How to print PE, PP bottles with light (UV) curing ink
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The surface printing of the bottle is printed on the container and can be done with a curved screen printer. For bottles processed with non-polar PE and PP materials, such as shampoos, shower gels, cosmetics, insecticides, etc., generally use light-curing inks, so the surface of PE and PP must be treated before silk screen printing. In order to solve the adhesion of the ink on it. So, what is the best curing condition for photocurable inks? How to polarize PE and PP bottles to achieve a good inking effect, this issue is described below.
First, the basic knowledge of UV ink
1. The composition of UV ink and its role in the ink
1.1 reactive diluent (also known as diluent monomer): is a small molecular weight (low molecular weight) chemical, which can reduce the viscosity, play the role of dispersing pigment and dilute resin, determine the surface characteristics of the ink, such as viscosity, Hardness and softness. It does not volatilize and participates in the UV resin curing cross-linking reaction.
1.2 Auxiliaries: including pigments, lubricants, thickeners, fillers, accelerators, etc., which affect the color of the ink, the curing speed, the thickness of the ink layer, and the printability and the weatherability of the ink.
1.3 Photo-curing resin: It is the main part of UV ink film formation, that is, the connecting material of UV ink, which determines the gloss and adhesion of UV ink. Different inks use different resins to accommodate substrates of different materials.
1.4 Initiator: It is a chemical that excites the crosslinking reaction. In the photochemical reaction, the initiator is once After the photoexcitation absorbs photons, it becomes very excited and active, generates free radicals, transfers energy to other photosensitive polymers, causes chain reaction, and combines monomolecular substances, additives, and photo-solid resins to form a network-like stable structure. The ink conjunctiva is firmly attached to the substrate.
2. UV ink curing and requirements for curing system
2.1 type of reflector: focus type, non-focus type and multi-face reflection type. Focused, non-focused and multi-faceted types are commonly used to cure varnish (ink without coloring).
2.2 UV lamp: The power of the UV lamp should meet the requirements of being able to penetrate the UV ink to achieve curing. The maximum service life of UV lamps should not exceed 1200 hours. It is generally recommended to replace them after 1000 hours. Clean the lamp at regular intervals. After cleaning, rotate the lamp counterclockwise for 1/4 week (can be repeated three times). Be careful when installing and cleaning UV lamps.
2.3 Detection of the 3 system: The energy output of the UV lamp can be measured with an energy meter. Use UV, IMAP waveform display to draw UV map, according to UV map, detect the curing system; if the energy map is sharp, it means there is no problem in the curing system; if the top is hump, the reflector is dusty; if the top is round Indicates that the lamp is aging, and the replacement should be replaced according to the degree of aging.
2.4 light curing elements:
●The power of UV lamp: that is, the amount of UV light radiation, also called penetrating power, must meet the requirements of UV-curable ink. If the power of UV lamp is not enough, even if the illumination time is longer, or the number of times of curing is repeated, the ink cannot be made. Fully cured. Therefore, when the power of the UV lamp is insufficient, the ink needs to be solidified by the high-power lamp, because the UV light emitted by the low-power lamp cannot penetrate to the bottom of the ink layer, so that the bottom does not solidify.
● Curing speed: The curing speed should be considered from two aspects. One is fully cured; the other is curing at the optimum curing speed. The best curing speed is selected by first curing at a certain speed through a UV machine. If it is cured, the speed is increased until the ink layer that passes through the curing machine cannot be cured. At this time, the optimum curing speed can be determined. That is, the speed at which no curing starts × 0.8 = the optimum curing speed.
● The spectral wavelength range of the UV lamp: that is, the spectral sensitivity range of the UV lamp must match the UV ink. It is generally required that the UV lamp has a wavelength in the range of from 18 to 420 nm.
