CA2223916A1 - A method for the transfer of a toner image in powder form from a transfer member to a substrate - Google Patents

Abstract

The toner image (500) is heated to a temperature sufficient to reduce the viscosity thereof to less than 50 Pa s. The transfer member (594) carrying the toner image (500) is brought into contact with the substrate (541). The transfer member (594) is forcibly cooled to a temperature below the glass transition temperature Tg of the toner while the transfer member (594) remains in contact with the substrate (541). Thereafter the transfer member (594) is separated from the substrate (541).

Description

CA 02223916 1997-12-0~

A METHOD FOR THE TRANSFER OF A TONER IMAGE IN
POWDER FORM FROM A TRANSFER I t. t~t-:tt TO A S~BSTRATE

This invention is concerned with the transfer of a toner image in powder form from a transfer member to a substrate, and in particular with the fixing of toner images on a substrate in an electrostatographic printer or copier.
More particularly, it is concerned with the fixing of powder toner images obtained via electrophotographic, electLoyLd~hic~ ionographic or magnetic recording processes in which an electrostatic or magnetographic latent image is rendered visible by the deposition of a suitable toner composition on the latent image.

In present day copying machinPs or electronic printer devices, the fixing or permanent adherence of a toner image on a substrate in the form of a sheet is often carried out with a so-called hot roller or nip-roller fixing system.
This image-fixing device comprises a pair of rollers through the nip of which a sheet carrying a toner image is fed.
The surface of the roller contacting the toner image is heated above the softening temperature of the toner resin, which becomes tacky or molten and forms a per~rtnpnt bond with the surface of the sheet. The roller, contacting the toner image, is provided with a coating having abhesive (i.e. non-adhesive~ properties versus the toner image. In CA 02223916 1997-12-0~

order to increase the non-adhesive characteristics of the roller surface, use is often made of silicone oil. The rollers forming the image-fixing unit are pressed against each other. The roller contacting the backside of the sheet is generally covered with a silicone elastomer, capable of resisting the heat generated by the image-fixing roller.

Problems arise with hot roller image-fixing devices. In particular, in heavy duty printers where long periods between servicing are usual, it is difficult to maintain a constant image-fixing quality and a long roller lifetime.

A technique known as ~flash-fixing~ i8 also known in which a short intense burst of radiant energy is applied to the substrate carrying the toner image to be fixed. The wavelength of the radiant energy is chosen to be absorbed by the toner. Such a technique is unsuitable for multi-colour images, where toners of different composition are carried on the substrate, said toners having different absorption characteristics in the visible spectrum.

A number of constructions of image fixing devices using infra-red radiant fixing have been proposed in the art, including United States patent US 3449546 (Dhoble / Xerox Corporation) and United States patent US 5526108 (Billet et al / Xeikon NV).

CA 02223916 1997-12-0~

Infra-red fix1ng devices however cause a loss of moisture from the substrate, as a result of the high temperature to which the substrate is heated, i.e. the substrate becomes too dry. This loss of moisture can result in deformation of the substrate and the low moisture level can result in the generation and retention of electrostatic charges on the substrate, both of which effects can produce problems in subsequent handling of the substrate.

A number of proposals have been made for fixing toner images by the use of a belt. These include United States patents US 3948215 (Namiki / Ricoh Co., Ltd) and US 5483331 (Wayman et al. / Xerox Corporation). Other propos~ls concern; ng image fixing include European patent application EP 295901 (Canon KK) and United States patent US 4780742 (Canon KK).

Due to the fact that dry toner images have a high thickness (sometimes more than 10 ~m), the appearance of such images is sometimes unnatural and non-uniform and these images u~ually have a non-uniform colour saturation. While this appearance is acceptable for many applications, it is sometimes desired to provide an image having a different appearance or finish. By the term ~finish~ in the cont~t of the present invention, we mean either a surface characteristic which is glossy, i.e. highly reflective, and/or which provides high saturation of colours, this usually being achieved by reducing the scattering of light from the surface of the printed article, or both such -characteristics. For example, a glossy appearance is especially desirable where the receiving material itself has a glossy surface. A higher degree of colour saturation can be very desirable in high quality print work.

It has been proposed to provide glossy images by the use of a toner which incorporates a glossing agent, or by the application of a transparent glossing layer over the toner image. However, these methods are costly in terms of consumables. Examples include United States patents US
5521688 (Moser / Xerox Corporation), US 5319429 (Fukuchi et al. / Konica Corporation), US 5099288 (T~ -rk Tntern~tional~ and United States patent US 5258256 (Eastman Kodak Comr~ny).

