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Uvacure Series Inks Technical Information Pages.
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Dry offset printing is similar to offset lithography in that a rubber blanket is used to carry the image from the printing plate to the container surface. The plate used has, as in letterpress, the image area raised above the surface of the plate. Ink is distributed through a series of rollers and onto the raised surface of the plate. The plate transfers the image to the blanket, which then prints the entire multicolour copy taken from one to as many as eight plate cylinders and transferred on the container in one operation. The "dry" denotation of this offset system serves to differentiate it from the offset system which uses the incompatibility of water and inks to "dampen" the surface of the plate or substrate to prevent ink transfer.
One to ten colours can be printed in a single pass over the container, with all colours being applied simultaneously by the same blanket. The inking station on each colour head contains the ink fountain and the individual set of rollers. The number of rollers and their arrangement guarantees the finest most uniform distribution of ink to the individual plate cylinder. The ability to effectively distribute the ink as an even film across the full width of the design is the object of the system.
The offset blanket is inked in turn by each plate cylinder, each plate cylinder having all of the copy for one colour. The printing plate is prepared by a photochemical process, providing extremely fine reproduction of delicate artwork. The plate cylinder holds the printing plate, which is secured to the cylinder by clamps or through magnetism. Each plate cylinder has a very fine micrometer (.001 of an inch in any direction) adjustment. The inked printing plates deposit their image in sequence and in registration on a common printing blanket. The paste-type ink used in this process, whether ultraviolet or conventional in nature, allows wide latitude in choice of colours that are resistant to scuffing and moisture.
The blanket platen holds the rubber printing blanket, which is secured through the use of either "sticky back" material or ratchet clamps. The size of the blankets match the circumference of the container in length and is cut marginally wider than the height of the printed design. The blanket transfers all of the images and copy to the container in one pass. Various blanket materials and blankets with different thickness are available for varying printing requirements on different containers.
The container shape and its tolerance are important, not only for the mechanical nature of its handling, but also for the quality of print that can be transferred to it.
The material handling section of the decorating line is customized for the individual style container. The indexing or constant motion turret holds the specific container tooling, commonly called mandrels (or spindles), during all operations including printing. The material handling section or the individual container tool can be swiveled for container taper and positioned for printing pressure. Because of the pressure required for printing, containers must be supported by a mandrel or inflated with air pressure. Surface blemishes, uneven mold join lines and uneven container wall thickness should be avoided.
When required, containers can be cleaned by rotating them in front of a de-staticizer which uses ionized air to remove static and dirt particles. The plastic surface is prepared to promote the adhesion of ink to the substrate with either a flame or corona system.
The characteristics of the plastic substrate must always be considered as part of the printing operation because the nature of its surface is significant in determining suitable ink formulations for printing on it. Dry offset inks are available for printing all the common packaging plastics. Polyolefin surfaces are both inert and have a low surface energy with the result that inks printed on them tend to reticulate and possess poor adhesion. The problem is overcome by modifying the surface using either a corona discharge or flame treatment. However, it is essential to control this treating operation if optimum print performance is to be achieved. Corona discharge is a popular method for treating plastic films but is losing ground to flame treating for containers. Other substrates such as the vinyl's and styrene are much less inert than polyolefins and pretreatment is unnecessary.
Stations for orientation can be provided which use a container lug, side seam or bottom notch to register the container prior to the print station. After printing, the parts pass through the print curing unit (for "on mandrel" drying) or are transferred to a conveyor for transport through an appropriate dryer oven (ultraviolet or infrared).
Print production speeds are high: tapered plastic containers such as yogurt cups are handled by indexing equipment up to 500 parts per minute; constant motion two piece beverage decorating equipment operates at up to 2000 + cans per minute.
Auxiliary equipment for automatic operation such as feeds, take off devices and dryer ovens are all important adjuncts to an offset printing system for containers. No system is universal, and while most are designed for size changeover among product lines or families of containers, few will permit the modification required to handle various types of containers (such as jars, tubes and bottles).
Dry offset is used to decorate more preformed containers than any other process because it is a high volume, low cost per container process. A printing line is designed to handle a range of sizes of the same type or family of containers, such as cups, tubes, jars or cans and their respective closures.
Dry offset handles rounds, tapered rounds and flats best; with appropriate tooling, irregular shapes such as squares and rectangles can be handled at slower speeds. Ink lay down is minimal, so that opacity is best on a white container; light colour printing on a dark substrate is not recommended.
UVACURE Inks are the product of many years of experience and research in printing inks, using the latest raw material developments and manufacturing techniques.
UVACURE Inks are specially formulated so that under normal circumstances they may be used directly from the container. Because of the extreme reactivity of the system, Ultra Violet inks and varnishes have a limited shelf life. We guarantee a shelf life of 6 months under proper storage conditions, i.e., under 70°F (20°C) and protected from direct sun light. Specialty inks including fluorescent and metallic colours (once mixed) are more unstable and should be used as soon as possible and therefore are not guaranteed.
UVACURE Inks fulfill the Food, Health and Safety requirements of most countries, as well as California Proposition 65. Their optimum adhesion properties make them ideally suited for the printing of food packages and similar containers.
In general, when producing small containers (yogurt and margarine cups), pre-treating with Corona discharge is preferred, whereas with larger containers (buckets), gas flame pretreatment is more suitable. On PE and PP it is recommended to achieve a surface tension (pre-treatment intensity) of at least 40 mN per metre (40 dynes/cm), this guarantees maximum adhesion properties. Surface tension tests can be performed at press side with a dyne tension test kit, a relatively inexpensive item. Contact us if you need more information on treatment systems or test kits.
Last modified: March 29, 2004
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