|Nickel-plated 3D printed plastic models
3D printed objects definitely look great in their natural finish. However, there are a variety of ways to improve upon and modify the existing surface through coatings, finishes, and surface treatments that can be applied to printed objects to add another level of dimensionality with regard to a material's appearance in addition to the potential to develop new unique material properties. One system in particular that is compatible with 3D printed plastic models can effectively "transmute" the surface to metal--the electroless plating process.
Electroless plating is analogous to electroplating in that both are surface treatment processes. However, electroless plating does not require an electrical power source but instead is an auto-catalytic chemical reaction revolving around chemical reduction as opposed to an electrical reductive process. This key difference is what enables traditionally non-conductive materials like plastics, wood, etc. the ability to be plated with a thin layer of metal. Electroless plating is possible with a variety of metals including copper and gold. However, nickel is the most commonly used plating metal due to its exceptional hardness & corrosion and wear resistance. Nickel is also a very good base metal in preparation for subsequent surface treatments and coatings. Nickel coated plastic models can be also be soldered and applied toward unique electronics and lighting projects as well.
|Electroless nickel plating solids prior to dissolution
Realizing the potential of this technique, I wanted to try my hand at replicating (it) to learn more about this process. There are a seemingly limitless number of recipes for nickel-based electroless platings. However, using what materials I had on hand, I applied a formula using nickel chloride as the metal source and sodium hypophosphite as the reducing agent. A few other chemicals were added to the plating solution including sodium citrate (a complexing / chelating agent) and ammonium chloride (for pH adjustment & balance). The solids were dissolved in water, forming a nice teal green aqueous nickel solution. While the electroless plating solution is easily prepared, actually plating an object is bit more complicated as it involves more than just throwing the sample that you want plated into solution (which will lead to no result, unless your sample has a catalytically-active metal surface).
|Teal green aqueous electroless nickel plating solution
What I learned from my venture into electroless plating is that surface preparation and activation are absolutely key to achieving success with this process. The surface that you want to plate must be hydrophilic ("water-loving") and easily-wetted, which can be achieved by oxidizing the material. I ended up dipping my 3D prints in concentrated sulfuric acid for a few seconds followed by a thorough rinse in water. I believe my prints could have been oxidized a bit longer to achieve better results. However, this being a preliminary investigation, the achieved results were a good test. Following the surface preparation step, an activation step is required in which the surface of material is impregnated with essentially a catalytic seed metal atom from which nickel will crystallize and grow from the plating solution. I used a very dilute aqueous solution of palladium chloride for this step. Alternative activation solutions such as silver nitrate are also effective from what I've read as well. After allowing the palladium solution to dry onto the surface of the plastic 3D print, the samples were finally ready to be lowered into the plating solution. I did not suspend the models due to a lack of space and simply dropped them to the bottom of the heated solution.
The reaction proceeded very vigorously and appeared to get very hot. I question whether the plating solution may be going out of control (thermal overrun) and precipitating all the nickel out of solution--the plating solution should theoretically be reusable over many cycles by plating only the catalytically activated surface of the 3D printed model that is lowered into solution. This is something that will need to be examined more closely in the future.
|Surface prep is key to a quality finish
Regardless, the 3D printed plastic models were allowed to plate for about an hour before being removed from the plating solution--revealing a rather lustrous and durable silver-colored metallic finish. The texture of the model surface appears to be accentuated by the nickel coating and it seems that very smooth surfaces do not coat well (due to the metal plating being a mechanically-bonded material interaction / see owl in above photo) and may need to be roughened up or chemically etched to enhance nickel's adherence. The nickel plated 3D prints feel very metallic (as should be expected) and look great too. All-in-all, the electroless plating process shows a lot of potential for further optimization and application toward further surface treatment steps, such as chrome plating and more.