US2827935A - Method for treating wood surfaces - Google Patents

Description

March 25, 1958 G. E. ALEXANDER 2,827,935

METHOD FOR TREATING woos SURFACES Filed Oct. 10, 1956 INVENTOR. 6e07ye Z yiexarr$ r BY 3 a M United States Patent METHOD FOR TREATING WOOD SURFACES George E. Alexander, Bloomingdale, N. Y., assignor to General Plywood Corporation, Louisville, Ky., a corporation of Kentucky Application October 10, 1956, Serial No. 615,127

18 Claims. (Cl. 144309) This invention relates to certain new and useful improvements in wood products and in a method of and apparatus for treating wood surfaces; and this application is a continuation in part of copending application Serial No. 566,690, filed February 20, 1956.

In completing the manufacture of plywood wall panels, flush doors and the like, it is customary to sand the surface in order to remove blemishes and render the surface as smooth as possible. The sanded wood surfaces of flush doors, for example, are often sealed With alcoholdiluted shellac and ultimately finished with varnish. One application of each is desirable. Unfortunately, the sanded surface of the door absorbs large quantities of shellac, which raises the grain of the wood, producing a marked toughening of its surface. It is then necessary to resand this roughened surface, and resanding necessitates the application of another coat of sealer. After sealing is completed, the varnish coat is applied. :As often as not, it strikes into the surface and raises the grain again, necessitating a resanding operation which must be followed by another varnish coat. These resanding and refinishing operations, which obviously increase the cost of finishing the door, are often performed on the job usually by hand or with small sanders. Here, the quality of the final finish depends largely on the skill of the worker and again, as often as not, does not measure up to that desired.

The principal object of this invention is to provide a novel wood surface treating process which can be simply and inexpensively carried out and which results in the substantial improvement of that surface in one or more respects such as increased hardness, smoothness and reflectivity and reduced porosity and absorbency.

Another important object is to provide a wood glossifying process which can be performed at high speed on a straight through production line conveyor.

Another important object of this invention is to provide a novel and inexpensive frictional method for changing the structure of the wood along the surface without changing the underlying structure and, through such change, to effect a substantial increase in surface smoothness and a substantial decrease in its ability to absorb finishing or other materials.

Another object of this invention is to provide a new and novel process for producing a wood product having its surface wood structure of relatively greater density and finer texture than the density and texture of the underlying natural wood structure.

I have found that the objects of my invention may be achieved by frictionally heating the surface of the wood sufliciently'to soften the wood along that surface without softening it throughout its depth and by compressing the wood while the surface wood is softened so as to density the surface wood without appreciably densifying the wood throughout its depth. The surface should be frictionally heated to a temperature high enough to soften the surface wood (by softening the plastic binder of the surface wood) and it may be heated to a temperature sufiicient to char the surface wood provided the charring temperature is reduced below the lower char limit before charring begins. The chemistry of wood is very complex, difiicult to understand and not too well understood. However, it is known to contain interdispersed heat plasticizable materials and its content of these materials is believed to be largely embraced by the term lignin. The process is carried out on seasoned and dried wood without artificial moistening.

By frictionally heating and simultaneously compressing a commercially sanded surface in this manner, its properties can be so greatly enhanced as to render it highly useful in many difierent applications. For example, it may be advantageously employed in concrete forms since there is substantially less tendency of the concrete to stick to such surfaces. It is particularly suited for the reception of finishing materials ranging from transparent to opaque. For example, only one coat of sealer and one of varnish are required to produce a finish far superior to that heretofore obtainable even with the most exacting labor. This is due to the fact that the ,surface is twice as smooth as the initial sanded surface and there is little or no tendency for the grain or surface fibers to rise; hence, the application of these materials does not normally roughen the surface. I base my statement that the surface is twice as smooth, etc. on the fact that the reflectivity of the treated surface is approximately twice that of the commercially sanded surface.

The invention is illustrated in the accompanying drawing which is a diagrammatic View of the apparatus used to practice the method of this invention.

