Manufacturing traffic signs

Abstract

Manufacturing of a traffic sign wherein the traffic sign includes a decorated retroreflective substrate comprising the steps: applying in a print area of a multi- pass inkjet printer (100) a retroreflective substrate; and jetting on the applied substrate, according to a plurality of pixels of a halftone image, a plurality of droplets of a free radical curable inkjet ink-set (I1..M) for decorating the applied substrate by the printer; and curing the jetted plurality of droplets on the substrate with UV radition after 300 or more milliseconds for spreading the jetted plurality of droplets wherein the plurality of droplets, having a first volume, is jetted within two or less passes by a plurality of inkjet printhead boxes (180) (PB1..N) of the inkjet printer wherein each inkjet printhead box (PBi, i1..N) is capable of jetting the ink-set (I1..M).

Description

DescriptionManufacturing traffic signsTechnical Field

[0001] The present invention relates to the manufacturing of traffic signs wherein a retroreflective substrate is used which is decorated by a free radical curable inkjet ink-set with a multi-pass inkjet printer.Background Art

[0002] Traffic signs are commonly used along roads to display information to vehicle drivers and pedestrians. Said signs convey information for traffic control purposes, for example, a stop sign, a speed limit sign, a railway crossing sign,... but also direct the road users along streets and highways, inform them of intersecting routes, direct them to cities or other important destinations, identify nearby rivers, forests, and historical sites, and generally give such information to help them along their way in the simplest way. Such signs are also used in harbours or along rivers for informing captains on a boat; also used in railway stations or along rails for informing train drivers; also used in airports for informing pilots on a plane.

[0003] Traditionally screen printing has been used to manufacture traffic control and guidance signs. However, screen printing unique guidance signs is costly and inefficient because a separate screen needs to be made for each individual sign.

[0004] Inkjet printing systems, such as multi-pass inkjet printers, have now proven their reliability in an industrial environment and are being incorporated in production lines for signs. Although their flexibility in use, such as variable data printing making unique signs possible, has led to an economical and efficient production, the print result with radiation curable inkjet inks still needs further improvements compared to screen printing inks.

[0005] It is known that many traffic signs include an optically active (e.g. retroreflective) substrate that has characters or images printed thereon. The characters or images provide information that is of interest to thevehicle drivers or pedestrians, while the retroreflective substrate allows the information to be vividly displayed at night. The term "retroreflective" refers to the property of reflecting an obliquely incident light ray in a direction antiparallel to its incident direction, such that it returns to the light source or the immediate vicinity thereof. Light from motor vehicle headlamps is retroreflected by the signs, allowing the information to be read more easily by passing motorists, train drivers, pilots and pedestrians.

[0006] EP4227373 A1 (AGFA NV) discloses a specific UV inkjet ink and UV inkjet ink-set to improve the efficiency of providing information towards the motorists and pedestrians. The information is printed on retroreflective substrate by inkjet technology. It was found that by using inkjet technology for providing the information embedded in a halftone image on such retroreflective substrate the retroreflectivity is influenced badly due to the layer that is formed by the cured droplets on said substrate. By using the claimed ink / ink-set the retroreflective was improved and more improved by using delayed curing at the expense of production-speed. Still the retroreflectivity when using traditionally screen printing cannot be achieved yet.

[0007] So, there is still a need for improvements in the retroreflectivity by using inkjet technology in a fast way to speed up the production of manufacturing traffic signs.Summary of invention

[0008] In order to overcome the problems described above, preferred embodiments of the present invention provide a free radical curable inkjet ink as defined in claim 1.

[0009] It was surprisingly found that by using a multi-pass inkjet printer having more than one inkjet printhead boxes (PBI..N) wherein each inkjet printhead box (PBi, i->i ..N) is capable of jetting the same free radical curable inkjet ink-set (II .M) on a retroreflective substrate, the retroreflectivity of the manufactured traffic sign is improved but not at the expense of productionspeed.

[0010] Further advantages and preferred embodiments of the present invention will become apparent from the following description.Drawings

[0011] Fig. 1 illustrates a part of the steps of manufacturing of traffic signs. A retroreflective substrate is applied (10) on a multipass inkjet printer; jetted (20) with multiple droplets which are then cured (30). Hereby is the retroreflective substrate decorated which can be attached (not shown) to a metal, wood or plastic to form a traffic sign.