It would be desirable to use one and the same device to fix the toner images and to provide them with the desired gloss.
However, contact-less fixing devices are unable to provide a uniform glossing effect, while we have found that the use of known heated rollers or heated belt fixing devices suffer from toner offset problems and do not provide sufficient control over the gloss and colour saturation of the images.
In particular such known devices exhibit limited process parameters, with a narrow window of optimum performance.

OBJECTS OF THE INVENTION
It is an object of the present invention to provide a device and method whereby toner images in powder form can be CA 02223916 1997-12-0~

transferred to a substrate and provided with a desirable level of gloss in one single device, while widening the range of operating conditions without risk of offset occurring.

SUMMARY OF THE INVENTION
According to the present invention there is provided a method for the transfer of a toner image in powder form from a transfer member to a substrate, comprising:
heating the toner image to a temperature sufficient to reduce the viscosity thereof to less than 50 Pa s;
bringing the transfer member carrying the toner image into contact with the substrate;
forcibly cooling the transfer member to a temperature below the glass transition temperature Tg of the toner while the tran~fer member remA; nR in contact with the substrate; and thereafter separating the transfer member from the substrate.

Dry-development toners essentially comprise a thermoplastic bin~r consisting of a thermoplastic resin or mixture of resins including colouring matter, e.g. cArhon black or colouring material such as finely dispersed pigments or dyes.

The mean diameter of dry toner particles for use in magnetic brush development is conventionally about 10 ~m (ref.
IlPrinciples of Non Impact Printing~ by Jerome L. Johnson -CA 02223916 1997-12-0~

Palatino Press Irvine CA, 92715 U.S.A. (1986), p. 64-85).
For high resolution development, the mean diameter may be from 1 to 5 ~m (see e.g. British patent specification GB-A-2180948 and International patent specification WO-A-91/00548).

The toner particles contain in the resinous binder one or more colorants (dissolved dye or dispersed pigment) which may be white or black or has a colour of the visible spectrum, not excluding however the presence of infra-red or ultra-violet absorbing substances.

The thermoplastic resinous binder may be formed of polyester, polyethylene, polystyrene and copolymers thereof, e.g. styrene-acrylic resin, styrene-butadiene resin, acrylate and methacrylate resins, polyvinyl chloride resin, vinyl acetate resin, copoly(vinyl chloride-vinyl acetate) resin, copoly(vinyl chloride-vinyl acetate-maleic acid) resin, vinyl butyral resins, polyvinyl alcohol resins, polyurethane resins, polyimide resins, polyamide resins and polyester resins. Polyester resins are preferred for providing high gloss and improved abrasion resistance. Such resins usually have a glass transition point Tg of more than 54~C with a melt viscosity of at least 50 Pas up to no more than 1500 Pas. The presence of other ingredients in the toner particles, such as the colorant, usually have no significant effect upon the glass transition temperature.
The volume resistivity of the resins is preferably at least 1013 Q-cm.

Suitable toner compositions are described in European patent applications EP-A-601235, and EP-A-628883 and International patent applications WO 94/27192, 94/27191 and 94/29770 (all Agfa-Gevaert NV). The softening points of most common toner compositions are similar at about 60~C. Typical fixing temperatures are therefore about 120~C, deppn~;ng inter alia on the nature of the substrate and the pressure applied.

However, according to the present invention, the surface of the toner image should contact the substrate at a temperature above the fluid temperature of the toner, so as to ensure complete transfer of the toner image to the substrate and the fixing of the image on the substrate. The fluid temperature is the temperature at which the viscosity of the toner falls below 50 Pa s, such as from 10 Pa s to 40 Pa s. This temperature to which the multiple toner image is heated is above the glass transition temperature of the toner but below the degradation temperature thereof, that is below the temperature at which irreversible changes occur in the toner composition le~;ng to a significant change in its spectral properties. The fluid temperature is typically above 150~C, even above 200~C, depPn~;ng upon the composition of the toner. Viscosity is typically measured by the use of a cup viscometer (Ford cup, Shell cup or Z~
cup). ASTM D-1200 is an accepted standard for the measurement of viscosities of printing inks. Laray and CA 02223916 1997-12-Oj Churchill falling rod viscometers may also be used.

Where the substrate carries a number of different toners, as for example in the case of multi-colour images, the temperature should be raised to above the lowest fluid temperature of the toners present, most preferably above the highest fluid temperature of the toners present, so as to ensure mixing of the toner particles of different colours.