Apparatus for practicing the invention The drawing discloses one type of apparatus for practicing the process of this invention. This apparatus includes a pair of horizontally spaced and aligned conveyor sections or portions 1 and 2, each of which contains a plurality of freely rotatable rollers 3. The upper surface of the rollers 3 project slightly above the surface of the conveyor frame to support a piece of plywood or planar wood products such as a fiush door or wall panel 4. Thus, the wall panel 4 is freely movable on the rollers 3. The adjacent spaced ends of conveyor portions 1, 2 are mounted on correspondingly spaced brackets 5, 6 which are secured by any suitable means, such as welding, to upstanding supporting members 7, 8 of inverted L-shape. The supporting members 7, S are also provided with upwardly turned lip portions 7a, 8a, respectively, which are spaced from each other to define an opening 9 between them.

The opening 9 is arranged to receive an arcuate portion of a cylindrical member or drum 10 fixedly mounted on driving shaft 11, which is driven by any suitable driving means such as a motor (not shown). This drum 10 is preferably approximately 16 inches in diameter but may be of any diameter desired according to the size of the apparatus in the drawing, and is preferably covered with one or more cylindrical layers or sleeves 10a, composed of rubber or other suitable yieldable material so that the roll can compel the belt to conform to uneven or itregular surfaces in a manner known in this art. If rubber is used it may be vulcanized to the surface of the drum 10. The arcuate portion of the drum 10 which extends through the opening 9 is so positioned that it tangentially engages the undersurface of the panel 4, when it is positioned on the conveyor rollers 3.

In order to treat the underside of the panel 4 in accordance with this invention a continuous or endless surface treating belt 12, preferably composed of cork or containing a cork surface, is arranged around the peripheries of 3 the rotating drum 10, which functions as a backing member for belt 12, and an idler drum 13, which is fixedly mounted for rotating movement on a shaft 14 supported in any suitable manner. The drums 10, 13 are rotated by drive shaft 11 in the counterclockwise direction indicated by arrows D and I, the idler drum 13 being used to maintain the desired tension in the surface treating belt 12.

It should be noted that I preferably employ a surface treating belt of the type having a cork surface, e. g., as exemplified by the belts presently made under U. S. Patent No. 2,542,058, except that the very mild abrasive sometimes employed in such cork belts is unnecessary and may be omitted. Any other flexible, yielding, relatively nonabrasive frictional rubbing medium or frictional belt material, such as canvas, or other cloth made of natural or synthetic fibers, may be employed so long as it is capable of frictionally developing the requisite temperature when the requisite operating conditions are employed, and of withstanding both the operating temperatures and pressures. It will be observed that these belts provide a flexible, frictional, relatively nonabrasive buffing or rubbing and somewhat yielding surface composed of nonmetallic material. Since this material, in comparison with metal, is a relatively poor conductor of heat or a good heat insulator, it may be aptly described as a heat insulating material.

It should be noted also that my process does not make a rough surface smooth, nor does it involve the abrasive cutting or removal of any material. For example, if a surface is rough sanded, and then heated and densified in accordance with my invention, it will still remain a rough surface even though there has been some improvement in smoothness. But this rough sanded surface will nevertheless have the superior finishing properties which characterize surfaces treated in accordance with my invention. However, for the most uniform surface treatment, the process should be applied to wood surfaces which have been finish sanded.

The panel 4 is rapidly moved from left to right, along the predetermined path provided by the conveyor. This movement is effected by a feed roll 15, which is arranged above and in contact with the upper surface of the panel. The feed roll 15 is preferably conventionally covered with a spirally grooved or ungrooved cylinder of rubber to increase traction and is driven in the direction of the arrow F by means of a shaft 16 on which roll 15 is fixedly mounted. The shaft 16 is supported on any suitable type of resilient bearing pedestal (not shown) which permits a limited amount of vertical movement so that the feed roll 15 will exert a downward pressure on the work in the direction of the arrow P. The weight of the roll itself may be augmented if desired by auxiliary weights, springs or other means for exerting the required downward pressure, which should be substantial. The shaft is preferably driven by means of a belt 17 which is arranged around the shaft 16 in such a manner as to pull downwardly on the shaft 16 as it is being driven.

