WO2006032493A2

WO2006032493A2 - Apparatus and process for the printing of microstructures

Info

Publication number: WO2006032493A2
Application number: PCT/EP2005/010246
Authority: WO
Inventors: Johannes Matthiesen
Original assignee: Securis Limited
Priority date: 2004-09-23
Filing date: 2005-09-20
Publication date: 2006-03-30
Also published as: WO2006032493A3; GB0421169D0

Abstract

Surface relief microstructures such as holograms may be replicated rapidly and with accuracy on a web based in-situ polymerization replication (ISPR) printing apparatus. The apparatus may consist of a number of stations including a first radiation curing stage that at least partially cures a radiation curable coating and a second radiation curing stage which also comprises a microstructure embossing surface which imparts a microstructure to the partially cured radiation curable coating as it is in contact with the microstructure embossing surface. Other stations including heating units, cooling units, post-cure units and secondary UV sources associated with the microstructure embossing surface. The ISPR printing apparatus is arranged to easily and effectively accommodate these additional stations and is operated to attain maximum web speed whilst maintaining image quality.

Description

APPARATUS AND PROCESS FOR THE PRINTING OF MICROSTRUCTURES

[0001] The invention relates to an apparatus and process for the manufacture of surface relief microstructures e.g. holograms.

[0002] The manufacture of surface structures with dimensions between the nanometer and micrometer level e.g. surface relief holograms has been undertaken in the past via a number of different methods. One commonly used method is thermal embossing where a hard embossing cylinder is utilized typically with pressure and temperature to transfer the image from the cylinder to a suitable thermoformable plastic such as PVC. A further method utilizes solvent based casting where a plastic dissolved in a solvent is coated onto the master with surface relief hologram and allowed to dry by evaporation, and the resulting dry layer of plastic is peeled off the master surface relief.

[0003] A more recently developed method is that of in-situ polymerization replication (ISPR). With this technology a liquid polymer or resin, usually deposited on a substrate such as a polymer film or web, is cast or molded against the microstructure to be replicated e.g. holographic image or optically variable effect profile, in a continuous fashion. The molded profile is retained in the polymer or resin on or after removal from the microstructure mold by use of a curing stage. Examples of this approach are described in United States Patent Nos. 3,689,346, 4,758,296, 4,840,757, 4,933,120, 5,003,915, 5,085,514 and in DE-A-4, 132,476, WO88/09252 and WO94/18609. In most of these prior art techniques the microstructure to be molded is provided as a relief in a metal surface on a rotary cylinder. They all utilize radiation curable media as the liquid polymer or resin media for casting and therefore this technology is sometimes referred to as UV casting.

[0004] In WO94/18609 the curing of the radiation curable media is achieved by the use of a UV source that is located within the bore of a hollow quartz cylinder that is carrying the microstructu re relief image to be molded. In one embodiment the relief image is formed in a polymer sleeve that has been placed or cast on the outer surface of the quartz cylinder. The polymer sleeve Is substantially transparent to the UV radiation that is used to cure the cast radiation curable resin. In a further embodiment the microstructure relief image is cast on the cylinder using UV curable resin system. Typically the thickness of the cast layer containing the surface relief microstructure is in the range from 3μm up to 300 μm. Also, as the surface relief microstructure located on the quartz cylinder is a cured organic material it is relatively soft compared to the metal surfaces used in more conventional other ISPR processes.

[0005] ISPR processes are attractive processes as they are web based and lend themselves to continuous/semi-continuous operations for bulk manufacture of surface relief structures. One of the enduring problems with web based processes however is that there is always a limit to the speed at which the web can be processed and this limit usually manifests itself in the limit at which acceptable quality may be achieved or the limit of the casting technology used; this is usually the maximum curing speed for the radiation curing resin to be cast. UV curing technologies are known in the printing industry that can run at web speeds in excess of 100 meters per minute. However, it is difficult to achieve such high web speeds with the ISPR processes described in the prior art especially those as described in WO94/18609, as attempts to run at higher speeds result in a reduction on the quality of the surface relief microstructure images. Image quality is critically important in many applications as the resultant application relies upon good quality optical effects which are directly related to the quality of the transferred image, respectively microstructure of trie refraction grating. In addition many applications of ISPR technology require that the apparatus for ISPR is utilized in-line and in co-operation with other processes e.g. printing and finishing processes as used in the graphic arts industry. A further challenge is to provide ISPR processes that may be easily controlled and implemented with the minimum degree of wastage of materials; this is especially important for in-line applications. [0006] Thus there is a need for ISPR processes and equipment that may be operated at higher web speeds whilst maintaining image quality and which address as many as possible of the identified technical challenges for their effective and economic use.

