WO1981003306A1

WO1981003306A1 - Density control of jet printing droplets

Info

Publication number: WO1981003306A1
Application number: PCT/AU1981/000057
Authority: WO
Inventors: L Wills; A Stearn
Original assignee: Commw Scient Ind Res Org; L Wills; A Stearn
Priority date: 1980-05-16
Filing date: 1981-05-14
Publication date: 1981-11-26
Also published as: JPS57500871A; GB2095624A

Abstract

Variation in colour intensity when printing areas of a surface (13) by a jet printing technique is achieved by (a) varying the voltage increment of the ramp incremental voltage applied to the droplet charging electrode (12) of a jet printer, and (b) simultaneously varying the spacing between consecutive droplet scans of the jet printer. In this way, a uniform matrix of equi-spaced droplets is printed over the area of the surface (13) and striated or non-uniform colour intensity in the printed area is avoided.

Description

TI LE: "DENSITY CONTROL OF JET PRINTING DROPLETS" TECHNICAL FIELD

This invention concerns jet printing. More particularly it concerns the deposition of droplets of ink, dye, paint or the like on a fabric or paper web, a carpet, or the like that is being printed by a jet printing technique. BACKGROUND ART

Conventional jet printing equipment, such as that described in the specification of Australian patent No. 502,523 incorporates banks of droplet generating heads, each of which produces a constant stream of droplets of ink, dye, paint or the like. ^' At the point of separation of the droplets from the stream, they are given a charge by a droplet charging electrode. The charge applied o a droplet depends on the voltage (positive or negative) which is applied to the droplet charging electrode. The magnitude of the voltage applied to the droplet charging electrode is selected from values which are separated by uniform steps, so that, at any time, the charge on a droplet is one of a number of uniform values, ranging from zero to the charge that is acquired when the maximum voltage is applied to the charging electrode. The individual voltage steps for the droplet charging electrode are known as th incremental ramp voltage of the electrode. The charge acquired by a droplet from the droplet charging electrode determines the deflection the droplet will experience as it passes a deflecting electrode (or pair of electrodes) connected to a high voltage source (typically +4kV) . Because the charge on the droplets is one of the values determined by the incremental ramp voltage, a droplet may be deflected to strike the surface being printed at any one of a number of predetermined positions (typically between 16 and 64) across the surface. These positions are spaced from each other by a distance which is determined by the difference between the voltage values applied to the charging electrode. A droplet having zero charge, and thus not deflec by the deflecting electrode, is.collected by a gutter, or t (also called a catcher) , and does not contribute to the printing. Because the rate of generation of droplets is hig and the movement of the surface being treated is relatively slow, it is possible, when maximum colour saturation is required over a region of the surface, for droplets having each possible charge value to be applied to the surface fro a single droplet generating head while the surface is , effe tively,- stationary. When this occurs, a droplet strikes th surface being printed at every possible point. That is, a complete scan of the incremental ramp voltage is achieved while the surface being printed is, effectively,stationary. When programming the printing apparatus for such maximum colour saturation over an area of surface, the time between each complete scan of the incremental ramp voltage is selec¬ ted so that between the starts of consecutive scans, the sur face has travelled, relative to the printing head, a distanc corresponding to the separation of droplets which are charge to adjacent ramp values. Thus a uniform matrix of printing droplets is received by the surface being treated as it passes a printing head.

When less than maximum colour saturation is requir the interval between the scans is usually increased, and it is possible to arrange for droplets to be charged by only each second or third possible value of the incremental ramp voltage of the charging electrode. With this technique, it is also possible to arrange for a uniform matrix of printing droplets to strike the surface being printed as it travels past the printing head, so that a uniform, but diluted, colo intensity is achieved on the printed surface.