Operation In operation, a panel 4 is fed into the space between drums and 15. The substantial pressure of the feed roll against the panel forces the undersurface of that panel downwardly against the treating belt 12 as the panel passes therebetween. The panel is fed or propelled by the feed roll at a linear speed approximating the peripheral speed of the feed roll, there being relatively little or no slippage therebetween. The drum 10 drives the surface treating belt in a direction opposite to that of the panel and at a substantially higher speed. Under these conditions, a high temperature is momentarily produced on the surface of the panel along the line or band of contact between the belt and the panel. This temperature should be high enough to soften the surface wood momentarily, and more or less instantaneously, without softening the underlying wood structure of the panel. As a result of this surface temperature condition, the pressure exerted against the panel along the same line of contact becomes effective to compress and densify the softened surface wood without compressing or densifying the underlying wood structure. Naturally, since heat and pressure are thus applied to the panel progressively along its length, the surface of that panel is progressively compressed and densified in a like manner.

If desired a very mild abrasive or lubricating material in dry powdered form may be employed in conjunction with the process with some improvement in the surface sheen. The process, however, is essentially a nonabrasive process, as is clear from the fact that even a very light pencil mark made on a piece of wood before treatment is not removed therefrom during the surface treatment.

I am not able to explain, with complete certainty, precisely what happens in or to the wood during the practice of my invention. However, I believe that the following occurs: first, the heat, frictionally developed along the line or band of contact, more or less instantaneously softens the surface wood along the line of contact, by softening its heat plasticizable materials without softening such materials throughout the depth of the wood; second, the softening of such surface wood renders it compressible without appreciably affecting the compressibility of the rest of the wood; third, the pressure of the frictional rubbing medium compresses the softened surface wood without compressing the underlying wood; fourth, the compression of the surface wood not only densifies the wood, thereby decreasing its porosity and increasing the fineness of its texture, but also causes the softened heat plasticizable materials contained in that wood at least to flatten or spread out (and possibly to flow in some cases) and thereby fill or seal most if not all of the microscopic pores and like openings in the surface wood; and fifth, the frictionally developed heat is quickly dissipated, causing the said heat plasticizable materials to set to a hard or rigid state and thereby bond the densified surface wood permanently in its densified condition and also impart a semiglaze to the exterior surface of that wood.

Heated platens In understanding the present invention, it may be helpful to observe that a wood surface may be pressure smoothed and densified by a heated platen having a surface which is pressed against the surface of the wood. For example, a platen heated to a temperature of 475 F. to 500 F. and pressed against the wood surface for one second at 200 p. s. i. gives good results. In general, when a heated platen is used, the pressure should be p. s. i. or more, and the quality of the finished surface improves up to a pressure of 300 p. s. i. or more. The only upper limit on the pressure is that irrlposed by the crushing strength of the unheated body of the wood, which must not be reached.

The results obtainable on the treatment of birch plywood panels by direct pressure with a flat rigid platen are as follows:

In the above table the quality of the surface finish is indicated roughly. The surface produced by frictional treatment w1th the apparatus illustrated in the drawing is better than any produced with a rigid heated platen and would be rated excellent.