2.5 adhesion test:
After the printing is passed through the UV machine and cooled to room temperature, the printed matter can be cut into 6×6mm. Note that the ink film can only be scratched and can not be damaged. The adhesion is tested with 3M-600# tape, and the following adhesion can be demonstrated by testing the adhesion. problem:
● If there is no ink layer on the tape, it means good curing and good adhesion.
● If all the blots are transferred to the tape, there are four problems: the ink series is not selected (that is, it does not match the material of the substrate); the substrate contains a large amount of plasticizer or surface release agent; the surface tension of the substrate Too low, must be treated or processed without treatment; poor ink curing.
● The edge of the blot does not transfer, and the middle portion is transferred to the tape, indicating that it is not fully cured. This is because the power of the lamp is insufficient or the power of the lamp is satisfied, but the curing speed is too fast.
● The blot becomes a ghost, indicating that the ink is not fully cured.
●When 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 tested after overprinting, the first layer is insufficiently cured. The UV curing rule for overlay printing is 70/30 RULE, ie 70% cure, 30% uncured, and finally the ink layer should be 100% cured.
2.6 Comparative examples of factors affecting ink thickness (Table 1)
2.7 effects of additives.
Adding additives can affect some properties of the ink, such as adding an initiator, which can improve the adhesion, but it will affect the softness of the ink, and if the amount added is too much, it will immediately cause cross-linking and cannot be printed.
2.8 process conditions:
● Screens in UV ink printing generally use high tension, low elongation, S-grade plain weave or single-sided flattened wire mesh. The frame strength is good.
●The tension on the screen is consistent, the error is ±2N/cm, the tension is inconsistent, the ink layer is uneven, and the position deviation can be generated.
● The mesh distance is generally between 2-4mm, and the mesh distance should be adjusted according to the tension.
●The hardness of the scraper is 60°∽90°, generally 70° by hand and 80° by machine.
● Pretreatment of the substrate. Due to the different surface conditions of the substrate material, the surface treatment of the substrate is extremely important for the adhesion of the UV ink. For plastic film, use a cleaning cloth dampened with alcohol or isopropyl alcohol to remove plasticizers and impure chemicals such as dust and grease floating on the surface of the substrate; PP, PE, PU with low surface tension Materials such as PET can be flame treated or corona treated according to the thickness and thickness of the printed matter to increase the surface activity and improve the adhesion. Preferably, the surface tension of the substrate is ≥45 dynes/cm after surface treatment.
● The ink should be fully stirred before use. In particular, it must be stirred evenly after adding the additive. Otherwise, some curing adhesion is strong, and some have no curing or insufficient curing adhesion, and the printing will also cause adhesion.
● The power of the UV lamp must meet the requirements and adjust the curing speed.
● After curing, it was cooled to room temperature, and the cross-cut test was carried out with 3M-600# tape. The adhesion and solvent resistance were optimized 24 hours after curing.
● Wash the net, wash the net with ordinary washing net, but can not use alcohol, because alcohol can damage the mesh version.
2.9 The impact of climate on UV inks.
●The effect of temperature: The temperature is high, the viscosity of UV ink becomes low, and it is easy to produce bristles and paste after printing. The temperature is low, the viscosity is high, and it is easy to cause incomplete curing. Therefore, it is required that the summer ink should be refrigerated, 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 slow down the curing speed as appropriate.
● Humidity effect: Humidity affects the surface condition of the substrate and the silk screen performance of the ink on the substrate. 10 color effects.
Because the reflection of light varies from light to color, the UV ink has an effect on the UV curing of the ink depending on the color. The relationship between the reflection wavelength of the seven colors and the speed of light curing is shown in Table 2. White and black reflection wavelength is 575-490nm
Second, the pre-treatment requirements and parameters of bottles (plastic PE, PP, PET, etc.)
At present, the photocurable ink has not been directly used on untreated PE and PP bottles. Therefore, to print PE and PP bottles, PE and PP must be polarized before printing, otherwise the UV inks cannot be firmly attached. So how to deal with polarization?
