(UV Flexo, Technical Advancements)
In the past five years, the UV flexo market has continued to develop at a steady pace. Although this trend is more pronounced for narrow webs, medium- or wide-web printing technology is also continuously improving.
Throughout the development process, experts have done a lot of research and development to improve the workflow, reduce skin irritation, increase the speed of cure, reduce plate swell, and improve ink adhesion to plastic substrates.
This article describes some of the research and development work and discusses the technical requirements necessary to further expand the UV flexo ink printing market.
Although UV flexo inks, water-based flexo inks, and solvent-based flexo inks are used in the same printing process, each ink varies in chemical composition, rheology, and viscosity. Among them, UV flexo inks are particularly special in chemical composition, rheological properties, and viscosity. Their differences are as follows: (The chemical composition and rheology of flexographic inks)
UV Solvent Water Based Colorant XXX
Connecting material XXX
The binder composition UV solvent type water-based water X
Solvent X
Monomer X
Solid resin? XX
Oligomer X
Photoinitiator X
Additive UV Solvent Water Based Silicone/Wax/Flowability XXX
Curing/Drying UV Thermal Drying Thermal Drying Viscosity (cps) 500-4000 200-400 200-400
Jet Viscosity Measurement #3 Very High (40+) 20-30 20-30
Most non-Newtonian fluids are Newtonian fluids, mostly Newtonian fluids.
Unlike water-based and solvent-based inks, monomers used in UV ink systems do not volatilize. With an ink viscosity of 5 to 30 centipoise, acrylic monomers may be monofunctional, bifunctional, or multifunctional. It must be realized that monomers are not only non-volatile, but unlike water and solvents, monomers can also interact with light to form an interconnected polymer.
In recent years, the technology associated with curing equipment has been greatly improved, the choice of curing equipment has increased a lot, and at the same time, the technology associated with printing plates and metal anilox rolls has also greatly improved. The following is a detailed discussion of the development of chemical and rheological properties in UV flexo printing.
Monomer selection:
The following features are very important:
1. Fast curing;
Low viscosity
3. It is less likely to irritate the skin;
4. Good liquidity;
5. The negative effect on the printing plate is small;
6. Has a good adhesion to a variety of film base.
Although there is no solvent in the UV ink, the monomer and some photoinitiators will act as a solvent and diffuse into the polyester molecular arrangement of the printing plate, resulting in swelling and softening. It is well known that some fast curing, low viscosity monomers, such as HDODA, have a very significant negative effect on the photosensitive resin printing plate, which often leads to swelling and softening of the printing plate, especially long lived printing. As a result, printers and ink makers are looking for new solutions for long-lived work. Of course, speeding up the frequency of plate change is also a method, and another feasible method is to reduce the monomer that negatively affects the printing plate in the ink formulation. This technical change will likely affect the ink viscosity and curing efficiency.
In general, people use inks with lower surface tension to dispose inks. This method improves the adhesion of inks to different substrates, including plastics. It is worth noting that the surface energy of the base can be increased by corona treatment to improve the adhesion of the ink. The surface energy of the plastic substrate is usually raised to 40-45 dyne/cm to obtain satisfactory adhesion.
Rheology of Ink From the above table, we can see that UV inks have higher viscosities and mostly non-Newtonian fluids.
This rheology will affect the flow of the ink on the press and how the ink is involved in printing. The rheology studies in the above table show that even if an ink initially "resisted" to flow, the ink could flow well in a Newtonian fluid once sufficient force was applied to it.
An obvious advantage of this system is that the ink can flow well when it is pressed, and deliver high-quality, clear dots. If a temperature-controlled squeegee is used to measure the ink and transfer ink through it, the best results can be obtained. There is no doubt that this approach will lead to better predictability and a more stable image reproduction process. It has been found that the ink flowability can be greatly improved if an appropriate agitation system and a micro-motion pump are used to absorb the ink on the printing press.
Most narrow web presses use an open ink fountain. In an ink supply system, the ink is pushed toward the edge of the ink fountain by each stirring. Poorly fluid inks can pile up ink on the edge of the ink fountain and do not normally flow to the anilox roller. At this time, the printer can adjust or adjust the ink recipe to solve this problem. Interestingly, a short body is useful when using permeation or semi-permeable substrates. The excessive penetration of the ink is minimized, thus also reducing the possibility of curing problems.
