GB2119681A - Impact blasting system for etching metal surfaces - Google Patents

Abstract

Apparatus and methods for etching a metal surface 32 utilize a shrouded centrifugal throwing wheel to effect a uniform, non-pulsating application of very fine, lightweight, impact blasting media. The resulting etched finish is of a fine-grained quality suitable for use in lithography. A replaceable, wear-resistant liner 152, 154 is provided within the shroud which has side walls 112, 114 which converge towards a discharge opening 200 to funnel and dampen the blasting media as it is discharged towards the workpiece. <IMAGE>

Description

SPECIFICATION Impact blasting system for etching metal surfaces The present invention relates generally to apparatus and methods for impact blasting lithographic sheet material and the like with a very fine, lightweight blasting media to provide a finegrained, uniformly etched finish.

Metals sheets used in lithographic printing processes require finely textured, uniformly etched surfaces. A preferred material for use in lithography is aluminium and alloys thereof, but other materials may also be used. Surface etching of lithographic sheet material is normally conducted using chemical etching techniques.

Chemical etching is effected by applying one or more chemicals to the surface to be etched. A chemical reaction results which causes portions of the exposed metal surface to be eaten away; thereby creating a uniform array of fine surface indentations which give the resulting product fine-grained, satin-like finish.

While chemical etching techniques are capable of providing suitably uniform finishes, these techniques are expensive to perform. Moreover, the resulting spent chemicals include hazardous substances which present environmental disposal problems. In view of these drawbacks, industry has long sought a suitable, less expensive, nonpolluting method of uniformly etching the surfaces of metal sheets for use in lithography.

The type of etched finish required for use in lithography is quite different from the type of surface treatment normally obtained using impact blasting techniques. If an etched finish is to be.

acceptable for lithographic purposes, it must be quite smooth, typically about as smooth as the types of surfaces which result from some honing, cold rolling, drawing or grinding operations. In many instances, the surface must be as smooth as that which is achieved with drilling or milling operations. Moreover, the etched surface profile must be held to what is known in the art as a specified "AA" etch number, with a toierance of only plus or minus 3AA over the entire etched surface. Typically, the specified "AA" number is within the range of about 1 7AA to about 40AA.

Impact blasting techniques previously have been found to be incapable of providing the type of etched surface required for lithographic purposes. The etching surface consistency required for lithographic sheets is far more stringent than that obtainable through the use of conventional impact blasting techniques.

The present invention overcomes the foregoing and other drawbacks of prior proposals by providing apparatus and methods for impact blasting a metal surface to consistently provide a fine-grained, uniformly etched finish which is well suited for use in lithographic operations and the like.

In accordance with the preferred practice of the present invention, a conventional centrifugal throwing wheel is provided with a specially configured shroud for ducting impact blasting media from the throwing wheel toward a sheet of metal which is moving along a feed path. A very fine, commercially available, mineral sand is used as an impact blasting media. The specially shrouded throwing wheel is capable of providing an extraordinarily uniformly etched surface finish of approximately 40AA on a sheet or web of metal moving along a feed path at the relatively high speed of about 21.3 metres (70 feet) per minute.

In working with extremely fine, extremely lightweight blasting media, as is required to provide a finely-grained etched surface of the type needed in lithography, a problem of non uniformity in delivery of the blasting media has been encountered. This problem previously has not been detected in conventional blasting operations where heavier shot or grit is used.

Where very fine-grained, lightweight blasting media is used, non-uniformities, having the appearance of elongate, shadow-like streaks, have been found to be present in the resulting etched surface. While these shadowy non-uniformities may be relatively inconspicuous to the human eye, -theV render an etched surface totally unsuitable for use in lithography. Streaks extend across the etched surface in directions extending substantially perpendicularto the direction of the feed path along which the workpiece moves relative to the throwing wheel.

Through careful experimentation, it has been found that a properly configured blastirrg wheel shroud, positioned at an appropriate distance from the workpiece, will overcome the natural tendency of the throwing wheel to deliver fine-grained, lightweight blasting media in a pulsating, nonuniform pattern. It is believed that the type of pulsating application pattern which is encountered where a conventional centrifugal throwing wheel is used to apply very lightweight, fine-grained blasting media results from what is akin to the "beat frequency" phenomena encountered in centrifugal fans. Inasmuch as a centrifugal throwing wheel can be thought of as constituting a modified form of centrifugal fan, the general type of beat frequency phenomena encountered in centrifugal fans undoubtedly has a counterpart type of phenomena in centrifugal throwing wheels.

