PRINTING FORME FOR ROTARY SCREEN PRINTING MADE FROM FIBRE-REINFORCED PLASTICS MATERIAL

Abstract: A printing forme (10) for rotary screen printing comprises a thin-walled hollow cylinder (12) made from fibre-reinforced plastics material, the cylinder wall of which is provided with continuous printing openings (14) which define an image (16, 18) which is to be printed. According to the invention, the cylinder (12) comprises at least one layer (20) having fibres (22) with at least one oriented direction of the fibres. Examples comprising, inter alia, multi-layer laminates (30, 40) made from carbon/epoxy having at least one layer with an oriented direction of the fibres which is parallel to the longitudinal axis of printing forme (10) are preferred.

Description: Printing forme for rotary screen printing made from fibre- remforced plastics material The present invention relates to a printing forme for rotary screen printing, comprising a thin-walled hollow cylinder made from flbre-remforced plastics material, the cylinder wall of which is provided with continuous printing openings which define an image which is to be printed. A printing forme of this nature is described, for example, in NL-A-8802144. The cylinder used in this printing forme is made from plastics material, to which solid particles and/or fibres may be added in order to increase the mechanical stability and strength. The wall of the cylinder is provided with a large number of small perforations which are in the form of a grid of dots, i.e. printing openings, which define the image which is to be printed. The printing openings may be made with the aid of a laser. US-A-3 981 237 has likewise disclosed a printing forme made from plastics for rotary screen printing, the cylinder wall of which may comprise one or more layers. This patent states that, by suitably selecting the plastics material, such as polyester, it is possible to obtain the mechanical properties which are demanded for use as a printing forme. In this known printing forme, the printing openings may likewise be made with the aid of a laser. The plastics material may be filled with a reinforcing material such as glass fibres. Such a rotary printing screen is manufactured from a tubular blank made by conventional extrusion methods, which is stretched biaxially afterwards. These filler materials are (short) chopped fibres, which are distributed randomly in the plastics material, even after biaxially stretching. Accordingly the volume concentration of the fibres is 40% at maximum. However, the strength and rigidity of a screen thus manufactured leaves much to be desired. The abovementioned printing formes made from plastics material, in which printing openings which define the printed image are provided, are also known in the art as plastics "galvano", even though there is actually no electrodeposition of metals involved in the production of these printing formes. US-A-4 959 260 discloses a mesh fabric useful for manufacturing a flat printing screen and consisting essentially of conjugate filaments. Each of the conjugate filaments has a sheath made from a material having a high adhesive property to an emulsion and resin used for preparing a stencil from a screen, and a core made from a material having high dimensional stability and elastic recovery property. Furthermore the mesh fabric should meet well defined mechanical requirements. In such a fabric or a screen produced therefrom no plastics matrix is present, wherein the filaments are incorporated. Nickel stencils for rotary screen printing are difficult to handle and are fragile, in particular if the stencil (still) contains printing paste. The thickness of the nickel stencils, which is of decisive importance for the rigidity, is limited by the size of holes required and the mesh number, and can only be increased to a limited extent using the standard electroformmg techniques. In the case of plastics materials in which perforations can be made with the aid of high-energy radiation, it is in principle possible to make more robust printing formes, which are therefore easier to handle. However, these known printing formes made from plastics have not attained commercial success. One of the possible reasons for this is the low rigidity of plastics in general. Plastics are less rigid than nickel, which is the preferred material for conventional screen-printing formes, by a factor of approximately 100. A nickel stencil has a modulus of elasticity E of approximately 90 GPa; a cylinder provided with perforations which is made from polyester, which is one of the more rigid plastics, has a modulus of elasticity E of approximately 2 GPa. The elastic deformations which occur, inter alia, when the printing forme is used for rotary screen printing, are dependent on the modulus of elasticity and also on the wall thickness of the cylinder. A PET galvano must have a thickness of approximately 0.89 mm in order to attain the same level of rigidity as a nickel stencil with a thickness of 0.1 mm. A large wall thickness of this nature has an adverse effect on the printing properties and the possibility of making printing openings with the aid of a laser. In addition, thick-walled printing formes of this nature are less easy or even altogether impossible to push together for transport and/or storage, as is customary with the thin-walled metal screen-printing