How does UV curable ink screen print PE and PP bottles

The surface printing of bottles belongs to container printing, which can be completed with a curved screen printing machine. For the printing of bottles made of non-polar PE and PP materials, such as shampoo, shower gel, cosmetics, insecticides and other bottles, light curing inks are generally used, so the surface of PE and PP must be treated before screen printing Only then can the ink be firmly attached to it. So, what are the best curing conditions for photo-curable inks? How to polarize PE and PP bottles to achieve a good inking effect is described below.

1. Basic knowledge of UV ink

1. Composition of UV ink and its role in ink

â‘ Reactive diluent (also called diluent monomer): It is a chemical compound with a small molecular weight (low molecular weight), which can reduce the viscosity, play the role of dispersing pigments and diluting resin, and determine the surface characteristics of ink, such as viscosity, Hardness and flexibility. It is non-volatile and participates in UV resin curing crosslinking reactions.

â‘¡Auxiliaries: including pigments, lubricants, thickeners, fillers, accelerators, etc., it affects the color of the ink, curing speed, ink layer thickness, printability and weather resistance of the ink.

â‘¢Photocurable 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 adapt to substrates of different materials.

â‘£Initiator: It is a chemical substance that stimulates the cross-linking reaction. In the photochemical reaction, the initiator becomes very excited and active as soon as it absorbs photons after being excited by light, generates free radicals, transfers energy to other photosensitive polymers, and causes a chain reaction to combine single molecules, additives, and photo-curable resins It forms a mesh-like stable structure, and the ink conjunctiva firmly adheres to the substrate.

2. UV ink curing and requirements for curing system

â‘  The type of reflector: focusing type, non-focusing type and multi-faceted reflection type. Generally used focusing type, non-focusing type and multi-faceted reflection type are commonly used to cure varnish (ink without coloring material).

â‘¡UV lamp: The power of the UV lamp should meet the requirements of penetrating UV ink to achieve curing. The maximum service life of the UV lamp cannot exceed 1200 hours, it is generally recommended that it should be replaced after 1000 hours of use. Clean the lamp at regular intervals. After cleaning, turn the lamp counterclockwise 1/4 turn (can be repeated three times). Be careful when installing and cleaning UV lamps.

â‘¢ Detection of the chemical system: the output energy of the UV lamp can be measured with an energy meter. Use UV, IMAP waveform display to draw a UV map, according to the UV map, detect the curing system; if the drawn energy map is sharp, it means that the curing system is no problem; if the top is a hump, it means that the reflector is dusty; if the top is round , Explain that the lamp tube is aging, which should be replaced according to the degree of aging.

â‘£The elements of light curing:

● Power of UV lamp: That is, the amount of UV light radiation, also called penetrating power, must meet the requirements of UV curing ink. If the power of UV lamp is not enough, even if the illumination time is longer, or the number of repeated curing times, the ink cannot be used Completely cured. Therefore, when the power of the UV lamp is not enough, the ink needs to be cured by the high-power lamp, because the UV light emitted by the low-power lamp cannot penetrate the bottom of the ink layer, so that the bottom is not cured.

● Curing speed: The curing speed should be considered from two aspects. One is full curing; the other is curing at the best curing speed. The method of selecting the best curing speed is: firstly cure with a UV machine at a certain speed, if cured, then speed up until the ink layer coming out of the curing machine cannot be cured, at this time, the best curing speed can be determined, That is, the speed at which curing does not start × 0.8 = the optimal curing speed.

● Spectral wavelength range of UV lamp: That is, the spectral sensitivity range of UV lamp must match UV ink. The wavelength range of UV lamps is generally required to be between 180-420nm.

⑤Adhesion test:

After the printed piece passes through the UV machine and cools to room temperature, the printed piece can be scratched 6 × 6mm. Note that only the ink film can be scratched without damage. Use 3M-600 # tape to test the adhesion. The test of adhesion can explain the following problem:

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

● If all prints 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, it must be processed or processed but not processed well; the ink is not cured well.

● The imprinted edge does not transfer, and the middle part is transferred to the tape, it means that it is not fully cured, because the power of the lamp is not enough or the power of the lamp is satisfied, but the curing speed is too fast.

● The imprint becomes a ghost image, indicating that the ink is not completely cured.

● When overprinting, if the adhesion of the second color is not good, only the second layer is dropped during the tape test, indicating that the first color is over-cured; if both layers are dropped after overprinting, the first layer is insufficiently cured. The rule of UV curing during overprinting is 70 / 30RULE, that is, 70% cured, 30% uncured, and finally the ink layer should be 100% cured.

⑦ Influence of additives.

Adding additives will affect some properties of the ink, such as adding an initiator, which can improve the adhesion, but it will affect the flexibility of the ink, and if the amount is too much, it will immediately cause cross-linking and cannot be printed.

