US 3580771 A
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Description (OCR text may contain errors)
May 25, 1971 UTILIZING DIRECT CONTACT '1 NSFER Filed April 15, 196
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CLEAN TAPE SAMPLE 1 Fla 2 DEPOSIT THERMOPLASTIC MATERIAL ON TAPE SURFACE BOND TAPE TO THERMOPLASTIC MATERIAL PRESS TAPE ONTO CASTING PLATE INVENTOR.
KENT N. MAFFITT CAUSE RELEASE OF COATED TAPE FROM CASTING PLATE Alto/nay United States Patent METHOD FOR RENDERING AN IRREGULAR SURFACE SMOOTH UTILIZING DRECT CONTACT TRANSFER Kent N. Maflitt, Minneapolis, Minn, assignor to Litton Systems, Inc., Beverly Hills, Calif. Filed Apr. 13, 1967, Ser. No. 630,704 Int. Cl. 1344c 1/00 US. Cl. 156235 5 'Claims ABSTRACT OF THE DISCLOSURE A method for smoothing minute irregularities in the surface of magnetic recording tape. A thermoplastic material is applied to the tape oxide surface and the tape then is disposed against a gold-coated fiat casting surface while heat and pressure are applied. Thermal shock from applied liquid nitrogen causes release of the coated tape from the casting surface. A weighted roller on the apparatus applies pressure to the tape.
BACKGROUND OF THE INVENTION This invention relates in general to a method and apparatus for coating and in particular, to a method and apparatus for applying a coating having a highly smooth surface to a magnetic recording tape.
It is often desired in the course of working with or evaluating a material to provide at least a portion of such material with a highly smooth surface. The presence of such a smooth surface may be required to facilitate inspection of the material, or the smooth surface may be necessary to minimize abrasive wear of an object which contacts the surface of the material.
As one example of an area wherein excessive surface roughness of a specimen undergoing examination can be troublesome, one can look to the field of electron mirror microscopy. In one mode of operation of an electron mirror microscope, a collimated beam of electrons is directed toward a target or a specimen being examined. This specimen normally carries an electrical bias potential slightly negative with respect to the electron gun producing the electron beam. Because the target specimen potential is slightly negative with respect to the electron gun, the electrons comprising the beam do not have enough energy to reach the surface of the specimen and are slowed to zero axial velocity at some small but finite distance in front of the specimen. The electrons are then re-accelerated away from the specimen and back through a suitable magnetic lens system onto a phosphor screen to produce a visual pattern for observation or photographic recording.
Since the electrons are mirrored from a potential surface in front of the specimen and not from the physical surface of the specimen, it follows that the contours of this potential surface are a function, among other things, of any magnetic fields above the specimen physical surface and on the extent of physical smoothness or roughness of the surface. Other variables being equal, the pattern observable on the phosphor screen of the electron mirror microscope is indicative of the magnetic field on the sample being investigated, and so the magnetic patterns recorded on magnetic recording tape, for example, can be examined with the electron mirror microscope. Since the surface roughness of the tape or other specimen being examined also produces a distinctive pattern on the electron mirror microscope, however, this surface roughness pattern may be so prominent as to mask or obliterate the electron deflection by the magnetic field above the sample.
An exemplary sample of magnetic recording tape of the kind commercially available for recording data can 'be found with the aid of a suitable measuring instrument to have a measurable surface roughness characterized by a mean radius of curvature of up to, for example, 60 microns. The magnitude of the problem presented by this sort of roughness can best be appreciated by recognizing the fact that the typical separation in an electron mirror microscope between the physical specimen surface and the bias-produced potential mirroring surface may be only about 60 angstroms, where one microns equals 10,000 angstroms. Experience with the electron mirror micro scope has shown that the mean radius of curvature of the hillocks on the surface of a specimen being examined must be at least 1 cm. to avoid undue distortion of the image caused by surface roughness of the specimen. Accordingly, it can be seen that magnetic recording tape as described above is in need of treatment to render the tape surface smooth before the oxide coating surface of the tape can be examined in an electron mirror microscope.
