EP0544925B1 - Process and apparatus for applying and incorporating dyes into a plastic-bearing substrate - Google Patents

Description

The invention relates to a device for applying and introducing dyes or a decoration made of dyes onto or into a plastic substrate with the features of the preamble of claim 1. Such a device is known from DE-C-39 04 424. There, the color carrier is heated for the purpose of color transfer from the color carrier to the substrate (the object) by means of heat radiation sources arranged on the back of the color carrier.

A method for applying dyes to plastic is known from US Pat. No. 2,721,821. There, a colored decor is printed on a polyethylene substrate, heating to 120 to 185 ° C. The heating takes place with infrared radiation, which is radiated through the substrate onto the ink carrier, i.e. the ink carrier is heated on its side facing the substrate by means of IR radiation. It is provided that the ink carrier is designed to be reflective to the IR radiation on its surface not covered with dyes, so that this radiation is absorbed only where paint is applied.

GB-A 1 107 401 describes a process for colored printing on plastics, in which the plastic is heated in a heated glycerol bath at 175 to 180 °. The surface of the plastic is gelled.

From EP 0 098 506 A2 a method and a device for generating permanent images on substrates are known, in which a carrier with dye is arranged between the substrate and a heated matrix, which has raised sections whose shape corresponds to the image to be transferred . After the transfer of the dyes to the substrate has taken place by means of this heated matrix, the substrate is heated so that the dye diffuses from an ink into the substrate.

DE 24 38 723 A1 describes a method for the dry transfer of organic compounds to webs, in which transfer temperatures between 100 and 200 ° C are provided.

GB 2 127 747 A1 describes a transfer printing process in which the transfer printing partners are positioned by means of electrostatic charging.

Furthermore, methods for applying decorations from dyes to plastic substrates are known from DE 37 08 855 C1 and DE 39 04 424 C1.

From DE patents 17 71 812, 23 37 798, 24 36 783 and from 24 58 660 it is known to print textile fabrics with the so-called transfer printing process. Here, an ink carrier (also called auxiliary carrier) is printed with printed images (decors) made from sublimation printing inks. The ink carrier (auxiliary carrier) can in particular consist of paper. Printing takes place, for example, by means of offset or rotary printing processes. The printed images are transferred by sublimation from the ink carrier to the textile fabric to be colored (so-called transfer printing).

The printing inks mentioned are produced from sublimable disperse dyes using binders and oxidation additives. In the state of the art, the printed ink carriers (also called transfer papers) are placed with the color printed side on the textile side to be printed and heated by means of a printing plate heated to 170 to 220 ° C (using the cycle method) or by means of a rotating cylinder (using the continuous process). As soon as the temperature of approx. 170 to 220 ° C reaches the dyes, they sublime into the textiles made of synthetic fibers.

A known method (EP-A 0 014 615), which is primarily intended for decorating eyeglass frames, is carried out in such a way that an eyeglass frame with its surface to be decorated is placed on a base with each working cycle, the arranged within a vacuum chamber and movable up and down by means of a piston-cylinder unit. The vacuum chamber has a side opening for inserting the eyeglass frame, which can be closed with a door. On its upper side, the vacuum chamber has a horizontal, stationary frame which delimits a slot with a frame which is also arranged above it and is horizontal but can be moved up and down. A carrier foil is inserted through the slit, which is unrolled from a reel and is provided on its underside with the decor that is to be applied to the spectacle frame. The decor has been applied to the carrier film, for example, as a multicolor print or as a decal and consists of colors which can be sublimed at a temperature below the destruction temperature of the carrier film. As soon as an eyeglass frame is placed in the vacuum chamber and its door has been closed, the upper frame is lowered so that it clamps the carrier film between itself and the lower frame and the vacuum chamber is thereby sealed and can be evacuated. The carrier film is over heats the upper frame arranged to the sublimation temperature of the decor and then moves the spectacle frame upwards by means of its base which can be raised and lowered within the vacuum chamber and on which it has been placed, and is pressed against the carrier film. The vacuum causes the carrier film to nestle tightly against the surfaces to be decorated on the front and in lateral areas of the spectacle frame. This state is maintained for a period of time sufficient for a transition of the colors forming the decor away from the carrier film into the structure of the material of the spectacle frame. The vacuum is then released, the spectacle frame is lowered and thereby separated from the carrier film and finally removed from the vacuum chamber.

