It is generally believed that the quality of the mold is closely related to the quality of the parts. Many molds (such as die-casting molds) must be electroplated on the surface to improve their corrosion resistance, wear resistance, oxidation resistance and hardness. Although the traditional chromium plating technology is mature and the quality is relatively stable, because hexavalent chromium is a toxic substance, it seriously pollutes the environment. Therefore, nickel-tungsten alloy electroplating on molds has attracted increasing attention.
Electroplating nickel-tungsten alloy is a good choice to improve mold quality, extend mold life, and clean production. Nickel and tungsten metals have high hardness, good wear resistance, and high adhesion temperature to the molten matrix. The nickel-tungsten alloy coating has fine and bright crystals, good wear resistance, strong adhesion to the substrate, and high hardness. The Vickers hardness can reach more than 1000 at high temperatures. This technology has received attention from all parties in recent years and will gradually replace chromium plating on molds. However, according to the editor of HC.com, this technology currently has defects such as rough coating, uneven coating, and pitting, which seriously restricts its application and development. Let the editor analyze the formula and process flow of nickel-tungsten alloy electroplating on molds for you.
Plating solution formula and process flow
The plating solution is mainly composed of sodium tungstate, nickel sulfate and sodium citrate, and its contents are respectively 40～45gL, 20～30gL and 40～50gL.
Process flow:
Sandblasting → inspection → degreasing → cleaning → electroplating → inspection
Quality Defects and Cause Analysis of Coating
Common defects in the quality of nickel-tungsten coatings are more pitting, the side and spherical coatings are rough with white particles, the central coating is severely uneven from the side, corners and R, and even micro cracks appear at the corners and R.
1. Pockmark
Pockmarks are tiny white and black dot-like defects on the coating. They have various shapes, some are obvious and regular, easy to find, and relatively easy to control; a small amount of “invisible” to the naked eye can only be checked with an instrument.
Cause Analysis:
Blast holes, pores, pitting, adhesions, etc. of the mold matrix are caused by defects; the mold adheres to sand particles during sandblasting, or the sand quality is poor, or the sand quality is caused by impurities, oil, foreign matter, dust, etc.; the mold is degreasing Emulsions adhered at the time, impurities in the water during cleaning, etc.; contaminated by compressed air, pure water and its pipelines, tools and impurities in the environment used during liquid preparation; undissolved salts in the plating solution and activation solution Particles, acidified electrode metal objects, etc. can cause pitting.
The above pitting can be seen directly, and some of the thin layer of adhesion on the surface of the mold, after analysis, is mainly the carbon compound in the contaminated sand before plating. It is firm, strong adhesion will gradually become acid and alkali resistant, anti-vibration high-viscosity jelly. It is not easy to remove during degreasing and cleaning. Once neglected, it will flow into the electroplating process, and it will be covered by the plating layer, which is not easy to be found by the naked eye and cannot be compensated.
2. Rough coating
Cause Analysis:
The basic composition of the plating solution is nickel sulfate, sodium citrate, sodium tungstate, etc., as well as salts and organic complexes. The test found that the defects of mold electroplating are mainly related to the impurity content in the plating solution. The purity of chemicals is not high, foreign matter in the plating solution, frequent use of the plating solution, and the content of metal impurities Cu, Fe, Cr, Co in the plating solution beyond the allowable range, etc., can cause the coating to be rough, pitting, and white. Particles.
3. The thickness of the coating is severely uneven
The center and sides, corners, and R areas of the coating are severely uneven, and even small cracks appear at the corners and R areas.
Cause Analysis:
After the plating solution is used for a long time, the probability of defects in the coating increases. The white particles on the side and spherical surface, the corners, and the micro cracks at the R are particularly obvious. The test found that when the impurities Cu, Fe, Cr, and Co content in the plating solution were significantly increased to 20, 20, 20, and 50 mgl, the plating quality was significantly reduced.
The test also found that the distribution of the electroplating current directly affects the uniformity of the coating. The current density at the edges, corners, and R of the mold is obviously higher than that of other parts, and the corresponding metal deposits are large, and the coating thickness is large. Under normal circumstances, the difference in thickness between the center and the sides, corners, and R of the plating layer is about 4μm. Uneven current distribution will cause the coating thickness difference to reach more than 15μm. In severe cases, the coating will fall off due to excessive local metal deposition.
In addition, the shape, structure, and material of the mold also affect the quality of the coating.
Countermeasure
(1) Before sandblasting, check and eliminate blisters, pores, pitting, adhesions, etc. of the mold matrix.
(2) Check the sand quality first during sandblasting. If there is contamination by impurities, foreign matter, oil, etc., it should be replaced immediately and completely. After sandblasting, clean the remaining sand particles and other adherents on the surface of the mold in time.
(3) Check the surface of the degreasing liquid before degreasing. If oily emulsifier accumulates, it should be sprayed and washed away in time; if there is oily emulsion on the surface of the mold after degreasing, it must be cleaned.
(4) Strictly control the time during acid activation to avoid over-corrosion of the mold; at the same time, control the depth of the lower groove of the mold to prevent corrosion of the electrode plates and connectors. The corrosives will contaminate the plating solution and cause poor plating.
(5) The purity of the chemicals should be as high as possible when preparing the plating solution, and the impurity content should be analyzed in time. In addition, the plating solution should be periodically analyzed, filtered, and replaced regularly.
(6) According to the structure and shape of the mold, scientifically design a reasonable current balance plate, which can effectively prevent the local metal deposition from being too thick and ensure the quality of the coating.
(7) Optimizing the design of the mold under the premise of ensuring that the mold meets the production process is particularly conducive to improving the quality of the coating.
(8) Replenish anodes in time, repair accessories, clean the sludge of plating tank regularly, and carry out comprehensive treatment of “three wastes”.
(9) Purify and protect the working environment, abide by the process regulations, strengthen the operator’s sense of responsibility, and prevent environmental and mechanical impurities from entering the plating solution.
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Link to this article：Mold application nickel-tungsten alloy electroplating process
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