formes, which can be transported and stored pushed together in the shape of a kidney. Furthermore, it is known that under load plastics are deformed more readily as a result of creep than metals, such as nickel. As has been already been suggested above in accordance with NL-A-8802144, plastics can generally be made more rigid by the addition of fillers and/or fibres. However, this Dutch patent application does not provide any insight into how the intended increase in mechanical stability can be achieved. The object of the present invention is to provide a printing forme for rotary screen printing made from fibre- reinforced plastics material, in which the wall thickness of the cylinder is such that the printing properties and the possibility of making printing openings with the aid of a laser, as well as the possibility of transporting and/or storing the formes pushed together, are not adversely affected, or are scarcely adversely affected. Another object of the invention is to provide a printing forme for rotary screen printing made from fibre-reinforced plastics material, the mechanical properties of which are improved compared to the printing forme made from (fibre- reinforced) plastics material described in the prior art. According to the invention, the printing forme for rotary screen printing of the type described above is characterized in that the cylinder comprises at least one layer having fibres with at least one oriented direction of the fibres. A single plastics layer with fibres incorporated therein which are oriented in one direction (also referred to below as unidirectional fibres) has anisotropic elastic properties, i.e. the properties are dependent on the direction in which the load is acting. The addition of fibres to plastics generally imparts an improved rigidity. Fibres may be added in the form of so-called short fibres (also known as "chopped fibres") , as long fibres which are arbitrarily distributed in the plastics material, and as unidirectional fibres. The use of unidirectional fibres in principle provides the highest rigidity which can be achieved. Furthermore, the highest fibre content can be achieved with layers having unidirectional fibres, and consequently it is with these fibres that the highest modulus of elasticity can be attained. As has already been stated, a single plastics layer having unidirectional fibres has anisotropic elastic properties. According to the invention, this anisotropy is used to limit the deformation in the direction of the load. In this specification a fibre-reinforced plastics material is meant to be a plastics matrix layer, wherein reinforcing fibres are incorporated. According to the invention these fibres are oriented in one direction. In addition to the customary use of the printing forme according to the invention for rotary screen printing, in which printing ink or printing paste is applied to a substrate which is to be printed through the continuous printing openings, the printing forme is also suitable for applying other coating materials to a substrate of this nature. One example of a coating material of this nature is a hot-melt adhesive. In other words, the expression "for rotary screen printing" is not restricted to using printing ink or printing paste to print a substrate, but rather, in a broad sense, comprises the application of a material to a substrate through the continuous openings while the printing forme is rotating. The printing screen according to the invention may be a conventional screen from which a stencil is prepared using standard photoresist techniques, so that only those openings in the screen remain open, which define the image to be printed, the so called "continuous printing openings", while the other openings in the screen are covered and/or filled by the cured portions of the photoresist. The printing screen according to the invention is preferably a galvano, comprising only continous printing openings, which may have been made by a laser. During operation, a plurality of loads act on a printing forme simultaneously but in different directions. According to one embodiment, the printing forme according to the invention therefore comprises a fibre-reinforced layer having fibres with two differently oriented directions of the fibres, such as a woven or braided fabric of fibres, a fibre mat or a fibre cloth. To reduce the total amount of bending, it is preferable for one oriented direction of the fibres to run parallel to the longitudinal axis of the printing forme, i.e. in the axial direction . According to another embodiment of the printing forme according to the invention, the cylinder comprises a first layer having fibres in a first oriented direction of the fibres, and a second layer having fibres in a second oriented direction of the fibres, with the first and second directions of the fibres not being identical. As has already been stated, the separate layers having unidirectional fibres may have a high fibre content (compare a fibre content of 63% by volume for a unidirectional carbon fibre in epoxy resin with a fibre content of approximately 35% by volume for a nonwoven in epoxy resin) , which is of benefit to the elastic properties. A cylinder of this type for the printing forme according to the invention is composed of at least two layers of fibre-reinforced plastic, of which the fibres in a layer