US 3227136 A
Description (OCR text may contain errors)
Jan. 4, 1966 L. c BARTLETT ETAL 3,227,135
EXTRUSION COATING APPARATUS Filed Oct. 26, 1961 Fig./
LAWRENCE C BARTLETT WALKER F HUNTER JEROME S. OSMALOI/ INVENTORS By Fwd/4 QM fa-war ATTORNEYS United States Patent This invention relates to an extrusion coating apparatusespecially intended for use in the manufacture of magnet1ctape of the type wherein a dispersion of magnetic particles in a suitable binder is coated onto the surface of a carrier web or strip.
In the manufacture of magnetic tape of the type described, it is desirable to coat wide strips of the base or carrier material and to then slit it into tapes of the desired narrow width. However considerable difiiculty has been heretofore encountered, when attempting to coat webs or strips of appreciable width, in maintaining the desired uniformity of coating over the entire width of the web. One of the factors that tends to produce undesired variation in the coating arises from the tendency of the magnetic particles to agglomerate so that the dispersion varies in consistency as it is deposited on the base strip. Furthermore the final coating tends to reflect, in variations of its thickness and magnetic characteristics, every slight vibration of the coating apparatus.
It is an object of this invention to provide an extrusion coating apparatus which will very effectively coat a uniform layer of magnetic dispersion material onto a base or backing web of considerable width while eliminating the above difiiculties.
; t A further object is to prevent settling-out of the magnetic dispersion by maintaining the dispersion under high shear rate conditions from the time when it leaves the supply hopper until it is actually coated onto the base tively simple construction but which will maintain uniform coating thickness over a wide web of material and which will not be subject to clogging or other dithculties in commercial use.
Further objects will become apparent from the following description and claims particularly when viewed in the light of the accompanying drawing wherein:
FIG. 1 is a diagrammatic view of an extrusion coating apparatus particularly suited for coating of magnetic tape; FIG. 2 is a cross-sectional view through an extrusion 1 hopper used in the system shown in FIG. 1;
FIG. 3 is an enlarged view of the extrusion orifice of the hopper of FIG. 2 showing its relation to the coating roller; and a FIG. 4 is a side view of the extrusion hopper with parts broken away better to show the interior construction thereof.
As indicated above, FIG. 1 shows the general arrangement which we have found to be especially well adapted for coating magnetic dispersion by the extrusion method onto a suitable web orbacking support. In this figure no attempt has been made to show parts to proper scale 2 since it is primarily diagrammatic in character. The material to be coated consists of a dispersion f magnetic particles in a suitable binder, of the general type well known in the art. This dispersion is stored in a suitable supply tank or hopper 2 provided with suitable stirring apparatus 3 to maintain the dispersion properly mixed together. The dispersion is drawn off from the tank 2 by means of a circulating pump 4 which forces the material through a suitable filter 5 and thence, by way of a return pipe 6, back to the hopper or tank 2. It is well known that, when a fluid or fluid-like material flows through a conduit, that-portion of the fluid flowing along the wall of the conduit will have a lower velocity than that spaced further from the wall, and that this elfect is most pronounced immediately adjacent the wall. The term shear rate is a numerical expression of this effect and, as used herein,,is equal to the rate at which the fluid velocity in the area near the boundary surface of the conduit or passageway changes with the distance from such surface. It is expressed in terms of velocity change per unit distance (e.g. cm. per sec. per cm.) or in simplified form, as reciprocal seconds (secf The size of the conduits in the above-described circulating system and the rate of movement given the fluid material by means of pump 4 are preferably such that the material is maintained throughout the circulating system under relatively high shear rate conditions of about 200 cm. per sec. per cm. (or 200 S6074).
The supply of dispersion for the actual extrusion operation is tapped off from the circulating system, as indicated at 7, and is fed at a predetermined uniform rate and, again, under high shear rate conditions, to the extrusion hopper 8 by means of a suitable metering pump 9. The rate of rotation of pump 9 is, of course, correlated with the desired rate of extrusion, which in turn depends upon and is correlated with the rate of movement of the base material 9 about the backing roller 10 at the coating station. In order to prevent the strip 30 of material extruded by the hopper 8 from pulling away too soon from the lower lip 31 of the extrusion orifice 32 a suitable vacuum baflle, diagrammatically indicated at 11, may be provided adjacent the extrusion orifice. The pressure within bafiie 11 is preferably maintained slightly (.05 to 2.0 inches of water) below atmospheric pressure by means of a suction line 13 adapted to be connected to a suitable vacuum pump (not shown).