Proofing It can be seen from the above table that all the components of the UV ink are solid, and the viscosity of the UV ink is also higher than the general flexo ink. For these reasons, it is not possible to reduce the amount of ink with a Meyer bar or a flexo manual proofer because these techniques require thicker films resulting in improper ink film density on the sheets. Therefore these techniques cannot be used to measure the ink density obtained from the anilox roll.
Under the leadership of Ronald Saltzman of NAPIM, people got a better proofing method. Before this, people tried and evaluated a variety of methods, including: Prufbau, Little Joe, Pamarco with a squeegee, Cavanaugh squeegee ink proofer. Although no one method can completely match the printing density, the author believes that the Cavanaugh Squeegee Inkjet Proofer has certain advantages over other methods. Using this method, one can not only better simulate the printer density, but also match the color within a small range of color differences.
Ink Curing UV flexo originated in label printing. Initially, most low-speed narrow web presses were used to print stickers, and a 300 W/line inch medium-pressure mercury vapor lamp was used to obtain a satisfactory cure.
As the speed of printing presses continues to increase and the web size continues to increase, the curing ability of inks has also been challenged more fiercely. In order to keep up with the increasing demand for curing speed, the curing lamp technology has also been continuously improved.
The problem is the curing of UV flexo inks on different substrates, including flexible films for general flexo printing (without support). Can the new technology be adapted?
Before discussing the curing problem, first of all pay attention to the medium pressure mercury vapor lamp will generate a lot of heat, probably in the vicinity of its 50% spectral output. There is no doubt that these heat will cause the film to shrink or deform.
Using one or more of the following methods will minimize the amount of heat generated, resulting in a satisfactory cure.
1. Special heat-removing "two-color" filters;
2, cold water type quartz filter;
3, air cooling;
4, low-power high-efficiency lamps;
Using a low-power, high-efficiency lamp (D-type) allows for a thicker ink with better curing. Although the overall spectral output of this lamp is similar to the spectral output of a standard mercury vapor lamp, its spectral band shifts toward long wavelengths. As a result of this movement, the spectral window partially blocked by the absorption of the colorant is also effective for the light absorption of the photoinitiator, thereby initiating the photochemical reaction.
Many people believe that the demand for low-calorie lamps will increase as people's demand for printing on flexible films grows. The "Excited Atom" lamp received a better recognition because it was considered to be completely heat-free. There are many photoinitiators that work effectively with this type of "excited atom" lamp. Their absorption range is generally 280-310 nm. Experimental data shows. It is best to reduce the normal amount of photoinitiator to obtain the curing effect of the surface and the inner layer.
End-use applications UV flexo printing is now widely used in a variety of printing applications, including packaging. There are different narrow, medium, large format rotary presses and CI printers to produce the final print. The question often asked is: "Can UV flexo be used in food packaging?" "Is it checked by the US Food and Drug Administration?"
The answer to the last question is negative, and the answer to the first question is more complex and requires further discussion.
Regardless of whether it is a rotary printing or a sheet-fed printing, the front side of the printed sheet may always come into contact with the back of the printed sheet. Therefore, the ink may be transferred to the back surface, such as the inner layer of the actual package, resulting in the ink directly or indirectly contacting the food. Whether the ink will come into contact with food depends to a large extent on the type of functional barrier between the food and the residual ink. The purpose of a functional barrier is to prevent the ink from coming into direct contact with the food while also preventing any transfer of material through the barrier.
As we all know, photoinitiators and photochemical by-products can cause odors. These odors will come into contact with food and change the original flavor of food, making it unacceptable. Phenol and other sulfur containing photoinitiators are no longer used because they produce odorous by-products. Even the use of the BDK is denied by the fact that it produces a fruity taste of methyl benzoate through a decomposition method accompanied by thermal decomposition.