When a colour intensity value is required which falls between the maximum value and the first diluted colour intensity that is achievable with this technique, the curren practice in jet printing is to omit one or more of the possible voltage values of the droplet charging electrode. This approach, however, particularly when applied to textiles and especially when applied to woollen fabrics, results in a striated or banded colour intensity of the region being printed in a single colour. Consequently, the selection of patterns to be printed, or the quality of the printed pro¬ duct, is restricted. DISCLOSURE OF THE INVENTION It has now been found that this limitation of jet printing can be overcome by (a) varying the size, and hence the number, of the voltage steps in the incremental ramp voltage applied to the droplet charging electrode (that is, varying the ramp characteristic of the droplet charging electrode) , thus varying the number of printing droplets in a scan of droplets between the "start droplet" (the droplet possessing the minimum non-zero charge) and --the droplet possessing maximum charge, and (b) at the same time, varying the interval between consecutive start drops, to achieve a uniform matrix of printing droplets striking the surface being printed.

Thus, according to the present invention, a method of controlling colour intensity in the printing of an area of a surface by a jet printing technique which involves applying an incremental ramp voltage to a droplet charging electrode arrangement of a jet printer is characterised in that it comprises the steps of:

(a) varying the size of the voltage increment of the incremental ramp voltage; and (b) simultaneously varying the spacing between the impact positions on the surface of consecutive start droplets of the printing process to effect a uniform matrix of print¬ ing droplets over the area of the surface. The present invention also encompasses jet printing apparatus to perform this method of controlling colour intensity.

An embodiment of the present invention will no be described in more detail with reference to accompanyi drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates, schematically, the forma of a regular matrix of printing droplets on an area of s being printed by a single head of a jet printer.

Figure 2 shows the formation of a similar matr of droplets to that depicted in Figure 1, which produces uniform printed colour intensity on the surface, which c intensity is less than that achieved with the matrix of droplets -in Figure 1. Figure 3 is a schematic signal diagram for a continuous print head controller of a jet printer. DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The printing droplet matrix depicted (schemati in Figure 1 is the result of five scans of droplets from printing head 10 and the start droplet 11 of the sixth s

As shown in this illustrative example, the ramp of the v that is applied to the droplet charging electrode 12 is effective to separate consecutively charged droplets, af they have passed through the deflecting field, by a dist a on a surface 13. The ramp voltage increment is one si of Vmax - Vmm. , where

is the modulus of the maximum voltage applied to the charging electrode 12 and V . is modulus of the voltage applied to the charging electrode enable the droplets to just clear the collector. Drople have struck surface 13 after being deflected the full ex possible (in the illustrated example) because the chargi electrode was, at the moment these droplets were separat at the droplet generating head 10, at a voltage of + or -V , depending on the nature of the jet print apparatus. During each complete scan of droplets from the start droplets 15 to the corresponding maximum deflected droplets 14, the surface 13 being printed has moved a distance b in the direction of arrow A. All the droplets from the generating head 10 that are uncharged are collected in the gutter 16 associated with droplet generating head 10 and are preferably returned to the reservoir feeding the ink, dye, paint or the like to the droplet generating head. In the illustrated example, the charged droplets are deflected in a direction which is at 45 to the direction of travel of the surface 13. That is, angle θ of Figure ^• 1 is 45°. However, angle θ need not be 45 . In an experimental jet printing arrangement used to demonstrate the present invention, the angle θ was tan 0.5. Irrespective of the magnitude of the angle θ, provided a uniform matrix of droplets is deposited over the scan width w of the surface 13 which receives droplets from generating head 10, a uniform colour intensity will be achieved over the area^' of surface which is printed.

Figure 2 shows that a different, but nevertheless uniform, matrix of droplets striking surface 13 can be achieved using the same droplet generating head 10 of the jet printing apparatus used to produce the matrix of printing droplets illustrated in Figure 1. For the formation of the .matrix of droplets illustrated in Figure 2, the increment of the ramp voltage applied to the charging electrode 12 is one fourth of V_mma,x„ -Vmm. , and the interval between start drops 25 is adjusted to be such that the distance d is the same as distance c_. The matrix of printing droplets illustrate schematically in Figure 2 will produce a uniform colour printing over scan width w of material 13, but the intensity of colour will be less than that observed with the matrix of printing droplets shown in Figure 1. One arrangement -for effecting the simultaneous change in ramp increment and start drop interval of a jet printer droplet generating head is shown in Figure 3. In this arrangement, two microprocessors 31, 32 control, respectively, motor M (which controls the transport of th surface being printed through the printing apparatus) and the droplet charging electrodes E, to E_n of the n droplet generating heads of the apparatus. The microprocessors 31 and 32 are linked to communication interface 34 of a computer 33, through communication cables 35, 36 respecti The pattern data for the printing is supplied to decoder 3 through high speed interface 39, over high speed serial li 40.

