USRE27617E - Louvered transparent sheeting made by skiving - Google Patents

Abstract

SKIVING OF A LAMINATED BILLET HAVING OPAQUE LAYERS WHILE HEATING THE SURFACE PROVIDES A WEB WITH LONGITUDINAL LOUVER-LIKE ELEMENTS. THE SURFACE IS POLISHED.

Description

April 17, 1973 F. o. OLSEN Re. 27,617

LOUVERED TRANSPARENT SHEETING MADE BY SK'IVING Original Filed Aug. 15, L967 I k H6. 4 W m NW 5y I Aggy/V575 United States Patent f 27,617 LOUVERED TRANSPARENT SHEETING MADE BY SKIVING Frederick 0. Olsen, Oakdale Heights, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn.

Original No. 3,524,789, dated Aug. 18, 1970, Ser. No. 660,754, Aug. 15, 1967. Application for reissue Jan. 21, 1971, Ser. No. 108,665

Int. Cl. B32b 31/00; 1344f 1/00 U.S. C]. 161-6 2 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE Skiving of a laminated billet having opaque layers while heating the surface provides a web with longitudinal louver-like elements. The surface is polished.

This invention relates to and has as its object a process for the production of web materials for use in achieving optical effects and particularly for uses where limited angles of visibility are sought so that there is a restricted optical aperture and the products of the process. Examples of uses for such web material are in television screens, trafiic signals, prevention ofrefiection and glare from instrument panels, one way glass and other such uses.

This invention is embodied both in processes for production of such webs and in the products of the processes displaying the desired properties in extremely long webs as compared to previously described products. The invention includes several steps of the processes.

The prior art has contemplated the production of sheets of plastic made by slicing blocks of alternating transparent and opaque sheets cemented together, for example, U.S. Pats. 2,053,173, 2,689,387, and 3,031,351. It is not known that such products have been introduced into commerce presumably because of the very considerable difiiculty of cutting polymeric materials in the manners contemplated in the prior art.

An alternative method for the cutting of one polymeric material, polytetrafluoroethylene, has been borrowed from the veneer and leather working industries namely skiving. Illustrative of the application of this is the disclosure of U.S. Pat. 2,642,625 for polytetrafluoroethylene, of 2,245,- 170 for wood, and 2,827,403 for leather. It will be recognized that the inherent properties of polytetrafluoroethylene are so unusual that it occupies an almost unique position among polymers and it and processes using it cannot be equated to other polymers or processes using them. For example, in its inability to wet or to be wet by most materials, which prevents its sticking to a skiving knife, it must resemble materials such as wood or leather which do not adhere to metal spontaneously.

It is surprising to find that the process of skiving can be adapted to the cutting of webs of polymers which can adhere to metals provided certain modifications are made. Although the process is probably generally adaptable to thermoplasts, it is particularly useful for the production of webs of thermoplastic polymers having structures not readily produced by extrusion or casting processes.

Such a structure is one in which relatively wide clear strips and relatively narrow opaque strips alternate across the face of the web and extend therethrough so that there is restricted optical aperture. Although panels of such material of a few square centimeters may have been produced before, webs of substantial widths, say 50 cm. or more, and lengths of or more meters are believed entirely new. 3

Re. 27,617 Reissued Apr. 17, 1973 These webs are more or less readily wound up into supply rolls from which desired pieces may be cut. The ease of winding will depend on thickness, stiffness and other characteristics of the sheet as skived. Because of the much greater size than heretofore available novel utilities can be realized in which relatively large expanses are covered. For example, vision can be provided in a large window without permitting the nearly overhead sun to shine in. Such a shading effect can be especially useful in low latitudes where the sun always passes almost overhead. The elfect is thus of having very narrow Venetian blinds built into the window.

It is a peculiar feature of webs produced by skiving that, as formed, one surface will necessarily be linearly shorter than the other although subtending the same arc. If cooled immediately after skiving plastic memory retains this difference as a curvature and straightening the web longitudinally transforms the difference to a transverse curvature.

It has been found that the skiving of thermoplastic materials is best performed using a billet having a rigid shaft with the polymer molded therearound and heating the surface of the billet immediately before contact with the skiving knife to a temperature sufiicient to permit softening of the polymer at least to the depth contacting the skiving knife. Methods of heating will be discussed herein elsewhere. A steel shaft with one end shaped to be gripped by a chuck is particularly appropriate for the billet. Differential shrinkage of polymer around the shaft can be enhanced by conventional means.

