Process design of the hottest paper pulp mold curv

2022-08-23
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Pulp mold surface printing process design

in today's society, people pay more and more attention to environmental protection. EPS foamed plastic products, which once brought great convenience to people, are being phased out due to its environmental pollution, and replaced by a packaging material called pulp mold. At present, it is mainly used in the cushioning packaging of disposable tableware and household appliances in China. In addition, it is also used in the outer packaging of fragile goods (such as small packaging of eggs) in foreign countries. Pulp mold is a new packaging material that integrates environmental protection, resource saving and energy saving, and can be naturally degraded. Its appearance will eliminate the pollution of white garbage. But this brings us a new problem, that is, how to print on this rough three-dimensional packaging material. Therefore, we put forward the research topic of "Research on the printing process of pulp mold curved surface", in order to solve the technical problems such as how to use and how to print high-quality color samples on the pulp mold box, and the selection of which will also affect the experimental results

I. printability of pulp mold

the pulp mold used in our experiment uses waste newspaper as raw material. After secondary beating, it is attached to the copper on the mold surface by adsorption. After the above process, the pulp mold has formed its unique properties. Its uniformity is poor. When looking at the light, you will find that there is light transmission with different degrees of brightness and darkness; The surface cleanliness of the pulp mold is not high, with bright and dark colors and different reflective spots; The flatness is low, and there are folds and wrinkles by hand; In addition, after twice beating, the fiber morphology in the pulp mold is worse than that of the raw material newspaper, and its ink absorption is uneven; In addition, no fillers and adhesives were added during the processing, and the surface strength was not very high

II. Selection of printing methods

after the above analysis of the suitability of pulp mold printing, combined with the special shape of pulp mold box, we selected the following methods suitable for curved surface printing and compared them

1. Silk printing

silk printing itself has no requirements for the substrate, but for pulp molded products, due to a certain thickness, it is necessary to change the height of the workbench. What is more complex is to print bar codes on the side, and there is an outer edge on the side box cover, which brings some difficulties to silk printing. If this printing method is used, it is necessary to make special frames and workpiece molds

2. Pad printing

as the name suggests, is the process of transferring the ink of the graphic part on the gravure plate to the elastic silicone rubber head first, and then to the substrate. This printing method is especially suitable for irregular shaped surfaces, but if the printing area is too large, the cost of silicone rubber head will be greatly increased. In addition, chromatic printing requires special equipment with pneumatic devices

3. Heat transfer printing

heat transfer printing is to first print the printed text on special paper, and then print a layer of hot melt adhesive on the surface of the text. When developing efficient and economical heat preservation and insulation materials and practical construction technology, transfer printing relies on hot pressing to transfer pictures and texts on paper to pulp mold. At present, this printing method is mostly used in textiles, such as color patterns on T-shirts. There are the following problems in heat transfer printing on pulp mold: ① the surface of substrate is not smooth; ② The pulp mold is a three-dimensional object, and its interior is hollow, so it is not easy to bear large pressure

therefore, we think it is more appropriate to choose the silk printing method, which has a wide range of adjustment, wide adaptability and low requirements for equipment

III. silk printing process

1 Selection of originals

(1) the color block pattern starts from a simple one, and the decorative pattern composed of color blocks is first selected as the original. This kind of manuscript can be designed by yourself on the computer according to the characteristics of the package

(2) continuous color adjustment manuscript. This kind of manuscript has bright colors and rich layers, which makes it difficult to copy, and it is also the focus of our research

(3) bar code now most of the outer packaging of goods should be printed with bar code. Bar code is equivalent to the ID card of goods. The printing quality directly affects the reading rate of photoelectric scanning reading device

2. The production of positive negative

continuous adjustment manuscript is proposed to adopt two adding methods, one is the traditional amplitude modulation point; Second, frequency modulation point

(1) key points of making am dot film

① the relationship between the number of wire mesh and the number of wires added to the negative film

generally, the number of wire mesh is 3 ~ 4 times or higher than the number of wires added to the negative film. If 45 Wires/inch is used, the number of wire mesh should be 135 mesh or 180 mesh, or even higher

