US2869511A - Apparatus for propelling particulate matter - Google Patents

Description

Jan. 20, 1959 R. C. DICKEY ETAL 2 Sheets-Sheet 1 INVENTORH v.61. 6. 3 W 2 M im J My A rr Jan. 20, 1959, R. c. DICKEY ETAL 2,869,511

APPARATUS FOR PROPELLING PARTICULATE MATTER Filed Oct. 27, 1954 2 Sheets-Sheet 2 O O O 0 o 0 4/ United States Patent 2,869,511 APPARATUS FOR PROPELLING PARTICULATE MATTER Robert C. Dickey, Mount Clemens, Frederick P. Hauck,

Detroit, and Hugh M. Archer, Dearborn, Mich., assignors to Michigan Abrasive Company, Detroit, Mich., a corporation of Michigan Application October 27, 1954, Serial No. 464,93d 6 Claims. (Cl. 118-638) The present invention relates generally to the propulsion of particulate matter by electrostatic means. More specifically, this invention relates to apparatus and the method of making abrasive-coated'sheeting material by electrostatic deposition, that is, by the process wherein the abrasive is electrostatically propelled into contact with an adhesive-coated sheeting material. Most specifically, this invention relates to a method and apparatus for electro" static deposition wherein the abrasive grains and their support are electrically conditioned before introduction to the electrostatic field and the field itself is protected against the deleterious effects of stray or unwanted charges.

In the electrostatic propulsion of particulate matter, difficulties have been encountered heretofore in maintaining the high potential gradients required across the field electrodes for efiicient propulsion. Difi'iculties also have occurred in the dispersion of very fine, and the propulsion of the coarser, particle sizes of particulate matter. The former usually are agglomerated and are propelled to the adhesive surface as such. The potential gradient required for the propulsion of the larger grains has been higher than is safe or easily obtained with known apparatus.

Known forms of electrostatic propulsion or deposition machinery have been subject to erratic operation due to humidity, static electrical charges and other difficult-tocontrol atmospheric variables. Moreover, difiiculties have been encountered with arcing and tracking when using the high voltages required in known forms of apparatus, these voltages frequently being of the order of 10 to 50 kilovo-lts or more. Even when using such high voltages, it has been found that there is a barrier efiect resulting in an effective potential gradient in the active volume of electrostatic propulsion devices very much less than that pre sumed to be available from the impressed voltage.

It is among the objects of this invention, therefore, to provide a method and apparatus for the electrostatic propulsion or electrostatic deposition of particulate matter which will operate without the above-indicated dilficulties, which will more efiiciently propel particulate matter in a wider range of particle sizes and shapes, and particularly in the finer and larger sizes, Which will permit the use of lower impressed voltages yet provide higher effective potential gradients in the active propulsion volume of the apparatus, which will operate predictably even under adverse atmospheric and other conditions, which will more effectively disperse the particulate matter, and which will operate with greater safety to personnel.

In accordance with the present invention, it has been discovered that unwanted charges on the particulate matter itself, on the conveyor belt or on other parts of the apparatus which are near, or which actually enter the field, can be effectively removed and the effective potential gradient in the active propulsion volume of the apparatus increased by electrically conditioning those parts of the apparatus entering the field and by electrically preconditioning the particulate matter itself. By electrical conditioning is meant the removal of unwanted electrical charges which decrease the propulsive effect of the field. It also, in some instances, may include humidification or dehumidification of the particulate matter in order to render it more amenable to the electrical condi- Patented Jan. 20,

tioning and to the propulsive field. In those forms of apparatus wherein portions of the apparatus enter the field, such as for example a conveyor belt, it has been found easier and more effective to condition the belt or other apparatus member which enters the field than the hardto-handle particulate matter. In some instances, as will be pointed out herein, conditioning of both are required.