are oriented in one direction and the directions of the fibres do not run parallel to one another. Preferably, the oriented direction of the fibres of the first layer forms an angle α with the longitudinal axis of the printing forme, and the oriented direction of the fibres of the second layer forms an angle -α with the longitudinal axis of the printing forme. The cylinder advantageously also comprises a further layer having fibres in a third oriented direction of the fibres, the direction of the fibres of which further layer runs parallel to the longitudinal axis of the printing forme. More preferably, the further layer is arranged between the first and second layers. Examples of the angle α are 45° and angles >60°. A 3- layer laminate for use in the printing forme according to the invention therefore comprises 3 layers with differently oriented directions of the fibres of, for example, 0° and ± 60° or 0° and ± 45° or 0° and 90° (x2) with respect to the longitudinal axis of the printing forme. According to another embodiment of the printing forme according to the invention, the first layers with a first oriented direction of the fibres and the second layers with a second oriented direction of the fibres alternate with one another. One example of an embodiment of this nature is a printing forme which is made from a 4-layer laminate, with the oriented direction of the fibres of the first and third layers and of the second and fourth layers being identical. According to a particularly preferred embodiment, the cylinder is of symmetrical structure in the thickness direction, and the oriented direction of the fibres of outermost, first layers forms an angle α with the longitudinal axis of the printing forme, and the oriented direction of the fibres of second, intermediate layers forms an angle -α with the longitudinal axis of the printing forme. In this embodiment too, the cylinder advantageously comprises a further layer having fibres in a third oriented direction of the fibres, which direction of the fibres runs parallel to the longitudinal axis of the printing forme. Just as in the cylinder comprising 3 layers which has been discussed above, the further layer is preferably arranged between the second, intermediate layers, i.e. in the centre as seen in the thickness direction of the cylinder wall. In the multilayer laminates described above, which are used to make the printing forme according to the invention and which comprise a further layer having fibres in an oriented direction of the fibres which is parallel to the longitudinal axis of the printing forme, the thickness of this further layer is advantageously greater than the thickness of the other layers. In this case, the other layers with an identically oriented direction of the fibres generally have an identical layer thickness. Depending on the thickness (diameter) of the fibres and the desired final thickness of the cylinder of the printing forme according to the invention, this printing forme may also comprise a plurality of adjacent layers having the same direction of the fibres. The presence of at least one layer having unidirectional fibres with at least one oriented direction of the fibres or a combination of layers of this nature in the printing forme according to the invention allows a low total wall thickness to be maintained, preferably in the range from 80 - 300 micrometres, so that the printing properties are good. A small thickness of this nature also allows the printing openings to be made using a laser, in accordance with a design which is to be printed. In addition, the printing formes can be bent into the kidney shape which is suitable for transport and can therefore be pushed together. The mechanical properties of the printing forme according to the invention are also good, as will be apparent from the following more detailed consideration of the load which acts on a screen-printing forme during printing. In rotary screen printing, it is customary for a printing forme - galvano or stencil - to be clamped on a printing machine above a printing belt. A printing belt of this nature is an endless belt to which the substrate to be printed is temporarily attached. The printing belt is supported by means of a counter roller at each printing position in the printing machine. In the printing forme there is a squeegee which supplies the printing paste and presses it through the printing openings in the cylinder wall of the printing forme on to the advancing substrate. The load to which the printing forme is subjected during the printing process is caused by the contact between the printing forme, on the one hand, and, on the other hand, the squeegee the printing paste and the substrate to be printed. The printing forme moves past the stationary squeegee, resulting in a frictional force directed oppositely to the direction of rotation of the printing forme. The shear forces of the printing paste also act in this direction. To partially compensate for these forces, the speed of the printing belt is set to be a few tenths of a per cent higher than the peripheral velocity of the printing forme; in other words, the printing belt is said to have a lead over the stencil. Therefore, the substrate exerts a frictional force, which is oppositely directed to that of the squeegee, on the printing forme. The resulting maximum load on the printing