The extrusion hopper itself is conveniently constructed of a pair of body members 14 and 15 rigidly clamped together by suitable bolts 16. Members 14 and 15 are hollowed out as shown in FIG. 2 to conjointly form a generally cylindrical chamber 17, the forward walls 17' of which converge toward an extrusion slot 18 terminating in an orifice 32. It should be noted that the vertical width or thickness of slot 18 has been greatly exaggerated in FIG. 2 for purposes of illustration. In an actual hopper having a chamber of 2 /2 inches diameter, the vertical width or thickness of slot 18 might be only about .003 to .004 inch, for producing a layer of about .002 inch thick ness (prior to drying) as shown in enlarged view FIG. 3. As shown in FIGS. 2 and 4 material to be coated is supplied to the chamber 17 by way of a suitable port 19 preferably located in the median plane widthwise of the coating hopper to facilitate uniform distribution of the dispersion throughout the chamber. To assist in maintaining the hopper at a constant temperature the hopper sections 14 and 15 are preferably provided with a plurality of inter-connected passageways 20 for circulation of a temperature regulating fluid therethrough as diagrammati cally indicated at 20'.
Mounted for rotation within the chamber 17 is a cylindrical rotor 21, the periphery of which is spaced slightly from the interior walls of the chamber 17. Rotor 21 preferably has an extremely smooth outer periphery and is rotatably driven as by a motor 29 at a predetermined constant rate. In any particular case the rate will depend upon many factors such as the consistency of the dispersion, rate of extrusion, etc., but will ordinarily be between 200 and 2000 rpm. Rotation of this rotor tends to cause the dispersion within the chamber to constantly swirl about the chamber, maintaining it under high shear rate conditions and thereby preventing separation of the solid particles from the binder. Rotor 21 not only serves to maintain the dispersion within the chamber 17 under high shear rate conditions but also tends to greatly improve the distribution of the material throughout the entire length of this chamber, thereby causing the extruded layer to be of uniform quality throughout the width of the carrier strip.
However, the length of this rotor, particularly when it is intended for coating relatively wide strips, as contemplated in the present case, tends to cause a problem. The rotor is so long that, unless provisions were taken to guard against this effect, the rotor would sag slightly due to its own weight and would therefore tend to set up vibrations which would be transmitted to the dispersion being coated and would be reflected as small wave-like variations in the thickness of the coated dispersion on the final tape. However, we have effectively prevented this trouble by so constructing the rotor that it has a weight substantially equal to that of the displaced dispersion. The construction, of course, will depend upon the actual composition of the magnetic dispersion, but, in any particular case, this will be known and will be maintained constant for any particular coating operation. Thus the rotor effectively floats without any particular weight in the dispersion being utilized and there is no tendency of the rotor to sag or to set up vibrations which would, as previously described, tend to cause improper coating action.
The type of rotor construction shown in FIG. 4 has been found most effective to accomplish this result. Thus the rotor is conveniently formed of a tubular member 22 of suitable thickness mounted on a pair of end hub members 23 and 24 respectively. A suitable seal 25 is provided to prevent the dispersion (which is quite abrasive) from getting into the righthand bearing 26 by which the rotor is mounted in the hopper end plate 27.
Slot 18 and orifice 32 are preferably made sufficiently narrow (in the direction corresponding to the thickness of the extruded strip) that the dispersion passing therethrough will be maintained under the desired high shear rate conditions until it is finally extruded from the orifice. Thus there is never an opportunity for unwanted separation to occur.