Experiments with heated platens indicate clearly that the best results are achieved when the operation is carried out so rapidly that only a very thin surface layer of the wood is heated and with a pressure sufiicient to produce a high degree of permanent compression of the thin surface layer. This superiority may be seen by comparing specimen 1 and specimen 3, above. On specimen 1, the platen heated to 400 F. was held in contact with the panel at a pressure of 35 p. s. i. for 4 /2 minutes (5 minutes would scorch the wood). A permanent compression of the panel of .0019" was obtained but the quality of the surface was inferior to that obtained with specimen 3 above. Possibly the pressure was insufiicient to produce the desired degree of surface densification at the temperature used although, due to the long exposure, the wood was heated and softened to a greater depth and consequently the total permanent compression (i. e., reduction in thickness due to treatment of one surface) was higher than that of specimen 3. It should be noted further that the platen temperatures given above will necessarily be higher than the temperature reached by the wood due to the necessity of transferring heat by conduction from the platen to the wood.

While a thin surface layer of the wood may be densitied by the use of heated platens, their use in commercial operations is subject to the difficulty that the heat must be transferred from the platen to the wood by conduction, and it is seldom practical to provide a uniform contact pressure between the platen and all portions of the wood surface due to irregularities in the wood surface and the practical difiiculty of constructing a platen having sufficient flexibility to conform to an uneven wood surface. Accordingly, a greater time of exposure is required when platens are used, so that the high points of the wood surface will be compressed enough to allow subsequent heating and compression of the low points. By the same token, somewhat greater pressures are required than would be effective if all portions of the surface were contacted with the same pressure at the same time, and the temperatures must be lower than those which would be permissible for shorter time exposures. In addition, a platen type apparatus is necessarily more cumbersome and difficult to operate at high speed. While the generation and use of frictional heat is an essential part of my invention, hot platens are mentioned because they facilitate an understanding of my invention.

Searching and charring In order to heat and densify only a very thin layer of wood, it is found that high temperatures are required and that, except in those instances where scorching would not be objectionable (i. e., on wood that is to be painted or used for concrete forms), the time of exposure is usually limited by the tendency to scorch the wood. Since scorching and charring are functions of time and temperature, it may be interesting to note, by way of example, that woods such as yellow birch, yellow poplar and Douglas fir may be expected to scorch and char under the approximate time and temperature conditions indicated in the following table when subject to pressure under stationary heated platens:

Yieldably covered backing member However, vastly superior results are achieved by the apparatus illustrated in the drawing wherein the surface of the wood is heated directly by friction and it is a simple matter to obtain relatively uniform pressure between the friction member and all portions of the wood surface and hence uniform heating and densification even though the wood surface is not perfectly flat. In that connection, the use of a rubber or other yielding sleeve on drum 10 helps to distribute the pressure uniformly over the surface even though the surface is not perfectly flat and the sheet of wood is'relatively rigid. If the sheet is thin and flexible, the rubber sleeve is not necessary but may be employed. Moreover, with this apparatus the process may be carried out economically at high speed and in conjunction with prior continuous sanding operations.

Temperatures and pressures It is extremely difiicult to determine the precise temperatures and pressures which obtain in the practice of this invention with the apparatus of the drawing. I am well satisfied that the minimum temperatures employed should fall within the softening-melting range (400 F. to 525 F.) of the heat plasticizable content of the wood. I am equally well satisfied that the momentary operating temperatures employed may and preferably do greatly exceed that range so long as the contact time is reduced sufficiently to prevent charring of the surface Wood in all instances and scorching in those instances where it is objectionable.

Correlation of rubbing speed, pressure and time In the apparatus of the drawing, the work feed rate, the belt speed and the pressure must be correlated to produce the desired densification of surface wood without charring and in most cases without scorching the wood. For the purposes of this invention beneficial results are achieved when a permanent compression of the wood surface ranging from about .0005 to about .005" is obtained provided the treatment time with the apparatus of the drawing is sufliciently short to restrict the heating effect to a very thin surface layer. However, the preferred range of permanent compression is from about .0005" to about .0025", since within that range the best results are achieved.

Since the desired results are achieved when the temperatures developed are high and only surface wood is heated it is desirable to complete the treatment with the apparatus of the drawing within a time period of no more than /5 of a second, and preferably within a period not exceeding of a second. The period of treatment is determined by the width of the wood surface contacted under pressure by the friction member and the rate of feed of the work. With a 16-inch rubber covered drum it is estimated that the width of the band of contact is no more than A when the pressure applied by feed roller 21 is 7 /2 pounds per lineal inch of roller length.