In this known method, the carrier film is greatly stretched in individual areas so that it nestles sufficiently against the spectacle frame. It is inevitable that the decor will be distorted in the particularly stretched areas of the carrier film. The distortions can be compensated to a certain extent by applying an appropriately corrected decor to the carrier film from the outset. Moreover, distortions in objects such as glasses frames, whose surfaces to be decorated are relatively narrow, are hardly noticeable. It is different with objects that are to be decorated over a large area. In the case of such objects, it is not always possible to avoid disturbing distortions of the decor if the decor has been applied by the known method. In addition, as the size of the surface to be decorated increases, the risk that the decor will be adversely affected by air pockets.

In another known method for applying decorations to objects (DE-A-32 28 096), the objects, for example tin cans, are first of all applied by a coating system guided, which applies a layer of dye-affine, migration-preventing plastic on the outside of the objects. After the coating has been chemically or physically dried, the coated objects are fed to a labeling machine, in which decor carriers in the form of printed banderoles are removed from a stack or endless strips, placed around each object and fixed with an adhesive strip, glue line, electrostatic field or the like become. The objects are then heated, for example by means of hot air, to a temperature of 200 ° to 350 ° C., preferably 250 ° to 300 ° C. at these temperatures, which produce an extreme heat shock, the water contained in the banderoles evaporates suddenly, so that each banderole is shrunk onto the associated object in a fraction of a second and generates an autogenous pressure required for the transfer of the decoration from the banderole to the object . When heated further, the dyes that form the decor sublimate into the plastic coating underneath.

In this process, it is of crucial importance that the relative movement inevitable when shrinking a banderole relative to the associated object is completed before the dyes which form the decoration are heated to such an extent that their migration into the plastic layer begins. If this difficult condition cannot be met, it must be expected that at least parts of the decor will be blurred on the object.

From US 4 178 782 a device for printing a textile web with sublimable dye is known, which is fed on a carrier film. The device has a drivable drum which can be heated from the inside, around the bottom of the carrier film with the radially outwardly facing layer of dye and above that the textile web to be printed, and above this an endless pressure belt made of metal mesh guided over rollers to run. The region of the drum that is wrapped in this way can be covered by a hood, within which a negative pressure is maintained. In this way, when the ink is sublimated, gas is sucked out through the textile web to be printed and the pressure belt made of metal mesh lying thereon. The pressure forces exerted by the pressure belt on the textile web are generated exclusively by the mechanical tension of the pressure belt and somewhat reduced by the negative pressure inside the hood.

From DE 25 42 350 C1 the attempt has become known to also print certain plastic products in the transfer printing process, which had previously been used successfully with textiles, which poorly accept the sublimable dyes in question. Attempts have been made there to coat such bodies with thermoplastic films which absorb the dyes, and then attempts are made to print on the films using the transfer printing process explained above. However, the process has not proven itself, in particular because the migration resistance of the dyes (that is, the positional stability of the dyes after transfer printing) was only guaranteed for medium to high molecular dyes (with molecular weights between 300 and 1,000). Temperatures of over 180 ° C or 200 to 220 ° C for a period of at least 25 seconds have been used for sublimation. At these relatively high temperatures, however, most thermoplastic films melt or they become so soft that the ink carriers (paper etc.) used in transfer printing stick or damage the surfaces of the films so that the product did not meet the aesthetic requirements. The migration resistance of the dyes required for good image reproduction was also not achieved.

Therefore, noteworthy results in the sublimation printing transfer process have so far only been achieved in the prior art with thermosetting films and lacquers (DE-A 24 24 949, GB-A-1 517 832). However, these methods did not lead to satisfactory reproducible results. Both the materials and the sublimation process are not described with sufficient precision. The results achieved left a lot to be desired, particularly due to yellowing and low migration resistance and color blurring.

Because of the extensive use of thermoplastic films and sheets, which are thermoformed into three-dimensional bodies, e.g. Components for interior fittings, furniture parts (especially fronts), household appliances, office machines, light bodies, car molded parts, etc., can be molded, there has long been a need for a way to provide good quality thermoplastic substrates with colored decorations.