⑧Process conditions:

● The screen in UV ink printing generally uses high tension, low elongation, S-class plain screen or single-sided flat screen. 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 position 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 selected from 60 ° to 90 °, generally 70 ° by hand and 80 ° by machine printing.

● Pretreatment of substrates. Due to the different surface conditions of the substrate material, it is extremely important for the adhesion of the UV ink to perform surface treatment on the substrate. For plastic film, it can be wiped with a cleaning cloth dipped in alcohol or isopropyl alcohol to remove plasticizers and impure chemicals such as dust and grease floating on the surface of the substrate; for PP, PE, PU with low surface tension , PET and other materials can be flame treated or corona treated separately according to the thickness and thinness of the print to increase its surface activity and improve its adhesion. After the surface treatment, the surface tension of the substrate is preferably ≥45 dynes / cm.

● Ink must be fully stirred before use, especially after adding additives, it must be stirred evenly, otherwise it will produce some strong adhesion, some not cured or insufficiently cured, the adhesion will be poor, and the printing will also cause adhesion.

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

● Cool to room temperature after curing, and use the 3M-600 # tape for cross-cut test. 24 hours after curing, the adhesion and solvent resistance are optimal.

● Wash the net, wash the net with ordinary net wash water, but can't use alcohol, because alcohol can damage the net template.

⑨The influence of climate on UV ink.

● The influence of temperature: the temperature is high, the viscosity of UV ink becomes low, and it is easy to produce hairy teeth and paste phenomenon after printing. Low temperature and high viscosity, easy to produce incomplete curing. Therefore, it is required that the ink should be refrigerated in summer, and the temperature of the silk screen workshop cannot be too high; when it is brought in a frozen environment in winter, it should be placed at the temperature of the printing environment in advance, and the curing speed should be appropriately slowed down.

● The influence of humidity: Humidity affects the surface state of the substrate and the screen printing performance of the ink on the substrate.

â‘© Influence of color.

Because various colors reflect light differently, the UV ink has an effect on the UV curing of the ink with different colors. The reflection wavelength of the seven colors on light is related to the speed of light curing. [next]

2. Pre-press processing 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, the PE and PP must be polarized before printing, otherwise the UV ink cannot be firmly attached. So how to polarize?

There are many methods of polarization treatment. In PE, PP plastics and its bottle industry, most of the flame treatment methods are used. The result of this flame treatment is the use of free radicals such as O, N0, OH, and NH in the flame. The surface oxygen of polymer such as PE, PP is extracted, and then the surface oxidation is carried out according to the free radical mechanism, and some polar oxygen-containing groups are introduced to undergo chain scission reaction, which changes the polarization of non-polar PE and PP The wettability of the surface ensures the fastness of the ink.

Water surface tension is 72.5 dynes / cm, oil surface tension is 25-50 dynes / cm.

Flame treatment parameters: Most of the flame treatment units currently use gas as a combustion source, and the air should be controlled: gas = 7: 1 to reach the blue-violet flame, then the flame temperature can reach 1270. Around C, the length of the flame 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), the rotation speed is 100 -150 rpm, the surface depth that can be processed is 40-90A, so that the surface of PE and PP can reach 45 dynes / cm to meet the fastness requirements of ink adhesion.

Detection of the polarization effect of PE and PP plastic bottles after flame treatment: After PE and PP are flame treated, the original surface tension value of> 45 dyne must be reached to ensure that the UV ink is firmly attached, so the detection of flame treatment effect It is very important, its methods include Dyne test pen (using 35% ethylene glycol ether + 65% formamide mixed solution) detection and water immersion detection. Currently, immersion is commonly used for detection. All flame-treated PE and PP bottles are immersed in clean water. Then remove the bottle from the water and stop for a while (about 10 seconds) to allow excess water to run off, and then watch the bottle being processed. Whether the surface of the part is covered with a uniform water film, if it is covered with hydrophilicity lasting more than 15 seconds, it means that the flame treatment has reached the printable requirements, if the water film on the bottle surface is discontinuous, even like rain After the water droplets on the rear lotus leaf, the flame treatment parameters must be adjusted to meet the requirements, such as the combustion ratio of air to gas, the time and distance between the effective flame components and PE, PP, etc.

3. Screen printing 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. Silk screen and screen version. Wire mesh: yellow with mesh> 355 mesh / inch, high strength, low extension mesh. Screen: Tension 18-25N / cm, using direct photosensitive paste or capillary film, the film thickness is 15-25 microns.

3. Glue scraping: the hardness is 80 ° polyurethane sharp scraping.

4. Use common net water for screen cleaning.

5. Curing conditions: medium / high pressure mercury lamp, wavelength: 280-380nm, the peak value is better at 365nm. Power: about 80-120W / cm. Over UV speed: adjust the demand for light curing energy.