The temperature developed during the treatment for any given rate of work feed and width of pressure contact depends on the pressure and the belt speed, increasing as either or both of these factors increase and vice versa. The best results are achieved when the temperature developed closely approaches the scorching temperature for the treatment time employed. The process cannot be applied to green wood or to wet wood since the evaporation of moisture absorbs the heat and prevents the wood surface from attaining the requisite temperature.

While the maximum pressure exerted may be varied widely, it must be suflicient to produce the above specified degree of densification within the time indicated. The actual maximum pressure exerted by the apparatus of the drawing is difficult to determine since it varies over the narrow band of contact from zero at the edges of the band to a maximum at the center. However, it is believed that relatively high maximum pressures are deas a'noas veloped and are necessary to obtain a useful densification of the wood surface in the very short treatment times employed. In any event excellent results have been obtained when the diameter of drum 10 is 16 inches and the downward pressure exerted by roller 15 ranges from 7 /2 pounds to 15 pounds per lineal inch of contact be tween the belt and the wood surface along the length of the roller. Under proper conditions of belt speed and work feed the pressures may be greater or less than that range.

- Rapidity of operation It will be apparent from the above that the essential requirements of the invention as practiced by a friction member are that the desired degree of permanent compression be obtained in a short time and that the pressures, the speed of the friction member and the rate of work feed may be varied widely, if properly correlated to meet those requirements. However, as a guide in determining any desired proper combination of pressures, speeds and feeds in accordance with the above principles, it may be stated that excellent results on birch plywood have been obtained with the apparatus of the drawing under the following conditions:

Diameter of drum 10 16".

Work feed 90 ft. per minute. Belt speed 4000 ft. per minute. Force exerted by roller 15 7 /z# per lineal inch.

Under the above conditions the contact time is of a second or less for a contact band of one inch or less and the temperature closely approaches the scorching temperature which is believed to be in the neighborhood of 620 F. for that treatment time. This is indicated by the fact that under the conditions stated a very slight indication of scorching is noted at rare intervals. This can be avoided, if desired, and excellent results achieved by increasing the feed rate to 125 feet per minute.

The extreme rapidity with which the present invention can be practiced is emphasized by the foregoing, which makes clear that excellent results can be obtained from a contact time of a fraction of a second per inch of contact band width. The reason why good results can be obtained in a fraction of a second is largely due to the fact that the belt is compressed between a backing member on one side and the product on the other. The rubbing p. s. i. is thus determined by the compression imposed on the belt by the backing member and the area involved in such compressed contact. Consequently, by controlling the magnitude of the rubbing pressure over the band of contact, one can readily obtain a rubbing p. s. i. high enough to insure the production of a high plasticizing temperature in a short rubbing time.

If, for example, the drum 10 is of larger diameter than 16 inches, thus increasing the width of the band of contact, the maximum pressure may have to be increased by increasing the force exerted by roller in order to obtain the desired degree of permanent compression and if this results in objectionable scorching the work feed will have to be increased to reduce the treatment time or the belt speed reduced to reduce the temperature developed. If any one of the other conditions is changed from the values stated in the above table, its effect, if deleterious may be compensated for by modifying one or more other conditions in accordance with the principles outlined above to produce the desired permanent compression within the stated time limits without objectionable scorching.

Since the best results are obtained when the temperature of the wood approaches its scorching temperature and since the time of treatment has an important hearing on the temperature reached and the results achieved, it is important to maintain the rate of work feed uniform. This, as a practical matter, requires a mechanical work feeding mechanism, such as a conveyor or the rollor 15, operating at constant speed.

