagnetic wiping phenomena in a galvanised wire link. FIG. 5 depicts four cases of electromagnetic wiping on various wire links and joints. FIG. 6 depicts a preferred wiping arrangement for 'Hexagonal Mesh' in normal vertical exit from the bath. 5 FIG. 7 depicts a preferred wiping arrangement for 'Chain Link Mesh' and 'Expanded Wire or Sheet Mesh' in normal vertical exit from the bath. FIG. 8 depicts a preferred wiping arrangement for 'Square Mesh' or 'Rectangular Mesh' in normal vertical exit from the bath. FIG. 9 depicts a preferred wiping arrangement for 'Square Mesh' or 'Rectangular Mesh' in an 10 oblique vertical exit from the bath. FIG. 10 depicts in elevation a preferred modified wiping device and its arrangement for a long and very wide square or rectangular mesh in normal vertical exit from the bath. DETAILS OF FIGURES AND RELATED ELECTROMAGNETIC WIPING PHENOMENA 15 1. Conventional Mesh Galvanising FIG. I depicts a vertical sectional elevation of the hot dip bath and the galvanised mesh, the section being longitudinal through the bath and mesh in normal galvanising practice. In this 20 figure, the mesh is shown at '1'. This mesh runs through the bath at a constant velocity shown by an arrow marked 'V'. The mesh enters the bath level '3' at an entry point, runs through the bath '2' around a sunk roller '40' or a similar such device and exits the bath level 3 either at an exit point 'El' in a vertical take up mode or at another exit point 'E2' in the so called horizontal take up mode. The darkened menisci at El and E2 are indicative of liquid coating mass returning to 25 the bath under the action of gravity. In the vertical take up mode, the exiting net, shown at '1-', generally subtends a right angle with the bath level 3; while in the horizontal take up mode, the exiting net, shown at '1-2', subtends an acute angle with the bath level, as shown at 'a', which is generally less than 45 degrees for operational convenience on the plant. 30 Also in FIG. 1, the coating layer on the galvanised net is shown at '4', and the backward and forward directions on the galvanise line are shown by arrows marked 'Lb' and 'Lf respectively at any arbitrary point marked 'P". The gaseous atmosphere covering the exiting net is marked '5' 8 and the action of gravity is indicated with arrows marked 'g'. As can be expected, the coating layer 4 remains in a liquid state for some time from the exit of the wire net from the bath and all the wiping needs to be carried out during this time. The spatial zones in which the coating layer remains liquid above the bath are shown by the region marked 'RI' for the vertical take up mode 5 and a similar region marked 'R2' for the horizontal take up mode; both regions being shown by broken outlined rectangles. As per this invention, the wiping device needs to be located anywhere in the regions RI and R2 for the respective take up modes. In the vertical take up mode, the liquid coating layer 4 on the exiting net is acted upon by gravity g directly in the backward direction and in the horizontal take up mode, a component of gravity acts on the liquid 10 coating layer in the backward direction, which component is shown by the arrow marked 'gc'. Both g and gc act to facilitate draining off of excess liquid coating back to the bath on all the constituent wire links of the running net that are not horizontal or parallel to the bath level 3. The horizontal and vertical take up modes are basically similar, as far as wiping and recovering 15 the excess coating mass to the bath is concerned. Therefore, for brevity, reference will be made mainly to the vertical take up mode in the rest of this specification, assuming that similar action can be expected in case of the horizontal take up mode. Also in FIG. 1, the onward and backward directions of travel of the net are shown by arrows 20 marked 'Ow' and 'Bw' respectively. As stated earlier, yet a third mode of exit exists in the manufacturing practice for some rectangular or square nets, wherein the travelling net makes an oblique sideways turn from the normal line direction at its exit from the bath. In this case, the onward and backward directions of travel shown at Ow and Bw respectively would be oblique to the vertical. This arrangement is shown in greater detail in FIG. 9. 25 2. Details of Zinc Pick Up Phenomena FIG. 2 shows a partial sectional elevation of a vertically oriented coated wire link at an arbitrary region marked 'R3' in FIG.1, wherein the coating layer is in a liquid state. In this figure, the 30 interface between the constituent wire I and the coating layer 4 or the chemically bonded layer is shown at '1/4' and the interface between the coating layer 4 and the surrounding atmosphere 5 or the coating layer outer surface is shown at '4/5'. The coating outer surface 4/5 behaves like an 9 elastic sack or membrane under tension. The tension is generally known as surface tension and is shown by arrows marked 'ST' at some arbitrary point marked 'Sip'. Depending on its reaction with the atmosphere 5 and the prevalent temperature, the tension on the coating layer outer surface 4/5 changes from place to place in its course of onward travel. 