GB2319264A

GB2319264A - Printing process and printing paper

Info

Publication number: GB2319264A
Application number: GB9623467A
Authority: GB
Inventors: Peter Kevin Owen
Original assignee: Rexam Coated Products Ltd
Current assignee: Rexam Coated Products Ltd
Priority date: 1996-11-12
Filing date: 1996-11-12
Publication date: 1998-05-20
Also published as: GB9623467D0; WO1998021047A1

Abstract

In a method of printing, a hotmelt ink is applied to the surface of a sheet of recording paper as a series of molten droplets that solidify on the surface of the paper and are pressed into the surface of the paper by means of a pressure roller. The recording paper has a smoothness represented by an R q value of 2.45~m c 0.46~m and a formation index of at least 100.

Description

PRINTING PROCESS PRINTING PAPER AND METHOD OF NANUFACTURING PAPER The present invention relates to a printing process, to a paper for use in the process and to a method of maufacturing a printing paper.

In the printing process known as the Hotmelt Ink jet process (also known as phase change Inkjet or solid Inkjet), an ink that is solid at room temperature is applied to a substrate in a molten state by means of an inkjet print head. The drops of ink cool and solidify on the substrate surface.

The Hotmelt Inkjet process is employed in a printing machine designed by Tektronix Inc. and sold under the Trade Mark Phaser 600. In this printer the solidified droplets of hotmelt ink that have been applied to the surface of a sheet of recording paper are mechanically treated by a pressure roller that traverses the paper in synchronisation with the print head. The pressure applied by the roller causes the ink droplets to flatten, spread and penetrate into the surface of the paper. This process is referred to as fusing.

It is important that the colour of a specific area on a number of identical printed images does not show significant variation from print to print or from the colour required. It is an object of the present invention to allow good colour control within the hotmelt inkjet printing process.

According to the present invention there is provided a method of printing in which a hotmelt ink is applied to the surface of a sheet of recording paper as a series of molten droplets that solidify on the surface of the paper and the solidified droplets are pressed into the surface of the paper by means of a pressure roller, characterised in that the recording paper has a smoothness represented by an Rq value of 2.45m+0.46 and a formation index of at least 100.

Smoothness is a surface property that relates to the evenness and flatness of the paper surface. Paper smoothness is sometimes determined by measuring the rate of flow of air between the paper surface and a smooth standard surface. Instruments for measuring this property include (RTM) (RTM) those kno(n by the names Bendtset, Marker Print Surf and Sheffiel. The inventors have found these instruments to be insufficiently accurate for the purposes of the present invention and have 4 ed instead an instrument known as the (.r1.

Mitutoyo Surftes , hich is a stylus type profilometer.

This type of instrument is well known in engineering applications and can be used to measure a surface profile.

Several standard measurements can be measured and % was selected for the present invention.

According to the present invention the recording paper has a smoothness represented by an P, value of 2.45 m +0.46, preferably 2.45m #0.23 and more preferably 2.45pm +0.09.

Although these figures cannot be related directly to smoothness values as determined by measuring air flow rates, they correspond to values in the region of approximately 24ml/min ilOml/min, +5ml/min and # 2ml/min respectively. A definition of the term "smoothness" is given in the "Pulp and Paper Dictionary" by John Lavigne, Miller Freeman, 1993 (page 350), the contents of which are incorporated herein by reference.

Formation relates to the physical distribution and orientation of fibres and other solid constituents in the structure of the paper, which affect its appearance and other physical properties. It is also referred to as look through because the formation can sometimes be observed by looking through the sheet. Formation can be measured using image analysis techniques and apparatus and may be expressed in terms of a uniformity index or formation index. For the purposes of the present invention, the formation index was measured using a Kajaani formation analyser.

According to the present invention the recording paper has a formation index of at least 100, preferably at least 110 and more preferably at least 115. A definition of the term "formation index" is given in the "Pulp and Paper Dictionary" by John Lavigne, Miller Freeman, 1993 (page 187).

The inventors have discovered that the area of the applied ink dots after fusing is related to the surface profile or smoothness of the paper, and that a variation in the smoothness has a significant affect on the area of the fused dots within the printed image. A consequence of a variation in the dot area is to give a change in the colour of a printed image, and a change in the smoothness of the paper will change the colour gamut of the printing process.

The inventors have thus discovered that the smoothness of the paper must be carefully controlled for consistent and reproducible printed colour images.

The inventors have also discovered that the formation of the paper has a significant affect on the area of the fused dots, with a significant increase in the standard deviation of the dot area being observed for papers with poor formation. Good formation is therefore essential for even print quality across the printed image.

