Apparatus For Graining Surfaces Of Planographic Printing Plates

Abstract

An apparatus for graining the surface of a strip of sheet material useful for making planographic printing plates comprising die means having a slot-like opening therein and gas and liquid chambers converging at one of their longitudinal sides to form a mixing channel ending at said slot-like opening, whirling niche structure extending along one of the sidewalls of the mixing channel, the distance between the slot-like opening and whirling niche being at least as long as the slot-like opening is wide, elements for conveying the strip of sheet material across the slot-like opening, apparatus for feeding a gas into the gas chamber, and for feeding a liquid suspension of solid graining means into the liquid chamber.

Parent Case Text

This is a division of application Ser. No. 97,945, filed Dec. 14, 1970, now abandoned.

Description

This invention relates to graining surfaces of a material useful for planographic printing plates made of metal or plastic material.

It is known that, in most cases, planographic printing plates must be provided with a roughened surface. This roughness is often called the grain of the surface. For the production of the grain, the surfaces of metal sheets or foils, inter alia, have been roughened by sand-blasting, but it is difficult with this method to achieve a uniform grain. Furthermore, sand-blasting involves dust, the removal of which is complicated and, moreover, recovery of the graining means, particularly its separation from the grits is difficult. Furthermore, the metal surface must be degreased before sand-blasting and washed after sand-blasting. Another known graining method consists of grinding the surface by means of rotating steel wire brushes. This method has the advantage that it can be performed in a relatively simple and continuous manner so that it is not necessary to grain each printing plate separately, but possible to grain a metal strip which is then cut into printing plates. The brushing method, however, yields a comparatively coarse grain and easily leads to the formation of grooves in the brushing direction and extraordinarily careful preparation of the brushes is necessary in order to avoid those and other errors. Another known method is electrochemical roughening of the surfaces for planographic printing plates. By this method, it is also possible to roughen strips but with the disadvantage that it is not certain that scratches or grooves present in the material and caused, for example, by rollers or metal foils can be removed, sometimes they become even deeper.

The present invention provides an apparatus for graining surfaces of planographic printing plate material, which produces uniformly roughened surfaces, can be performed continuously in a relatively simple manner and with relatively little space, consumes little auxiliaries, and produces little waste.

The known process for graining surfaces of a material for planographic printing plates, in which graining of the surface is performed by roughening by means of a jet of a graining means, is used as a basis and the present invention includes the additional process steps: moving a strip of the material in the direction of the strip, suspending the graining means in a fluid, and causing the suspension of graining means to impinge upon the travelling strip as a wide jet which extends over the entire width of the strip. The fluid used in the process for suspending the graining means usually is water. Suitable graining means are, for example, pulverized iron, corundum, aluminum oxide, and other grinding and lapping means known from grinding and lapping techniques. The grain size of the graining means depends upon the desired effect. For a relatively fine grain, a correspondingly fine grain size is selected, e.g., of 0.01 mm and, for a coarser grain a coarser grain size, e.g., of 0.1 mm. For producing a roughness of a depth of about 0.002 to 0.004 mm, as is normally desired for aluminum plates, silicon carbide powder of an average grain size of about 0.018 to 0.020 mm may be used with good results, for example. Furthermore, the roughness depth achieved also depends upon the impact energy of the graining means particles contained in the jet suspension. Therefore, the speed of impact of the suspension jet onto the strip must be adapted to the desired result, which, when a slot die is used, may be achieved by adjusting the pressure under which the jet of suspension is ejected by the die.

The jet of suspension produced by the apparatus must be at least sufficiently wide that it covers the entire width of the strip to be grained. Advantageously, a slot die having an orifice of a corresponding width is used for producing the jet. The "aperture" of the slot, i.e., the dimension of the slot in the direction of the strip, generally is from 3 to 10 mm. The wider the aperture of the slot, the more effective, under otherwise comparable conditions, is the influence of the graining means onto the strip or the faster is the feed speed of the strip at which a particular degree of graining is achieved. For practical reasons, the maximum values for the slot aperture are limited because the larger the aperture of the slot die, the more difficult it is to produce a jet having a uniform effect.

The fluid pressure necessary for achieving a particular effect may be produced and maintained in the slot die by means of a rotary pump, for example. A sufficiently high speed of the wide jet may be achieved, without employing a high fluid pressure, by mixing the suspension of the fluid and graining means with an expandable gaseous or vaporous medium, preferably compressed air or steam, before causing it to impinge upon the surface of the strip of material. This method is by far superior. It is performed by means of slot dies which are constructed as mixing nozzles or injector nozzles.

When using the injector nozzles, the invention can be performed without a pump for imparting the necessary speed of the wide jet. This is a very important advantage since pumps conveying graining means suspensions are very rapidly worn.