5 In this figure, the velocity distribution in the liquid zinc layer 4 dragged or entrained by the travelling wire I is shown at an arbitrary line 'M-N' which is normal to the onward travelling wire and coating layer. The outer surface 4/5 and the wire/coating interface 1/4 together support and drag the liquid coating mass 4 within against gravity in the onward direction by virtue of the 10 liquid coating viscosity. It can be envisaged that the pattern of velocity within the free zinc layer is of a magnitude equal to or less than V at the outer surface 4/5, which progressively attenuates to a certain minimum, shown by arrow 'Vm', from the surface inwards and again progressively increases to V at the interface 1/4. This velocity distribution flattens as the wire and the coating mass travel onwards (due to increased viscosity as the coating mass cools) and eventually all the 15 coating mass attains the velocity V upon solidification. Before solidification, at a height or distance where the weight of the supported liquid coating column exceeds the surface tension ST, the coating surface 4/5 ruptures, shedding off its supported coating mass, which tends to return to the bath under gravity. Depending upon the 20 cooling and oxidising conditions outside the bath, which mainly control the coating outer surface tension ST and which can be heterogenous or unsteady, the rupturing and reformation of the coating outer surface 4/5 can happen in an unsteady but repetitive manner in steady state operations of mesh galvanising, resulting in an array of coating mass lumps on the galvanised product. This phenomenon is commonly known as "Bambooing Effect". 25 As per this invention, a considerable amount of heat is generated in the substrate and the coating mass in the vicinity of the Electro-Magnetic Wiping device due to eddy current heating and magnetic hysteresis. As a result, the temperature of the coating layer and its outer surface 4/5 is higher within the wiping zone. This, together with the atmosphere 5 around the coating being 30 neutral and oxygen free, results in lowering of tension on surface 4/5 within the wiping zone. Therefore, the coating outer surface 4/5 is preferentially created after the wiping action is complete (where the surface tension is the least) and hence does not contribute to any support for 10 additional coating pick-up from the bath. This avoids the "Bambooing Effect" and renders the final coating surface smooth. 3. Basic Electromagnetic Wiping System 5 FIG. 3 is an isometric view of the basic wiping system as per this invention for any hot dip galvanised wire net exiting the zinc bath in a vertical take up mode. For clarity, a rectangular portion of the running net immediately above the zinc bath 2 is marked at its four corners as ' ', '12', '13' and '14'. To clarify further, the line joining the corners 11 and 12 lies in the bath level 10 3, and this line would appear as the exit point El in FIG. 1. The basic wiping device consists of two electrical conductors marked '6' and '7', and two sets of 'Horse Shoe' or 'C' shaped soft ferromagnetic cores, marked '8' and '9', which substantially and adequately cover the conductors 6 and 7 on their outer sides except directly in between the conductors and the running net. The conductors preferably have rectangular cross sections, as shown, to snugly fit the inner profiles of 15 the C cores and also preferably have through bores, shown at 'Br', in order to facilitate passage of a coolant like water, which provides adequate cooling for the conductors as well as the cores in normal use. The conductors 6 and 7 are parallel to each other and the net and are symmetrically located on opposite sides of the plane of the running net. The conductors run the entire width of the net to be wiped and are electrically connected together on one side beyond the width of the 20 net through a piece of conductor, as shown at 'Ctn' or in any other suitable way, and their remaining pair of terminals is connected to the output terminals, marked 'Tl' and 'T2', of a high frequency, variable power source, marked '100', through suitable electrical cables marked 'Cbl 1' and 'Cbl-2'. This power source 100 should be capable of maintaining the required variable alternating voltage at the required frequency, waveform and magnitude, marked 'Vac', at the 25 cable connection terminals TI and T2. Such maintained voltage maintains the required alternating current, shown by arrow 'I', of required frequency, waveform shape and magnitude through the conductors 6 and 7 to achieve sustained, stable electromagnetic wiping action on the running wire net. 30 FIG. 3 also shows the directions of forces of electromagnetic interactions acting on two molten zinc particles at two arbitrary points, shown at 'Pl' and 'P2', in the wiping zone or the space between the C cores 8 and 9. For reference, the entire set up has three mutually perpendicular 11 axes, marked 'XX', 'YY' and 'ZZ'. XX is the central longitudinal axis of the basic wiping device constituted by conductors 6 and 7 and cores 8 and 9, as shown at '10'. YY is also central to the wiping device and perpendicular to XX, and ZZ is perpendicular to both XX and YY. Accordingly, the plane XX-ZZ is the central plane, which cuts the wiping device 10, as shown by 5 the rectangle 'ABCD'. Also, the plane XX-YY is central to the wiping device 10 but