For the production of a polymeric web with restricted optical aperture, a billet is formed around a shaft as described above. Discs with center holes larger than the shaft by from about 0.1 to about 0.5 mm. are cut from a primary laminate of clear and contrasting polymeric material and are then molded around the shaft. Many thermoplasts include small amounts of volatile materials such as moisture and the discs are then advantageously dried thoroughly before subsequent use to make a billet.

The primary laminate may be produced by any convenient means or, less conveniently, discs of clear and contrasting material can be used alternately. In the latter instance there is, however, more opportunity to obtain a distubed sequence, for example, of several clear discs superposed. Conveniently the primary laminate is prepared from an extruded film of thermoplast of one type by coating with a layer of contrasting type either by extrusion or other convenient process, e.g., rotogravure or other method of printing, vapor coating, etc.

It will be recognized that the contrasting layer may be polymer which has been colored by dissolved dyestufis or by pigments. For illustrative purposes reference will usually be to a pigmented layer, for example, including 3% by weight of carbon black as the pigment, but it is within the scope of the invention to include a bicolored contrasting layer in which the side next to the clear layer is one color and the outer layer is a different color or the clear polymer may be coated on opposite surface with different color. For example, one could produce a web which in addition to the restricted optical aperture from in front would appear green from one side and red from the other side.

By whatever method selected, clear and pigmented layers are provided and the discs are placed on a shaft using suitable retaining means. The inclusion of extraneous bits and pieces either of foreign materials or of the material of the laminae is avoided. The assemblage of discs is built up, until the pile slightly exceeds the desired width of the web. The entire assemblage is then placed in a chamber of just slightly greater (by an amount sufficient to compensate for thermal effects) diameter for gradual warming and application of pressure until the 3 aillet is fully consolidated and molded around the shaft and to the diameter of the chamber. Vacuum may be applied simultaneously. The method of heating and the temperature necessary will depend on the particular thernoplast used. A steam jacket is convenient because after :omplete consolidation, cooling water can be introduced nto the jacket and the billet cooled rapidly. Normally the aillet will be ready for skiving immediately after cooling.

When the thermoplast is quite flaccid, polyvinylbutyral :eing exemplary, it may be difiicult to skive. Such a situaion is handled by chilling the billet to a temperature at which the polymer is sufiiciently rigid, e.g., using solid :arbon dioxide, etc.

The invention is now further explained by reference to he drawings attached hereto, wherein:

FIG. 1 shows in somewhat diagrammatic cross-sec- :ional veiw the shaping of a billet.

FIG. 2 is a perspective view of billet, skiving knife assembly, heating lamp and web in the relationships which hey occupy on the bed of a conventional large metal working lathe.

FIGS. 3, 4 and 5 show cross-sections of webs in which here is respectively no added cover, transparent coverings an both sides of the same thermoplast and with glass tttached to each face of the web.

Referring to FIG. 1, which is somewhat diagrammatic 1nd not to scale, there is shown the manner in which a Jillet is formed on shaft 12 fromdisks 14 having the )paque surfaces (represented by the lines) alternating with clear portions. One end of theshaft 16 is squared :0 permit grasping in the chuck of a lathe (not shown) and is provided withcentral hole 20 for the admission of :team or cooling water. [Holt]Hole 20 is threaded for at- :achment of an assembly permitting placement ofshaft 16 with relationship todisks 14 during the molding operation. Pube 11, attached to adapter 13 which fitshole 20 and retains O-ring 15, passes through upper retainingplate 46 9y sliding seal 19 and O-ring 21 and is held in place against gravity by retainingclamp 23 which is split on one ride and is tightened by a screw to preventshaft 16 from aottoming in the well ofram plate 54.