In a centrifugal fan, the beat frequency is the frequency at which bursts or pulses of air are discharged from the fan. It is known that a beat frequency phenomena can result where a relatively large duct section is connected to a centrifugal fan by means of a relatively small duct section. It is believed that, with a centrifugal throwing wheel, the wheel housing is substantially equivalent to a small duct, and the chamber into which impact blasting media discharges is substantially equivaient to a large duct. If this theory is correct, it offers an explanation as to why a non-uniform, pulsating discharge occurs when a centrifugal throwing wheel is used to propel very fine-grained, lightweight blasting media, for a flow of air containing such media will undoubtedly behave very much like a flow of air which contains no particles at all.

Stated in another way, this theory, if correct, offers an explanation as to why workpiece surfaces etched with fine-grained, lightweight media discharging from a conventional centrifugal throwing wheel are found to have streaks. The streaks result from something akin to a beat frequency phenomena in that the flow of media is a pulsating type of flow which cannot achieve uniform application. Unfortunately, however, this theoretical explanation does nothing to suggest a solution to the problem of streaking.

While conventional teachings dealing with the art of centrifugal fans suggest that beat frequency pulsations of a fan can be relieved by dampering the fan's inlet, efforts to dampen the inlet of a centrifugal throwing wheel were found to provide no improvement. Similarly, while centrifugal fan technology suggests that dampering the outlet of a fan may help to reduce beat frequency problems, fans are not used to provide a uniform application pattern of impact blasting media, and the information to be gleaned from centrifugal fan technology was not found to suggest a suitable solution.

The solution presented by the present invention utilizes a specially configured shroud which connects with the housing of a conventional centrifugal throwing wheel. The shroud alters the flow of impact blasting media which passes through the shroud to eliminate the pulsations which characteristically occur when a conventional centrifugal throwing wheel is used to apply very fine lightweight blasting media. A shroud embodying features of the present invention utilizes opposed side walls which are inclined relative to each other to effect a slight funneling or narrowing of the width of the path of discharge of media, whereby the flow of media is caused to interact with itself in such a manner that it dampens and eliminates flow pulsations.

A further aspect of the invention lies in a determination that the outlet of the shroud must be positioned at an adequate distance from the workpiece surface being etched to prevent the occurrence of an air-spring action which can feed back into the shroud and prevent its proper operation. Different minimum shroud-toworkpiece distances are required with different throwing wheel parameters. By way of example, a 35.56 cm (14 inch) diameter throwing wheel operating at 1250 RPM and having a shroud outlet positioned at about 40.64 cm (16 inches) from a workpiece surface will produce a flawlessly etched surface.However, a 49.53 cm (19z inch) throwing wheel operating at 900 RPM will give about the same blasting media velocity as the above-described situation involving a 35.56 cm (14 inch) wheel, but the 49.53 cm (1921 inch) wheel will pump nearly twice as much air in the process; accordingly, the shroud outlet used with the 49.53 cm (193 inch) wheel must be positioned farther from the workpiece to prevent air from becoming "trapped" between the shroud and the workpiece thereby creating an air spring action that feeds back into the shroud and prevents the shroud from properly performing its pulsation-dampening function.

Inasmuch as it is not fully understood precisely why the present invention functions as it does to overcome the problem of non-uniform application of lightweight impact blasting media issuing from a centrifugal throwing wheel, the apparatus and methods of the invention are described and claimed in terms of such parameters as have been found by tests to contribute to and provide features for an operable system.

According to the present invention there is provided an apparatus for treating a workpiece, comprising: a) structure defining a treating chamber; b) positioning means for positioning a workpiece within the treating chamber; c) centrifugal throwing wheel means for receiving and projecting impact blasting media along a path of discharge; and d) shroud means extending about the discharge path for ducting impact blasting media from the throwing wheel means toward the workpiece, for discharging such media within the treating chamber at a location spaced from the workpiece, and for effecting such discharge in a uniform, nonpulsating pattern of application, the shroud means including:: i) first and second side wall means extending in overlying but non-parailel relationship along opposite sides of the discharge path; ii) first and second end wall means interconnecting the first and second side wall means and cooperating therewith to define a closed-wall duct which is open at one end to receive impact blasting media from the centrifugal throwing wheel means, and which defines a discharge opening at the other end thereof for discharging impact blasting media toward the workpiece; iii) the first and second side wall means being inclined relative to each other in a converging relationship for funneling and dampening the flow of blasting media as it moves along the discharge path between the first and second side wall means to narrow the width of flow of the media in one dimension as the media travels through the duct; and, iv) the first and second side wall means having opposed portions which extend in spaced, parallel relationship to define opposite sides of the discharge opening.

