# Fabrication of a Conductive Pattern on a Photo-Polymerized Structure Using Direct Laser Sintering
## 2 Materials and Methods
### Composite Resin  
In order to form a conductive pattern on a 3D shape using direct laser sintering, it was essential to prepare a suitable metal composite resin. Composite filaments containing metal powder used in ME-type 3D printers are commercially available in various types, but metal composite resins that are easy to use in PP-type 3D printers are extremely limited and their content is also very low. Therefore, in this study, experiments were conducted  
Figure 1: Schematic representation of the experimental process flow for the suggested fabrication method: (**a**) 3D printing using commercial resin, (**b**) 3D printing using metal composite resin, (**c**) direct laser sintering for fabricating a conductive pattern. with various combinations, from the type of metal powder to the base resin for mixing. In the case of the metal powder, the most widely used for forming conductivity is copper. In addition, nickel has also been used as a metal seed for deposition in previous studies. In the case of these two materials, they are most widely used because they have a good chemical reactivity with electroless copper plating solutions [14; 15]. However, as the laser used in this study had to be able to perform both curing and sintering, there was a limit to the use of high-power lasers. When a high-power UV laser was used, it became difficult not only because of an increase in the equipment price, but also to set precise conditions for the curing process in which the reaction occurred at a relatively low energy. In addition, excessive energy irradiation caused damage to the base material structure, so it was not preferable. Therefore, in this study, various metal materials with a low melting point and excellent conductivity were reviewed, enabling the process to be performed even with a low laser power. Among them, as shown in Table 1, it was confirmed that aluminum had a relatively low melting point compared with copper or nickel and, at the same time, had excellent conductivity. In addition, as its price is low, it is a material widely used for actual conductive wiring. Therefore, in this study, an experiment was conducted using aluminum powder. In general, when mixing powder and resin to produce a composite resin in the form of a slurry, a bimodal particle size distribution type is recommended. This minimizes powder agglomeration, thereby minimizing the abnormal viscosity increase in the suspension and facilitating the resin supply. In addition, better results can be obtained by increasing the powder behavior and efficiency during sintering [18; 19]. Therefore, in this study, two types of aluminum powders with different size distributions (4-10 \(\upmu\)m and 15-25 \(\upmu\)m in a spherical form with 99.5% purity, Avention, Incheon, Korea) were mixed and these were used at a 1:1 weight ratio, respectively. In the case of commercial resins that are the base of metal composite resins, various products are being sold. However, in the case of mixing with a metal powder, the curability is generally greatly lowered, so it is inevitable that the UV energy increases during curing. In this case, if the shrinkage rate during the curing of the resin is excessive, warping or surface peeling may occur easily and, in severe cases, the material itself may be vaporized. For this reason, among the most widely used acrylic compound-based resins, the products of three companies, which are known to have relatively excellent thermal stability and cure shrinkage, were mixed with aluminum powder to compare the curing degree. As a result, as shown in Table 2, when only the base resin was cured, there was a difference in the curing thickness between the products, but it was to such a degree that there was no major problem in the actual use.