substantially coincides with the plane of the running net or mesh. Accordingly, the point P1 is located on the plane XX-YY and below the plane XX-ZZ within the wiping zone, and the net or effective force of electromagnetic interactions acting on the zinc 10 particle at P1 is in the YY direction acting away from the plane XX-ZZ or towards the zinc bath, and is shown by arrow 'Fl'. On the other hand, the zinc particle at P2, located on the plane XX YY and above the plane XX-ZZ within the wiping zone experiences an effective force of electromagnetic interaction that acts away from the plane XX-ZZ and the zinc bath and is shown by arrow 'F2'. Since the coating layer is liquid and not enclosed by a rigid medium on the 15 outside, the force F2 cannot act to nullify or oppose the force F1 and hence effective wiping of excess coating mass takes place below the plane XX-ZZ. The wiping action for various commonly occurring links of various wire nets in various orientations is shown in greater detail in FIG. 4 and FIG. 5. As per this invention, the wiping 20 force F1 is effective on all kinds of links and their junctions in the net, in their various orientations and directions of travel through the wiping device, except on those links that lie entirely in the XX-ZZ plane while they pass through the wiping zone. Accordingly, the wiping device is suitably oriented so that no links of the net are perpendicular to the YY axis. This guiding principle can be seen in FIGS. 6 - 10. 25 4. Details of Electromagnetic Wiping Action/Phenomena FIG. 4 depicts a sectional elevation of the entire basic wiping device 10, the zinc bath 2 and a constituent galvanised wire link, of circular cross section, of the mesh centrally located in the 30 device and oriented and running along the YY direction, the section being axial through the link and in the YY-ZZ plane. 'I' denotes the alternating current in the conductors 6 and 7. Likewise, 'J' denotes the induced current density at any points of interest in the liquid coating layer. The 12 directions of I and J are indicated using the well known "Dot and Cross" notation system for vectors and phasors, wherein a dot in a circle indicates a vector normal to the paper and coming out towards the viewer, and a cross in a circle indicates a vector normal to the paper and going away from the viewer. The velocity of the running wire link is shown by the arrow 'V'. The 5 loop current I sets up alternating magnetic flux in the surrounding space including the cores 8 and 9, the gaseous atmosphere 5 surrounding the travelling net, the liquid coating layer 4 and the substrate 1. Such flux is shown by solid line closed loops with arrows marked 'flx-l'. In a similar manner, the induced currents also set up alternating magnetic flux through the surrounding space, which is shown by dashed line closed loops with arrows marked 'flx-2'. Such 10 alternating magnetic flux can be said to exist in the entire wiping zone of the wiping device 10, and the resultant vector sum of flx-i and flx-2 maintains the alternating magnetic flux in the said zone as required for the squeezing and wiping action in the liquid coating layer. Important dimensions on the wiping device to achieve efficient and well-controlled wiping are 15 shown in FIG. 4. These are: e 'G' - the gap between the conductors 6 and 7 or cores 8 and 9, allowing a rectangular prismatic passage for the coated mesh to pass through e 'H ' - the height or width of conductors 6 and 7 facing the running mesh * 'H2' - the span height or width of the C cores including the conductors facing the mesh 20 The squeezing and wiping action within the liquid coating layer below the plane XX-ZZ is schematically shown in FIG. 4 at two arbitrary points, marked 'Pl' and 'Ql', symmetrically situated on opposite sides of the plane XX-YY within the wiping zone. J denotes the alternating induced current densities at points P1 and Qi. 'Bz' denotes the components of the existent 25 alternating magnetic flux densities along ZZ at PI and Q1. 'By' denotes the components of the existent alternating magnetic flux densities along YY at P1 and Q1. Interactions between By and J at PI and Q1 generate effective mechanical forces, marked 'Fz', which are normal to the wire surface andact from outside inwards, cumulatively developing pressure in the liquid coating layer 4. Owing to the configuration of the wiping device about the running net, both, By and J 30 increase in intensity along YY from the bath level up to the plane XX-ZZ, and so does the said pressure due to Fz, which is the product of By and J. The incremental pressure in 4 along YY 1. effectively causes a differential force acting on the liquid mass in the YY direction but away from the plane XX-ZZ, and this differential force is marked 'dFz' in FIG. 4. The interactions between the horizontal components of the magnetic flux densities Bz and the 5 current densities J at P1 and QI create effective mechanical forces, as marked 'Fy', which act along YY and away from the plane XX-ZZ and directly contribute to the wiping action below the plane. Both, dFz and Fy are vibratory or pulsatory at high frequencies, with their net impulsive actions in directions as shown, and substantially act around the entire periphery of the coated wire in cross section