In the molding operation anoutlet 22 is provided for :he escape of steam fromhole 20 inshaft 16 and throughipper retaining plate 46. Heat is provided externally toJillet mold 28 during the molding operation by steam iacket 26 (having inlet and outlet not shown). Billet mold 18 haspolytetrafluoroethylene liner 30.Tie rods 42 andlpper retaining plate 46secure billet mold 28 onstand 40 which latter is attached bytie rods 45 to the base )f pressure means (not shown) havingpiston 60. The itack ofdisks 14 is separated from upper retainingplate 16 byinsulation 48 andupper pressure plate 50. Pressure .s exerted from below bylower pressure plate 52 separated byfurther insulation 49 from ram plate'54 which s also separated frompiston 60, acting through opening 44 instand 40, by insulating shim 58. The pressure plates it tightly aroundshaft 16 andlower pressure plate 52 is prevented from slipping from the lower end ofshaft 16 9y retaining pin 18. Gas tight means (not shown) may 9e provided as part ofstand 40 aroundpiston 60. Ports nay be provided through upper retainingplate 46 and in the lower wall ofbillet mold 28 so that vacuum can be applied and retained during the molding of a billet if iesired.

it will be seen that, at temperatures at which the polyrner ofdiscs 14 becomes thermoplastic, force applied frompiston 60 will consolidatedisks 14 into a billet around shaft 12. The lining ofcylinder 28 is not wet by he thermoplastic polymer and withdrawal of the billet from the cylinder is thus possible after cooling as a result 3f the differential shrinkage of the polymer.

Referring to FIG. 2, the billet, 10, prepared as described above is seen to have a central shaft 12 (with squared end and central hole as above described) rotating in a clockwise direction (arrow) because engaged in the chuck of a driving lathe of suitable design (not shown). The far end of the shaft bears against the tailstock of the lathe (not shown). Skiving knife, 114, rigidly attached in support, 116, to the cross-feed mechanism of the lathe is advanced horizontally toward shaft 12 by the cross-feed mechanism at a constant rate such that after contacting the billet surface the knife will cut a web of the desired thickness. As the Web is formed it is wound up into a roll (not shown). The edge ofknife 114 must be parallel to and in the same horizontal plane as the axis ofbillet 10.

The significant feature of this equipment is heater shell which extends overbillet 10 both lengthwise and for about A to /2 the circumference.Heater shell 120 provides means for heating the skin ofbillet 10, for example, to a temperature in the range of 250 to 350 F. (120 to 180 C.) for cellulose acetate butyrate, so that steady even skiving becomes readily possible without enormous expenditure of power and/or production of gross irregularities. The heating means is controllable to provide more or less heat as needed for a particular polymer or for a particular depth of cut. It may be adjusted manually or automatically during the course of skiving to counteract too high or too low temperatures. Too high temperatures may result, for example, in excessive necking or constriction of the web immediately after skiving and before it is cooled or taken up on a supply roll. On the other hand, too low temperatures may result in knife chatter and irregular cutting as well as the necessity for extreme power and probably more rapid dulling of the knife.

The heating means shown comprises a bank of individually controllable heating lamps, 122, such as General Electric Heat Lamps No. 3M/T3/CL/ 1, or 3M/T3/CL/2, each capable of radiating 1200 watts per lineal foot. Other means may be employed if more convenient. The panel is spaced away from the billet slightly and the distance increases as the skiving progresses. This generally creates no problem as the heat fluxis normally more than sufficient to effect the desired degree of skin softening of the billet. The panel extends beyond the ends of the billet. There is some tendency for overheating the ends and the center of the billet. This is counteracted by application of less heat absorbent coatings to the end of the billet, e.g., light color or metallized coating and further by providing flat reflectors parallel to the ends of the billet so that radiant heat passing the ends is returned. The overall effect of these measures is to equalize the temperature lengthwise of the billet and give more uniform heating and hence improved webs. Other means for heating may be used alternatively. It will be recognized that normal ventilation measures will be necessary in multiple and large scale operations to remove any fumes.

Althoughknife 114 and itssupport 116 are mounted so as to advance toward shaft 112,heating panel 120 is initially positioned with relation to the billet and not moved.

'Web is formed from the skiving operation and is seen to be composed of alternating clear, 132, and opaque, 134, strips. The web is taken up by any suitable means which preferably is of a type which merely prevents accumulation of slack in the web Without placing more than slight tension on the web. It may be desirable to introduce chilling or polishing rolls or to have a takeup reel provided so that a lining sheet is taken up at the same time to prevent a still relatively hot and soft web from adhering to itself.

It is contemplated that control of variables may be manual or by suitable automatic devices.