EP 0 014 901 describes an attempt to achieve constant, traceable and stable transfer printing results by specifying the molecular weights of the sublimable disperse dyes, the temperatures used and the composition and nature of the plastic substrates. It has come to the conclusion that heating to temperatures of 220 ° C. and more is necessary for the application of the transfer printing process to plastics. This excludes a large number of thermoplastics. The process remained limited to certain thermosetting plastic coatings and certain substrates made of inorganic materials.

The state of the art also teaches as a prejudice that the molecular weight of the dyes used is important in sublimation printing. The aforementioned EP 0 014 901 teaches the use of high molecular weight disperse dyes with molecular weights between 300 and 1,000, in particular with a view to the required resistance to migration.

The German patents 37 08 855 and 39 04 424 already mentioned at the beginning bring progress in that they sublimation printing are based on the use of heated printing plates or heated cylinders and instead suggest heating with thermal radiation (infrared radiation). This prior art does not give details of the materials used or of the sublimation temperatures.

From the unpublished European patent application EP-A-455 849 a method and an apparatus for applying and introducing dyes onto or into a plastic substrate are known, in which the ink carrier on its side facing away from the substrate depending on the plastic of the Substrate is heated to temperatures below 170 ° C, the ink carrier is brought into full-surface contact with the substrate by electrostatic charging and the substrate is heated so that the side of the substrate to which the dyes are applied or introduced, is heated less than the opposite other side (back) of the substrate. The present invention is based on this not prepublished prior art.

The invention has for its object to develop the generic device for applying and introducing dyes on or in a plastic-containing substrate so that the quality of the product produced is further improved.

According to the invention, this object is achieved in that a plurality of first channels for receiving a flowing heating fluid and a plurality of second channels, which can be connected to a vacuum source, are provided in the heating plate of the device, the second channels having holes in the surface of the Plate are connected.

Preferably, the first and second channels extend parallel to each other, in a plane that is parallel to the surface of the plate of the device.

In this case, the first and second channels are preferably arranged alternately offset from one another, although a first channel does not necessarily follow a second channel, but can be arranged between one type of channel of several channels of the other type.

According to a preferred embodiment of the invention, it is provided that the density of the first channels, which carry the heating fluid, is greater in outer areas of the plate than in inner areas. This serves in particular to achieve a particularly homogeneous temperature distribution on the plate.

Furthermore, according to another preferred embodiment of the invention, it is provided that individual ones of the second channels which carry vacuum can optionally be connected to the vacuum source, so that depending on the size of the substrate which is provided with dyes, individual channels of the device are optionally rendered ineffective can be.

Fig. 1

in Drauf- und Seitenansicht die Vorbereitung eines Substrates und eines Farbträgers für den Transferdruck;

Fig. 2 und 3

schematische Ansichten des Transferdruckes;

Fig. 4

Einzelheiten des Transferdruckes in stark vergrößertem Maßstab,

Fig. 5

eine Farbverteilung eines zu druckenden Bildes und eine zugehörige Steuerung der Intensität von Infrarot-Strahlern und

Fig. 6

schematisch eine Draufsicht auf eine Heiz-Platte einer erfindungsgemäßen Vorrichtung, wobei die Anordnung der ersten und zweiten Kanäle dargestellt ist, und

Fig. 7

einen Teil-Schnitt durch eine Platte gemäß Fig.6.

The invention is explained in more detail below with reference to the drawing. It shows:

Fig. 1

in top and side view the preparation of a substrate and an ink carrier for transfer printing;

2 and 3

schematic views of the transfer printing;

Fig. 4

Details of the transfer print on a greatly enlarged scale,

Fig. 5

a color distribution of an image to be printed and an associated control of the intensity of infrared emitters and

Fig. 6

schematically shows a plan view of a heating plate of a device according to the invention, the arrangement of the first and second channels is shown, and

Fig. 7

a partial section through a plate according to Figure 6.

A substrate 10 is to be decorated with an image of sublimable disperse dyes by transfer printing. The term substrate is intended in particular to encompass films, foils or plates, the films or foils having thicknesses of 25 to 1000 μm and the plates having thicknesses of 1 to 10 mm. The films, foils or plates can be extruded from a plastic granulate, granulate mixtures as well as from several types of plastics or mixtures. Inorganic particles (powder, flour) can also be mixed in, the proportion of plastics on the surface of the substrate preferably being more than 50%. The following plastics are particularly suitable for the application of the invention: PC (polycarbonates), ABS, PMMA, PET and PDT. The process parameters are set depending on the material used (see below).