forme, which is also known as the process force, is a uniformly distributed load in the tangential direction on the underside of the printing forme at the location where the printing forme is m contact with the squeegee and the substrate to be printed. The result is that an image (design) which is to be printed is applied to the substrate in a curve, i.e. curvature arises if there are differences in the deformation. The above entioned resultant maximum load, for a nickel stencil having a thickness of 0.1 mm and with a print width of 1850 mm is approximately 0.1 N/mm. Above this load, torsional bending occurs. To prevent this bending of a printing forme, an axial pre-stress of 1000 N is imparted. For longer stencils, the maximum permissible load is proportionally lower. Curvature means that a straight line in the image to be printed appears as a curve on the substrate. Factors which contribute to this curvature include local deformation of the printing forme, (pure) torsion of the printing forme and (pure) bending of the printing forme. Local deformation arises as a result of the fact that the printing forme is only held at the two ends, by end rings. Torsion of the printing forme is corrected by adoption of a skew position, likewise resulting in a curve being formed on the substrate. Of these factors, local deformation is the biggest contributor to the curvature. In the printing forme according to the invention, local deformation is counteracted by the use of unidirectional fibres in one or more layers having one or more differently oriented directions of the fibres. The oriented directions of the fibres, layer thicknesses and material can be varied, although there are preferred directions and materials, as has already been stated above and will be illustrated in more detail below. If desired, in the printing forme according to the invention, it is possible for the material layers of the cylinder to be combined to form a quasi-isotropic laminate, in order to reduce the anisotropy of a single layer having unidirectional fibres. As has already been stated above, it is possible for the fibre materials and the plastics of the binder matrix to be selected as a function of the desired properties. Examples of suitable fibre materials include carbon fibres, inorganic fibres such as glass fibres and boron fibres, metal fibres and organic plastic fibres, such as stretched fibres, for example aramid fibres and fibres of high-strength stretched polyethylene, as well as combinations thereof. The carbon fibres and inorganic fibres are particularly preferred, and of these carbon fibres are most preferred. The binder material used is not critical, since it makes little contribution to the mechanical properties compared to the fibres, and may be selected from the known thermoplastics, such as polyesters and thermosetting plastics, such as epoxy resins. Of customary combinations, carbon/epoxy is preferred, on account of the excellent relationship between cost price and rigidity. The rigidity of this combination is virtually twice as great as that of kevlar/epoxy and three times as great as that of glass/epoxy. Other combinations with a still higher rigidity include carbon/polyimide, graphite/epoxy and silicon carbide/ceramic. However, these combinations are expensive. The above entioned single-layer materials are commercially available. The elastic parameters for a number of fibre/binder matrix single-layer combinations are described, inter alia, in "Engineering Mechanics of Composite Materials", I.M. Daniel et al . , Oxford University Press, 1994, and are compiled below in Table 1. Table 1 The elastic parameters for quasi-isotropic laminates are given in Table 2 below. Table 2 The materials from which the layers are made may, in addition to the abovementioned binders and unidirectional fibres, also contain other conventional additives, such as lubricants, adhesion-promoting agents, fillers, pigments and the like, if desired. Furthermore, if desired, it is possible for the printing forme accordxng to the invention to be provided with one or more additional surface layers which do not contain any fibres and have desired properties, in particular plastics layers, on the inner and/or outer side of the cylinder. Examples of desired properties of this nature include, inter alia, reduced friction, wear resistance, scratch resistance, hydrophobic properties, an increase in contrast, and strength. An increase n contrast may be advantageous in order to make the fine printing openings and the design which they form readily visible under defined viewing angles, for example by the application of a white surface layer of polyethylene to the outer circumference of a black cylinder made from epoxy resin containing carbon fibres. A surface layer of polytetrafluoroethylene, such as TEFLON, may be provided with a view to reducing friction. The invention also relates to a rotary screen-printing device provided with a printing forme according to the invention. The invention is explained below with reference to the appended drawing in which: Fig.l shows a perspective, diagrammatic view of a rotary screen-printing forme; Fig. 2 shows a diagrammatic view of a fibre-reinforced plastics layer with an oriented direction of the fibres; Fig. 3 shows an exploded, diagrammatic view of a symmetrical 3-layer laminate; and Fig. 4 shows an exploded, diagrammatic