We prefer to use toners having a composition comprising a thermoplastic binder and from 10% to 50~ by weight, based on the weight of the toner composition, of a pigment. We also prefer that the toner composition in powder form has a weight average particle size of between 0.5 ~m and 5 ~m, preferably between 1 ~m and 4 ~m. The use of toner compositions having a higher level of pigment therein enables images with a higher density to be printed.
Alternatively, for the same image density, smaller toner particles can then be used. The use of smaller toner particles has the advantage that the height of the toner image above the surface of the substrate is lower. The advantages of a lower toner image height include (a) irregularities in the surface of the substrate have less of an effect upon the gloss of the image, (b) the total usage of toner is reduced - this is important because the cost of the toner may be significant in the total cost of the printed product, (c) the t~n~ncy of the printed page to curl is reduced, (d) the stacking of printed pages, for CA 02223916 1997-12-0~

example in the preparation of a book, is more even, and (e) there is a flatter feel to the printed page, a characteristic which is of advantage to some users.

The method according to the invention may be used for transferring an unfixed toner particle image from a transfer member to a substrate and fixing the image on the substrate, the method comprising feeding the substrate through a contact zone which extends continuously from an entrance to an exit thereof and is defined by the transfer member and a reaction surface in face-to-face pressure contact with the transfer member, heating the transfer member adjacent the entrance to a temperature sufficient to reduce the viscosity thereof to less than 50 Pa s, forcibly cooling the transfer member intermediate the entrance and the exit to a temperature below the softening point Tg of the toner, and applying pressure between the transfer member and the reaction surface intermediate the entrance and exit, thereby to fix the toner image on the substrate.

While not wishing to be bound by theory, we believe that, where toner images are fixed on a substrate by means of a heated surface such as a roller or heated belt, there is a risk of molten toner becoming transferred to the heated surface as the substrate separates therefrom, to be subsequently deposited on a following section of substrate, resulting in the phPnorPno~ of ~ghost images". Even if the characteristics of the heated surface are so chosen as to reduce the risk of such ~hot-offset~, the separation of the heated surface from the substrate tends to distort the toner particles into a somewhat non-flat shape, le~;ng to low gloss and colour saturation. Forcibly cooling the substrate on the other hand, while pressure is applied thereto, tends to flatten the toner particles, leading to an increase in colour saturation or alternatively enabling the quantity of toner used during printing to be reduced by, for example, 20~ to 30~. Thus, it is essential according to the invention to cool the transfer member to a temperature below the softening point Tg of the toner while the transfer member is in pressure contact with the reaction surface.
There is therefore a temperature gradient within the cnnt~ct zone, from a temperature above the fluid temperature of the toner adjacent the entrance of the contact zone to a temperature below the softening point Tg of the toner before the exit from the zone.

While not wishing to be bound by theory, it is our underst~n~ing that it is generally preferred to transfer toner images from a material of relatively low surface energy to one of relatively high surface energy. This reduces the possibility of toner particles shearing during transfer which reduces the efficiency of the transfer process and leaves residual toner on the donor surface.
Ideally therefore, the surface energy of the donor surface should be lower than that of the receiving surface. This can be achieved for the transfer of the image from the CA 02223916 1997-12-0~

transfer member to the substrate, since the surface energy of the substrate, such as paper, is generally more than 45 dyne/cm. The transfer process is more efficient when the donor surface is at a higher temperature than the receiving surface. Thus the present invention requires heating of the toner image on the transfer member so as to m~;m; se the efficiency of the transfer to the substrate.

The transfer member will usually be the surface of an endless member, such as an endless belt. While the transfer member may be the surface of a belt, it is also possible for the transfer member to be constituted by the surface of a drum. As used in the following general description, the term ~'belt~ is intended to embrace other forms of transfer member, such as a drum, except where the context ~- -n~
otherwise.

The transfer member may comprise an outer surface formed of a material having a low surface energy, for example silicone elastomer (surface energy typically 20 dyne/cm~, polytetrafluoroethylene, polyfluoralkylene and other fluorinated polymers. The transfer memher is preferably in a form having a low mass, so that the surface thereof can be easily heated prior to the transfer of the multiple toner image to the substrate and easily cooled after transfer cooled before the transfer thereto of a further multiple toner image from the primary belt. For this reason, while the transfer member can be in the form of a transfer roller CA 022239l6 l997-l2-0 or drum, it is preferably in the form of a transfer belt, for example an endless metal belt of 40 ~m thickness coated with 40 ~m thickness silicone rubber.

For the production of glossy images, it is advisable that the surface of the transfer member be as flat as possible.

