UV curing ink how to screen printing PE, PP bottles

The bottle surface printing belongs to the container printing and can be completed with a curved screen printer. For bottles printed with non-polar PE and PP materials, such as shampoos, shower gels, cosmetics, and insecticides, generally light-curing inks are used. Therefore, PE and PP must be surface-treated before screen printing. In order to solve the adhesion of ink on it firmly. So, what are the best curing conditions for photo-curable inks? How to polarize PE and PP bottles in order to achieve a good ink effect, this problem is described in the next.

First, the basic knowledge of UV ink

1. Composition of UV ink and its role in ink

Reactive diluent (also known as diluent monomer): It is a kind of chemical with small molecular weight (low molecular weight), which can reduce the viscosity, act as dispersing pigment and diluting resin, and determine the surface characteristics of ink, such as viscosity, Hardness and softness. It is non-volatile and participates in UV resin curing cross-linking reactions.

2 Additives: including pigments, lubricants, thickeners, fillers, and accelerators, which affect the color of the ink, the curing speed, the thickness of the ink layer, the printability, and the weatherability of the ink.

3 light-curing resin: It is the main part of UV ink filming, that is, the connection material of UV ink, determines the gloss and adhesion of UV ink. Different inks use different resins to accommodate substrates of different materials.

4 Initiator: It is a chemical that stimulates cross-linking reactions. In the photochemical reaction, once the initiator has absorbed photons by light excitation, it becomes very excited and generates free radicals, transfers energy to other photosensitive polymers, and makes them chain reaction, combining single molecules, additives, and photo-curable resins. As a result, a net-like stable structure is formed and the ink conjunctiva is firmly attached to the substrate.

2. UV ink curing and curing system requirements

1 The type of reflector: focus type, non-focus type and multi-faceted reflective type. Focusing, non-focusing, and polyhedral reflections are commonly used to cure varnish (inks without colorants).

2UV lamp: The power of the UV lamp should meet the requirement of penetrating UV ink to cure. The maximum lifetime of a UV lamp cannot exceed 1200 hours. It is generally recommended that it should be replaced after 1,000 hours. Clean the lamp at regular intervals. After cleaning, turn the lamp counter-clockwise for 1/4 of a week (can be repeated three times). Be careful when installing and cleaning UV lamps.

3 Detection of the system: The energy output of the UV lamp can be measured with an energy meter. Use a UV, IMAP waveform display to draw a UV map. According to the UV map, detect the curing system. If the drawn energy diagram is sharp, it indicates that the curing system has no problem; if the top is a hump, the reflector has dust; if the top is round, , indicating lamp aging, according to the extent of the aging of the replacement should be replaced.

4 light curing elements:

● UV lamp power: the amount of UV light radiation, also known as penetrating power, must meet the requirements of UV curing ink, if the UV lamp power is not enough, even if the light time is longer, or the number of repeated curing, can not make ink Completely cured. Therefore, when the power of the UV lamp is insufficient, the ink needs to be cured by the high-power lamp because the UV light emitted from the low-power lamp cannot penetrate to the bottom of the ink layer, so that the bottom is not cured.

Curing speed: Curing speed must be considered in two ways. One is full cure; the second is cure at the best cure speed. The choice of the best curing speed is: first curing at a certain speed through the UV machine, if it is cured, then speed up until the ink layer through the curing machine can not be cured, then the best curing speed can be determined, That is, the rate of non-curing starting x 0.8 = the optimum rate of curing.

● Spectral wavelength range of the UV lamp: The spectral sensitivity range of the UV lamp must match the UV ink. General requirements UV lamp wavelength range is between l80-420nm.

5 adhesion test:

After the print is passed through a UV machine and cooled to room temperature, the print can be cut to a width of 6×6mm. Note that only the ink film can be scratched and cannot be damaged. Use 3M-600# tape to test the adhesion. problem:

● If there is no ink on the tape, it means good curing and good adhesion.

● If the blots are all transferred to the tape, there are four problems: the ink series is not selected correctly (ie does not match the material of the substrate); the substrate contains a large number of plasticizers or surface release agents; surface tension of the substrate Too low to be processed or treated without handling; poor ink curing.

• The imprinted edge does not transfer, and the middle part is transferred to the tape, indicating that the curing is not complete. This is because the lamp power is insufficient or the lamp power is satisfied, but the curing speed is too fast.

● Imprinting becomes ghosting, indicating that the ink is not completely cured.

● During overprinting, if the adhesion of the second color is not good, only the second layer will fall when the tape is tested, indicating that the first color is over-cured; if both layers are lost after the overprinting, the first layer is not sufficiently cured. The rule for UV curing at the time of stack printing is 70/30 RULE, ie 70% curing, 30% uncured, and the final ink layer should be 100% cured.

7 additive effects.

The addition of additives can affect some properties of the ink, such as adding an initiator, can increase the adhesion, but it will affect the softness of the ink, and adding too much will immediately lead to cross-linking and can not be printed.

8 process conditions:

● Screens in UV ink printing are generally made of high-tension, low-elongation, S-grade plain screens or single-sided and double-sided screens. Frame strength is good.

● The tension on the screen is the same, the error is ±2N/cm, the tension is inconsistent, the ink layer is not uniform, and positional deviation can occur.

