Production of rotary screen printing cylinders and other fine-apertured sheet materials

A method of producing a rotary screen printing cylinder or other fine-apertured sheet material which comprises electro-deposition of a plastic material onto an electrically conductive fine-apertured cylinder or other sheet substrate. When applied to wire mesh cylinders, the method is a fast and effective way of reducing the aperture size. The method can also be operated so that plastic is deposited on one side only of the substrate to increase the substrate strength with little or no aperture size reduction.

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 screen cylinder is covered 
with a tubular fine-mesh fabric, is also described in Swiss Patent 
Specification Nos. 545692 and 7205/72. 
In another method which is widely used, the screen cylinder is formed by 
electrodeposition of metal on a mandrel. The so-called electroforms 
produced by this method can have very small aperture sizes and a high 
aperture density (e.g. up to about 2000 per cm.sup.2); however, the 
mandrels required are expensive and the electroforms are brittle and 
require very careful handling. 
I have now found a new method of producing fine-apertured sheet materials, 
e.g. having aperture sizes ranging from 50 to 500 .mu.m and particularly 
those for use in rotary screen-printing, which involves forming a coating 
of plastic material on a fine-apertured sheet substrate by 
electrodeposition of the plastic material. 
This method has many advantages. The plastic coating has the advantage of 
increasing the strength of the substrate and reducing its brittleness. The 
method can be operated with or without reduction in the aperture size, and 
if desired some of the apertures can be closed completely. The plastic can 
be coated onto the substrate quickly, evenly and accurately, and the 
deposition can be accurately terminated. The deposition is thus easily 
controllable and allows consistent standards to be achieved at low cost. 
The method is primarily applicable to the treatment of fine-apertured 
sheet for rotary screen printing cylinders, and is a fast and inexpensive 
way of making such cylinders. 
As indicated above, in a principle embodiment of the invention, the method 
is operated to reduce the aperture size. This is particularly useful when 
the substrate apertures are relatively coarse (e.g. 100-500 .mu.m), as for 
example in wire mesh rotary printing screens. A sheet can be made with a 
very small aperture size and a good aperture density that is sufficiently 
strong for continuous use without being brittle. For example, if a 
nickel-plated phosphor bronze wire mesh cylinder (of the type normally 
intended to be covered with a tubular screen fabric) is coated by my 
method, the aperture size can be reduced in 1 to 3 minutes to such a 
degree that the fine-mesh fabric cover is unnecessary. Not only does the 
coating operation take less time to perform than it takes to fit a screen 
cover over a cylinder, but the expensive covers can be dispensed with 
altogether. Moreover, a range of printing screen cylinders having 
different aperture sizes can easily be produced for use with different 
print pastes and fabrics from one quality of wire mesh, thus eliminating 
the time and expense of fitting a selected quality of fabric cover over 
the same cylinder according to the particular paste being used and fabric 
being printed. 
Printing cylinders produced by this method are of particular advantage, on 
account of their strength, for use on wide textile printing machines and 
carpet printing machines, in place of the electroformed nickel screens 
normally used. 
In comparison with the alternative technique of reducing the aperture size 
by electrodeposition of metal, this method is again much quicker and 
cheaper. Plastics are less expensive materials than nickel or copper, and 
the deposition time of 1 to 3 minutes compares very favourably with times 
of 0.5 to 3 hours required for electroplating. The method is also more 
accurate and, unlike electroplating, does not give a brittle product. 
The fine-apertured substrate which can be coated by this method may be any 
suitable electrically conductive material. It may for example be either 
metallic or non-metallic, and it may be woven or non-woven. The substrate 
is preferably made of metal, such as phosphor bronze, which may be lightly 
electroplated with for example nickel or copper, or it may be made of 
stainless or non-stainless steel, copper, aluminium, nickel, brass or 
Monel. These metallic substrate sheets are preferably used in the form of 
a wire mesh. 
