Patent Application
20100101440
The invention relates to printing and in particular to registering printing plates in an imaging system such as a computer-to-plate system.
Contact printing using high volume presses is commonly employed to print a large number of copies of an image. Contact printing presses utilize printing plates to sequentially apply colorants to a surface to form an image thereon. The surface can form part of a receiver medium (e.g. paper) or can form part of an intermediate component adapted to transfer the colorant from its surface to the receiver medium (e.g. a blanket cylinder of a press). In either case, a colorant pattern is transferred to the receiver medium to form an image on the receiver medium.
Printing plates typically undergo various processes to render them in a suitable configuration for use in a printing press. For example, exposure processes are used to form images on an imageable surface of a printing plate that has been suitably treated so as to be sensitive to light or heat radiation. One type of exposure process employs masks. The masks are typically formed by exposing highly sensitive film media using a laser printer known as an “image-setter.” The film media can be additionally developed to form the mask. The mask is placed in area contact with a sensitized printing plate, which is in turn exposed through the mask. Printing plates exposed in this manner are typically referred to as “conventional printing plates.” Typical conventional lithographic printing plates are sensitive to radiation in the ultraviolet region of the light spectrum.
Another conventional method directly forms images on printing plates through the use of a specialized imaging apparatus typically referred to as a plate-setter. A plate-setter in combination with a controller that receives and conditions image data for use by the plate-setter is commonly known as a “computer-to-plate” or “CTP” system. CTP systems offer a substantial advantage over image-setters in that they eliminate film masks and associated process variations associated therewith. Printing plates imaged by CTP systems are typically referred to as “digital” printing plates. Digital printing plates can include photopolymer coatings (i.e. visible light plates) or thermo-sensitive coatings (i.e. thermal plates).
In order to provide printed materials of suitable quality during a printing operation the images formed on the printing plate must be accurately registered. Typically, in computer-to-plate imaging systems, one or more edges of a printing plate are used for registration purposes during the formation of the images. For example, during an image forming procedure, a printing plate is aligned on an imaging support surface of a computer-to-plate system by bringing one of its edges known as a “registration edge” into contact with various registration members. Conventional computer-to-plate registration systems typically have a number of registration pins or stops fixedly attached to the imaging support surface. Various groupings of fixed registration pins are often employed to register printing plates of different sizes or to register multiple printing plates.
Although these conventional fixed pin registration systems are relatively simplistic in nature, various problems are associated with them. For example, limited surface contact between a printing plate's registration edge and the fixed pins is usually established as the printing plate is moved into engagement with the pins. Ever increasing throughput demands placed on the computer-to-plate system require that the printing plate be conveyed with increasing speeds. These increased conveyance speeds can increase loading conditions between the printing plate's registration edge and the fixed pins and impart deformations or other damage onto the registration edge of the printing plate.
Edge deformations or damage can lead to various problems. For example, once the printing plate is registered against the registration pins it is imaged typically in accordance with various offsets from the various printing plate edges. Deformations such as small dents in the vicinity of the contacted registration pins can cause shifts in a desired image placement with respect to the registration edge. Additional printing plate preparation steps can include punching and bending procedures which are used to impart various features onto the printing plates to facilitate the mounting and registration of the printing plates on press. If these features are added by equipment that uses a registration system that engages with deformed areas of the registration edge, the desired positioning of these features can be adversely impacted. In some systems, punching capabilities are incorporated in the computer-to-plate system itself.
Other factors can also lead to the formation of deformations on various edges of a printing plate. For instance, there is an increasing demand for computer-to-plate systems that can accommodate larger plate sizes. The increased size and weight associated with these larger printing plates requires larger conveyance forces to move the printing plate into engagement with conventional registration pin systems. These increased forces can further lead to the formation of registration edge deformations.