in directions as shown for all frequencies, waveform shapes and magnitudes of 10 the alternating loop current I. The resultant of forces dFz and Fy, marked 'Fl' at the points P1 and QI, retards the motion of the entrained liquid coating mass under the plane XX-ZZ, causing effective squeezing and wiping. Similar analysis of the electromagnetic phenomena at arbitrary points, marked 'P2' and 'Q2', in the liquid coating layer above the plane XX-ZZ shows that the resultant forces of electromagnetic interactions thereat, marked 'F2', act away from the said plane 15 and hence in the onward direction. However, since the liquid coating layer is free of any contact with solid or rigid bodies on the outside, the forces F1 and F2 cannot nullify each other, and squeezing and wiping progresses smoothly and unhindered below the plane XX-ZZ. FIG. 4 depicts a symmetrical set up for a straight wire link of round cross section, with the two 20 conductors at the same distance from the mesh. However, non-circular wire or asymmetrical placement of the conductors does not alter the effectiveness of the invention. Since the zinc layer is in a molten state, asymmetrical wiping forces still result in a substantially smooth and uniform coating mass distribution owing to the pulsatory nature of the forces of electromagnetic interactions as described above and as the coating outer surface always tries to follow the profile 25 of the inner substrate to minimise surface energy. Therefore, the above and the following discussions apply equally well to asymmetrical configurations and to non-circular wire links. The flow of liquid coating mass is indicated by arrows marked 'If'. FIG. 4 depicts the liquid mass flow pattern in the vicinity of the wiping device 10. Below the XX-ZZ plane, the liquid 30 layer close to the running substrate flows in the onward direction while the wiped away liquid coating mass flows backwards on the outer sides and eventually returns to the zinc bath. Above the XX-ZZ plane, the liquid mass flow is only onwards. This indicates that the wiping action 14 occurs completely below the plane XX-ZZ. The backward flowing coating mass causes the coating outer surface 4/5 also to travel backwards, which, in turn, marginally enhances the wiping action by virtue of its surface tension. 5 Owing to the elevated temperature of the coating mass travelling through the wiping device, its viscosity reduces. This reduces the onward viscous drag exerted by the running wire on the coating mass and indirectly enhances the wiping action by forces of electromagnetic interactions as described above. 10 The force of gravity on the entrained coating mass acts downwards and supports the wiping action by the forces of electromagnetic interactions below the plane XX-ZZ. The actions of the forces FI and F2 are pulsatory, which further help in smoothing out the coating layer by distributing it uniformly all over the surfaces of wire links and junctions of the running 15 net. Since the returning coating mass flows along the coating outer surface, the surface tension on the coating/atmosphere interface 4/5 plays an important role in the wiping phenomena under the plane XX-ZZ, and also in smoothing the coating layer, which is an important quality requirement 20 on the finish product. Oxygen, if present in the atmosphere 5, readily attacks the pure zinc surface to form zinc oxide, which, being in solid state, has very high surface tension compared to pure liquid zinc at the operating temperatures. As explained above in subsection 2 on "details of zinc pick up phenomena", higher surface tension increases its support to the coating mass within and reduces the wiping action, thus resulting in thickening of the coating layer due to inefficient or 25 inadequate wiping. Oxygen in any form, either free or in the combined forms such as water vapour, carbon dioxide, carbon monoxide, sulphur dioxide etc. readily and preferentially combines with pure zinc to form zinc oxide at the operating temperatures. The speed of zinc oxide formation depends on the form and concentration of oxygen in the atmosphere 5. The oxidation of zinc is progressive over time and disturbs the stabilized wiping action in a cyclic 30 manner, thereby causing lump formation in the coated layer on the finish products. As per this invention, the best way to avoid lump formation and achieve smooth finish on the wiped product is to eliminate the presence of oxygen as much as possible in the atmosphere surrounding the 15 mesh during the wiping action in the wiping zone. While complete elimination of oxygen is not practicable, reducing its concentration to below 20 parts per million gives good finish on the electromagnetically wiped product. 5 All of the forces and effects as discussed above tend to support and actuate effective backward motion of the liquid coating mass under the plane XX-ZZ. For the sake of convenience, the effective combined force causing the said backward motion is termed "the wiping force" and is denoted and marked as 'F' in the following analysis and figures depicting some practical cases of mesh galvanising and wiping. This force has a finite or non-zero component acting along the 10 length of any wire link, which is oriented at an acute angle with YY and such a component is shown by an arrow marked 'Fe' in the following figures. For links oriented parallel to the plane XX-ZZ (or perpendicular to the YY axis), the force component Fc reduces to zero, and for this reason orienting the plane of the wiping device (the plane XX-ZZ) parallel to any of the wire links is avoided. 