[0007] The present invention addresses these technical challenges by providing a new design of ISPR printing apparatus that is more flexible and compact compared to such prior art arrangements as that described for example in WO94/18609.

[0008] Therefore in a first aspect the present invention provides for an

ISPR printing apparatus comprising a) mechanism for feeding a substrate through the apparatus, b) a coating station comprising a source of liquid curable resin and means for applying liquid resin from the source to a surface of the substrate, d) a printing/curing station comprising means for imprinting a surface relief microstructure into the surface of the applied resi n on the substrate, and means for curing the resin having surface relief microstructures imprinted therein such that these microstructures are retained in the cured resin, characterized in that, the apparatus is arranged such that the resin is applied to the top surface of the substrate; that the means for imprinting the surface relief microstructure comprises a hollow transparent cylinder having a surface relief microstructure and two nip rollers which contact the back surface of the substrate and which have an axis of rotation that is alo ng the same axis as the axis of rotation of the hollow transparent cylinder and trie means for curing the resin is a UV source located in the bore of the hollow transparent cylinder.

[0009] Application of the resin to the top surface of the web is important as this enables the resin to flow-out across the web to a uniform thickness; this is highly desirable in order to obtain high quality surface relief microstructures. In prior art arrangements such as that described WO94/18609 the resin is often metered directly onto the printing cylinder. [0010] In a further aspect of the present invention the ISPR printing apparatus further comprises a means for heating the substrate with uncured resin coated thereon, which is located between the coating station and the printing/curing station. It has been found that the heating station is advantageous when high web speeds are desired or when high viscosity resins are used to provide the resin coating. The heating station assists in ensuring that there is uniform flow-out of the thin resin layer after application to the top of the substrate. This ensures that when the coated substrate enters the printing station the resin layer is highly uniform in thickness. This uniformity assists in ensuring that surface relief microstructures of high quality are produced. One suitable heating means is a temperature controlled oven , tunnel or an IR heating station. If an IR heating station is used it may apply heat directly to the resin on the top surface of the substrate or it may apply heat to the back surface of the substrate or a combination of both methods. Preferably the heat is applied via an IR source to the resin on the top surface of the substrate, or by heated air.

[0011] In a further aspect the ISPR printing apparatus of the present invention further comprises a cooling station located proximate to the printing/curing station. In some modes of operation the temperature of the surfaces in the printing/curing station and the ambient temperature around the printing/curing station may be relatively high. Whilst this is tolerable for certain substrates and applications that use the ISPR printing apparatus in some applications the substrate may be heat sensitive. The main source of heat is the UV lamps located in the bore of the quartz cylinder. The cylinder itself is quartz, and heat builds up in the cylinder during use. Such relatively large increases in heat during use can also affect maintenance of surface relief microstructure quality during use of the ISPR printer. However, in previous ISPR printer technologies it has either not been necessary to cool the printing station or the arrangement or layout of the printing station has been too complex to enable access for efficient cooling. In the present invention the arrangement of the ISPR printing apparatus lends itself to easy access for efficient cooling and especially for the location of a cooling station proximate to the printing cylinder to enable cooling to be applied directly to the cylinder surface during use. In a preferred embodiment the cooling means is a bank of air coolers or fans located above the printing cylinder and between the guide rollers in the printing station. The present arrangement is such that in this orientation the cooling may be applied directly to the cylinder surface. This surface cooling in combination with the inherent cooling associated with the UV lamp and an additional cool air flow inside the cylinder, being independent from the lamp and lamp housing cooling system and optionally the use of additional cooling beneath the printing cylinder, which impinges on the back of the substrate web, results in a significant reduction in the temperature of the printing station. The temperature may be controlled to be within the range of 40 to 80⁰C and more preferably within the range of 40 to 5O⁰C. Without this effective additional cooling the temperatures may easily exceed 80⁰C. A further benefit of the effective cooling of the surface relief microstructure on the cylinder is that this aids release of the cured resin from the surface of the cylinder; this enables higher web speeds to be achieved whilst maintaining image quality.

