The four key components required for silk-screen printing are: The print medium, the screen with emulsion areas (defining the pattern to be printed), the surface of a substrate to be imprinted, and a doctor blade that squeezes the print medium through the screen.
Masking of the screen 3 is accomplished by applying a photosensitive emulsion over a large area of the screen and then exposing it through a photomask. The emulsion may be a positive or negative photoresist. In the case of a positive photoresist the developing process will leave intact those areas that were not exposed, whereas those regions will be ablated that were exposed through the photomask. In the case of the negative photoresist the exact opposite applies. In either case the result is a fabric that contains emulsion-occluded regions through which no print medium can be squeezed, whereas the print medium can penetrate through the areas that are devoid of any emulsion. That emulsion as well affects the thickness of the medium applied on the surface of the object substrate. The emulsion causes that thickness to increase by up to 50%. This process permits the implementation of patterns whose smallest components may be about three times the mesh size of the screen employed. For smaller patterns the mesh would interfere with the printed image at least in some particular applications.
As a suitable adhesive print medium 9 according to the invention, epoxy resin is mixed with spacers. Alteratively, the adhesive component may be a UV-hardening, thermally hardening or multi-component chemically curing cement, or one that hardens through the evaporation of solvents. The example shown employs glass-bead spacers 5 μm in diameter. Other spacer sizes up to 80% of the mesh size are reasonably employable. Preferably, however, the maximum dimension of the spacers will not exceed 30% of the smallest dimension of the gaps defined by the mesh. The answer to the question of how high a concentration of spacers should be added is that it must be remembered that too high a spacer concentration will lead to a lumping of the spacers and thus to an occlusion of the screen. Desirable concentrations are between 0.5% and 80%. The preferred amount in the case of spherical spacers is 5%.
For the silk-screening process the screen 3, clamped onto the frame 7, is positioned about 5-10 cm above the target surface of the substrate 13. The screen 3 is aligned with the substrate 13 with the aid of a camera (not shown) that is moved between the substrate 13 and the screen 3, making it possible for instance by means of a beam-splitter prism to control and adjust the position of the screen 3 relative to the substrate 13. Once the alignment has been made, the camera is removed, and the screen is brought up to within a distance of between 0.5 mm and 5 mm, and preferably 2 mm.
Next, the print medium 9 in the form of the spacer-containing cement, preferably epoxy, is placed on the screen 3. Exerting pressure, a doctor blade 11 is then moved across the screen, squeezing the cement with its intermixed spacers through the mesh. Enough pressure must be applied to cause the part of the screen on which the doctor blade is bearing down to make contact with the object surface underneath that is to be imprinted, as shown in
There are different ways in which the doctor blade can be moved across the screen. A one-time pass of the doctor blade across the screen is usually sufficient. However, there are also many reciprocating dual doctor blade systems in use.
After a structured film of cement has been deposited on the surface of an element, the two surfaces to be bonded must be brought together. When a surface is to be cemented along a specific pattern, the technician usually faces the requirement of creating precisely defined adhesive layer segments, meaning that the width and the thickness of the cement layer must be defined. Moreover, especially in the case of optical elements, bubble inclusions must be avoided. Bubble inclusions are usually caused by the silk-screen printing process itself, as well as by the conditions under which the two elements are joined. The inventors have found that bubble inclusions cannot be avoided merely by heating the applied cement layer to between 30° C. and 80° C., preferably 60° C. There is another condition to be met, whereby the ratio between the width of the applied cement film and its thickness must not exceed 20:1 at least along one dimension. This means that it is possible to deposit very long strips, for as long as the width of the strip does not exceed 20 times the thickness of the strip. Given the surface tension, heating the cement will then cause a degassing of the bubbles. Moreover, the above-described geometry will lead to a convexity in one dimensional direction so that, when the second element that is to be attached is brought up, there will essentially be no formation of bubble inclusions. Now when the two elements are brought together in a precisely defined manner and are ultimately pressed together, they will be joined up to a distance beyond which the spacers will not allow them to come in contact. In the example shown that is the 5 μm mentioned above.
To be sure, when the elements are brought together, the pressure must be applied as evenly as possible. When precision-optical elements with an optical surface are to be cemented together, it will not be possible in many cases to simply press the object elements together with a tool. Another aspect of this invention is therefore dedicated to a method whereby the two elements can be joined in a desirable manner. A method of that nature is feasible when only one of the two elements is of a design whereby a relatively homogeneous channel distribution permits access to the surface of the other element.
This pressure system, of course, can be modified. For example, the base unit 107 can serve as the bottom section of a pressure chamber in which the structured element 103, the second element 110 and perhaps the gasket 113 can be pressurized, while the channel 115 on the base unit opens up to the ambient atmosphere, thus providing air pressure in the channels.
Number | Date | Country | |
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60680811 | May 2005 | US |