CA1243896A - Pin register system for flexographic printing plates - Google Patents

Abstract

PIN REGISTER SYSTEM FOR FLEXOGRAPHIC PRINTING PLATES

Abstract A method for accurately positioning photopolymer color plates onto print cylinders of a rotary press is disclosed. Alignment accuracy is provided by producing multiple color separated film negatives from a camera film negative on which an image field and reference marks have been photographically transferred from mechanical artwork.
Continuity of alignment accuracy is provided by center scribe reference marks utilized together with index holes which are precision located and punched, respectively, onto and through carrier sheets, masks, duplicating film and color separated negatives. Various combinations of pre-punched carrier sheets, masks, contact duplicating film, color separated negatives, and unexposed printing plates are mechanically coupled in precise alignment by pin register tabs throughout the various stages of the plate production process. Finished photopolymer plates are mounted onto assigned print cylinders with the aid of index pins.

Description

"PIN REGISTER SYSTEM FOR FLEXOGR~PHIC PRINTING PL~TES"

This invention relates generally to the art of color printing, and in particular to method and apparatus for precisely loca-ting a flexographic printing plate onto a print cylinder.
In the art of color printing, photo-engraved plates are used in multiple processes. The first step in preparing color printing plates is to make a camera film negative from camera ready art (mechanical art). Multiple color separated film negatives are then produced from -the camera film with the aid of a mask for line processes, or with the aid oE
color filters for half tone processes. ~fter the color separated negatives are developed, the color separated image is photographically transferred to a photosensitive plate, with a separate printing plate being produced for each color. The exposed plate is then etched to defin~ the printing surface. Non-printing areas on the plate are removed by the etch solution, leaving the printing surEace standing in relieE.
The finished plates are thereafter separately mounted onto print cylinders. The plates are inked and applied to separate sheets or a moving webb of printing ~3~

paper, with the print fields of the ~arious plates being superimposed upon one another. Thus, the picture is succes.sively built up step-by-step, and when the impressions are properly superimposed, the resulting print is a ~aithful reproduc-tion of the mechanical art.
To achieve good results, it is essen-tial that the successive print fields register exactly one upon the other, with the separate color impressions being spaced with respect to each o-ther so that overlapping does not occur.
The complementary printing plates must be precisely aligned to obtain appropriate posi-tioning oE the multiple color impressions. ThereEore, it is essential tha-t the print fields of the flexographic plates be aligned within very close tolerances at corresponding locations on each print cylinder.
According to conventional practice, the location and mounting of a printing plate onto a print cylinder is carried out manually by aligning index marks on the plate with corresponding index marks which are scribed onto the surface of the print cylinder. Plate alignment accuracy depends largely upon personal judgment; significant alignment errors can be induced by the ~Iparallax~ eEEect.
The alignment procedure is complicated by the presence of a bonding agen-t which secures the prin-ting plate onto the cylinder.
Typlcally, double-sided adhesive tape is u-tilized for securely bonding the photopolymer pla-te onto the surface of the prin~ cylinder. That is, the adhesive is a contact adhesive which bonds rapidly thereby limiting the amount of adjustment displacement of the pho-topolymer plate after initial contact. It is usually necessary to perform one or more trial printing runs and adjust the position of one or more of the color plates to achie~e an acceptable color print. The adjusting process t~y take as many as 12 to 20 man-hours. It will be appreciated that the foregoing conventional plate mounting procedure requires a high level of skill and experience to be carried out effectively.
According to the invention, plate alignment accuracy is provided in part by producing each color separated film from the same camera film negative on which center scribe marks and registry marks have been photographically trans~erred from mechanical artwork.
Continuity oE alignment accuracy is provided by a system of center scribe refexence marks utilized together with index holes which are precision located and punched, respectively, onto and through carrier sheets, masks, duplica-ting film and color separated negatives. Various combinations of pre-punched carrier sheets, masks, contact duplicating film, co]or separa-ted negatives, and unexposed printing plates are mechanically coupled in precise alignment by index pin tabs throughout the various stages of the plate production process. In particular, superimposed sheets are coupled toge-ther with corresponding scribe maxks being held in alignment by index pin tabs which are received in registration in the precision punched index holes.

