What are the common problems of ABS plastic electroplating?

ABS plastic is the most widely used 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 impact on electroplating, which should be controlled at 18%~23% generally.
High butadiene content, good fluidity, easy forming, and good adhesion with coating. Because ABS is non-conductive, a conductive layer must be attached before electroplating. The formation of conductive layer requires several steps, such as coarsening, neutralization, sensitization, activation, and chemical plating, which is more complex than metal plating and is prone to problems in production.
Starting from the ABS plastic electroplating process, we analyzed the causes and found solutions.
1. Problems and solutions
1.1 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 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 concave, and there are two small holes on the side. At first, only one copper wire is used to clamp two small holes for electroplating.
Due to the release of gas during electroplating, the lampshade is easily separated from the copper wire, and the copper wire is also light, which is not enough to make the lampshade immersed 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 is scorched and the plastic is exposed, which is caused by poor conductivity.
In order to solve the floating and conductive problems of the workpiece, we designed a special fixture. The clamp has a certain weight and will not float after the lampshade is put on, and then two wider conductive pieces are clamped on the hole of the lampshade to make the current uniform everywhere, so that the contact point will not be scorched.
1.2 Bubbles appear during electroless copper plating on the lampshade, and the bubbles become larger after electroplating, and the process flow of plastic electroplating can be uncovered:
Degreasing → water washing → roughening → water washing → sensitization → tap water washing → deionized water washing → activation → water washing → chemical 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 lead to blistering. There are many reasons for poor adhesion, and the oil removal process and coarsening process are often prone to problems. Incomplete oil removal will cause peeling and peeling.
The lampshade adopts chemical degreasing (plastic parts are not suitable for degreasing with organic solvent). During operation, the temperature rises to 65~70 ℃, and the workpiece is continuously shaken until there is no water bead after washing. Coarsening is a very important process in ABS plastic electroplating. Coarsening is insufficient, and the binding force decreases; Excessive coarsening will make the hole larger and deformed, and the binding force will also be reduced.
Due to the extremely unstable divalent tin in the sensitized solution, the sensitized solution is easy to fail. If it is not adjusted, it will lead to activation failure. Insufficient activation will cause incomplete deposition of electroless plating; However, if the activation is excessive, the active metal will be reduced too much on the surface to form a discontinuous film, which will also reduce the binding force.
From the beginning of oil removal, we strictly followed the formula and operating conditions of the oil removal solution, and checked the time and temperature of the roughening process. A new sensitizer and activator solution were prepared. 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 doubtful whether there is a problem with the composition and molding process of ABS plastics, because the composition and molding process of ABS plastics are directly related to electroplating.
ABS particles are easy to absorb water, and the water content before pressure injection is required to be less than 0.1%. They must be dried in 80 ℃ hot air drying oven for 2~4h, and the surrounding environment must also be dry. The surface of ABS parts after bright nickel plating is matt, and there are black spots. After adding brightener, the problem is still not eliminated. The content of each component in the tank solution is within the range.
After placing the bath solution overnight, pour all the supernatant into the standby bath, and it is found that there is yellow-brown muddy sediment at the bottom of the plating bath. According to analysis, it is caused by the high temperature of nickel plating solution and the decomposition of brightener. During electroplating, the air stirring device turns up the sludge at the bottom of the tank and deposits it on the coating with nickel ions, causing burrs and black spots.
Later, it was also found that the same set of conductive rods were used for nickel plating and chromium plating, and the conductive rods had chromic anhydride left during chromium plating, which was easy to bring chromium into the nickel bath during nickel plating; And because the hanger is not insulated, copper impurities are brought into the nickel plating solution, which are the reasons for blackening.
Copper impurities can be removed by electrolysis with current density of 0.5A/dm2. To remove hexavalent chromium, first use sulfuric acid to adjust the pH of the bath to about 3, then add 0.2~0.4g/L sodium sulfite, stir to reduce hexavalent chromium to trivalent chromium, and then remove trivalent chromium with low current density.
Finally, use activated carbon to remove organic impurities. After the bath solution is treated, there is no black spot after nickel plating. During chrome plating, the concave part of the lampshade adopts the pictorial anode, and the lampshade finally plated is bright and detailed, meeting the product requirements.
2. Conclusion
1) The composition and molding process of plastic parts cannot be ignored, which is often a problem that electroplating workers are not easily aware of.
2) Special hangers shall be designed during electroplating.
3) Pay attention to the maintenance of the bath solution to keep all components within the process specification.
No other ingredients can be mixed in the plastic. Through investigation, we found that the injection molding plant piled a large number of 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 injected shall be dried at 80 ℃ for 2~4h, and the injection can be carried out after they meet the electroplating requirements. The blistering problem of the improved lampshade after electroplating did not occur.
1.3 Black spots appear on the surface of the lampshade after electroplating. After electroless copper plating on the matt lampshade, it is transferred to the electroplating process. The process we adopt 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 easy to produce copper powder (Cu2O) during electroplating, and the copper powder entering the plating solution will cause the coating to be rough. We use corrosion-resistant anode cloth to wrap the anode and then put it into the anode sleeve. After electroplating, we often open it for cleaning. Therefore, the surface after copper plating is bright and fine without any problems.