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This invention relates to a blanket for offset lithographic printing and
more particularly to a blanket and carrier plate for web offset printing
and to a method of assembling the blanket with the carrier plate. The
blanket may have a ferromagnetic carrier plate for use in a press with
magnetic cylinders, or a nonmagnetic carrier plate for use with a cylinder
having a mechanical lockup mechanism.
BACKGROUND OF THE INVENTION
In offset printing an image is transferred from an inked plate to a blanket
having an elastomer printing surface, and from the blanket to the paper
being imprinted. Typically, an offset printing blanket has an elastomer
body with one or more layers of fabric reinforcing and a fabric backing.
The blanket is stretched around a cylinder and the ends are secured by a
locking mechanism located in a longitudinal gap in the cylinder surface.
Due to the planographic nature of offset printing, high pressure is
required between the blanket and the plate or paper to insure transfer of
the ink image. The discontinuity of the cylinder gap causes problems in
high speed web printing, affecting the quality of the printed product and
the design and maintenance of the press. For example, cylinders typically
have bearer rings which minimize impact and vibration attributable to the
locking mechanism gap, but which require a substantial mechanical load,
introducing bearing and wear problems. Moreover, the gap leaves an
unprinted area on the paper web, resulting in paper waste.
A magnetic gapless cylinder for web offset presses is shown in Peekna
application Ser. No. 736,062 filed May 20, 1985, now U.S. Pat. No.
4,625,928, and Peekna et al. U.S. application Ser. No. 763,128, filed Aug.
6, 1985, now Pat. No. 4,676,161 assigned to R. R. Donnelley & Sons
Company. This application discloses and claims a printing blanket on a
carrier plate of ferromagnetic material suitable for use with the Peekna
cylinder in web offset printing, and a method of assembling the blanket.
Banike U.S. application, Ser. No. 642,080, filed Aug. 20, 1984, and
assigned to R. R. Donnelley & Sons Company, now U.S. Pat. No. 4,577,560,
discloses a gapless lockup mechanism for a plate cylinder. A blanket
mounted on a plate can be used with a cylinder which has the Banike lockup
mechanism.
Adhesive coated "sticky back" blankets which mount directly on a cylinder
are used in offset form printing. The adhesive is dissolved by fountain
solution and the "sticky back" blankets are not suitable for web offset
printing. Others have mounted an elastomeric printing sheet on a steel
plate, but not for the severe physical and chemical environment to which a
blanket is subjected in offset web printing. For example, Faust U.S. Pat.
No. 4,040,351 shows a rubber printing mat cemented to a steel base of shim
stock, mounted on a magnetic cylinder, in a flexographic labeler or
addresser. McKay U.S. Pat. No. 3,180,259 shows a molded rubber printing
plate cemented to a steel base plate held on a magnetic printing wheel as
used in a coding or dating machine. Jenkins U.S. Pat. Nos. 3,885,497 and
3,885,498 show molded magnetic cylinders, on which printing plates are
mounted, and methods for molding the bases. Stromme U.S. Pat. No.
2,982,207 has a flexible printing plate secured, as by bonding, to a
corrugated plate.
None of these blankets or resilient plates is subjected to the pressures,
speeds or chemical exposure of web offset printing.
SUMMARY OF THE INVENTION
The printing blanket and carrier plate disclosed herein are usable with the
severe physical and chemical environment of web offset printing.
One feature of the invention is an offset printing blanket used on a
cylinder having a magnetic surface, in an offset web press, comprising an
elastomer blanket sheet secured by an adhesive to a carrier plate of
corrosion resistant ferromagnetic material. More particularly, the carrier
plate is a ferritic stainless steel.
Another feature is that the blanket sheet has a printing surface and a base
layer, which may be fabric reinforced. The based layer is a resilient
closed cell foamed elastomer. The closed cell material prevents a
structural adhesive, as an epoxy, from impregnating the blanket base and
destroying its resilience, and minimizes the opportunity for blanket wash
and fountain solution to attack the adhesive bond between the blanket and
plate.