When web, 130, is formed as described from a billet of thin black and relatively thick clear layers, the crosssection of a small portion of the width is essentially as illustrated in FIG. 2 in which theclear portions 132 are separated from one another by theblack portions 134. The optical aperture is illustrated by the angle included between the broken construction lines. It will be evident that this can be readilv controlled bv alterin either the clear portions, 132, or by changing the thickness of skiving. Inclination of the opaque portions also affects the optical aperture and angle for transparency. Accordingly, by proper programming of a skiving machine, i.e. a heavy duty lathe, it would be possible to produce webs having changing numerical aperture. For example, sensing means immediately above the line of cutting could control and vary the depth of cut as desired.

The web as produced possesses some utilities depending on the particular thermoplast employed and upon the fact that cutting with a knife cannot give a completely clear surface. In addition to longitudinal lines due to slight irregularities in the knife edge there are greater or less transverse irregularities due to the lack of complete smoothness in cutting by the knife possibly arising from the operation of the machinery. Both types of irregularities presumably have always existed in previously skived materials such as veneer and leather and in sliced plastic materials but have been overlooked as of no consequence for the contemplated uses or were accidentally relieved.

A positive effort at removal of surface imperfections is highly desirable for most purposes and necessary Where best optical effects are desired. When the web or portions thereof are cemented between two sheets of glass 140 a cross-sectional structure is as shown in FIG. 5. This may be a safety glass with polyvinylbutyral as the thermoplast of the web. Another method of smoothing surfaces is to coat the web on each side with a layer of the same clear thermoplast so that a small piece in cross-section will be as illustrated in FIG. 4.

Yet another method of providing clarity is to polish the surfaces by heating to the softening point in contact with a highly polished non-adhering surface such as a polyester film which is stripped away after cooling.

As a more specific ilustration of the best mode contemplated for practicing the invention, a detailed description is provided of the preparation of a louvered web in which cellulose acetate butyrate is employed as the thermoplast.

The primary laminate is made by first extruding a film of the butyrate into which 3% by weight of carbon black has been milled in a Banbury mixer or other such mill. This initial film, approximately 0.0015 inch (or 0.004 mm.) thick, is then coated by extrusion with a layer of clear butyrate about 0.01 inch (0.25 mm.) thick.

Molding of the billet is carried out using 21 containing cylinder lined with polytetrafluoroethylene having internal diameter of 10.058 inches (255.47 mm.) on a shaft with external diameter 2.500 inches (63.50 mm.). Disks are stamped from the primary laminate so as to have clearances of about 0.02 inch (0.51 mm.) externally and 0.006 inch (0.15 mm.) at the center, i.e. disks about 10.038 inches (254.96 mm.) outside diameter with 2.506 inches (63.65 mm.) central opening. Punching may be performed of one or several disks at a time, usually three. It is important that there be sufficient clearance so that there is no tendency to hang-up externally or internally during compression which results in introducing slightly irregular canting of laminae. Canting due to excessive thermal expansion of laminae is corrected by using laminae having larger central holes and smaller outer diameters. Waste from the punching operation may be reprocessed by addition of suitable amounts of carbon black to give new black film.

After a sufficient supply of disks has been punched, they are heated in stacks of about 80 to 200 in an oven under forced draft for four or more hours at about 65 C. (150 F.) in order to expel moisture and other volatile materials and so that the slight curvature, induced by storage of the primary laminate in rolls, is removed.

The shaft (36 inches or 92 cm. long) is holow. It may also be so that steam or cooling water can be introduced as shown in FIG. 1. The shaft is secured in a vertical position with the lower plates in place. Disks still hot from the drying oven are placed on the shaft imparting a spinning motion manually or by an air stream sufficient to throw out extraneous matter and particularly the center punch-outs. The disks are worked as low as possible on the shaft. This operation also randomizes inherent unevenness in the discs, for example, a stripe of greater or lesser thickness running lengthwise of the rolls of primary laminate. In the apparatus of FIG. 1, the weight ofdisks 14 and upper retainingplate 46 particularly is permitted to provide initial compaction of the disks by action againstlower pressure plate 52 and retaining pin 18. This raises shaft-16 as high as possible through the stack of disks and it is held in that position by tightening retainingclamp 23.