In these forms of the apparatus wherein the particulate matter is spread out on a conveyor belt and then con ducted into the field, unwanted charges acquired by the belt so greatly reduce the ettective potential gradient operative on the particulate matter as to act as an electrical barrier. it has been found that when a conditioning electrode or electrodes are placed in electrical proximity to, or in contact with, a conveyor belt in accordance with this invention, and a high voltage of the same polarity as that of the propelling field electrode is applied thereto, the barrier efiect substantially disappears, the unwanted charges are neutralized and the belt is charged. It has been found also that unwanted charges occurring on the particulate matter have a very pronounced effect on the effective propulsive potential gradient. In this invention, however, it has been found that an electrode or electrodes suitably located outside the field in electrical charging proximity will electrically condition the particulate material before it enters the field. The charges supplied by the electrode or electrodes also assist in breaking up agglomeration in the particulate material and significantly improve the dispersion, especially of the finer particle sizes. It has also been found, especially when using a belt or support or" semi-conducting material, that electrical charges leak back into the electrostatic field from the exit side of the belt (i. e., with respect to the direction of travel of the belt or support). In accordance with this invention, a buffer electrode is used to protect the field against unwanted charges entering from the discharge side of the apparatus and allow the establishment of the higher potential gradients in the field.

The invention will now be more fully described with reference to one embodiment of apparatus adapted to pro duce abrasive-coated sheet material such as sandpaper, emery cloth, etc. It is to be understood that the method and apparatus disclosed herein have application to other electrostatic propulsion or deposition methods. Other ob jects and advantages of the invention will be apparent, or will become apparent, in the following detailed descrip tion of the invention when taken in conjunction with the accompanying drawings, in which:

Figure l is a perspective view of an abrasive-making machine of this invention;

Fig. 2 is a fragmentary sectional view through the electrode mounting appearing at the right-hand end of the machine, the section being taken along the line 22 of Fig. 1;

Fig. 3 is another sectional view of the electrode housing of Fig. 2, the view being taken along the line 3-3 of Pig. 2; and

Fig. 4 is a schematic representation of the electrical charges on a typical electrostatic abrasive deposition apparatus not equipped with conditioning electrodes, the view representing a transverse section through the electrodes and conveyor belt.

The apparatus shown in Figs. 1 to 3 comprises an end less conveyor belt 10 trained over a pair of rolls 11, one of which is driven by means not shown. The rollers 11 are insulated from the remainder of the apparatus and preferably from connection with the ground. The belt 10 preferably is constructed of a suitable semiconducting material, that is, a material which is not an insulator but which will ofier considerable impedance to the flow of electrical currents. A preferred material of construction is flexible, natural or synthetic plastic material which contains a controlled proportion of carbon black or other similar filler productive of a desired conductivity. By so compounding the plastic, a belt may be obtained which has the desired current-carrying and tracking resistance properties. When suitably insul ted from ground, however, a belt of metal or other conductmaterial may sometimes be used in connection with the conditioning electrodes of this invention. A belt of insulating material may also be used if a conditioning electrode is disposed in contact with both the upper and lower belt surfaces. it is to be understood that the conductance or drain current of the belt and its resistance to ground are to be adjusted to the conditions, voltages, proportions and locations of structural parts obtaining in each apparatus. if made of a plastic material, the belt should have good ozone resistance and a high resistance to tracking.

At the right-hand end of the apparatus of Fig. 1, there is provided an abrasive grain hopper 12, a spreading screen or screens 13 and a plurality of charging wires which pass beneath the screen 113 so as to be in the path of the abrasive grain shower in its passage to the belt. The hopper 1?. has an adjustable s ide member 15 which is moved up and down to regulate the flow rate of the abrasive grains. Any other form of feed mechanism can be employed to deposit a uniform layer of abrasive grains on the belt.

The charging wires 14 are connected to a common conductor 16 which in turn is connected to a terminal 17. A high voltage applied to wires 14 effectively conditions the abrasive grains. The charges of varied polarity existing on the grains thus are neutralized and a common charge im arted thereto. This tends to make the grains mutually repulsive and break up or explode agglomerates thereby assisting the screen 13 in laving down a more uniform layer A on the belt 1W.v The charging wires 14 are not always required but are sometimes of assistance under ditficult operating conditions. if the belt 10 moves in the direction of the arrow, the abrasive layer A may be passed close by an adjustable wire-like electrode which is supported in a pair of electrode housings l9, l9 constructed of insulating material. The electrode 18 is provided with a connection to the high voltage supply in such a way that its potential may be varied between that of ground or any value between that of ground or that of the high voltage propulsive electrode. Where the potential impressed on wire 13 is suitably adjusted in this range, the grains are further conditioned in mechanical and electrical fashion. A so supported in housings 19 is a roller or rod-like electrode Ztl which is in close electrical proximity with the underside of belt Zlt As shown, it is preferred that the beltconditioning electrode 20 be in actual contact with the belt.