A further feature is the method of assembling the printing blanket and
carrier plate including the steps of providing an elastomer blanket sheet
having a printing surface and a closed cell base layer, providing a
carrier plate of corrosion resistant ferromagnetic material, applying
adhesive to at least one of the surface of the base layer or carrier plate
and adhering the surface of the base layer of the elastomer blanket sheet
to the carrier plate.
Yet another feature of the assembling method, in which the carrier plate is
stainless steel, is the inclusion of a step of roughening the surface of
the plate before adhering the elastomer blanket sheet thereto, as by hand
sanding.
And a further feature of the assembling method is a step of cleaning the
surface of the base layer of the elastomer blanket sheet, as by washing
with acetone, before adhering the sheet to the carrier plate.
Further features and advantages of the invention will readily be apparent
from the following specification and from the drawings, in which:
FIG. 1 is an exploded perspective illustrating a magnetic cylinder with
printing blankets;
FIG. 2 is an end view of the cylinder of FIG. 1 with the printing blankets
mounted thereon;
FIG. 3 is an enlarged fragmentary section through the blanket and carrier
plate;
FIG. 4 is an enlarged fragmentary section of a portion of the base layer of
the blanket sheet, the surface of the stainless steel carrier plate and
the adhesive bond therebetween; and
FIG. 5 is a fragmentary section illustrating a blanket and carrier plate
mounted on a cylinder with a gapless lockup mechanism.
A blanket cylinder 10, FIGS. 1 and 2, for an offset web press has a magnet
and pole piece surface structure 11 of the character disclosed in Peekna
et al. U.S. Pat. No. 4,676,161.
Two identical 180.degree. blanket and carrier plate assemblies 12, 13 are
curved to fit on the cylinder. Each assembly has a printing blanket 14, an
elastomer sheet secured to a ferromagnetic carrier plate 15, removably
mountable on the cylinder. The magnetic structure of the cylinder forms no
part of the present invention and is not illustrated or described in
detail. A two-around blanket construction is illustrated for printing two
pages with each rotation of the blanket cylinder. Other configurations, as
four-around with each blanket subtending 90.degree. of the cylinder, are
possible. In a typical press, a two-around blanket cylinder has a diameter
of 7.5 inches and length of 40 inches. The longitudinal gaps between
adjacent edges 16, 17 and 18, 19 of the two blanket assemblies 12, 13 are
of the order of 0.005 inch or less.
The elastomer blanket sheet 14 is made up of multiple layers as shown in
FIG. 3. The printing surface 22 is provided by a layer 23 of nitrile
rubber, as Buna-N. The printing surface transfers ink from an image
carrying plate (not shown) to a paper web (not shown). The nitrile rubber
layer 23 which has the printing surface 22 cannot be secured directly to
the steel carrier plate 15 as the rubber with a rigid support would not
withstand the physical stresses encountered in web offset printing. A
composite structure is necessary to provide additional strength. A
suitable blanket, as illustrated in FIG. 3, has a base layer 25 of closed
cell foamed elastomer and an intermediate layer 26 with a woven
reinforcing material impregnated with closed cell foamed elastomer. Two
layers 27a, 27b of woven reinforcing material are shown. The inner layer
27a is of cotton and nylon fibres and is relatively coarse. The outer
layer 27b is of cotton and polyester fibres and is a finer weave. One
layer or more than two layers might be used. Typically, the outer layer of
reinforcing fabric has a finer weave when more than one reinforcing layer
is used. If appearance of the coarse weave image on the printed web is a
problem with a blanket having a single reinforcing layer, a blanket with
multiple layer reinforcing should be used. The three layers 23, 25, 26 are
bonded together in the manufacture of the blanket sheet. The illustrated
blanket is a Vulcan type 714 from Reeves Brothers, Inc. The Vulcan 714
blanket is sold commercially with a pressure sensitive adhesive on the
base layer 25 and is mounted directly on the cylinder of a press for
printing business forms. The blanket is provided by Reeves Brothers, Inc.
without adhesive for use in accordance with the present invention.