In order to reduce energy loss to the enviL~ SSt, we prefer that the means for heating the toner image on the tr_nsfer member is in heat exchange relationship with the means for cooling the transfer member after transfer. For example, the means for heating the multiple toner image on the transfer m~m~r comprises a pre-heating roller and the means for cooling the transfer member comprises a pre-cooling roller, the pre-heating roller and the pre-cooling roller being in heat exchange relationship with each other. This heat ~XchAnge relationship can be achieved for example by each of the heating and cooling rollers being hollow rollers ts~srough which a heat ~xchA~ge fluid, such as water, is caused to flow. In this way heat extracted by the cooling roller is transferred to the heating roller and contributes to the heating of the toner image on the transfer member.

The heating means may comprise a heating surface in contact with the belt, such as a roller, or a heated stationary body over which the belt passes. Heating may be achieved, for example, by passing a heating fluid (e.g. steam or hot oil) at an elevated temperature through the roller or stationary CA 02223916 1997-12-0~

body, or by the provision of radiant heating means positioned within the roller or stationary body. It is also possible to use radiant heating means for directly heating the belt, and this may be especially beneficial where the belt is formed primarily of heat non-conductive material.
Generally, the belt will be heated from the side thereof opposite to its contact with the reaction surface and the substrate. Generally, the belt contacts the substrate with a dry surface, i.e. there is no need to apply a liquid release agent to the belt surface.

In place of, or in addition to, the intermediate pressure, the transfer member may be heated adjacent the exit to a temperature above the softening point Tg of the toner. The advantage of this second heating is to raise the temperature of the flattened surface of the toner, thereby lowering its surface energy. This eases the release of the toner from the belt, without raising the temperature of the bulk of the toner so much that the toner loses its flatness as it separates from the belt or even breaks down leaving toner deposited on the belt. The second heating means may be constructed in a similar mAnn~r to the heating means at the entrance to the contact zone, for example as a second heated roller over which the belt passes. Where second heating means in the form of a second heated roller is provided adjacent the exit of the contact zone, it is preferable to arrange the geometry such that the belt wraps partially around the second heated roller within the contact zone, to CA 022239l6 l997-l2-0 enhance the heating effect thereof.

Preferably both the intermediate pressure and the second heating are used together to gain mA~imllm advantage from the invention.

The cooling means may comprise a cooling surface in contact with the belt, such as a cooling roller over which the belt passes. Cooling may be achieved, for example, by passing a cooling fluid (e.g. water at room temperature or reduced temperature) through the roller or stationary body. It is also possible to direct cold or cooled air directly at the belt. Generally, the belt will be cooled from the side thereof opposite to its contact with the reaction surface and the substrate.

The heat extracted from the belt by the cooling means may be used to pre-heat the belt on its return run, in advance of the heating which takes place at the entrance to the contact zone. Thus, the cooling means may be constituted by the cold region of a heat pump, the hot region of which is in contact with the belt on its return run. Alternatively, heat extracted from the belt by the cooling means may be used to pre-heat the substrate.

The belt may comprise a heat conductive hAck;ng carrying a coating of non-adhesive material, preferably a silicone rubber. In any event, the belt should have a low thermal CA 02223916 1997-12-0~

capacity, to ensure the rapid heating and cooling thereof.
Such rapid temperature changes enable the apparatus to be smaller in size than would otherwise be necessary. The belt should also be formed primarily of a heat conductive material, if heating from the "back-sideN thereof is to be used. A heat-conductive belt has the advantage of distributing a more even temperature, as "hot spots" are avoided. The belt, or at least the coating carried thereon, should be seamless, especially if substrates in web-form are to be used. The belt is preferably impermeable. The reaction surface is also preferably impermeable. The use of an impermeable belt and reaction surface leads to a particular advantage of the present invention. Although the substrate temperature rises in the contact zone, even to above 100~C, any moisture in the substrate cannot escape and cnn~nqes on the belt to be returned to the substrate by the second heating means. The disadvantages of open radiant fixing referred to above, resulting from the substrate becom-ng too dry, are therefore avoided.

The reaction surface may be constituted by a further belt.
In this embodiment, further heating means may be provided for heating the further belt adjacent the entrance to a temperature above the fluid temperature of the toner, and further cooling means may be provided for forcibly cooling the further belt intermediate the entrance and the exit to a temperature below the softening point Tg of the toner.
Alternatively, the reaction surface may be constituted by a surface of a stationary body, which may include means for cooling the stationary body.