Thus, there is a need for an imaging apparatus with improved plate registration capabilities. There is also a need for a computer-to-plate imaging system adapted to improve the positioning printing plates to form images accurately thereon. In addition, there is a need for a computer-to-plate system with a printing plate registration system that reduces the potential to form undesired deformations on the edges of printing plates during the handling thereof.
Briefly, according to one aspect of the present invention a method for positioning a printing plate includes supporting the printing plate on a support surface. A first force is applied to the printing plate to move the printing plate over the support surface along a path. A second force is applied to the printing plate to alter the movement of the printing plate along the path. The printing plate is pivoted on the support surface while applying the first force and the second force to the printing plate, wherein the printing plate is pivoted about a pivot point located on the printing plate at a location different from each of the locations on the printing plate to which the first and second forces are applied.
Embodiments and applications of the invention are illustrated by the attached non-limiting drawings. The attached drawings are for purposes of illustrating the concepts of the invention and may not be to scale.
Throughout the following description specific details are presented to provide a more thorough understanding to persons skilled in the art. However, well-known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive sense.
Controller 20 can comprise a microprocessor such as a programmable general purpose microprocessor, a dedicated micro-processor or micro-controller, or any other system that can receive signals from various sensors, and from external and internal data sources and that can generate control signals to cause actuators and motors within imaging apparatus 10 to operate in a controlled manner to form imaged printing plates 24.
Image recording system 14 comprises an imaging head 22 adapted to take image-forming actions within an image forming area of an imaging support surface 28 so that an image can be formed on each of one or more printing plates 24 loaded within the image forming area on imaging support surface 28. In the embodiment illustrated, the plurality of printing plates 24 loaded on imaging support surface 28 comprises a first printing plate 24A and a second printing plate 24B. However, this is not limiting and in other embodiments imaging support surface 28 may be capable of holding a different number of printing plates 24 in a manner that allows imaging head 22 to form images on each of printing plates 24 held thereby. First and second printing plates 24A and 24B can include different sizes or substantially the same size as shown in the illustrated embodiment.
Imaging head 22 generates one or more modulated light beams or channels that apply image modulated energy onto first and second printing plates 24A and 24B. Imaging head 22 can move along a sub-scanning axis SSA while a motor 36 or other actuator moves the imaging support surface 28 along a main scanning axis MSA such that image forming actions can be taken over an image forming area of imaging support surface 28 on which first and second printing plates 24A and 24B are located.
Imaging head 22 is illustrated as providing two light emission channel sources 30 and 32 which can each comprise, for example, a source of laser light and laser modulation systems of a kind known to those of skill in the art (not illustrated) each capable of taking image forming actions on printing plates 24 located within the image forming area. In some embodiments, light emission channel sources 30 and 32 can be independently controlled, each source applying modulated energy to first and second printing plates 24A and 24B. In yet other embodiments of this type, a single light emission channel source can be used to generate a modulated light beam that can be directed across the entire image forming area.
In various embodiments, not illustrated, various types of imaging technology can be used in imaging head 22 to form an image pattern on first and second printing plates 24A and 24B. For example and without limitation, thermal printing plate image forming techniques known to those of skill in the art can be used. The choice of a suitable light emission source can be motivated by the type of printing plate 24 that is to be imaged.
In the embodiment of
During imaging operations, controller 20 causes image modulated beams of light from imaging head 22 to be scanned over the imaging forming area by a combination of operating a main scanning motor 36 to rotate imaging support surface 28 along main scanning axis MSA and translating imaging head 22 in the sub-scanning direction by causing rotation of a threaded screw 38 to which light emission channel sources 30 and 32 are attached in a manner that causes them to advance in a linear fashion down the length of threaded screw 38 as threaded screw 38 is rotated. In some embodiments, light emission channel sources 30 and 32 can be controlled to move independently of one another along sub-scanning axis SSA. Other mechanical translation systems known to those of skill in the art can be used for this purpose. Alternatively, other well-known light beam scanning systems, such as those that employ rotating mirrors, can be used to scan image modulated light across the image forming area of imaging support surface 28.