31~36 Multiple photosensitive plates are finished with the aid of the specially prepared color separated films, and the finished plates are precisely aligned and ~ecurely moun-ted onto print cylinders with the aid of a pair of index pins. The index pins are mounted onto each print cylinder at a precise spacing dis-tance "L". Each print cylinder of the color print se~uence is equipped with the index pins mounted at mutually corresponding locations. A pair of index holes are punched at a corresponding distance "L" through each color plate along a major axis thereof. Each color plate is thereafter mounted onto a print cylinder with the index pins received in registration with the index holes.
Becau-~e of the precision provided by the method of the invention, no further adjustment of one plate relative to another is required. With all plates and print cylinders being set up in this manner, the successive impressions produced by the superimposed printing fields are properly spacedt thereby providin~ faithful reproduction. Moreover, the entire plate mounting procedure, including the punching of the index holes and exposure of the photosensitive plate, can be carried out by one person in less than an hour per plate.
From another aspect, the present invention provides an improved print cylinder assembly including a roller having a tubular sidewall for supporting a flexographic printing plate, characterized in that: said roller having first and ~econd bores formed in said tubular sidewall at separate location~
which are spacecl apart by a distance of L; first and second ~3~
- ~a -index pins received within the first and second bores, respectively, each index pin having a head portion projecting outwardly from the tubular sidewall external surface; a layer of adhesive material tdisposed about the external surface of said tubular sidewall; a flexographic printing plate mounted on said roller sidewall having first and second index holes formed thereon at separate locations which are substantially spaced apart by a distance L corresponding to the spacing L of the first an.d second bores in said roller sidewall, said first and second index pins being received within said first and second index holes of the flexographic printing plate, and said flexographic printing plate being bonded to said tubular sidewall by said adhesive layer; and, said flexographic printing plate having a foundation sheet of flexible material contacting said adhesive material and a layer of photosensitive material bonded to said flex1ble foundation sheet, said first and second index holes being formed through said flexible foundation sheet, with a portion of the photosensitive material of said flexographic plate being removed from first and second areas coincident with said first and second index holes, respectively, thereby defining first and second windows coincident with said first and second index holes, respectively, and said first and second index pins projecting into said first and second windows, respectively.
The advantages o~ the invention will be further appreciated by those skilled in the art upon reading the detailed description which follows in conjunc~ion with the attached drawings, in ~hich:

3i3~6 FIGURE l is a simplified perspective view of a print cylinder on which index pins have been mounted;
FIGURE 2 is a perspective view of a cylinder index pin;
FIGURE 3 is a front elevational view of the print cylinder shown in FIGURE l;
FIGURE 4 is a simplified top plan view oE camexa ready mechanical artwork;
FIGURE 5 is a simplified top plan view of a camera film negative prepared from the mechanical artwork of FIGURE 4;
FIGURE 6 is a top plan view of a color separation mask superimposed over an assembly o:E camera film and unexposed duplicating film;
FIGURE 7 is an exploded view of the film assembly shown in FIGURE 6;
FIGURE 8 is a side elevation view of the film assembly of FIGURE 6 undergoing exposure;
FIGURE 9 is a top plan view of a color separated negative prepared pursuant to the mask exposure step of FIGURE 8;
F'IGURE 10 is a perspective view of an exposure table on which a color separation negative and photopolymer plate are mounted in superimposed relation for exposure;
FIGURE ll is a simplified view, partly in sec-tion, of the exposure table shown in figure 6;

~3~

FIGURE 1~ is a -top plan view oE a flexographic printing plate having a color separated print surface exposed through the color separation negative of FIGURE 9;
FIGURE 13 is a sectional view of a print cylinder onto which the photopolymer plate of FIGURE 12 is mounted according to the teachings of the present invention;
FIGURE 14 is a top plan view of a precision punch table having a work surface onto which horizontal and vertical scribe marks are formed; and, FIG~RE 15 is a perspective view of a printing press cylinder having a photopolymer plate mounted with the aid of index pins.
In the description which follows, like parts are indicated throughout the speci~ication and drawings with the same reEerence numerals, respectively. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to clarify certain aspects o-E the present invention.
Referring now to FIGURES 1, 2, 3 and 13, a print cylinder lO of a ro-tary press having multiple print cylinders for producing multi-color prints is modi:Eied by the a-ttachment of a pair of index pins 12, 14. ~s can bes-t be seen in Figures 2 and 13, the index pins 12 r 1~ are mounted onto the print cylinder in an upright orientation, and at separate locations which are precisely spaced at a distance L. The index pins may be attached by slip socket engagemen-t as shown in FIGURE 13~ or by threaded engagement.