The plastic films, sheets or foils can also be composed of several types and layers of plastic.

The method according to the invention is not only suitable for substrate surfaces that are smooth, but also for structured, porous, matt and rough surfaces. The substrate material can be clear or colored.

A substrate is also to be understood as a plastic layer which is applied in the form of lacquer to a material surface of, for example, wood, ceramic or artificial stone, possibly with crosslinking.

If plastics which are sensitive or are less resistant to chemical and mechanical stresses or to light are used, they can, after printing according to the invention, be coated with more permanent curls or coatings of other types of plastics known as such.

The substrate 10 is printed in color with the aid of an ink carrier 12. For this purpose, the image to be transferred to the substrate is printed on the ink carrier 12 using sublimable disperse dyes. In particular, sheets of paper come into consideration as ink carriers 12, which on the one hand absorb the image to be transferred from sublimable colors well and on the other hand have sufficient air permeability so that air can be sucked through the ink carrier 12 during sublimation transfer printing. Good results are achieved with paper weights from 30 to 120 g. The paper surfaces can be of any size, in particular they can be 1 m² or larger.

Sublimable disperse dyes of a conventional type are processed into printing inks using binders and, if appropriate, oxidation additives. The images, patterns, individual colors or motifs with which the substrate 10 is to be provided are printed on the ink carrier 12 by means of offset, rotary, gravure, flexographic or screen printing processes.

According to FIG. 1, the substrate 10 which is to be printed is placed in a loading station 14 and an ink carrier 12 is placed on the substrate. As shown, the printed ink carrier 12 made of paper is substantially larger than the substrate 10, so that the ink carrier clearly overlaps the substrate on all edges. The overlapping area in the illustrated embodiment is at least 20%, preferably at least 30%.

In a first step, the substrate and ink carrier layers, which are superimposed in this way in a first step, are transferred to a station 16 for electrostatic charging. Here, the substrate 10 is so electrostatically charged with respect to the ink carrier 12 that the ink carrier 12 lies snugly over the entire surface of the substrate 10. This is achieved in the third step according to FIG. 1. In the fourth step, the arrangement on substrate 10 and ink carrier 12, which is glued to it, is conveyed to a sublimation station, which is shown in more detail in FIG.

A conveyor belt 18 transfers the arrangement of substrate 10 and ink carrier 12 produced as described above to a table 20 with a heating plate, which is provided with air-permeable vertical channels (not shown), so that air can be sucked through the table top from top to bottom is. For this purpose, a vacuum chamber 22 is provided under the table top, which is connected to a vacuum pump, not shown.

According to FIGS. 2 and 3, the substrate 10 is conveyed onto the table 20 with the ink carrier 12 firmly attached thereto, and then vacuum is applied in the chamber 22. Neither a cover film over the ink carrier 12 nor a base between the substrate 10 and the table 20 is required.

A housing 24 is arranged above the table 20, in which a multiplicity of infrared emitters 36 are accommodated side by side. The housing 24 with the infrared emitters 36 overlaps the entire area of the substrate 10 and the ink carrier 12. A temperature measuring device 26 measures the temperature on the surface of the substrate 10 facing the infrared emitters and on the side of the ink carrier 12 lying thereon with the sublimable ones Emulsion paints. Another temperature device 27 measures the temperature of the heating plate of the table 20 and thus the temperature on the surface of the substrate 10 which lies directly on the table 20, that is to say the side of the substrate 10 which is not decorated.

A controller 28 controls the individual infrared radiators 36 in the housing 24 to different temperatures, as will be explained in more detail below with reference to FIG. 5.

As is shown schematically in FIG. 3, air is sucked into the vacuum chamber 22 through the porous ink carrier 12, specifically through the channels (not shown) in the heated table top 20. The edge space 30 between the overlapping ink carrier 12, the substrate 10 and the table 20 pumped air-free, so that the ink carrier 12 attracts evenly over the entire surface of the substrate 10. There are no folds or warps in the ink carrier 12 and air or gas bubbles are removed. This applies in particular to heating during sublimation with the formation of steam.

In this state, the table 20 and the infrared radiators 36 are heated. The infrared radiation 32 generated by the infrared radiators 36 is used to heat the sublimable dyes arranged on the underside of the ink carrier 12, while the heating of the table 20 serves to heat the substrate 10 on the non-printed side. This heating of the substrate 10 not only has the purpose of achieving a dimensional stability of the substrate, but also has a significant impact on the penetration of the color molecules into the substrate 10.