The contact zone extends from the initial point of contact between the belt and its reaction surface to the point of separation between the belt and its reaction surface. It is important to maintain contact within the contact zone, although the pressure need not be constant throughout the zone. The pressure may be generated by virtue of the geometry of the belt and its reaction surface, but it is helpful to provide a pair of intermediate pressure rollers located one on either side of the contact zone, upstream of the cooling means. The pressure which is applied intermediate the entrance and exit of the cnntAct zone is preferably applied at the same region as, or immediately before, the region of application of the forced cooling. It is also preferred to apply pressure between the belt and the reaction surface adjacent the entrance to the contact zone.
Thus, in the contact zone at least two pressure points are realised, one adjacent the entrance and the other intermediate the entrance and the exit. We have found that an average contact pressure at the pressure points of between 2 to 20 N/cm2, such as from 5 to 10 N/cm2 is preferred, dep~n~;ng on the absorbency of the substrate, the temperature and the viscosity of the toner at that temperature.

Where the cooling means and the further cooling means are CA 02223916 1997-12-0~

both constituted by cooling rollers, these cooling rollers should be so positioned as to ensure more than tangential contact between each cooling roller and its associated belt.
By ensuring that each belt partially wraps around its as60ciated cooling roller, the forcible cooling effect is thereby obtained.

The substrate may be in the form of a web, but the invention is equally applicable to substrates in sheet form, the device then being provided with suitable sheet feeding means. The geometry of the device may be such as to define a substantially straight path for the substrate. This can be of advantage for heavier, especially thicker or less flexible, substrates.

The belt may be driven directly, for example by applying drive to a heating rolier at the entrance of the contact zone, to a second heating roller at the exit of the contact zone or to an intermediate pressure roller. It is important to arrange for the belt to be driven in synchronism with I ve..._nt of the substrate to prevent slippage which may distort the toner image. Alternatively, where the substrate i8 in the form of a web, the belt may be driven by III~V~_ t of the web itself, means being provided to compensate for the torque resistance of the belt(s). This aL ~y~_ -nt ensures that the substrate web and the belt(s) move in synchronism.

CA 022239l6 l997-l2-0 The belt may return from the exit of the contact zone to the entrance thereof via an adjustable tensioning and alignment roller. Where an intermediate pressure roller is in contact with the belt within the contact zone, this intermediate pressure roller may be in heat exchange relationship with the alignment roller, for example by way of a heat ~h~nge fluid passing through hollow interiors of both rollers. The energy requirements of the device can thereby be reduced.

The device according to the invention may be part of a printer, advantageously an electrostatographic printer, having at least one imaging station, where a latent image is formed upon a rotatable endless surface member such as an electrostatically chargeable photocnn~nctive drum or belt and an array of image-wise modulated light-emitting diodes is used as an exposure source. The latent image is then developed at a toner development station to form a toner image on the surface member. The toner image is transferred at a toner transfer station from the surface member onto a moving substrate, or onto a moving transfer member for later transfer to a substrate. The printer may also be equipped with cutting means in order to cut the printed web into sheets. The cutting means is preferably positioned downstream of the fixing device.

The development station uses a developer which cnnt~;nc toner particles cont~;nlng a mixture of a resin, a dye or pigment of the appropriate colour and normally a charge-. CA 02223916 1997-12-0~

controlling compound giving triboelectric charge to the toner. In dual-component developers which are normally used, carrier particles are also present for charging the toner particles by frictional contact therewith. The carrier particles may be made of a magnetizable material, such as iron or iron oxide. Developing technologies other than magnetic brush development, such as mono-component developers, can be used.

The printer may be a colour printer, cont~lnlng a plurality of imaging stations each asgociated with a development and transfer station and the image-fixing station is located downstream of the last toner transfer station before cutting the printed web. In one embodiment of such a colour printer, the development stations contain respectively cyan, magenta, yellow and optionally black toner particles.

The web of substrate may be fed through the printer from a roll. If desired, the substrate may be conditioned (i.e.
its moisture content adjusted to an optimum level for printing), prior to entering the printer.

The device according to the invention may also be part of an electrostatic copier, working on similar principles to those described above in connection with electrostatic printers.
In copiers however, it is common to expose the rotatable endless surface member by optical means, directly from the original image to be copied.

The invention also provides a method of single pass electrostatographic printing comprising:
- moving a transfer member along a continuous path;
- depositing a toner image in powder form onto the moving transfer member to form an unfixed toner particle image thereon;
- feeding substrate through a contact zone which extends continuously from an entrance to an exit thereof and is defined by the moving transfer member carrying the unfixed toner particle image and a reaction surface in face-to-face pressure contact with the moving transfer member whereby the toner image is transferred to the substrate;
- heating the transfer m~mh~r adjacent the entrance to a temperature sufficient to reduce the viscosity thereof to les6 than 50 Pa s (such as 10 to 40 Pa s); and - forcibly cooling the transfer member intermediate the entrance and the exit to a temperature below the softening point Tg of the toner.

