Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
  • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
  • B41J25/00—Actions or mechanisms not otherwise provided for
  • B41J25/001—Mechanisms for bodily moving print heads or carriages parallel to the paper surface
  • B41J25/003—Mechanisms for bodily moving print heads or carriages parallel to the paper surface for changing the angle between a print element array axis and the printing line, e.g. for dot density changes

Definitions

• This invention relates to droplet deposition apparatus and in particular to ink jet printers.
• an ink jet printer For a wide variety of applications including the printing of word processing output, it is desirable for an ink jet printer to be able to deposit the drops at a high density, currently standardised at between 300 and 360 dpi. This corresponds approximately to between twelve and fourteen drops per millimetre (dpmm).
• This invention is particularly concerned with ink jet print heads in which ink is ejected through electrical actuation, usually, of a piezoelectric ink chamber wall.
• the problem with angling the print head at such a small angle to the scanning direction is that the overall width of the printer is increased beyond the width of the paper by approximately twice, or sometimes three times, the width of the print head, which may amount to 50mm or more.
• a further problem with inclined print heads is that the closer the angle of the print head to the scanning direction, the greater is the amount of print data requiring to be held in buffer storage.
• EP-B-0 277 703 and EP-B-0 278 590 achieves high densities by utilising transverse displacement of a wall separating two channels to effect droplet ejection from one of the channels.
• the wall is usually formed of piezo ⁇ electric material deflected in shear mode.
• Such a print head has a maximum frequency of operation related to the time taken for a pressure pulse within an ink channel to result in the ejection of an ink droplet. It is an object of one aspect of the present invention to provide, in simple manner, for an increase in the maximum operating frequency.
• the maximum operating frequency can be increased by reducing the length of the channels, and making a corresponding increase in the channel thickness. Whilst this necessarily reduces the linear density of channels, the specified print density can be achieved by angling of the print head. It is convenient to refer to a matrix which represents all possible positions at which drops can be deposited on the substrate. Within this matrix can be defined the direction of scanning movement of the print head relative to the substrate. For convenience, this will be referred to as the horizontal matrix direction. The spacing of matrix points in this horizontal direction is related to the frequency of droplet ejection and the scanning speed.
• the present invention consists in a method of increasing the operating frequency F of a multi-channel droplet deposition head adapted for scanning movement in a scanning direction, comprising parallel channels having a length L and a channel thickness d e ; side walls of piezoelectric material having a thickness d w dividing the channels one from another; a series of nozzles which are uniformly spaced at the spacing p of the channels, where p equals the sum of channel thickness d c and wall thickness d w , and which communicate respectively with the channels for ejection of droplets of liquid from the channels; connection means for connecting the channels with a source of droplet deposition liquid; and electrical actuating means for effecting, on selection of a channel for droplet deposition, transverse displacement of at least one channel separating side wall of that channel to cause pressure change therein and thereby to effect droplet ejection therefrom, and droplet deposition thereby in lines from said nozzles at a line spacing of p orthogonal to the scanning direction
• the invention provides a further, important advantage. It has been recognised by the present inventors that the maximum optical density at which drops can be printed is not ordinarily increased by increasing the number of channels per unit length of the print head. This in fact merely results in the deposition of a larger number of smaller drops, with the print density remaining unchanged for the same operating frequency. However, angling the print head in accordance with the present invention increases the number of channels per unit length in the vertical matrix direction without reducing the spacing of the channels. There is accordingly a real increase in the print density for the same scanning velocity.
• the present invention consists in a further aspect in a multi-channel droplet deposition head adapted for scanning movement in a horizontal matrix direction relatively to a substrate upon which droplets of liquid are to be deposited along a line in a vertical matrix direction transverse to the scanning direction with a closest spacing in the vertical matrix direction of p and with a closest spacing in the horizontal matrix direction of q, the head comprising parallel channels having respective side walls which extend in the lengthwise direction of the channels and separate one from the next of the channels in an array direction, a series of nozzles which are spaced at the spacing of the channels and which communicate respectively with the channels for ejection of droplets of liquid from the channels; connection means for connecting the channels with a source of droplet deposition liquid; and electrically actuable means for effecting, on selection of a channel for droplet deposition, transverse displacement of at least one
