Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1980-05-26
1983-05-24
Williams, Howard S.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204181C, C25D 1306, C25D 1314
Patent
active
043849458
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with a method of producing fine-apertured sheet materials and sheets produced by the method. It is in particular concerned with the production of rotary screen printing cylinders and will be described principally by reference to such cylinders. It is however to be understood that the invention has a wider applicability, for example in the production of flat fine-apertured sheets for other purposes, e.g. filtration or sieving.
In rotary screen printing, a cylinder of a fine-apertured sheet material is first engraved with the desired pattern to be printed by forming on the screen areas of blocked apertures forming a negative image of the pattern. There are thus areas on the screen whose apertures are unblocked and the print medium can be forced through these apertures to apply a positive image of the pattern onto the fabric.
The development of wire cloth rotary screen printing cylinders is described for example in British Patent Specification Nos. 756,315, 830,506, 1,050,649 and 1,208,109. The problems and advantages associated with these screens can be summarized as follows.
Woven wire mesh cloths which are sufficiently fine to give reasonably good definition in engraving and printing on textile fabrics (for example 60, 80 or 100 mesh per inch, usually woven from phosphor bronze wire or occasionally from Monel or stainless steel wire) invariably have too wide apertures and deposit too much print paste on the fabric for the printing conditions under which they have to work. In their normal loom state they also have low dimensional stability which can cause distortion and damage in printing as well as bad pattern registration in multi-colour printing; this is due to the different weights of print paste that each screen may contain at any given moment. In certain cases the apertures can be reduced to a suitable size by electrodeposition of copper and/or nickel, but as the amount of copper or nickel is increased the wire mesh becomes increasingly brittle and very easily damaged in printing or handling. Additionally, towards the end of the electroplating process, and as the apertures become smaller, it becomes more difficult to control accurately the termination of the process and to achieve high standards consistently.
A major advance in overcoming these difficulties was brought about by the introduction of a two-ply wire mesh/fabric screen as described in British Patent Specification No. 1050649. In this method, greatly improved strength and dimensional stability is obtained by first making a very strong cylinder from heavy gauge phosphor bronze wire and then giving this cylinder a lightly electroplated coat of nickel. This makes the cylinder chemically resistant to print-paste constituents and gives it extra dimensional stability. By covering this cylinder with either flat or tubular screen fabrics, (e.g. of polyester or polyamide) possessing small aperture sizes (e.g. between 50 and 150 microns), strong printing screens can be obtained with good dimensional stability. These screens are suitable for engraving and printing on textile fabrics to give well defined patterns. This type of screen has been used, with advantages in many cases over electroformed nickel perforated screens, since 1965.
Some disadvantages of two-ply wire mesh/fabric screens include: the high cost of fitting flat or tubular fabric onto a wire mesh cylinder; the possibility of the tubular fabric not being stuck firmly to the wire mesh cylinder; the possibility of damage to the tubular screen fabric, and the engraved detail, where it is worn by the fabric selvedge, or damaged by adhesive tape, which may be used to temporarily "mask out" and narrow the pattern width; and the necessity of using either special engraving techniques, or when using conventional photo-sensitive resin emulsions for engraving, of being unable to bake at high temperature to obtain maximum durability, on account of the poor heat stability and heat resisting qualities of the fabric material.
A similar method, in which a relatively coarse scr
REFERENCES:
patent: 3591466 (1971-07-01), Heiman
patent: 4210507 (1980-07-01), Davidson et al.
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