[0012] Fig. 2 illustrates a multi-pass inkjet printer (100) having a row (185) of multiple inkjet printhead boxes (180), each having the same ink-set having two colors (C1 , C2) which are arranged mirror wise in each inkjet printhead box (180), respectively with colour order C1 ;C2 and with colour order C2;C1 . The row (185) of multiple inkjet prinhead boxes has at both ends an UV radiation device (170) for the curing step of the jetted droplets. While the droplets are jetted the whole row (185) of multiple inkjet printhead boxes (180) is moved (300) above the retroreflective substrate (200) (300) for forming swaths (also known as fast-scan movement) and the retroreflective substrate (200) is also moved (400) during the jetting of the droplets (also known as slow-scan movement).Description of embodimentsManufacturing of a traffic sign

[0013] The present invention is a manufacturing method of a traffic sign wherein the traffic sign includes a decorated substrate wherein the substrate is a retroreflective substrate which is decorated by inkjet technology, more specific by a multi-pass inkjet printer (100).

[0014] It is important for the transport industry that legible traffic signs are manufactured for traffic safety (highways, harbours, railways, airports...), especially in low-light conditions and at night.

[0015] So, the use ^of retroreflective .substrates is herein a very big improvement to enhance the retroreflectivity of traffic signs to provide positive visual guidance to assist driver’s awareness and to provide a safer driver environment. The retroreflectivity of traffic sign is measured by: the coefficient of retroreflection RA (expressed in cd / m2 / lx) represents the ratio of the luminance (cd / m2) to the illuminance (lx). The coefficient RA was determined using a Retrosign® GRX reflectometer from DELTA (Denmark) according to the norm EN 12899 with an illumination angle of 5° and an observation angle of 0.33°. The latter simulates the viewing of a traffic sign at night by a standard car using tungsten lamps at a viewing distance of 100 m, wherein the tungsten lamps and the driver are 0.65 m respectively 1.20 m above the road.

[0016] The retroreflective substrate is preferably a substrate having prismatic air- backed construction or prismatic metalized construction or an embedded glass bead construction or an encapsulated glass bead construction to enhance the retroreflectivity of the substrate. The prismatic constructions are most preferred. They are also called cube corner constructions. Such prismatic constructions return light more efficiently than glass bead constructions. Incoming light bounces namely off all surfaces in the prismatic construction and better returns to its source.

[0017] Examples of retroreflective materials can be found e.g. in W02000043813 A1 (Reflexite Corporation); US6247818 B1 (3M Innovative Properties Company); US6024455 A (3M Innovative Properties Company); and EP1756632 B1 (Orafol Americas Inc).

[0018] An other example of a retroreflective substrate is Oralite™ 9910 from ORAFOL Europe GmbH.

[0019] The decorated retroreflective material is then applied on the base of a traffic sign which is mainly made of metal, plastic or wood. In a preferred embodiment, the retroreflective substrates have an adhesive back-layer containing a permanent pressure sensitive adhesive for application on said base such as aluminium and zinc coated steel plate. Such an adhesive layer facilitates the manufacturing of traffic signs.

[0020] As effectiveness of traffic signs, having a retroreflective substrate, tends to deteriorate over time when used, regular replacement is important. Traffic signs lose namely their colour, legibility, and retroreflectivity in time because of corrosion caused by exhaust fumes and industrial pollution, sun exposure, dirt, ice and wind. Hereby is the use ^of jnkjet Technology for decorating the retroreflective substrate an improvement because fast and manufacturing traffic signs especially comprising specific information becomes possible for said replacement of deteriorated traffic signs and so provide a safer driver environment.

[0021] Inkjet technology gives the possibility to make unique traffic signs cheaper than when traditional screen printing is used for decorating the retroreflective substrate. If e.g. temporary detours have to be organised said unique traffic signs can be manufactured fast and cost-effective.

[0022] In the present invention the inkjet technology of a multi-pass inkjet printer (100) is used. An example of a multi-pass inkjet printer is the Anapurna H 1650i LED from AGFA NV which is capable of forming images in bidirectional printing mode 540x1080 dpi and UV LED curing. The retroreflective substrate is int the present invention applied on a print area of a multi-pass inkjet printer (100) which is located between a substrate support and the multiple inkjet printhead boxes of the multi-pass inkjet printer (100).