view of a 5-layer laminate . Fig. 1 shows a rotary screen-printing forme 10 which comprises a thin, hollow cylinder 12 made from fibre-reinforced plastics material. Perforations are made m this cylinder m accordance with the design to be printed using high-energy radiation, such as a C0 2 laser. These perforations form the printing openings 14, which define the image 16 and 18 to be printed, in the embodiment shown the full area of a circle 16 and the full area of a rectangle 18. Fig. 2 shows a planar fibre-reinforced plastics layer 20 having fibres 22 (diagrammatically indicated by relatively thin, continuous lines) , of which the oriented direction of the fibres runs parallel to the longitudinal axis of a cylinder which is to be made from this layer. This axial direction of the fibres is advantageous for reducing the total bending of the printing forme 10. For the sake of simplicity, in the following figures identical components are denoted by identical reference numerals . Fig. 3 diagrammatically depicts a 3-layer laminate 30, in which the middle layer 32 consists of the fibre-reinforced plastics layer shown in Fig. 2, i.e. the fibres 22 run parallel to the longitudinal axis (not shown in Fig. 3) . On both sides of the middle layer 32 there is an outermost layer 34, the fibres 22 of which have an orientation which is perpendicular to the longitudinal axis. A laminate with unidirectional fibres of this nature may, for example, be formed by means of customary techniques, such as pultrusion, injection-moulding, lamination and winding, and may be machined, if necessary, by grinding, polishing and the like. A suitable method for preparing a tubular body is disclosed in US-A-5 071 506, which body is perforated thereafter in order to provide the continuous printing openings. In this method a composite material is applied on a mandrel having an inflatable bladder. The mandrel is inserted into a single press mould. Upon pressurizing the inflatable bladder the composite material is forced against the wall of the mould. Subsequently the composite material is cured. Fig. 4 shows a diagram illustrating the layer structure of an example of a 5-layer laminate 40 according to the invention m planar form. The laminate 40 with a symmetrical structure comprises a middle layer 32, in which fibres 22 are oriented in the longitudinal direction of the printing forme which is to be made from this laminate. On either side of the middle layer 32 there are two intermediate layers 44, of which the direction of the fibres forms an angle α -70° with respect to the longitudinal axis. The outermost, first layers 46 have fibres 22 with an orientation of +70° with respect to the longitudinal axis. Table 3 below shows the deformation data for carbon/epoxy laminates (Ex=142 GPa; Ey=10.3 GPa; Gxy=7.2 GPa and v=0.27; repeat = 64 cm), with variations in the layer thickness and the direction of the fibres. For comparison purposes, the deformation data of a nickel stencil and a printing forme made from isotropic carbon/epoxy (E=56.7 GPa; Gxy=22.0 GPa; v=0.286) are shown, as are deformation data for epoxy on its own (E=4 GPa) and PET (E=3 GPa). Cf. Examples 1 and 20-21. The maximum deformation (in mm) in the centre on the plane of the substrate on which the image is being printed is given for two print widths of 1620 mm and 1850 mm. Table 3 Table 3 continued Table 3 continued : angle (°) of direction of the fibres with respect to the longitudinal axis of the printing forme : layer thickness (mm) For a 7-layer laminate, in which each layer has a thickness of 0.028 mm (total thickness 0.169 mm), and the direction of the fibres in the successive layers is -60, 60, -30, 0, 30, -60, 60° with respect to the longitudinal axis, the maximum deformation is 1.38 and 2.00 for print widths of 1620 mm and 1850 mm, respectively. If each layer thickness is 0.032 mm (total thickness 0.224), these deformation values are 1.13 and 1.61, respectively. It is apparent from the above examples, inter alia, that with printing formes which are made from 3-layer laminates and have a middle layer which is 0.084 mm thick with a direction of the fibres running parallel to the longitudinal axis (=0°), the maximum deformation decreases as α increases, with the layer thickness of the outermost layers remaining constant, cf. Examples 6, 10, 12 and 13c. It should be noted that in practice, the selection of a suitable direction of the fibres will partly be determined by the reproducibility of the manufacturing technique employed and the total cost. For example, it has been found that it is more difficult to produce a printing forme according to the invention with one or more layers which have unidirectional fibres at an angle of 90° with respect to the longitudinal axis of the printing forme by means of winding, and therefore additional costs are involved, than winding a layer of this nature with a direction of the fibres of less than 90°, for example 85°, with respect to the longitudinal axis. The preferred position of the layer with a direction of the fibres which is parallel to the longitudinal axis can be seen by comparing Examples 6 and 11. Furthermore, the above examples clearly show the influence of the layer thicknesses (cf. for example Example 3) given constant angles and of the number of layers (cf. Examples 15 and 16, 3 and 10) on the maximum deformation.

