EP2072255A1

EP2072255A1 - Reflector assembly for uv radiation

Info

Publication number: EP2072255A1
Application number: EP07150209A
Authority: EP
Inventors: Anton Andrusier; Ran Vilk
Original assignee: Hewlett Packard Development Co LP
Current assignee: Hewlett Packard Development Co LP
Priority date: 2007-12-20
Filing date: 2007-12-20
Publication date: 2009-06-24

Abstract

A reflector assembly (36) for reflecting UV radiation from a UV-radiation source (30) onto a substrate (12) movable relative to the source (30) and having a surface at least partially covered with a UV-curable ink or coating. A first curved reflector (38a, 38b) focuses UV radiation from the source (30) onto the surface in a first area (56) for providing a peak intensity in that area (56). At least one second reflector (40a, 40b) reflects UV radiation from the source (30) onto the surface in at least one second area (62a, 62b) for providing a relatively lower intensity in that area (62a, 62b). The second reflector (40a, 40b) has a curved reflecting surface (58). The cross sectional profile of the curved reflecting surface (58) in the relative-motion direction is described by a polynomial of second or higher order.

Description

FIELD OF THE INVENTION

The present invention is directed to curing of ink or coatings with UV radiation. For example, it is directed to a reflector assembly for reflecting UV radiation from a UV-radiation source onto a substrate movable relative to the source and having a surface at least partially covered with a UV-curable ink or coating.

BACKGROUND OF THE INVENTION

The curing of photosensitive coatings and inks with ultraviolet (UV) radiation is being used increasingly in a number of technical fields, for example in inkjet printing.
In general, for treating an UV curable ink on a printed matter substrate in the form of a continuous web or an individual sheet with UV radiation in an inkjet printer, either the substrate is moved past a UV radiation source, generally in the form of a high intensity UV lamp, or a UV radiation source is moved past the substrate in order to irradiate the substrate with direct and indirect radiation from the source, the indirect radiation being reflected by a reflector or reflector assembly before it reaches the ink. When reaching the ink the UV radiation will catalyze polymerization reactions in the ink thereby curing or drying the ink. In order to allow immediate use of the printed matter the ink should be completely cured or dried before the printed matter leaves the printer.
However this is difficult to achieve because for a specific thickness of the ink layer the curing speed on the surface of the ink layer can be quite different from the curing speed in the bulk of the ink layer depending on the intensity of the UV radiation used for curing. If the intensity of the UV radiation is too high, only the ink on the surface of the layer is cured, while the ink in the bulk of the layer still remains uncured. If the intensity is too low, only the ink in the bulk of the layer is cured, while the ink on the surface of the layer still remains uncured. This will complicate the printing and will delay the delivery of user ready printed matter, especially printed multicolor images.
In order to solve this problem in some prior approaches a number of UV radiation sources were used in order to achieve a simultaneous curing of the ink on the surface and in the bulk of the layer. However when used in connection with inkjet printers this solution would raise the cost of the printers and complicate printer service.
US Patent No. 3,983,039 , proposes a lamp unit for curing photosensitive inks and coatings, comprising a UV lamp mounted in a reflector. The reflector has a first curved reflecting surface partially surrounding the UV lamp for providing a region of peaked relatively high intensity illumination and a second plane reflecting surface for providing a region of relatively lower intensity illumination. It is said that the region of relatively low intensity illumination was effective for pre-curing the ink while the region of peaked relatively high intensity illumination beneath the UV lamp was well suited for performing the main curing operation.