The invention also provides shroud means for use with a centrifugal throwing wheel (1) for positioning about a path of discharge which is followed by impact blasting media as the media discharges from the throwing wheel, (2) for ducting such media toward a workpiece surface to be etched, and (3) for discharging such media at a location spaced from the workpiece in a uniform, non-pulsating pattern to uniformly etch the workpiece surface, the shroud means comprising:: a) first and second side walls extending in overlying but non-parallel relationship along opposite sides of the discharge path; b) first and second end walls interconnecting the first and second side walls and cooperating therewith to define a closed-wall duct which is open at one end to receive impact blasting media from the centrifugal throwing wheel, and which defines a discharge opening at the other end thereof for discharging impact blasting media toward a workpiece; c) the first and second side walls being inclined relative to each other in a converging relationship for funneling and dampening the flow of impact blasting media as it travels between the first and second side walls to narrow the width of flow of the media as the media travels through the duct; and, d) the first and second side walls having opposed portions which extend in spaced parallel relationship to define opposite sides of the discharge opening.

It is a feature of the invention to provide a wear-resistant lining for a centrifugal throwing wheel shroud wherein the shroud is configured to extend about a path of discharge followed by impact blasting media as the media discharges from the throwing wheel (1) for ducting such media toward a workpiece, (2) for funneling and dampening the flow of media so that, when the media is discharged at a location spaced from the workpiece, the media is applied to the workpiece in a uniform, non-pulsating pattern to effect a uniform etch on an exposed surface of the workpiece, the wear-resistant lining comprising wear-resistant means configured to protectively line at least a part of such interior surface portions of the shroud as serve to funnel and dampen the flow of media, and fastening means for removably connecting the wear-resistant means to the shroud.

According to another aspect of the invention there is provided a method of etching a workpiece surface, comprising the steps of: a) providing structure defining a treating chamber; b) providing a centrifugal throwing wheel assembly for projecting impact blasting media along a discharge path; c) positioning a workpiece in the treating chamber at a location along the discharge path; d) providing shroud means connected to the throwing wheel assembly and extending about the discharge path (1) for funneling and dampening the flow of impact blasting media as it travels through the shroud means, and (2) for discharging the dampened flow of blasting media in the treating chamber at a selected distance from the workpiece; and, e) operating the throwing wheel assembly to discharge impact blasting media through the shroud means wherein the flow of media is funneled to narrow the width of its path of flow and to effect a dampening of pulsations in the flow so that the media which discharges from the shroud means is delivered in a uniform, nonpulsating pattern of application when it impacts with the workpiece surface.

A further aspect of the invention provides a method of etching a side surface of an elongate sheet of metal to provide a uniform, fine-grained etch suitable for use in lithography, comprising the steps of: a) providing structure defining a treating chamber; b) providing means for positioning the elongate sheet of metal for traveling along a feed path extending through the treating chamber; c) providing a centrifugal throwing wheel assembly at a location housed within the treating chamber, with the throwing wheel assembly oriented to project relatively fine-grained, lightweight impact blasting media along a discharge path toward a side surface of a metal sheet when the sheet is moving along the feed path through the treating chamber;; d) providing shroud means connected to the throwing wheel assembly and extending about the discharge path for funneling and dampening the flow of impact blasting media as the media travels through the shroud means for discharge at a selected distance from the side surface of the metal sheet; and, e) operating the throwing wheel assembly to discharge impact blasting media through the shroud means and on to the side surface of the metal sheet as the metal sheet is moved through the treating chamber along the feed path whereby the shroud means operates to narrow the width of the path of flow of the media and to cause the media to partially interfere with its own flow to effect a dampening of pulsations in media flow rate, with the result that a uniform, non-pulsating flow of media is applied to the side surface of the sheet of metal to uniformly etch the side surface.