Claims: CLAIMS 1. Printing forme (10) for rotary screen printing, comprising a thin-walled hollow cylinder (12) made from fibre- reinforced plastics material, the cylinder wall of which is provided with continuous printing openings (14) which define an image (16, 18) which is to be printed, characterized in that the cylinder (12) comprises at least one layer (20; 32, 34; 32, 44, 46) having fibres (22) with at least one oriented direction of the fibres. 2. Printing forme according to claim 1, characterized in that the cylinder (12) comprises a fibre-reinforced layer having fibres with two oriented directions of the fibres. 3. Printing forme according to claim 1 or 2, characterized in that an oriented fibre direction lies parallel to the longitudinal axis of the printing forme (10) . 4. Printing forme according to claim 1, characterized in that the cylinder (12) comprises a first layer having fibres in a first oriented direction of the fibres, and a second layer having fibres in a second oriented direction of the fibres, the oriented directions of the fibres not being identical. 5. Printing forme according to claim 4, characterized in that the oriented direction of the fibres of the first layer forms an angle α with the longitudinal axis of the printing forme (10) , and the oriented direction of the fibres of the second layer forms an angle -α with the longitudinal axis of the printing forme (10) . 6. Printing forme according to one of the preceding claims 4 or 5, characterized in that the cylinder (12) also comprises a further layer (32) having fibres (22) in a third oriented direction of the fibres, the direction of the fibres running parallel to the longitudinal axis of the printing forme (10) . 7. Printing forme according to claim 6, characterized in that the said further layer (32) is present between the first layer (34) and the second layer (34) . 8. Printing forme according to one of the preceding claims 4 or 5, characterized in that first layers having a first oriented direction of the fibres and second layers having a second oriented direction of the fibres alternate with one another . 9. Printing forme according to claim 1, characterized in that the cylinder (12) is of symmetrical structure in the thickness direction, and the oriented direction of the fibres of outermost, first layers (46) forms an angle α with the longitudinal axis of the printing forme (10), and the oriented direction of the fibres of second, intermediate layers (44) forms an angle -α with the longitudinal axis of the printing forme (10) . 10. Printing forme according to claim 9, characterized in that the cylinder comprises a further layer (32) having fibres (22) in a third oriented direction of the fibres, which direction of the fibres runs parallel to the longitudinal axis of the printing forme (10) . 11. Printing forme according to claim 10, characterized in that the further layer (32) is arranged between the second, intermediate layers (44) . 12. Printing forme according to one of claims 6-7 or 9-11, characterized in that the layer thickness of the outermost, first layers (46) is identical. 13. Printing forme according to one of claims 6-7 or 10-11, characterized in that the thickness of the further layer (32) is greater than the thickness of the other layers (34; 44, 46). 14. Printing forme according to one of the preceding claims, characterized in that the fibre-reinforced plastics material consists of unidirectional carbon fibres in an epoxy matrix. 15. Printing forme according to one of the preceding claims, characterized in that the total thickness of the cylinder wall of the printing forme (10) lies in the range from 80-300 μm. 16. Printing forme according to claim 1, characterized in that the cylinder (12) is made from a quasi-isotropic laminate. 17. Printing forme according to one of the preceding claims, characterized in that a surface layer which does not contain any fibres is provided on the inner side and/or outer side of the cylinder (12) . 18. Rotary screen-printing device provided with a printing forme (10) according to one of the preceding claims.

Inventors: ELEMANS NORBERTUS FRANCISCUS J, JONKERS THOMAS MARIA, VAN WEPEREN KARST JAN, ELEMANS, NORBERTUS, FRANCISCUS, JACOBUS, JONKERS, THOMAS, MARIA, VAN WEPEREN, KARST, JAN
Applicants: ELEMANS NORBERTUS FRANCISCUS J, JONKERS THOMAS MARIA, STORK SCREENS BV, WEPEREN KARST JAN VAN, ELEMANS, NORBERTUS, FRANCISCUS, JACOBUS, JONKERS, THOMAS, MARIA, STORK SCREENS B.V., VAN WEPEREN, KARST, JAN
Application year: 2000
Main classification: B41N1
IPC codes: B32B1, B32B5, B41N1, C08J5