Three-dimensional printing is based on optical principles, the use of lenticular plate to make the image memory of a three-dimensional sense of a printing method. Three-dimensional printing than flat printing, its core technology is grating technology. Grating is three-dimensional printing to form a three-dimensional image of the viewer, is the basis of three-dimensional color printing technology.
Grating is a layer of optical material composed of bar-shaped cylindrical lenses, in accordance with the principles of human vision rules arranged on a transparent plane material to form the division of image information and aggregation of a synthetic optical components (special lenses). This special lens is arranged in an orderly manner on the substrate printed by the three-dimensional printing process to form a transparent material body.
Printing grating according to the special lens form is divided into plane lens grating, column lens grating and spherical lens grating three. At present, three-dimensional printing imaging are used column lens grating, which is currently the most widely used and most mature grating.
Lenticular lens grating imaging is based on the principle of lens refraction to realize the image of three-dimensional reproduction. Column lens grating is composed of many column lens transparent plastic plate (sheet), the surface of the grating lines by many structural parameters and performance is identical to the small semi-cylindrical lens linear arrangement composed of the back of the plane for the focal plane of the column lens element, each column lens element is equivalent to the convergence lens, play the role of concentrated light imaging. Therefore, the two-dimensional images obtained at different viewpoints can be used to reconstruct the three-dimensional model of the original space object. The stereoscopic image produced by it can be viewed normally without backlight or stereo glasses, and its imaging principle is shown in Figure 3-8.
Fig. 3-8 Schematic diagram of the imaging principle of the cylindrical lens grating plate
With the wide application of UV printing technology and the improvement of grating materials, began in the grating material directly printed on the back of the reverse grating image production of three-dimensional prints, printing and grating composite imaging synchronization is completed. This process not only eliminates the grating composite process, but also due to the use of UV ink curing technology, offset printing ink can be instantly cured to ensure the accuracy and efficiency of printing, is the current main process and way of printing three-dimensional images, UV offset printing technology constitutes the basis of modern three-dimensional printing. In recent years, the emergence of three-dimensional lenticular ink technology, the process is first printed on paper according to the traditional flat color printing requirements of the lenticular image, and then screen printing in the image of the lenticular ink, and finally through the online lenticular thermoforming technology made of three-dimensional lenticular prints. The combination of this technology and UV fast curing process, can realize fully automatic production line, is the development direction of high line number lenticular three-dimensional printing process. With the diversified development of modern printing technology and three-dimensional printing product applications continue to expand, inkjet printing technology and digital printing technology will also be used in the field of three-dimensional printing.
“Offset + lenticular composite” three-dimensional printing technology in the printing after the need to print images and lenticular plate for composite, lenticular composite imaging there are mainly three ways.
① Flat press lamination method. The flat press is used to attach the PVC film to the substrate while the cylindrical lens is being formed, thus realizing the composite imaging of the printed matter and the lenticular plate.
② Roll lamination method. The rolled polyvinyl chloride film is fully heated, then overlapped with the substrate and passed between the cooled negative mold and the pressure roller, and pressurized to fit it while the column lens is being formed, realizing the composite imaging of the printed matter and the lenticular plate.
③ Molding and laminating method. By the flat press will be molded rigid cylindrical lens grating plate with adhesive attached to the surface of the print, the print and grating plate composite imaging. Currently used more is the first molding after the lamination method.
Grating parameters include grating line number, grating thickness, grating distance, grating transmittance, grating deviation value. Currently the grating commonly used line number of 62 lines / in, 75 lines / in, 100 lines / in and 141 lines / in. grating thickness directly affects the stereo image can express the depth of field range. Grating expression of the depth of field range of the ability to focus the depth of field coefficient to express, usually 75 lines / in more than the grating focus depth of field coefficient of 2 to 3; 30 to 75 lines / in the grating focus depth of field coefficient of 2 to 4; 30 lines / in below the grating focus depth of field coefficient of 3 to 5. grating viewing distance and grating line number has a certain correspondence (as shown in Table 3-29). The higher the grating transmittance, the clearer the image. The higher the grating deviation value, the better, in order to ensure that the image grating distance and grating grating distance to achieve an accurate match.
