Printing – Stenciling – Stencils
Patent
1980-12-23
1983-02-15
Heinz, A. J.
Printing
Stenciling
Stencils
1011284, B05C 1706, B05C 1708
Patent
active
043734413
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to screen printing frames, and more particularly to printing frames for use with thermoplastic ink.
BACKGROUND OF THE INVENTION
Screen printing processes are known in which, for using thermoplastic printing inks, a chrome-nickel steel screen is used which is electrically heated by joining its opposite sides to a current supply. For functioning, such steel screens have to be insulated at the positions where they are supported and fixed in position, this being done by having an insulating material, such as rubber or plastics, between the frame and the screen, this making for a complex screen fixing system designed for pulling the screen tight on the one hand and at the same time stopping any danger of electric shocks. For this reason it is normal for non-conducting lengths of fabric to be stitched on to the two sides of the edges of the steel screen and these strips are then fixed to the frame mechanically with or without a system for pulling the screen tight on later use, while the two other opposite sides of the steel screen or fabric are loosely rested or guided over insulating bridges on the frame and run to points on which they are joined with an electrically insulated system for pulling the screen tight. A screen fixing system, with which the screen may be pulled tight later as desired, is better, because, when the screen is heated, it becomes extended, that is to say less tight. In a simple system the metal screen is directly fixed to a wood frame. However screens have been bedded or seated in thermosetting (that is to say not thermoplastic) synthetic resin screen frames. No suggestion has been made so far for using adhesives without a mechanical join at the same time, because adhesives are generally thermoplastic and the heated screen would, it would seem, then become loose.
SUMMARY OF THE INVENTION
The problem stated above is taken care of in the present invention because the screen fabric is bedded in thermoplastic synthetic resin on one or more sides, for example by ironing it on, injection molding and the like, and for stopping the danger of the screen becoming loose at overhigh temperatures and/or on pulling forces acting on the screen, certain measures are put forward in the invention.
(1) If the resin/wire connection is in the direction of the current, the current input and output connections are made narrower than the space between the connections,
(2) if the resin/wire connection is normal to the direction of the current, the steel screen is decreased in resistance from the current terminal as far as the said connection, for example by copper-plating, so that it is not heated or is only hardly heated at these points.
(3) The two ends of the current path are within the thermoplastic screen fixing system on the frame and the current does not go across the fixing system.
Using these measures screen printing frames are put forward by the invention which have a thermoplastic, insulating casing or inlay, for example made of polyethylene, and in which (for making the said wire/resin connection) the screen fabric is bedded, for example by using heat and pressure.
The casing may be produced by a shrinking process, spraying on or fluidized bed sintering. The inlay may have the form of a plastics section which may be then designed so as to be undone from the frame, even when the screen is fixedly joined up with it. This part of the invention is more specially useful for frames which have a system for pulling tight the screen for purposes of adjustment. For heatable screens of great size it is furthermore possible to make use of a thermoplastic electric frame or edging as in the German Pat. No. 1,671,636 on making use of the measures noted.
An account will now be given of the invention in more detail using working examples.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a horizontal sectional view of a frame and screen assembly according to the present invention.
FIGS. 1A, 1B and 1C are enlarged cross-sectional views of different frames provided with
REFERENCES:
patent: 3167004 (1965-01-01), Miller
patent: 3800697 (1974-04-01), Sullivan
patent: 4028230 (1977-06-01), Rosenblum
Gordon Herman L.
Heinz A. J.
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