## 3 Results and Discussion
### Pre-Curing Criterion for Printing Process  
Material conversion from liquid to solid state is crucial in the hollow connectors direct printing process. Not only does material conversion relate to intrinsic material properties, but it is also influenced by processing-related parameters such as UV intensity, exposure length, extrusion and printing speed. According to UV radiation simulations and rheological characterizations, the adhesive chosen in the study was predicted to reach the solid-like state with a gelation time of 7 \(\pm\) 1 s during the printing process. Thus, it was proposed that the material was pre-cured to solidification before the  
Figure 5: The relationship between gelation time and UV intensity. deposition to succeed in hollow connectors printing. Figure 7a schematically describes the material conversion mechanism during the process. Two fluids were extruded out from the tip of the coaxial nozzle and then pass through the transparent jacket. UV adhesives constrained in the transparent jacket were illuminated by UV light with a length of 6 mm. The length is also called the exposure to deposition length. With enough UV exposure, materials can be converted to solid-state and then deposited. When the nozzle moving upwards, stabilized hollow structures would be printed on the substrate under a thread-jetting co-flow behavior regime. Pre-cure time is defined here as \(T_{p}\). It was estimated by the exposure to deposition length dividing to the printing speed, assuming the material extrusion speed at the tip of the transparent jacket was close to zero caused by the significant friction. The curing criterion for the printing process was determined as the minimum pre-cure time required for successful printing. Certain parametric studies for the pre-cure time were performed under the flow rates \(Q_{i}\) and \(Q_{o}\) at 4 and 8 mL/min. The printing substrate was PMMA with matching holes (\(\O\) 2.5 mm). Figure 7b,c shows some typical printing results. When the pre-cure time was set to 2 s, the material was still in liquid state and could not be printed on the substrate. When the pre-cure time was adjusted to 4 s, a hollow structure could be printed on the substrate but with a thinning behavior. At the same time, the bottom part of the printed connector was swelling due to insufficient UV curing. When the pre-cure time was increased to 6 s, a uniform-shaped printed connector with a well-constrained inner channel was successfully fabricated. The outer diameter of uniform-shaped printed connectors is around 1 mm, which is well  
Figure 6: Co-flow behavior regimes and region: (**a**) different co-flow behavior regimes including dripping, outer flow, undulant jetting, and threading jetting; and (**b**) concluded co-flow behavior region. Figure 7: (**a**) Schematic of material conversion mechanism in printing. Typical printing results in pre-cure time investigation: (**b**) hollow structure with thinning and swelling when pre-cure time at 4 s; and (**c**) uniform-shaped hollow structure with a well constrained inner channel when pre-cure time at 6 s.  
restricted by the transparent jacket. All the inner channels were filled with black colored water for straightforward observation. Table 4 lists the success rate of uniform-shaped hollow connectors printing with multiple printing trials of 10 times under different pre-cure time. The success rate was defined as the the times of uniform-shaped connectors (OD 1 mm) printing divided by the total printing trials under a particular pre-cure time. It was determined that the pre-cure time of 6 s was the minimum pre-curing criterion with the success rate at 100%. In addition, dimension clarification and magnified view of the successfully printed hollow connector are presented (Figure 8). The length of the printed connector was 5 mm. The inner diameter was around 170 \(\upmu\)m while the outer diameter was around 1 mm. The inner diameters were measured at each position with an interval of 1 mm.