Note: Descriptions are shown in the official language in which they were submitted.
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TRANSFER PRINTING PROCESS
Technical Field of the Invention.
This invention is concerned with a transfer printing process.
More particularly, this invention is concerned with a transfer
printing process of particular use in the preparation of security
documents, such as passports and identity cards.
Background of the Invention.
On identity cards, passports and the like there are generally at
least two levels of printing, which may also include features
which are not normally visible to the unaided eye. In this
context, "printing" is not limited to readable alphanumeric
information, and includes any form of indicia which can be placed
onto a surface of the document. The base sheet is often a so-
called security paper, or may even be a synthetic material other
than paper, such a Teslin(Trade Mark) sheet.
The first level of printing occurs when the base sheet for the
document is prepared. This may involve background printing of,
for example, a security pattern, the identity of the issuing
agency, numerical data, a watermark, an encodable magnetic strip,
and a holographic image. Further, some or all of this printed
material may be applied to one or both sides of the base sheet.
The second level of printing occurs when personalised information
identifying the person to whom the document is to be issued is
applied to the base sheet. In the past this has usually involved
text information, such as name, address, date of birth, a
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personal identification number, a document serial number, either
of which may be in alphanumeric format or as a bar code, and date
of issue. Although images of the holder have been used for many
years in passports particularly, these have been in the form of
a photograph as such, which has been attached in some way,
usually by a gluing step, to the base sheet, which is often
followed by a laminating step to protect the photographic image.
In more recent practise, this pictorial information identifying
the holder, such as a photograph or a fingerprint, is printed
directly onto the base sheet, using computer based imaging
systems.
Several techniques have been described both for preparing a base
sheet carrying printed indicia at both levels, and for enhancing
the inherent security of the resulting document. One example of
this is the discontinuance of the use of a conventional
photographic print which is attached in some way to the base
sheet. Experience has shown that it is very difficult indeed to
protect such a photograph adequately to prevent tampering with
the document to alter it. However, although replacement of a
photograph, and even all of the second level printing, by a
computer generated image overcomes many of the recognised
problems with the older methods of preparing security documents,
it also has certain disadvantages. The chief problem is that in
the printing techniques that have been used to date either a
special printer is required, or an existing printer is
significantly modified. In addition, a special printing
technique is often used which has properties that are
significantly different to the standard printing media, such as
conventional printing ink and laser printer toners. One example
of a special printing technique is the use of a laser which
literally burns information into the document. Another example
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is described by Waller, in US 5,890,742. In this process, the
second level printing is applied directly onto a silicone coated
surface of a transfer sheet. The printed release sheet is then
positioned adjacent an adhesive layer on a security laminate, and
transferred onto the adhesive layer. After removal of the
transfer sheet, the security laminate is combined with a data
receiving page carrying any desired first level printing. It can
thus be seen that this is a complex process, which actually forms
the second level printed image on the laminate, not the receiving
page.
There is therefore a need for a document preparation process
which both provides an acceptable level of security to the
finished document, thus rendering it resistant to tampering and
alteration, and which also only requires the use of standard
printing equipment and printing techniques. Further, there is a
need for such a process which additionally is able to print
computer generated images including both pictorial information
such as a photograph or fingerprint, and the more conventional
alphanumeric information. This will allow the issuing authority
to use current printing equipment meeting the standard
requirements for document production with no more than a minimal
level of adjustment.
Additionally, there is a need for a security document printing
process which can be used to print material directly into a pre-
prepared multi-page document, such as a passport, without using
either a specially designed printer or any other special
equipment. This need arises since conventional printing
equipment uses single sheet (or roll) feed, and cannot print into
a pre-prepared booklet, such as a passport. Although this
difficulty can, to some extent, be avoided when the document is
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initially prepared, it cannot be avoided when additional
information has to be added; a particular example is the addition
of a printed visa to a passport.
Summary of the Invention.
This invention seeks to provide a process whereby the
difficulties with the known process are effectively avoided. By
the use of a transfer printing process, a printing process is
provided which both uses conventional equipment, and which can be
used to add second level printing to an existing document,
especially a multi-page one such as a passport. With the process
of this invention it is therefore possible to assemble the
document, such as a passport, completely and to the highest
security standards before any second level printing is included
into it. Additionally, this printing process can readily be used
to apply second level printing to both sides of the same base
sheet. Further, in this process all of the printing equipment
and materials used, particularly in producing the second level
printed image, are standard commercially available equipment and
materials requiring no modification.
In the process of this invention a multi-step procedure is used.