Alternatively, a fine-apertured non-metallic substrate may be used, for 
example of a plastics material. Substrates of this kind are not naturally 
electrically conductive and they therefore have to be coated or otherwise 
treated with an electrically conductive material to enable them to be used 
in the electrodeposition coating step. Plastics may for example be 
rendered conductive by coating with graphite or an electrolyte, as is well 
known in the electroplating and electrodeposition arts. Electroplated 
plastic (e.g. nickel-plated polyester) may also be used, as described in 
British Patent Specification No. 1,332,046. Suitable plastics for the 
substrate are synthetic or natural polymeric materials, e.g. polyesters, 
polyamides, polyolefins such as polypropylene, or regenerated cellulosic 
materials. These materials are again preferably used in mesh form. 
The aperture size of the sheet substrate may for example be from 100 to 500 
microns, usually 150 to 300 microns, and the size may be reduced by the 
electrophoretic coating to, for example, 40 to 200 microns. However, 
sheets having smaller aperture sizes (e.g. down to 35 .mu.m) can also be 
used. 
My electrodeposition method can also be used to increase the strength of 
the substrate material with litle or no reduction in aperture size. This 
is of particular value in connection with electroforms for rotary screen 
printing, which have a satisfactory combination of aperture size and 
aperture density in their original state but lack strength and structural 
stability. 
In this embodiment of the method, the plastic material is electrodeposited 
onto one side only of the electrically conductive fine-apertured sheet 
substrate. 
The substrate sheet may again be in the form of a cylinder, e.g. a rotary 
screen printing cylinder, and is preferably an electroformed rotary 
printing cylinder. If desired however the substrate may be a flat sheet, 
which can either be formed into a cylinder after being coated by the 
method of the invention or be left flat for other uses, e.g. as a filter 
or sieve. 
A number of different techniques can be adopted to ensure that only one 
side of the substrate is coated. 
For example, one side of the substrate (the side which is not to be coated 
by electrodeposition) may be temporarily protected by a coating or film of 
a non-conductive material. The non-conductive coating can then be removed 
after electrodeposition of the plastic material. The non-conductive 
coating material should be water-insoluble and easily removable after 
electrodeposition, for example with an organic solvent; examples of 
suitable materials are esters of poly (methylvinyl ether/maleic acid) such 
as the monobutyl ester sold under the trade name Gantrez ES 435 (GAF), 
bitumen or ultra-violet hardened polyvinyl alcohol. Gantrez ES 435 can for 
example be removed with an organic solvent such as isopropanol. Hardened 
polyvinyl alcohol is removable with sodium hypochlorite solution. Another 
material which may be used is "Pro-peel" (a polyvinyl chloride solution 
made by TAK Chemicals Ltd.) which can be peeled off after the 
electrodeposition process. 
When a cylindrical substrate is used, the inside of the cylinder can be 
protected during electrodeposition by inflating a bag or tube (e.g. of 
rubber) inside the cylinder. The bag or tube can be simply deflated and 
removed after electrodeposition, leaving the inner surface of the cylinder 
uncoated. 
Another alternative technique can be used when applying my method to an 
electroform. Electroforms are produced by electrodeposition of metal 
(usually nickel) on a mandrel, and the plastic material can simply be 
electrodeposited into the outer surface of the electroform whilst it is on 
the mandrel. Thus, after deposition of the metal onto the mandrel and 
washing with water, the mandrel can be transferred to the tank for 
electrodeposition of the plastic material; the electroform remains on the 
mandrel and the mandrel prevents coating of the inner surface of the 
electroform. A particular advantage of this method is that reduction of 
the aperture size is prevented by the resist already on the mandrel, the 
electrodeposited plastic material acting only to strengthen the cylinder. 
Electroform printing cylinders are of very light construction and normally 
have for example a metal thickness of 60-200 .mu.m, aperture diameters of 
50-400 .mu.m and from 250-1800 apertures per cm.sup.2. The thinner 
cylinders generally have the larger aperture densities and smaller 
aperture sizes, and they are easily damaged in handling both on and off 
the printing machine. Extra strength and durability can be obtained by 
increasing the metal thickness, but the apertures then become coarser and 
much design detail is lost in engraving and printing. For example, a 
cylinder 80 .mu.m thick may have about 1800 apertures/cm.sup.2 and 
aperture diameters of about 60 .mu.m; this is a good combination of 
aperture sizes and diameters for detailed printing, but the screens are 
very delicate. On the other hand, screens 90 to 110 .mu.m thick (having 
aperture diameters of 120 or 150 .mu.m and aperture densities of about 
1000 or 600) are noticeably stronger, but are less suitable for detailed 
work. 
