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Application Techniques
More important than the method of application of various print finishes is the procedures used to apply them. We at Box Line Technology have amassed a significant amount of information gained from various installation around the world and our own development work in the UK.
Hot Foil Stamping
Hot foil stamping requires heat and pressure. A metal or polymer die is used in the shape of the impression to be made, it is essential that the die is of the best quality and to achieve this is is paramount that the quality of the negative film used in the manufacture of etched dies is of the highest quality without pinholes or blemishes. CNC engraved dies are also widely used. In general photopolymer dies are used for low impact work on manual equipment. The most popular metal die remains sensitised magnesium produced by etching with nitric acid. However magnesium does not etch to very fine details and for such cases copper is etched using ferric chloride. Both processes are resonable quick and several dies can be made at one time. Brass dies are made by CNC engraving which needs an up front design software to produce the tool path for the engraving machine. In general the manufacturing process for brass dies is slower than etching but the resultant dies will have a longer life. Mulitlevel sculptured dies are made in brass by engraving as are combination or fluted dies. These dies are produced as females and require a counter or male die in order to work. Counter dies are made by several different processes, KV, epoxy and photoplymer and are in essence molded from the female die and are a mirror image of it. For processing thicker substrates there is sometimes a reduction factor on the counter die to allow for the additional thickness of the material. Curved dies are made in brass.
Hot stamping foil is made in 5 layers on a polyester carrier which becomes a waste item at the end of the process.
The release coat and the adhesive backing react to heat, the first by melting and releasing the foil from the polyester carrier and the second by adhering the foil to the substrate. There are many types of release coats from hard (difficult) to release to soft(very easy to release) for different types of substrates and applications. For instance, fine lines generally require a harder release as a soft release can fill the areas where foil is not wanted. There are also many different adhesives depending on the substrate. In all cases heat is required in the general range of 90 degrees to 120 degrees Centigrade.
Hot foil application machinery differs considerably in style from a Heidelberg platen operating at 1,000 sheet per hour to the Steuer Foiljet operating at 12,000 sheet per hour, however whilst Heidelberg platens are still widely used, even though Heidelberg stop making them in the 1970's, Steuer's are few and far between. The most common middle market machines are Bobst type originally based on automatic platen diecutting presses. This type of press generally operates between 3,000 to 6,000 sheet per hour and requires some special features. In this type of press the foil is advanced, normally in the opposite direction to sheet travel, but also across the machine, and is indexed by multiples of the design being applied to the sheet. This normally takes the form of several small indexes followed by a clearling pull to bring new foil into the press. Also it is common to refer to the index as PULL although on most machines the foil is released against a faster running take up roller and is in effect PUSHED though the machine. The normal minimum allowence for an index is 6mm between impressions so the foil is used very effiently. Several foil widths can be used across the machine and most machine have between 2 to 4 different foil advancing rollers, each capable of advancing several rolls of foil. An important issue in this type of press is the introduction or air after each impression to release the polyester carrier completly from the foil, particularly at the higher running speeds. The introduction of the blast of air needs to come from the area immediately in front of the platen so that the air travels towards the feeder. The air is only of any use if its delivery is whilst the platen is opening from top dead centre but before the gripper bar starts to move, in the timimg cycle of the platen this is genrally brtween 33 and 66 degrees, after this the air is wasted and only serves to cool the dies. It is of paramount importance that the voulme of air is correct as well as the pressure and to achieve the volume it is normally required to have an air receive very close to the machine.
As mentioned above the dies cool after each impression as heat is taken out by the substrate and the air blast. To achieve constant high speed running it is necessary to get heat back into the die as fast as possible without overheating. The fast acting, self teaching digital heat controllers used on modern machines will achieve this. The maximum speed of any press is always govenered by the job in hand and the coverage of foil.
Combination stamping is foil stamping and embossing in one operation using a textured die and male counter die. The female die is set on the honeycomb chase in the normal way whilst the counter is place directly underneath it on the stamping plate. On each impression the substrate is foiled and embossed. The emboss depth is normally very shallow.