Initially, all of the information to be applied in the second
level printing procedure to the base sheet is assembled as a
complete image, preferably using computer based imaging
technology. The information can then be reviewed for both
accuracy and completeness on a suitable screen. The image is
then printed in reverse, as its mirror image, onto the silicone
coated surface of a silicone release paper. The image can again
be checked for accuracy and completeness. A conventional laser
printer is used for this printing step. The release paper
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carrying the image and the base sheet are then placed into a
heated press, with the image adjacent the base sheet. By the use
of heat and pressure the image is transferred effectively
completely from the silicone release surface onto the base sheet.
After removal from the press, and allowing the document to cool,
the release paper is removed, leaving the second level printed
image on the base sheet.
Thus this invention seeks to provide a transfer printing process
for applying an image to a document base sheet including the
following steps:
(a) assembling the image;
(b) converting the image into its mirror image;
(c) printing the mirror image onto a silicone coated surface
of a release substrate by means of a laser printer or photocopier
to provide a stable toner particle copy of the mirror image;
(d) combining the release substrate and the base sheet, with
the toner particle copy of the mirror image in contact with the
base sheet;
(e) applying sufficient heat and pressure to the combined
release substrate and the base sheet to transfer the toner mirror
image onto the base sheet; and
(f) removing the release substrate to provide a base sheet
carrying the image.
Preferably, both image preparation and conversion in step (a)
into its mirror image are carried out within a computer imaging
system.
Preferably, a laser printer is used in step (b).
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Preferably, a twin platen single sheet press in used in step (d),
having one unheated platen, and one heated platen, and the base
sheet is located adjacent the unheated platen. More preferably,
a twin platen single sheet press is used in step (d), having one
unheated platen, and one platen constructed and arranged to be
both heated and cooled, and the base sheet is located adjacent
the unheated platen.
Conveniently, the base sheet comprises one sheet of an existing
document. Most conveniently, the base sheet comprises a sheet
within a passport or equivalent multi-page document.
Alternatively, the base sheet comprises the inside face of the
cover page of a multi-page document.
Conveniently, in order to obtain as constant an applied pressure
over the whole surface of the base sheet, a layer of deformable
material, such as an elastomeric layer or cardboard, is located
in the press between the base sheet and the unheated platen.
Detailed Description of the Invention.
In the following description, a passport is used as a typical
document to which the process of this invention can be applied.
However it is to be understood that the invention is not limited
to multi-page documents with a comparatively stiff cover, and can
be used with either single pages which are later assembled into
a multi-page document, or with single page documents.
Additionally, it is to be understood that the invention is not
limited to creating a second level printed image on one side only
of a sheet: it can be applied to both sides, by repeating the
procedure for the second side.
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In step (a) of the process, the information required to be in the
second level printed image is assembled. Although this can be
done by manual techniques, it is most convenient to prepare this
image using computer imaging techniques. These are well known
and do not require detailed description. The use of a computer
imaging technique allows for the inclusion of both alphanumeric
and pictorial information in the image. It is also possible to
include other standard information within the image, such as, for
example, the identity of the issuing agency.
In step (b) the assembled image is converted into its mirror
image for printing in step (c). This conversion can be either
left-to-right, or top-to-bottom. The conversion is required to
provide an image which when reversed again in the transfer
process of step (e) will result in the same image being
reproduced on the base sheet. If a reversed final printed image
is required, step (b) can be omitted.
In step (c) the reversed image is printed with either a
photocopier if the image was assembled manually, or with a laser
printer if a computer imaging technique is used. Preferably, a
high resolution laser printer is used; the choice will depend on
whether the image is in monochrome (also known as black and
white), or is coloured. The following printers have been found to
be suitable: for colour, Tektronix Phaser 560(trade mark) and
Tektronix Phaser 740(trade mark), and for monochrome, Hewlett-
Packard Laserjet-4M(trade mark) and Hewlett-Packard Laserjet-
6P(trade mark).
The prepared and reversed image is printed onto the silicone
coated surface of a release substrate, which is usually a paper
material. The release paper desirably has as smooth a surface as
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possible, in order to obtain as good an image as possible for
later transfer. A suitable release paper has been found to be
the commercial product known as MACTAC (trade mark) Starliner
(trade mark). This material is commonly used as a carrier for
self adhesive labels; other similar products are available.
If desired, the reversed printed image can also include
registration marks for image alignment in the next step.