The coating of one side of the sheet enables the strength of the thin 
electroform cylinders to be increased without reducing the aperture size 
or density. The electrodeposition method can for example be used to 
deposit a coat of plastic 5-40 .mu.m thick on the electroform, and this 
considerably increases the strength and resistance to tearing and 
creasing. 
One-side coating can also be applied to coarser electroforms (e.g. those 
90-200 .mu.m thick), and here some reduction in aperture size can be 
allowed to occur and in some circumstances is positively desirable. This 
is achieved by applying an extra thick coating of plastic. This not only 
reduces the amount of colour which can pass through the screen but gives a 
smoother-edged aperture as compared to the irregular edged apertures 
frequently present on such screens. This allows for better registration of 
multicoloured designs, as the screens are again stronger, less flexible 
and less brittle than uncoated cylinders. The reduction in aperture 
diameter can for example be from 10-20 or 30 .mu.m, depending on 
circumstances. 
The one-side coating method can also be applied to the relatively coarse 
substrates described above, of both the metallic and non-metallic kind. It 
can for example be applied to wire meshes for rotary printing cylinders, 
which generally have aperture diameters of 150-300 .mu.m. Some aperture 
size reduction normally occurs during electrodeposition with such 
substrates, but as indicated above this is desirable. 
The coating of the sheet substrate in my method may be performed by known 
electrodeposition techniques. Such methods are for example described in 
British Patent Specification Nos. 482,548, 972,169, 933,175, 970,506, 
998,937, 1,003,238, 1,419,607 & 1,382,512, U.S. Pat. No. 3,200,057, and 
Dutch Patent Specification Nos. 6407426, 6407427, 6407428 and 6407429. 
Thus in general the clean substrate may be connected to an electrical 
supply and immersed in a tank containing an aqueous dispersion, emulsion 
or solution of the plastic material which is to form the coating. The 
substrate will usually be connected as the anode, but it can also be used 
as a cathode with cathodically-depositable plastics. When current is 
passed through the bath, a coating of the plastic is rapidly and evenly 
built up on the substrate. Currents of for example 2-20 amps/ft.sup.2 
(2-20 mA/cm.sup.2) at 30 to 150 volts may be used at temperatures of 
20.degree. to 45.degree. C., using coating times of 0.25-3 minutes. The 
coating operation can be accurately terminated by appropriate choice of 
coating time, voltage and current. In some cases, the process can be 
self-terminating as the coating itself is non-conductive. 
A cylindrical substrate is preferably rotated during the coating operation 
to ensure uniform coating and the current may be reversed when coating is 
complete to allow the sheet to be lifted out cleanly from the tank. The 
fluid in the tank is preferably continuously agitated, and may be 
continuously circulated and filtered to remove undesirably large 
particles. The coating tank is also preferably thermostatically 
controlled. On completion of the coating process, the substrate may be 
rinsed and air-dried to remove excess water. 
When the coated sheet is to be used in rotary screen printing, the 
substrate sheet is preferably first formed into a cylinder and then 
coated. The method can however be also applied to flat substrate sheets on 
either a continuous or discontinuous basis; the coated sheet can then be 
formed into cylinders or left flat for other uses, e.g. as filters or 
sieves. 
The plastics material used for the coating may in general be any type of 
synthetic or natural polymeric material which can be used in 
electrodeposition methods, for example epoxy, acrylic, polyester, 
polyurethane or alkyd resins, and cross-linkable vinyl polymers and 
non-hardenable resins and polymers. A thermosetting plastic is preferably 
used, particularly if the coated sheet is subsequently to be subjected to 
baking (e.g. at 120.degree. to 200.degree. C.) during an engraving 
process. Non-hardenable thermoplastics may however also be used. The 
choice of resin may be varied according to particular circumstances; for 
example, modified alkyd resins give more flexible films, whereas butadiene 
and acrylic resins give harder films which are more resistant to abrasion. 
