The present invention relates generally to thermal transfer printing,
sometimes referred to as sublimation, thermostatic, or dye diffusion printing.
More particularly, the present invention relates to thermally transferring printing
onto a metal substrate such as, for example, thermally transferring a colored image
onto a cigarette lighter.
In the dye diffusion printing process, color images are preprinted
onto a transfer media in reverse using thermal transfer inks. The conventional
process is illustrated in Fig. 1. The substrate 12 to be printed is placed on a
block 16 of resilient heat resistant material. The printed media 10, commonly
referred to as a transfer, is placed on the substrate 12 and subjected to heat
and pressure, as illustrated at 14, by means of rigid heated platen 18. The transfer
10 is thus sandwiched between the platen 18 and the substrate 12 for thermal transfer
of the printing from the transfer 10 onto the substrate 12. When the transition
temperature of the printed dyes is reached, the dyes either gasify or liquify and
migrate from the transfer 10 to the substrate 12 being printed and are absorbed
into the substrate surface.
The dye diffusion printing process is commonly employed in the printing
of wearables by garment manufacturers and specialty shops. Unlike other surface
printing methods, the suitability of substrates for dye diffusion printing is limited
by their absorption characteristics and receptivity to the dyes being used. As
a result, the process historically has been limited to the transfer of printing
When the transfer is sandwiched between the platen 18 and substrate
12 for dye diffusion printing, as illustrated in Fig. 1, the transfer paper 10
undesirably acts as an insulative thermal barrier between the heat source (platen)
and the part (substrate) being printed. See U.S. patents 5,260,127 to
Umise et al and 5,322,751 to Chou et al, which are hereby incorporated
herein by reference, relative to this thermal barrier effect.
U.S. patent 5,336,658 to Edwards, which is hereby incorporated
herein by reference, discloses a thermal transfer printing receiver comprising
a substrate containing particulate metal salts or metal oxides, a protective polymeric
interlayer of an acidic polymer composition, and a receiver coat of dye-receptive
polymer containing a crosslinked silicone release system and having a softening
temperature below the temperature used during printing. The purpose of the interlayer
is stated to be to alleviate problems with the effectiveness of the release system
when the substrate contains particulate metal oxides or metal salts. None of the
disclosed substrates are metallic. In one example, a substrate has a white coating
of rutile titanium dioxide dispersed in a polyester urethane binder.
Since the dyes in the Edwards process penetrate the pigment-containing
coating, any amount of pigment therein would be in competition with the dyes.
As a result, color density may be sacrificed and results washed out.
The polyester urethane binder of Edwards would require a solvent
system so that it could be applied in a liquid form. Solvents compatible with polyester
urethane resins are generally considered to be very aggressive and hazardous.
Other art which may be of interest are U.S. patents 5,096,877; 5,262,231;
and 5,314,862. The latter patent discloses a transparent layer and a white opacifying
layer laminated on a thermal transfer sheet.
The absorption characteristics and receptivity to dyes of metals
are such that they must first be coated with typically a transparent coating before
they can be dye diffusion printed. The prints on such coated metals have undesirably
been metallic in appearance and with limited color density and definition, and
the full color range has been limited.
It is accordingly an object of the present invention to provide a
high quality thermal transfer of printing onto a metallic object without resulting
in the metallic appearance to the printing.
It is another object of the present invention to produce high resolution,
high density, full color prints on metallic objects.
It is a further object of the present invention to eliminate or reduce
the use of hazardous solvents during such a process.
It is yet another object of the present invention to provide such
In order to provide a high quality printed metal object without the
metallic appearance to the printing, in accordance with the present invention,
a white coating then a transparent coating is applied to the metal object after
which the printing is thermally transferred onto the object. The white coating
as well as the transparent coating are applied preferably electrostatically whereby
a hazardous solvent is not required. Heat is transferred from the heat source through
the metallic object to the transfer media whereby the metallic object may reach
a suitable temperature before heat is supplied to the transfer sheet for effective
and consistent results.
The above and other objects, features, and advantages of the present
invention may be found in the following detailed description of the preferred embodiments
thereof when read in conjunction with the accompanying drawings wherein the same
reference numerals denote the same or similar parts throughout the several views.
In the Drawings:
Fig. 1 is a schematic sectional view illustrating a thermal transfer
printing process in accordance with the prior art.
Fig. 2 is a schematic sectional view illustrating a thermal transfer
printing process in accordance with the present invention.
Fig. 3 is a schematic enlarged partial sectional view of the metallic
substrate to be printed and of the transfer media.
Detailed Description of the Preferred Embodiments
Referring to Figs. 2 and 3, there is illustrated at 20 a metal substrate
which is being dye diffusion printed in accordance with the present invention.
The printed image, illustrated schematically at 22, is contained in reverse on
a surface 24 of a transfer media 26 which surface 24 is brought into contact with
the rigid substrate surface 28 to receive the printing 22. By "printing" is meant
to not only include letters and numbers and other symbols but also other images
such as provided by pictures. The substrate 20 and transfer 26 are placed between
a rigid heated platen 30 and a resilient heat resistant block 32 and heat and pressure
suitably applied, as illustrated at 34, for transferring the printing 22 onto the
substrate 20. When the transition temperature of the printed dyes is reached,
the dyes gasify (or may be such as to liquify) and migrate from the transfer 26
to the substrate surface 28 to be printed for absorption into the substrate surface
The printability of substrates by the use of dye diffusion printing
is limited by their absorption characteristics and receptivity to the dyes being
used. This has historically resulted in the dye diffusion printing process being
limited generally to the printing of cloth substrates. The application of transparent
coatings to metals to allow absorption and receptivity of the dyes has still undesirably
resulted in the prints on the metals being metallic in appearance with limited
color density and definition and limited full color range. Referring to Fig. 3,
in order to provide a higher quality printing onto the metal substrate 20 without
the metallic appearance, in accordance with the present invention, a white opacifying
base coat, illustrated at 40, is first applied to the substrate surface 28 to provide
maximum adhesion and coverage over the metal, the white coating providing opaqueness
to eliminate the metallic appearance and allowing a full range of color to be
printed thereon with the coating serving as the color white. The pigment for the
white base coat 40 may be titanium dioxide or other suitable pigment. A transparent
finish coat, illustrated at 42, is then applied over the white coat 40 to provide
maximum receptivity to the thermal transfer dyes with minimum color loss and bleed.