As pointed out hereinabove, the conditioning of the belt is the critical feature of this invention. Under favorable operating conditions with intermediate grain sizes, charging Wires lid and wire electrode 13 are not required. However, the use of either or both of the charging wires 1 and wire 18 eliminates the occasional erratic field operation encountered even under favorable conditions. With the very fine and larger grain sizes, the use of one or both types of wire electrodes is usually required. The charging wires l4, 18 to be effective must be brought very close to the grain layer, the distance flequently being to /2 inch or less. in the drawings, the wires appear to be spaced at greater distances from t B As is shown schematically in Fig. 2, the. wire 13 is connected by lead portions 21 and. 21 to a resistance element 74} which has one end connected to high voltage terminal 17 and its other end connected to ground at '71. The lead 2i has a movable contact which permits 4 the adjustment of the voltage impressed on wire it; at any potential between ground and that impressed on roller electrode 2%.

As shown in greater detail in Figs. 2 and 3, the electrode housings 19 are molded of an insulating material such as Lucite, phenolic laminated paper, etc. with an electrical lead 21 integrally molded therein. The lead is extended upward (see Fig. 3) so as to form a sliding contact for an adjustable shoe or slider element 22 supported in a three-sided cavity 2.3. The slider element 22 is of insulating material but has an internal rod or tube 22a of metal which contacts lead wire El. The wire electrode 18 enters cavity 23 through an aperture and is secured to a wire tensioning device comprising a nut 25 and an inwardly projecting screw 27 threaded inside slider 22. The nut and screw have an internal passageway 22b terminating in a conical bottom cavity 26. The wire 18 is threaded through a tight fitting hole in electrode 22a (in slider 22) and its end is knotted so as to be retained in the conical end cavity 26. The nut 25 and screw 27 are secured together so as to be capable of movement as a unit in or out of slider member 22, in order to adjust the wire tension. Slight vertical movement or the slider element 22, the nut 25 and sleeve 27 is achieved by turning adjusting screw 28, the nut 25 and sleeve 27 moving vertically a small distance in a small slot 27:: in the face of the housing 19.

The rod-like belt electrode 2% is mounted in a vslot 3G in the baclt side of each of the housings is. The high voltage input to el ctrode it) is supplied through a corn ductc-r 31 supported in an insulating bracket i smaller electrode Contact element 33 inside hollow conductor 31 is forced upwardly into contact with the rod ilce electrode by means of a spring or other means not shown.

As is illustrated in Fig. l, a continuous length of adhesive-coated sheeting material 45? brought. down under a pair of rolls 4-1 with adhesive-coated surface facing downward. The rolls 41, like belt rolls ill, are completely insulated from the apparatus and may be grounded, if desired. The sheeting as preferably is moved in the direction of the arrows and in .irncd -c. tion with the belt in in order to eliminate streaks in the abrasive coating. If desired, the conveyor belt may more in opposite direction to the sheeting. After tional sandpaper or abrasive cloth process. Locat..r proximately intermediate the runs of thebelt 1 sheeting 4th is a pair of large flat field electrodes 12, Electrode 5-2 is located beneath belt It) and a sheet, layer, or completely-enclosing envelope 44 of insulating material disposed between it and the belt it). Electrode 43 is located above the sheeting 4G and parallel to the latter, to the belt 10 and the lower electrode 42. lf desired, an envelope similar to envelope 44- of insulating or semi-conducting material may be interposed between upper electrode 4-3 and the sheeting ill. Near the lefthand end of the belt 16 there is located a second electrode housing 4-5 (the external metallic elements of which are connected to ground by conductor 45 for purposes of: safety) which carries a second rod or roller electrode as adjacent or in electrical contact with the underside of: belt 1h. The function of electrode 46 is to protect or butter the field from the effects of in-leakage of upwanted electrical charges along the belt from its dis charge end. Butler electrode 46 is not'always required under very favorable conditions such as prevail with optimum humidity, etc. With the very fine and the larger grain sizes, or adverse atmospheric conditions. however, the use of butter electrode leis necessary in order to maintain the required conditioning or charge conditions on thebelt.