Bufiing belts and wheels While the use of a belt to develop heat by friction is preferred, the panel may be frictionally heated by other means such as, for example, a cylindrical cotton bufiing wheel having an axial length equal to or exceeding the width of the surface to be treated. In any event, the rubbing surface moves through a concavo-convex path with its convex side intimately and firmly pressed by a backing memberinto flat-faced rubbing contact with the fiat product surface to maintain in the rubbing zone a transversely-extending band of rubbing contact.

Prior burnishing operations The process should be distinguished from known wood burnishing operations which involve frictionally heating wood with cloth. Prior burnishing operations have not been applied to fiat wood surfaces of any substantial area, and the techniques employed are incapable of giving a uniform treatment over a large flat area.

Results obtainable with present "Microseal Process It will be appreciated that my process, which has become widely known as the Microseal process, provides a simple and inexpensive way of obtaining an extremely smooth densified surface on a panel of wood. The process so hardens the wood and increases the fineness of its texture that it may be viewed, in the door, furniture and remainder of the wood finishing field, for example, not only as a method for preparing a wood surface to receive one or more coats of finishing material and to provide a superior finished appearance at reduced cost, but also as a method of sealing or filling the wood. In this latter connection, it should be noted that my process can be performed to provide a surface which does not require the use of sealing or filling materials and which may, without using a sealer or a filler, be finished to produce .a surface far superior in quality to that heretofore obtainable on a commercially sanded surface with either or both sealer or filler. When the process is applied to Douglas fir plywood, the results are startling in that it substantially eliminates the very troublesome tendency of either or both the spring and summer grain and surface fibers of Douglas fir to raise after painting or other surface coating, and it also reduces the tendency of the wood to check. Some way of overcoming these difficulties, which are quite aggravated in the case of Douglas fir, has long been sought.

Summary From the foregoing, it will be understood or observed: 1) that the Microseal process is useful in finishing wood panels, flush doors and other flat-faced cellulosic products presenting a smooth porous firm heat-insulating fiat surface which contains plasticizable material having the properties of being plasticizable at temperatures elevated substantially above room temperature and of being in a hardened state at room temperature after it has been heat plasticized; (2) that the term lignin is used in this application in a general way to designate the interdispersed heat plasticizable binder of natural wood; (3) that the product is bodily moved edgewise along a predetermined path extending longitudinally through a rubbing zone; (4) that fast product speeds are facilitated by the movement of the belt or rubbing surface toward and away from the product surface through a concavo-convex path with its convex side pressed against said product surface by a backing member; (5) that by pressing the belt against the product surface, the backing member maintains a transversely extending band of rubbing contact in the rubbing zone as the product moves through that zone; (6) that the magnitude of the product speed is controlled to provide a rubbing contact time of the order of a fraction of a second for each inch of contact band width; and (7) that the rubbing speed and pressure are correlatively controlled at values required to cause the rubbing medium to glossify the surface, such control apparently causing frictional heat to be generated by the rubbing medium on the product surface at the elevated temperature level required for plasticizing the heat plasticizable material on the rubbed surface.

What is claimed is:

1. The process of treating a relatively smooth Wood surface comprising: frictionally heating and compressing the Wood surface by a relatively nonabrasive heat-insulating friction member which, while a backing member presses it into contact with the wood surface, rubs it at one speed and relatively traverses it at a lower speed such that it does not remain in contact with any given portion of the wood surface for more than one-fifth of a second, the rubbing speed and the pressure and time of contact between the friction member and the wood surface being correlated to heat the surface to a temperature which will soften the heat plasticizable material in the wood and cause a permanent compression of the surface of the wood ranging from .0005" to .0050 during the period of contact.

2. The process of treating a relatively smooth wood surface comprising: frictionally heating and compressing the Wood surface by a relatively nonabrasive heat-insulating friction member which, while pressed against the wood surface, rubs it at a high speed and traverses it at a relatively low speed such that it does not remain in contact with any given portion of the wood surface for more than one-eighteenth of a second, the rubbing speed and the pressure, and time of contact between the friction member and the wood surface being correlated to heat the surface to a temperature which will soften the heat plasticizable material in the wood and cause a permanent compression of the surface of the wood ranging from .0005 to .0025 during the period of contact.