The transfer member plays the role of transferring the multiple toner image to the substrate. It is not necessary therefore that the transfer member has a photocnn~nctive surface. Indeed, the need to heat and cool the transfer member in the apparatus according to the invention means that the use of conventional photocnn~nctor materials is to be avoided, since the photoconductive properties of such materials are sensitive to temperature changes.

CA 02223916 1997-12-0~

Where the printer is a multi-colour printer, the method will include depositing a plurality of toner images of different colours in powder form in register with each other onto the moving transfer member to form a multiple toner image thereon. A plurality of toner images of different colours may be electrostatically deposited onto the mo~ing transfer member to form a charged multiple toner image thereon. For example, the multiple toner image is firstly formed on another member and then deposited as such onto the transfer member.

Thus, the transfer m~mber is an intermediate transfer member and the means for forming a multiple toner image on the transfer member comprises:
- a primary transfer member;
- means for guiding the primary transfer member past a set of toner image producing stations whereby a plurality of toner images of different colours are formed on the primary transfer member in register with each other to form the multiple toner image on the primary transfer member, the intermediate transfer m~mber being in contact with the primary transfer member downstream of the image producing stations, whereby the multiple toner image i8 electrostatically transferred from the primary transfer member to be deposited on the cooled intermediate transfer member. In this embodiment, the primary transfer member i8 preferably constituted by a primary belt.

CA 022239l6 l997-l2-o~

In order not to disturb the multiple toner image on the transfer member between the deposition of the image thereon and the transfer of the image to the substrate, we prefer that the surface of the transfer member which carries the image is free of contact with any other member. Thereby, undesirable transfer of the image, or a part thereof, from the transfer member is avoided. Thus, where for example the transfer member is in the form of a belt, rollers or other guide means, contact the belt on the surface thereof opposite to that carrying the image, at least between the deposition of the image and its transfer to the substrate.

The invention wiil now be described in further detail, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a single pass, multi-colour duplex electrostatographic printer according to the invention, incorporating a simultaneous fixing and glossing device.

Figure 1 shows a single pass, multi-colour duplex electrostatographic printer 510. The printer comprises a first primary seamless belt 512 passing over guide rollers, including a guide roller 514. The primary belt 512 moves in a substantially vertical direction past a set of four toner image producing stations 518, 520, 522, 524. At the four toner image producing stations 518, 520, 522, 524, a plurality of toner images of different colours are transferred by transfer coronas (not shown) to the primary belt 512 in register with each other to form a first multiple toner image.

An intermediate transfer member in the form of an earthed seamless transfer belt 594, is in contact with the primary belt 512 downstream of the last image producing station 524.
The intermediate transfer belt is in the form of a metal band of 70 ~m thickness carrying a 2S ~m thickness silicone rubber coating. The intermediate transfer belt 594 passes over spaced guide rollers 526, 528, 536 and 542 which are so positioned as to bring the transfer belt 594 into contact with the toner image carrying belt 512 as it passes over its upper guide roller 514. The guide roller 542 acts as a first stage heating roller, being formed as a hollow roller through the hollow interior of which a heat transfer fluid such as water at an elevated temperature is passed. The hard metal guide roller 526 acts as a second stage heating roller, being formed for example with an internal r~ nt heater. The resilient guide roller 528 acts as a cooling roller, being formed with a hollow interior through which cooling fluid, such as water at a controlled temperature close to room temperature, passes.

The heated guide roller 526 is driven by a motor 27. Drive i8 transmitted in turn from the drive motor 27 to the guide roller 526, via the transfer belt 594 to the primary belt 512 downstream of the toner image producing stations and to the toner image producing statlons themselves. The guide roller 514 and the intermediate transfer belt 594 are positioned in opposition to each other to form a contact region therebetween, through which the primary belt 512 passes. Adherent contact between the primary belt and the intermediate transfer belt causes the primary belt and the intermediate transfer belt to move in synchronism with each other.

The multiple toner image 500 adhering to the surface of the primary belt 512 is transferred to the moving intermediate transfer belt 594 by a second function of guide roller 514 acting as an electrostatic transfer roller connected, for example, to -1000 V.

The printer shown in Figure 1 is adapted for duplex printing. To achieve this, the printer further comprises a second primary belt 540 which moves past a second set of four toner image producing stations 519, 521, 523, 525. At the four toner image producing stations 519, 521, 523, 525, a plurality of toner images of different colours are transferred to the primary belt in register with each other to form a second image. A second intermediate transfer belt 596 is in contact with the second primary belt 540 downstream of the last image producing station 525 of the second set.

The second intermediate transfer belt 596 paggeg over spaced CA 022239l6 l997-l2-0~

guide rollers including a first stage heating roller 544, a hard metal second stage heating roller 527, a resilient cooling roller 529 and a guide roller 538 which are so positioned as to bring the second transfer belt 596 into contact with the second toner image carrying belt 540 as it passes over its upper guide roller.