② relationship between wire angle and film plus angle

the wire itself has an angle. If the wire angle is maintained at 0 °, the film plus cannot use the traditional plus angle, otherwise, moire will occur. It is generally believed that when the wire is at 22 ° to the screen, it can be corrected

color 15 ° = silk - 7 ° (counterclockwise)

color 75 ° = silk + 7 ° (clockwise)

③ the gradient range should be short

the high tone cannot be avoided (refers to the colorless place), and the tone on the plate should be compressed by 20% - 25% compared with offset printing. Subject to the intermediate tone (50% dot), the tone value on the plate should be less than the original color value. After printing, due to the dot expansion, the ideal tone restoration effect can be achieved

(2) printing characteristics of FM dot film

FM dot is a new addition method in the digital era, which depends on the number of dots per unit area to show the level of the picture. In silk printing, there are many advantages, mainly to eliminate the moire phenomenon caused by the use of amplitude modulation points. Because of the moire problem, it has always been a bottleneck problem that puzzles the quality of color halftone printing. Because the frequency modulation point does not need to add line number and angle in plate making, it will not conflict with the number of wire mesh, and there will be no moire. This is conducive to improving the quality of color silk printing. The diameter of frequency modulation point is generally 35 ~ 85 μ m. The diameter should be as large as possible

3. Selection of silk materials

(1) materials

at present, most chemical fiber materials (such as nylon or polyester) are used

(2) performance comparison

advantages of nylon: good resilience, good inking, low static electricity, used for bright and beautiful printing products

disadvantages: slightly poor acid resistance, large elongation, not suitable for fine product printing

advantages of polyester: better chemical resistance than nylon, low elongation, suitable for printing fine products

disadvantages: poor resilience

therefore, nylon should be selected as much as possible in monochrome screen printing with low dimensional accuracy requirements. When solvent based ink is used for printing plastic products, solvent resistant polyester yarn JTG d30 (2) 015 highway subgrade design code should be preferred. Polyester yarn should be used for color silk printing of pulp mold

(3) mesh selection

according to the previously mentioned relationship between the mesh number and the number of threads, it is better to buy 135 mesh and 180 mesh polyester yarn

(4) color selection of silk

from the reflection analysis of the light source: it is best to choose yellow silk, but considering that the price of yellow silk is relatively expensive, we will not choose it in the experiment for the time being

4. Stretching and printing process

(1) stretching

the stretching tension of the four-color plate must be consistent, otherwise there will be ghosting during printing overprint. The millimeter tensiometer is used in the laboratory, and the value is generally 2 ~ 3mm, which is more appropriate

(2) printing process

① coating: the direct method is adopted. The coating of photosensitive solution should be as thin and uniform as possible, and the thickness should be controlled at 10 ~ 30 μ m. The film thickness can be measured with a film thickness gauge. Generally, when coating, you can apply it twice on the back and once on the front

② exposure: a self-made exposure box is used, in which five 220V, 20W cold light source ultraviolet tubes are installed. The exposure time is generally controlled within 5 ~ 8 minutes. The exposure time should be determined by the experimental equipment such as universal experimental machine and pressure experimental machine according to the type of photosensitive adhesive, the thickness of coating, and the area of graphics and texts

③ development: wash the layout in the development pool or under flowing water, remove the photosensitive resin that has not seen light, and then dry it

5. Design and manufacture of workpiece mold

according to the overall dimensions of the pulp mold, make a mold with wood or foam plastic to fix the pulp mold box during printing

6. Printing process

① selection of ink

silk printing itself has no excessive requirements for ink, but depends on the nature of the substrate. The pulp mold itself has strong adsorption, but the surface smoothness is poor. Cardboard printing ink can be used, but the viscosity is not easy to be too high, and the permeability of the ink is good

② the choice of scraping angle

is generally between 45 ° ~ 80 °. The larger the scraping angle is, the smaller the imprinting force is, and the smaller the ink output is. The distance is less than or equal to 2.5mm

③ overprint method

since the manual screen printing machine does not have an automatic registration device, you need to find a way to manually register. There are many methods: A. when printing, register, position on the exposure box, and sun the four-color negative to the same position of the four silk plates; b. Positioning on the workbench: when printing, we use negative positioning, which is more suitable for our existing conditions

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