Prior art smoothing techniques have been found to be deficient in some respect in overcoming the foregoing problem. Polishing of the tape oxide surface by means of an abrasive or butting surface produced fine scratches on the surface of the tape and often removed excessive amounts of the oxide coating. Furthermore, polishing was found to give poor reproducibility on successive tape specimens. Another technique that was attempted involved depositing an excessive amount of a flowable coating material on the tape surface and then thinning this coating material by drawing a straightedge over the surface of the tape. This method, however, produced coatings which were too thick or which tended to follow the existing contours of the tape surface. Still another attempt, representative of many different techniques employed in the hopes of overcoming the tape surface roughness problem, involved the use of carnauba wax and standard bufiing techniques to attempt to impart a high-gloss surface to the tape. It was found, however, that even such a surface is not sufficiently smooth to overcome the problems presented here.
SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide an improved method and apparatus for providing a coating having a high degree of surface smoothness.
It is another object of this invention to provide a method and apparatus for producing a very smooth coating on the oxide surface of the magnetic tape.
It is still another object of this invention to provide a method and apparatus for obtaining a very smooth surface on a magnetic tape without affecting either the tape structure or any magnetic information present thereon.
It is a further object of this invention to provide a method and an apparatus for obtaining a sufficiently smooth surface on a magnetic tape to enable such tape to be effectively examined in an electron mirror microscope without undue distortion of the microscope image resulting from the tape surface roughness.
Generally speaking, the objects and aims of this invention are accomplished through a tape coating process wherein the tape, after first being thoroughly cleaned, has deposited on the oxide coating thereof a thin filament, droplets, or particles of a thermoplastic coating material having suitable viscosity characteristics. The temperature of the tape and the coating material thereon is controlled to cause the coating material to have the desired viscosity, and the coated tape then is carefully pressed, coated oxide surface down, onto a smooth casting plate onto which a thin layer of a material such as gold or the like has previously been deposited. After the tape, the coating material, and the casting plate have cooled sufficiently, thermal shock is imparted to these items to cause the coated tape to be released from the casting plate. Because the gold film of the plate was bonded more strongly to the thermoplastic material than to the casting plate the gold film releases from the casting plate but remains bonded to the thermoplastic coating surface. The resulting tape surface is a gold-coated thermoplastic surface, with the exposed surface of the gold coating having substantially the same smoothness characteristics as the flat casting plate onto which the gold coating originally was deposited. The hillocks and other surface irregularities present in the tape surface have been substantially filled by the thermoplastic coating material.
Also disclosed herein and comprising a part of the invention is an apparatus for coating a magnetic tape sample according to the above method or according to a somewhat different technique as described below.
The exact nature of this invention, as well as other objects and advantages thereof will be readily apparent from consideration of the following summary.
DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross-section view of the oxide portion of a tape which has been coated according to the present invention;
FIG. 2 shows a flow diagram of the basic steps in the coating method of this invention; and
FIG. 3 shows a schematic view of apparatus according to the present invention.
DESCRIPTION OF THE DISCLOSED EMBODIMENTS FIG. 1 shows a cross-section view of an exemplary segment of magnetic recording tape indicated generally at 10, and showing at 11 the metallic oxide surface on which information is magnetically impressed during the recording operation, it being understood, of course, that the oxide layer 11 is disposed on a thin flexible web typically made of a plastic material and not shown in FIG. 1. Although oxide layer 11 may appear smooth to the unaided eye, this layer in reality consists of a series of hillocks 12 and valleys 13, with these valleys as seen in section representing a network of crevices interspersed between the hillocks in the tape surface. The hillocks 12 and valleys 13 thus produce the uneven tape surface, referred to above, which makes observation of the tape with the electron mirror microscope difficult and which, incidental- 1y, causes wear of the recording and reproducing heads in tape recording and play-back equipment, particularly at the high tape-head velocities found in recording at high information rates.