The first and second transfer belts 594 and 596 constitute reaction surfaces in face-to-face pressure contact with each other to form an extended contact zone therebetween, thereby to define a substrate path ext~n~;ng through the contact zone from an entrance 516 to an exit 517. Each transfer belt is impermeable, comprising a 70 ~m metal hAck;ng carrying a 30 ~m coating of non-adhesive silicone material such as DOW 200 Series (ex Dow Corning Corporation).

The first transfer belt 594 passes over, and is in contact with the hard metal heated roller 526 which directly heats the first transfer belt adjacent the entrance 516 to a temperature above the fluid temperature of the toner sufficient to reduce the viscosity of the toner to less than 50 Pa s. Similarly, the second transfer belt 596 passes over a heated roller 527 which directly heats the second transfer belt 596 adjacent the entrance 516 to a temperature above the fluid temperature of the toner.

Each transfer belt 594, 596 also passes over, and is in contact with, the respective resilient cooling roller 528, 529 which directly cools the respective transfer belt intermediate the entrance and the exit of the contact zone to a temperature below the glass transition temperature Tg of the toner. The cooling rollers 528, 529 are so positioned as to ensure more than tangential contact between each cooling roller 528, 529 and its associated transfer belt. Thus each transfer belt 594, 596 partially wraps around its associated cooling roller 528, 529 to increase the forcible cooling effect achieved thereby to a temperature below the glass transition temperature Tg of the toner.

Each transfer belt 594, 596 also passes over a respective s~con~ heated roller 530, 531 which heats the transfer belt adjacent the exit 517 of the contact zone to a temperature at least 10 C~ above the glass transition temperature Tg of the toner.

A pair of intermediate pressure rollers 532, 534 are located one on either side of the extended contact zone, upstream of the cooling rollers 528, 529, exerting a pressure of, say, 7.5 N/cm2.

The device shown in Figure 1 operates as follows. Substrate in the form of a paper web 541, unwound from a supply roll 430 is fed by a pair of downstream web drive rollers 366 along the substrate path between the first and second transfer belts 594, 596 from the entrance 516 to the exit 517 of the extended contact zone. Tension in the web 541 is controlled by the application of a brake (not shown). The substrate is fed at a speed such as to spend from 2 to 10 seconds in the contact zone. In a typical embodiment, the first-stage heating rollers 542, 544 raise the temperature of the multi-colour toner image on the transfer belts 594, 596 to about 9ooc. The transfer belts 594, 596 are then heated by the heating rollers 526, 527 adjacent the entrance 516 to 160~C, which is above the fluid temperature of the toner to reduce the viscosity thereof to between 10 Pa s and 40 Pa s. This is the optimum temperature for final transfer to the paper web 541. The transfer belts 594, 596 are forcibly cooled by the cooling rollers 528, 529 intermediate the entrance and the exit to 50~C, which is below the glass transition temperature Tg of the toner. The toner images on the transfer belts 594, 596 are transferred to the substrate and become fixed to the substrate, and their appearance is rendered glossy, with high colour saturation. The second heated rollers 530, 531 heat the belts to 70~C, to ease the release of the toner from the belts. Following transfer of the images to the substrate 541 the cooling rollers 536, 538 reduce the temperature of the transfer belts to about 20~C, ideal for electrostatic transfer of a further image onto the transfer belts.

Downstream of the drive roller pair 366, the paper web passes to a cutting station 466 where the web is cut into sheets.

CA 02223916 1997-12-o~

The present invention provides a number of advantages compared with known devices:
(i) the consumption of toner powder may be reduced;
(ii) the moisture content of the substrate is retained;
(iii) where the substrate is a transparent material, such as an over-head projector sheet, the contrast of the image is improved;
(iv) gloss can be deeper than can be achieved with known devices, because the first roller can be very hot;
(v) there are no additional consumables; and (vi) better coverage of the substrate by the toner particles leads to the possibility of a greater range of hues obtAinAhle from combinations of toners of different colours, 8ince the colour of the substrate itself plays a les6 important role to the spectral character of the image.