[0023] To provide a safer driver environment multiple ^colours are used to inform the driver. According national traffic regulations, some colours indicate a danger e.g. red and other specific colours indicate an obligation towards the driver e.g. blue. Said multiple colours are achieved by the present invention by using an inkjet ink-set (II. M), having a plurality of inks wherein preferably each ink different in colour towards the colour of the other inksof the inkjet ink-set (II .M) by a colour difference more than 10 AEciELab,76 which is calculated by the well-known formulawherein both colours are specified in CIELAB colour space.The colours of inks of the inkjet ink-set (II .M) are preferably selected from the group of colours: cyan, magenta, yellow, black, brown, green, blue, white. It is important to use colours that results in a good contrast between each other to make the information on traffic signs fast readable by the driver.An example of an inkjet ink-set used in the manufacturing of traffic signs is ORALITE® 5019i UV Digital Printing Ink-set

[0024] By using colour ^halftone ^techniques, preferably colour error diffusion halftone techniques, several grades of colours can be achieved on a decorated retroreflective substrate. It is found that a dependent colour halftone technique is a preferred halftone technique. Dependent colour halftoning method which also controls the dot placement over the entire colour image and avoid the dot-on-dot printing as much as possible influences positive the retroreflectivity because a more uniform flat layer of cured droplets is formed. An example of dependent colour halftone technique is disclosed in ‘ Dependent Color Halftoning: Better Quality with Less ink fromlSasan Goorari published in ‘ Journal of imaging Science and Technolog , on July 2004.

[0025] The inkjet ink-set (II. M) defines a colourant space having a plurality of colours that can be used for decorating the retroreflective substrate by mixing the inks of the inkjet ink-set (II .M) on the retroreflective substrate. But it is found that a big mixture of said inks influences badly the retroreflectivity so the total ink limit should be lower than 180%. Said total ink limit may be achieved by using a printer colour profile, e.g. an ICC profile having a format defined by International Color Consortium, wherewith colours are converted with a maximum total ink limit lower than180% within the colourant space. Such maximum total ink limit is also called Total Area Coverage, abbreviated as TAC.

[0026] The inkjet ink-set (II..M) may also comprise a clear ink to achieve a varnish or protective coating on top of the decorated retroreflective substrate or to achieve a coating layer on the retroreflective substrate before applying the droplets from the inkjet ink-set (II..M) on the coated retroreflective substrate.

[0027] In the present invention the information on a traffic sign is applied by inkjet technology which applies droplets from the inkjet ink-set (II..M) on the retroreflective substrate but said droplets are in the present invention jetted from more than i^one Li kjet printhead box (PBI..N) wherein each inkjet printhead box is capable of jetting said inkjet-set (II..M) whereby the production-time of manufacturing traffic signs becomes faster and surprisingly the retroreflectivity of a decorated retroreflective substrate becomes higher and thus the traffic sign having such decorated retroreflective substrate guarantees a safer driver environment. The inkjet printhead boxes (180) are forming a row (185) of consecutive inkjet printhead boxes (180) (PBI..N) within the multi-pass inkjet printer (100). The use of said multiple inkjet printhead boxes (180) seems to produces more uniform flat layer of cured droplets on the retroreflective substrate which effects the retroreflectivity positive wherein the traffic signs guarantees a safer driver environment. The multiple inkjet printhead boxes (180) are transported together in a first direction and a second direction opposite to the first direction over the applied retroreflective substrate while jetting and / or curing. The applied retroreflective substrate may during the jetting and / or curing also be transported by a transport device such as a conveyor belt. A movement of the inkjet printhead boxes (180) in one direction is called a swath.

[0028] The use ^of free radical curabje inkjet nk-set (H. M) in the present invention guarantees a longer effectiveness of the traffic signs which tends to deteriorate over time. Said inks makes that weatherability of the traffic signs, which are used mostly used out-side, becomes higher and thus guarantees a safer driver environment.

[0029] Said inks are cured by one or more UV radiation devices (170) such as UV Led modules of manufacturer IISHIO Europe BV. In the present invention the curing js delayed after a droplet is jetted on the retroreflective substrate. It is found that such delay seems to produce a more uniform flat layer of cured droplets on the retroreflective substrate because the droplets spreads on the substrate during the delayed curing whereby the retroreflectivity of the decorated retroreflective substrate becomes higher. The delayed curing is preferably higher than 300 milliseconds, more preferably higher than 1000 milliseconds, most preferably higher than 1500 milliseconds. More than 2500 milliseconds of delayed curing may enlarge the print speed to much for having a fast production of traffic signs.

[0030] Two UV radiation devices are normally mounted on both side of an inkjet printhead box such as constructed in the Anapurna H 1650i LED of manufacturer AGFA NV. But in a preferred embodiment they be mounted between each inkjet printhead box (180) of a row (185) of multiple inkjet printhead boxes (180), additionally on both sides of the row (185) of multiple inkjet printhead boxes (180).

[0031] An example disclosing of an UV radiation device for multi-pass inkjet printers is US9757960B2 (AGFA NV). Said UV radiation device has a movable shutter to make delayed curing in a multi-pass inkjet printer (100) possible.