If a tape section as thus far described were treated to produce a surface of ideal flatness for electron mirror microscope examination or for other purposes by the process of the present invention, the valleys 13 would be filled with a thermoplastic material 14 as described below and the upper surface 15 of the thermoplastic material would be coincident with the top of the highest one of hillocks 12 in a particular region of the tape segment 10. In addi tion to the coating of thermoplastic material 14 the present invention also includes an additional layer 16 of an electrically conductive material such as gold or the like, so that the outer surface of the coated tape can be electrically biased for examination in the electron mirror microscope. It has been experimentally determined that a practical tape coating which enables the tape to be satisfactorily examined with electron mirror microscopes of the present state-of-the-art must have a top surface 17 sufficiently smooth that the radius of curvature of any deviation from a fiat plane is greater than about 1 cm. or 10,000 microns. Moreover, the thickness of such coating as seen at the peaks of hillocks 12 of the tape should be no greater than 0.2 micron at maximum and preferably not greater than one-tenth the density of information recorded on the tape. Assuming a not unlikely situation where magnetic information is contained on the tape at a density in the order of one line per micron, it is seen 4 that the desirable maximum peak to top surface 17 coating thickness should not be more than about 0.1 micron.
The general steps comprising the coating method of this invention are set forth diagrammatically in FIG. 2 of the drawing. Briefly, these steps include cleaning the oxide surface of the tape to remove any loose particles of oxide or any other unwanted foreign substances on the surface thereof and then depositing on the oxide surface a quantity of a thermoplastic material. The tape and the thermoplastic material next are heated to a temperature which provides the desired viscosity of the thermoplastic material without producing unwanted etfects on the tape. Following the steps of heating the tape is pressed, thermoplastic coated surface down, onto a casting plate, the surface of which has previously been coated with a thin layer of gold. The tape and the casting plate next are cooled to approximately room temperature and then are subjected to thermal shock by immersion in a very cold substance, such as liquid nitrogen, to cause separation of the casting plate from the coated tape. As a final step, if necessary or desired, the coated surface of the tape, which now contains the gold coating formerly on the casting plate, may be coated by evaporation or another suitable process with an additional metallic layer. The process now is complete, and the coated tape produced through this process has a sufiiciently smooth surface to permit examination in an electron mirror microscope, or to be used for other purposes where an extremely smooth surface is required.
Turning now to a closer examination of the details of the method steps outlined above, it has been found that proper cleaning of the tape surface is essential to insure that all of the gold coating on the casting plate which is in contact with the thermoplastic film on the tape will adhere to this film and will release from the casting plate surface. One cleaning method found to be effective is the agitation of the tape sample 10 in an ultrasonic cleaner with a suitable cleaning liquid. By way of example, methanol and isopropanol have both proven to be effective cleaning agents for use in an ultrasonic cleaning process.
Following the step of cleaning, a quantity of thermoplastic material is deposited on the oxide surface of the tape. The thermoplastic material used may be any suitable substance so long as this material has the desired viscosity at the temperatures used in the coating method, since maintaining the proper viscosity is essential to obtain a tape coating of the requisite thickness. With recording tapes presently commercially available, it has been experimentally determined that severe curling of the tape does not allow the tape temperature to be raised above 100 C. during the coating process. Moreover, it has been additionally determined that tape samples coated at temperatures substantially above 80 C. are significantly more difficult to release from the casting plate, and so a practical compromise is desirable for an effective rcproducible coating process. Assuming a maximum temperature for the process of 80 C., the thermoplastic material to be used should be one having a viscosity not greater than about 27,000 poises at that temperature.