15 In all the practical cases, the wiped away liquid coating mass flows in the direction of the wiping force F or its component Fc along the length of a wire link. A similar flow of the liquid coating mass can be expected around junctions of wire links in the mesh also. When the wiping device is properly oriented so that no links of the coated mesh are parallel to the plane XX-ZZ, a smooth 20 and continuous unobstructed backward flow of the wiped away liquid coating mass can be expected alternately along the wire links and around the junctions in sequence until the rejected coating mass returns to the bath under the action of gravity. In all the following figures, i.e. FIGS. 5 - 10, representative junctions of wire links are marked 25 'Jn' and arbitrary points of interest on the wire links are marked 'P'. Both the points are indicated by dots. Arrows 'V' mark the onward velocity of the wire net or links. This velocity or its appropriate component drags the liquid coating mass onwards through and across the wiping device 10 and the wiping force F or its component Fe acts against the onward drag thus causing effective wiping of the excessively picked up coat weight and returning it back to the bath. The 30 backward flow of the wiped away liquid coating mass is indicated by arrows marked 'If'.
16 5. Wiping Action on Various Types of Links and Joints in Mesh Galvanising FIG. 5 depicts in elevation various situations wherein some typical links and joints in any mesh are likely to pass through the wiping device and undergo electromagnetic wiping. The figure is 5 subdivided into four cases. In all cases except CASE-4, effective wiping is achievable. CASE-I depicts a single wire link oriented and travelling at a velocity V in the YY direction through the wiping device. The wiping action and phenomena on such link have already been described above in case of FIG. 4. An arbitrary point of interest on this wire link below the plane 10 XX-ZZ is indicated by a dot and marked as P. The effective wiping force F is along the length of the wire link backwards as shown and the flow of the wiped away liquid coating, as shown with an arrow marked lf, is also in the same direction as F. The backward flowing liquid coating mass eventually returns to the bath under gravity. 15 CASE-2 depicts an assembly of two or four wires linked together through twisting, welding or partial shearing and expanding. All the wire links are at acute angles to YY and the said assembly remains substantially flat and rigid and in the plane XX-YY while travelling through and across the wiping device. The travel direction can be parallel or oblique to YY, and in either case, the assembly can be thought of as travelling at a relative velocity V in the YY direction as 20 shown. The wiping action and phenomena at two arbitrary points P on two wire links and their junction Jn, all situated below the plane XX-ZZ, are similar to what have been described in FIG. 3 and FIG. 4. The effective wiping forces at P and Jn are as shown by arrows marked F, their directions being normal to and away from the plane XX-ZZ. These forces F have finite or non zero components as shown by arrows marked Fc acting along the lengths of the links backwards, 25 i.e. towards the bath. The eventual flow of the wiped away liquid coating mass, as shown by arrows marked If, is around the junctions Jn and along the lengths of the links backwards and towards the bath. CASE-3 depicts a link, consisting of two or more wires stranded or twisted together to form a 30 rope like structure, oriented and travelling at a velocity V in the YY direction. The wiping action and phenomena are similar to what have been described in FIG. 4, except that a closely twisted link acts as one wire link and a loosely twisted link with gaps between the constituent wires 17 behaves like separate wires with several junctions subjected to wiping, thus causing effective wiping on the entire link in a manner similar to what has been shown for CASE-2. The effective wiping force F and liquid flow If at an arbitrary point P below the plane XX-ZZ are as shown, i.e. away from the plane XX-ZZ and towards the bath, causing effective wiping of the excess pick up 5 on the entire link. CASE-4 depicts a link, oriented transverse or at right angles to the YY direction and travelling at a velocity V in the YY direction. The wiping force F at the arbitrary point P is normal to the link and has no finite components (Fc = 0) along the length of the link. Therefore, wiping actions and 10 consequent liquid flows are absent on and along such links. The above cases deal with various geometries of wire links and junctions in the XX-YY plane. It is important to note that any links and junctions with geometries lying in the YY-ZZ plane are affected the same way. It is also important to note that all practical mesh patterns can be resolved 15 into some of the four cases illustrated above or combinations thereof, and an adequate electromagnetic wiping device and system can be designed to suit all commonly manufactured galvanised meshes. The most commonly used mesh patterns are dealt with below. 