3. A method of treating the surfaces of wood and other firm cellulosic materials, which are coherent enough to resist destructive Wear under rubbing pressures, comprising: providing a product presenting a relatively smooth regular porous firm cellulosic surface containing interdispersed heat plasticizable material having the properties of being heat plasticizable at temperatures elevated substantially above room temperatures and of being in a hardened state at room temperatures after it has been heat plasticized; and rubbing said cellulosic surface with a relatively fast moving rubbing medium having a flexible relatively nonabrasive frictional heat insulating rubbing surface which is intimately and firmly pressed by a backing member against said cellulosic surface while traversing it at a controlled speed providing momentary rubbing contact at successive areas of said cellulosic surface; the magnitudes of the rubbing pressure against the cellulosic surface and the rubbing speed relatively to the cellulosic surface being correlatively controlled to cause said medium, during its momentary time of contact with a given area of said cellulosic surface not only to generate frictional heat on said cellulosic surface at the elevated temperature level required for heat plasticizing the said heat plasticizable material interdispersed in the cellulosic surface of said given surface area without plasticizing the underlying cellulosic material throughout its depth but simultaneously to pressure smooth and densify the cellulosic surface of such area without densifying the underlying cellulosic material throughout its depth.

4. A method of treating wood, comprising: providing a product having a wood member presenting a relatively smooth regular porous wood surface containing interdispersed heat plasticizable material having the properties of being heat plasticizable at temperatures elevated substantially above room temperatures and of being in a hardened state at room temperatures after it has been heat-plasticized; rubbing said wood surface with a relatively fast moving rubbing medium having a flexible relatively non-abrasive frictional heat insulating rubbing surface which is intimately and firmly pressed by a backing member against said wood surface while traversing it; controlling the magnitude of the traversing speed at which the rubbing medium traverses said surface to provide rubbing contact of predetermined momentary order at successive areas of said wood surface; and correlatively controlling the magnitudes of the rubbing pressure against the surface and the rubbing speed relatively to the surface to cause said medium, during its momentary time of contact with a given area of said wood surface, not only to generate frictional heat on said wood surface at the elevated temperature level required for heat plasticizing the said heat-plasticizable material interdispersed in the surface wood of said given surface area without plasticizing the underlying wood throughout its depth but simultaneously to pressure smooth and densify the surface wood of such area without densifying the underlying wood throughout its depth.

5. The method of claim 4 wherein: the second mentioned controlling step is performed with the magnitudes of said rubbing pressure and rubbing speed controlled to produce plasticizing, smoothing and densifying without charring said surface wood.

6. The process of claim 5 wherein: the second mentioned controlling step is performed with the magnitudes of said rubbing pressure and rubbing speed controlled to produce said plasticizing, smoothing and densifying without scorching said surface wood.

7. A method of treating wood, comprising: providing a product having a relatively smooth and fiat porous wood surface; moving that product edgewise relatively along a predetermined path extending longitudinally through a transverse work plane; moving an endless flexible rubbing medium, having a yielding relatively nonabrasive frictional heat insulating rubbing surface, at a relatively high rubbing speed toward and away from said surface at and along said transverse work plane; pressing said rubbing medium with a backing member into firm intimate frictional engagement with said wood surface to maintain a transversely extending band of rubbing contact therewith at and along said work plane; controlling the magnitude of the relative product speed through said work plane at a value providing momentary rubbing contact time of the order of a fraction of a second for each inch of contact band width; and correlatively controlling the magnitudes of the rubbing pressure against said surface and the rubbing speed relatively to the surface at values causing said medium, during its momentary time of contact with a given area of said wood surface, not only to generate frictional heat on said surface at the elevated temperature level required for heat plasticizing said wood surface along said work plane without plasticizing the underlying wood throughout its depth but simultaneously to pressure smooth and densify the surface wood without densifying the underlying wood throughout its depth.