[0012] A further disadvantage with prior art ISPR printing equipment is that it has been difficult to provide high levels of UV energy at the point of contact between the surface relief microstructure of the cylinder and the uncured resin located on the substrate. When the printing cylinder is a solid material such as metal this is because the UV curing station is usually located after the printing station; this means that the surface relief microstructure surface of the cylinder is released from the resin before it is cured, with inherent problems in resulting image quality. In other arrangements the UV source is located such that irradiation occurs through the back of the transparent substrate surface and into the uncured resin whilst the resin is in contact with the printing cylinder. In the present arrangement it is now possible to utilize effectively and easily at least two UV sources; one located within the bore of the hollow cylinder and the other located beneath the cylinder and proximate to the back surface of the web as it passes around the hollow cylinder. In this arrangement the uncured resin when in contact with the surface relief microstructure of the cylinder is irradiated from above and below through the transparent substrate. It is possible with the present arrangement to utilize two additional UV sources outside of the hollow cylinder in the printing station. The benefit of additional UV irradiation at the contact point between the uncured resin and the surface relief microstructure on the hollow cylinder is that the resin may be cured faster and more thoroughly when in contact with the hollow cylinder surface relief ensuring high image quality and faster web speeds. This arrangement results in maximum UV irradiation in the contact region between the two nip rollers. Thus in a further aspect the present invention provides for an ISPR printing apparatus, which comprises at least two UV sources associated with the printing/curing station. One of the UV sources is located within the bore of a hollow printing cylinder and the second is located beneath the cylinder irradiating the resin in contact with the surface relief microstructure of the cylinder through the transparent substrate upon which the resin is coated. In a preferred embodiment there are two additional UV sources located beneath the cylinder.

[0013] Even though the ISPR printing apparatus of the present invention enables useful increases in web speed to be achieved via the use of various additional components or stations as described above there is still the need for additional means of increasing web speed whilst maintaining the quality of the surface relief microstructure image. It has been found that if a UV pre-curing station is utilized in the ISPR printing apparatus to partially cure the curable resin before it enters the printing/curing station then it is possible to increase web speed without loss of quality of the microstructures produced. It has been found that if the station for applying the resin material into which the microstructure is to be cast is located in advance of the printing/curing station, and a UV pre-curing station is utilized, which partially cures the radiation curing resin before the image is cast into the resin, then the overall speed of the imaging line may be increased without loss of image resolution. This is surprising as the radiation curable material has been partially cured prior to the imaging process yet despite this the image is still transferred into the partially cured resin with high quality. Thus in a further aspect the present invention provides for an ISPR printing apparatus further comprising a UV pre-curing station. In this aspect of the present invention the ISPR printing apparatus may also utilize means for applying pressure to the first nip roller of the printing/curing unit. The application of additional pressure at this nip roller forces the partially cured resin coating against the surface relief microstructure on the printing cylinder and this pressure aids in deforming the partially cured resin in order to ensure that it conforms to the profile of the surface relief microstructure. The use of additional pressure at this point enables the ISPR printer to be run at higher web speeds as the partially cured coating will be imprinted at higher speeds than would be achieved without the additional pressure. The exact pressure used will depend on the nature of the resin used and the extent of partial curing and thus the deformability of the partially cured resin.

[0014] It has also surprisingly been found that if the radiation sources of the first radiation curing station and the second radiation curing station are operated in a co-ordinated way then it is possible to exercise a high degree of control over the imaging process.

[0015] The present invention therefore provides a process for the manufacture of a surface relief microstructure by ISPR, which process comprises the steps of:

(a) applying a radiation curable liquid resin coating to a substrate,

(b) at least partially curing the radiation curable liquid resin coating with a pre-curing station, (c) bringing the partially cured radiation curable liquid resin coating into contact with a cylinder having a surface relief microstructure such that the surface relief microstructure is imprinted into the partially cured radiation curable liquid resin coating and

(d) exposing the partially cured radiation curable liquid resin coating when in contact with the cylinder having a surface relief microstructure to a further source of radiation to cure the coating.

[0016] In a further aspect the present invention provides for a ISPR printing apparatus, comprising:

a) means for continuously moving a substrate through at least three stations in succession, b) the first station including means for applying a radiation curable liquid resin coating onto the substrate, c) the second station comprising radiation curing means to at least partially cure the radiation curable liquid resin coating on the substrate, and d) the third station comprising means for imparting a surface relief microstructure into the partially cured radiation curable liquid resin coating and radiation curing means for fully curing the coating.

