Gravure printing surface treatment

I. Introduction

Gravure is promising for its advantages such as good print quality, high print-resistance, and long-term imprints. The method of gravure platemaking has been carbon paper, direct screening, electronic engraving, laser engraving and gravure conversion. Although different methods, but with the application of surface treatment process. The continuous improvement and innovation of the surface treatment process will inevitably make up for the shortcomings of the gravure plate production cycle, increase the resistance to printing force, reduce the cost, and provide a strong guarantee for the wide application of the gravure plate.

Second, the application of surface treatment process

1. Electrodeposition

1 deposition of copper plate

Prior to making a copper plate, the base roll is deposited with a pre-plated layer. For example, dark nickel plating, cyanide copper plating, and Ballard treatment as required. The quality of the plated copper layer must meet the following requirements: The purity of copper is high and the grains are fine; the surface of the plated layer should be smooth without dents, no stripes, etc.; the coating has sufficient hardness and toughness. For this reason, the mature copper plating methods such as sulfuric acid, copper plating, bismuth borate, and cyanide plating can be selected. According to the consideration of cost and ease of solution management, the sulphate copper plating process is accepted by most organizations. At present, the advanced high-speed acid copper electroplating method has a cathodic current density in the range of 13-20 A/dm2. The hardness of the obtained copper layer is Vickers (210±10) HV, which can meet the requirements of electric engraving. . Of course, it is also applicable to the plate-making process except for laser engraving because they only need Brinell hardness of 60-90 HB. Anodes used in high-speed electroplating consume relatively fast. Based on the requirement of increasing the actual surface area of ​​the anode to prevent anode passivation, the anode is preferably spherical. Copper anodes do not dissolve well, and copper powders easily affect the quality of the copper layer. However, copper anodes containing 0.1% of phosphorus, due to the presence of Cu3P (non-stoichiometric components), are uniformly dissolved and have higher dissolution rates under the same conditions. Sulfuric acid concentration, impurity iron ion, and chloride ion content have a great influence on the quality of the copper layer, and the solution management should be given special attention.

In recent years, the sulfate electroforming copper method, which is one of electrodeposition processes, has been paid attention to and improved by the technicians because of its advantages of quick deposition speed and low cost. Electroforming is the process of electrodeposition on a mandrel and then separating to make or reproduce metal products. The main difference between electroforming and electroplating lies in the different process methods and implementation requirements. The replating of copper by the drum of the barrier layer (Ballard process) is actually the same as that defined by electroforming. Sulfate electroforming copper electrolyte general process:

Copper sulfate 150～300g/l
Sulfuric acid 40～110g/l
Temperature 20°C～50°C
Cathode current density 10 ~ 30A/dm2
Additive amount

The anode and cathode current efficiency is close to 100%.

Electrodeposition of copper to achieve considerable speed and quality requirements, additives are essential. The choice must be specific to the process specification and must not be taken care of.

The copper layer obtained by the process and other reasons may inevitably cause blisters, short scratches and other defects. The remedial measures afterwards include: filling with glue and copper powder; soldering iron solder spot welding or tin-mercury soldering; use a 5-15V rectifying power source to repair the simplest form of the brush plating process. Practice has proved that brush copper plating repair method is the most convenient and reliable. The operation method is as follows: clean the correction area, use the drum as the cathode, lead a wire from the rectifier cathode, and wrap the bare part with cotton, and immerse the plating solution as an anode and wipe lightly back and forth on the area to be corrected until the copper is completely plated. Adjustment of the voltage to prevent scorching of the coating causes the binding force between the brush plating layer and the copper plate making layer to be weak and the work is abandoned. Finally, sanding with water can be used.

2 Hard chrome plating

Hard chrome plating has a high hardness and a high degree of wear resistance and chemical stability. To improve the resistance to printing, the hardness of the chromium layer is mainly increased. If the number of imprints reaches 500,000 impressions, the hardness of chromium needs to be 750-950 HV. Although the quality of ordinary hard chromium plating process is stable, the low deposition rate of chromium can not meet the fierce market competition.