The transition temperature of the finish coat 42 is preferably close
to the gasification temperature of the dyes in the transfer 26 so that the coatings
may be softened during the transfer printing process to promote transfer and improve
color density. For example, the finish coat 42 may be a polyester resin having
a transition temperature of about 395°F which is close to the typical gasification
temperature of about 400°F of the dyes.
A polyester urethane binder for the finish coat would require a solvent
system so that it could be applied in a liquid form. The solvents typically compatible
with polyester urethane resins are considered to be very aggressive and hazardous.
In order to eliminate the safety and environmental problems inherent in applying
conventional solvent based polyester urethane coatings as well as other solvent
based coatings, in accordance with the present invention, the coating 42 as well
as coating 40 is preferably electrostatically applied in accordance with principles
commonly known to those of ordinary skill in the art to which this invention pertains,
the metal substrate 20 being electrically conductive to allow such electrostatic
application. The transparent coating 42 is preferably a transparent polyester
resin which has been ground into a powder. The electrostatic application of the
white base coating 40 has the added advantage of providing an even application
over the substrate surface 28 so that a higher quality printed image may be achieved.
The white base coating 40 may alternately, however, be applied as a water-based
It should be noted from Fig. 1 that the conventional dye diffusion
printing process shown therein requires the thermal conductive path of the heat
in the rigid heated platen to be through the transfer media then into the substrate
being printed. This heating of the transfer sheet from the backside results in
the paper of the transfer sheet disadvantageously acting as an insulative thermal
barrier between the heat source and the part being printed. In addition, the rigid
platen will not conform to an irregular substrate surface thereby inhibiting heat
transfer and transfer of the printing. Referring to Fig. 2, in order that the
metal substrate 20 may advantageously reach a suitable temperature before heat
is supplied to the transfer sheet 26, in accordance with a preferred embodiment
of the present invention, the heat source 30 is applied directly to the metal substrate
20, i.e., the substrate 20 is positioned between the heat source 30 and the transfer
sheet 26 so that heat is transferred from the heat source 30 through the substrate
20 to the transfer media 26. This also advantageously positions the resilient heat
resistant block 32 next to the transfer 26 so that it conforms to an irregular
substrate surface 28 for improved heat transfer and transfer of the printing.
The use of the conventional silk screen process for providing the
printing directly on the substrate using wet inks (no transfer sheet being involved)
requires a screen for each color with the screens requiring registration which
is difficult and results in rejects if the screens are not adequately registered.
Such a process also does not allow shades or halftones and does not provide images
which appear photographic. In order to achieve improved color densities and line
screen definitions while eliminating such registration problems, in accordance
with a preferred embodiment of the present invention, the transfer 26 is printed
using four-color lithographic offset printing, which is a process commonly known
to those of ordinary skill in the art to which this invention pertains. In this
process, the image to be applied is scanned and then broken down into color dots
through computerization. A printing plate is then generated for each of the three
primary colors and the color black. Each color is then printed in turn at respective
stations. The printing plates are held captive so that there are no registration
problems. The printing inks used may be the same as commercially available inks
conventionally used for dye diffusion printing onto fabric. For example, such inks
may be sublimation inks commercially available from Superior Printing Inks, Inc.
of Cheektowaga, New York. The carrier media 26 is suitably a conventional paper
material which maximizes the yield and eliminates sticking to the coated substrate
surface 28. The resulting high receptivity of the coating and maximum yield characteristics
of the carrier media are desired to allow transfers to be printed at improved color
densities and line screen definitions.
In order to provide a finer grain so that a clearer and sharper image
may be printed, in accordance with an alternative embodiment of the present invention,
the transfer sheet 26 is printed by use of a process commonly known as a "continuous
tone" process which prints dashes rather than dots. Such a process is commonly
known to those of ordinary skill in the art to which this invention pertains. A
full color continuous tone printer is commercially available from Eastman Kodak
Company of Rochester, New York and identified as 1525+ copier/printer. Such a printer
preferably includes digital imaging so that the printing cost may be reduced.
It should be understood that, while the invention has been described
in detail herein, the invention can be embodied otherwise without departing from
the principles thereof, and such other embodiments are indeed meant to come within
the scope of the present invention as defined in the appended claims.
According to a second aspect of the present invention there is provided
an article of manufacture comprising a metal substrate having a surface, a white
coating on the substrate surface, a transparent coating over the white coating
and printing on the coated substrate surface wherein the printing has been applied
in accordance with the method of the above first aspect.
It will be understood that the metal substrate may be any suitable
substrate including knives, jewellery, boxes and other containers of metal car
body panels, body panels for scooters, motor bikes, mud guards, bicycle frames,
handle bars, pedals, metallic accessories, cigar cutters, cigarette boxes, cigarette
lighters, smokers' accessories, key rings, badges, bottle openers, hand tools,
culinary utensils, metal table wear, metal cook wear, small domestic utensils and
containers made of metal, sporting articles made of metal, and the like.