Supports – Props and braces – Vacuum hold-down
Reexamination Certificate
1999-11-22
2002-04-16
King, Anita (Department: 3632)
Supports
Props and braces
Vacuum hold-down
C248S363000, C271S276000
Reexamination Certificate
active
06371430
ABSTRACT:
FIELD OF THE INVENTION
This invention generally relates to vacuum surfaces, such as drums, plates, or surfaces of other configurations, which use suction to secure a workpiece or flexible media positioned over suction openings located on the vacuum surface, and more particularly to configurations that allow the vacuum surface to respond to the shape and placement of the workpiece by providing suction only suction openings under or near the workpiece to the vacuum. The invention further relates to the use of vacuum surfaces used to secure printed circuit boards during the manufacturing thereof.
BACKGROUND
Vacuum tables, vacuum plates and vacuum drums, collectively referred to herein as “vacuum holders,” are workpiece holding and restraining devices having a vacuum surface. A common approach for manufacturing such a vacuum holder is to have many suction openings terminating at a vacuum surface. The application of a vacuum produces a pressure difference across the workpiece, which it turn imparts a suction force on the workpiece towards the vacuum surface. The position, size and shape of the suction openings relative to the workpiece determine the required amount of vacuum and the suction force per workpiece area.
The uses for vacuum holders include a variety of industrial and commercial applications that require locating, restraining or transporting pieces. For vacuum plates and tables, workpieces are commonly positioned on the plate and the suction force is then engaged. Vacuum plates are used, for example, to transport a printed circuit board (PCB) from a stack to a direct imaging exposure device such as the exposure device described in co-assigned U.S. patent application Ser. No. 60/107,842. Vacuum drums are commonly configured as rotating cylindrical surfaces and are used for transporting flexible media, such as film and paper. Vacuum holders are used to position, secure and transport paper or film for scanning or printing/exposing. Paper processing and printing machines, for example, use vacuum holders to transport film or paper from one part of the machine to another. Other machines, such as imagesetters, laser printers and rotary data scanning and recording devices, use vacuum drums to support flexible media during exposure or scanning. In these devices the drum rotates with the suction engaged. Suction attaches the media to the drum at the point of first contact with open suction openings, and then pulls the media around the drum as it rotates.
All vacuum holders with vacuum surfaces incorporate a series of channels and suction openings in the vacuum plate or drum, allowing for one or a few vacuum connections to provide suction over an area of the surface. In many early and some contemporary prior art systems, the internal vacuum plumbing is configured so that suction is applied to all suction openings simultaneously. Another approach for manufacturing vacuum holders is to use a “porous” surface instead of suction openings. Regardless of the approach taken, there is a balance between the flow restriction of the surface and the vacuum source. A large suction force over a large area usually requires a low restricted porous material with a high capacity vacuum source.
There are several problems inherent in prior art configurations that result in a vacuum pumping requirement larger than the minimum needed to secure the workpiece. Consider the operation of a vacuum plate in which the workpiece is placed on the surface, covering some of the suction openings. Uncovered suction openings have a constant pumping requirement, and thus there will always be an excess capacity whose amount is determined by the minimum workpiece size. Covered suction openings will have a large pumping requirement until a vacuum seal between the workpiece and surface is formed, at which time the pumping requirement diminishes, theoretically approaching zero for a perfect vacuum seal. The pumping requirement will decreased from an initial value which must accommodate all of the covered suction openings, to nearly zero as a vacuum seal is formed. Thus it is seen that prior art vacuum holders require vacuum pumps that are oversized relative to the minimum capacity needed to restrain the workpiece.
Similar problems also occur in vacuum drum applications. The drum first makes contact with and picks up the leading edge of a flexible media. As the drum rotates, the media wraps about the dram and is held in place at the point of contact with the drum. In this application, the number of uncovered suction openings, and hence the pumping requirement, decreases as the rotation proceeds and suction openings are covered. The vacuum system must be capable of accommodating all of the initially uncovered suction openings.
Having several suction openings not covered by the workpiece may also produce undesirable noise and vibration.
Thus the application of suction simultaneously to all of suction openings on the vacuum surface produces several problems in earlier prior art systems. These can be characterized as requiring vacuum pumping overcapacity due to 1) uncovered suction openings, and 2) exposing all covered suction openings simultaneously. The problems due to uncovered suction openings has been previously acknowledged but only partially addressed in several U.S. Patents. Thus in U.S. Pat. No. 5,716,048, Morrisette describes a drum mask placed over the drum, where the mask is configured to cover those suction openings not covered by the media. This solution effectively tailors the vacuum drum to a media size as determined by the available masks. As noted in Morrisette, a mask must be produced for every media size, and the operator or machinery involved must adapt to changes in media size by changing masks. While that invention improves the performance by lowering the pumping requirement for each media size due to changes in the number of covered suction openings, this prior art invention requires intervention by either the operator or some machinery to choose the appropriate mask size. Furthermore, the suction force may be different for different masks because the area kept uncovered and number of free suction openings may differ.
Both U.S. Pat. Nos. 5,183,252 and 4,202,542 describe various methods for allowing vacuum drums to accommodate a few different media sizes through valving mechanisms that applying vacuum to pre-selected patterns of suction openings. These solutions do not require the additional mask hardware required by Morrisette, but do require complex, externally switchable vacuum plumbing if many different media sizes are to be accommodated. As with Morrisette, these references must also incorporate means to detect the size or orientation of the media. Each of these prior art solutions adapts the vacuum drum to a predetermined number of media sizes and orientations, and thus is not easily adaptable to sizes, shapes or orientations not considered in the initial machine design. In addition, none of the prior art addresses the excess pumping requirement due to applying suction to all of the suction openings simultaneously.
U.S. Pat. No. 5,374,021 to Kleinman includes a vacuum chamber which is divided into several sub-chambers each connected via a control passageway to one or more suction openings on a vacuum surface. Each control passageway includes a valve which is biased to keep the passageway open, and configured to close when the sub-chambers openings are not covered by a workpiece and a vacuum is applied to the vacuum sub-chambers. The valves of the passageways to openings that are covered by a workpiece remain open so that a vacuum is applied to hold the workpiece. The Kleinman system thus in effect provides a “self adapting mask” comprised of all the valves that are of the passageways to openings that are not covered by the workpiece. This offers advantages over the Morrisette and systems of U.S. Pat. Nos. 5,183,252 and 4,202,542 in that the Kleinman system adapts to all sizes, shapes or orientations.
The Kleinman system, however, still has several shortcomings. In addition, none of the prior art ad
Inventek
King Anita
Mania Barco N.V.
Rosenfeld Dov
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