[0017] In a further aspect the present invention provides a continuous process for the manufacture of a surface relief microstructure which process comprises:

(a) applying a radiation curable liquid resin coating to a web based substrate,

(b) fully curing the radiation curable liquid resin coating by exposure to a first source of radiation, (c) bringing the fully cured radiation curable liquid resin coating on the substrate into contact with a cylinder having a surface relief microstructure, whilst exposing the fully cured liquid resin coating in contact with the surface relief microstructure to a further source of radiation,

(d) reducing the power of the first source of radiation so that radiation curable liquid resin coating becomes partially cured, and

(e) bringing the partially cured radiation curable liquid resin coating into contact with the cylinder having a surface relief microstructure, whilst exposing the partially cured liquid resin coating in contact with the surface relief microstructure to a further source of radiation, so that the surface relief microstructure is transferred into the partially cured radiation curable liquid resin coating, which is then fully cured.

[0018] The apparatus, methods and processes of the present invention may be used in combination with any of the known ISPR methods and apparatus in the art such as those described in United States Patent Nos. 3,689,346, 4,758,296, 4,840,757, 4,906,315 4,933,120, 5,003,915, 5,085,514 6,214,443 6,344,245 6,436,483 and in DE-A-4, 132,476, EP 0,338,378, WO88/09252, WO94/18609, W 099/38704; the whole contents of each of these foregoing reference are all hereby incorporated by reference.

[0019] In a preferred embodiment the apparatus of the present invention further comprises a control unit which is capable of controlling the UV energy output of the pre-curing UV source and the UV source associated with the printing/curing station. This control unit enables the energy of each UV source to be varied independently of each other or to be varied in a coordinated fashion. In the preferred process the pre-curing station is set at the required energy to ensure full curing of the resin coating before the radiation curable liquid resin is applied to the substrate. The UV source associated with the printing/curing station may be set at minimum power or at power that would be sufficient to ensure full curing of the radiation curable liquid resin. At this point the radiation curable liquid resin is applied to the substrate and is fully cured at the pre-curing station. The substrate with fully cured coating then passes into the printing/curing station and passes through the station without having any surface relief microstructure imprinted into its surface. This is the start up phase of the ISPR printing apparatus. At this point the energy of the UV pre-curing station is reduced whilst either maintaining the optimum energy of the UV source associated with the printing/curing station or increasing its energy up to the optimum level for curing the resin. As the energy of the UV lamps are altered in this way the radiation cured coating leaving the pre-curing station becomes partially cured and on passage to the printing/curing station is able to be imprinted with the surface relief microstructure and fully cured in the imprinted state. When the printing run is complete the process is reversed to ensure that no uncured resin passes through the printing/curing station. This technique may be optimized to provide partially cured resin to the printing/curing station that on imprinting and full curing provides high quality surface relief microstructures in the cured resin at optimum speed.

[0020] In a further aspect of the present invention the ISPR printing apparatus may further comprise a UV-post-curing unit with or without a heating unit, or just an IR-heating unit, or combined UV/IR, which may be especially recommanded in order to support and speed up the curing of typically post curing cationic varnish systems. This post curing unit may be used when the coated substrate leaving the printing/curing unit although successfully imprinted is not full cured. The post curing unit ensures that the coating is fully cured. When the post-curing unit comprises a heating unit e.g. an IR heating unit or oven then this may be useful in ensuring full cure of resin coatings based on cationic systems as these systems are living polymerizations that are accelerated to completion by the action of heat. [0021] In one embodiment the apparatus of the present invention may be an off-line or stand alone unit or in an alternative embodiment this may be an in-line or integrated system with other further conventional printing, laminating, cutting, slitting and other converting stations as part of an integrated manufacturing process. In one embodiment the apparatus and processes of the present invention may be configured and used to provide partial holographic printing of a web based substrate. This may be achieved by partially printing the radiation curable lacquer as for example graphic elements onto the web based substrate and replicating the microstructure only in that areas where the radiation curable lacquer has been printed.