~;~43~

In some instances, it may be desirable to remove the pins after a Clexographic pla-te has been secured. It may be desirable to utilize such pins for hold-down purposes as well as for indexing purposes, in which case they are retained during the printing process. However, the primary function o~ the pins 12, 1~ as utilized herein is for indexing purposes, as described in greater detail hereinafter.
Referring to FIGURE 12 and FIGURE 13, a flexographic photopolymer plate 16, prepared according to the teachings of this invention, is mounted onto the print cylinder 10 with the index pins 12, 14 of the print cylinder being received in registration with index holes 18, 20. The index holes 18, 20 are formed in the plate 16 at a precise spacing distance "L" which corresponds with the spacing distance of the cylinder index pins. The index holes 18, 20 formed in the photopolymer plate 16 are carefully located according to the following method.
The locations of -the plate index holes 18, 20 correspond exactly with the location of a corresponding set of index holes 2~, 26 formed in a color separated film negative 22 (FIGURE 9). ~ccording to the method of the invention, the index holes 2~, 26 are precisely located with respect to center scribe marks 28, 30 which are photographically t:ransferred from corresponding scribe marks on camera ready mechanical artwork 32 (FIGURE 4).

~ ~3B~

The index holes 24, 26 are spaced at a distance '~L"
and are formed on a precision punch table 33 (FIGURE 14).
The precision punch table 33 has a work surface 35 on which a corresponding set of horizontal and vertical center scribe 5 marks 28, 30 are ~ormed. Multiple sets o-E die openings 37 are formed in -the punch table at symmetrically-spaced locations about the vertical center scribe line 30, and are coincident with the horizontal center scribe line 28. The center scribe marks 28, 30 of the col.ox separated negative 22 are superimposed in alignment with the corresponding center reference marks on the punch table 33, and the holes 24, 26 are punched at equally spaced locations with respect -to the vertical scribe mark 30, and coincident w.ith the horizontal scribe mark 28.
~ preliminary step is to prepare a film negative 34 (FIGURE 5) from the camera ready mechanical artwork 32 (FIGURE 4). The camera ready artwork 32 is attached to a base carrier sheet 36 on which hori~ontal scribe marks 28 and vertical scribe marks 30 have been formed.
Corresponding horizontal and vertical center scribe marks 28, 30 are also formed onto the camera ready mechanical artwork 32. The mechanical artwork 32 is taped onto -the base carrier sheet 36 with the corresponding horizontal and vertical center scribe marks in alignment. Additionally, cross-hai.r register marks 38 are applied to each corner of the camera ready mechanical artwork.

3~
g The mechanical artwork 32 includes three separate image fields designated ~t B and C. For purposes of this description, it is assumed that each image ~ield is intended to be reproduced in different colors. Accordingly, for this example, it will be necessary to prepare three separate color separated films Erom which three diEferent printing pla-tes are produced.
For purposes oE illustra-tion, the description which follo~s is directed -to the method for preparing a printing lO plate for reproducing the image field ~. After the camera film negative 34 has been prepared from -the mechanical artwork 32, the camera negative 34 is attached to a base carrier sheet 40 onto which horizontal and vertical scribe reference marks 28, 30 have been formed. Additionally, index holes 42, 44 are 15 precision punched a-t equally spaced locations with respect to the vertical scribe mark 30, and coincident with horizon-tal scribe mark 28. The index holes are spaced at a distance 'IL", which corresponds exactly witn the cen-ter-to center spacing of the index pins 12, 14 which are mounted onto the print cylinder lO.
The side portions of the :Eilm nega-tive 34 are notched to provide clearance Eor the index holes and to reveal the underlying center scribe marks. Addi-tionally, the film 34 is mechanically a-ttached to the base carrier sheet 40 by small strips of tape 46 which are applied along the edge portions of the film negative at locations outside oE the image fields.