Therefore, the ink used for food packaging must fully comply with the following formula:
1. A monomer or oligomer that is very light in nature;
2. High purity ingredients;
3. Light-inducing photoinitiators (eg, phenols, phenol derivatives, and derivatives of sulfur-containing ketones cannot be used);
4. The ink has good curability;
Multi-color and monochrome printing proves that under the ideal conditions, the effect of UV flexo printing can be comparable to that of offset printing. Ideal conditions include the use of the most suitable plate, anilox, chemical composition, and binders to configure the correct ink, press, and operating environment.
Today's photosensitive resin plates can provide very high print quality. There are many coated and non-coated photosensitive resin plates with different hardness readings available. It is best to consult the manufacturer of the media about the choice of media.
As with the plate, there are many options for the anilox roller's engraving mode, number of ink holes, and ink volume. In general, 200-400 lines/2.5-10bcm Anilox roller is suitable for monochrome printing, and 700-1000 lines/0.8-2.5bcm Anilox roller is suitable for multi-color printing.
Although the choice of plate and transfer roller is very important for the print quality, the effect of the viscosity of the ink on the final quality of the print is very important. Whenever possible, higher viscosity inks should be used for four-color printing for minimal dot gain. On the other hand, lower-viscosity inks should be used for printing on the field, because when the pinhole and cellulite phenomena are controlled to a minimum, a solid-looking effect can be obtained.
Health As a general category, acrylate monomers are considered to stimulate the skin. Most acrylics are irritating to the skin because they contain a variety of impurities, including acids. These products are no longer used by the industry. Although no chemicals can prove to be "safe," UV/EB curing monomers are safely used by many people and do not produce any skin-related problems. However no matter how safe the product is considered to be, the supplier always reminds the customer to follow good chemical hygiene practices.
For many years, customers have always relied on the relevant index of skin irritation to decide whether to use or how to use a monomer. Skin irritation is obtained by experimenting on rabbits. However, for the protection of animals, similar routine experiments are their biggest hidden danger. Large chemical companies do not use animals for experiments, and more and more experiments and processes no longer use animals. Our industry has a responsibility to find a way to not only continue to provide customers with credible information on safety and health, but also to meet the humanitarian needs of our society.
UV flexo will continue to grow, mainly
In the past five years, the UV flexo market has continued to develop at a steady pace. Although this trend is more pronounced for narrow webs, medium- or wide-web printing technology is also continuously improving.
Throughout the development process, experts have done a lot of research and development to improve the workflow, reduce skin irritation, increase the speed of cure, reduce plate swell, and improve ink adhesion to plastic substrates.
This article describes some of the research and development work and discusses the technical requirements necessary to further expand the UV flexo ink printing market.
Although UV flexo inks, water-based flexo inks, and solvent-based flexo inks are used in the same printing process, each ink varies in chemical composition, rheology, and viscosity. Among them, UV flexo inks are particularly special in chemical composition, rheological properties, and viscosity. Their differences are as follows: (The chemical composition and rheology of flexographic inks)
UV Solvent Water Based Colorant XXX
Connecting material XXX
The binder composition UV solvent type water-based water X
Solvent X
Monomer X
Solid resin? XX
Oligomer X
Photoinitiator X
Additive UV Solvent Water Based Silicone/Wax/Flowability XXX
Curing/Drying UV Thermal Drying Thermal Drying Viscosity (cps) 500-4000 200-400 200-400
Jet Viscosity Measurement #3 Very High (40+) 20-30 20-30
Most non-Newtonian fluids are Newtonian fluids, mostly Newtonian fluids.
Unlike water-based and solvent-based inks, monomers used in UV ink systems do not volatilize. With an ink viscosity of 5 to 30 centipoise, acrylic monomers may be monofunctional, bifunctional, or multifunctional. It must be realized that monomers are not only non-volatile, but unlike water and solvents, monomers can also interact with light to form an interconnected polymer.
In recent years, the technology associated with curing equipment has been greatly improved, the choice of curing equipment has increased a lot, and at the same time, the technology associated with printing plates and metal anilox rolls has also greatly improved. The following is a detailed discussion of the development of chemical and rheological properties in UV flexo printing.