The electrodeposition bath may also contain additives (such as pigments, 
dyes or extenders), as in conventional practice. 
One advantage of the electrodeposition method is that the coating can 
easily be removed if desired, before baking. Stripping can be effected 
with ammonia solution, amines such as diethylamine, polyvinylpyrrolidone, 
paint stripping solutions or caustic soda. This can be useful if for some 
reason the coating is imperfect. 
After electrodeposition, the sheet may be engraved for use in screen 
printing by conventional screen engraving techniques to give the desired 
pattern of permeable and impermeable areas. This can for example be done 
using photographic techniques by means of the photosensitive resin 
emulsions normally available for screen engraving, using in this case a 
positive film for the light exposure process. 
Alternatively, the cylindrical screen can be engraved before 
electrodeposition so that only the open permeable parts of the screen have 
the apertures reduced in size whilst the apertures in the non-permeable 
parts are completely blocked by light hardened resin emulsion. 
To obtain the reverse effect when engraving before electrodeposition, the 
wire mesh cylinder can be engraved photographically with a negative film 
of the design by coating the cylinder with light-sensitised polyvinyl 
alcohol so that the image formed acts as a resist during electrodeposition 
which is conducted to give complete closure of apertures. The 
light-hardened polyvinyl alcohol is then removed by stripping agents such 
as sodium hypochlorite to leave the permeable parts of the screen with 
completely open apertures.

A cylindrical wire cloth screen (1) is placed on a mild steel shaft (2) 
each end of which rests on two brass V-block contactors (3) which are 
connected to the positive terminal of a DC power source (4). The 
cylindrical screen (1) is fixed at each end to mild steel rings (5) by 
means of either steel screws, or by a Jubilee Clip firmly clamping the 
ends of the screen to the rings. The two rings (5) are in turn firmly 
bolted to the steel shaft (2) thereby completing contact to the positive 
terminal of the power source (4) so that the cylinder for the purpose of 
the coating process is the anode. 
The cathode can be either the sides of the mild steel coating tank (6) 
providing that it has not been lined with an insulating material, or 
preferably, the mild steel plates (7) corresponding to the length and 
diameter of the wirecloth cylinder. The mild steel plates are supported 
opposite each side of the cylinder, e.g. about 3 cms or more away, as may 
be desired. 
The coating solution (8) is pumped into the inner section of coating tank 
(6) and the level maintained by pumping the solution continuously over the 
weirs (9) from an adjacent overflow tank (not shown). A thermostatically 
controlled heater is fitted to maintain the desired solution temperature, 
normally for convenience 20.degree. C., and a filter unit is placed 
between the coating tank and the pump to ensure solution clean lines. 
Before commencing the coating process the cylindrical screen (1) is rotated 
on the steel shaft (2) by means of spur gears fitted to the steel shaft 
and the driving motor (10). 
A direct current is then passed between the cylindrical anode and the 
cathode steel plates. Under the influence of the electric field, 
negatively charged particles come into contact with the positively charged 
cylinder. The particles then lose their charge and deposit as a coating on 
the cylinder. 
After completion of the required coating time, the cylinder is removed from 
the coating tank and any undeposited coating solution is washed from the 
cylinder by a spray of cold water. 
The process is completed by drying, e.g. by first drying at low temperature 
and then at 120.degree.-200.degree. C. The following Examples illustrate 
the invention. In each case the apparatus shown in the drawings was used. 
EXAMPLE 1 
Substrate: Phosphor bronze plain weave wirecloth cylinder (lightly nickel 
plated) with 60 apertures per 25.4 mm, aperture size of 250 microns and 
wire diameter of 170 microns. 
Coating solution: A melamine-modified alkyd resin (Code X6126; Macpherson 
Industrial Coatings Limited, Lancashire, England) which is a water soluble 
polymer neutralised with an organic base containing small amounts of 
coupling solvent (specifically butyl alcohol) or alternatively butyl 
glycols, plus a small amount of phthalocyanine blue pigment dispersion in 
low concentration added as a sighting agent. 