By way of example, one thermoplastic material successfully used in the practice of this invention is an epoxy resin made by the Ciba Products Company and designated No. 6060. At 80 C. this resin has a viscosity of approximately 11,000 poises. It should be noted, however, that the viscosity of this resin is highly dependent upon temperature changes of only a few degrees C. from the 80 C. level, thus making temperature control an important factor in the use of this resin.
Another thermoplastic resin used in the practice of this invention is made by the Pennsylvania Industrial Chemical Corporation and is designated resin A-; this resin has a viscosity of about 27,000 poises at C.
After the desired thermoplastic material is chosen, a quantity of thermoplastic material must be used to insure This may be done by fabricating the thermoplastic material into a thin filament or into a number of droplets, particles, or spheres, and then placing the desired number thereof onto the tape surface. Since the thickness of the coating applied by this process is basically a function of coating material viscosity at a particular temperature, this coating thickness is not significantly afiected by the quantity of thermoplastic material initially deposited on the tape, so long as an obviously excessive amount of material is not used. However, it is clear that at least a sufi'lcient quantity of thermoplastic material must be used to insure complete coverage of the tape area to be coated.
After the tape area of interest has received a quantity of thermoplastic material, the tape and the material thereon are heated to a desired temperature, for example, 80 C. to bond the thermoplastic material to the tape.
Following the foregoing preparation of the tape, the next step requires a flat casting surface covered with a relatively thin layer of a material such as gold, palladium, or the like. Since the material selected for coating the casting surface must adhere more strongly to the thermoplastic material than to the casting surface, and since this material must be electrically conductive and/or must be capable of having a conductive coating deposited thereon, a choice of coating material should be made with these characteristics in mind. In an exemplary practice of this invention the thickness of the gold or other coating on the casting plate has been chosen to be approximately 500 angstroms thick, although experimental use of this process on a series of tapes cast against gold coatings having a thickness range producing resistances between 5000 and 1 ohm/sq. cm. revealed no significant difference in the manner in which the gold coating released from the casting plate surface.
Glass casting plates produce good results because such plates can be obtained having the desired surface smoothness characteristics, and also since layers of gold or palladium deposited thereon readily release from a glass casting plate upon adherence to the thermoplastic resin.
Once the tape has been prepared by bonding a quantity of the desired thermoplastic material thereto and the casting plate with, for example, a gold casting thereon also has been prepared, the actual casting process may be undertaken. The tape sample is placed on the casting plate with the thermoplastic material on the tape facing the gold coating on the casting plate surface. The tape, the thermoplastic material thereon, and the casting plate then are heated to the desired casting temperature, for example, 74 C., preferably by placing the tape, the casting plate, and any casting apparatus such as described below into a controlled environment such as an oven. Because the viscosity of the thermoplastic material may vary significantly with a temperature change of only a few degrees C., close temperature ontrol should be maintained at this time to insure that the coating obtained is uniformly of the desired thickness.
The thermoplastic material now is cast to a thin film by pressing the tape firmly against the casting plate. One way in which this may be done requires that the casting plate and the tape disposed therein be sandwiched between a pair of intermediate members such as two smooth plates, and the resulting sandwich combination have pressure applied thereto, such as with a press or simply by being clamped in a vice, for a period of time, for example, minutes and at the desired maintained temperature. Better results have been obtained, however, by using a weighted roller to press the tape onto the gold coated casting plate. For example, a roller weighing one kilogram and moving across the tape at a speed in the range of 4 to millimeters per minute has produced good results, with the best results being obtained at the lower roller speeds. The roller diameter should be chosen to produce the desired coating quality with the viscosity of the thermoplastic material chosen, at the particular chosen casting temperature, and additionally at the roller rate selected. It
has been found that a roller having a radius as small as 0.5 millimeter tends to cause unwanted permanent deformation of the tape, whereas good results are obtainable with a roller of about 3 millimeters radius.