The closed cell structure of the foam preferably has a cell size between 10
and 25 microns. Foam material with smaller cell size is stronger. Closed
cells restrict migration of fountain solution and blanket wash which may
attack the adhesive bond to the carrier plate and cause blanket
deterioration. In addition, the closed cells prevent the adhesive from
penetrating the foam. This is particularly important when a structural
epoxy is used as epoxy in the foam destroys the foam resilience and
shortens the blanket life.
The closed cell foam elastomer of base layer 25 may alternatively be a
polysulfide or an epichlorohydrin material.
The base layer 25 of the blanket is adhesively secured to the surface of
the steel plate at the interface 28. A structural epoxy adhesive, H. G.
Fuller FE-7007, has been found satisfactory.
Offset blankets for web printing generally have a fabric backing of one or
more layers or plys which provides mechanical strength to allow the
blanket to be stretched around the cylinder. A fabric backed blanket
cannot be adhered to a plate with a structural adhesive if the adhesive
impregnates the fabric and solidifies, destroying the resiliency of the
blanket. The absence of the fabric backing contributes to a longer blanket
life as the backing is the most likely blanket component to fail in the
event of a smash. Alternatively, a blanket with fabric backing which
either is not impregnated by the adhesive or is so thin relative to the
blanket that the blanket resiliency is not impaired may be used.
The nature of the bond between the blanket 14 and steel plate 15 is
illustrated in FIG. 4. The under surface of the blanket base layer 25 is
very irregular as compared with the surface of the steel plate 15. The
valleys of the blanket surface are filled with the structural epoxy
material forming a bonding interface 28. Voids which would contribute to a
structural deficiency in the bond and which would permit press room
chemicals to enter and attack the bond are filled with epoxy. The
application and curing of the epoxy adhesive are described in more detail
in Peekna et al. Ser. No. 123,192 filed Nov. 19, 1987 and assigned to R.
R. Donnelley & Sons Company.
Before applying the epoxy adhesive and adhering the blanket 14 to the
carrier plate 15, surfaces of both the blanket and the carrier plate are
prepared so that the bond between the blanket and plate is more reliable.
The blanket base surface is paper finished and has a talc coating. This
coating must be removed before applying the adhesive. The preferred
procedure is to wash the blanket surface with acetone, taking care to
minimize the time during which the rubber is exposed to acetone. Excessive
acetone contact with the rubber causes the rubber to become tacky.
The surface of the carrier plate is roughened as by sanding. Chemical
etching does not roughen the surface sufficiently to achieve a reliable
bond. Sandblasting removed excessive metal and the temperature resulting
from sandblasting relieved residual stresses in the carrier plate, causing
warping. Accordingly, it is preferred to sand the carrier plate, in a flat
configuration, with a fine abrasive, as a paper designated
"K622-FINE-5725", from Norton Company. The paper is used in a hand
manipulated power sander. Following sanding the carrier plate is curved to
fit the cylinder before the blanket is adhered thereto.
Many of the advantages of the gapless blanket described above can be
achieved without a magnetic cylinder by mounting the blanket on a carrier
plate and securing the carrier plate to a cylinder with a minimal gap
lockup mechanism. FIG. 5 illustrates a blanket 30 with a carrier plate 31
on a cylinder 32 with the lockup mechanism 33 of Banike U.S. Ser. No.
642,080 now U.S. Pat. No. 4,577,560. The blanket 30 is secured to carrier
plate 31 utilizing a suitable adhesive as an epoxy. The plate 31 may, for
example, be aluminum or stainless steel so that it will not corrode. The
plate ends 31a are formed inwardly and engaged by the lockup mechanism.
The nonprint gap 30a has a width of the order of 0.060 inch for an
aluminum plate 0.012 inch thick or 0.030 inch for a steel plate 0.005 inch
thick.
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