In step (d) the reversed printed image from step (c) is
transferred onto the base sheet intended to receive it. The
printed release sheet is entered between the two platens of a
transfer press with the image correctly aligned into position,
together with the base sheet onto which the image is to be
transferred. In the press, one platen, usually the fixed lower
one, is heated, and the other platen, usually the upper moveable
one, is not heated. The release paper and the base sheet are
arranged so that the toner image on the silicone release surface
is in contact with the base sheet, and with the release paper
adjacent the heated platen. It is highly desirable that the
press platens be rugged enough to provide an effectively constant
applied pressure over the entire surface of the release paper and
the base sheet between which image transfer takes place in this
step, bearing in mind that the pressures used in this step are
relatively high, in the range of 400-500kPa. It is also
desirable to provide a layer of deformable material between the
base sheet and the unheated platen, as this aids in obtaining
good contact and a constant applied pressure between the release
sheet surface and the base sheet over the surface of the base
sheet, especially when the base sheet is one page of a passport
or similar booklet. Such a layer will also serve to provide
constant applied pressure to a base sheet having a level of
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surface irregularity, such as one including watermarks or raised
printing. Where the base sheet is part of a booklet such as a
passport it is preferred that the booklet be opened out, so that
only the base sheet enters between the press platens. The choice
of deformable material is quite wide. As the pressures involved
are quite high, any material which will deform to some extent and
thereby even out the applied pressure will suffice. In practise,
both a thin elastomeric layer, and a layer of cardboard have been
found satisfactory. Other suitable materials can easily be
identified by simple experimentation.
The temperature of the heated platen depends on a number of
factors.
In both a laser printer and a photocopier, the toner image is
fused onto the paper by the application of heat. For the process
of this invention, the same toner particles are subjected to two
heating steps, in the initial printing in step (c) onto the
silicone coated release surface of the release paper, and in the
transfer in step (d) onto the surface of the base sheet. It is
therefore necessary that the printing conditions in step
(c)provide a stable image on the silicone surface of the release
paper which is stable enough to be handled, but which also
retains some latent fusing capability, so that in step (d) the
toner particles transfer onto and fuse to the surface of the base
sheet, to provide the desired fully fused toner image. It has
been found that with most currently available toner materials,
for both monochrome and colour printers, the wax component in the
toner particles which provides fusing capability can be heated
and cooled satisfactorily at least twice. If a toner is used
which does not include a fusible wax component, some adjustment
of the heaters in the printers may be found to be necessary. If
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the image on the base sheet is not fully fused onto the base
sheet by the combination of time, temperature and pressure used
in the press, image damage may result when the release paper and
the base sheet are separated. Some experimentation may therefore
be required to obtain the best temperature combinations between
the laser printer and the press platens. It should also be noted
that if the press temperature is too high, image damage can also
happen. With the printers mentioned above, a platen temperature
of about 95 C appears to be sufficient for monochrome images, and
of about 105-110 C appears to be sufficient for coloured images.
The residence time in the heated press depends to some extent on
the material used as the base sheet to receive the second level
printing. A single sheet, such as an internal page of a
passport, will typically require about 5 seconds. A thicker
page, such as the inside face of the outside cover of a passport,
will typically require about 10 seconds at the temperatures
mentioned previously.
In step (f) the still warm document is removed from between the
press platens, with the release paper adhering to the base sheet.
After allowing enough time for the base sheet and release paper
to cool to a temperature at which it an be handled, which is
usually quite short, the release paper is peeled away from the
base sheet, thus providing a visible image on the base sheet
which is now the correct way around, as a second mirror image
reversal takes place in the transfer step.
In a modification of this step, a press is used in which one
platen can be both heated and cooled, so that the document can be
both heated and cooled, for example at least to about 70 C,
before removal from the press. It appears that cooling the
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combination of base sheet and silicone coated release paper at
least somewhat in contact under pressure improves the quality of
the transferred image. It is also then possible to include a
suitable temperature sensing device in the press, so that it
opens automatically when a desired preset temperature has been
reached.
In the preceding description a two platen press is used. It has
been found that although a twin roller press could be used, its
use is not recommended. Although the required temperature and
pressure can be obtained, it appears that without the flattening
effect of the press platens adequate image transfer does not take
place. It is observed that with a roll press, the release paper
and the base sheet tend to curl away from each other before
adequate toner transfer and adhesion to the base sheet has
occurred. Consequently, adequate transfer of the toner image from
the release paper onto the base sheet is not always obtained.
If desired, after completing the printing process, the face of
the base sheet carrying the second level printing, or both faces
of the base sheet, can be protected with a security laminate,
using conventional laminating procedures and materials. Further,
if desired, the printing process can be repeated to apply another
image to the other side of the base sheet before the lamination
is applied to the base sheet.
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