[0032] The spreading of the droplets can also be improved by using a corona treatment on the retroreflective substrate or partly decorated retroreflective substrate. Said spreading produces a more uniform flat layer of cured droplets on the retroreflective substrate which effects the retroreflectivity of the decorated retroreflective substrate in positive way and so guarantees a safer driver environment with such manufactured traffic signs. The corona treatment may be applied during a swath (= a movement of the multiple inkjet printhead boxes) by a corona treatment device which is attached to said inkjet printhead boxes (180). The corona treatment is preferably done before the droplets are landed on the retroreflective substrate.

[0033] It is found that also the adhesion of the cured droplets which forms the printed halftone image on the retroreflective substrate becomes better whereby delamination and deterioration is less.

[0034] Each inkjet printhead box (PBi;j-> 1..N), which are constructed in a row of multiple inkjet printhead boxes (180) within the multi-pass inkjet printer (100), is able to jet droplets from the inkjet-set (II .M) which is achieved by a plurality of inkjet printheads in a certain consecutive order of colours. Said order identifies which ink is printed first and which ink is printed later in a swath of the multi-pass inkjet printer (100). It is found the consecutive order of colours of an inkjet printhead box of the multi-pass inkjet printer (100) (e.g. CMYK) is mirrored opposite the consecutive order of colours of an other inkjet printhead box of the multi-pass inkjet printer (100) (e.g. KYMC).

[0035] After decorating the decorated retroreflective substrate, a protective layer may be attached on top of the cured droplets. Said protective layer may be applied by inkjet technology with a clear free radical curable inkjet ink or applied by lamination transparent laminate on top of the cured droplets. The advantage of applying a protective layer is that not only the scratch resistance of the sign is improved, but also the light stability of the pigments in the cured image as the transparent film filters out UV light that is very harmful for bleaching pigments.

[0036] Suitable transparent films are readily available from commercial sources, such as Orafol® 5061 , 5062 and 5051 from ORAFOL GmbH.Examples:Multi-pass inkjet printer: Anapurna H1650i LED of manufacturer AGFA NV adapted for jetting 30 pl droplets and splitting the single inkjet prinhead box in two inkjet printhead boxes (180) where each of the printhead boxes is capable of printing the same ink namely the red ink of ORALITE® 5019i UV Digital Printing Ink-set.On Oralite™ 9910 from ORAFOL Europe GmbH large red squares are printed with said red ink. The halftone image was formed by halftoning a digital image having a 96% toned square with an error-diffusion halftone technique. The jetted plurality of droplets on the retroreflective substratewere cured with UV radiation (UV LED) after more than 300 milliseconds and less than 1000 seconds.Example 1 : the red square is formed with cured droplets from only one of said printhead boxes by jetting droplets with said red ink within 4 passes at a resolution of 720x720. The example is measured on three locations (MEA1 ; MEA2, MEA5; MEA6) within the red square.Example 2: the red square is formed with cured droplets from both printhead boxes by jetting droplets with said red ink within 2 passes at a resolution of 720x720. Around 50% of the jetted droplets came from one of the inkjet printhead boxes (180) and the other jetted droplets came from the other inkjet printhead box. The example is measured twice (MEA3; MEA4; MEA7; MEA8) on another location within the red square.

[0037] Table 1:

[0038] Table 2:The parameters P1 ; P2; P3; P4; P5; P6: P7 are measured with a pSurf mobile confocal microscope from Nanofocus and gives an idea how the topography is formed with the cured droplets with the red ink. The topography was herein quantitatively evaluated by software from MountainsMaps which results in measurement data delivered conformingto norms I3D parameters defined in ISO 25178 including height (Sa, Sq, Ssk, Sku, Sz, etc.,) and functional (Smr, Smc, Sxp) parameters defined in ISO 4287 and new ISO 21290 primary and roughness parameters (Ra, Rq, Rsk, Rmr, Rdc, Rdq, RPc, etc.). The parameters show that that Example 2, according to the present invention, has a smoother surface which results in a better retroreflection.

[0039] P1 = Surface RoughnessP2 = Surface RMS SlopeP3 = Developed interfacial area ratioP4 = Mode[Polar angle (alpha)]P5 = Mean[Polar angle (alpha)]P6 = Circular mean[Polar angle (alpha)]P7 = Main[Polar angle (alpha)]

[0040] For retroreflectivity a retroreflectometer (e.g. from manufacturer Zehnter) is used that measures the retroreflectivity of the retroreflective substrate and the decorated retroreflectivity substrates to ensure optimal road driving conditions wherein the following standards are used ASTM E1710-18, ASTM E2177, ASTM E2302, CIE 54.2, EN 13197, EN 1436. The retroreflectivity may be measured at several angles.