Table 3-29 Relationship between grating line number and viewing distance
Note: 1in = 2.54cm.
Stereoscopic printing grating material to plastic as raw material, mainly PET (polyester), PP (polypropylene), PVC (polyvinyl chloride) three. Usually there are two methods to make lenticular plate. One is the hot compression molding method, injection molding is an important method of thermoplastic optical lens processing, grating plate is a kind of plastic lens, but also the focus of the optical components used by people. With the grating plate mold and plastic sheet close to the heating and pressure, so that the plastic is made into a convex spherical type cylindrical mirror grating stripes. Another is the screen printing method, with transparent UV three-dimensional grating ink directly on the plastic sheet net printed grating stripes, and then cured by ultraviolet irradiation, you can get the grating plate. Screen printing grating process is not only fast, low cost, and can be partially printed grating stripes, so that the image to achieve local stereoscopic, you can produce the whole picture both three-dimensional image and flat image of the product.
Screen printing grating is the grating on the stripes as a graphic to print, Figure 3-9 for the grating plate stripes on the structure of the schematic.
Fig. 3-9 shows the structure of the stripes on the lenticular plate.
The ink layer of screen printing can be as thick as 30~100μm, so it has strong covering power. In recent years the development of thick version of the gum, screen thickness of up to 800 ~ 1000 μm. Due to the grating view of the thickness of the face of the requirements of particularly high, such as 200 lines of fine grating thickness of 0.3mm, 100 lines of the thickness of 0.4mm or more, so, from the formation of the ink layer thickness of the ability to analyze the printing of the grating screen technology is the first choice of the method of on-line lenticular (local stereoscopic printing) production.
UV three-dimensional lenticular ink is based on UV screen printing varnish material, in order to adapt to the special needs of online printing lenticular and a special ink. In addition to meet the screen printing UV ink requirements, it must also meet the following three requirements of the three-dimensional lenticular UV ink:
① Ultra-thickness. General UV screen printing ink layer thickness of 30 ~ 60μm, while the three-dimensional lenticular UV ink requires ink layer thickness of 200 ~ 500μm, and should be dried in an instant UV curing, relying on the ink layer buildup to reach the required thickness is not scientific.
② plasticity. The plasticity referred to here refers to screen printing, without UV radiation curing before the stereotypes, UV ink layer only film is not stereotypes, can accept a certain amount of external pressure and produce arbitrary deformation characteristics, which can be molded. This is the key to realize the production of online grating. After molding can be UV radiation curing stereotypes, and its plasticity disappeared.
③ highly transparent. Because the grating is the formation of a three-dimensional image of the viewer, must be an optical lens elements, so there must be both good optical surface and strong transparency, otherwise it is difficult to observe the three-dimensional image.
UV three-dimensional lenticular ink formula is based on screen printing UV varnish ink blending, screen printing UV varnish is a colorless transparent ink, in which to add a certain amount of high-level transparent rosin powder, can be used after mixing. Such as rosin 40% + ordinary varnish 18% + screen printing UV varnish 42% mixed evenly, it can be adjusted to make UV lenticular ink. As the ink has not yet UV radiation curing, the use of grating mold hot press molding, rosin powder in the ink due to heat soluble and volume expansion, the formation of a transparent plastic sheet on its surface composed of a strip of columnar mirrors, and then UV curing, made of three-dimensional grating.
In recent years, with the development of three-dimensional print production methods, UV printing grating method has been the rapid development of the process is a three-dimensional image printed directly on the back of the columnar lens grating, one-time completion of the three-dimensional image, this process not only eliminates the need for composite process, and three-dimensional effect is better, it is a three-dimensional printing process of the direction of development.
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