6. Wiping Action on Hexagonal Mesh 20 FIG. 6 depicts a sectional elevation of a preferred electromagnetic wiping device arrangement for what is known as 'hexagonal mesh' in its normal galvanising practice, wherein the mesh exits the zinc bath in a vertical take up mode. The net has hexagonal holes or voids constituted by twisted or stranded wire links, which are vertical, and single wire links, which are oblique to the vertical. 25 Accordingly, in a preferred embodiment, the wiping device 10 is oriented with the XX-ZZ plane substantially horizontal and located close to the bath level 3 in a convenient position wherein the coating layers on the travelling net remain in the liquid state. The wiping actions on the vertical stranded links, oblique single wire links and their junctions or 30 intersections are similar to those shown in CASE-3 and CASE-2 of FIG. 5, and are briefly explained further. In FIG. 6, arbitrary points of interest on various links below the plane XX-ZZ are indicated with dots and marked as 'P'. Also, various representative junctions of links below 18 the said plane are indicated with dots and marked as 'Jn'. The wiping forces on such points P and junctions Jn act in directions normal to and away from the plane XX-ZZ, and are shown by arrows marked 'F'. On oblique wire links, the forces F have non-zero components, as indicated with arrows marked 'Fc', along the lengths of the links. The wiped away liquid coating flow, as 5 indicated with arrows marked 'If, is away from the plane XX-ZZ along the links and around the junctions Jn and eventually to the bath under the action of gravity. Depending on the constituent wire sizes and the geometry of the hexagons of the mesh, the relative wiping intensities on the stranded links (F) and oblique links (Fc) can differ in a practical situation. Eventually, the retained coating thicknesses on the two links may also differ. Nevertheless, the average retained 10 coat weight can be expected to be fairly constant and uniform with smooth surface all over the mesh in steady state operations. 7. Wiping Action on Chain Link Mesh and Expanded Sheet Mesh 15 FIG. 7 depicts a sectional elevation of a preferred electromagnetic wiping device arrangement for what are known as 'Chain Link Mesh' and 'Expanded Sheet Mesh' in their normal galvanising practice, wherein the mesh exits the zinc bath in a vertical take up mode. (These meshes are prefabricated differently but are similar in behaviour as far as electromagnetic wiping is concerned; hence they are classified together in this specification). In this mesh, the voids are 20 square or rhombic in shape and all the links are oblique to the horizontal and vertical. Hence, in the preferred embodiment, the wiping device 10 is oriented with the XX-ZZ plane substantially horizontal and located close to the bath level 3 in a convenient position wherein the coating layers on the travelling net remain in the liquid state. 25 The wiping actions on all the links and their junctions or intersections are similar to those shown in CASE-2 of FIG. 5, and are briefly explained further. In FIG. 7, arbitrary points of interest on two links below the plane XX-ZZ are indicated with dots and marked as 'P'. Also, a junction below the said plane is indicated with a dot and marked as 'Jn'. The wiping forces on points P and junction Jn act in directions normal to and away from the plane XX-ZZ, and are shown by 30 arrows marked 'F'. All the forces F have non-zero components, as indicated with arrows marked 'Fc', along the lengths of the links. The wiped away liquid coating flow, as indicated with arrows marked 'lf', is everywhere away from the plane XX-ZZ along the links and around the junctions 19 Jn and eventually to the bath under the action of gravity. All the links are generally identical in shape and size and are equally oblique to the vertical; hence the wiping intensities and coat weights retained on them after wiping can be expected to be equal. The average retained coat weight also can be expected to be fairly constant and uniform with smooth surface all over the 5 mesh in steady state operations. 8. Wiping Action on Square or Rectangular Mesh in Normal Vertical Exit From Bath FIG. 8 depicts a sectional elevation of a preferred electromagnetic wiping device arrangement for 10 what is known as 'Square Mesh' or 'Rectangular Mesh' in its normal galvanising practice, wherein the mesh exits the zinc bath in a vertical take up mode. In this situation, the longitudinal wires or links are vertical and the transverse wires or links are horizontal or parallel to the bath level 3. Hence, in the preferred embodiment, the wiping device 10 is oriented with the XX-ZZ plane oblique to the horizontal as well as vertical and located as close as possible to the bath level 15 3 in a convenient position wherein the coating layers on the entire travelling net remain in the liquid state while passing through the wiping device. This way, no links of the mesh are parallel to the plane XX-ZZ while passing through the wiping device. The acute angle subtended by the XX axis with the horizontal is