## 3 Primary or Polymer Metallization
#### Alternative Methods of PM  
Alternative PM methods have been emerging that address some of the drawbacks of common PM techniques. [PERSON] et al. [16] proposed a unique solution using a paste made of aluminum powder, charcoal, enamel, and distilled water mixed at a weight ratio of 40:3:36:21 for plating onto fused deposition modeling (FDM) parts. This mixture was then applied to a degreased acrylonitrile butadiene styrene (ABS) FDM part and allowed to dry completely. Electroless deposition was then performed at room temperature for 48 h. After the electroless deposition process, a comparison of surface resistivity was made with a sample that was conventionally etched with chromic acid. There was a noticeable difference in the surface resistivities, with the conventional chromic acid etching resulting in a surface resistivity of 920,000 \(\Omega\), while the aluminum-charcoal paste achieved a surface resistivity of 0.09 \(\Omega\)[16]. The lower surface resistivity of the metallized polymer increases the efficiency of the further electroplating process and increases the durability and uniformity of the electrodeposited metal.  
In an extension of this idea of applying a conductive layer over the polymer with only minor changes to the as-printed surface, a silver conductive layer was used by [PERSON] and [PERSON] [17]. Applying a silver conductive paint (SCP) to the as-printed polymeric substrate yielded all the desired properties. The surface resistivity of the painted samples after two applications was 0.015 \(\Omega\)[17]. A comparison of surface resistivity values can be seen in Table 2. With a single application of SCP having a thickness of 19 microns, the total thickness of the coating is ~38 \(\upmu\)m, which is well within tolerances for most engineering applications [44; 45]. SCP can also be applied quickly as opposed to the electroless methods, which take around 48 h to achieve a relatively low surface resistivity [16]. Furthermore, the SCP eliminates the need for environmentally harmful chemicals as used in electroless plating, and does not require specialized equipment used in other forms of PM.  
Due to the highly conductive nature of silver, SCP provides a highly conductive seed layer allowing the nickel ions to be uniformly deposited. The paint also provides an adequate level of adhesion of the deposited metal to the substrate, as too much adhesion would cause problems removing the polymer substrate, while too little adhesion would cause delamination during the plating process. Lastly, silver is chemically stable in most nickel-plating solutions and does not react adversely during the electroplating process [46].  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Primary Metallization Process** & **Surface Resistivity (\(\Omega\))** & **Required Time for Similarly Sized Parts** \\ \hline Chromic Acid Etching & 920,000 & \textasci{-48 h} \\ Aluminum–Charcoal Paste & 0.09 & \textasci{-48 h} \\ Silver Conductive Paint & 0.015 & \textasci{-0.5 h} \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of primary metallization methods.  
Figure 4: Illustration of the stages of etching on ABS polymer.