Plastic plating-ABS plastic is widely used in plastic plating processes
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Plastic plating has been widely used in decorative plating of plastic parts. ABS plastic is the most widely used type in plastic plating. ABS plastic is a ternary copolymer of acrylonitrile (A), butadiene (B), and styrene (S). For electroplated ABS plastics, the content of butadiene has a great impact on electroplating and should generally be controlled at 18% to 23%. High butadiene content, good fluidity, easy forming, and good adhesion to the coating. Since ABS is non-conductive, a conductive layer must be attached before electroplating. The formation of the conductive layer requires several steps such as roughening, neutralization, sensitization, activation, and electroless plating, which is more complex than metal plating and prone to problems in production. Starting from the ABS plastic plating process, we analyzed the reasons and found solutions.
1. problems and solutions
1.1 The plated part is easy to float, and the place in contact with the hanging tool is easy to be burnt. Because the specific gravity of plastic is small, it is easy to float in the solution. The shape of the lampshade is like a small plate. The inner surface is recessed and there are two small holes on the side. At first, only a piece of copper wire was used to clamp the two small holes for electroplating. Due to the release of gas during electroplating, the lampshade is easily separated from the copper wire. In addition, the copper wire is also light, which is not enough to immerse the lampshade in the solution. Later, heavy objects were attached to the copper wire to solve the floating problem. The contact point between the copper wire and the lampshade was burnt and the plastic was exposed, which was caused by poor conduction. In order to solve the problems of workpiece floating and conduction, we designed a special fixture. The fixture has a certain weight and no longer floats after putting on the lamp shade. Two wider conductive sheets are then clamped on the holes in the lamp shade to make the current uniform everywhere and the contact points will not be burnt.
1.2 Bubbles appear during electroless copper plating of the lampshade. After electroplating, the bubbles become larger and can be lifted. The process flow of plastic plating is: degreasing → water washing → roughening → water washing → sensitization → tap water washing → deionized water washing → activation → water washing → chemical copper plating → water washing → electroplating → water washing → water washing → drying. It can be seen from the above that problems in any step before electroless copper plating will lead to blisters. There are many reasons for poor binding force, but problems often occur in the degreasing process and roughening process. Incomplete oil removal may cause peeling and peeling. The lampshade uses chemical degreasing (plastic parts are not suitable for degreasing with organic solvents). During operation, the temperature is raised to 65 - 70 ° C, and the workpiece is constantly shaken until no water droplets are hung after washing with water. Roughening is a very important process in ABS plastic plating. Insufficient coarsening will reduce the binding force; excessive coarsening will cause the hole to become larger and deformed, and the binding force will also be reduced. Because the divalent tin in the sensitizing solution is extremely unstable, the sensitizing solution is prone to failure. If it is not adjusted, activation failure will occur. Insufficient activation will cause incomplete deposition of the electroless coating; excessive activation will cause excessive reduction of the active metal on the surface to form a discontinuous film, which will also reduce the binding force.
Starting from degreasing, we strictly followed the formula and operating conditions of the degreasing fluid, and checked the time and temperature of the roughening process to prepare a new sensitizing fluid and activation fluid. As a result, bubbles still appeared after electroless copper plating. After several repeated tests, the results were the same, and it was finally concluded that bubbling was not caused by degreasing, coarsening, sensitization, and activation. At this time, it is suspected that there is a problem with the composition and forming process of the ABS plastic, because the composition and forming process of the ABS plastic are directly related to electroplating. ABS particles are easy to absorb water, and the moisture content before pressure injection is required to be less than 0.1%. They must be dried in a hot air drying oven at 80 ° C for 2 to 4 hours, and the surrounding environment must also be dry. No other ingredients can be mixed into ABS plastic. Through investigation, we found that the injection molding factory piled a large number of molded ABS plastic parts on the wet warehouse floor, and the raw materials before injection molding were not dried. Under our guidance, the raw materials to be injection molded are dried at 80 ° C for 2 to 4 hours, and injection molding is carried out after being inspected to meet the plating requirements. The problem of bubbling after electroplating of the improved lampshade no longer appeared.
1.3 Black spots appear on the surface of the lampshade after electroplating. After electroless copper plating, the matte lampshade is transferred to the electroplating process. The process we use is: nickel plating → copper plating → bright nickel plating → chromium plating. The copper plating solution is relatively stable. The main problem is that the copper anode is prone to producing copper powder (Cu2O) during electroplating. The copper powder entering the plating solution will cause the coating to be rough. We wrap the anode with a corrosion-resistant anode cloth and place it in the anode sleeve. After electroplating, we often open it for cleaning. Therefore, the surface after copper plating is bright, meticulous, and no problems occur. The surface of the parts after being plated with bright nickel is dull and has black spots. After adding brightener, the problem has not been eliminated. Analysis of the bath liquid shows that the content of each component is within the range. After leaving the bath solution overnight, all the supernatant was poured into the spare bath, and a yellow-brown mud-like precipitate was found at the bottom of the plating bath. After analysis, it was caused by the high temperature of the nickel plating solution and the decomposition of the brightener. During electroplating, the air stirring device turned up the mud at the bottom of the tank and deposited it on the coating together with nickel ions, causing burrs and black spots. Later, it was discovered that because nickel plating and chromium plating used the same set of conductive rods, the conductive rods contained chromium anhydride left during chromium plating. When nickel plating, chromium was easily brought into the nickel tank; and because the hanging device was not insulated, copper impurities were brought into the nickel plating solution, which were all the reasons for blackening. Copper impurities can be removed by electrolysis with a current density of 0.5A/dm2. To remove hexavalent chromium, first adjust the pH of the plating solution to about 3 with sulfuric acid, then add 0.2 - 0.4 g/L sodium sulfite, stir, reduce hexavalent chromium to trivalent chromium, and then remove trivalent chromium with low current density. Finally, the organic impurities are removed by treatment and filtration with activated carbon. After the bath solution was treated, no black spots appeared after nickel plating. When applying chromium, the concave part of the lampshade uses pictographic anodes, and the final plated lampshade is bright and detailed, meeting product requirements.
2 Conclusion
1)The composition and forming process of plastic parts cannot be ignored, which is often a problem that is not easy for electroplating workers to notice.
2)Special hanging tools should be designed during electroplating.
3)Pay attention to the maintenance of the bath solution to keep the ingredients within process specifications.
4)Strengthen cleaning between processes and do not bring in other impurities.