Microprocessor 32 is loaded by computer 33, with the control functions for the particular pattern to be printed. These are to control buffer units 38,,...,38 to output data to each droplet head, to determine the cloc rate for data (which governs-the distances b and d of Figures 1 and 2) and the other necessary functions for printing the required pattern (such as determining when mo printing data is required) .

Those who are familiar with the programming of j printing apparatus and formulating pattern data will readi understand the operation of the droplet generating head controller illustrated in Figure 3. Such skilled persons will also recognise that alternative droplet head control arrangements can be used to perform the technique of the present invention. INDUSTRIAL APPLICABILITY The present invention is particularly suitable f full-width, single colour printing using a jet printer whe a full range of colour intensities in the printing is required, without the occurrence of striations or uneven colour distribution.

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Claims

1. A method of controlling colour intensity in the printing of an area of a surface (13) by a jet printing tech¬ nique which involves applying an incremental ramp voltage to a droplet charging electrode arrangement (12) of a jet printer, is characterised in that it comprises the steps of:

(a) varying the size of the voltage increment of the incremental ramp voltage; and

(b) simultaneously varying the spacing between the impact positions on the surface (13) of consecutive start droplets (15, 25) of the printing process to effect a uniform matrix of printing droplets over the area

. of the surface (13) .

2. A method as defined in claim 1, in which the drop¬ lets forming the uniform matrix are deflected, on leaving the charging electrode arrangement (12) , at an acute angle (θ) relative to the direction of travel (A) of the surface (13) .

3. A method as defined in claim 2, in which the acute angle (θ) is 45°, or tan^" 0.5.

4. A method as defined in any preceding claim, further characterised in that step (a) is effected by a first micro¬ processor (32) adapted to control the voltage applied to a plurality of charging electrodes (E, , ... E ) of the jet printer, and step (b) is effected by a second microprocessor

(31) adapted to control a motor (M) which controls the trans¬ port of the surface (13) through the jet printer.

5. A method as defined in claim 4, in which the first microprocessor (32) and the second microprocessor (31) are both linked by respective communication cables (35,36) to a communication interface (34) of a computer (33) which is pro¬ grammed to control the pattern to be printed by the jet printer.

6. Jet printing apparatus adapted to print on a sur¬ face (13) , said apparatus comprising at least one droplet generating head (10) , droplet charging electrode means (12) associated with each,droplet generating head (10) , a drop¬ let deflecting electrode associated with each droplet chargin electrode means (12) , and means to supply a ramp incremental voltage to each droplet charging electrode means (12) , char¬ acterised in that it includes:

(a) .means to vary the size of the voltage increment of the incremental ramp voltage; and

(b) means to simultaneously vary the time between the occurrence of consecutive start droplets (15, 25) of the printing process to cause the production of. a uniform matrix of printing droplets over the printed area of the surface (13) .

7. Jet printing apparatus as defined in claim 6, furth characterised in that said means to vary the size of the vol¬ tage increment comprises a first microprocessor (32) adapted to control the voltage applied to said or each droplet charg¬ ing electrode (12) , and said means to vary the time of occurrence of consecutive start droplets (15, 25) comprises a second microprocessor (31) adapted to control a motor (M) which governs the transport of the surface (13) through the jet printing apparatus.

8. Jet printing apparatus as defined in claim 7, in which the first microprocessor (32) and the second micro¬ processor (31) are both linked by respective communication cables (35, 36) to a communication interface (34) of a com¬ puter (33) which is programmed to control the pattern to be printed by the jet printing apparatus.