SUMMARY OF THE INVENTION

According to a first aspect, a reflector assembly is provided for reflecting UV radiation from a UV-radiation source onto a substrate movable relative to the source and having a surface at least partially covered with a UV-curable ink or coating. The reflector assembly comprises at least one first curved reflector and at least one second reflector. The first curved reflector focuses UV radiation from the source onto the surface in a first area for providing a peak intensity in that area. The second reflector reflects UV radiation from the source onto the surface in at least one second area for providing a relatively lower intensity in that area. The second reflector has a curved reflecting surface; the cross sectional profile of the curved reflecting surface in the relative-motion direction is described by a polynomial of second or higher order.
According to a second aspect, a reflector assembly is provided for reflecting UV radiation from a UV-radiation source onto a substrate movable relative to the source and having a surface at least partially covered with a UV-curable ink or coating. The reflector assembly comprises means for casting UV radiation from the source onto the surface in a first area for providing a peak intensity in that area; and means for casting UV radiation from the source onto the surface in at least one second area for providing a relatively lower intensity in that area.
According to a third aspect, a printing apparatus is provided that comprises a reflector assembly for reflecting UV radiation from a UV-radiation source onto a substrate movable relative to the UV-radiation source and having a surface at least partially covered with an UV curable ink or coating. The reflector assembly comprises at least one first curved reflector and at least one second reflector. The first reflector focused UV radiation from the UV radiation source onto the surface in a first area for providing a peak intensity distribution in said first area. The second reflector reflects UV radiation from the UV radiation source onto the surface in at least one second area for providing a relatively lower intensity distribution in said at least one second area. The second reflector has a curved reflecting surface. The cross sectional profile of said curved reflecting surface in a direction of the relative motion is described by a polynomial of second or higher order.
Other features are inherent in the devices disclosed or will be apparent to those skilled in the art from the following description of the embodiments and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

FIG. 1 is an elevational side view of an ink jet printer having an UV curing apparatus for curing UV curable ink on a printed substrate;
FIG. 2 is a perspective top view of the UV curing apparatus;
FIG. 3 is a perspective bottom view of the UV curing apparatus;
FIG. 4 is a cross-sectional side view of the UV curing apparatus showing a UV radiation source and a reflector assembly;
FIG. 5 is a cross-sectional side view of the reflector assembly of the UV curing apparatus;
FIG. 6 is a graphical representation of the intensity of UV radiation on the surface of the printed substrate located below the reflector assembly of Fig. 6;
FIG. 7 is a schematic side view of a UV radiation source and a reflector for explaining a collection angle of the reflector.

The drawings and the description of the drawings are of embodiments of the invention and not the invention itself.