Fourth, the current plastic bottle printing better UV ink

1. American Nuogu Light-curing Ink

â‘ PPC series: can be screen-printed on PE and PP bottles, and can also be screen-printed on PET bottles without processing, which can achieve high adhesion effect, high gloss, water-resistant ink layer, if screen printing colored PET bottles such as amber , Adding a hardener to the ink can make the ink adhesion higher.

â‘¡PCI series: high gloss, screen printing can process PE, PP bottle ink, high adhesion strength, good water resistance.

2. German Digao curing ink

â‘  Serit 985UV has a high gloss and good fluidity, such as adding a curing agent is more chemical resistant, suitable for processed PE, PP and PVC.

â‘¡Serie 988V has good viscosity, fluidity and chemical resistance on different materials, suitable for processed PE, PP, PVC, PS and PET

3. Ten light curing inks in Japan

RCPERG / RCPEP-2 (HV) has flexibility, chemical resistance, good adhesion, and is used on processed 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 and good adhesion of treated PE and PP, to enhance water resistance , Can add special hardener and solvent-based two-component epoxy resin screen printing ink (8500 series), the ink is suitable for glass, metal, spray plate processing PE, PP and other substrates. [next]

Fifth, light curing ink failure and handling opinions

1. No adhesion

â‘ Recommendations for handling incompatible ink and substrate:

Illumination treatment: ultraviolet light with a wavelength of 170-200nm is selected to irradiate the surface of the substrate before screen printing;
Flame treatment: polarization treatment method for substrates with thickness greater than 0.6mm;
Corona treatment: Polarization treatment method of thin substrate or thin film;
Wrong ink selected: The adhesive resin in the ink does not match the material of the substrate.

â‘¡Recommendations for the treatment of contaminated surface of substrate:

Remove the dirt on the surface of the substrate;
Alcohol wipes plasticizers, release agents and other softeners on substrates;
Add special adhesion enhancer for ink.

2. During the overprinting process, the adhesion between the ink layers is poor

â‘ Operation suggestions on the over-curing of the previous layer and blocking:

Reduce the degree of photo-curing of the previous ink, as long as it reaches 70% curing, so that the UV ink surface has good wettability;
Add special interlayer adhesion enhancer to the ink.

â‘¡ Excessive added additives:

Control the amount of additives, reduce the additives by l / 2;
Control UV curing parameters such as UV power, increase the speed of UV and so on.

3. Can not be cured or the surface is sticky

â‘ Suggestion that the ink layer is too thick to cure:

Change the screen parameters, such as the use of plain weave or flattened screen, reduce the screen distance, and reduce the thickness of the photosensitive layer of the screen; increase the hardness of the rubber scraping> 75 ° and sharp rubber scraping;
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;
Add curing enhancer (solidification) agent to the ink.

â‘¡Recommendations for insufficient UV lamp power (energy):

Wipe the UV lamp tube and lampshade to improve the reflection effect of UV light;
The aging of the UV lamp should be replaced, and the general UV lamp should be replaced after 1200 hours of service life;
Increase the power within the unit length of the UV lamp to ensure 80-120W / cw;
After using the UV lamp for 25 / 100-30 / 100 hours, the lamp should be rotated 1/4 before use.

â‘¢ Reasons and suggestions for insufficient light solidity caused by black, white, yellow or a mixture of these colors:

Adjust light curing parameters, such as UV lamp power, to slow down the time after UV;
Add curing enhancer to the ink to speed up the reaction time of the ink;
Add appropriate varnish to the ink to make the UV light easier to penetrate.

4. Low gloss

â‘  The reason for the incomplete curing of the ink, the suggestions for the treatment are the same as "3";
â‘¡The ink must be fully stirred before use. If it is not evenly mixed, it will cause low gloss.

5. The ink becomes patchy after curing

â‘  Insufficient curing of the ink film, the treatment suggestions are the same as "2" in "3";
â‘¡Due to uneven ink film, handling suggestions:
The rubber blade is not flat;
The tension of the screen and its uniformity are not good;
The heat generated during UV curing is concentrated, so the speed of the UV machine must be increased.

6. Screen printing ink appears small "fish eyes" on the substrate

â‘ Incompatible ink and substrate. At this time, the surface performance of the substrate should be improved. The generation of air bubbles can be controlled by adding an appropriate amount of defoaming agent to the ink to moisten the surface of the substrate to improve its surface printing state.

â‘¡ There is debris on the surface. At this time, the surface of the substrate must be cleaned, and it can be washed with absolute ethanol and other solvents to improve the affinity of the ink and the substrate.

In short, in the process of using UV ink, you must pay attention to control the printability of the ink and the substrate, and control several factors of UV ink photocuring and ensure good polarization treatment on the surface of the bottle made of PE and PP. The silk screen printing of the bottle will definitely achieve a very good printing effect!

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