[0022] The radiation curable composition used for the manufacture of the cylinder surface and for the radiation curable lacquer will be a composition that is one of three types. The first are free radical polymerized or cured resin systems which are unsaturated resins or monomers, pre-polymers, oligomers etc that contain vinyl and/or acrylate unsaturation for example and which polymerize and/or cross-link through use of a photo initiator activated by the radiation source employed e.g. UV. These are typically referred to as rad ϊcal systems. The second is cationic polymerizable or cured resins in which ring opening (e.g. epoxy rings) is effected using photoinitiators or catalysts wh ich generate ionic entities under the radiation source employed e.g. UV. The ring opening is followed by cationic polymerization and/or intermolecular cross- linking. Also used as cationically curable systems are resins that contain one or more groups with vinyl unsaturation. These systems are typically referred to as cationic systems. The third type is a hybrid cured systems in which both free radical cured resins and cationic cured resins are combined. These systems are typically referred to as radical/cationϊc systems or combined systems.

[0023] Typical resins useful in UV curable coatings are styrented polyesters and acrylics, such as vinyl copolymers of various monomers and glycidyl methacrylate reacted with acrylic acid, isocyanate prepolymers reacted with an hydroxyalkyl acrylate, epoxy resins reacted with acrylic or methacrylic acid, and hydroxyalkyl acrylate reacted with an anhydride and subsequently reacted with an epoxy.

[0024] The radiation used to effect curi ng of the radiation curable medium will typically be UV radiation, alternatively or in combination the radiation source could include electron beam, visible, infra-red or higher wavelength radiation, depending upon the material, its absorbance and the process used.

[0025] The radiation sources useful in the apparatus and processes of the present invention include Electron Beam radiation units that are readily available and typically consist of a transformer capable of stepping up line voltage and an electron accelerator. In one type of machine the electrons are generated from a point source filament and then scanned electromagnetically like a television set to traverse the coated object. In another type of machine, the electrons are generated in a curtain from an extended filament which can irradiate the entire width of the surface without the need for scanning. While commercial machines are available with accelerating voltages of over a million electron volts, the range for this and similar coating applications is typically fro m 150-300 KV (kiloelectron volts). It is common when curing coatings with electron beam radiation units to take steps to eliminate oxygen from the surface of the coating. A further useful source of radiation are UV curing stations that are well known in the art. It is within the scope of the present invention that the radiation sources may be both EB or both UV or any combination of UV and EB.

[0026] In one embodiment it is preferred that the cylinder comprising trie micro-structure is manufactured using a UV curable silicone resin or a two-part silicone elastomer. With the UV curable silicone resins the silicone compounds for use in the present invention are silicone compounds that contain either vinyl and/or acrylate unsaturation that may be co-polymerized or cross-linked via free radicals or they contain vinyl and/or oxirane functionality e.g. epoxy groups that may be incorporated into cationic polymerized systems via cationic coploymerization and/or cross-linking. Such silicone compounds are known in the art and are commercially available. Silicone compounds that may be incorporated via free radical mechanisms include products that are available from Degussa AG Goldschmidt Industrial Specialities and marketed under the trade name TEGO^®and as described in "UV Curable Silicones", S. Oestreich, paper presented at Radtech Europe Conference 2001 , held at Basel, Switzerland from 8^th to 10^th October 2001. Silicone compounds that may be incorporated by cationic polymerization mechanisms are commercially available from Rhodia-Silcones under the trade name SILCOLEASE^® and as described in "UV/EB Silicone Release Coating - Global Approach" J-M Frances et al, paper presented at Radtech Europe Conference 2001 , held at Basel, Switzerland from 8^th to 10^th October 2001.

[0027] The silicone compound may be utilized in the formulation for the radiation curable medium for the cylinder at any concentration. Preferably it is present in the formulation at a concentration of at least 1 wt% based on the total weight of composition, more preferably at least 2 wt%, even more preferably at least 3 wt% and most preferably at least 5 wt%. In one embodiment the silicone compound may be a complex oligomeric material and may constitute 100% of the polymerizable component of the formulation, the remainder of the formulation being made up of photoinitiators and/or other additives.

[0028] In a preferred embodiment the surface relief microstructure is manufactured on a transparent hollow cylinder. The cylinder may be manufactured from any material that is substantially transparent to UV radiation. The cylinder may be manufactured from a polymer material or quartz. When quartz is used the cylinder may be manufactured from synthetic/processed or natural quartz. Preferably the cylinder is manufactured from synthetic/processed quartz. The cylinder comprising the surface relief microstructure may be manufactured by the general process as described in WO94/18069. In manufacturing the cylinder comprising the surface relief microstructure surface in a first step an original surface relief microstructure e.g. hologram is prepared by well-known means. To produce the original hologram, an object is first recorded in a first hologram by standard off-axis recording techniques. A Benton hologram is then recorded from the first hologram onto a surface relief medium such as photoresist, thus producing the original surface relief hologram. The next step is the manufacture of the hologram master. From the original surface relief hologram, a hologram master may be made by any of various techniques. One well-known way technique is to electroform nickel onto the original surface relief hologram, thereby producing a reversed metal replica of the original. This master may then be used to manufacture the surface relief microstructure using the general process as described in WO94/18069.