There are many methods for polarization treatment. Most of the PE, PP plastic and bottle industries use flame treatment. The result of this flame treatment is to use free radicals such as O, N0, OH and NH in the flame. The surface oxygen of high polymers such as PE and PP is extracted, and then surface oxidation is carried out according to the free radical mechanism, and some polar oxygen-containing groups are introduced to undergo chain scission reaction, and the polarization of non-polar PE and PP is changed. The wettability of the surface ensures the adhesion fastness of the ink.
Table 3 Several plastic surface tension values for plastic bottles
The water surface tension is 72.5 dynes/cm and the oil surface tension is 25-50 dynes/cm.
Flame treatment parameters: Most units that currently have flame treatment use gas as the combustion source, and should control the air: gas = 7:1 to reach the blue-violet flame, then the flame temperature can reach 1270. Around C, the inner core length of the flame is controlled at 6-19mm, the top end of the flame core 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 rpm, the surface depth of the treatment can be 40-90A, so that the surface of PE and PP can reach 45 dynes/cm to meet the fastness of ink adhesion.
Detection of polarization effect after flame treatment of PE and PP plastic bottles: When PE and PP are treated by flame, the original surface tension value of >45 dyne must be reached to ensure that the UV ink adheres firmly, so the flame treatment effect is detected. It is very important to have a Dyne test pen (with glycol ether 35% + formamide 65% mixture) for detection and water immersion testing. At present, water immersion is commonly used for testing. The flame-treated PE and PP bottles are all immersed in the purified water, and then the bottle is taken out of the water for a short time (about 10 seconds) to let the excess water flow away, and then the bottle is treated. Whether the surface of the part is covered with a uniform water film. If it is full and lasts for more than 15 seconds, it means that the flame treatment has reached the requirements of printing. If the surface of the bottle is not continuous, even like rain. After the water is on the lotus leaf, the flame treatment parameters must be adjusted to meet the requirements, such as the ratio of air to gas combustion, the time and distance of the flame active component in contact with PE and PP, and so on.
Third, silk screen process parameters
1. The ink should be thoroughly stirred before printing, and all the materials splashed on the ink tank cover should be scraped into the ink and stirred evenly.
2. Screen and screen. Screen: Yellow with a mesh size > 355 mesh/inch, high strength, low stretch mesh. Screen: Tension 18-25N/cm, with direct photographic paste or capillary film, film thickness 15-25 microns.
3. Squeegee: The hardness is 80 ° polyurethane tip scraping.
4. Screen cleaning with ordinary washing net water.
5. Curing conditions: medium/high pressure mercury lamp, wavelength: 280-380 nm, peak value is preferably 365 nm. Power: 80-120W/cm or so. Over UV speed: The need to adjust the photocuring energy.
Fourth, the current plastic bottle printing better UV ink
1. US solid color curing ink
1PPC series: can be screen printed on PE, PP bottles, and can also be screen printed on the PET bottle without treatment, can have high adhesion effect, high gloss, ink resistant layer, if silky amber and other colored PET bottles Adding a hardener to the ink can make the adhesion of the ink higher.
2PCI series: high gloss, silk screen printed PE, PP bottle ink, high adhesion, good water resistance.
2. Germany Di high light curing ink
1Serit 985UV has a high gloss and good fluidity, such as the addition of curing agent and more chemical resistance, suitable for treated PE, PP and PVC.
2Serie 988V, has good viscosity, fluidity and chemical resistance on different materials, suitable for processed PE, PP, PVC, PS and PET.
3. Japan's ten light-curing inks
RCPERG/RCPEP-2 (HV) has flexibility, chemical resistance and good adhesion for use on treated PE and PP bottles.
4. French Dolby light curing ink
POIYCOLOR/POLYPLUS series, suitable for high-gloss UV inks on treated PE, PP and PET bottles, with fine particles, high color density, good printability, good adhesion to treated PE and PP, and enhanced water resistance It can be added with special hardener and solvent-based two-component epoxy screen printing ink (8500 series), which is suitable for glass, metal, spray-coated PE, PP and other substrates.