Embodiments of the present invention will now be described by way of example, reference being made to the accompanying drawings, in which: Fig. 1 is a schematic front elevational view of an impact blasting apparatus embodying features of the present invention, with portions of the apparatus broken away; Fig. 2 is a schematic side elevational view thereof; Fig. 3 is a front elevational view, on an enlarged scale, of one of the shrouded centrifugal throwing wheel assemblies which is utilized in the apparatus of Figs. 1 and 2; Fig. 4 is a view, on an enlarged scale, of a shroud structure used on the throwing wheel assembly of Fig. 3, with portions of the shroud broken away, as seen from a plane indicated by a line 4-4 in Fig. 3; and Fig. 5 is a sectional view, on an enlarged scale, as seen generally from a plane indicated by a line 5-5 in Fig. 4.

Referring to Figs. 1 and 2, an impact blasting apparatus for etching opposed side surfaces of a traveling web of metal is indicated generally by the numeral 10. The system 10 includes an enclosure 12 having four upstanding walls 14, 16, 18, 20 which define a treating chamber 22. The treating chamber 22 surrounds portions of a vertically-oriented feed path indicated by arrows 24. A web or elongate sheet of metal 30 is fed upwardly along the feed path 24. Sets of rolls 26 or other suitable positioning means are provided to position the web of metal 30 for travel along the feed path 24. The metal 30 typically comprises aluminium, or an alloy thereof, or some other metal which, when properly etched, may be used in lithography. The web of metal 30 is typically about 1 27 cms (50 inches) in width.

The feed path 24 extends substantially centrally through the chamber 22 defined by the enclosure 12. The web of metal 30 has opposed side surfaces 32, 34 and opposed edge surfaces 36, 38. The opposed side surfaces 32, 34 of the web of metal 30 parallel the upstanding walls 14, 1 6 of the enclosure 1 2. The opposed edge surfaces 36, 38 of the web of metal 30 parallel the upstanding walls 1 8,20 of the enclosure 12.

Eight centrifugal throwing wheel assemblies 60, 61, 62, 63, 64,-65, 66, 67 are housed within the enclosure 12. The throwing wheel assemblies 60, 61, 62, 63 are located on the left side of the web of metal 30, as viewed in Fig. 1 , for directing impact blasting media along paths indicated by the numerals 70 toward the metal side surface 32.

Similarly, the throwing wheel assemblies 64; 65, 66, 67 are located on the right side of the web.of meta! 30, as viewed in Fig. 1, for directing impact blasting media along paths indicated by the numerals 72 toward the metal side surface 34.

The throwing wheel assemblies 60-67 are arranged in a staggered array within the enclosure 12 so that the patterns of impact blasting media 70, 72 which discharge from the assemblies 60-67 onto the web of metal 30 cooperate to thoroughly expose the opposed surfaces 32, 34 of the sheet 30 to an etching action.The assemblies 60-67 are paired such that: (1) the assemblies 60, 61 cooperate to cover the full width of the sheet surface 32 with one application of impact blasting media 70; (2) the assemblies 62, 63 cooperate to cover the full width of the sheet surface 32 with another application of impact blasting media 70; (3) the assemblies 64, 65 cooperate to cover the full width of the sheet surface 34 with one application of impact blasting media 72; and, (4) the assemblies 66, 67 cooperate to cover the full width of the sheet surface 34 with another application of impact blasting media 72.

The staggered array of the assemblies 60-67 is provided so that the pattern of impact blasting media discharging from any one of the assemblies 60-67 will not interfere with the pattern of impact blasting media discharging from others of the assemblies 60-67. Moreover, as is shown somewhat schematically in Fig. 2, divider plates 80 are provided between such ones of the assemblies 60-67 as are positioned relatively closely to each other. The plates 80 further assure that impact blasting media issuing from any one of the assemblies 60-67 does not interfere with media issuing from another of the assemblies 60-67.

Impact blasting media is supplied to the assemblies 60-67 through a suitable system of conduits (not shown) which connect with inlet structures 82 provided on each of the assemblies 60-67. Once the blasting media 70, 72 has impacted with the web of metal 30, the media is collected within the enclosure 1 2 using conventional apparatus (not shown) so that the media can be recycled and reused. U.S.-A- 3,731,432 issued May 8, 1973 to James H.

Carpenter, Jr., et al., describes one typically suitable system for delivering and collecting impact blasting media. The disclosure of this patent is incorporated herein by reference.

The assemblies 60-67 are supported within the enclosure 12 by suitable mounts (not shown) which position and orient the assemblies 60-67 to discharge impact blasting media 70, 72 as shown and described. The assemblies 60-67 include conventional centrifugal throwing wheel units of the general type described in U.S.-A- 3,290,827 issued December 13, 1966 to J. E.