[0029] In manufacturing the cylinder comprising the surface relief microstructure in a first step an original surface relief microstructure e.g. hologram is prepared by well-known means. To produce the original hologram, an object is first recorded in a first hologram by standard off-axis recording techniques. A Benton hologram is then recorded from the first hologram onto a surface relief medium such as photoresist, thus producing the original surface relief hologram. The next step is the manufacture of the hologram master. From the original surface relief hologram, a hologram master may be made by any of various techniques. One well-known way technique is to electroform nickel onto the original surface relief hologram, thereby producing a reversed metal replica of the original. This master may then be used to manufacture the cylinder with surface relief microstructure using the general process as described in WO94/18069 and as described above.

[0030] In one embodiment the radiation curable liquid coating or lacquer is a composition which comprises a radiation curable composition diluted with a solvent. In a preferred embodiment the radiation curable lacquer comprises up to 60% by weight of solvent with the balance being radiation curable composition. The solvent may be any solvent that is compatible with the radiation curable composition and which may be removed after coating of the radiation curable lacquer on the substrate. In a preferred embodiment the solvent is a material which is not designed to react with the radiation curable components on radiation curing. Lacquers comprising solvents have relatively low viscosities e.g. water viscosities at ambient temperature and on application to the web based substrate provide relatively thin smooth and almost mirror like lacquer surfaces. The thickness is typically 1 to 2.5 μm. This provides a perfect varnish surface for high quality and high efficient replication of microstructures especially very fine microstructures. In a preferred embodiment therefore apparatus and various processes according to the present invention utilize means for removing the solvent preferably by evaporation from the coated lacquer prior to contact with the cylinder comprisi ng surface relief microstructures. This evaporation may be achieved through use of for example a hot air drying tunnel.

[0031] In a further embodiment solvent free lacquers may be used, which are approaching 100% reactive material. In this embodiment it is therefore preferred that apparatus and various processes of the present invention further comprise means for coating on the web based substrate that utilizes curtain coating. Preferably curtain coating means that are capable of providing a relatively uniform thickness of from 1 to 80 μm to be achieved with a relatively smooth surface.

[0032] While the methods disclosed herein are primarily directed toward replication of surface relief microstructures, it is clear that the methods are also useful for replication of any kind of surface relief pattern. In the claims which follow, the word microstructure relief surface is used to mean holograms, diffractive patterns and any structure that may provide an optical effect. The term also encompasses structures at the nanometer to micrometer scale that are not designed to provide optical effects. [0033] It is further to be understood that in relation to the description of several aspects of the invention and various specific embodiments that any of the technical features described herein may be combined with one or more of the technical features in any aspect of the present invention, In particular in relation to the described apparatus any combination of two or more of the various stations described and/or modifications suggested to the ISPR printing apparatus may be used in the present invention.

[0034] The invention will now be illustrated by way of example only with reference to the following drawings in which:

[0035] Figure 1 is a schematic representation of an ISPR printing apparatus according to the present invention,

[0036] Figure 2 is a schematic representation of an ISPR printing apparatus according to the present invention wherein the radiation curable liquid casting resin coating is heated with an IR source prior to entering the printing station,

[0037] Figure 3 is a schematic representation of an ISPR printing apparatus according to the present invention which incorporates a cooling station proximate to the printing cylinder,

[0038] Figure 4 is a schematic representation of an ISPR printing apparatus according to the present invention which incorporates additional UV sources within the printing/curing station,

[0039] Figure 5 is a schematic representation of an ISPR printing apparatus according to the present invention which incorporates a UV-pre- curing station in advance of the printing/curing station, and [0040] Figure 6 is a schematic representation of an ISPR printing apparatus according to the present invention which incorporates a pre-curing station in advance of the UV-printing/curing station and a UV-post-curing station.