Cold Foil Application
The sytem requires a cold metallic foil and a lithographic adhesive. Cold foil has a very similar structure to hot foil in that it consists of 5 layers of material, however in this case the release coat reacts to sheer and not heat and the hot melt adhesive on the back of hot foil is replaced with a laquer coat which has an affinity for cold foil adhesive. Cold foil adhesives are availabe as conventional or UV formulations. Cold foil adhesive has more in common with ink than glue in its constitution and applied to the substrate in the same manner as ink through the normal ink train with an application amount of approximately 2 g/m2. The adhesive can, in general be treated like ink and used neat into the ink ducts, reducers may be used if necessary and wash up is the same as ink. In use the dampening system is reduced from normal printing and attention has to be paid to the wetting agents used as some of these can cause scumming or beading of the adhesive. The pH value and the hardness of the water needs to be monitored.
Two towers of a lithographic press are used. On the first the adhesive is printed onto the substrate with a standard lithographic plate in the pattern desired. The adhesive cannot be used on uncoated stock as the absorbtion rate is too high. On the second tower the cold foil is passed between the blanket cylinder and the impression cylinder and runs at the speed of the press.The exit angle of the foil to the sheet is critical. As the sheet arrives at the second tower so the foil adheres to the adhesive in the printed pattern and is transfered to the sheet, the remaining foil stays on the polyester and is rewound or disposed of, depending on the operational system. The foil is overprinted in the remaining units of the press to produce the desired finished effect. With cold foil the colour now comes from the process and not the material and so predominatly silver foil is used.
Either conventional printing or UV printing can be used and in both cases it is necessary to coat for drying, UV obviously dries or cures instantaniously whilst conventional ink requires standing time. Attention has to be paid to the dyne levels through the process to ensure that the foil sticks to the substrate and the ink sticks to the foil. Where UV adhesives and inks are used it is possible to cure the adhesive through the foil and in many cases the first curing is also after the first ink on the foil, in this way it is possible to use only a couple of lamps through the press plus the delivery lamps for the coating.
The application of the foil to the substrate is, of course, directly propotional to the accuracy of the printing of the adhesive but also highly dependent on a very soft release chacteristic from the foil and the use of the correct printing blanket. Several different blankets are commercially available.
Cold foil is laid onto the
substrate as opposed to stamped in as with hot stamping, the resultant
product having the surface of the particular substrate, however with
correct processing cold foil can achieve the brilliance of hot foil on
all types of substrate. An added advantage is that if a textured
substrate is used, the resultant foil surface with be equally textured
giving a brand new effect not possible with hot stamping. This process
also looks almost the same as the very old process of "Bronzing", also
not possible with hot stamping.
UV Film Casting
UV film casting is also refered to as "Cast & Cure" as a layer of UV varnish is cured whilst impressed into a micro embossed polyester film which leaves a holographic effect pattern in the cured varnish once the film is removed.
The micro etched film is produced using a polyester carrier which is coated on one side with an emulsion which is dried. The laminate is then passed between two heated roller which are being pressed together. On one roller are a series of metal shims into which a pattern has been micro embossed by laser. As the laminated passes the rollers so the pattern is transferred into the emulsion layer and dried. The pattern can be bespoke or a wallpaper design of which there are several standards.
For the in-line application on the lithographic printing press two units of the press are required, normal the last 2 units or the last printing unit and the coater. On the first printing unit a duct varnish is applied to the substrate by a standard lithographic plate in the pattern desired. On the next unit the film passes between the blanket cylinder and the impression cylinder and down around the first quadrent of the impression cylinder at which point a UV source is placed which can be the normal interdeck if this is positioned so. As the sheet with the wet varnish arrives at the second unit the printing pressure of the press pushes the micro-emboss in the varnish which is then cured as it passes under the UV light sourse. After curing the film is removed from the varnish and rewound for further use. The film can be used up to 10 times.
The micro-embossed pattern is now dried into the varnish giving a holographic effect.
It is also possible to do this with a piece of equipment external to the press.
For futher information on any of these application please make an enquiry via the Contacts page.
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