Bowling et awl., the disclosure of which is incorporated herein by reference. The assemblies 60-67 additionally include shroud structures 100 which, in accordance with features of the present invention, serve to influence the flow of impact blasting media which issues from their associated throwing wheels to dampen pulsations and thereby enable the assemblies 60-67 to effect uniform applications of impact blasting media on opposed side surfaces 32, 34 of the web of metal 30.

Referring to Fig. 3, the throwing wheel assembly 60 and its shroud structure 100 are depicted in greater detail. The assembly 60 has a motor enclosure 90 and a wheel housing structure 92. The shroud structure 100 cooperates with the housing structure 92 to enclose a rotary blast wheel (not shown), which tends to project impact blasting media along a discharge path which centres about a plane indicated in Figs. 3 and 5 by the numeral 94. The shroud structure 100 is configured, in accordance with features of the present invention, to funnel the flow of impact blasting media 70 toward the web of metal 30 in such a manner as will effect a uniform, nonpulsating application of the media 70 to the surface 32 of the web of metal 30.

Referring to Figs. 4 and 5, the shroud structure 100 includes a pair of spaced, parallel-extending side wall plates 102, 104 which define opposite side walls of a chamber 106 which houses a portion of a vaned centrifugal throwing wheel of conventional configuration (not shown).

Depending portions 112, 114 of the side wall plates 102,104 are inclined inwardly toward the plane 94. The plate portion 112 is inclined relative to the plane 94 at an angle "A" of about 22 degrees. The plate portion 114 is inclined relative to the plane 94 at an angle "B" of about 30 degrees. Outwardly turned flanges 122,124 are formed at the lower ends of the plate portions 112, 114. The flanges 122, 124 have inner corners 132, 134 (1) which extend parallel to each other, (2) which parallel the plane 94, and (3) which are located at equal distances on opposite sides of the plane 94.

The shroud structure 100 additionally includes a pair of end wall plates 108, 110 which define opposite end portions of the chamber 1 06. The end wall plates 108, 110 extend between and join opposed end portions of the side wall plates 102, 1 04, and cooperate with the side wall plates 102,104 to define a closed-wall conduit for directing impact blasting media toward tHe web of metal 30.

A replaceable wear-resistant liner is preferably provided on the inner surfaces of the plate portions and on the inner surfaces of the end wall plates 108; 110. Referring to Figs. 4 and 5, the liner includes wear plates 152, 1 54 which are secured by threaded fasteners -156 to the wall portions 112, 114, and wear plates 158, 160 which are secured by threaded fasteners 1 62 to the end walls 108, 110.

The wear plates 152,154 have edges 1 72, 1 74 which define opposed sides pf a discharge opening, indicated generally by the numeral 200. The discharge opening 200 extends substantially within a plane indicated in Fig. 5 by the numeral 96.-The plane 96 is perpendicular to the plane 94.

The wear plates 158,160 have edges 178, one of which is shown in Fig. 5, which define opposite ends of the discharge opening 200. In the event no wear plates are used to line the inner surfaces of walls and wall portions 108, 110, 112-, 114, these walls and wall portions will then define the sides and ends of the discharge opening 200.

A feature of the invention lies in the discovery that there is a minimum distance between the discharge opening 200 and the workpiece surface 32 which must be maintained if the shroud structure 100 is to function properly. The best way to determine this minimum distance for each set of impact system parameters (i.e., throwing wheel size, throwing wheel velocity, angle of the throwing wheel plane 94 relative to the plane of the workpiece surface 32, etc.) is by experimentation. If the discharge opening 200 is located too close to the workpiece surface 32, air apparently becomes "trapped" between the workpiece surface 32 and the shroud structure 100, and can feed back into the shroud structure 100 inhibiting its normal operation in dampening media flow pulsations.When the discharge opening 200 is located too closely to the moving workpiece surface 32 shadowy streaks or other non-uniformities will become apparent in the resulting etched surface. It is believed that the maximum distance which can be utilized between the opening 200 and the surface 32 is limited not by a problem of non-uniform media application, but rather by such problems as a failure of impact media momentum to properly carry the media into contact with the workpiece surface 32. In short, so long as the required minimum distance is observed, the opening 200 may be positioned at any reasonable distance from the workpiece surface 32.