Imaging apparatus 10 has a transfer support surface 60 and a positioning system 62. Transfer support surface 60 is sized to receive, hold and/or deliver a plurality of printing plates 24 at the same time. In this example embodiment, positioning system 62 is connected between frame 12 and transfer support surface 60 and defines a movement path for transfer support surface 60 between a transfer position shown in
As schematically shown in
First and second registration members 40A and 40B are arranged to help control the position of registration edge 52 of first printing plate 24A along main scanning axis MSA. Registration members 40C and 40D are arranged to help control the position of registration edge 54 of second printing plate 24B along main scanning axis MSA. Alignment along sub-scanning axis SSA in either case can be provided in various ways. In a preferred embodiment, imaging head 22 has an integral edge detector (not shown) that is adapted to sense lateral edges 25A and 25B of first and second printing plates 24A and 24B as imaging head 22 is moved past the printing plates during imaging operations. Controller 20 receives signals from the edge detector and adjusts imaging operations so that images are formed on first and second printing plates 24A and 24B in precise relation to the sensed lateral edges 25A and 25B of first and second printing plates 24A and 24B respectively. Typically, integral edge detectors include an optical sensor that detects an edge based upon differences in an amount of light reflected thereby. However, integral edge detectors can take other forms known to those of skill in the art including magnetic field detectors, electrical sensors, and contact detectors.
In the embodiment illustrated, a support surface 90 is provided and is adapted to exchange various printing plates 24 (e.g. first and second printing plates 24A and 24B) with imaging support surface 28. Printing plates 24 can be provided to support surface 90 for subsequent transfer to imaging support surface 28 in various ways. For example, plate handling mechanism 33 can be used to pick each printing plate 24 from one or more printing plate stacks 35 and transfer each printing plate 24 to support surface 90 by various methods as are well known in the art. Printing plate stacks 35 can be arranged or grouped in various manners, including by plate size, type, etc. Cassettes, pallets and other containing members are regularly employed to group a plurality of printing plates 24. The printing plates 24 in printing plate stack 35 are shown separated from one another for clarity.
Once a printing plate 24 is transferred to support surface 90, a plate positioning system 64 is operated to engage with a surface of the printing plate 24 and move it at least in part from support surface 90 onto imaging support surface 28. In this regard, it is desired that the printing plate 24 be transferred to imaging support surface 28 such that one of its edges is in contact and aligned with each of an associated set of registration members.
Regardless of the reason for the skewed orientation, printing plate 24C is brought into register with registration pins 108 and 110 by engaging one of the registration pins 108 and 110 first and then pivoting about the engaged registration pin to engage the other one of registration pins 108 and 110. Typically, plate positioning system 104 continues to move printing plate 24C as it pivots about one of the two registration pins 108 and 110. In this illustrated case, printing plate 24C pivots about a point of contact with registration pin 108. In this regard, the point of contact acts as a pivot point about which the printing plate 24C pivots about on support surface 102. As a printing plate 24 is pivoted about a given pivot point, the pivoting motion can cause the speed of various portions of printing plate 24C relative to support surface 90 to vary from one another. The pivoting dependant speed is referred to as the “pivoting speed.” The pivoting speed of various portions of printing plate 24C will be related to a distance from the pivot point to a location of each of the portions and the angular speed (i.e. typically expressed in units of radians/sec) with which printing plate 24C is pivoted about the pivot point. Accordingly, portions of the printing plate 24 positioned further from the pivot point will have higher pivoting speeds than portions of the printing plate 24 that are positioned closer to the pivot point. When a pivot point is directly located on a printing plate 24, the location of the pivot point will correspond to a location of a portion of the printing plate 24 that has substantially a null pivoting speed as the printing plate 24 pivots.