This is explained in more detail in FIG. 4, where the individual parts are shown in greatly distorted magnification in order to illustrate the process of sublimation and the penetration of the color molecules into the To illustrate substrate. As said, the air flow 34 from the outside through the ink carrier 12 and the air flow 36 below the ink carrier 12 creates a uniform, tension-free, bubble-free and saturated application of the ink carrier 12 on the substrate 10. To achieve a particularly migration-resistant print on the substrate 10, this is now The substrate 10 with the table top 20 is heated to a temperature which is higher than the temperature at the surface of the substrate 10 to be printed. In FIG. 4, an increasing temperature gradient is thus produced from top to bottom. The consequence of this temperature gradient is that the dye molecules 40 penetrate relatively far into the substrate after sublimation. In FIG. 4, the area of the substrate 10 into which the molecules 40 penetrate is designated by 10b, while the area which is essentially free of dye molecules is designated by 10a.

The degree of heating on the surface of the substrate 10 and correspondingly on the lower surface of the ink carrier 12 (measured with the temperature measuring device 26 according to FIG. 2) depends on the material of the substrate 10. The heating is between 60 ° C (for e.g. ABS) and 150 ° C (for e.g. PBT). For PC, heating to 130 ° C has proven to be beneficial. The temperature on the lower surface of the substrate 10 (measured with the temperature sensor 27 according to FIG. 3) should in each case be about 3 to 30 ° C., in particular 5 to 15 ° C. higher, depending on the type and thickness of the material.

In particular, temperature values of 120 to 135 ° C in the sublimation area (ie on the substrate surface and the substrate surface) have proven to be favorable for ABS, for ABS 90 to 100 ° C, for PBT 150 to 160 ° C and for PET 80 to 90 ° C.

The problem of back-sublimation is solved by the relatively low temperatures used according to the invention.

Depending on the strength of the substrate 10 and its material, the sublimation process is completed in 10 to 30 seconds.

The quality of the product produced can be further increased by controlling the intensity of the individual infrared emitters 36 as a function of which color is to be sublimed by the emitter in question. Different colors require different energies per unit area for sublimation. The energy requirement (and accordingly the temperature to be generated by the infrared radiation) increases from yellow to red and cyan to black by around 20%. This is taken into account by individual control of the individual infrared emitters according to the predominant color component directly under the emitter. As a result, uniform sublimation is achieved for all colors, and at the same time the surface of the substrate to be printed is heated sufficiently uniformly, which promotes image quality. 5 shows an example of an image to be printed on the ink carrier 12 on the left, lighter colors (increasingly from yellow to red) being provided in the outer regions 12a, 12b and 12c, while the image becomes increasingly darker towards the center until it is in the center Section has a black area. Correspondingly, FIG. 5 on the right schematically shows a view of the infrared radiators 36 in the housing 24 from below (for example with reference to FIG. 4), 100% of a predetermined IR power with the in the middle area corresponding to the black area of the image Infrared emitters is generated, while the infrared power is reduced to the outside as indicated.

The ink carrier 12 is preferably blackened on its rear side.

The penetration depth of the sublimable dye molecules into the substrate of 100 to 300 μm achieved with the method described above does not cause any fading of the Picture but surprisingly on the contrary an improvement of the picture quality; the picture appears more intense and spatial. Migration is negligible. The product produced as described can be subjected to a brief shock heating of, for example, 200 to 300 ° C. for 2 to 3 minutes for the purpose of thermoforming without impairing the image quality. Continuous heating from 100 to 200 ° C is also possible (depending on the type of plastic, e.g. 145 ° C for PC and 200 ° C for PBT).

There is no damage to the substrate surface The plastic surface retains its structure without any change, regardless of whether the surface is polished to a high gloss, matt, semi-gloss, curved, coarse or finely structured. Due to the electrostatic attraction forces and the vacuum forces acting at the same time, the image quality is also good for coarse, rough and finely structured surfaces.

With the described method, relatively large amounts of dye can be transferred (10 to 20 g wet or 3 to 7 g dry). This and the large diffusion depth described enable thermoforming up to 250% expansion without fading or brightening of the colors.