Chromium hardness is closely related to temperature, current density, chromic anhydride, and sulfuric acid content. Adding the right amount of additives to hard chrome plating baths is common. It has been reported that the addition of a low-chromium bath containing a rare earth addition penalty increases the current efficiency of high-chromium baths from 60% to 15% of high chromium to 20% to 31%; the deposition rate increases by 600% to 110%; and the hardness increases. 30% to 60% for wear-resistant hard chrome plating. The chromium hardness obtained from chromium plating waves containing organic additives may be higher than normal. Chromium added Liu summed up into 4 categories:

1) Inorganic anionic additives (eg SO42-, F-, etc.)

2) Organic anionic additives (eg carboxylic acids, sulfonic acids, etc.)

3) Rare earth cationic ion additives (such as La3+, Pr3+, etc.)

4) Non-rare earth cations from additives (eg Sr2+Mg2+, etc.)

Research on the addition of rare earths in China is quite active, and some have gone out of the laboratory and applied to production practices. It plays a major role in solving the deficiencies of chrome plating. However, the stability and reliability of quality have also been exposed in trial production. Some manufacturers have weak technical strength, and the plating caused by additives is difficult to interpret with them. Therefore, when selecting additives, we must take the principle of advancement, stability, and economy. Conditions can visit the relevant application manufacturers to understand. There are several domestic plants that produce non-rare earth additives.

Chromium anhydride is expensive and toxic. It is a type of environmental control chemicals. It is particularly important to strengthen solution management. The impurities that affect the chromium layer quality are mainly copper ions and nitrogen ions. The source of copper is mainly the copper dissolution on the surface of the hangers and rollers. If copper exceeds the standard (>8g/l), the chromium layer in the middle of the drum will be black, making the product unusable. Excessive chlorine will make the drum grey. The method for removing impurities in the hard chrome plating bath includes electrolysis and ion exchange using DJ001 macroporous resin in a countercurrent manner. French patent FR 2607489A (1988.6.3) provides a method for recovering chromic anhydride from waste liquor. But for most plate manufacturers, it is always a headache for the scrapping of the chromium solution.

2. Chromium removal

The gravure must be proofed before leaving the factory. If there is a quality problem, the drum is usually treated as a back-chrome. Therefore, the chromium removal process, especially the speed of chromium removal, has attracted increasing attention. At present, there are two kinds of de-chroming solutions, one is a hydrochloric acid solution with a volume ratio of 1:1; the other is a sodium hydroxide electrolytic solution with a concentration of 80 g/1, and the current density is 10 to 13 A/dm2. For a 4.5V process. The latter is an improvement of the former. However, the rate of chromium removal of the two is relatively invasive, such as 0.01mm thick chromium layer, the two will take 20 ~ 25min before retiring clean. A desorption process with dilute sulfuric acid and additive has emerged in foreign countries. The desorption time is only 4 min and there is no over-corrosion to meet the need of high efficiency, but the additive is more expensive. Based on the principle of electrochemistry, the author developed an additive that is equivalent to its function. The Hull cell test showed that the hard chrome layer with a thickness of 0.01 mm was removed within 2 to 4 minutes and the copper plate was bright. The LD-D4 digital display depth meter was used to measure the net value of the test piece. There is no change before and after chromium.

3. Surface modification

The laser-engraved engraving base is a specially formulated epoxy layer. If the number of prints is more than 500,000, the hard-edged plate will be hard-chrome plated. Since the epoxy resin is non-metal and the electroless plating of the hard chromium process is still in the research stage, only the surface modification can hard chromium plating. Electroless copper plating or electroless silver plating is a relatively mature process and the solution composition is simple. Easy to operate. The coating can be used as a conductive layer to meet the current requirements for hard chromium plating.

Third, the conclusion

Gravure printing is a systematic project. When involved in several disciplines, the surface treatment technology is more complex and has the most potential for development. How to take advantage of new achievements in surface treatment is an important issue before us.

Source: Chinese Graphic Arts Master