In operation, a very fine-grained, commercially available, impact blasting media is fed in a conventional manner to the assemblies 60-67, and each of the assemblies 60-67 is operated in the conventional manner to discharge the media along a path of discharge which centres about such a p!ane as the plane 94. The shroud structures 100 duct the media toward the moving web of metal 30, and narrow-the width of the flow of the media as the media passes between the relatively inclined.plate portions ilk, 114. During this funneling process, the flowing media interacts with itseif to dampen such flow pulsations as tend to occur where a conventional centrifugal throwing wheel is used to propel very fine; very lightweight media.The dampening action which takes place assures that a uniform, non-pulsating flow of media is applied to the surfaces of the web of metal 30, whereby an exceptionally uniform, fine-grained etching operation is performed on these surfaces.

In one test, the throwing wheel assemblies 60-67 each employed a conventional 35.56 cm (14 inch) diameter throwing wheel rotating at about 1 600 RPM and throwing approximately 14,061 kg (31,000 pounds) per hour of media having a bulk density of about 2947 kg/m3 (184 pounds per cubic foot). Wheel rotation speeds within the range of 900 RPM to 2000 RPM were also tried and found to be operable. The web of metal 30 to be etched was 127 cm (50 inches) wide and moved along the feedpath 24 at a speed of 21.3 metres (70 linear feet per minute).The media was zircon sand comprised by weight of 1 7 percent at 149 micron 100 mesh (.0059 inch squares), 59 percent at 105 micron 140 mesh (.0041 inch squares), 21 percent at 74 micron 200 mesh (.0029 inch squares), and a 3 percent remainder of smaller particles. The assemblies 60-67 were inclined with respect to the feed path 24 such that their respective planes 94 intersected the feed path 24 at angles of about 30 degrees; however, it was found that angles within the range of 150 to worked equally well. Angles outside this range were not tested. The distance between the discharge openings 200 and the workpiece surfaces being etched, as indicated in Fig. 5 by the dimension "C", was about 40.64 cms (16 inches), but was found to work equally well when increased to about 55.88 cms (22 inches). Less than about a 40.64 cm (16 inch) distance brought an onset of streaking. Utilizing these parameters, a uniformly etched surface finish of approximately 40AA was obtained satisfactorily and consistently.

No problems with such non-uniformities as streaking were observed.

Although the invention has been described in its preferred form with a certain degree of particularlity, it will be understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the scope of the invention as hereinafter claimed.

Claims (32)

1. An apparatus for treating a workpiece, comprising: a) structure defining a treating chamber; b) positioning means for positioning a workpiece within the treating chamber; c) centrifugal throwing wheel means for receiving and projecting impact blasting media along a path of discharge; and, d) shroud means extending about the discharge path for ducting impact blasting media from the throwing wheel means toward the workpiece, for discharging such media within the treating chamber at a location spaced from the workpiece, and for effecting such discharge in a uniform, nonpulsating pattern of application, the shroud means including:: i) first and second side wall means extending in overlying but non-parallel relationship along opposite sides of the discharge path; ii) first and second end wall means interconnecting the first and second side wall means and cooperating therewith to define a closed-wall duct which is open at one end to receive impact blasting media from the centrifugal throwing wheel means, and which defines a discharge opening at the other end thereof for discharging impact blasting media toward the workpiece; iii) the first and second side wall means being inclined relative to each other in a converging relationship for funneling and dampening the flow of blasting media as it moves along the discharge path between the first and second side wall means to narrow the width of flow of the media in one dimension as the media travels through the duct; and, iv) the first and second side wall means having opposed portions which extend in spaced, parallel relationship to define opposite sides of the discharge opening.

2. Apparatus according to claim 1 wherein each of the first and second side wall means has an inner surface facing toward the inner surface of the other, and removable wear liner means protectively covers at least portions of these inner surfaces.

3. Apparatus according to claim 1 or 2 wherein each of the first and second end wall means has an inner surface facing toward the inner surface of the other, and removable wear liner means protectively covers at least portions of these inner surfaces.

4. Apparatus according to claim 1,2 or 3 wherein: a) the throwing wheel means includes a throwing wheel which rotates about an axis of rotation; b) the discharge path followed by media discharging from the throwing wheel is elongate in cross-section and extends along a first plane which is substantially perpendicular to the axis of rotation; c) the discharge opening is of elongate configuration; and, d) opposite sides of the discharge opening extend substantially parallel to the first plane and are located on opposite sides of the first plane.

