Specification of Electroplating Process - Chrome Plating Process (I)

Fourth, standard chrome plating process

Chrome surface is a face of the gravure plate making company, which can directly win the trust of the customer. The smooth chromium surface, excellent resistance to printing force, and satisfactory scraper consumption all have a great appeal to customers. When the chrome plating layer is directly contacted with the printing material during printing, the copper layer must be effectively protected from being scratched or scratched. Therefore, the chromium layer is required to have a certain hardness.

The high hardness and surface hardness of the chromium layer is the key to improving the printing roll resistance of the gravure cylinder. If the number of prints reaches 500,000 prints, the hardness of the chromium layer should be HV750 to HV950, and the resistance force should be 800,000 to 1,000,000 times. At present, the industry generally adopts the hard chrome plating process, and some companies also use the Swiss chrome plating process to ensure that the chromium layer has a high hardness, good wear resistance and chemical stability, thereby enhancing the printing roller printing force, so that It can withstand the frequent rubbing of the squeegee and the pigment in the ink.

At present, gravure platemaking companies all over the country have deeply felt that the quality of chrome plating is an eternal problem and a major factor in rework. There are many factors that affect the quality of chromium plating, and it is also very complicated. The chromium layer hardness is closely related to the temperature, current density, the content of chromic anhydride and sulfuric acid. To sum up, I believe that there are three main factors: First, poor conductivity, and second, the chromium plating fluid is not stable, and third, cleaning is not clean, and some companies although the development of quality standards and control data, but employees are not serious in the implementation, Not strict, causing rework. Therefore, we must strengthen management and strictly manage it.

(I) Chrome Plating Process

Carved roller → inspection (qualified) → assembly → drum cleaning → chrome plating → polishing → self-test (qualified) → submit the total inspection (unqualified chromium removal).

(B) The basic principle of chrome plating

The chromic acid in the chrome plating solution is generally in the form of dichromic acid (H2Cr2O7) and can be present in the form of trichromic acid (H2Cr3O10) and tetrachromic acid (H2Cr4O13) in a very high chromium plating solution. When only the chromic acid and the catalyst such as sulfuric acid are present in the plating solution, direct current is supplied, only hydrogen is precipitated on the cathode, and no chromium layer is deposited, which is equivalent to electrolyzed water.

After addition of a suitable sulfuric acid catalyst (CrO3:H2SO4 = 100:1), the following reactions occur in succession on the cathode:

Cr2O72-+8H+ +6e → Cr2O3+4H2O 1
2H++2e → H2↑ 2
Cr2O72-+ H2O2CrO42-+2H+ 3
CrO42-+ 8H+ +6e → Cr↓+4H2O 4

From the above reaction, it can be seen that the chrome plating cathode reaction is very complicated. The mechanism of chrome plating is briefly described using the colloidal film theory and the chrome-plated cathode polarization curve. At the beginning of energization, the first reaction that occurs is the reduction of hexavalent chromium to trivalent chromium (Reaction Equation 1), as shown by the ab-section of the polarization curve in Figure 1. As the potential shifts negatively, the current density increases dramatically. Reaction 1 produces trivalent chromium very quickly. The potential is negatively shifted to point b and the current reaches its maximum value. After point b, the precipitation potential of hydrogen ions is reached, and the reaction formulas 1 and 2 are performed simultaneously. Looking at the bcd segment of the polarization curve, as the potential shifts negatively, the current density gradually decreases, indicating that the surface state of the electrode has changed. Because reaction 1, 2 consumes a large amount of hydrogen ions, the pH value at the electrode interface increases, and a layer of basic chromium chromic colloid film (Cr(OH)3·Cr(OH)CrO4) is formed, covering the surface of the electrode. As the resistance increases, the current density drops. The increase in pH near the surface of the cathode created conditions for the conversion of Cr2O72- ions into CrO42- ions, and reaction 3 proceeded to the right, and the CrO42- concentration increased rapidly. When the potential is negatively moved to point d, the potential corresponding to this point is the reduction potential of chromium ions. Reaction 4 begins. The de segment is the true polarization curve of the chrome plating. The reactions 1, 2, 3, and 4 are performed simultaneously. With the negative shift of the potential, the reaction 4 is rapidly accelerated.

Under the action of the catalyst sulfate ion, the colloidal membrane covering the surface of the electrode dissolves:
This dissolution first takes place locally and then gradually erodes. This reveals a small substrate area, a high true current density, and a large polarization, and the reduction of chromium can proceed at a certain rate. A colloidal film will be formed on the surface of the newly formed chromium layer, and the dissolution and generation of the colloidal film will continue to play an important regulatory role.

Although the SO42- in the plating bath and the trivalent chromium produced in the cathode do not directly participate in the electrode reaction, their presence and content are critical to the quality of the chrome plating layer. Trivalent chromium is an important component of the film. If its content is low, the colloidal film is difficult to form or is thin and porous after being formed, and is easily dissolved by sulfuric acid. At this time, the exposed area of ​​the substrate is large and the current density is lower. Precipitation potential of chromium, so less trivalent chromium, poor coverage; If the concentration of trivalent chromium, colloidal film is thick and dense, difficult to be dissolved by sulfuric acid, chromium layer can only grow on the original grain, resulting in rough crystals, dark plating And dull. The sulfuric acid content is high, the film is easily dissolved, the chromium-free layer in the low current density area is the same as when the trivalent chromium is low, and the sulfuric acid is insufficient, and the chromium layer is rough as in the case of high trivalent chromium. So chrome plating must strictly control their content, especially the ratio of chromic anhydride to sulfuric acid.

(III) Strengthened cleaning management

The drum should be cleaned before chrome plating. The cleaning must be thorough. The oxidized impurities on the copper surface should be washed away. There can be no rust marks. This is an important part of guaranteeing the quality of chrome plating. If the cleaning is not complete, it will easily lead to chromium off failure. Manual cleaning of the roller is divided into the following steps.

1. Add magnesium alcohol, warm water-washed laundry detergent and dishwashing liquid to magnesium carbonate paste (MgCO3), make into a paste, and scrub with a sponge (5 to 10cm thick).

2. Rinse with tap water.

3. Neutralize with 3% to 5% sulfuric acid (H2SO4) to increase activity.

4. Rinse with distilled water.

5. Electro-relieve oil, that is, the combination of anode and cathode degreasing, is an important part. The anode current density is 1 to 5 A/dm2, and the cathode current density is 5 to 7 A/dm2. The degreasing liquid has three components, one is sodium hydroxide (NaOH) 7g/L; the second is sodium carbonate (Na2CO3) 20g/L; and the third is trisodium phosphate (Na3PO4) 20g/L. The time was: 1 minute for the anode, 6 minutes for the cathode, and 1 minute for the anode, for a total of 8 minutes. The specification changes the bath solution once a week.

6. Finally neutralize with 3% to 5% dilute sulfuric acid (H2SO4), then rinse with distilled water, and then perform chrome plating.
(to be continued)