DETAILED DESCRIPTION OF EMBODIMENTS

Some of the embodiments show a reflector assembly for reflecting UV radiation from a UV radiation source onto a surface of a substrate. The substrate is movable relative to the UV radiation source. The surface is at least partially covered with an UV curable ink or coating. A first curved reflector is provided that, in some embodiments, focuses UV radiation from the UV radiation source onto the surface in a first area opposite from the UV radiation source for providing a peak intensity distribution in said first area. At least one second reflector, in some embodiments, reflects UV radiation from the UV radiation source onto the surface in at least one second area adjacent to said first area for providing a relatively lower intensity distribution in said at least one second area. In some embodiments, the at least one second reflector has a curved reflecting surface. In some of the embodiments, the cross sectional profile of the curved reflecting surface in a direction of the relative motion can be described by a polynomial of second or higher order.
In some embodiments, a printing apparatus, e.g. an inkjet printer, is equipped with the reflector assembly. This permits a more efficient transfer of UV radiation to the printed ink and therefore a significantly reduced curing time and an increased motion speed of the substrate with respect to the UV radiation source. Furthermore the lesser the time the printed matter is exposed to the UV radiation, the lesser is the heat absorption and heat rise in the printed matter.
With the reflector assembly and apparatus of the aforementioned type, UV radiation from a single UV radiation source can be configured such that the curing speed of bulk curing is equal to the curing speed of surface curing in order to achieve a complete curing of the surface and of the bulk of the ink layer while the substrate is treated with the UV radiation. The UV radiation from a single UV radiation source is portioned into a dose for bulk curing and a dose for surface curing at which doses for a given UV intensity of the UV radiation source the curing speed of bulk curing is equal to the curing speed of surface curing.
The above-mentioned UV curing apparatus and reflector assembly are efficient, particularly for inkjet printing, where it is possible to increase the speed of the relative motion of the substrate with respect to the UV radiation source in order to achieve a complete curing of the printing ink concurrently with the printing of the substrate.
Furthermore, it is possible to minimize the curing dose, i.e. the UV radiation to be emitted by the UV radiation source in order to provide a complete curing of the ink.
In some embodiments, the first area is opposite from the UV-radiation source, and the second area is adjacent to said first area.
In one of the embodiments the at least one second reflector is arranged on a side of the UV radiation source facing away from the substrate in order to reflect stray radiation from the radiation source, i.e. the radiation that is emitted by the source on a side facing away from the substrate, onto the surface in the second area. This allows achieving bulk curing of the ink with the relatively lower intensity distribution of the stray radiation in the second area before the ink reaches the first area where the peak intensity distribution resulting from the focusing of the UV radiation and direct UV radiation from the UV radiation source will result in surface curing.
In some of the embodiments, the intensity distribution of the UV radiation across said second area is essentially constant.
Some of the embodiments provide for at least one third reflector for reflecting UV radiation from the UV radiation source onto the surface in the second area in an overlapping or overlying relationship with the UV radiation reflected by the at least one second reflector. This will make it easier to provide a desired UV radiation intensity distribution in the second area.
In some of the embodiments the UV radiation from the UV radiation source reflected by the at least one second reflector and the UV radiation from the UV radiation source reflected by the at least one third reflector cover the same areas.
In some of the embodiments the calculation of the curvature of the cross sectional profile of the reflecting surfaces of the at least one second reflector and/or the at least one third reflector is done based on desired intensity and distribution of the UV radiation in the at least one second area, a distance between the UV radiation source and the substrate, a minimum distance between the UV radiation source and the at least one second and/or third reflector and a collection angle defined by rays reflected from two opposite ends or edges of the at least one second and/or third reflector. The desired intensity distribution in the at least one second area may be such that the intensity distribution is essentially constant or homogenous throughout the second area.
In order to focus the UV radiation from the UV radiation source onto the first area having a relatively high UV intensity distribution the reflecting surface of the at least one first reflector may have an elliptical cross section in the direction of relative motion.
In some embodiments, the at least one first reflector is located between the UV radiation source and the substrate in order to shield a part of the second area from direct UV radiation from the UV radiation source and to reflect and focus this UV radiation onto the first area of the substrate.
In some embodiments the reflector assembly has two first reflectors for reflecting UV radiation from the UV-radiation source onto said surface in said first area in an overlapping or overlying relationship, wherein in the direction of relative motion one of the first reflectors is located upstream of said UV-radiation source and the other one of said first reflectors is located downstream of said UV-radiation source.