The electrodes 42, 2t} and 14 are connected to the same high voltage lead 47, electrode 42being connected at ter inal electrode 46 terminal 4%, wires 3-; at terminal 37 and roller electrode 20 at terminal 17. Wire electrode 18, however, preferably should be connected through a resistance element 70 as shown. In some cases, it could be connected to a pole of the same polarity as lead 47. All that is necessary is that sufficient electrical stresses be produced around wire 18 to cause a transfer of charge to or from the particles. For this reason the voltage applied to all these electrodes need not be the same. In the usual circuit electrodes 14, 2d, 42 and 46, however, would need to be of the polarity. In a split circuit where one field electrode is above, and the other below, ground potential there may be a situation in which all such electrodes are not of the same polarity.

The upper field electrode 43 is connected by means of lead wire 50 to ground, or to a direct current source of polarity opposite that of lower electrode 42. Connection to ground is generally preferred. in some cases, it also may be desirable to wipe unwanted charges from the sheeting material, in which case conditioning electrodes would be disposed in proximity to, or contact with, the sheeting material itself.

The field electrodes 42, 43 should be of a material of high conductance. They may be of carbon, copper, aluminum, or silver and other metals. Since the magnitude of the current in the field electrodes is small, and electrode area large, a suitable electrode construction is a core of wood or other suitable insulating material with a uniform surface coating or layer of metal.

After passage through the field, the abrasive grains not projected upwards into the adhesive coating of sheeting 40 fall back onto the belt It) and drop into a hopper 61 on the left-hand end of the belt ill. In actual practice, a slight excess of abrasive grains are deposited on the belt by the feed mechanism in order to insure a complete abrasive coating on the sheet material. By regulating the amount of abrasive grains and the field potential coatings of both the closed and open types can be obtained.

The operation of the apparatus proceeds by first adjusting the gravity hopper-delivery mechanism to deliver a uniform stream of abrasive grains which is dispersed and distributed to a considerable extent by the screen 13. The grains are conditioned, that is, neutralized or charged by wires 14. The belt and its layer A of grains then advances to the region of the electrode 18 and the belt electrode 26 The belt electrode 215 neutralizes or charges the belt and the wire 18 does the same for the abrasive grain layer A where necessary. The height of wire 18 is preferably adjusted so as to be just above the grain layer but not in actual contact therewith. The conditioning of the grains can be regulated to some extent by adjusting the distance between the wire 18 and the grains. When wire 18 is at potential between that of the ground and roller 20, the latter at the same polarity as lower field electrode 42, the belt and grains are neutralized, charged or otherwise conditioned to secure the proper potential gradient between the grains and the adhesive-coated sheeting. If required, the polarity of electrode 18 and electrode 26 can be varied and their voltage independently increased or decreased to efiect proper conditioning.

' The properly conditioned, that is, electrically conditioned, belt and abrasive grain layer A then enter the electrostatic field existing between the field electrodes 42, 43. The grains on the belt almost immediately are noted to lift ofi the belt and form a cloud B in which the individual grains are in extremely rapid random movement due to repeated collision and mutual repulsion one with the other. Since the field is unidirectional, the particles tend to align themselves on end with their major axes perpendicular to the field electrodes. A much higher percentage of particles are thus aligned than is possible to obtain by gravity deposition or mechanical d propulsion. Some of the grains strike the adhesive coat ing with suflicient force to be embedded therein. The repeated bombardment of 51mins against the adhesive coating during its passage through the field insures a complete uniform coating in which the grains are firmly embedded in the adhesive. After passage through the field, the abrasive-coated sheeting is conducted away to the drier or curing oven or other subsequent step.