The wide jet of the graining means suspension also may be centrifuged by a rotating body and the wide centrifuged jet may be so directed that it impinges transversely upon the travelling strip.

In the invention, the wide jet of suspension may be directed either vertically or obliquely upon the travelling metal strip. The arrangement of the jet depends upon the effect desired and also on the other process conditions, such as the impact speed of the jet and the feed speed of the strip. It is also possible, of course, to cause several wide jets of suspension to successively impinge upon the travelling metal strip.

In a preferred embodiment of the invention, graining is accompanied by a further feature in that the graining means is suspended in a fluid containing, in addition to the graining means, another agent in a dissolved or dispersed form. The invention is performed, for example with an aluminum strip as it is supplied from the rolling mill by the impingement of a suspension of a graining means in water, which contains a degreasing agent either dissolved or emulsified therein, in such a manner that, in one working step, a degreased and grained aluminum strip is obtained, as is required for the production of offset printing plates. The degreasing agent is a fatty alcohol or an alkylaryl sulfonate, for example. Another example is that the aqueous suspension of the graining means used in the invention contains a substance in the dissolved state which imparts to the treated surface of the strip a better adhesion with respect to light-sensitive reproduction layers to be later applied, as is effected, for example, by water glass or polyvinyl phosphonic acid in the case of aluminum surfaces or by trichloroacetic acid alone or in combination with chromic acid in the case of a biaxially stretched film of polyethylene terephthalate. It is possible to achieve, in one working step, a roughness as well as an improved adhesion of the surface of the treated strip of material. In the case of films of a saponifiable plastic material, such as polyvinyl acetate, the surface may be roughened and saponified simultaneously.

In some cases, the strip of material grained by the impact of the suspension of the graining means will not require cleaning or other treatment before being cut into printing plates. If, however, it is desired or required to subject the strip between the impingement of the graining means suspension and the cutting into printing plates to cleaning or another treatment, they may be performed in any known manner. The grained strip may be freed from any residues of graining means adhering thereto by spraying with water, for example. Other working steps between graining and cutting of the printing plates may be, for example: adhesion improvement of the surface of the grained metal strip, chemical or electrochemical production of an aluminum oxide layer on a grained aluminum strip, sensitization of the grained strip by applying a lightsensitive reproduction layer thereon, or several of these or other working steps.

The invention will be further illustrated with reference to the accompanying drawings.

FIG. 1 is a diagrammatic sectional view of an apparatus for the performance of the invention,

FIG. 2 is a larger view of a detail of FIG. 1, and

FIG. 3 shows another embodiment of the detail shown in FIG. 2. The views are not to scale.

In the apparatus shown in FIG. 1, a material strip M, of which one surface is roughened in the apparatus, is conveyed over an inlet roller 1 to a rotating drum 2 in such a manner that it contacts the lower half thereof. The material strip then leaves the apparatus via an outlet roller 3. The lower half of the drum 2 extends into a chamber 4 through an opening in the chamber wall 5. The lower part of the chamber is a trough 7 which holds the graining means suspension S. A slot die 8 with the slot 9 thereof is positioned beneath the drum 2, its width extending along the lowest surface line of the drum. The slot die 8 is a mixing nozzle having an air chamber 10 and a fluid chamber 11. The graining means suspension in the trough 7 is circulated by means of a pump 12 via a circulatory suction line 13 and a circulatory pressure line 14, the circulation speed being adjustable by means of a circulation valve 15. From the pressure line 14 between the pump 12 and the circulation valve 15, a pressure line 16 branches off through which the graining means suspension can be pumped into the fluid chamber 11 of the slot die. A suspension valve 17 serves for adjusting the quantity pumped into the chamber 11. The air chamber 10 of the slot die receives compressed air from a compressed-air source (not shown) via a compressed-air line 18 in a quantity adjustable by a compressed-air valve 19.

The wide spray jet, produced in the slot die 8 from the supplied graining means suspension and the supplied compressed air, impinges upon the material strip M, which is on the drum 2, and is advanced thereby above the die 8 from right to left, and effects roughening.

After impingement upon the material strip, a part of the graining means suspension falls back into the trough 7 as is shown by the bent arrow lines T. As is shown by the multiply bent arrow lines R, the air charged with the floating residue of the graining means suspension is forced to flow to and fro by the baffle plates 20, 21, and 22 mounted in the chamber 4 and thus a great part of the residue is caused to separate from the air and to return into the trough 7. The air charged with the last residue of the graining means suspension passes through the separators 23 where the last residue is removed. From the separators 23, the cleaned air is drawn off through the suction lines 24 by means of the fans 25 and fed into exhaust-air ducts 26.