[0041] Referring to the Figure 1 a roll (1) of plastic film substrate is unwound and passed through a coating station (2) that applies a curable resin (3) to the top surface (4) of the substrate. This coated substrate then passes to the printing/curing station (5), which comprises a hollow quartz cylinder (6) upon which is located a layer comprising a surface relief microstructure (7), which is transparent to UV. The coated substrate under tension is guided by guide rollers (8) and (9) around the cylinder (6) and passes through two nip sections provided by nip rollers (10) and (11), which are in contact with the back surface (12) of the substrate. The cylinder and nip rollers (8) and (9) are in indirect contact with each other (the substrate is located between them) along an axis "X", which is horizontal in the apparatus of this embodiment. As the coated substrate passes through the printing/curing station (5) the resin is imprinted with the surface relief microstructure and simultaneously cured by the action of the UV radiation, which passes through the transparent cylinder (6) and the layer (7) to cure the resin on the substrate thus ensuring that the surface relief microstructure is retained in the surface of the cured resin layer. The layout of this apparatus is compact and highly adaptable for the inclusion of other stations and process features.

[0042] Referring to the Figure 2, the ISPR printing apparatus shown is almost identical to that described in relation to Figure 1 , with the exception that an IR heating station (20) is located above the coated substrate (21) and between the coating station (22) and the printing/curing station (23).

[0043] Referring to the Figure 3, the ISPR printing apparatus shown is almost identical to that described in relation to Figure 1 , with the exception that a cooling station (30) is located between guide rollers (31) and (32) and above the surface (33) of the printing cylinder (34). In this arrangement it is possible for the cooling station (30) to provide cooling air directly to the surface (33) of the printing cylinder (34) during use. Also shown is optional additional cooling station (35), which is located beneath the printing cylinder (34) and which cools the back surface (36) of the web substrate (37). An additional cool air flow is used from inside the cylinder, being independent from the lamp and the lamp housing cooling system.

[0044] Referring to the Figure 4, the ISPR printing apparatus shown is almost identical to that described in relation to Figure 1 , with the exception that additional UV sources (40) and (41) are located between the nip rollers (42) and (43) and proximate to the back surface (44) of the polymer web (45). In combination with the UV source (46) located within the bore of the transparent hollow cylinder (47), these additional UV sources (40) and (41) provide high intensity of UV irradiation to the contact region between the coated polymer web (45) and the surface relief microstructure (48) of the cylinder C47) between points A and B.

[0045] Referring to the Figure 5, the ISPR printing apparatus shown is almost identical to that described in relation to Figure 1 , with the exception that there is a UV-pre-curing unit (50) located after the coating station (51) and before the printing/curing station (52). The pre-curing unit (50) irradiates the radiation curable resin coated on the web substrate (53) so that it is at least partially cured before it enters the printing/curing station (52). In the preferred start-up mode for this ISPR printing apparatus the pre-cure unit (50) will operate at full power to ensure full cure of the resin at start-up. This is in o rder to avoid uncured varnish closing and blocking the microstructure on the printing cylinder due to unsufficient curing energy during the start-up phase. When the web substrate (53) is transparent to UV the UV pre-curing unit may be located below the web irradiating the UV curable resin through the substrate (53). It is preferred that the UV pre-curing unit (50) irradiates the surface of the UV curable resin. The lamp of the UV pre-curing unit (50) and the printing/curing station UV lamp (54) may be controlled by the same control unit (not shown) so that their power outputs may be coordinated and balanced as described above. The careful use of the pre-curing unit in combination with the main LJV curing unit of the cylinder enables the apparatus to operated cleanly so that after completion of the printing process with termination of UV lacquer coating the apparatus may be operated to ensure that all uncured UV lacquer is removed from the apparatus in what may be referred to as a "self-cleaning" mode of operation.

[0046] Referring to the Figure 6, the ISPR printing apparatus shown is almost identical to that described in relation to Figure 1 , with the exception that there is a UV-pre-curing unit (60) located after the coating station (6"I) and before the UV-printing/curing station (62) and a UV-post-curing unit <63).

Claims

1. An ISPR printing apparatus comprising a) mechanism for feeding a substrate through the apparatus, b) a coating station comprising a source of liquid curable resin and means for applying liquid resin from the source to a surface of the substrate, d) a printing/curing station comprising means for imprinting a surface relief microstructure into the surface of the applied resin on the substrate, and means for curing the resin having surface relief microstructures imprinted therein such that these microstructures are retained in the cured resin, characterized in that, the apparatus is arranged such that the resin is applied to the top surface of the substrate; that the means for imprinting the surface relief microstructure comprises a hollow transparent cylinder having a surface relief microstructure and two nip rollers which contact the back surface of the substrate and which have an axis of rotation that is along the same axis as the axis of rotation of the hollow transparent cylinder and the means for curing the resin is a UV source located in the bore of the hollow transparent cylinder.