~3~

It should be noted that the subject matter appearing on the camera ready artwork 32, including the center scribe marks, cross-hair register marks and image field ~ is reproduced in photographically negative relation on the camera film 34. ~hat is, the let-ter A appears in solid black -format surrounded by a transparent field in the mechanical artwork 32, whereas in the negative Eilm 34, the letter "A" appears in transparent form, surrounded by an opaque field~ Likewise, the center scribe marks and cross-hair register marks also appear lO as a clear transparency area on the negative, with all other areas of the negative being opaque. However, to simplify illustration, such other opaque areas o~ the film negative 34 have been omi-tted.
The hori.zontal and vertical center scribe marks 28, 30 formed onto the base carrier sheet 40 should be accurately located. To improve accuracy, a photo-mechanical scribing device such as a computer plotter or an opto-cop~ machine can be used to apply the center scribe marks onto the mechanical art, the base carrier sheets and the unexposed photopolymer 20 plate.
Prior to taping the camera film nega-tive 34 onto the base carrier sheet 40, -the inde~ holes 42l 44 are precision punched -through the base carrier sheet along the horizontal center scribe line 28. ~fter the index holes have been 25 punched, the film negative 34 is taped onto the carrier sheet 40 with the respective cross-hair register marlcs and horizontal and vertical center scribe marks in alignment. The alignment of the film negative 34 is per-formed on a light table which enables the operator to see through the film and the carrier sheet to line up the reference marks.
Re:Eerring now to FIGURES 6, 7, 8 and 9, a mask 48 of opaque sheet material is punched to produce index holes 50, 52 at a spacing distance "L". The mask 48 is placed on top of the film negative 34 with its index holes 50, 52 being in registration with corresponding index holes 42, 44 of the film carrier 40. The mask 48 is mechanically coupled to the base carrier/film negative assembly by pin register tabs 54, 56. The pin register tabs 54, 56 carry upright index pins 58, 60, respectively.
The rnask 48 is made of an optically opaque material which completely obscuxes the underlying film negative 34.
~ccordingly, a window 62 is cut into the mask 48, thereby revealing the image field ~. Additionally, small windows 64 are cut into the -mask to reveal the underly.ing horizontal and vertical center scribe marks 28, 30. Other windows 66 are cut in corner loca-tions to reveal the cross-hair scribe 20 marks 38. rrhe foregoing arrangement serves as a color separation technique for the film. That is, the large window exposes the area of the negative which is to be printed in a certain color. The sma].ler windows allow the center scribe marks and register ~marks to be printed onto the color separated negative.
The film negative 34, the mask 48 and the pin register tabs 54, 56 are used in combina-tion with a sheet 68 ~3~

of unexposed contact duplica-ting film as shown in FIGURES 7, . The contact duplicating film 68 is taped onto a base carrier sheet 70 having holes 72, 7~ pre-punched on a precision punch table at corresponding locations which are spaced at a distance "L". The con-tact duplicat,ing film 68 is placed in "emulsion-up" orientation and in con-tact with the under side of -the transparent base carrier sheet ~0.
The carrier sheets together with the film neyative, the contact duplicating film and the mask are coupled together in superimposed relation by the pin regis-ter tabs 54, 56, with the index pins 58, 60 being received wi-thin the vertically aligned index holes at each end of the assembly.
The coupled assembly is then placed on a vacuum hold-down table 76 where it undergoes exposure to a source of ultraviolet light 78. ~fter exposure for a predetermined period, the contact ~ilm is removed rom the assembly and developed to produce the color separated negative 22 as shown in FIGURE 9 ~ It should be unders-tood that the Eoregoing procedures are followed for each additional color as desired. For example, for a six-color job, six contact film duplicates are made with the aid oE six diEferen-t masks, one for each oE the six colors. The result in each case would be a separate color separated negative of the type illustrated in FIGURE 9, but with a different print field exposure.
It should be noted that the color separation negative 22 as shown in FIGURE 9 is identical to the original camera film negative 3~, with the e~ception that print fields B ana C do not appear. Otherwise, the color separated film 22 includes -the same horizontal and vertical center scribe marks 28, 30 and corner cross-hair register 5 marks 38 which are identically located wi-th respect to the corresponding reference marks on the original camera ready mechanical artwork 32.
The next step is to prepare an unexposed photopolymer printing plate by punching index holes 18, 20 at the same center-to-center spacing "L" as the indexing pin holes on the contact duplicating film 68. The photGpolymer plate is larger than the field of reproduction so that the location of the index holes with respect to a single exposure axis is not critical. Preferably, however, the index holes should be located along and coi.ncident with the horizontal exposure axis of the plate. ~Eter the index holes have been punched, the unexposed photopolymer plate is placed in an exposing :Erame with the index pins of pin register tabs projecting through the index holes. Next~ the color separated film negative 22, which also has index holes punched, is placed over the pins, thereby locating the color separated image field ~ precisely on the photopolymer plate 16 in relation to the index holes.
The assembly is then transEerred to an exposure table 80 as illustrated in FIGURE 10. ~ccording to this arrangement, the developed color separated Eilm 22 is superimposed over the unexposed photopolymer plate 1~, with ~3~