The present invention has determined that relatively large frictional moments between printing plate 24C and support surface 102 are required to be overcome to permit a conventional pivoting movement about a registration pin such as shown in
Frictional characteristic between printing plate 24C and support surface 102 can be simulated by dividing printing plate 24C into fifteen (15) frictional cells 122 shown in broken lines. The number of frictional cells 122 employed in this simulation are selected for illustration purposes only and those skilled in the art will realize that different numbers can also be employed. Portions of printing plate 24C corresponding to each friction cell 122 are assumed to contact support surface 102 in a uniform manner and a frictional force FFA associated with each friction cell 122 can be estimated by the following relationship:
F
FA
=μ*ρ*L*W*b*g; where: (1)
μ is coefficient of friction associated with printing plate 24C and support surface 102;
ρ is the mass density of printing plate 24;
L is a first size of each frictional cell 122;
W is a second size of each frictional cell 122;
b is a thickness of printing plate 24C; and
g is a gravitational acceleration constant.
In this case, the frictional force acting on each frictional cell is determined to be FFA=0.0573 N for the following conditions: μ=0.3, ρ=2700 kg/m3, L=W=0.19 m, b=0.0002 m and g=9.81 m/s2.
The positioning of each of the frictional cells 122 is arranged according to a matrix grid coordinate system comprising five (5) rows identified by row index i=1, 2, 3, 4 and 5 and three (3) columns identified by column index j=1, 2, and 3. Accordingly, as shown in
The total frictional moment MTOTA that resist pivoting about pivot point 116 can be estimated by the following relationship:
M
TOTA
=ΣD
i,j
*F
FA, where i=1, 2, 3, 4 and 5, and j=1, 2 and 3. (2)
When this summation is completed for the previous example, the total frictional moment MTOTA is determined to be 0.475 Nm.
The magnitude of the plate movement force FA required to overcome the total frictional moment MTOTA and rotate printing plate 24C about pivot point 116 can be estimated from the following relationship:
F
A
=M
TOTA
/X; where: (3)
X is a moment length associated with the application of plate movement force FA.
In this example X≃2*W or 0.38 m and the plate movement force FA is estimated to be equal to 1.06 N. A summation of forces shows that reaction force RA is equal to plate movement force FA (i.e. RA=FA=1.06 N).
Reaction forces RA of this magnitude can lead to formation of high contact stresses between registration pin 108 and the engaged edge portion of printing plate 24C. These contact stresses can lead to the formation undesired deformations in the engaged edge of printing plate 24C.
Further analysis of relationship (3) that plate movement force FA can be reduced by reducing frictional moment MTOTA. Reductions in plate movement force FA in turn correspond to reductions in reaction force RA.
The present invention has determined that the total frictional moment acting between a printing plate 24 and a surface onto which it is supported can be reduced by pivoting the printing plate 24 about a pivot point that is located at a different location than those of the application points of the various applied forces (e.g. applied force FA and reaction force RA). The present invention has additionally determined that the total frictional moment acting between a printing plate 24 and a surface onto which it is supported can be reduced by pivoting the printing plate 24 about a pivot point that is positioned inboard from the perimeter of printing plate 24 as defined by its edges. In particular, the present invention has determined that the total frictional moment can be significantly reduced by pivoting the printing plate 24 about a pivot point that lies between the locations of the applied forces, especially in proximity to the geometric center of printing plate 24 or in the vicinity of the center of mass of the printing plate 24 or in the vicinity of a centroid of one or more areas of contact between the printing plate 24 and the support surface onto which it is pivoted.
Frictional characteristic between printing plate 24C and support surface 90 are again simulated by dividing printing plate 24C into fifteen (15) frictional cells 132. The number of frictional cells 132 employed in this simulation are again selected for illustration purposes only and those skilled in the art will realize that different numbers can also be employed. In this embodiment, frictional cells 132 are substantially the same in form as frictional cells 122 that were previously analyzed. The frictional force FFB associated with each friction cell 132 is therefore estimated by relationship (1).