5. Apparatus according to claim 4 wherein the first and second side wall means, and the first and second end wall means are configured such that the discharge opening extends in a second plane which is substantially perpendicular to the first plane.

6. Apparatus according to-claim 4 wherein at least one of the first and second side wall means is inclined relative to the first plane.

7. Apparatus according to claim 6 wherein the angle at which said at least one side wall means is inclined relative to the first plane is within the range of about 22 to about 30 degrees.

8. Apparatus according to claim 4 wherein the first side wall means is inclined at a first angle relative to the first plane, the second side wall means is inclined at a second angle relative to the first plane, and the first and second angles are unequal.

9. Apparatus according to claim 8 wherein the first angle is approximately 22 degrees, and the second angle is approximately 30 degrees.

1 0. Apparatus according to any one of the preceding claims wherein: a) the centrifugal throwing wheel means is housed within the structure which defines the treating chamber; b) the positioning means supports the workpiece for movement through the treating chamber; c) the workpiece is in the form of an elongate sheet having opposed side surfaces; and, d) the centrifugal throwing wheel means is oriented to project blasting media toward at least a portion of one of the opposed sides of the sheet.

11. Apparatus according to claim 10 wherein the centrifugal throwing wheel means includes two centrifugal throwing wheels housed within the structure.

12. Apparatus of claim 11 additionally including barrier means interposed between the respective discharge paths of the two throwing wheels to prevent blasting media projected from one of the throwing wheels from interfering with the operation of blasting media projected from the other of the throwing wheels.

13. Apparatus according to claim 11 wherein each of the two centrifugal throwing wheels is oriented to project blasting media toward separate portions of one of the opposed sides of the sheet.

14. Apparatus according to claim 11 wherein each of the two centrifugal throwing wheels is oriented to project blasting media toward a separate one of the opposed sides of the sheet.

1 5. Apparatus according to claim 1 constructed, arranged and adapted to operate substantially as herein described with reference to the accompanying drawings.

1 6. Shroud means for use with a centrifugal throwing wheel (1) for positioning about a path of discharge which is followed by impact blasting media as the media discharges from the throwing wheel, (2) for ducting such media toward a workpiece surface to be etched, and (3) for discharging such media at a location spaced from the workpiece in a uniform, non-pulsating pattern to uniformly etch the workpiece surface, the shroud means comprising:: a) first and second side walls extending in overlying but non-parallel relationship along opposite sides of the discharge path; b) first and second end walls interconnecting the first and second side walls and cooperating therewith to define a closed-wall duct which is open at one end to receive impact blasting media from the centrifugal throwing wheel, and which defines a discharge opening at the other end thereof for discharging impact blasting media toward a workpiece; c) the first and second side walls being inclined relative to each other in a converging relationship for funneling and dampening the flow of impact blasting media as it travels between the first and second side walls to narrow the width of flow of the media as the media travels through the duct;; and, d) the first and second side walls having opposed portions which extend in spaced parallel relationship to define opposite sides of the discharge opening.

1 7. Shroud means according to claim 1 6 wherein each of the first and second side walls has an inner surface facing toward the inner surface of the other, and removable wear liner means protectively covers at least portions of these inner surfaces.

1 8. Shroud means according to claim 1 6 or 17 wherein each of the first and second end walls has an inner surface facing toward the inner surface of the other, and removable wear liner means protectively covers at least portions of these inner surfaces.

19. Shroud means according to claim 16, 17 or 1 8 wherein each of the first and second side walls is inclined with respect to the discharge path.

20. Shroud means according to claim 19 wherein the angle at which the first side wall is inclined with respect to the discharge path differs from the angle with which the second side wall is inclined with respect to the discharge path.

21. Shroud means according to any one of claims 1 6 to 20 wherein the angles at which the first and second side walls are inclined with respect to the discharge path are within the range of about 22 to about 30 degrees.

22. Shroud means according to any one of claims 1 6 to 21 wherein the opposite sides of the discharge opening are positioned at substantially equal distances from the centre of the discharge path.

23. Shroud means according to claim 1 6 constructed, arranged and adapted to operate substantially as herein described with reference to the accompanying drawings.