The billet mold, as noted, is cylindrical and lined with polytetrafluoroethylene. It is provided at the ends with means which minimize application of pressure to the shaft and aluminum disks thermally insulated from the recessed caps by which pressure is transmitted to the billet. If desired the mold may be sealed and evacuated to a pressure of about 10 mm. of Hg throughout the molding cycle. Pressure of about 30 pounds per square inch is aplied mechanically (hydraulic press) to the stack of disks at a temperature of about 50 C. (residual temperature of the disks after drying) and this pressure is maintained throughout the following operation and until the molded billet has finally cooled enough to be removed from the mold. Concomitant with the application of hydraulic pressure steam at about C. is introduced into the heating jacket and the shaft and the temperature of this steam is maintained for 6 hours and then the temperature is raised to C. for a further 6 hours until the laminae have fused to one another. Heating is discontinued and the billet is permitted to cool, with application of hydraulic pressure, for 16 hours at which time the temperature is about 35-40 C. The billet has shrunk away from the polytetrafiuoroethylene lining and is removed. Provision for external cooling is also feasible and shortens the cooling time. The billet is molded around the shaft and is ready to be skived.

The billet is mounted in a powerful lathe having sufiicient clearance to handle a workpiece of this size with one end in the chuck and the other rotatably supported at the tailpiece. The skiving knife assembly is essentialy a steel block about 20 cm. square by 80 cm. long with one side recessed to receive the special knife and in which the knife is securely fastened. This assembly is mounted on the bed of the lathe and aligned so that the knife is parallel to the axis of the lathe. The upper edge of the knife makes an angle of 20 with horizontal and the forward face slopes back by 8 from vertical. Variations in these angles from 0 up to at least 50% higher figures are fully permissible. The point of the blade is thus approximately 50 to 90. More acute angles than 45 may be somewhat more easily damaged. The heating panel of eight 3 kva. heating tubes 85 cm. long and wired to be controlled separately is brought over the billet and about 3 to 7 cm. away from it initially. A take-up reel with a constant torque drive is provided that is essentially parallel to the axis of the lathe.

The lathe is started at 12 r.p.m. and cross-feed for the knife-blade is set to advance about 0.044 inch (1.1 mm.) per revolution but not started. About half the bank of heating lamps are turned on and the temperature of the surface of the billet is followed by use of commercially available crayons that show temperatures by melting or failing to melt. Heating is continued without advancing the knife-blade until the crayons show a temperature from 300 to 320 F. to C.) on the surface of the billet. This temperature is maintained by increasing or decreasing the amount of heat as needed and the cross-feed for the knife-blade is engaged. Cutting starts by removal of high spots followed very rapidly by removal of web to the full width of the billet. The end is quickly brought to the take-up reel and attached. Skiving continues, as-

nrning proper mechanical function, until the cross-feed s disengaged before the knife-blade comes so close to he metallic shaft as to be possibly damaged. Necessary tdjustments in heating are made during the cutting if it tppears that the temperature of the billet is too high or .00 low. Too low temperatures leading to a rough surface tI'C the source of trouble when very thick webs are made, For example, above about 1 to 2 mm. Cuts less than about ).05 mm. are so thin as to be of little interest for narrowng of optical aperture although potentially useful in some Jther fields.

Slight irregularities in the knife-blade produced parallel ongitudinal scoring of the skived film and together with ther surface imperfections make the skived film transucent rather than transparent. In addition there is a very ;light curvature in the web resulting from the skiving vhich cuts a slightly shorter linear distance on each suczeeding helix cut from the billet. Although this curvature produced in the length of the web, an attempt to remove t longitudinally produces stresses such that the curvature ippears transverse of the web. In other words there is nherently a recognizable curvature to a skived web of polymer.

These surface elfects are corrected by pressing between Jolished flat plates under about 3.5 atmospheres pressure and above about 130 C., the softening point of the polyner, and cooling in contact with the polished surface. Jther methods of molding the smooth surface are also apalicable and smooth clear sheets of the same polymer or )f glass may be laminated to the surface thereby providng fiat sheets having cross-sections as shown in FIGS. 4 1nd 5 respectively.

