Several factors affecting ink adhesion in bottle screen printing process

Silk screen printing is one of the most commonly used graphics and text printing processes in cosmetic packaging materials. It has simple equipment, easy operation, simple printing and plate making, low cost, and strong adaptability. If it is poorly adhered and fixed, does not have a certain fastness, is not resistant to friction, and is not resistant to impact, it means poor printing.

Screen printing Screen printing process is that the ink leaks onto the substrate after passing through part of the mesh of the screen. The remaining mesh of the screen is blocked and the ink cannot pass through. When printing, the ink is poured on the screen. The ink will not leak onto the substrate through the mesh without external force. When the scraper scrapes the ink with a certain pressure and tilt angle, it will transfer through the screen. onto the underlying substrate to replicate the image.

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1.?? Bottle material

If the printing material is impure, it will have a considerable impact on the adhesion. In the current market, impure printing materials are mainly manifested in synthetic materials and nozzle materials. Furthermore, due to the differences in the processes of different companies during injection molding, the surface of the plastic material presents different conditions. There are many materials with grease on the surface, which will affect its adhesion. The color of the bottle material also affects UV absorption and therefore its adhesion.

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2.?? Prepress processing

In order to obtain good adhesion of the ink to the substrate, the bottle must be polarized so that the surface tension of the bottle reaches not less than 3.8×10-2N/m (generally speaking, the ink has good adhesion to the substrate). The most basic condition for applying force is that the surface tension of the substrate should reach at least 50 dyn/cm).

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The main method of polarizing bottles is flame treatment. The principle of flame treatment is to decontaminate the plastic surface through high temperature and melt the film surface, thereby increasing the surface energy of the plastic bottle and improving its surface adhesion to the ink. All in all, the surface of the substrate must be clean and effectively polarized before printing. If the surface of the bottle is not thoroughly treated, the adhesion will inevitably be affected.

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Flame treatment

The flame treatment machine is composed of a conveyor belt, a conical insert, a flame nozzle and a gas tank. When in use, the plastic bottle is placed on the conical insert, and the conveyor belt moves forward at a constant speed. Before reaching the flame nozzle, the conical insert begins to rotate, which also drives the bottle to rotate. The blue flame ablates the surface of the plastic bottle, thus Improve the surface energy of plastic bottles and increase the binding force with ink.

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The intensity of the flame, the running speed of the conveyor belt, and the smoothness of the bottle rotation all directly affect the processing effect, among which the smoothness of the bottle rotation is the most critical. The smoothness is directly proportional to the size of the bottle. The shorter the bottle, the smaller the radius and the better the treatment effect. For larger bottles, we must take measures to increase its stability. The common method is to attach a nylon sleeve to the tapered insert. The inner diameter of the sleeve is also tapered, and the outer diameter is consistent with the size of the bottle mouth. The sleeved bottles cannot fall off on their own and must be picked up manually. Plastic bottles that have been flame-treated should be printed immediately, otherwise the surface energy of the plastic bottle will be reduced and it will be easy to get dusty again, which will affect the adhesion of the ink.

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Detection of Flame Treatment Effect

The easiest way is to use a surface tension test pen for testing. Use a surface tension test pen with a specification of 3.8×10-2N/m. If the liquid traces spread evenly and do not shrink where the test pen has passed, it indicates that the treatment effect is good; if the liquid traces shrink, or part of the liquid traces shrinks and part does not shrink. Shrinkage indicates insufficient processing. Or you can use "splashing water treatment" to detect it. If it is handled well, a dense "water film" will be formed on the surface of the plastic bottle, which is often referred to as "water splashing" by screen printing colleagues.

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Timeliness of bottle treatment surface the effect of polarization treatment has a certain timeliness. The longer the time, the worse the effect, and the worse the bonding force with the ink. To ensure the adhesion of the ink, it should be printed in time after treatment. The investigation found that due to limitations of the customer's production process, polarization processing and printing are not in the same department in most printing plants, which will cause printing lag and affect the adhesion between the ink and the bottle.

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3.?? Curing equipment

UV lamp curing

The ink should be cured under a 280-320nm UV lamp immediately after printing. The curing speed depends on the thickness of the ink layer and the power of the lamp. The conveyor belt speed is neither too fast nor too slow.

UV curing lamp

The curing lamp must ensure that the wavelength, power, and energy meet the curing requirements of UV ink. All three are indispensable. Otherwise, it will cause non-curing, incomplete curing or embrittlement of the ink layer, further affecting the adhesion and softness of the ink. UV lamps have insufficient ultraviolet energy. Different lamps will present different ultraviolet energy. The energy emitted by the lamp contains ultraviolet and infrared rays. The more ultraviolet energy the light source emitted by the lamp contains, the more effective it will be in curing the ink. The better.

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Furthermore, the spectral intervals of ultraviolet rays vary in size. A good lamp has a small spectral interval and high ultraviolet purity. The spectral range of low-quality lamps is generally wider than the specific efficiency. The ultraviolet rays are not concentrated and appear diffuse. The purity is low, so the curing effect is not good. The usage time of a lamp is about 1000 hours (depending on the quality of the lamp and the environment in which it is used). Improper operation of UV equipment will greatly affect the adhesion.