Operation of the extiusion apparatus shown is believed to be obvious from the previous description. The dispersion in tank 2 is, as previously described, continually being circulated by pump 4 through filter and return pipe 6. Some of this extrusion, during the actual coating operations, is drawn off at a predetermined rate by metering pump 9 and supplied to the extrusion hopper 8. The material within the chamber 17 of hopper 3 is continually being swirled about the axis of the chamber at a relatively high rate due to the frictional drag produced by the rotor 21, as a result of which the dispersion will be maintained under high shear rate conditions, particularly adjacent the opposed peripheral walls of the chamber 17 and rotor 21 so that any tendency to separate is minimized. Some of the material is constantly being forced through slot 18 and out of the orifice 32 whereupon it is laid down in smooth fashion onto the surface of the backing material or web 9 passing about the roller 10. Throughout this entire operation the material is maintained under high shear rate conditions with the resulting effect that separation of the magnetic particles is effectively avoided. Thus it is possible to coat extremely wide webs of backing material witha layer of magnetic dispersion of uniform thickness and with substantially no variations therein. The actual thickness of the coating will, of course, depend upon the width of the slot 18 at the orifice 32, the rate at which the material is extruded therefrom, and the rate at which it is carried away by the web of backing material. Preferably the rate of movement of the backing material is somewhat greater than the rate of extrusion so that the extruded material is drawn out slightly as it leaves the orifice. For example, as previously stated for a coating thickness (before drying) of .002 inch we utilize a hopper having a slot width of from .003 to .004 inch.
While only one form of the extrusion apparatus has been shown in the drawing it will be obvious to those skilled in the art that many variations in the specific construction can be made without departing from the spirit and scope of the invention as defined by the appended claims.
1. An extrusion hopper comprising a body having formed therein a generally cylindrical elongated chamber having a peripheral wall, said body having a slot extending generally radially outwardly through said peripheral wall of said chamber and terminating in an extrusion orifice at the exterior of said body, a substantially smooth cylindrical rotor having its axis extending axially of and generally coaxial with said chamber and mounted for rotation therein, the outer periphery of said rotor being spaced radially inwardly from said wall of said chamber, means for supplying material to be extruded to said chamber at a predetermined rate and means for rotating said rotor about its axis with a uniform angular velocity.
2. An extrusion hopper as in claim 1 wherein said rotor is supported from said body solely by a pair of spaced bearings located adjacent opposite ends of said chamber.
3. An extrusion hopper as in claim 2 wherein said rotor is so constructed that the weight of that portion thereof between said bearings is substantially equal to the weight of a volume of corresponding size of the material to be extruded.
4. An extrusion hopper as in claim 1 wherein said rotor comprises a pair of end hub members and a cylindrical tubular member connecting said hub members.
5. An extrusion hopper comprising a body having a generally cylindrical elongated chamber formed therein and having a slot extending generally radially outwardly from said chamber and terminating in an extrusion orifice at the exterior of said body, said slot and said orifice being substantially coextensive in length with said chamber, a cylindrical rotor having a substantially smooth exterior periphery of a diameter somewhat less than that of said chamber, spaced bearing means carried by said body at opposite ends of said chamber and mounting said rotor for rotation within said chamber substantially coaxially therewith, means for supplying material to be extruded to said chamber and means for rotating said rotor with uniform angular velocity about its axis.
6. An extrusion hopper as in claim 5 wherein at least the major portion of said rotor between said bearings is so constructed that its weight is substantially equal to the weight of the volume of extrusion material which is displaced thereby within said chamber.
7. An extrusion hopper as in claim 6 wherein said portion of said rotor is of hollow tubular construction.
8. An extrusion hopper comprising a body having an elongated chamber formed therein, and having a slot extending outwardly from said chamber and terminating in an extrusion orifice at the exterior of said body, the major portion of the inner wall of said chamber being substantially cylindrical about an axis extending lengthwise of said chamber and said slot extending substantially radially with respect to said axis, a substantially smooth cylindri- 5 6 cal rotor positioned within said chamber with its axis References Cited by the Examiner substantially coinciding with said first-mentioned axis, and UNITED STATES PATENTS means mounting said rotor on said body for rotation about its axis, means for supplying extrusion material to 2 Egg h; 5513 said chamber for extrusion through said slot and orifice, 5 2474691 6/1949 Roehm 118410 and means Mating Said mm 2:681:294 6/1954 Beguin IIIZ IlS-AIO X 9. An extrusion hopper as in claim 8 wherein the portions of the inner wall of said chamber adjacent said CHARLES WILLM H, P imary Examiner.
slot converge toward said slot substantially tangentially JOSEPH R, LLIAM R MARTIN, to the cylindrical portion of said chamber wall. 10 Examiners.