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 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.
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.
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.
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
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 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.
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.
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.
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.
6 Comparative examples of factors affecting ink thickness
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.
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 floating on the surface of the substrate, such as dust and grease; for 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.