After the entire tape sample has been pressed against the casting plate, the sample and casting plate are allowed to cool to room temperature. Following this cooling, release of the coated tape from the casting plate is obtained by subjecting the tape and plate, as temporarily bonded together, to severe thermal shock. One way of doing this is to subject the tape and casting plate to a substance, such as liquid nitrogen, which is very cold and which does not contaminate or otherwise adversely effect the coated tape. This may be accomplished either by placing the casting plate and the tape bonded thereto in a quantity of liquid nitrogen; alternatively, the liquid nitrogen can be poured over the tape and the plate to cause the desired separation. If the tape sample is not released from the casting plate during the first subjection to liquid nitrogen, release still may be effected by repeated alternate applications of the liquid nitrogen. If tape release still is not obtained, it has been found that removal of the tape sample and the casting plate from the liquid nitrogen and placing the tape sample on a metal block at room temperature can produce sufiiciently rapid heat transfer to the sample to cause the desire to release from the casting plate.
Once release of the coated tape from the casting plate is obtained, the tape sample then may be used for whatever purpose desired. If the layer of gold previously deposited on the casting plate and now forming the top surface 17 of the tape sample is not sufliciently thick or continuous to permit the electric bias required of the sample for electron mirror microscope examination to be provided, an additional layer of gold can be coated onto top surface 17 by evaporation or by any other suitable technique. It should be understood, however, that this last mentioned step of providing the additional layer of gold is optional depending on the intended use of the coated sample and does not comprise an essential element of the method of this invention.
Turning now to FIG. 3, there is shown an embodiment of an apparatus which can be used to facilitate the practice of the method described herein. The apparatus of FIG. 3, as indicated generally at 21, includes a base member 22 having a fixed clamp 23 secured intermediate the ends thereof, with this clamp being readily actuatable by any appropriate mechanism to effect clamping of the end 25 of a tape sample 24 to the upper surface of the base member 22. Alternatively, and as described below, the end 25 of the tape sample 24 can be secured at a second clamp location 26 a predetermined distance above the upper surface of base member 22. Clamp 23 could simply comprise a pair of block-like members releasably fastenable to the base member 22.
Positioned on base member 22 at a spaced apart distance from fixed clamp 23 is a slidable clamp 30 having a clamping location 31 spaced above the top surface of the base member for clamping engagement of the other end 32 of the tape sample 24. Slidable clamp 30 has freedom of movement in at least the path indicated by double arrow 33, and the slidable clamp is normally biased toward the end 34 of the base member 22 by means of a spring or other resilient device 35 anchored adjacent the end 34.
The tape sample 24 shown inserted between the fixed clamp 23 and the slidable clamp 30 passes over a casting plate 38 which previously has been coated with gold, for example, as described above. A roller 39 is shown passing over the tape sample 24, with this roller as depicted in FIG. 3 being approximately midway between the ends of the casting plate 38 and thus retaining about one-half of the tape sample disposed over the casting plate in contact therewith. The desired Weight for the casting operation is imparted to roller 39 by means of a weight 40 which rests on the roller which additionally rests for sliding movement along support block 41.
Attached to the unsupported end of weight 40 is a line 42 which passes over a suitable drive pulley 43 and terminates in a counterweight 44. Pulley 43 is connected by any suitable mechanism to a motor or other drive device so that the pulley can pull the line 42 and the weight 40 at the desired speed. Movement of the weight 40 in this manner frictionally engages roller 39 to cause the roller to depress the tape sample 24 into casting engagement with the casting plate 38. With the apparatus shown it will be understood that the rotational speed of pulley 43 must be adjusted to produce the desired rate of forward movement of roller 39 for the particular roller diameter and pulley diameter being used. As set forth above, good results have been obtained at a roller forward speed of about 4 millimeters per minute.