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Different ink colors require different energy during curing

Generally speaking, during the curing process of UV bottle ink, black, white, yellow, and red inks require higher energy. Therefore, the track speed of the UV curing equipment should be appropriately reduced during curing to increase the curing time of the ink. In addition, the curing time of mixed ink oils of different colors must be tested and confirmed.

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4.?? Silk printing materials and processes

Wire mesh

For bottle screen printing, a polyester or nylon screen with a mesh size of 140 to 165 mesh/cm is generally selected to ensure the covering power of the ink layer and complete curing of the UV ink during printing.

Screen selection

In bottle printing, especially when printing with UV ink, the selection of screen is relatively strict. The mesh number of the screen is ideally between 140 and 165 mesh/cm. This is to ensure that during UV curing, UV light can completely penetrate the ink layer and ensure that the ink is fully cured.

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Squeegee

The temperature of the squeegee is generally between 75 and 85 degrees. A 5×25 flat squeegee or a pointed squeegee can be used. The squeegee must be wear-resistant and solvent-resistant. Squeegee hardness and angle selection the squeegee hardness and angle have a great impact on the fineness and ink volume of screen printing. For fine pattern printing, it is generally required to choose a squeegee with higher hardness. As for the angle of the squeegee, the theoretical optimal angle is 45°. At this time, the edge of the squeegee contacts the screen in a straight line. Considering the pressure during printing, the angle of the squeegee should be greater than 45°. If the hardness of the squeegee is too small and the angle is improperly selected, it will increase the amount of ink deposited, affect the degree of solidification of the ink, and thus affect the adhesion of the ink.

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Craftsmanship

The screen-printing ink layer should not be too thick. If it is too thick, the UV penetration will be weak and the light will not be completely cured. Naturally, the adhesion will be affected.

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5.?? UV ink

Before using ink

Ink must be fully stirred before use. Ink must be fully stirred before use. This is the most basic principle when using ink. This is because the proportions of resins, solvents, pigments, and additives in the ink are different, and delamination will inevitably occur after the ink is left for a period of time. If the ink is not stirred sufficiently before use, the components with light specific gravity will be on the upper layer and the components with heavy specific gravity will precipitate. In this way, customers will often react to phenomena such as unstable ink quality and different covering power and adhesion of inks used before and after. Leaving it for 24 hours after printing will deepen the curing effect and various resistances.

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From a physical perspective, UV curing is divided into surface curing, inner layer curing, deep curing and complete curing. In the general printing process, most of the UV ink only solidifies the inner layer because the UV rays form a thin film on the surface of the printing body at the moment of exposure, so that the ultraviolet rays cannot penetrate the inner layer of the ink in a short time. It failed to achieve the deep light-curing effect, thus failing to achieve the best adhesion of the ink. Leaving it for 24 hours after printing will deepen the curing effect and various resistances, and will achieve the best adhesion effect.

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Prepared ink

The prepared ink must be used up on the same day to avoid contamination and affect the quality of screen printing and thus the adhesion.

The adhesion of the tinted ink is generally worse than that of the original basic color because a variety of pigments are mixed together, which affects the ink's absorption of ultraviolet rays.

Extra white ink has worse adhesion than ordinary white ink because it has more toner and greater friction resistance. Its adhesion is not as good as ordinary white ink, and it is relatively difficult to light cure.

The adhesion of colored ink is worse than that of white, so its wear resistance will be relatively poor. Due to the large number of pigments in the toner ink, it is often difficult to achieve a deep curing effect when exposed to ultraviolet rays. Therefore, under normal circumstances, its adhesion is worse than that of a single pigment, and the wear resistance coefficient will also become worse. Therefore, for the toner ink part, if a high wear resistance coefficient is required, only by enhancing its UV absorption intensity can the deep curing effect be achieved to improve adhesion.

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UV ink reacts easily when exposed to light

When printing with UV ink, avoid direct sunlight and keep away from fluorescent lamps and mercury lamps.

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Ink storage

of UV ink is 1 and a half to 2 years in the environment of 22 ℃ -25 ℃ in a dark room without opening the can. After opening the can, please tighten the cap and store it in the environment of 22 ℃ -25 ℃. The storage time is half a year. Respectfully Please be careful not to let moisture spill into the bottle after the ink is opened.

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6.?? Printing workshop environmental requirements

Printing shop requirements

Because UV ink is ultraviolet light-setting ink, and sunlight contains ultraviolet rays, it is recommended that the indoor sunlight in the UV ink printing workshop should not be too bright. If the light is too bright, please install curtains to block the light. For example, if there are many fluorescent lamps in the indoor printing workshop, please note that the silver powder on the surface of the fluorescent lamp falls off and causes ultraviolet transmission. Too strong indoor light and ultraviolet transmission of the fluorescent lamp will cause the UV ink to solidify on the screen during the printing process, thus blocking the screen. (In production lines where the sunlight is too bright, please pay special attention to the production line near the window, and please take back the UV ink on the screen in time after printing to avoid curing).

Control the temperature and humidity in the workshop

If the temperature is low and the humidity is high, the photocuring reaction of UV ink will be slow and the binding ability of the ink and substrate will be reduced. Therefore, the ambient temperature and humidity of the workshop must be controlled during UV printing. The general temperature is about 18 ℃ ~ 25 ℃, and the relative humidity is about 50% ~ 65%.

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