GB2081181A - Cleaning offset printing blankets - Google Patents

Abstract

A blanket of an offset printing machine is cleaned to remove debris by passing through the machine either in web or sheet form a material having at least one surface which is tacky and removes the debris from the blanket, the surface not adhering to the feed mechanisms of the machine, or to adjacent sheets when stacked in sheet form, before or after passing through the machine. The adhesive is preferably an acrylic ester polymer which is pressure-sensitive, and the sheet of web preferably carries a pattern of ribs to give the non- adherent properties in the feeding and stacking mechanism.

Description

SPECIFICATION Cleaning device for printing machine This invention relates to cleaning means for printing machines.

Many printing machines suffer quality problems due to the existence of or creation of paper and paper board debris which adheres to parts of the image, thus producing defective printing and in some extreme cases such debris will find its way into the ink pigment.

Frequently when such circumstances arise it is necessary to clean the printing plates or other parts of the machine to restore the original appearance.

In some cases this cleaning is done manually and the machine is stopped. In other cases mechanical devices have been developed and introduced to perform this function.

These problems are common to machinery that is fed either with sheets of paper or paper board or to machinery that is fed with reels of paper or paper board.

A particular form of.printing machine in which this problem is apparent is an offset printing machine.

The offset printing process fundamentally creates the printed image by printing the ink from a printing plate onto a resilient material covering a cylinder, which is commonly formed of rubber or a synthetic material, and is known as a blanket. This resilient blanket transfers the ink onto the material to be printed.

By the nature of the process the printing blanket tends to have a certain "tack" to retain the printing ink.

It is this tack, plus the tack normally present in most printing inks, which accepts paper dust and other contraries from the paper or paper board and, in the case of offset printing, retains and accumulates such contraries.

Thus, after a certain number of sheets have been printed the contraries will have accumulated to the point where the image deteriorates, and in this case the machine stopped and the blankets washed, normally with solvents, to clear the contraries away.

Thus, for example, US Patent No. 3 467 011 describes a device which includes a supply of wiping tape which can be moved into engagement with selected portions of the blanket cylinder. The wiping tape is moistened by cleaning fluid. While the use of cleaning fluid can be successful in removing debris from a printing cylinder, it is inconvenient to use and results in a break in the printing run while the cleaning fluid, which has been applied to the cylinder, is worked off.

In all prior art solutions to this problem production is interrupted and a time lag whilst normal ink charge is restored to the blanket is created before printing can be restarted. As much as up to about 20% printing time can be lost in this way.

It is an object of this invention to provide a means for cleaning the blanket cylinder of an offset printing machine substantially without interrupting production and to eliminate waste material as result of either poor quality printing or as a direct result of stopping the printing cycle of the press.

We have now discovered that this object can be achieved by the present invention which provides a blanket cleaning means for an offset printing machine comprising a substrate for feeding through the offset printing machine, the substrate having surfaces that will substantially not adhere to the feed-in mechanism and feed-out mechanism of the offset printing machine, but having at least one surface which, in use, contacts the blanket surface of the blanket cylinder, which is adapted to adhere to debris on the blanket cylinder and remove the debris therefrom.

In the case of a web-fed offset printing machine, the blanket cleaning means is most suitably in the form of a web. However, the invention is most applicable to a sheet fed offset printing machine where the cleaning means is preferably in the form of a sheet, having surfaces that will substantially not adhere to adjacent sheets in the paper supply stack or in the finished printed paper stack.

The invention also provides a method of cleaning debris from the blanket of an offset printing machine comprising stopping the ink addition to the plate roll, continuing the operation of the machine to remove liquid ink from the blanket cylinder and feeding one or more blanket cleaning sheets through the machine, the cleaning sheets having a permanently tacky adhesive coating on one surface adapted to contact the blanket surface and adhere to and remove debris therefrom without adhering to other parts of the machine or the supply of unprinted material to the machine.

In use, the cleaning sheet is introduced into the normal printing cycle and the passing of this sheet through the press cleans the blanket, therefore greatly reducing the interruption of the normal printing cycle.

The cleaning sheet should have certain characteristics: 1. It must have no adhesive qualities either immediately before or immediately after the cleaning process has been carried out. This is so that the sheet can be introduced to the pile of paper or paper board sheets without sticking to the immediately adjacent sheets in particular the sheet immediately above it. Therefore, this dry (untacky) characteristic enables the sheet to go through many of the mechanical parts prior to the moment of the impression to the rubber blanket without adhering to these mechanical parts.