In the operation of the apparatus shown in FIG. 3 the tape sample 24 first is prepared as set forth above by cleaning and by bonding a quantity of thermoplastic material to the oxide surface of the tape. The tape sample then is mounted in the apparatus as shown in FIG. 3 with end 25 retained by clamp 26 at or adjacent the top surface of base member 22 and with the other end 32 of the tape sample being clamped at location 31 of slidable clamp 30. The gold coated casting plate 38 next is suitably positioned on the base member 22 underneath a portion of the tape sample 24, and the entire apparatus or at least the tape sample and the casting plate are placed in an oven or other controlled environment for elevation to the desired casting temperature.
At the time that the heating of the apparatus is commenced, the roller 39 and the weight 40 are positioned such that the bottom of the roller contacts the casting plate 38 at or adjacent the left side 45 thereof, thus causing contact between the tape sample 24 and the casting plate 38 only at this left side. Sufiicient time should be allowed for the temperature of the apparatus to stabilize at the desired casting temperature, after which the pulley 43 is actuated to cause the weight 40 to move the roller 39 across the tape sample. This movement of the roller 39 presses the tape sample into casting engagement with the gold coated casting plate 38 as the roller progresses on its path and retains in place on the casting plate that portion of the tape sample over which the tape has passed. The resilient tension exerted on the slidable clamp maintains the end 32 of the tape sample under constant tension as the roller 39 progresses, so that the portion of the tape sample as yet uncontacted by the roller does not prematurely engage the casting plate.
After the roller has completed its traverse of the casting plate, the tension on the tape sample 24 is released by disconnecting the resilient member and the weight and roller 39 are removed from the surface of the tape sample. The apparatus is permitted to cool, for example, to room temperature, after which tension is restored on the tape sample 24 by reconnecting the resilient member 35. Liquid nitrogen then is poured over the tape sample 24 and the casting plate 38, causing the casting plate to be released and to fall from the tape. The coating of the tape sample is now completed, and the coated tape sample now can be removed from the clamps 23 and 30 for additional gold coating or for utilization as desired.
In some instances it is required that a tape sample have a smooth coating thinner than can be applied by the foregoing procedure. By causing repeated passes of the roller 39 over the tape sample and by supporting the tape sample differently, however, an even thinner coating according to the present invention can be applied to the tape sample. Referring once more to FIG. 3 and noting the clamp location 26 on the fixed clamp 23, it can be seen that by securing the end 25 of the tape sample at clamp location 26 and with the other end 32 of the tape sample secured as before at location 31, then the entire tape sample is supported out of contact with the casting plate 38. When the rolling procedure is commenced from the left side 45 of the casting plate 38 as described above, the
tension on the tape sample imparted by the resilient member 35 retains the tape sample completely out of contact with the casting plate except for that portion of the tape sample under the roller 39 at any particular time. Thus, the rolling process may be repeated several times and with each successive pass of the roller a portion of the thermoplastic material present on the tape sample adheres to the casting plate rather than to the tape. It will be understood, of course, that a new gold coated casting plate must be used for each pass of the roller. For the final pass of the roller the tape sample must be supported in the apparatus in the normal manner with the tape end 25 retained by the fixed clamp 23 adjacent the upper surface of the base member 22, so that as the roller 39 passes over the sample the sample is allowed to adhere to the casting plate in the normal manner. After this is accomplished, the removal of the casting plate from the coated tape sample is accomplished as above by pouring liquid nitrogen thereover. It has been found experimentally that about four passes of the roller over the tape sample can be made before the residual film of thermoplastic material on the tape becomes discontinuous.
From the foregoing it can be seen that there is disclosed and described herein a new and improved method for applying smooth coatings to articles such as recording tape or the like. Although the major emphasis of this specification is directed toward such tapes, this is by way of example only and without the intention of limiting the invention thereto, the only limiting factors being that the article to be coated must be susceptible of receiving the thermoplastic material used so that this material separates from the casting plate or the coating placed thereon more readily than it separates from the article being coated. There also is described and disclosed herein an apparatus by which the coating method can be carried out without the problems of smearing the thermoplastic material and improperly tensioning the tape sample that may introduce awkwardness into manual practice of the method.