From the exhaust-air ducts 26, there branch off blast-air lines 27 which lead to air knives 28. They produce air jets extending over the entire width of the material strip M and one of them serves for blowing off contaminations, particularly dust, from the material strip, before they enter the working zone of the slot die 8. The other one serves for blowing off residues of the graining means suspension from the material strip and for drying the material strip after it has left the working zone of the slot die 8. The distribution of the quantity of air blown from the fans 25 to the carrying-off parts 26a of the exhaust-air ducts 26 and the blast-air lines 27 is controlled by the throttle valves 29 and 30.

When the slot die used in the process, as shown in FIG. 1, is a mixing nozzle, its mixing channel is advantageously equipped with a whirling niche 31 (FIG. 2). The niche 31 is so arranged that the ribbon of air issuing from the chamber 10 through the wide-slotted air channel 32 enters the whirling niche 31 after fusion with the ribbon of graining means suspension pumped through the wide-slotted suspension channel 33 and is whirled up therein before the ribbon of air and graining means suspension suspended therein emerges from the slot 9 of the die 8. In the stretch of the slot die extending between the orifice of the slot 9 and the niche 31, the suspension is accelerated. The length of the accelerating distance must be at least equal to the "aperture" of the slot.

When the slot die used in the process illustrated in FIG. 1 is an injector nozzle, the slot die used may be of the construction shown in FIG. 3. The body of the die 8 contains an air chamber 10 with a wide-slotted air channel 32 through which a ribbon of air is blown into the channel 34 of an inverted channel-type funnel 35 surrounding the body of the die 8. Due to the Venturi effect, a certain quantity of graining means suspension, in which the body of the die 8 and the channel-type funnel 35 are immersed, is entrained. The graining means suspension is in trough 36. Generally, the graining means suspension in the trough 36 reaches to the outlet cross-section of the wide-slotted air channel 32, but it also may be some millimeters above or beneath this level. In the case of an injector nozzle, the accelerating distance extending between the orifice of the wide-slotted air channel 32 and the orifice of the channel 34 also must have a minimum length, which can be determined by tests. The known hole-type injector nozzles of corresponding cross-section also have this accelerating distance.

The above-illustrated apparatus for the performance of the invention includes various modifications. It is possible, for example, to continuously take part of the graining means suspension S from the trough 7 and to feed it again to the trough after removal of grits and replenishment of used graining means by fresh graining means and replenishment of used water, possibly of used cleaning agent, adhesion-improving agents or other used constituents of the graining means suspension.

It is particularly advantageous in some cases to perform the process illustrated in FIG. 1 in several steps and to coordinate the individual stations of the apparatus to one another in such a manner that fresh graining means suspension is continuously introduced into the trough 7 of the last apparatus station and a corresponding quantity of more or less used graining means suspension flows into the stepwise arranged lower trough 7 of the preceding stations down to the first station of the apparatus. The graining means suspension flowing from the first station of the apparatus is either removed as waste or, after regeneration, introduced into the last station of the apparatus.

With an apparatus as illustrated in FIG. 1, it is possible, for example, to roughen an aluminum strip over a relatively short stretch and at a relatively high feed speed of the strip in a manner suitable for lithographic purposes. An extraordinarily uniform roughness is achieved with a 10 percent by volume aqueous graining means suspension at a feed speed of the strip of 2 to 6 m/minute and with a die pressure of 2 to 8 kg/cm.sup.2. When using silicon carbide of an average grain size of 0.02 mm, the depth of roughening is approximately 0.002 to 0.003 mm and the number of peaks from about 300 to 600 per cm.sup.2.

One of the most important advantages of the invention is that it is possible to achieve a relatively high process speed with relatively little space. Another advantage lies in the fact that the performance of a second treatment, e.g., degreasing or an improvement of the adhesion, can be performed simultaneously with the graining operation. Further advantages are that a very high uniformity of graining can be achieved, that the waste produced during the process can easily be collected, and that the process involves only little waste of treating agents.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

Claims

What is claimed is:

1. An apparatus for graining the surface of a strip of sheet material useful for making planographic printing plates comprising die means having a slotlike opening therein and gas and liquid chambers converging at one of their longitudinal sides to form a mixing channel ending at said slot-like opening,

whirling niche means extending along one of the sidewalls of said mixing channel, the distance between said slot-like opening and said whirling niche being at least as long as said slot-like opening is wide,

means for conveying the strip of sheet material across said slot-like opening,

means for feeding a gas into said gas chamber,

and means for feeding a liquid suspension of solid graining means into said liquid chamber.

2. An apparatus according to claim 1 wherein the width of said slot-like opening is between 3 and 10 mm.