The present invention further provides a recording paper for use in a hotmelt inkjet printing process, characterised in that the paper has a smoothness represented by an Rq value of 2.45?m +0.46 and a formation index of at least 100.

The recording paper may be an opaque, machine finished, woodfree paper having a weight of between 709sum and 2OOGQm.

Such papers are considered particularly suitable for use in the hotmelt inkjet printing process. The invention may also be applicable to other types of paper.

The present invention yet further provides a method of manufacturing a recording paper for use in a hotmelt inkjet printing process, characterised in that the smoothness and formation of the paper are controlled such that the finished paper has a smoothness represented by an P, value of 2.45m +0.46pm and a formation index of at least 100.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying graphs, of which: Fig. 1 is a flow diagram illustrating the steps of the paper manufacturing process; Fig. 2 is a graph showing the relationship between black dot area and smoothness; Fig. 3 is a graph showing the relationship between the colour gamut achieved and paper smoothness; Fig. 4 is a graph showing the relationship between colour brilliance C and paper smoothness, and Fig. 5 is a graph showing the relationship between formation index and the standard deviation of dot area.

The main steps of the paper manufacturing process are shown diagrammatically in the flow diagram of Fig. 1. The steps of the manufacturing process are in themselves conventional, the novel aspects of the manufacturing process lying in the control of the process to ensure that the finished paper has the required smoothness and formation characteristics. The steps of the paper manufacturing process, which are described briefly below, are explained more fully in various standard paper making text books, for example, "Paper and Board Nanufacture", Editors Julius Grant, James Young, Barry Watson; Technical Division, British Paper and Board Industry Federation, 1978, and "Handbook for Pulp and Paper Technologists", G.A.

Smook; Angus Wilde Publications, 1992, the contents of which are incorporated herein by reference.

The steps of the paper making process are as follows: in step 1, carried out in the pulper, the fibre furnish is selected to give the required formation. In step 2, the refining process is controlled to give the required paper strength and formation characteristics. In step 3, the refined pulp is cleaned to give a clean, dirt-free paper.

In step 4, the paper is formed from the treated fibre slurry. The desired formation of the paper is achieved during this sheet formation stage, but is influenced by the pretreatment of the fibre slurry and by the fibre type.

In step 5, the formed paper sheet is passed through a press section in which the sheet is consolidated. The sheet is then dried in step 6 and the surface of the paper is treated in a size bath, step 7, to provide surface strength and to give the paper the required chemical characteristics. The paper is then dried again, step 8, and is passed through calendering rollers, step 9, to provide the required smoothness. Finally, the finished paper is reeled, step 10, before being converted into rolled or cut sheet products, according to the customer's requirements.

The required formation is achieved through careful control of the paper making process, which includes the choice of fibre furnish used, the mechanical treatment applied to the fibre within the refining section of the stock preparation system, and the manner in which the fibre is formed into a sheet once it has been prepared. The smoothness is achieved by careful control of the steel calender section of the paper machine.

Further details of these processes and definitions of the various terms used herein are given in certain well-known paper making text books, for example, "Pulp and Paper Dictionary", John Lavigne, Miller Freeman, 1993 (see, for example, the following definitions: page 187 "Formation", page 350 "Smoothness", page 317 "Refining" and page 192 "Furnish").

The papers used in the tests described below were all opaque, machine finished, woodfree papers having weights between 70gsm and 200gsm. These are considered the most suitable papers for use in the hotmelt inkjet printing process. It will be appreciated that the invention may also be applicable to other types of paper.

A number of papers with similar structures but of different smoothness having an P, in the range 2.235 m to 3.433;m were printed using a Tektronix Phaser60 printer. The dot size for black dots was measured using image analysis techniques, utilising a ccd camera to capture an image of the fused, printed dots and VISILOGw software to carry out the measurement. The results obtained are shown in the following table.

Smoothness R (i) j 2.235 2.729 2.297 j 2.626 3.060 3.433 lean of dot area (n2) 0.02655 0.02488 0.02638 0.02503 0.02428 0.02321 Standard Standard deviation 0.00217 0.00205 0.00235 0.00213 0.00221 0.00202 As may be seen from the above results and the graph shown in Fig. 2, there is a clear relationship between the smoothness of the paper and the average dot size of the black dots. Similar results were seen for dots of the other primary colours (cyan, magenta and yellow).

With the change in dot size an apparent change in colour was seen in the printed image. The same papers were printed using a COREL DRAWN software package. A number of 25% area fill blocks of colour were printed using the colours cyan, magenta, yellow, red, green and blue. The colour of the blocks was determined using the L.a.b. scale.

