CONFORMAL PROJECTION PRINTING METHOD FOR 3D NAIL ART PRINTING 173 at 100 m3/min. The focus was on setting three different ranges of angles, fi nding dif- ferent optimized printing conditions accordingly, and printing within a certain amount of time. RESULTS AND DISCUSSI O N The microscopic phen omena of the ink deposition due to the difference in adhesion force because of the surface energy difference between materials when the ink reaches on the nail tip are shown in Figure 4 by measuring the surface tension and energy change. When droplets of water about 100 μm diameter are dropped on a nail tip made of ABS or acrylic material, the contact angle changes according to the surface energy of the tip material and the water. As shown in Figure 4A, the contact angle is close to 90°, which is seen when using hydrophobic substrates that dislike each other because of the large surface tension of the two materials. In the plane of high incidence angle, this type of material property can be used to increase the precision. However, if the angle of incidence is small, the ink fl ows down from the nail tip (Figure 4B). On the other hand, if the nail tip is made hy- drophilic by laser treatment, then the contact angle is close to 0°. There is a disadvantage of spreading (Figure 4C), but if the angle of inclination is small, the ink does not fl ow down from the nail tip (Figure 4D). In CPPM, the ratio of s e ttling degree  ¬ žWP ­ ž ­ I W varies depending on the range of different θ (theta) values. Based on the amount of projected ink per unit area, the standard degree of theta is Rb 0 90°, ° and the ratio is b1. 0 WP WI Through the geometric simulations and repetitive experiments, the comparative analysis is conducted based on the experiments under the conditions shown in Table I. The experiments tried to fi nd the optimal number of prints by outputting high accuracy results by repeating experiments to enable effi cient and accurate printing according to the angle of incidence. The experimental method that shows the difference between the incidence angles of dripping ink and the conformal surfaces is as explained in Figure 1. The conformal surface has a steady output when the angle of incidence of the ink falling from the inkjet print head is perpendicular to the surface, so it does not fl ow down the surface. However, when the theta value decreases, the ink does not settle on the conformal surface and fl ows down. Spreading in unstable sur- face causes DPI decreases when the absolute amount projected per unit area is diluted. Accuracy and clarity have been increased by changing the number of print layers accord- ing to theta value changes. Table I shows the optimal number of printing layers from 1 to 3 which depends on the different theta range. There are different numbers of printing layers depending on the theta to increase accuracy. The ratio of settling degree should be greater than 1.00 to obtain accurate results. Various sizes of nail tips a re classifi ed into different angles of incidence in three ranges. After laser marking on the substrate for improving accuracy, different layers are printed according to the theta value. In addition to the UV-curable inkjet printing method and laser treatment, the technology of printing three-dimensional shapes by ink printing is introduced. When the ink height is accumulated about 2–5 μm, the difference of accu- mulation in the shade gives a three-dimensional effect. A combination of printing tech- nologies that use the color of the ink can give a visual effect and rapidly print and apply in three dimensions with other materials like ceramic or polymer. However, the experi- ment mainly focuses on the CPPM.

JOURNAL OF COSMETIC SCIENCE 174 As the angle of incidence bet ween the nail surface and projected ink decreases, the actual projected area increases because of the fl ow of ink. It reduces the amount of spraying printed per unit area. The problem of decreasing the amount of projection per unit area according to the angle of incidence b R b 15 30° ° can be solved by repeated printing up to three times while increasing the precision and accuracy for the projection correction. It expands up to 150% of the maximum amount of projection. Incidence angles from 30° to 70° are supposed to be projected twice in the same position to get high precision thus, the actual projected amount increased from 114% up to 181% of the ideal pro- jected amount of the minimum incident angle. By repeating experiments, the number Table I Different Numbers of Printed Layers Depend on the Angle of Incidence θ (the angle of incidence) P I W W (the ratio of settling) Number of printing (layers) 70°–90° 0.94–1.00 1 30°–70° 0.50–0.94 2 15°–30° 0.26–0.50 3 Figure 4. Distribution of water and ink per unit area.