In recent years, plastic electroplating has been widely used in decorative electroplating of plastic parts. ABS plastic is the most widely used kind of plastic electroplating.
We often hear about metal plating, but what is plastic plating and how is it applied?
Because of its structural advantages, ABS plastic not only has excellent comprehensive properties, is easy to process and shape, but the surface of the material is easy to corrode to obtain a higher coating bonding force. Therefore, it is widely used in electroplating.
Compared with normal metal parts, plastic electroplated products can not only achieve a good metal texture but also reduce the weight of the products. While effectively improving the appearance and decoration of the plastic product, it also improves its electrical, thermal, and corrosion resistance properties, thereby improving the strength and stability of its mechanical surface.
For the selection of plastic materials for electroplating, it is necessary to comprehensively consider factors such as the processing performance of the material, the difficulty of electroplating, and the dimensional accuracy.
Overview of Plastic Plating Process
Plastic electroplated products have the characteristics of both plastic and metal. It has a small specific gravity, good corrosion resistance, simple molding, metallic luster and metallic texture, and electrical conductivity, magnetic permeability, and welding characteristics. It can save complicated mechanical processing procedures, save metal materials, and is beautiful and decorative. At the same time, it also improves the mechanical strength of the plastic. Since the metal coating has high stability to external factors such as light, atmosphere, etc., after the plastic is plated with metal, it can prevent the plastic from aging and prolong the service life of the plastic product. With the rapid development of industry and the increasing application of plastic plating, it has become one of the important means of surface decoration in plastic products. At present, it has been widely used at home and abroad for electroplating on the surface of ABS, polypropylene, polycarbonate, polycarbonate, nylon, phenolic glass fiber reinforced plastic, polystyrene, and other plastics. Among them, ABS plastic is the most widely used electroplating and the electroplating effect is the best.
Principle of ABS Electroplating
Since ABS plastic is not conductive, it cannot be directly electroplated with metal/alloy on its surface. Therefore, electroless plating is required before electroplating. ABS plastic will form a thin noble metal film on the surface of the object after roughening, sensitizing, reducing, and accelerating degelling. This layer of the precious metal film can play the role of active catalysis, also called catalytic film; it can accelerate the reduction reaction of electroless plating. The practice has proved that precious metals such as silver and palladium have this catalytic ability, which makes the electroless plating process smoother.
Common problems and solutions in ABS 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%. Engineering plastic has 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 electroplating. The formation of the conductive layer must go through several steps such as roughening, neutralization, sensitization, activation, electroless plating, etc., which is more complicated than metal electroplating and is prone to problems in production.
Common problems and solutions
The plated parts are easy to float, and the place in contact with the hanger is easy to be scorched. Because the specific gravity of the 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 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. After plastic processing, 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 the holes of the lampshade to make the current everywhere even, and the contact points will not be burnt.
Bubbles appear during chemical copper plating of the lampshade, and the bubbles become larger after electroplating, and the process flow of plastic electroplating can be lifted: 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 cause bubbling. There are many reasons for the poor adhesion of plastic parts. 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 there are no water droplets 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. Because the divalent tin in the sensitizer is extremely unstable, the sensitizer is easy to fail. 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. Plastic injection started with degreasing, strictly following the degreasing liquid formula, and operating conditions, and checking the time and temperature of the coarsening process, and newly prepared sensitizing liquid 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 has a direct relationship with electroplating. ABS particles are easy to absorb water. The moisture content before an 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 be dry.
No other ingredients can be mixed into ABS plastic. Through investigation, we found that the injection molding factory piled many 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 performed after the inspection meets the electroplating requirements. The bubbling problem after electroplating of the improved lampshade no longer occurs.
Dark spots appear on the surface of the lampshade after electroplating. 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 a 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. After the parts are plated with bright nickel, the surface is dull, and there are dark spots. After adding a 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, all 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.
After analysis, the reason is that the temperature of the nickel-plating solution is too high and the brightener is decomposed. During electroplating, the air stirring device turns up the sludge at the bottom of the tank and deposits it on the plating layer together with nickel ions, causing burrs and black spots. Later, it was also discovered that because nickel plating and chromium plating use a set of conductive rods, the conductive rods have chromium anhydride left during chromium plating. When nickel plating, chromium is easily brought into the nickel tank; and because the hanger is not insulated, the plating Copper impurities are brought into the nickel solution, which are the causes of 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, use activated carbon to filter and remove organic impurities. After the bath solution was processed, no black spots appeared after nickel plating. When chrome is used, the concave part of the lampshade adopts a pictograph anode, and the lampshade plated finally is bright and detailed, which meets the requirements of the product.
To sum up:
- 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.
- Special hangers should be designed during electroplating.
- Pay attention to the maintenance of the tank liquid to keep the components within the process specifications.
- Strengthen the cleaning between processes, and don't bring other impurities in.