The results are shown in the following table.

Snootimess R, (p) 2.235 2.729 2.297 2.626 3.060 3.433 1 a a a a a 1 a Cyan -14.95 -14.52 -15.26 -14.27 -13.69 -12.92 Magenta 38.59 38.10 38.25 38.15 35.47 34.86 Yellow -7.9 -7.35 -7.65 -7.15 -6.89 -6.65 Red 34.24 33.28 33.69 33.91 31.94 31.51 Green -31.43 -30.63 -31.79 -29.89 -29.42 -28.24 Blue 14.80 14.30 14.49 15.50 13.66 13.18 b b b | b | b | b Cyan -20.21 -19.47 -20.01 -19.03 -18.60 -18.02 Hagenta -17.53 -17.69 -17.76 -16.05 -15.84 -15.65 Yellow 37.24 35.63 36.48 35.28 34.43 33.70 Red 14.61 14.19 14.74 13.74 14.35 14.09 Green 12.25 11.93 12.48 11.88 11.86 11.49 Blue -31.80 -32.16 -31.90 -30.57 -29.78 -29.19 The "a" and "b" values were plotted for the different papers. The graph, shown in Fig. 3, shows the difference in the colour gamut achieved for the different papers.

The brilliance or clarity of a colour may be represented by the value C', which is calculated by taking the square root of a2+b2. A graph showing the value of C plotted against the smoothness P, of the paper is shown in Fig. 4. This demonstrates that there is a significant change in colour brilliance with a change in smoothness.

The inventors also looked at the affect of paper formation on dot size. Three papers with different formation values were printed with a 25% area fill black and a large number of dots were measured. The standard deviation of the dot areas is shown in the following table and in the graph shown in Fig. 5. The standard deviation of the dot areas was found to correlate closely with the formation index.

Formation index Standard deviation of dot area Paper 1 56.0 0.00271 Paper 2 73.7 0.00267 Paper 3 121.0 0.00254 The applicants have discovered that for an acceptable printed image, the recording paper should have a smoothness represented by an R, value of 2.45pm+0.46, preferably 2.45pm+0.23 and more preferably 2.45pm+0.09, and a formation index of at least 100, preferably at least 110 and more preferably at least 115.

Claims

1. A method of printing in which a hotmelt ink is applied to the surface of a sheet of recording paper as a series of molten droplets that solidify on the surface of the paper and the solidified droplets are pressed into the surface of the paper by means of a pressure roller, characterised in that the recording paper has a smoothness represented by an R, value of 2.45Mm +0.46cm and a formation index of at least 100.

2. A method according to claim 1 in which the recording paper has a smoothness represented by an P, value of 2.45pm +0.23cm.

3. A method according to claim 2 in which the recording paper has a smoothness represented by an P, value of 2.45Zm +O.O9pm.

4. A method according to any one of claims 1 to 3 in which the recording paper has a formation index of at least 110.

5. A method according to claim 4 in which the recording paper has a formation index of at least 115.

6. A recording paper for use in a hotmelt inkjet printing process, characterised in that the paper has a smoothness represented by an R, value of 2.45m +0.46Mm and a formation index of at least 100.

7. A recording paper according to claim 6, in which the paper has a smoothness represented by an P, value of 2.45pm +0.23cm.

8. A recording paper according to claim 7, in which the paper has a smoothness represented by an R, value of 2.45Am +O.O9pm.

9. A recording paper according to any one of claims 6 to 8, in which the paper has a formation index of at least 110.

10. A recording paper according to claim 9, in which the paper has a formation index of at least 115.

11. A method of manufacturing a recording paper for use in a hotmelt inkjet printing process, characterised in that the smoothness and formation of the paper are controlled such that the finished paper has a smoothness represented by an R, value of 2.45m +0.46pm and a formation index of at least 100.

12. A method according to claim 11, in which the finished paper has a smoothness represented by an P, value of 2.45pm +0.23cm.

13. A method according to claim 12, in which the finished paper has a smoothness represented by an P, value of 2.45Mm t0.09m.

14. A method according to any one of claims 11 to 13, in which the finished paper has a formation index of at least 110.

15. A method according to claim 13, in which the finished paper has a formation index of at least 115.

16. A method of printing, the method being substantially as described herein with reference to and as illustrated by the accompanying figures.

17. A recording paper for use in a hotmelt inkjet printing process, the paper being substantially as described herein with reference to and as illustrated by the accompanying graphs.

18. A method of manufacturing a recording paper for use in a hotmelt inkjet printing process, the method being substantially as described herein with reference to and as illustrated by the accompanying graphs.