the corresponding index holes being maintained in registra-tion by pin register tabs 54, 56. The upright index pins 58, 60 mechanically couple the color separa-ted negative 22 in superimposed relation with -the underlying unexposed photopolymer plate 16. The pins 58, 60 maintain the respective holes exactly in registration during the exposure process.
The photopolymer plate 16 is thereafter exposed to ultraviolet light 82 which is directed through the color separation film from a suitable ultraviolet light source 84.
As a result of this procedure, the color separation image A
is transferred to the photopolymer plate 16. Previous to the exposure, the photopolymer plate is pre-exposed on the back side of the plate, a process commonly employed in conventional techniques of photopolymer plate making.
~ eferring now to FIGURES 13 and 15, the finished photopolymer 16 is thereafter moun-ted onto the print cylinder 10 with the index pins 12, 14 of the print cylinder being in registration with the index holes 18, 20.
simplified represen-tation oE the finished photopolymer plate is illustrated in FIGURE 13, with etch cavities 84 separating print surfaces 86. In the various views in which the Eilm and plate appear, the thicknesses have been exaggerated Eor illus-tration purposes. The thickness of the 25 color separation Eilm is typically .004 inch, while -the thickness of the photopolymer plate 16 is typically .065 inch.

~Z~ i6 The photopolymer plate 16 is bonded onto a polyester film base 88 through which the holes 1~, 20 are punchedO The upper end 12~ o~ the index pin 12 projects above the polyester film base 88 by a distance of approximately .035 inch, but lies below -the level of the print surface 86. Thus, the upper end 12~ of the index pin projects approximately .040 inch above the exterior surface o~ the print cylinder 10, with its root shaft 12s extending approximately 0.50 inch into -the cylinder side wall. The bore 90 forms a pocket in which the root portion 12B is received. Cylindrical bores 90 are positioned at corresponding locations on each print cylinder with the aid of a template. Each bore 90 defines a slip pocket into which the index pins can be inserted and withdrawn.
The upper end 12~ of the index pin is approximately 0.25 inch diameter, and the lower root end 12B is approximately 0.125 inch diameter. The height of the large diameter head portion is .090 inch and the length of the shaft portion 12s is approximately 0.50 inch. The boxe 90 is enlarged near the surface of -the cylinder by a 0.25 inch counter bore.
~ fter the print cylinder 10 has been drilled and the index pins 12, 14 have been installed, a layer 92 of double-sided contact adhesive tape is applied to the 25 exterior surface of the print cylinder. The thickness of the adhesive tape is approximately 20 mils. ~fter the layer 92 of the contact adhesive tape is applied to the surface oE