6 shows the design of the table top 20 according to the invention in a schematic plan view. Figure 7 shows a vertical section (perpendicular to the paper plane of Figure 6) of a detail of Figure 6.

As can be seen from FIGS. 6 and 7, in this exemplary embodiment the table plate 20 is formed from a lower plate 20b and an upper plate 20a (FIG. 7). On the upper plate 20a, to which the top view according to FIG. 6 relates, the substrate 10 and then the ink carrier 12 are deposited (not shown in FIGS. 6 and 7).

As FIG. 6 shows, a multiplicity of channels extending parallel to one another in a plane are formed in the table plate 20 directly below the upper plate 20a. A distinction is made between two types of channels: a first type of channels 44, 44 'is used to guide a heating fluid, such as, in particular, heated oil. This heating fluid heats the table top 20 and thus the substrate lying thereon on the side facing away from the ink carrier. The heating device for heating the oil is conventional and is therefore not shown here.

Furthermore, the table top 20 has the function of sucking air through the ink carrier 12 via a multiplicity of suction channels, so that it lies snugly over the entire surface of the substrate 10. A second type of channels 46, 46 'is used for this purpose, each of which can optionally be connected to a vacuum source (not shown). Depending on whether a particular channel 46, 46 'is connected to the vacuum source, it generates a vacuum suction via the associated holes 48, 50, 52 via channels 54, 56, 58 (FIG. 7) connected to it, so that at the relevant point of the ink carrier 12 is sucked.

The combination of heating channels 44, 44 'and vacuum channels 46, 46' shown in FIGS. 6 and 7 enables an optimal combination of the two functions "heating" and "suction" of the table top 20.

It has been shown that a completely homogeneous temperature distribution on the plate 20 is by no means easy to achieve, even if the plate is made of solid metal. The plate 20 is namely not only heated by means of the heating fluid 60 (oil) flowing through the channels 44, 44 ', but also by the infrared radiators 36. In particular, the infrared radiators 36 can cause an uncertainty with regard to the temperature distribution on the plate 20 .

The individual channels 44, 44 'and 46, 46' are each formed in blocks 62, 64, ... 68, which are clamped between the upper and lower plates 20a, 20b. This results overall in a compact heating plate which can be controlled with relatively little delay and completely homogeneously in terms of its temperature. Because individual vacuum channels 46, 46 'can be connected to the vacuum source by means of valves 70, 72, ... 74, 76 (only a part of the valves shown in FIG. 6 are provided with references), the plate 20 can be more easily Be adapted in terms of their vacuum effect to the size of the substrate 10 to be printed. If the substrate 10 has relatively small dimensions, then vacuum channels 46, 46 'of the associated valves 70, 72, 74, 76 located in the outer regions of the plate 20 can be shut off, so that only the inner vacuum channels are effective, to the extent that they are direct cause air suction through the ink carrier 12 lying on the substrate.

A venturi pump (air suction by means of a nozzle effect) is preferably provided as the vacuum source, the suction effect of which can be easily adjusted over a wide range.

Claims (7)

1. An apparatus for applying and incorporating dyestuffs or a dye pattern to/in a substrate (10), which comprises plastics, including a plate (20) on which the substrate and a dye carrier (12) on top of the same are placed and which comprises a plurality of passages (46, 46') adapted for connection to a vacuum source and communicating with holes (48, 50, 52) in the surface of the plate (20), characterized in that a plurality of further passages (44, 44') are formed in the plate (20) to receive a flowing heating fluid (60).
2. The apparatus as claimed in claim 1, characterized in that the passages (46, 46') and the further passages (44, 44') extend parallel to one another.
3. The apparatus as claimed in any one of claims 1 or 2, characterized in that the passages (44, 44', 46, 46') lie at least approximately in one plane.
4. The apparatus as claimed in any one of the preceding claims, characterized in that the passages (46) and the further passages (44) are disposed alternatingly offset with respect to each other.
5. The apparatus as claimed in any one of the preceding claims, characterized in that indidivual ones of the evacuatable passages (46, 46') are connectable selectively to the vacuum source.
6. The apparatus as claimed in any one of the preceding claims, characterized in that the closeness of the passages (44, 44') through which the heating fluid flows is greater in outer regions of the plate (20) than in inner regions thereof.
7. The apparatus as claimed in any one of the preceding claims, characterized in that a venturi pump is provided as the source of vacuum.

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