shown at 'b', which can be designed in order to achieve equal wiping intensities on all the wire links of the mesh. The wiping action is briefly explained 20 further. The mesh has square or rectangular holes, and is generally prefabricated with longitudinal and transverse wires joined together at the intersections through processes like spot welding, crimping and weaving. The wiping actions on all the links and their junctions or intersections are similar 25 to those shown in CASE-2 of FIG. 5. In FIG. 8, two arbitrary points of interest, one on a longitudinal link and one on a transverse link, both under the plane XX-ZZ are indicated with dots and marked as 'P'. Also, an arbitrary junction below the said plane is indicated with a dot and marked as 'Jn'. The wiping forces on points P and junction Jn act in directions normal to and away from the plane XX-ZZ, and are shown by arrows marked 'F'. The forces F have non 30 zero components, as indicated with arrows marked 'Fc', along the lengths of the links. The wiped away liquid coating flow, as indicated with arrows marked 'lf', is everywhere away from the plane XX-ZZ along the links and around the junctions Jn and eventually to the bath under the 211 action of gravity. For properly designed wiping devices and systems, the wiping intensities and coat weights retained on all the links after wiping can be expected to be equal. The average retained coat weight also can be expected to be fairly constant and uniform with smooth surface all over the mesh in steady state operations. 5 9. Wiping Action on Square or Rectangular Mesh in Oblique Vertical Exit From Bath FIG. 9 depicts a sectional elevation of a preferred electromagnetic wiping device arrangement for the 'Square or Rectangular Mesh' in its galvanising practice, wherein the mesh exits the zinc bath 10 in an oblique manner in a vertical plane. The acute angle subtended by the long side of the mesh with the horizontal is shown at 'c'. Depending on the angle c, the wiping device can be oriented to achieve equal wiping intensities on all the wire links. However, in the preferred embodiment, the wiping device 10 is oriented with the XX axis substantially horizontal and located as close as possible to the bath level 3 in a convenient position wherein the coating layers on the travelling 15 net remain in the liquid state. The wiping action is briefly explained further. The wiping actions on all the links and their junctions or intersections are similar to those shown in CASE-2 of FIG. 5. In FIG. 9, two arbitrary points of interest, one on a longitudinal link and one on a transverse link, both under the plane XX-ZZ are indicated with dots and marked as 'P'. 20 Also, an arbitrary junction below the said plane is indicated with a dot and marked as 'Jn'. The wiping forces on points P and junction Jn act in directions normal to and away from the plane XX-ZZ, and are shown by arrows marked 'F'. The forces F have non-zero components, as indicated with arrows marked 'Fc', along the lengths of the links. The wiped away liquid coating flow, as indicated with arrows marked 'if, is everywhere away from the plane XX-ZZ along the 25 links and around the junctions Jn and eventually to the bath under the action of gravity. For properly designed wiping devices and systems, the wiping intensities and coat weights retained on all the links after wiping can be expected to be equal. The average retained coat weight also can be expected to be fairly constant and uniform with smooth surface all over the mesh in steady state operations. 30 21 10. Wiping Action on Wide Square or Rectangular Mesh in Normal Vertical Exit From Bath FIG. 10 depicts in sectional elevation a preferred modified embodiment and arrangement of the 5 wiping device in a situation wherein a very wide square or rectangular mesh runs through and exits the zinc bath in a vertical take up mode without undergoing any turn in the oblique direction. Having long oblique conductors or device similar to FIG. 8 would not be practical here because of the likely progressive cooling and solidification of the zinc coating layer before undergoing wiping. Therefore, the preferred wiping device 10 consists of two or more short 10 wiping devices arranged along the width of the mesh in an alternately ascending and descending manner, and successively connected in series to form a single loop covering the entire width of the mesh to be wiped. This way, each section of the wide mesh running through a short device gets effectively wiped as shown in FIG. 8. 