Fig. 4 is a schematic representation of the electrical conditions believed to obtain in the field of apparatus of the general type shown in Fig. 1 but not having electrodes 14, 18, 20 and 46. The following discussion is an attempt to explain the function of the additional electrodes used in this invention. The invention, however, is not to be limited by this theory. For convenience, the elements in Fig. 4 are giventhe same numbers as like elements in Figs. 1-3. When a high voltage potential is impressed on electrode 42, which for purposes or" illus tration is taken to be positive (indicated by O symbol to indicate the deficiency of electrons), the electrodes (indicated thusly in insulating envelope d4 are attracted to electrode 42. Likewise, electrons in the belt 10 and abrasive grains are attracted to insulating envelope 44 and pass through semi-conducting belt without too much dificulty. All these electrons begin a highly impeded journey through envelope 44 to reach electrode 42. However, since the number of electrons arriving at the surface C of the envelope .4 from belt It) and the abrasive grain layer per unit time is greater than the number being transmitted therethrough, a great surplus accumulates thereon, thereby effectively neutralizing a substantial part of the force field or potential gradient normally existent between the grains and the adhesivecoated web or sheet 4t]. This dense electron layer acts as a barrier. A somewhat similar exists between the adhesive-coated web and upper field electrode In the method and apparatus of this invent n, however, the ionizing wires 14 and corona electrode drain off or neutralize this excess of electrons from the abrasive grains and electrode 2% does likewise with the com c or belt 19. Since the quantity of unwanted charges on the belt is likely to be greater than that usually found in the abrasive grains, and the preferred belt is a semi-conductor, it is seen that under most conditions, the belt electrode 20 could remove or add enough electrons to the belt and grain layer to substantially destroy the barrier With extremely fine abrasive grains, however, the in ance in the grain layer probably is too great for the free flow of electrons. Additional discharge electrores such as wires 14, 13 are required under these conditions. With the larger sizes of abrasive grains, the greater individual grain weight requires a higher charge and potential gradient to propel them to the adhesive coating. In these circumstances, discharge Wires above the belt materially increase the elfective potential gradient between the grains and the adhesive-coated sheeting and enables the efficient propulsion of the heavier particles using field.

voltages well within the insulating resistance of common materials. Also, the buffer electrode 46 protects the field against the effects of in-leakage of undesirable electrical charges from the discharge end through semi-conducting belt 19. The net result of the use of the charging wires 14, belt electrode 20, wire electrode l8 and electrode 4-6 is the more consistent and predictable operation of the apparatus at lower applied voltages, the production of a more uniform product, and a generally more satisfactory production process.

What is claimed is:

1. Apparatus for propelling particulate matter onto a traveling web member having an adhesive coated under surface from a traveling belt-like member which travels below the traveling web member and which carries the particulate matter on an upper surface thereof, the upper surface of said belt-like member facing and lying proximate the adhesive coated surface of the web member, comprising a pair of spaced-apart field electrodes one of which is at a high voltage relative to the other for establishing electrostatic field therebetween, first means for moving a first one of the members between said electrodes and through 'd '"eld closer to said high-voltage electro le than said other one ct said electrodes, second means for .o g the other one of the members between said electrodes and through said field and closer to said other one of said ole o 5 than to said high-voltage electrode, us for increasing the tive A prisng e condi'loning electrode sIru posed outside of said field and in direct electrical and mechanical engagement with first member for controlling the ootential of said first rnem for maintanug said conditioning tially the same potential as said high voltage 2. App tus for arc-pol 3 particulate matter onf traveling l er having an adhesive coate', unocr surface from a taveliug belt-like member which below the travel particulate o surface of s, ltmate comprxing which is lishing for me. trodes a voltage ele second between to said otl i or surface thereof, tie upper member facing and lying proxied surface of the Web member, part field electrodes one of ive to the other for estabeld therebctween, first means tlre bars between said elecfield a closer to said high- 0 said other one of said electrodes, one of the members said field and closer said for con .1 of said first member, and means to conditioning electrode at substantially the potential as said high voltage elecnode.