~L2~3~

the print cylinder, the photopolymer plate 16 is manually positioned about the cylinder with the index pins 12, 14 being received in registration with the printing plate index holes 18, 20, as illustrated in FIGURE 15. Pressure is applied to the photopolymer plate 16, forcing i-t into contact with -the layer oE adhesive -tape, thereby securing the position of the plate.
The index pins 12, 14 are then removed (if removable pins are used) and the printing cylinder is then ready for production. If drilled and tapped index pins are used, the index pins are left in the cylinder but the heads must be below the pitch line of the printing plate.
The foregoing procedure is employed for each printing station, thereby creating a precise registration from one color to -the next color.
~ s a result of the precision provided by the photographically aligned exposure Eield and index holes on each pla-te, and the corresponding identical locations of the index pins on each print cylinder, no Eurther adjustment of the photopolymer plate 16 will be required. ~fter bonding has been established, the index pins 12, 14 can be removed if desired. With each pla-te and print cylinder being set up in the foregoing manner, and assuming that the print cylinders are synchronized in rota-tion, the color impressions produced by the successive, superimposed prin-ting fields will be properly spaced, thereby providing faithful color reproduction.

:~L2~3~

It will be appreciated that the print cylinders of a conventlonal rotary press can be easily adapted -to accomodate the method of the invention. The bore pockets for receiving the index pins can be drilled by a machinist using commonly available drill equipment. ~ template is used for precisely locating the index pin holes at iden-tical locations on each of the print cylinders. Likewise, the flexographic plate index holes are formed with commonly available precision punch equipment.
Under initial conditions in which the print cylinders have been modified by installation of the index pins, and the color separation films have already been prepared, the remaining steps, including punching of index holes and acutal mounting of the finished photopolymer plate onto a print cylinder can be carried out by a single operator in less than an hour. Moreover, because of -the inherent precision provided by the pin register system, the exposure fields of the respective photopolymer plates are precisely aligned, so that alignment errors induced by the "parallax" effect are avoided, and personal judgment or skill of the operator is no longer a factor. Accordingly, once the photopolymer plate has been engaged by the index pins, further position adjustment is not likely to be re~uired, wi-th the result tha-t the set-up -time is reduced by a factor of 10 to 20, with the attendant savings in labor, 38~6 and wi-th reliable, faithful color reproduction being obtained.
Although a preferred embodiment of the invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spiri-t and scope of the invention as defined by the appended claims.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS: -

1. An improved print cylinder assembly including a roller having a tubular sidewall for supporting a flexo-graphic printing plate, characterized in that:

said roller having first and second bores formed in said tubular sidewall at separate locations which are spaced apart by a distance L;

first and second index pins received within the first and second bores, respectively, each index pin having a head portion projecting outwardly from the tubular sidewall external surface;

a layer of adhesive material disposed about the external surface of said tubular sidewall;

a flexographic printing plate mounted on said roller sidewall having first and second index holes formed thereon at separate locations which are substantially spaced apart by a distance L corresponding to the spacing L of the first and second bores in said roller sidewall, said first and second index pins being received within said first and second index holes of the flexographic printing plate, and said flexographic printing plate being bonded to said tubular sidewall by said adhesive layer; and, said flexographic printing plate having a foundation sheet of flexible material contacting said adhesive material and a layer of photosensitive material bonded to said flexible foundation sheet, said first and second index holes being formed through said flexible foundation sheet, with a portion of the photosensitive material of said flexographic plate being removed from first and second areas coincident with said first and second index holes, respectively, thereby defining first and second windows coincident with said first and second index holes, respectively, and said first and second index pins projecting into said first and second windows, respectively.

2. An improved print cylinder assembly as defined in Claim 1, characterized in that said flexographic printing plate is produced and adapted for mounting onto said roller sidewall as follows:

producing a camera film negative from mechani-cal artwork;

forming horizontal and vertical center scribe marks on the camera film negative;

producing a color separated film negative from the camera film negative;

forming first and second index holes in the color separated film negative at equally spaced locations with respect to the vertical center scribe mark and coincident with the horizontal center scribe mark, with the index holes being spaced apart by a distance corresponding with the roller sidewall bore spacing distance L;

forming first and second index holes in said printing plate at first and second plate locations which are spaced apart by a distance corresponding with the roller sidewall bore spacing distance L;

photographically exposing said printing plate through the color separated film negative with said plate and film negative being disposed in superimposed relation and with the corresponding index holes of said plate and film negative being maintained in registration by index pins; and, etching the exposed plate to produce a finished plate having a defined print surface.