• the present invention consists in still a further aspect in a multi ⁇ channel droplet deposition head adapted for scanning movement in a scanning direction relatively to a substrate upon which droplets of liquid are to be deposited along a print line transverse to the scanning direction with a closest spacing in the print line of p and with a closest spacing in the scanning direction of q, comprising parallel channels having respective side walls which extend in the lengthwise direction of the channels and separate one from the next of the channels in an array direction, a series of nozzles which are spaced at the spacing of the channels and which communicate respectively with the channels for ejectior of droplets of liquid from the channels; connection means for connecting the channels with a source of droplet deposition liquid; and electrically actuable means for effecting, on selection of a channel for droplet deposition, transverse displacement of at least one channel separating side wall of that channel to cause pressure change therein and thereby to effect droplet ejection therefrom, the channels being arranged in N groups which are successively enabled for droplet ejection in respective print phases such
• an ink jet printer having a number of sub-heads, each constructed and aligned in generally the same manner as the unitary heads discussed previously.
• One such instance is a colour printer where four sub-heads will typically be required, supplied respectively with black, cyan, magenta and yellow ink.
• Other examples lie in the use of plural sub-heads to increase print density or to produce a stationary print head capable of printing an increased width without scanning.
• the present invention consists in still a further aspect in multi-channel droplet deposition apparatus adapted for scanning movement in a horizontal matrix direction relatively to a substrate upon which droplets of liquid are to be deposited along lines in a vertical matrix direction transverse to the scanning direction with a closest spacing in the vertical matrix direction of p and with a closest spacing in the horizontal matrix direction of q, the apparatus comprising at least two heads each comprising parallel channels having respective side walls which extend in the lengthwise direction of the channels and separate one from the next of the channels in an array direction, a series of nozzles which are spaced at the spacing of the channels and which communicate respectively with the channels for ejection of droplets of liquid from the channels; connection means for connecting the channels with a source of droplet deposition liquid; and electrically actuable means for effectir.i, on selection of a channel for droplet deposition, transverse displacement of at least one channel separating side wall of that channel to cause pressure change therein and thereby to effect droplet ejection therefrom, the channels of each head being arranged
• Figure 1 is a diagrammatic representation of a prior art print head
• Figure 2 is a view similar to Figure 1 , showing a prior art modification
• Figure 3 is a diagrammatic representation of a print head according to the present invention.
• Figure 4 is a different diagrammatic representation of the print head of Figure 3;
• FIG. 8 is a block diagram showing certain signal processing aspects of apparatus according to this invention.
• Figure 9 is a plan illustrating a signal processing feature
• Figure 10 illustrates, in a diagrammatic form similar to that of
• FIG. 3 an alternative embodiment of the invention in which the printing apparatus comprises two or more sub-heads, with the array directions of the respective heads collinear;
• Figures 11 , 12 and 13 illustrate, in a similar manner to Figure 10, arrangements of two or more sub-heads where the respective array of directions are parallel but offset one from each other.
• a print head 10 in diagrammatic form.
• the print head is adapted to be scanned in a direction shown at arrow 12 which defines the horizontal matrix direction of a matrix of points at which droplets can be deposited.
• This matrix is depicted in Figure 1 as a grid.
• the pitch q of the matrix points in the horizontal direction is determined by the frequency of droplet ejection and the scanning speed.
• the pitch p in the vertical matrix direction is determined by the print head arrangement.
• the print head it will be seen to comprise a parallel array of channels 14, each channel being separated from the adjoining channels by active walls 16 of piezoelectric material. Each channel 14 communicates with a nozzle 18. To eject a droplet from the nozzle 18 of a selected channel, an electric field is applied to one and possibly both active channel walls causing them to displace relatively to the channel.
• an electric field is applied to one and possibly both active channel walls causing them to displace relatively to the channel.