Five, light curing ink failure and treatment advice
1. No adhesion
1.1 Ink and substrate incompatible processing recommendations:
Illumination treatment: ultraviolet light with a wavelength of 170-200 nm is used to illuminate the surface of the substrate before silk screen printing;
Flame treatment: a polarization treatment method of a substrate having a thickness greater than 0.6 mm;
Corona treatment: a thin film or a polarization treatment method for a film;
The wrong ink was selected: the bonding resin in the ink did not match the material of the substrate.
1.2 The treatment of the surface of the substrate is contaminated:
Remove dirt from the surface of the substrate;
a plasticizer, a release agent, and the like on the substrate;
Add a special adhesion enhancer for the ink.
2. In the process of overprinting, the adhesion between the layers of the ink is poor.
2.1 The previous layer of ink is over-cured and blocked.
Reduce the degree of photocuring of the previous ink, as long as 70% curing, so that the surface wettability of the UV ink is good;
A special interlayer adhesion enhancer is added to the ink.
2.2 added additive excess:
Controlling the addition amount of the auxiliary agent, reducing the auxiliary agent by l/2;
Control UV curing parameters such as UV power, increase the speed of UV over, and so on.
3. Can not be cured or the surface is sticky
3.1 Because the ink layer is too thick to be cured, it is recommended to:
Change the screen parameters, such as using plain mesh or flattening mesh, reduce the mesh distance, reduce the thickness of the screen photosensitive layer; improve the hardness of the squeegee >75 ° and sharp squeegee;
Increase printing speed, change scraping pressure and scraping angle, etc.;
Add a small amount of special thinner to properly reduce the viscosity of the ink;
A curing enhancement (promoting) agent is added to the ink.
3.2 UV lamp power (energy) is insufficient for reasons:
Wipe the UV lamp and lampshade to improve the reflection of UV light;
The aging of the UV lamp should be replaced. Generally, the UV lamp should be replaced after 1200 hours of service life;
Increase the power per unit length of the UV lamp to ensure 80-120W/cw;
After the UV lamp is used for 25/100-30/100 time, the lamp should be rotated 1/4 for reuse.
3.3 Black, white, yellow or a mixture of these colors causes the lack of light solidity and suggestions:
Adjust the light curing parameters, such as UV lamp power, to slow down the UV time;
Adding a curing enhancer to the ink to accelerate the reaction time of the ink;
Add appropriate varnish to the ink to make UV light easier to penetrate.
4. Low gloss
4.1 The reason for the incomplete curing of the ink is the same as the “3” question;
4.2 The ink must be fully stirred before use. If it is not stirred evenly, it will cause low gloss.
5. The ink becomes plaque after curing
5.1 Insufficient curing of the ink film, the treatment suggestion is the same as "2" in "3";
5.2 The ink film is not flat, and the treatment opinion:
The rubber scraping edge is not smooth;
The tension of the screen and the uniformity of its tension are not good;
The heat generated during UV curing is concentrated, so the speed of the UV machine must be increased.
6. Screen printing inks appear small "fisheye" on the substrate
6.1 Ink is incompatible with the substrate. At this time, the surface properties of the substrate should be improved, and the surface printing state can be improved by adding an appropriate amount of defoaming agent to the ink to control the generation of bubbles or moistening the surface of the substrate.
6.2 The surface is caused by debris dust. At this time, the surface of the substrate must be cleaned and washed with a solvent such as absolute ethanol to improve the affinity of the ink to the substrate.
In short, in the process of using UV ink, we must pay attention to control the ink and substrate printability and control several elements of UV ink light curing and ensure good polarization treatment of PE, PP material bottle surface, PE, PP plastic The screen printing of the bottle will definitely achieve a very good printing effect!