In this example embodiment the frictional force acting on each frictional cell is determined to be FFB=FFA=0.0573 N for the following conditions: ρ=2700 kg/m3, L=W=0.19 m, b=0.0002 m, g=9.81 m/s2 and μ=0.3 (i.e. assuming that the frictional characteristic of support surface 90 mimic those of conventional support surface 102).
The positioning of each of the frictional cells 132 is arranged according to a matrix grid coordinate system comprising five (5) rows identified row index r=1, 2, 3, 4 and 5 and three (3) columns identified by column index s=1, 2, and 3. Accordingly, as shown in
M
TOTB
=ΣD
r,s
*F
FB, where r=1, 2, 3, 4 and 5, and s=1, 2 and 3. (4)
When this summation is completed for the previous example, the total frictional moment MTOTB is determined to be 0.248 Nm or about half of the total frictional moment MTOTA that was previously calculated for the conventional pivoting arrangement.
The magnitude of the plate movement force FB required to overcome the total frictional moment MTOTB and rotate printing plate 24C about pivot point 130 can be estimated from the following relationship:
F
B=(MTOTB−(RB*Y))/Y; where: (5)
Y is a moment length associated with the application of each of plate movement force FB and reaction force RB about pivot point 130.
A summation of forces shows that plate movement force FB is substantially equal to reaction force RB and therefore relationship (5) can be rewritten as:
F
B
=M
TOTB/2Y. (6)
In this example Y≃1*W or 0.19 m and the plate movement force FB is estimated to be equal to 0.55N. Accordingly, and reaction force RB is also substantially equal to 0.55N or about half of the reaction force RA that was calculated previously for the conventional plate pivoting scenario. This reduced reaction force RB can be used to help reduce the chances of inflicting undesired deformations on an edge of printing plate 24C.
As shown in
It is desired that printing plate 24C be transferred from support surface 90 to imaging support surface 28 such that the registration edge 112 of printing plate 24C is registered against first and second registration members 40A and 40B. In this example embodiment, second registration member 40B is contacted by registration edge 112 after first registration member 40A is contacted by registration edge 112.
Plate positioning system 64 includes a first conveying member 150 and a second conveying member 152 which are adapted to engage edge 113 of printing plate 24C. In this illustrated embodiment, edge 113 opposes registration edge 112. First and second conveying members 150 and 152 are substantially identical in shape and form in this example embodiment.
Each of first and second conveying members 150 and 152 is pivotally movable to various locations between the two positions shown in
As shown in
As printing plate 24C is moved along first direction 138, contact is established between first registration member 40A and registration edge 112 at a contact position as shown in
As plate positioning system 64 continues to move printing plate 24C along first direction 138, first registration member 40A applies a reaction force to registration edge 112 which alters the movement of printing plate 24C along first direction 138. In this illustrated embodiment, printing plate 24C pivots about a pivot point 170 located on a surface of printing plate 24C that is substantially supported on support surface 90. Specifically, the location of pivot point 170 is inboard from the perimeter of the supported surface of printing plate 24C. In this illustrated embodiment, pivot point 170 is located on a portion of the printing plate that is not directly physically secured to, or constrained by support surface 90. That is, the portion of printing plate 24C in which pivot point 170 is located is separable from support surface 90.
As printing plate 24C pivots about pivot point 170, each of second conveying member 152 and unlocked first conveying member 150 maintain their contact with edge 113. In this illustrated embodiment, second conveying member 152 and unlocked first conveying member 150 move closer relative to one another as they pivot via their hinged members 154 to maintain contact with edge 113. In this illustrated embodiment, each of second conveying member 152 and unlocked first conveying member 150 are adapted to roll along edge 113 as printing plate 24C is pivoted. In some embodiments, each of second conveying member 152 and unlocked first conveying member 150 move with the same rotational direction. In some example embodiments, second conveying member 152 and unlocked first conveying member 150 can move in opposite directions as printing plate 24C is pivoted.