24. A wear-resistant lining for a centrifugal throwing wheel shroud wherein the shroud is configured to extend about a path of discharge followed by impact blasting media as the media discharges from the throwing wheel (1) for ducting such media toward a workpiece, (2) for funneling and dampening the flow of media so that, when the media is discharged'at a location spaced from the workpiece, the media is applied to the workpiece in a uniform, non-pulsating pattern to effect a uniform etch on an exposed surface of the workpiece, the wear-resistant lining comprising wear-resistant means configured to protectively line at least a part of such interior surface portions of the shroud as serve to funnel and dampen the flow of media, and fastening means for removably connecting the wearresistant means to the shroud.

25. A wear-resistant lining according to claim 24 wherein the wear-resistant means comprises a plurality of plate-like members, each being configured to protectively line a separate part of such interior surface portions as serve to funnel and dampen the flow of media.

26. A method of etching a workpiece surface, comrpising the steps of: a) providing structure defining a treating chamber; b) providing a centrifugal throwing wheel assembly for projecting impact blasting media along a discharge path; c) positioning a workpiece in the treating chamber at a location along the discharge path; d) providing shroud means connected to the throwing wheel assembly and extending about the discharge path (1) for funneling and dampening the flow of impact blasting media as it travels through the shroud means, and (2) for discharging the dampened flow of blasting media in the treating chamber at a selected distance from the workpiece; and, e) operating the throwing wheel assembly to discharge impact blasting media through the shroud means wherein the flow of media is funneled to narrow the width of its path of flow and to effect a dampening of pulsations in the flow so that the media which discharges from the shroud means is delivered in a uniform, nonpulsating pattern of application when it impacts with the workpiece surface.

27. A method according to claim 26 wherein the step of providing shroud means includes the steps of providing a shroud structure which has first and second side walls that extend in overlying but non-parallel relationship along opposite sides of the discharge path, and the step of operating the throwing wheel assembly to discharge blasting media through the shroud means includes the step of discharging the blasting media along a discharge path which causes at least a part of the blasting media to impact with the first and second side walls, whereby the blasting media is funneled by the first and second side walls to narrow the width of its path of flow, and is thus caused to interfere sufficiently with its flow to dampen pulsations in the flow.

28. A method of etching a side surface of an elongate sheet of metal to provide a uniform, finegrained etch suitable for use in lithography, comprising the steps of: a) providing structure defining a treating chamber; b) providing means for positioning the elongate sheet of metal for traveling along a feed path extending through the treating chamber; c) providing a centrifugal throwing wheel assembly at a location housed within the treating chamber, with the throwing wheel assembly oriented to project relatively fine-grained, lightweight impact blasting media along a discharge path toward a side surface of a metal sheet when the sheet is moving along the feed path through the treating chamber;; d) providing shroud means connected to the throwing wheel assembly and extending about the discharge path for funneling and dampening the flow of impact blasting media as the media travels through the shroud means for discharge at a selected distance from the side surface of the metal sheet; and, e) operating the throwing wheel assembly to discharge impact blasting media through the shroud means and on to the side surface of the metal sheet as the metal sheet is moved through the treating chamber along the feed path whereby the shroud means operates to narrow the width of the path of flow of the media and to cause themedia to partially interfere with its own flow to effect a dampening of pulsations in media flow rate, with the result that a uniform, non-pulsating flow of media is applied to the side surface of the sheet of metal to uniformly etch the side surface.

29. A method according to claim 28 wherein: a) a plurality of centrifugal throwing wheel assemblies are provided and housed within the treating chamber; b) each of the throwing wheel assemblies is oriented to direct impact blasting media toward separate portions of a metal sheet traveling along the feed path; c) each of the throwing wheel assemblies is provided with a separate one of said shroud means; and, d) the step of.operating the throwing wheel assembly includes the step of simultaneously operating all of the throwing wheel assemblies. -

30.A method according to claim 29 including the steps of, a) providing barrier means for shielding the path of discharge of blasting media projected from one of the throwing wheel assemblies from interfering with blasting media projected from another of the throwing wheel assemblies; and, b) the step of simultaneously operating.the throwing wheel assemblies includes the step of utilizing the barrier means to prevent blasting media projecting from one of the throwing.wheels from interfering with the path of flow of blasting media from another of the throwing wheel assemblies.

31. A method according to claim 28 wherein the step of operating the throwing wheel assembly includes the step of supplying to the throwing wheel assembly impact blasting media in the form of abrasive particles which will pass through a 1-49 micron 100 mesh screen having squares measuring about 149 x 1 0e metres (0.0059 inches) on each side.

32. A method of etching a workpiece surface substantially as herein described with reference to the accompanying drawings.