[0041] One of the parameters that can be measured is the attenuation of incoming light versus reflected light at the following angles: 0.2°; 0.33° and 0.5° which results in a percentage. The lower the attenuation, the better the retroreflectivity is.

[0042] Table 3:

[0043] The lower attenuation as shown in the retroreflectivity measurements onExample 2 and the smoothness of the topography of the cured droplets onthe Example 2 illustrates the benefit of the present invention, namely a safer driver environment.So, by using lesser swaths for forming the halftone image that contains the information for a traffic sign on the retroreflective substrate which is made possible by using a row of multiple inkjet printhead boxes (180) the retroreflectivity is much better. It was surprisingly found that lesser swaths results, as shown in the examples namely in smoother topography of the cured droplets.

Claims

ClaimsClaim 1. Manufacturing of a traffic sign wherein the traffic sign includes a decorated retroreflective substrate comprising the steps:- applying (10) in a print area of a multi-pass inkjet printer (100) a retroreflective substrate; and- jetting (20) on the applied retroreflective substrate, according to a plurality of pixels of a halftone image, a plurality of droplets of a free radical curable inkjet ink-set (II. M), having a plurality of inks, for decorating the applied retroreflective substrate by the multi-pass inkjet printer (100); and- curing (30) the jetted plurality of droplets on the retroreflective substrate with UV radiation after 300 or more milliseconds for spreading the jetted plurality of droplets; wherein the plurality of droplets, having a first volume, is jetted within two or less passes by a plurality of inkjet printhead boxes (180) (PBI..N) of the inkjet printer wherein each inkjet printhead box (PBi, ^I..N) is capable of jetting the free radical curable inkjet ink-set (II. M).Claim 2. Manufacturing of a traffic sign according to claim 1 wherein during a pass of the two or less passes a part of the plurality of droplets, having a second volume which is between 40% and 100% of the first volume, is jetted; and wherein 40% and 60% of the second volume is jetted by an inkjet printhead box (PBi) of the plurality of inkjet printhead boxes (180) (PBI..N).Claim 3. Manufacturing of a traffic sign according to claim 2 wherein the more than one inkjet printhead boxes (180) comprise:- a first inkjet printhead box (PBi) which applies the free radical curable inkjet ink-set in a first consecutive order of colours (Oi); and- a second inkjet prinhead box (PB2) which applies the free radical curable inkjet ink-set (II..M) in a second consecutive order of colours (O2) which is mirrored opposite the first order of colours (O1).Claim 4. Manufacturing of a traffic sign according to claim 3 wherein the halftone image is generated by halftoning a selected colour image for decorating retroreflective substrate with an error diffusion halftoning technique.Claim 5. Manufacturing of a traffic sign according to claim 4 wherein the selected colour image is converted before the halftoning by a printer colour profile to a colourant space of the multi-pass printer which is defined by the plurality of inks and wherein the printer profile defines a maximum total ink limit lower than 180%.Claim 6. Manufacturing of a traffic sign according to claim 4 or 5 comprising the step for spreading the jetted plurality of droplets:- applying a corona treatment on the retroreflective substrate before the plurality of droplets are jetted.Claim 7. Manufacturing of a traffic sign according to any of the previous claims wherein after step c) a protective coating is applied on the decorative retroreflective substrate.Claim 8. Manufacturing of a traffic sign according to any of the previous claims wherein the volume of each droplet of the plurality of droplets is 30 pL or more and a horizontal and vertical print resolution between 720 dpi and 2160 dpi.Claim 9. Manufacturing of a traffic sign according to any of the previous claims where an ink of the plurality of inks contains a colour pigment and a polymerizable composition, wherein the colour pigment is a mixture of colour pigments including at least one colour pigment selected from a quinacridone pigment, a diketopyrrolo pyrrole pigment, and a mixed crystal thereof.Claim 10. Manufacturing of a traffic sign according to any of the previous claims wherein the retroreflective substrates is having a prismatic air-backed construction and / or prismatic metalized construction and / or an embedded glass bead construction and / or an encapsulated glass bead construction.Claim 11. Manufacturing of a traffic sign according to any of the claims from 1 to 10 wherein the inkjet ink-set (II..M) comprises a clear ink for achieving a varnish or protective coating on top of the decorated retroreflective substrate.Claim 12. Claim 12: Manufacturing of a traffic sign according to any of the claims from 1 to 10 wherein the inkjet ink-set (II. M) comprises a clear ink for achieving a coating layer on the retroreflective substrate before applying the plurality of droplets from the inkjet ink-set (II. M) on the coated retroreflective substrate.

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