2. The sheet should have adhesive qualities when the blanket is impressed upon it through the rotation of the machine and this adhesive quality must be sufficient to remove debris, but insufficient to create permanent adhesion with the machine in order that it can follow the normal sheet path through the machine.

3. Thereafter, the sheet should maintain or retain its non-adhesive qualities so that it can again pass through the machine without adhesion and thus be delivered to the finished printing pile where it can be recovered.

For a multi-coloured machine it is envisaged that at least a sheet per colour will be required since it is presumed that two consecutive sheets, for instance, would create the situation of the first sheet cleaning the first blanket and the second sheet would retain its adhesive qualities whilst passing through the first blanket because that first blanket has already been cleaned and thus it is available to clean the second blanket, and so on.

Preferably the cleaning sheet has a flexibility, size and thickness similar to that of the paper or paper board being printed, so that use of the cleaning sheet does not require adjustment of mechanical parts of the machine. Thus the cleaning sheet may be made from paper or paper board, or from a plastics material. The sheet thickness may be about 250 , tm to about 1000 wm, preferably about 350 um to about 650 /zm.

In a first preferred embodiment of the invention, the necessary characteristics of the cleaning sheet are achieved by providing one surface of the sheet with an integral ribbed pattern. The ribbed pattern may be integral in the sheet itself (for example where the sheet is formed of extruded plastics material) or may be introduced to the sheet by some other means, for example embossing or by coating the sheet with a secondary material capable of retaining a ribbed pattern (for instance hot melt waxes).

These ribs, which may run longitudinally, diagonally or, less satisfactorily, laterally are of width, height and spacing which are determined by the nature of the printing process and the job itself. Embossed PVC or cold embossed foil laminated or conventional carton board are particularly suitable.

The height of the ribs may be from about 0.1 to about 0.5 mm, preferably about 0.3 mm, the width of the ribs may be about 0.2 mm to about 1.0 mm, preferably about 0.5 mm and the distance between the ribs may be about 1.0 mm to 15.0 mm, preferably about 3 mm.

The sheet then is coated with a permanently tacky adhesive in such a way that the adhesive falls in the valleys between the ribs but is not coated to the same height as the ribs themselves. The adhesive may be an aqueous dispersipn of an acrylic ester polymer and is (after drying) pressure sensitive and is of approximately 50% solid content (Acronal 80 D). The thickness of the coating may be about 5 to 100 /ism, preferably about 0.02 mm (0.04 mm when wet).

Substantially no adhesive is carried on the upper surface of the ribs so that the desired characteristic that the sheet will not adhere to adjacent paper sheets in the paper supply and will not adhere to the feeding mechanism of the machine is achieved.

The adhesive qualities of the sheet are only brought into play when the resilient blanket is pressed upon the sheet. The blanket will be deflected under normal printing pressure sufficient to touch the adhesive resting in the valleys between the ribs, thus taking the debris from the blanket to the surface of the sheet.

The height, width and frequency of the ribs is preferably calculated to give the optimum nonadhesive contact or to prevent permanent adhesion from the sheet to the blanket as is the adhesive value of the adhesive itself.

Therefore, immediately after leaving the blanket the sheet maintains its dry surface condition since only the upper surface of the ribs will normally come into contact with the machine feeding mechanism and the finished printed paper stack.

In some cases it will be advantageous to coat the reverse of the sheet with some inhibiting material (for instance a silicone material) to aid transportation and handling and further inhibit adhesion in the paper or paper board stacks where the paper or paper board itself may deflect into the valieys under weight pressure or to coat the ribs with an untacky material.

In a second preferred embodiment of the invention the ribs are not made integral with the sheet.

Instead, adhesive is applied over all of one surface of the sheet and thereafter a pattern of ribs is applied over the adhesive. The ribs may be formed from plastics material, metal wire, or a fibrous material.

Alternatively, a substance with an open weave, such as a gauze or Netlon could be laid over the adhesive layer. These applied ribs preferably have a thickness of about 100 to about 1000 iim, preferably about 1 60 to 500 Ism. The construction thus achieved works in a manner similar to the first described embodiment.