2. An ISPR printing apparatus as claimed in claim 1 which further comprises means for heating the substrate with uncured resin coated thereon, which heating means is located between the coating station and the printing/curing station.

3. An ISPR printing apparatus as claimed in either claim 1 or claim 2, which further comprises a cooling station located proximate to the printing/curing station.

4. An ISPR printing apparatus as claimed in any one of the preceding claims wherein the cooling is applied directly to the cylinder surface in the printing/curing station.

5. An ISPR printing apparatus as claimed in any one of the preceding claims wherein the printing/curing station comprises an additional UV source that is not within the bore of the cylinder.

6. An ISPR printing apparatus as claimed in any one of claims 1 to 4 wherein the printing/curing station comprises two additional UV sources that are not within the bore of the cylinder.

7. An ISPR printing apparatus as claimed in claim 5 or claim 6 wherein the additional UV sources are located beneath the cylinder and irradiate the resin in contact with the cylinder through the substrate.

8. An ISPR printing apparatus as claimed in any one of the preceding claims which further comprises a UV pre-curing station located between the coating station and the printing/curing station.

9. An ISPR printing apparatus as claimed in claim 8 which further comprises a control unit for the co-ordinated adjustment of the UV energy output of the pre-curing unit and the UV source located in the bore of the cylinder.

10. An ISPR printing apparatus as claimed in any one of the preceding claims which further comprises means for applying pressure to the first nip roller located within the printing/curing unit.

11. An ISPR printing apparatus as claimed in any one of the preceding claims which further comprises a UV-or IR-or UV/IR combined post-curing unit.

12. A process for the manufacture of a surface relief microstructure by ISPR, which process comprises the steps of: (a) applying a radiation curable liquid resin coating to a substrate, (b) at least partially curing the radiation curable liquid resin coating with a pre-curing station, (c) bringing the partially cured radiation curable liquid resin coating into contact with a cylinder having a surface relief microstructure such that the surface relief micro structure is imprinted into the partially cured radiation curable liquid resin coating and (d) exposing the partially cured radiation curable liquid resin coating when in contact with the cylinder having a surface relief microstructure to a further source of radiation to cure the coating.

13. A process as claimed in claim 12 wherein the radiation curable medium is a radiation curable acrylate based composition.

14. A process as claimed in claim 12 wherein the radiation curab le medium is a cationic or radical radiation curable medium.

15. An ISPR printing apparatus, comprising: a) means for continuously moving a substrate through at least three stations in succession, b) the first station including means for applying a radiation curable liquid resin coating onto the substrate, c) the second station comprising radiation curing means to at least partially cure the radiation curable liquid resin coating or the substrate, and d) the third station comprising means for imparting a surface relief microstructure into the partially cured radiation curable liquid resin coating and radiation curing means for fully curing the coating.

16. An apparatus according to claim 15 wherein said means for continuously moving said substrate includes means for handling a roll of a continuous web flexible material.

17. An apparatus as claimed in claim 15 wherein the means to at least partially cure the radiation curable liquid casting resin is a source of UV radiation.

18. An apparatus as claimed in claim 15 wherein the means to fully cure the coating is a source of UV radiation.

1 9. An apparatus as claimed in any one of claims 15 to 18 Λ/vherein said surface relief microstructure includes a hologram characterized by reconstructing a three-dimensional image when illuminated by light.

20. A continuous process for the manufacture of a surface relief microstructure which process comprises: (a) applying a radiation curable liquid resin coating to a web based substrate, (b) fully curing the radiation curable liquid resin coating by exposure to a first source of radiation, (c) bringing the fully cured radiation curable liquid resin coating on the substrate into contact with a cylinder having a surface relief microstructure, whilst exposing the fully cured liquid resin coating in contact with the surface relief microstructure to a further source of radiation, (d) reducing the power of the first source of radiation so that radiation curable liquid resin coating becomes partially cured, and (e) bringing the partially cured radiation curable liquid resin coating into contact with the cylinder having a surface relief microstructure, whilst exposing the partially cured liquid resin coating in contact with the surface relief microstructure to a further source of radiation, so that the surface relief microstructure is transferred into the partially cured radiation curable liquid resin coating, wh ich is then fully cured.