Solids content: 5% 
Conductivity: between 500 and 5000 Microsiemens but specifically 2500 
microsiemens. 
pH: 7.9 
Temperature: between 15.degree.-30.degree. C. (but specifically 20.degree. 
C.) 
Voltage: 30-150 volts (but specifically 50 volts). 
Current Density: 10 amps/square foot (10 mA/cm.sup.2) 
Coating Time: 13/4 minutes 
After coating the 250 micron square apertures had changed to 210 micron 
circular apertures. 
EXAMPLE 2 
Substrate: Phosphor bronze plain weave wirecloth cylinder (lightly nickel 
plated) with 80 apertures per 25.4 mm, aperture size of 186 Microns and 
wire diameter of 132 microns. 
Coating Solution: Resin Code X6126 
Solids content: 5% 
pH: 7.9 
Conductivity: 2500 Microsiemens 
Temperature: 20.degree. C. 
Current Density: 10 Amps/square foot (10 mA/cm.sup.2) 
Voltage: 50 
Coating time: 1 Minute 
After coating the 186 micron square apertures had changed to 140 micron 
round apertures. 
EXAMPLE 3 
Substrate: Phorphor bronze twill weave wire cloth cylinder (lightly nickel 
plated) with 66 apertures per 25.4 mm, aperture size of 145 microns and 
wire diameter of 240 microns. 
Coating solution: resin Code X6126 
Solids content: 5% 
pH: 7.9 
Conductivity: 2500 microsiemens 
Temperature: 20.degree. C. 
Current density: 9 amps/square foot (9 mA/cm.sup.2) 
Voltage: 50 volts 
Coating time: 11/4 minutes 
After coating the 145 .mu.m square apertures changed to 100 .mu.m rounded 
apertures. 
EXAMPLE 4 
Substrate: Phosphor bronze twill weave wirecloth cylinder with 66.times.50 
apertures per 25.4 mm, aperture size of 145.times.230 microns and wire 
diameters of 240 and 280 microns. 
Coating solution: Resin Code X6126 
Solids content: 5% 
pH: 7.9 
Conductivity: 2500 Microsiemens 
Temperature: 20.degree. C. 
Current density: 10 amps/square foot (10 mA/cm.sup.2) 
Voltage: 50 
Coating time: 3 minutes 
After coating the rectangular apertures had changed to elliptical apertures 
size 110.times.190 microns. 
EXAMPLE 5 
Substrate: Phosphor bronze twill weave wirecloth sheet (lightly nickel 
plated) with 66 apertures per 25.4 mm, aperture size of 145 microns and 
wire diameter of 240 microns. 
Coating solution: A butadiene base resin (Macphersons Industrial Coatings 
Limited) Code 11180 neutralized with an organic base and containing small 
amounts of coupling solvents plus phthalocyanine blue pigment dispersion 
in low concentration as a sighting agent. 
Solids content: 5.4% 
pH: 8.8 
Conductivity: 2050 microsiemens 
Temperature: 20.degree. 
Current density: 10 amps/ft.sup.2. (10 mA/cm.sup.2) 
Voltage: 60 
Coating time: 2 minutes 
After coating the aperture size had been reduced from 145 microns to 110 
microns. 
EXAMPLE 6 
Substrate: Electroformed fine apertured sheet with 60 apertures per 25.4 
mm, aperture size of 140 microns, half of one side being coated with the 
product Pro-peel (TAK Chemical Industries Limited, England) to act as a 
resist to the coating process. 
Coating solution: resin code X 6126 
Solids content: 5% 
pH: 9 
Conductivity: 2500 Microsiemens 
Temperature: 20.degree. C. 
Voltage: 30 
Coating Time: 3 Minutes 
Current density: 6.5 amps/square foot (6.5 mA/cm.sup.2) 
After coating the area covered by Pro-peel remained clear and the opposite 
side of the electroformed sheet was coated with resin. The apertures in 
this part retained their original size. In the part not coated with 
Pro-peel the electroform was coated on both sides with resin and the 
apertures reduced in size to 120 microns.