1. Field of the Invention
This invention relates to an apparatus for use in lithographic printing such as offset printing to automatically replenish dampening water. More particularly, this invention relates to an automatic dampening water replenisher with which the concentration of is conditioned to be within a predetermined range and can be automatically supplied into an associated tank or fountain in a lithographic printing press and which provides ease in troubleshooting.
2. Prior Art
Offset printing is a method of lithographic printing that makes use of the inherent immiscibility of water with grease. The surface of the printing plate used in this method consists of water-receptive areas that repel greasy ink and ink-receptive areas that repel water. The former are called "non-image" areas whereas the latter are "image" areas.
Dampening water used in lithographic printing wets the non-image areas to increase the difference in surface chemistry between the image and non-image areas, thereby widening the difference between the ink repulsion of the nonimage areas and the ink affinity of the image areas. Dampening water used for this purpose is prepared by diluting a stock solution, or an additive generally referred to as an "etch solution" (e.g. an acidic etch solution, an alkaline etch solution or an alcohol), with water to a concentration of 0.1-1 wt % or 0.1-5 wt %. To insure consistent printing operations, various chemicals are usually incorporated in the etch solution and they include a desensitizer for greases at the non-image areas of the plate, a pH modifier, a pH buffer, a cleaning agent, a wetting agent, a chelating agent, an antiseptic and an alcohol. Water that is used as a diluent may be tap water, industrial water or well water.
Specific examples of common dampening water are aqueous solutions consisting of water and small amounts of stock solutions containing alkali metal salts of bichromic acid or ammonium bichromate, phosphoric acid or salts thereof such as an ammonium phosphate, or colloidal materials such as gum arabic and carboxymethyl cellulose (CMC). Since etch solutions are used as additives for dampening water in very small amounts compared to the diluent water, it is considerably difficult to control their concentration to lie within an appropriate range. Even a slight variation in the concentration of each solutions will tend to upset the balance between printing ink and dampening water, causing various troubles in printing operations, including scumming, smudging, trapping on roller surfaces, roller stripping, delayed setting and drying, deteriorated gloss and poor dot reproduction.
An aqueous solution containing ca. 20-25% isopropyl alcohol (IPA) has also been used as dampening water. However, isopropyl alcohol is so volatile that maintaining its concentration at a constant level is very difficult unless an expensive device is used. In addition, isopropyl alcohol which has not only an unpleasant odor but is also toxic is not desirable today when there is a growing concern over environmental pollution caused by industrial activities.
Under these circumstances, various methods and apparatus have been used or proposed for adjusting the concentration of additives in dampening water to lie within appropriate ranges. Offset printing machines are usually equipped with an apparatus for cooling and circulating dampening water which is stored in a tank of its own. The tank of dampening water is supplied with a stock solution and diluent water which are mixed to prepare dampening water of a given concentration. Conventionally, the concentration of dampening water is maintained at a constant level by measuring the pH or electroconductivity of the dampening water in the tank or, if IPA is used as an additive, by adjusting its concentration with a hydrostatic controller using a float. However, these methods are incapable of precise performance in a consistent way.
To solve these problems, an apparatus that uses a metering pump to replenish the stock solution and mix it with diluent water has been proposed in Japanese Patent Publication (kokai) No. 39463/1980 but it involves a complicated mechanism and is not easy to use. An apparatus for mixing and feeding dampening water automatically by adjusting the quantity of the stock solution using both a metering pump and a timer that controls its working time has been proposed in Japanese Utility Model Application(kokai) No. 163535/1981 but the volume of neither diluent water nor stock solution can be adjusted with sufficient correctness to maintain the concentration of dampening water at a constant level. A method and apparatus that meters diluent water and chemical solutions on a weight basis have been proposed in Japanese Patent Application (kokai) No. 303739/1988 but they are also complicated in mechanism and are difficult to use.
The dampening water conventionally used in the various replenishing apparatus described above has a serious defect in that it does not wet the non-image areas of a printing plate uniformly, causing occasional smudging of the print. In addition, considerable skill has been necessary to properly adjust the amount of dampening water to be supplied.
With a view to solving these problems, the assignee proposed in Japanese Patent Application No. 243613/1989 a compact and inexpensive apparatus and method for replenishment that used novel dampening water that was free from the toxicity and safety problems, that was easy to handle and that was highly stable, which apparatus and method were capable consistently metering and feeding predetermined quantities of diluent water and stock solution into a dampening water tank in a device for cooling and circulating dampening water. The replenishing apparatus proposed in that patent application has the advantage that the dampening water it provides has better physical properties than what was previously used and that it maintains constant concentrations with fairly high precision. However, the apparatus uses a metering pump as a means of feeding the stock solution for dampening water and the metering of the solution which is performed by controlling the working time of the pump is subject to great variations, which limit the efforts to further improve the precision with which the concentration of dampening water can be held constant. Furthermore, the metering pump can sometimes idle and this has been another cause of the low precision of the apparatus.
As a further problem, none of the prior art apparatus for replenishing dampening water have means of calculating, displaying and storing the integrated (total) flow rates of water and stock solution or the concentration of dampening water as computed from those integrated flow rates. If the main power supply is cut off on account of either power failure or the occurrence of trouble in the apparatus or printing press, there is no way to know the integrated flow rates of water and stock solution or the concentration of dampening water, and this necessitates repeating the overall procedure of preparing dampening water in the tank or considerable difficulty is involved in identifying the cause of the trouble.