CA 02223916 1997-12-0~

REFERENCE NUMBER LIST

printer 510 2nd heated roller S30, 531 1st primary belt 512 p-.8~u-e rollers 532, 534 electrostatic transfer roller 514 paper web 541 toner image producing stations 518, supply roll 430 520, 522, 524 drive rollers 366 transfer belt 594 cutting station 466 2nd stage heating roller 526 1st stage heating roller 542 cooling roller 528 guide roller 536 motor 27 multiple toner image Soo 2nd primary belt 540 image producing stations 519, 521, 523, 525 2nd jnt~ ' Ate transfer belt 596 let stage heating roller 544 2nd stage heating roller 527 cooling roller 529 a guide roller 538 ~ 516 exit 517

Claims (10)

1. A method for the transfer of a toner image (500) in powder form from a transfer member (594) to a substrate (541), comprising:
(1) heating said toner image (500) to a temperature sufficient to reduce the viscosity thereof to less than 50 Pa s;
(2) bringing said transfer member (594) carrying said toner image (500) into contact with said substrate (541);
(3) forcibly cooling said transfer member (594) to a temperature below the glass transition temperature Tg of the toner while said transfer member (594) remains in contact with said substrate (541); and (4) thereafter separating said transfer member (594) from said substrate (541).

2. The method of claim 1, wherein said toner image (500) is formed of toner having a composition comprising a thermoplastic binder and from 10% to 50% by weight, based on the weight of the toner composition, of a pigment.

3. The method of claim 1, wherein said toner image (500) is formed of a toner composition in powder form, having a weight average particle size of between 0.5 µm and 5 µm.

4. The method of claim 1, wherein said multiple toner image (500) is heated to a temperature of more than the glass transition temperature Tg, but below the degradation temperature of said toner.

5. A method according to Claim 1, comprising:
- feeding said substrate (541) through a contact zone which extends continuously from an entrance (516) to an exit (517) thereof and is defined by said transfer member (594) and a reaction surface (596) in face-to-face pressure contact with said transfer member (594);
- heating said transfer member (594) adjacent said entrance (516) to a temperature sufficient to reduce the viscosity of the toner to less than 50 Pa s;
- forcibly cooling said transfer member (594) intermediate said entrance (516) and said exit (517) to a temperature below the softening point Tg of said toner; and - applying pressure between said transfer member (594) and said reaction surface (596) intermediate said entrance (516) and said exit (517), thereby to fix said toner image on said substrate.

6. A method according to Claim 1, comprising:
- feeding said substrate (541) through a contact zone which extends continuously from an entrance (516) to an exit (517) thereof and is defined by said transfer member (594) and a reaction surface (596) in face-to-face pressure contact with said transfer member (594);
- heating said transfer member (594) adjacent said entrance (516) to a temperature sufficient to reduce the viscosity of the toner to less than 50 Pa s;
- forcibly cooling said transfer member (594) intermediate said entrance (516) and said exit (517) to a temperature below the softening point Tg of said toner; and - heating said transfer member (594) adjacent said exit (517) of said contact zone to a temperature above the softening point Tg of said toner , thereby to fix said toner image on said substrate.

7. A method according to claim 5 or 6, wherein said transfer member means (594) contacts said substrate (541) with a dry surface.

8. A method of single pass, electrostatographic printing comprising:
- moving a transfer member (594) along a continuous path;
- depositing a toner image in powder form onto said moving transfer member (594) to form an unfixed toner particle image (500) thereon;
- feeding substrate (541) through a contact zone which extends continuously from an entrance (516) to an exit (517) thereof and is defined by said moving transfer member (594) carrying said unfixed toner particle image (500) and a reaction surface (596) in face-to-face pressure contact with said moving transfer member (594) whereby said toner image (500) is transferred to said substrate (541);
- heating said transfer member (594) adjacent said entrance (516) to a temperature sufficient to reduce the viscosity thereof to less than 50 Pa s; and - forcibly cooling said transfer member (594) intermediate said entrance (516) and said exit (517) to a temperature below the softening point Tg of said toner.

9. A method according to claim 8, including depositing a plurality of toner images of different colours in powder form in register with each other onto said moving transfer member (594) to form a multiple toner image (500) thereon.

10. A device for fixing an unfixed toner particle image on a substrate, comprising a transfer member (594), a reaction surface (596) in face-to-face pressure contact with said transfer member (594) to form a contact zone therebetween, extending continuously from an entrance (516) to an exit (517), means (366) for feeding a substrate (541) through said contact zone from said entrance to said exit, heating means (526) for heating said transfer member (594) adjacent said entrance (516) to a temperature sufficient to reduce the viscosity thereof to less than 50 Pa s, and cooling means (528) for forcibly cooling said transfer member (594) intermediate said entrance (516) and said exit (517) to a temperature below the softening point Tg of said toner characterised by means (532, 534) for applying pressure between said transfer member (594) and said reaction surface (596) intermediate the entrance and exit and second heating means (530) for heating said transfer member (594) adjacent said exit (517) to a temperature above the softening point Tg of said toner.