15 For clarity, the XX and YY axes of the four short wiping devices, as shown in FIG. 10 as an example, are marked in succession from left to right as XIXI & YlYl, X2X2 & Y2Y2, X3X3 & Y3Y3 and X4X4 & Y4Y4 while 'WW' indicates the horizontal centreline through the entire wiping device. The acute angles subtended by the ascending and descending XX axes with the horizontal are shown at 'd' and 'e' respectively, which are preferably equal. The entire wiping 20 device is preferably oriented with WW horizontal and located in a convenient position as close as possible to the bath level 3, wherein the coating layers on the travelling net remain in the liquid state. Adequate and complete coverage of the conductors 6 and 7 with C cores at the apexes or joints is preferable, as it ensures efficient wiping and helps in smoothening out of any likely lump formation on transverse link segments passing in the vicinity of the said apexes. The wiping 25 action is briefly explained further. The wiping actions on all the links and their junctions or intersections are similar to those shown in CASE-2 of FIG. 5. In FIG. 10, the wiping action is shown for each section of the mesh running through each ascending or descending wiping device separately. In each of the said sections, two 30 arbitrary points of interest, one on a longitudinal link and one on a transverse link, both under the corresponding XX-ZZ plane are indicated with dots and marked as 'P'. Also, an arbitrary junction below the said plane is indicated with a dot and marked as 'Jn'. The wiping forces on 22 points P and junction Jn act in directions normal to and away from the plane XX-ZZ, and are shown by arrows marked 'F'. All the forces F have non-zero components, as indicated with arrows marked 'Fc', along the lengths of the links. The wiped away liquid coating flow, as indicated with arrows marked 'If, is everywhere away from the plane XX-ZZ along the links and 5 around the junctions Jn and eventually to the bath under the action of gravity. For properly designed wiping devices and systems, the wiping intensities and coat weights retained on all the links after wiping can be expected to be equal. The average retained coat weight also can be expected to be fairly constant and uniform with smooth surface all over the mesh in steady state operations. 10 PRACTICAL DESIGNS & OPERATION OF ELECTROMAGNETIC WIPING SYSTEM Electromagnetic wiping systems described in this specification are workable for all contemporary mesh galvanising operations and for all design and control parameters and variables of the wiping 15 systems. However, control stability, accuracy and uniformity, and system practicability and efficiency are some of the important aspects to be considered in designing a wiping system for any application. While designing an electromagnetic wiping system, following important mesh and line parameters need to be considered and incorporated: 20 1. Vibrations and transverse bow on the running mesh at the exit from the bath 2. Bath temperature and cooling conditions immediately outside the bath 3. Nature of the take up from the bath - horizontal, vertical or oblique 4. Galvanising speeds and coat weight requirements on the finish products 5. Geometries of individual links in cross sections as well as that of the entire mesh 25 6. Electrical conductivities and magnetic permeabilities of the substrate and the coating metal at the operating conditions 7. Physical properties of the coating metal, such as, density, viscosity and surface tension at the operating conditions 30 Additional external heating can be suitably provided on the mesh running between the bath and the wiping device in order to prevent the liquid coating metal layers from premature freezing before undergoing wiping.
23 Having installed the wiping system on the mesh galvanising plant, the mesh is run in its normal course at its rated speed and the loop current (I) is varied and stabilized as required to vary, control and stabilize the obtainable coat weight on the entire mesh in steady state ongoing 5 operations. WIPING SYSTEM DESIGN AND PERFORMANCE IN MESH GALVANISING Following examples provide some details of electromagnetic wiping system design and 10 performance in mesh galvanising. Example 1. Type of Mesh Hexagonal 15 Wire Size 0.7 millimeter Material Low Carbon Steel Zinc Bath Temperature 450 degrees Celsius Take-up from Bath Vertical (Refer to FIG. 6) Galvanising Speed 50 Meters/Minute 20 Dimensions on Wiping Device Refer to FIG. 4 G - 10 millimeter HI - 10 millimeter H2 - 30 millimeter Waveform of Loop Current I Simple Sinusoidal 25 Frequency of Alternation of I 40 kilo Hertz Wiping Performance Coating Thickness Vs Current I: Loop Current Magnitude (Amperes) - 0 150 400 Coating thickness (microns) - >100 25 13 30 Example 2. Type of Mesh Square 24 Wire Size 2.0 millimeter Material Low Carbon Steel Zinc Bath Temperature 450 degrees Celsius Take-up from Bath Oblique, Vertical (Refer to FIG. 9) 5 Galvanising Speed 20 Meters/Minute Dimensions on Wiping Device Refer to FIG. 4 G - 30 millimeter HI - 30 millimeter 10 H2 - 90 millimeter Waveform of Loop Current I Simple Sinusoidal Frequency of Alternation of I 20 kilo Hertz Wiping Performance Coating Thickness Vs Current I: Loop Current Magnitude (Amperes) 0 150 400 15 Coating thickness (microns) >70 45 25 ADVANTAGES OF ELECTROMAGNETIC WIPING IN MESH GALVANISING 1. Accurate and arbitrarily controllable excess Coat Weight reduction 20 2. Smooth and Uniform Coatings 3. Effective on any mesh geometry and width 4. Effective at any galvanising speed 5. Reduced Wastage of Coating Metals resulting in substantial cost savings 6. Reduced Rejection of Finished Product 25 7. Enhanced Quality Control 8. Reduced Environmental Pollution 9. Silent Operation - Noise Reduction 10. Ability to Control Efficient Wiping of Vibrating Meshes and Also the Meshes With Transverse Bow 30 11. Easy Deployment - compatible with all galvanising plants 12. Suitability for All Production Conditions and Product Requirements.