In certain embodiments, for example, in case of the relative motion of the substrate and the UV radiation source being bidirectional, there are two of the first reflectors and two of the second and/or the third reflectors, and the reflecting surfaces of all of the reflectors may be symmetrical with respect to a plane extending through the UV radiation source and being transverse to the direction of relative motion.
In some embodiments, the reflector assembly has two second reflectors, wherein in the direction of relative motion one of the second reflectors is located upstream of said UV-radiation source and reflects UV radiation from said UV-radiation source to said second area upstream of said first area, and the other one of the second reflectors is located downstream of the UV-radiation source and reflects UV radiation emitted from said UV-radiation source to said second area downstream of said first area.
In some embodiments, the reflector assembly further has two third reflectors, wherein in the direction of relative motion one of the third reflectors is located upstream of said UV-radiation source and reflects UV radiation from said UV-radiation source to said second area upstream of said first area, and wherein the other one of the third reflectors is located downstream of the UV-radiation source and reflects UV radiation emitted from said UV radiation source to said second area downstream of said first area.
In some of the embodiments the first area is irradiated with direct UV radiation from the UV-radiation source.
In some embodiments at least a part of the at least one second area is irradiated with direct UV radiation from the UV-radiation source.
In some embodiments the UV-radiation source is an elongated UV lamp extending transverse to the direction of relative motion.
Turning now to Fig. 1, there is shown a perspective side view of an ink jet printing unit 10 for printing a UV curable printing ink on a flat substrate in the form of a large rectangular sheet of paper 12 placed on a flat upper surface of a support table 14. The printing unit 10 is movable in a plane parallel to the upper surface of the support table 14 along an X-axis and a Y-axis, which are orthogonal to each other. The printing unit 10 comprises in its central part two ink jet printing heads 16 and 18, the left one 16 for printing onto the sheet of paper 12 when the printing unit 10 is moved across the support table 14 in the direction of the arrow A in Fig. 1 along a straight line parallel to the X-axis, and the right one 18 for printing onto the sheet of paper 12 when the printing unit 10 is moved in the opposite direction of the arrow B in Fig. 1. The printing can be done alternately with both printing heads 16 and 18 during the forward and the backward motion of the printing unit, or alternatively with only one printing head 16 or 18 respectively during the forward or during the backward motion of the printing unit 10. After finishing the printing of one line having approximately the width of the printing unit 10 the printing unit 10 will be moved one step in the direction of the arrow C, i.e. in parallel to the Y-axis, and will then be moved again in parallel to the X-axis to print an adjacent line.
Adjacent to each printing head 16 and 18 respectively the printing unit 10 further comprises first and a second UV curing apparatus 20 and 22 for curing the UV-curable ink which has just been printed onto the sheet of paper 12 by the adjacent printing head 16 and 18 respectively. This means, when the printing unit 10 is moved in the direction of the arrow A the first UV curing apparatus 20 will be used to cure the ink just having been printed by the printing head 16, whereas when the printing unit 10 is moved in the direction of the arrow B the second UV curing apparatus 22 will be used to cure the ink just having been printed by the other printing head 18.
As can be best seen from Figs. 2 to 5 the UV curing apparatus 20 and 22 each comprise a rectangular housing 24 with a shallow roof shaped top 26 and a flat open bottom 28 the latter facing towards the sheet of paper 12 and being located in a small distance above the upper surface of the support table 14.
As depicted in Figs. 3 to 5 the housing 24 accommodates a UV radiation source in the form of a elongated UV lamp 30 mounted approximately in the middle of the housing 24 and extending transverse to the direction of motion of the printing unit 10 with respect to the support table 14, i.e. transverse to the direction of the arrows A and B. The UV lamp 30 comprises a cylindrical high-intensity UV light source 32 mounted in a transparent quartz glass tube 34 so that it will emit UV radiation in all directions radial to a center axis of the light source 32. The input power of the UV lamp 30 can be controlled by means not depicted in the drawing.
The housing 24 further accommodates a reflector assembly 36, comprising two first reflectors 38a and 38b located on both sides of the UV lamp 30 between the latter and the open bottom 28 of the housing 24, two second reflectors 40a and 40b located on both sides and above of the UV lamp 30 between the latter and the top 26 of the housing 24, and two third reflectors 42a and 42b located on opposite sides of the UV lamp 30, i.e. in the direction of motion of the printing unit 10 upstream and downstream from the UV lamp 30.
The UV lamp 30 and the reflectors 38a, 38b, 40a, 40b and 42a, 42b of the reflector assembly 36 are supported by a support structure 44 comprising a number of beams 46 made of extruded metal which are located inside the housing 24 and also serve to support the housing walls 48 and the housing top 26. A vertical ventilation tube 50 in the middle of the housing top 26 above the UV lamp 30 is for venting the housing interior and for discharging heated air from the vicinity of the UV lamp 30 when being in operation.