A number of samples of cleaning sheets according to the invention, and control sheets were prepared as follows.

SAMPLE 1 Paper board (Invercote D - 240 g/m2) was coated with a permanent tacky adhesive. namely Acronal 80 D, a 50% dispersion of an acrylic polymer. The dry adhesive thickness was 6 jim.

SAMPLE 2 This was similar to Sample 1, but with a dry adhesive thickness of 25 Iim.

SAMPLES 3 AND 4 These were similar to Samples 1 and 2 respectively, but using 75% Acronal 80 D and 25% Acronal 290 D which is a 50% dispersion of an untacky polymer.

SAMPLES 5 AND 6 These were similar to Samples 3 and 4 respectively, but using 50% Acronal 80 D and 50% Acronal 290 D.

SAMPLES 7 AND 8 These were similar to Samples 5 and 6 respectively, but using 25% Acronal 80 D and 75% Acronal 290 D.

SAMPLES 9 AND 10 These were similar to Samples 7 and 8 respectively, but using 100% Acronal 290 D.

SAMPLE 11 A polystyrene sheet with a honeycomb structure was coated with a 80 m thickness layer of Acronal 80 D. A doctor blade was used to control the coating thickness. The honeycomb structure had a cell wall thickness of 0.16 mm, i.e. twice the depth of the adhesive layer. The cross sectional area of each cell was 1 mm x 1 mm and the sheet thickness 350-650 ftm.

SAMPLE 12 A polystyrene sheet formed from a mixture of 92% standard -- PS Dow 364 and 80% Tufprene having a high impact strength was provided with ribs having a thickness of 0.05 mm giving a channel depth of 47,vim and a channel width of 3 mm. The sheet thickness was 500 from. The sheet was coated with an about 23 zm thickness layer of Acronal 80 D after having been electrically pretreated to ensure good spreading and good adherence of the adhesive on the sheet.

SAMPLE 13 This was a PVC sheet having an 18 Fim thickness layer of Acronal 80 D thereon with plastic monofilaments applied over the adhesive layer. The distance between the mid-points of the filaments was 3 mm. The filament thickness was 160 um.

SAMPLE 14 This was similar to Sample 13 with an adhesive thickness of 50 ,Çwm and a filament thickness of 300, lm.

SAMPLE 15 This was similar to Sample 14. The filament thickness was 500,vim.

The above samples were tested by taking 10 sheets of each sample and loading them into the paper stack of an offset printing machine, with 50 sheets of paper board above them. For cleaning, the machine-operator stops further ink addition by separating the ink-roll from the plate-roll; then the 50 waste board sheets are "printed", taking away all liquid ink from the rubber blanket; then the cleaning sheets are "printed", cleaning the rubber blanket. Then the inking roll is again contacted to the plate and normal printing goes on. Each sheet had a siliconised back to reduce adhesion to the next adjacent cleaning sheet.

The results of these tests are set out in the following table, where the expressions "Good" and "Bad" relate to whether or not the sample has the required characteristics for this invention.

TABLE Blocking against Cleaning of Permanent Sample Blocking against polystyrene dirty rubber adhesion No silicone sheet blanket ' to blanket 1 Bad - Very good (1) Bad 2 Bad - Very good (1) Bad 3 Bad - Medium Bad 4 Bad - Very good Bad 5 Good + bad - Medium Good 6 Good +bad Good Good 7 Good - Very bad Good 8 Medium - Medium Good 9 Very good - Very bad Very good 10 Good - Very bad Very good 11 Medium - Bad Good 12 Good Bad (2) Good Medium 13 - Very good Good Medium 14 - Very good Good Good 15 - Very good Medium Very good Notes on table (1) - but the adhesion to rubber is so high that sometimes the boards split.

(2) - but low blocking to siliconed surfaces instead of PS.

ANALYSIS OF RESULTS Samples 1 to 10 show that better cleaning is obtained with thicker, more tacky adhesive, but this in turn leads to permanent adhesion of the sheet to the blanket, other machine parts and adjacent sheets in the stack.

Further results: Thin coatings (6 um dried adhesive) had a significantly lower tack than thick coatings (25,us). Cleaning sheets made of plastic (instead of board) did not show the danger of splitting (see note 1).