3. The combination of claim in which said conditiorn electrode extends transversely of the line of trave ct said first member and supports the pot ion of said first member which is in engagement therewith at any instant.

4. Apparatus for propelling particulate matter onto a traveli web a adhesive coated under surface from traveling b e member which travels below the traveling web member and which carries the particulate matter on upper surface thereof, the upper surface of s sell-like member facin and lying proximate the adhesive coated surface of the web member, comprising pair of: spaced-apart field electrodes one of which is at a high voltage relative to the other for establishing an electrostatic field therebetween, first means for moving first one of the members between said elec trodes Jrough said field and closer to said highvoltage electrode than to said other one of said electrodes, second means for o-ving the other one of the members etween said electrodes and through said field and closer to said other one of said electrodes than to said highvoltage electrode, and conditioning means for reducing leakage of said field via said first member in a direction opposite to the direction of travel of said first member and for increasing the effective propelling potential gradient within said field comprising a metallic conditioning electrode structure disposed outside of said field and in direct electrical and mechanical engagement at any instant with the portion of said first member which will shortly thereafter during the travel of said first member enter said field for controlling the potential of said first memher, and means for maintaining said conditioning electrode at substantially the same potential as said high voltage electrode.

5. Apparatus for propelling particulate matter onto a traveling web member having an adhesive coated under surface from a traveling belt-like member which travels below the traveling web member and which carries the particulate matter on an upper surface thereof, the upper surface of said belt-like member facing and lying pro lmate the adhesive coated surface 0.: the web member, comprising a pair of spaced-apart field electrodes one of which is at a high voltage relative to the other for establishing an electrostatic field herebetween, first me as for moving a one of the members between said electrodes and through said field and closer to said high-- voltage electrode than to said other one of said electro I second means for moving other of the memb-rs between said electrodes and through said field an to said other one of said electrodes than to s voltage electrode, and conditioning means for r n leakage of said field via said first member bo h in the direction of travel of said first member and. in 21 91* ion opposite to the direction of travel of said first m and for increasing the efiective propelling peter .l g out within said field comprising a first metallic or tioni, g electrode structure spaced from said field in direction of the travel of said first me lb-er and in d electrical and mechanical engagement with said first me": her for controlling the poten'al of said first member it enters said field, a second metalli conditi ling elec trode structure spaced from said field. in a direction up posite to the direction of travel of said first member a in direct electrical and mechanical engagement with s first member for controlling potential of first member as it leaves said field, and means for maintaining both of 3, id conditioning electrodes at substantially the same potential as said high voltage electrode.

6. Apparatus for propellir particulate matter onto traveling Web member having adhesive 0. surface from a traveling belt-like member which travels below the traveling web member and which carries the particulate matter on an upper surface therecf, the upper surface of said belt-like member facing and lying prox mate the adhesive coated surface of the web member, comprising a pair of spaced-apart field electrodes one of which is at a high voltage relative to the other for establishing an electrostatic field thercbctween. first means for moving a first one of the members between said electrodes and through said field closer to said high-voltage electrode than to said other one of said electrodes, second i cans for moving the other one of the members between said electrodes and through said field and closer to said other one of said electrodes than to said high-voltage electrode, said first member being of semi-conducting material, and conditioning means for increasing the effective propelling potential gradient within said field. comprising a metallic conditioning electrode structure disposed outside of Said field and in direct electrical and mechanical engagement with said first member for controlling the potential of said first member, and means for maintaining said conditioning electrode at substantially the same potential as said high voltage electrode.

References Cited in the tile of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,869,511 January 20, 1959 Robert C Dickey et a1.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column, line 20, for "electrodes" read electrons Signed and sealed this 28th day of July 1959.

Attest KARL I-L, AXLINE ROBERT c. WATSON Attesting Officer Comnissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Robert C Dickey et al.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column .6, line 20, for {electrodesread electrons Signed and sealed this 28th day of July 1959.

Attest:

KARL H. AXLINE Comnissibner of Patents

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