• EP-B-0 277 703 and EP-B-0 278 590 It will be noted from Figure 1 that with the print head aligned with the vertical matrix direction, the pitch p is given by the sum of the channel thickness d c and the wall thickness d w .
• the frequency at which droplets can be ejected is related to the length L of the channel.
• the operating frequency which is the time taken for an acoustic wave to travel there and back along the length L of the channel.
• the length of the channel L is in turn governed by the volume of ink which is to be deposited in a droplet. The smaller the channel thickness d c the greater is the length L required to produce a droplet of a given volume.
• the present inventor has recognised that it is possible to provide the same matrix pitch p, with the same drop volume, yet at a higher operating frequency. This is achieved, as shown in Figure 3, by angling the print head away from an orientation parallel to the vertical matrix direction towards the horizontal matrix or scanning direction, whilst increasing the channel thickness from d c to a higher value d c '. This increase in channel thickness permits a decrease in channel length from L to a lesser value L' where L'/L equals d c /d' e . With an angle ⁇ between the array direction of the print head and the vertical matrix direction, the new nozzle spacing p' is given by:-
• N two groups of channels
• the channels are arranged in two interleaved groups with Group I comprising odd channels and Group II comprising even channels.
• the print head has channels and corresponding nozzles evenly spacec along a straight line.
• the nozzles are numbered 1 to 6.
• the channel * - extend in a direction normal to the plane of the Figure which may also be taken as the plane of the paper on which ink drops are deposited.
• the print head is scanned in the direction shown by arrow 12 with the velocity of scanning and the frequency of operation being such that th- minimum spacirg of drops in the scanning direction is q.
• Figure 5a there is shown in Figure 5a) at 50 the series of drops deposited from nozzle 5 if this were actuated at the corresponding maximum rate.
• the spacing p of drops along the print line, normal to the scanning direction 12 is determined by the physical spacing of nozzles in the print head and the angle of inclination ⁇ v
• the Table includes a "shift" value which is the number of horizontal matrix intervals q between adjacent channels in only one group. It can only be seen that this shift value takes the algebraic form iN ⁇ 1 where i is an integer.
• Figure 6 this illustrates the case in which nozzles are arranged in three interleaved groups. That is to say, of the nozzles shown in the Figures, numbers 1 ,4... belong to Group I, numbers 2,5... belong to Group II and number 3,6... belong to Group III. In this case, the smallest angle ⁇ , which meets the requirements of this invention shown in Figure 6a). It will be seen that if channel 4 is actuated, subsequent actuation in the next available cycle of channel 3 and then channel 2, will result in the deposition of a row of dots along the print line.
• Figure 6b), c) and d) show alternative orientations and the derivation of these angles together with the ratio R are set out below in Table II:- TABLE II
• Table II includes the orientation that arises from a shift of 7; this orientation is not shown in Figure 6.
• FIG. 8 illustrates in block diagram form the signal processing which serves to derive the firing signals required by a print head according to this invention.
• Print information is received in a Page Description Language (PDL) by a PDL interpreter 80.
• PDL Page Description Language
• a typical PDL is that known commercially as Postscript.
• the PDL interpreter 80 generates from the PDL input a bit map of the page to be printed, typically in a form which remains printer independent.
• the bit map is received by a geometric transform unit 82 which transforms the bit map to take into account the angling of the print head and the grouping of channels into successive phases.
• Figure 9 shows at 90 a print head angled as shown in Figure 6d) having nozzles 94 associated with respective channels that are arranged in three groups designating a,b,c-..
• the print head is moving in the arrowed direction 96 with respect to the paper or other substrate.
• the channels are enabled for firing in the sequence a,c,b,a,c,b-.. with the channels a being enabled in the position actually shown in the figure.
• the print data that is required to be supplied to the channels of the print head for the cycle of three print phases is illustrated in the drawing by cross-hatched blocks 98. These show the position in the un- ransformed bit map from which information is required to be taken for the specific channels.
• each channel c receives information from a matrix point spaced three intervals in the q direction and two intervals in the p direction.
• the channels b are enabled for firing, receiving information from a point spaced one interval in both the p and q directions from the channel a.
• the position is shown in which the print head is moving in the arrowed direction 102, relative to the paper.
• the channels b that are enahed for firing with the channels c being one third of an interval from a matrix point and the channels a two thirds of an interval from a matrix point.