The results with a honeycomb base (Sample 11) were poor presumably because air was being trapped in the honeycomb cells between the blanket surface and the adhesive. This was confirmed by Sample 12 where the rib structure provides channels for the escape of air. With Sample 12 good destacking -- lack of adhesion to adjacent sheets in the stack -- could only be obtained if the sheet had a siliconed back (see note 2). Samples 13 to 15 where the filaments were placed over the adhesive layer showed good results as in Sample 12, but the absence of any adhesive on the upper surface of the ribs achieved by this construction, ensured that very good destacking was achieved even where the sheet did not have a siliconed back.In place of ribs as described above, an adhesive layer on a flat cleaning sheet may be sprayed with a non-tacky powder or with particles of a known density and size.

The particles have the same function as the ribs in the previously described embodiments, namely to separate the adhesive from adjacent sheets in the paper stack while allowing the blanket surface to deform into contact with the adhesive.

A still further alternative embodiment of the invention provides the surface of the cleaning sheet with a layer of micro encapsulated quick drying adhesive. The micro encapsulation is sufficient to achieve the dry surface requirement of the cleaning sheet while it remains in the paper stack and in the normal feed-in path of the machine. The capsules break under the pressure at the blanket cylinder, the adhesive being such as will spread over the sheet surface. The adhesion time is limited to prevent permanent adhesion of the sheet to the blanket. It is essential that the adhesive be quick drying, e.g.

less than 1 second, so that no adhesion occurs between the cleaning sheet and the feed-out parts of the machine or the stack of finished printed sheets.

The invention is not restricted to the embodiments described.

Claims (14)

1. A blanket cleaning means for an offset printing machine comprising a substrate for feeding through the offset printing machine, the substrate having surfaces that will substantially not adhere to the feed-in mechanism and feed-out mechanism of the offset printing machine, but having at least one surface which, in use, contacts the blanket surface of the blanket cylinder, which is adapted to adhere to debris on the blanket cylinder and remove the debris therefrom.

2. A blanket cleaning means according to Claim 1 in the form of a web.

3. A blanket cleaning means according to Claim 1 in the form of a sheet having surfaces that will substantially not adhere to adjacent sheets in the paper supply stack or in the finished printed paper stack.

4. A blanket cleaning means according to Claim 2 or 3 having a flexibility, size and thickness similar to that of the paper or paper board being printed.

5. A blanket cleaning means according to Claim 4 made from paperboard having a thickness of between 250 and 1000 microns.

6. A blanket cleaning means according to Claim 4 made from plastics material having a thickness of between 250 and 1000 microns.

7. A blanket cleaning means according to Claim 5 or Claim 6 in which the surface which contacts the blanket surface of the blanket cylinder comprises a pattern of upstanding ribs having a permanently tacky adhesive in the valleys between the ribs, the adhesive lying below the tops of the ribs.

8. A blanket cleaning means according to Claim 7 in which the ribs are integral with the substrate.

9. A blanket cleaning means according to Claim 7 comprising adhesive applied over all of one surface of the substrate and a pattern of ribs applied over the adhesive.

10. A blanket cleaning means according to Claim 9 in which the pattern of ribs is formed by plastics monofilaments.

11. A blanket cleaning means according to Claim 8 or Claim 9 in which the ribs have a height of between 0.1 mm and 0.5 mm.

12. A blanket cleaning means according to Claim 11 in which the distance between adjacent ribs is between 1.0 mm and 15.0 mm.

1 3. A blanket cleaning means according to any one of Claims 7 to 12 in which the adhesive is an acrylic ester polymer.

14. A blanket cleaning means according to any one of the preceding claims in which the reverse of the sheet is provided with an adhesion inhibitor coating.

1 5. A method of cleaning debris from the blanket of an offset printing machine comprising stopping the ink addition to the plate roll, continuing the operation of the machine to remove liquid ink from the blanket cylinder and feeding one or more blanket cleaning sheets through the machine, the cleaning sheets having a permanently tacky adhesive coating on one surface adapted to contact the blanket surface and adhere to and remove debris therefrom without adhering to other parts of the machine or the supply of unprinted material to the machine.

1 6. A blanket cleaning means substantially as hereinbefore described.

1 7. A method of cleaning a blanket of an offset printing machine substantially as hereinbefore described.