The two first reflectors 38a and 38b are symmetrical to each other with respect to a plane 52 through the center axis of the light source 32 and each have an elliptical reflecting surface 54 in order to focus part of the UV light emitted from the UV lamp 30 onto a narrow strip 56 on the upper surface of the paper sheet 12 on both sides of the plane 52 directly below the UV lamp 30 so that together with direct light emitted from the UV lamp on this strip 56 an area of peak intensity distribution is generated. This peak intensity distribution can be seen in Fig. 6 depicting the intensity distribution as measured on the upper surface of the paper sheet 12 below the open bottom 26 of the housing 24 when the UV lamp 30 is in operation. In Fig. 6 the numbers on the horizontal axis are the distance from the plane 52 in mm and the numbers on the vertical axis are the light intensity in W/cm², which has a maximum of about 10 W/cm² in the middle of the strip 56.
Each of the pair of second reflectors 40a and 40b and the pair of third reflectors 42a and 42b also have reflecting surfaces 58 and 60 respectively which are symmetrical to the plane 52 through the center axis of the light source 32. Both pairs of reflectors 40a and 40b serve to reflect stray UV light from the UV lamp 30, which is not directed downwardly towards the sheet of paper 12 or towards the reflecting surfaces 54 of the pair of first reflectors 38a and 38b. The stray UV light is reflected by the reflectors 40a and 40b onto two broad strips 62a and 62b of the upper surface of the paper sheet 12 on both sides of the narrow strip 56, i.e. upstream and downstream of this strip 56 in the direction of motion of the printing unit 10 parallel to the X-axis, in order to generate in both of theses strips 62a and 62b an area of considerably lower light intensity with a desired intensity distribution. This can be seen in Fig. 6, where the desired light intensity distribution is a relatively constant or homogenous light intensity distribution throughout the whole length of the strips 62a and 62b in the direction of motion of the printing unit 10 being about 1-2 W/cm².
In order to create this desired light intensity distribution, after setting the desired intensity and distribution of the UV light, setting the height of the center axis of the UV lamp 30 above the upper surface of the support table 14, setting the minimum distance from the reflecting surfaces 58 and 60 of the two second and third reflectors 40a, 40b and 42a, 42b respectively to the center axis of the UV light source 34 and after setting a collection angle α of the two second and third reflectors 40a, 40b and 42a, 42b, the reflector geometry, i.e. the cross section of the two second and third reflectors 40a, 40b and 42a, 42b in the direction of motion of the printing unit 10 or the arrows A and B is calculated according to these settings, as have been listed above. The calculation is done by means of an algorithm f(x) describing the curvature of each reflecting surface 58, 60 wherein f(x) is a polynomial of second or higher order, x is the distance from the plane 52 through the center axis of the light source 34 in the direction of motion of the printing unit 10 and the settings mentioned above are parameters in the polynomial.
In the reflector assembly 36 depicted in Figs. 4 and 5 the curvature of the reflecting surface 58 of each of the second reflectors 40a and 40b is described by a polynomial of the second order. With other words, the second reflectors 40a and 40b are provided with a parabolic reflecting surface 58.
Next the collection angle α of the second and the third reflectors 40a, 40b and 42a, 42b will be explained with respect to the schematic diagram of Fig. 7 showing a single reflector with a curved reflecting surface, where the collection angle α is delimited by two rays from the center axis of the UV light source 32 to the upstream and downstream edge 66 and 68 of the reflecting surface respectively.
The reflecting surfaces 58 and 60 of each pair consisting of one second reflector 40a or 40b and one third reflector 42a or 42b, which is positioned either upstream or downstream from the plane 52 through the center axis of the light source 42, have such a configuration, i.e. such a curvature and length, that the limits on both sides of the surface area 62a and 62b illuminated by the UV radiation from these reflecting surfaces 58 and 60 coincide. Furthermore the configuration of the reflecting surfaces 58 and 60 of the second reflector 40a and the adjacent third reflector 42a is such that the rays from the UV light source 32 reflected by the upstream edge 66 of the second reflector 40a and the downstream edge 68 of the third reflector 42a will both be directed to the downstream limit of the strip 62a whereas the rays from the UV light source 32 reflected by the downstream edge 68 of the second reflector 40a and the upstream edge 66 of the third reflector 42a will both be directed to the upstream limit of the strip 62a. A similar condition applies for the rays reflected by the other pair of reflectors 40b and 42b. This leaves more possibilities of reflector design in order to create a homogeneous light intensity distribution in the areas 62a and 62b.
With the reflector assembly 36 described above it is possible to portion the UV radiation from the UV lamp 30 including the direct light from the UV lamp 30 and the light reflected by the reflecting surfaces 54, 58 and 60 of the first, second and third reflectors 38a, 38b, 40a, 40b and 42a, 42b into a dose for surface curing and a dose for bulk curing such that a balanced curing will occur, where the curing speed of the bulk curing is about equal to the curing speed of surface curing.