● The net distance is generally between 2-4mm, and the net distance should be adjusted according to the tension.

●The hardness of the scraper is 60°~90°, generally 70°, 80°.

● Pretreatment of substrates. Due to the different surface conditions of the substrate material, the surface of the substrate should be treated. This adhesion to UV ink is extremely important. For the plastic film, wipe it with a cleaning cloth of alcohol or isopropyl alcohol to remove plasticizers and impure chemical substances such as dust and grease floating on the surface of the substrate; for PP, PE, PU with low surface tension , PET and other materials, according to the thickness of the print, thin, respectively, flame treatment or corona treatment to increase its surface activity to improve its adhesion. After surface treatment, the substrate surface tension should be ≥ 45 dyne/cm.

● Ink should be fully stirred before use, especially after adding the additive must be stirred evenly, otherwise it will produce some strong adhesion, some without curing or insufficient curing adhesion, printing will produce adhesion.

● UV lamp power must meet the requirements, and adjust the curing speed.

● After curing, cool to room temperature. Use a 3M-600# tape for cross hatch testing. 24 hours after cure, adhesion and solvent resistance are optimized.

● Wash the net, wash the net with water, but you cannot use alcohol, because alcohol can damage the net template.

9 Climate impact on UV inks.

● Influence of temperature: The temperature is high, the viscosity of UV ink becomes lower, and the phenomenon of toothiness and stencil is easy to occur after printing. Low temperature, high viscosity, easy to produce incomplete cure. Therefore, it is required that the ink be refrigerated in the summer, and the temperature of the silk screen shop should not be too high; when it is taken in a frozen environment in winter, it should be placed in the temperature of the printing environment in advance, and the curing speed should be appropriately slowed down.

● Influence of humidity: Humidity affects the surface condition of the substrate and the screen printing performance of the ink on the substrate.

10 color effects.

Because various colors reflect light differently, UV inks have influence on the UV curing of inks depending on the color, and the relationship between the reflection wavelength of light and the speed of photocuring of seven colors is related.

Second, the bottle (plastic PE, PP, PET, etc.) prepress processing requirements and parameters

At present, photo-curable inks have not been directly applied to untreated PE or PP bottles. Therefore, PE and PP bottles must be printed. Polarization must be performed on PE and PP before printing. Otherwise, UV inks cannot be firmly attached. So how to polarize?

There are many methods for polarization treatment. Most of the PE, PP plastics and bottle industries use flame treatment. The result of this flame treatment is to use flames containing free radicals such as O, N0, OH, and NH. The surface oxygen of high polymers such as PE and PP is extracted, followed by surface oxidation by free-radical mechanism, and some polar oxygen-containing groups are introduced to undergo chain scission reaction. Polarization of non-polar PE and PP is changed. The wettability of the surface ensures the fastness of the ink.

The water surface tension is 72.5 dyne/cm and the oil surface tension is 25-50 dyne/cm.

Flame treatment parameters: At present, most of the flame treatment units use gas as the combustion source and should control the air: gas = 7:1 to achieve the blue-violet flame, then the flame temperature can reach 1270. Around C, the length of the flame inner core is controlled at 6-19mm, the top of the flame core to the surface is l 0-15mm, the contact time is l-4 seconds (LDPE is 1-2 seconds, HDPE is 3-4 seconds), and the rotation speed is 100 -150 rev / min, the surface depth can be treated at 40-90A, PE, PP surface energy to reach 45 dyne / cm to meet the ink adhesion fastness requirements.

The polarization effect of PE and PP plastic bottles after flame treatment: When PE and PP are treated by flame, the original surface tension value must be >45 dyne to ensure that the UV ink adheres firmly. Therefore, the flame treatment effect is detected. It is very important that the method has Dyne test pen (with ethylene glycol ether 35%+formamide 65% mixture) detection and immersion test. The current widespread application of flooding to detect, that is, the flame-treated PE, PP bottles are completely immersed in clean water, and then remove the bottle from the water and pause for a moment (about 10 seconds) to allow excess water to flow off, and then watch the bottle is processed Whether the surface of the part is covered with a uniform layer of water, if it is full of hydrophilicity and lasts for more than 15 seconds, it means that the flame treatment has reached the printable requirements. If the water film on the surface of the bottle is not continuous, it may even be like rain. After the lotus leaf is filled with water, it is necessary to adjust the flame treatment parameters to meet the requirements, such as the proportion of air and gas combustion, the time and distance of contact between the flame active ingredient and PE, PP, and so on.

Third, screen printing process parameters

1. The ink should be thoroughly stirred before printing, and the material splashed on the lid of the ink tank can be completely scraped into the ink and stirred evenly.

2. Screen and screen. Wire Mesh: Yellow, high strength, low stretch net with mesh number >355 mesh/inch. Screen: tension 18-25N/cm, with direct photosensitive or capillary film, film thickness 15-25 microns.

3. Squeegee: hardness 80 ° polyurethane tip scraping.

4. Screen cleaning with ordinary washing water.

5. Curing conditions: medium/high pressure mercury lamp, wavelength: 280-380nm, peak at 365nm. Power: 80-120W/cm or so. Over UV speed: The need to adjust the photocuring energy.

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