Printing – Stenciling – Processes
Reexamination Certificate
2001-05-18
2003-07-08
Yan, Ren (Department: 2854)
Printing
Stenciling
Processes
C101S123000
Reexamination Certificate
active
06588335
ABSTRACT:
This application claims priority to PCT/FR99/02088 filed Sep. 2, 1999 and to FR 98/11005 filed Sep. 3, 1998.
FIELD OF THE INVENTION
The present invention relates to a capillary surface injection squeegee for screen printing liquid products of a viscous liquid under optimum conditions through the openings of a screen printing stencil. This mechanism can be adapted to the object of FR 2754473, or any other existing device with an incorporated reservoir in which a pressure can be adjusted.
DESCRIPTION OF THE PRIOR ART
Conventionally, liquid products are deposited on a surface by spraying, by immersion, or by the use of a brush when there are no complex areas to be protected. Otherwise, that is to say if there are complex areas to be protected, then the screen printing process is used. Firstly, the screen printing process consists of a stencil, or screen printing mask, which is usually made of a piece of synthetic material or stainless steel. This screen printing stencil is produced in such a way that it has blocked areas and open areas corresponding to the patterns to be reproduced. Secondly, the screen printing screen is placed on the surface to be screen printed, and an inclined squeegee which is pressed simultaneously and moved in a direction parallel to the screen forces the liquid product to pass through the open areas of the screen. Squeegees are generally made either of a profile member of more or less hard rubber, or of stainless steel plates. In all cases, the transfer pressure transmitted to the product depends upon the angle which is formed between the plane of the screen and the active face of the squeegee. The smaller the scraping angle, the greater the transfer pressure. As for the masks, these can be made of a woven mesh of stainless steel, polyester, or silk, or even stencil plates. These screens can be either flat, or cylindrical, in the case of continuous screen printing. The screen printing process, as described hereinabove, is a very old process but it has a certain number of disadvantages, such as the rheological changes in the product which is to be transferred during the work, either by the evaporation of solvents contained in the product, or by interaction with the ambient environment. This rheological change is translated by an obligation to readjust working parameters or to replace the altered product by a good quality product.
Another disadvantage is the fact that the transfer pressure applied to the product is difficult to control. Indeed, it is not enough to control the scraping angle since the transfer pressure is interdependent on the scraping speed, on the quantity of product found to the front of the squeegee, and on the rheology of the product which is itself variable while work is being carried out, as explained hereinabove. Furthermore, since the maximum transfer force which can be applied is low, the resulting maximum scraping speed as well as the rate of product flow are also limited.
In order to partly eliminate this lack of control over the transfer pressure, it is possible to use a coating operation which consists in spreading the product uniformly over the surface of the screen, and in thus making it available for the scraping operation strictly speaking. Although this additional operation improves screen printing quality, it has the disadvantage of placing a large surface area of the product in contact with the ambient environment, thereby giving rise to rapid changes in rheology by evaporation of the solvents contained in the product.
In order to correct the problems related to the changes in rheology and poor control of the transfer pressure, a certain number of devices have been proposed. These devices all consist in confining the product to a chamber and in expelling it under pressure through a very narrow distribution slit in contact with the screen.
WO 96/20088 and U.S. Pat. No 4,622,239 describe devices of this type which are suitable for the transfer of pasty products, such as solder pastes. These devices are not used to transfer low to high viscosity liquid products because the sealing provided by the members which delimit the distribution slit is inefficient because it is very wide and therefore each time that the device is removed from the screen, e.g. to change series, the product contained in the device continues to flow out, resulting in significant losses of product.
FR 2754473 proposes another device which has better sealing but in which significant losses of product have also been noted as soon as an attempt is made to transfer liquid products, and in this case as well, the product contained between the sealing elements flows out from the device when it is removed from the screen. The invention described and claimed is also limited to use with pasty products, such as solder pastes.
U.S. Pat. Nos. 3,921,521, 4,023,486, GB 1,433,957 and DE 2250092 also relate to closed transfer devices, which, this time, are dedicated to liquid products, but in these cases too all of these devices have a distribution chamber which is delimited at the bottom by the sealing members which form a narrow slot closed by the printing screen, which means that the product contained inside that chamber flows out when the devices are removed from the screen.
In all the afore-mentioned patents, it is noted that product leaks particularly in the areas of the screen not supported by the substrate, e.g. at the sides when this latter is narrower than the length of the product distribution slit, or even at the start or end of the scraping operation when the screen printed substrate is replaced with a new substrate. All these instances of leakage, irrespective of their cause, are particularly harmful, because not only do they produce significant losses of product; they can also interfere with the quality of the deposits. Indeed, when the product which has been exposed to ambient air happens to be located on the surface of the screen, and is therefore unintentionally transferred onto the substrate during subsequent screen printing, this can cause deposits which lack precision or which are of poor quality. Moreover, excess product on the screen causes prolonged downtime for cleaning, and poses health and safety problems for the staff who come into contact with said products, particularly in the case of highly volatile and hazardous solvent-based inks like toluene.
Within a domain which differs significantly from that of the invention, namely the domain of photo-engraving and/or of flexographic printing, DE 43 30 681 proposes that to coat a product on an impression roller an applicator device is used which comprises a capillary element impregnated with coating product which is in direct and constant contact with the coating roller. The purpose of this capillary element is to provide homogeneous coating of the roller with which it is in contact. Although this device is interesting in its use of an intermediate capillary element it cannot be used within the domain of screen printing because this latter integrates the use of a screen printing screen which is responsible for transferring product properly onto the substrate for screen printing. Furthermore, the main inconvenience of the conventional processes and devices is the difficulty of preventing the product from flowing out when the applicator device is no longer in contact with the screen, if the product is particularly runny.
SUMMARY OF THE INVENTION
The present invention makes it possible for the various problems associated with prior art transfer devices to be remedied.
With respect to the device, according to the present invention, a member called a capillary surface injection member is placed between the closed reservoir containing the product to be transferred and between the screen printing screen in order to form deposits, by screen printing, of a liquid to liquid viscous product on a substrate through the openings in a screen which is resting on said substrate; the liquid product being available in a reservoir which can be pressurised and which is open on the side of the screen by a distribution s
Bourrieres Francis
Kaiser Clement
Novatec SA
Shanks & Herbert
Yan Ren
LandOfFree
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