• the firing sequence is b,c,a,b,c,a.
• the points in the bit map from which information is required to be taken are illustrated in the same manner. It will be seen that to allow for the switch in scanning direction, the transform operation is required to reverse the order and either to displace the starting point or to alter the pixel sequence, these being functionally equivalent.
• the transformed bit map is supplied to a wave form generator unit 84 which derives for the selected channels in each group in succession, the wave form necessary to produce the required displacement of the piezoelectric side wall in the print head shown in block form at 86.
• This wave form may generate a sequence of voltage pulses regions to create specific movements of the wall inwardly and outwardly of the chamber, to accomplish drop ejection and ink replenishment in the channel.
• the wave form supplied to a specific channel may also carry a correction factor depending in form upon which of the neighbouring channels are being, or are to be fired. The detail of the manner in which firing wave forms are generated forms no part of the present invention.
• Figure 10 shows a print head 10 having an array of nozzles in two sub-heads 20a and 20b, each sub-head having a corresponding array of parallel channels having the same dimensions and pitch.
• each sub-head 20a and 20b is supplied by a separate ink manifold.
• the sub-heads are arranged with the respective array directions coliinear and may be formed in a single coextensive ceramic wafer, or may be separate components assembled together in register by an assembly structure (not shown).
• the arrangement of Figure 10 is not limited to two sub-heads.
• One alternative configuration is a print head formed in a single ceramic wafer having four sub-heads for ink of the three primary colours cyan, magenta, yellow and black. Typical examples are angled print heads with four supply manifolds to the following number of actuable channels
• the print head has an array of nozzles in two sub-heads 30a and 30b where the respective array directions are parallel and offset with respect to each other across the direction of motion of the print head (or motion of the print substrate) as illustrated by the arrow.
• a head may, for example, consist of separate components assembled together in register by means of an assembly structure.
• Such an arrangement serves to provide a print head of increase width and the print head may advantageously be arranged in the form of a stationary print head consisting of a series of component print heads assembled across a page width.
• Figure 12 A further configuration is illustrated in Figure 12, in which the print head has an array of nozzles in two sub-heads 40a and 40b offset with respect to each other along the direction of motion as illustrated by the arrow.
• the print head comprises four components print heads assembled in register by means of an assembly structure, each component supplied with a different ink colour, i.e. cyan, magenta, yellow and black.
• Image quality in each of the arrangements of Figures 10, 11 and 12 above requires that drops are printed in the same dot matrix, the achievement of which places constraints in both design and manufacturing tolerance on the print heads. Where more than one sub-head is used, the constraints are advantageously that:-
• a single, common nozzle plate provides the nozzles for each of the sub-heads so as to ensure the nozzles of all sub-heads are in register.
• the dimension h is an integral multiple of q.
• nozzle manufacture is straightforward in that they are formed along a straight line at the channel pitch, without nozzle offset corresponding to the phase of operation.
• Th e data sequence is also offset to a degree depending on the line number i, being caused to lead or lag by i/N matrix vertices or by i firing pulses, the offset being advanced or delayed depending on the pass direction.
• i/N matrix vertices or by i firing pulses the offset being advanced or delayed depending on the pass direction.
• the data is also delivered by the drive chip with the specified offset also taking the pass direction into account.
• a further configuration shown in Figure 13 shows a print head with sub-heads offset by h, in which the channels in one sub-head interleave the channels in the second sub-head, so that printing with higher print density may be obtained.
• the "firing" of a channel will not be a binary event and will comprise the application of a defined firing signal to eject a corresponding number of droplets or a droplet of a corresponding size.

Landscapes

• Particle Formation And Scattering Control In Inkjet Printers (AREA)

Priority Applications (2)

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Publications (1)

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Cited By (7)

Citations (9)

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• 1993
  • 1993-09-09 GB GB9318749A patent/GB9318749D0/en active Pending
• 1994
  • 1994-09-09 EP EP94925591A patent/EP0719215A1/en not_active Withdrawn
  • 1994-09-09 JP JP7508554A patent/JPH09501622A/ja active Pending
  • 1994-09-09 WO PCT/GB1994/001966 patent/WO1995007185A1/en not_active Application Discontinuation

Patent Citations (9)

Non-Patent Citations (3)

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