Essentially the same process is employed, making allowlnces for the difierences in materials, to produce a web 1f polyvinylbutyral resin. Primary laminate and discs are prepared as above using lower temperatures for extrusion ll'ld laminating at about 200 F. (93 C.). The discs of )olyvinylbutyral do not require drying. The billet is nolded around the shaft by a similar procedure using lomewhat lower operating temperatures.

Skiving of the polyvinylbutyral billet requires that the illet be prechilled to a temperature low enough that it is irm. Cooling in solid carbon dioxide for about 16 hours s fully satisfactory although such a low temperature is rot imperative. Skiving is carried out as described above ising a lower surface temperature of about 95-110" C. [his web is laminated to glass surfaces by the procedures :onventionally used in the manufacture of safety glasses from polyvinylbutyral film.

What is claimed is:

1. A process for the production of a web of substanzially transparent plastic film material having optical aper- ,ure restricted in one linear direction and having relative- ,y narrow opaque elements regularly spaced across and :onstitutiiig iiot more than one-fourth the width of said web, comprising the steps of (I) providing an integral cylindrical billet of thermoplastic polymer having longitudinally regularly alternating relatively thick clear and relatively thin contrasting laminae unified by mechanical pressure at elevated temperature around a central shaft and (II) rotating said billet by means of said shaft therethrough While (A) advancing a knife parallel to said shaft against said billet throughout the length thereof at a rate maintained essentially constant with respect to the radial speed of said billet and (B) simultaneously heating the surface of said billet in advance of the line of contact with said knife to a temperature sufiicient to soften the outer portion of said billet at least to the depth of the cut being made therein.

2. A process according to claim 1 wherein the thermoplastic polymer is cellulose acetate butyrate and the surface in advance of the knife is heated to about 150 to 160 C.

3. A process according to claim 1 wherein the thermoplastic polymer is polyvinylbutyral and the surface in advance of the knife is heated to about to C.

[4. The product of the process of claim 1.]

[5. The product of the process of claim 2.]

[6. The product of the process of claim 3.]

[7. A Web of substantially transparent plastic film material having restricted optical aperture according to claim 4 having surfaces freed at least from scoring to an extent sufficient to permit vision with no more than minor distortion through clear portions of said web] [8. A web according to claim 7 wherein the film material is cellulose acetate butyrate.]

[9. A web according to claim 7 wherein the film material is polyvinyl butyral.]

10. The combination of a longitudinally laminated cylindrical billet of contrasting laminae of thermoplastic polymer and shaft centered therein adapted for skiving by the process of claim 1.

11. A process according to claim 1 wherein the relatively thin contrasting laminae are colored by dissolved dyestufis or by pigments.

12. A process according to claim 11 wherein the relatively thin contrasting laminae are bicolored.

13. A process according to claim 1 wherein the integral cylindrical billet of thermoplastic polymer is initially prechilled to a temperature low enough that it is rigid firm.

14. The combination of a longitudinally laminated cylindrical billet of colored contrasting laminae of thermoplastic polymer and shaft centered therein adapted f r skiving by the process of claim 11.

15. The combination 09 a longitudinally laminated cylindrical billet of bicolored contrasting laminae of thermoplastic polymer and shaft centered therein adapted for skiving by the process of claim 12.

16. A unified web comprising a clear web of thermoplastic polymer having disposed therein, in a direction substantially normal to the plane thereof, thin bicolored opaque parallel louvers extending through the web and spaced at such a distance from each other as to provide a restricted optical aperture for light passing through said web, said louvers being of a thickness which is relatively small compared with the space between them and being colored by dyes or pigments so as to confer upon said unified web difierent colors when viewed from different sides of said louvers and through said web.

17. A unified web according to claim 16 wherein the clear web is composed of cellulose acetate butyrate.

18. A unified web according to claim 16 wherein the clear webuis composed of polyvinyl butyral.

References Cited The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,053,173 9/ 1936 Astima 161-3.5 3,031,351 4/ 1962 McIlvaine 161-6 3,444,031 5/ 1969 Schrenk 161-6 2,158,086 5/1939 Roberts et al. 264158 X 2,827,413 3/1958 Friedman 156255 3,223,568 12/1965 Alderfer 156255 FOREIGN PATENTS 548,027 7/1941 Great Britain 156-400 319,806 4/1929 Great Britain 264-158 DANIEL J. FRITSCH, Primary Examiner US. Cl. X.R.

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