8. The method of claim 7 wherein: the second mentioned controlling step is performed with the magnitudes of said rubbing pressure and rubbing speed controlled to produce plasticizing, smoothing and densifying without charring said surface wood.

9. The method of claim 8 wherein: the second mentioned controlling step is performed with the magnitudes of said rubbing pressure and rubbing speed controlled to produce said plasticizing, smoothing and densifying without scorching said surface wood.

10. The method of claim 7 wherein the product speed, the rubbing pressure and the rubbing speed are so controlled that: the frictionally generated heat softens the surface layer along the work plane momentarily without softening the underlying wood; the pressure of the rubbing medium compresses and densifie-s the softened wood without densifying the underlying wood; and the frictionally developed heat in the softened surface wood is quickly dissipated as that wood leaves the work plane causing the softened surface to set and bond the densified surface wood in a densified condition.

411. The method of providing a substantially flat wood surface with a glossy and relatively non-porous finish comprising: nonabrasively bufling the surface with a buffing medium having a flexible yielding relatively nonabrasive frictional heat insulating rubbing surface moving at a predetermined bufiing speed, compressing the buffing medium between a backing member and said wood surface to press the bufiing medium into intimate and firm contact with said surface; and correlatively controlling the buffing speed, pressure and time to compress, pressure smooth and densify the surface wood without scorching the wood.

12. A method of finishing wood panels, flush doors and other fiat-faced products presenting a smooth porous firm heat-insulating flat surface, containing plasticizable material having the properties of being plasticizable at temperatures elevated substantially above room temperatures and of being in a hardened state at room temperatures after it has been heat plasticized, comprising: moving such a product progressively through a rubbing zone; rubbing said product surface in said zone with a fast moving flexible yielding unabrasive heat-insulating rubbing surface, which moves toward and away from said product surface through a concavo-convex path with its convex side intimately and firmly pressed by a backing member into frictional rubbing contact with said product surface to maintain in said zone a transversely-extending band of rubbing contact; controlling the magnitude of the product speed to provide a rubbing contact time of the order of a fraction of a second for each inch of contact band width; and correlatively controlling the magnitude of the rubbing pressure against, and the rubbing speed relatively to, said product surface at values causing said rubbing medium to glossify said product surface during said rubbing contact time.

13. The method of claim 12 wherein: the product speed is controlled to provide a rubbing contact time not greater than /5 of a second per inch of contact band width.

14. The method of claim 12 wherein: the rubbing pressure and rubbing speed are controlled to cause said rubbing medium to generate frictional heat on said product surf-ace at the elevated temperature level required for plasticizing the said heat-plasticizable material during said rubbing contact time without plasticizing the underlyng material throughout its depth.

15. The method of claim 12 wherein: the rubbing pressure and rubbing speed are controlled to cause said rubbing medium to pressure smooth said product surface during said rubbing contact time.

16. The method of claim 12 wherein: the rubbing pressure and rubbing speed are controlled to cause said rubbing medium to densify surface material of said product during said rubbing contact time without densifying the underlying material throughout its depth.

17. The method of claim 12 wherein: said rubbing operation is performed with a rubbing medium in the form of an endless belt which, in the rubbing zone, is compressed between the product surface and a backing member which rotates with the belt; and the product speed is controlled to restrict the rubbing contact time of said rubbing medium with any given surface portion of the wood to a fraction of a second.

18. The process of claim 17 wherein: said rubbing operation isperformed with said backing member presenting a yieldable surface to the belt; and the rubbing pressure and rubbing speed are controlled to cause said rubbing medium to generate frictional heat on said product surface at the elevated temperature level required for plasticizing the said heat plasticizable material during said rubbing contact time.

References Cited in the file of this patent UNITED STATES PATENTS 2,118,763 Mason May 24, 1938 2,412,523. Lundstrom Dec. 10, 1946 2,488,301 Lundstrom Nov. 15, 1949

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