As printing plate 24C pivots about pivot point 170, first registration member 40A maintains contact with registration edge 112. In this illustrated embodiment, initial contact is established between first registration member 40A and printing plate 24C at a contact location 171 on registration edge 112 and this contact location 171 does not substantially change as printing plate 24C is pivoted. That is, there is substantially no relative movement between first registration member 40A and the contacted registration edge 112 as printing plate 24C is pivoted about pivot point 170. In this illustrated embodiment, first registration member 40A moves along substantially a straight path along a second direction 172 that intersects first direction 138 as printing plate 24C is pivoted. The movement of first and second conveying members 150 and 152 against edge 113 cause a reaction force to be created between first registration member 40A and a contacted portion of registration edge 112 which in turn causes first registration member 40A to move under the influence of the generated reaction force. In this illustrated embodiment, first registration member 40A commences moving after it has contacted registration edge 112. In this illustrated embodiment, first registration member 40A moves along second direction 172 away from second registration member 40B as printing plate 24C pivots. First registration member 40A can rotate about shaft 169 to maintain contact with registration edge 112 as printing plate 24C is pivoted. A rotation axis of first registration member 40A intersects a plane of support surface 90 in this example embodiment. In this example embodiment, first registration member 40A moves along a path defined by the straight line linkage it is coupled to. In other embodiments, first registration member 40A can move along other paths in conjunction with constraints imposed by other linkages or guide mechanisms.
First conveying member 150, second conveying member 152 and first registration member 40A each move in a way that allows printing plate 24C to pivot about inboard pivot point 170 to a desired registered position in which contact with second registration member 40B is additionally established as shown in
The position of inboard pivot point 170 may vary slightly as printing plate 24C is pivoted on support surface 90. Slight variations can occur for various reasons which in this illustrated embodiment can include deviations in the approximated straight line path that first registration member 40A is constrained to move along by the employed straight line linkage. Nonetheless, these minor deviations still maintain pivot point 170 within the perimeter of printing plate 24C and still advantageously allow for reduced registration forces.
The position of inboard pivot 170 can vary among different printing plates 24, especially if the printing plates have different sizes. The printing plates 24 can be differently sized along their registration edges and/or lateral edges for example. This effect can be observed when different sized printing plates 24 are sequentially registered against the first and second registration members 40A and 40B. The distance between each of the respective pivot points and contacted first registration member 40A can be seen to vary when each differently sized printing plate 24 is pivoted to a common position in which both of the first and second registration members 40A and 40B are contacted. In some embodiments of the invention each of the differently sized printing plates 24 include an inboard pivot point.
In this illustrated embodiment, each of first registration member 40A and second registration member 40B includes a substantially planar surface adapted to further reduce contact stresses when contacted by associated portions of registration edge 112 in addition to the reduced applied forces. Other example embodiments of the invention may employ registration members that have other forms of contact surfaces.
In an example embodiment of the invention shown in
In this illustrated embodiment, movement of first registration member 40E is substantially confined rotate only about shaft 181. As shown in
Pivot point 173 remains inboard of the perimeter of printing plate 24D throughout this motion thereby advantageously allowing for reduction in the applied forces required to register printing plate 24D. In this example embodiment, pivot point 173 will translate relatively between printing plate 24D and support surface 174 but will remain positioned within the perimeter of printing plate 24D as printing plate 24 is pivoted to contact second registration member 40F. A component of this movement can be parallel to first direction 178.
In this illustrated embodiment, reduced edge deformations in printing plate 24D can be achieved by a combination of the relatively large sized rotating cylindrical contact surface of first registration member 40E and the reduced loading that accompanies the inboard pivoting.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.
Reference is made to commonly-assigned copending U.S. patent application Ser. No. ______ (Attorney Docket 95291/NAB), filed herewith, entitled MOVEABLE PRINTING PLATE REGISTRATION MEMBER, by Funk et al., the disclosure of which is incorporated herein.