Claims

Reflector assembly (36) for reflecting UV radiation from a UV-radiation source (30) onto a substrate (12) movable relative to the source (30) and having a surface at least partially covered with a UV-curable ink or coating, comprising
at least one first curved reflector (38a, 38b) for focusing UV radiation from the source (30) onto the surface in a first area (56) for providing a peak intensity in that area (56); and
at least one second reflector (40a, 40b) for reflecting UV radiation from the source (30) onto the surface in at least one second area (62a, 62b) for providing a relatively lower intensity in that area (62a, 62b), wherein said second reflector (40a, 40b) has a curved reflecting surface (58), wherein the cross sectional profile of said curved reflecting surface (58) in the relative-motion direction is described by a polynomial of second or higher order.
Reflector assembly (36) according to claim 1, wherein the polynomial of second or higher order is a function of at least one of a desired intensity and distribution of said UV radiation onto said at least one second area (62a, 62b), a distance between said UV-radiation source (30) and said substrate (12), a minimum distance between said UV-radiation source (30) and said at least one second reflector (40a, 40b) and a collection angle (α) defined by rays reflected from two opposite ends or edges (66, 68) of said at least one second reflector (40a, 40b).
Reflector assembly (36) according to claim 1 or 2, comprising at least one third reflector (42a, 42b) for reflecting UV radiation from the UV-radiation source (30) onto said surface in said second area (62a, 62b) in an overlapping or overlying relationship with said UV radiation reflected by said second reflector (40a, 40b).
Reflector assembly (36) according to claim 3, wherein the UV radiation from the UV-radiation source (30) reflected by the at least one second reflector (40a, 40b) and the UV radiation from the UV-radiation source (30) reflected by the at least one third reflector (42a, 42b) cover essentially the same areas (62a, 62b).
Reflector assembly (36) according to any one of claims 1 to 4, wherein the intensity distribution of the UV radiation across said second area (62a, 62b) is essentially constant.
Reflector assembly (36) according to any one of claims 1 to 5, comprising two of said second reflectors (40a, 40b), wherein in the direction of relative motion one of said second reflectors (40a) is located upstream of said UV-radiation source (30) and reflects UV radiation from said UV-radiation source to said second area (62a, 62b) upstream of said first area (56), and wherein the other one of said second reflectors (40b) is located downstream of the UV-radiation source (30) and reflects UV radiation emitted from said UV-radiation source (30) to said second area (62a, 62b) downstream of said first area (56).
Reflector assembly (36) according to any one of claims 3 to 6, comprising two of said third reflectors (42a, 42b), wherein in the direction of relative motion one of said third reflectors (42a) is located upstream of said UV-radiation source (30) and reflects UV radiation from said UV-radiation source (30) to said second area (62a, 62b) upstream of said first area (56), and wherein the other one of said third reflectors (42b) is located downstream of the UV-radiation source (30) and reflects UV radiation emitted from said UV radiation source (30) to said second area (62a, 62b) downstream of said first area (56).
Reflector assembly (36) according to any one of claims 1 to 7, comprising two of said first reflectors (38a, 38b) for reflecting UV radiation from the UV-radiation source (30) onto said surface in said first area (56) in an overlapping or overlying relationship, wherein in the direction of relative motion one of said first reflectors (38a) is located upstream of said UV-radiation source (30) and the other one of said first reflectors (38b) is located downstream of said UV-radiation source (30).
Reflector assembly (36) according to any one of claims 1 to 8, wherein said first area (56) is irradiated with direct UV radiation from the UV-radiation source (30) and/or at least a part of said at least one second area (62a, 62b) is irradiated with direct UV radiation from the UV-radiation source (30).
Printing apparatus (10), comprising a reflector assembly (36) according to any one of claims 1 to 9.