Although the method and apparatus set forth above is primarily directed to the preparation of a tape specimen for examination under laboratory conditions, it is emphasized that the scope of the present invention is not so limited. For example, a tape or other workpiece of indefinite length could be coated by this method to have a smooth surface less destructive to the heads used in magnetic recording equipment. Similarly, the casting surface could comprise, e.g., a cylinder on which the gold or other coating was applied and against which the tape being coated was pressed in either a circular or a helical manner.
Finally, although numerous specific values of temperature, physical dimension, and the like are contained herein, it should be understood that this is done by way of example only and without intent to limit, except where such values are indicated as being critical to the opera tion of the invention. Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced other than as specifically described.
What is claimed is:
1. The method of coating an article surface having minute irregularities therein to render the surface smoother, comprising the steps of:
depositing a quantity of thermoplastic material onto the surface to be coated;
providing a casting surface having deposited thereon a thin layer of material which will adhere more strongly to the thermoplastic material than to the casting surface;
heating the surface to be coated and the flat casting surface to a desired casting temperature;
pressing the surface to be coated into engaging contact with the casting surface, said pressing being at sufiicient pressure to cause the thermoplastic material to flow between the surface to be coated and the casting surface and into any irregularities in the surface to be coated;
cooling the surface to be coated and the casting surface to ambient temperature; and
separating from the casting surface that portion of the layer of deposited material against which the surface to be coated was pressed, so that the article surface coating comprises a layer of thermoplastic material covered by a layer of said material deposited on the casting surface.
2. The method as in claim 1, wherein:
said desired casting temperature is less than the minimum temperature at which one or more predetermined physical qualities of the article being coated becomes adversely aifected; and
said thermoplastic material deposited on the surface to be coated is chosen to have at said desired casting temperature a viscosity which promotes said flow during said step of pressing the surface to be coated into engaging contact with the coating surface.
3. The method as in claim 1, wherein said steps of pressing comprises the steps of:
disposing the article in close proximity to and out of contact with the casting surface;
establishing a zone of pressing contact between said article surface and said casting surface, the area of said zone being substantially less than the total area of the article surface to be coated;
traversing said zone of pressing contact along the article so that the entire article surface to be coated is serially pressed against the casting surface; and
retaining the portion of the article traversed by said zone in engagement with the casting surface until said step of traversing is completed.
4. The method as in claim 1, wherein said step of pressing comprises the steps of:
disposing the article in close proximity to and out of contact with the casting surface;
establishing a zone of pressing contact between said article surface and said casting surface, the area of said zone being substantially less than the total area of the article surface to be coated;
traversing said zone of pressing contact along the article so that the entire article surface to be coated is serially pressed against the coating surface;
disengaging each portion of the article from the casting surface immediately after said zone has traversed such portion;
providing in close proximity to and out of contact with the article, another fiat casting surface having a said thin layer of material deposited thereon; and
repeating at least one time and in sequence each of said steps of establishing a zone of pressing contact, traversing said zone, and disengaging each portion, while providing another new flat casting surface for each subsequent ones of said steps of repeating, so that the coating applied to the article surface is of the desired thinness, the final one of said repetitive sequence of steps being varied to retain the portion of the article traversed by said zone in engagement with the casting surface until the final step of traversing is completed.
5. The method as in claim 1, wherein said steps of separating comprises:
subjecting the casting surface and the article temporarily adhered thereto to thermal shock sufficient to cause separation of the casting surface from the coated article.
References Cited UNITED STATES PATENTS 3,043,728 7/ 1962 Stauffer 15 6233X 3,306,798 2/1967 Gaenge 15623 3X 3,458,376 7/1969 Malik l56233X 3,480,500 11/1969 Hotter 156-233X CARL D. QUARFORTH, Primary Examiner S. J. LECHERT, Jr., Assistant Examiner U.S. Cl. X.R.