Cause analysis and solutions of ABS plastic electroplating problems

In recent years, plastic plating has been widely used in decorative plating of plastic parts. ABS plastic is one of the most widely used in plastic electroplating. ABS plastic is a terpolymer of acrylonitrile (A), butadiene (B) and styrene (S). For electroplating grade ABS plastics, the content of butadiene has a great influence on electroplating, and should generally be controlled at 18% to 23%. High butadiene content, good fluidity, easy to shape, and good adhesion to the coating. Since ABS is a non-conductor, a conductive layer must be attached before plating. The formation of the conductive layer has to go through several steps such as roughening, neutralization, sensitization, activation, electroless plating, which is more complicated than metal plating and is prone to problems in production. We started from the process of ABS plastic electroplating, analyzed the reasons and found a solution.

2　Problems and solutions

2.1　The plated parts are easy to float, and the place in contact with the hanger is easy to be burnt because the specific gravity of the plastic is small, so 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 the beginning, only a copper wire is used to clamp the two small holes for electroplating. Due to the release of gas in electroplating, the lampshade is easily separated from the copper wire, and the copper wire is also light, not enough to make the lampshade immersed in the solution. Later, a heavy object was attached to the copper wire to solve the floating problem. The contact point between the copper wire and the lampshade is scorched and the plastic is exposed, which is caused by poor conduction. In order to solve the problem of workpiece floating and conductivity, we designed a special fixture. The clip has a certain weight and no longer floats after the lampshade is put on, and two wider conductive sheets are used to clamp on the hole of the lampshade to make the electric current evenly everywhere, and the contact points will not be burnt.

2.2　Lampshade chemistry copper plating when bubbles appear, the bubbles become larger after electroplating, and the process flow of plastic electroplating can be revealed: degreasing→washing→roughening→washing → Sensitization → tap water washing → deionized water washing → activation → water washing → electroless copper plating → water washing → electroplating → water washing → drying. It can be seen from the above that any problem in any step before electroless copper plating will cause bubbling. There are many reasons for the poor binding force, and the degreasing process and the coarsening process are often prone to problems. Incomplete degreasing can cause peeling and shedding. The lampshade uses chemical degreasing (plastic parts are not suitable for degreasing with organic solvents). During operation, the temperature rises to 65～70℃, and the workpiece is shaken continuously until no water droplets hang after washing. Roughening is a very important process in ABS plastic electroplating. Insufficient coarsening will reduce the binding force; excessive coarsening will make the hole larger and deformed, and the binding force will also decrease. As the divalent tin in the sensitizer is extremely unstable, the sensitizer is prone to failure. If it is not adjusted, the activation will fail. Insufficient activation will result in incomplete deposition of the electroless plating layer; excessive activation will cause excessive reduction of the active metal on the surface to form a discontinuous film, which will also reduce the bonding force. We started with degreasing, strictly according to the degreasing liquid formula and operating conditions, and checked the time and temperature of the coarsening process, and newly prepared sensitizer and activating liquid. As a result, bubbles still appeared after electroless copper plating. After several repeated tests, the results were the same. Finally, it was concluded that the bubbling was not caused by degreasing, coarsening, sensitization, and activation. At this time, it is suspected whether there is a problem with the composition and molding process of the material, because the composition and molding process of ABS plastic have a direct relationship with electroplating. ABS particles are easy to absorb water, and the moisture content before injection is required to be less than 0.1%. It 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 our 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 then injection molded after being inspected to meet the electroplating requirements. The bubbling problem after electroplating of the improved lampshade no longer occurs.

2.3　Black spots appear on the surface of the lampshade after electroplating, and after the non-gloss lampshade is electrolessly plated with copper, it is transferred to the electroplating process. The process we use is: Nickel plating→Copper plating→ Bright nickel plating→chrome plating. The copper plating solution is relatively stable. The main problem is that the copper anode is prone to produce copper powder (Cu2O) during electroplating, and the copper powder entering the plating solution will cause the coating to be rough. We wrap the anode with corrosion-resistant anode cloth and put it in the anode sleeve. After electroplating, it is often opened and cleaned. Therefore, the surface after copper plating is bright and detailed without any problems. The surface of the parts is dull after bright nickel plating, and there are dark spots. After adding brightener, the problem has not been eliminated. Analysis of the bath solution shows that the content of each component is within the range. After the bath was left overnight, the supernatant was poured into the spare bath, and it was found that there was yellow-brown muddy sediment at the bottom of the plating bath. The analysis is due to the high temperature of the nickel plating solution and the decomposition of the brightener. During electroplating, the air stirring device turns the sludge at the bottom of the tank up and deposits on the plating layer together with nickel ions, causing burrs and black spots. Later, it was also discovered that since nickel plating and chrome plating use a set of conductive rods, the conductive rods are left with chrome plating.

The chromic anhydride of

, when nickel plating, is easy to bring chromium into the nickel tank; and because the hanger is not insulated, copper impurities are brought into the nickel plating solution, which are the reasons for the blackening. Copper impurities can be removed by electrolysis at a current density of 0.5A/dm2. To remove hexavalent chromium, first adjust the pH of the bath to about 3 with sulfuric acid, then add 0.2～0.4g/L of sodium sulfite, stir to reduce the hexavalent chromium to trivalent chromium, and then use low current density to remove the trivalent chromium. Finally, it is treated with activated carbon and filtered to remove organic impurities. After the bath solution was processed, no black spots appeared after nickel plating. When chromium is applied, the concave part of the lampshade adopts pictographic anode, and the lampshade plated finally is bright and detailed, which meets the requirements of the product.

3　 Conclusion

1) The composition and molding process of plastic parts cannot be ignored, which is often a problem that is not easy to notice for electroplating workers.

2) Special hangers should be designed during electroplating.

3) Pay attention to the maintenance of the bath liquid to keep the components within the process specifications.

4) Strengthen the cleaning between processes and do not bring other impurities in.