Monomer selection:
The following features are very important:
1. Fast curing;
Low viscosity
3. It is less likely to irritate the skin;
4. Good liquidity;
5. The negative effect on the printing plate is small;
6. Has a good adhesion to a variety of film base.
Although there is no solvent in the UV ink, the monomer and some photoinitiators will act as a solvent and diffuse into the polyester molecular arrangement of the printing plate, resulting in swelling and softening. It is well known that some fast curing, low viscosity monomers, such as HDODA, have a very significant negative effect on the photosensitive resin printing plate, which often leads to swelling and softening of the printing plate, especially long lived printing. As a result, printers and ink makers are looking for new solutions for long-lived work. Of course, speeding up the frequency of plate change is also a method, and another feasible method is to reduce the monomer that negatively affects the printing plate in the ink formulation. This technical change will likely affect the ink viscosity and curing efficiency.
In general, people use inks with lower surface tension to dispose inks. This method improves the adhesion of inks to different substrates, including plastics. It is worth noting that the surface energy of the base can be increased by corona treatment to improve the adhesion of the ink. The surface energy of the plastic substrate is usually raised to 40-45 dyne/cm to obtain satisfactory adhesion.
Rheology of Ink From the above table, we can see that UV inks have higher viscosities and mostly non-Newtonian fluids.
This rheology will affect the flow of the ink on the press and how the ink is involved in printing. The rheology studies in the above table show that even if an ink initially "resisted" to flow, the ink could flow well in a Newtonian fluid once sufficient force was applied to it.
An obvious advantage of this system is that the ink can flow well when it is pressed, and deliver high-quality, clear dots. If a temperature-controlled squeegee is used to measure the ink and transfer ink through it, the best results can be obtained. There is no doubt that this approach will lead to better predictability and a more stable image reproduction process. It has been found that the ink flowability can be greatly improved if an appropriate agitation system and a micro-motion pump are used to absorb the ink on the printing press.
Most narrow web presses use an open ink fountain. In an ink supply system, the ink is pushed toward the edge of the ink fountain by each stirring. Poorly fluid inks can pile up ink on the edge of the ink fountain and do not normally flow to the anilox roller. At this time, the printer can adjust or adjust the ink recipe to solve this problem. Interestingly, a short body is useful when using permeation or semi-permeable substrates. The excessive penetration of the ink is minimized, thus also reducing the possibility of curing problems.
Proofing It can be seen from the above table that all the components of the UV ink are solid, and the viscosity of the UV ink is also higher than the general flexo ink. For these reasons, it is not possible to reduce the amount of ink with a Meyer bar or a flexo manual proofer because these techniques require thicker films resulting in improper ink film density on the sheets. Therefore these techniques cannot be used to measure the ink density obtained from the anilox roll.
Under the leadership of Ronald Saltzman of NAPIM, people got a better proofing method. Before this, people tried and evaluated a variety of methods, including: Prufbau, Little Joe, Pamarco with a squeegee, Cavanaugh squeegee ink proofer. Although no one method can completely match the printing density, the author believes that the Cavanaugh Squeegee Inkjet Proofer has certain advantages over other methods. Using this method, one can not only better simulate the printer density, but also match the color within a small range of color differences.
Ink Curing UV flexo originated in label printing. Initially, most low-speed narrow web presses were used to print stickers, and a 300 W/line inch medium-pressure mercury vapor lamp was used to obtain a satisfactory cure.
As the speed of printing presses continues to increase and the web size continues to increase, the curing ability of inks has also been challenged more fiercely. In order to keep up with the increasing demand for curing speed, the curing lamp technology has also been continuously improved.
The problem is the curing of UV flexo inks on different substrates, including flexible films for general flexo printing (without support). Can the new technology be adapted?
Before discussing the curing problem, first of all pay attention to the medium pressure mercury vapor lamp will generate a lot of heat, probably in the vicinity of its 50% spectral output. There is no doubt that these heat will cause the film to shrink or deform.
Using one or more of the following methods will minimize the amount of heat generated, resulting in a satisfactory cure.
1. Special heat-removing "two-color" filters;
2, cold water type quartz filter;
3, air cooling;
4, low-power high-efficiency lamps;
Using a low-power, high-efficiency lamp (D-type) allows for a thicker ink with better curing. Although the overall spectral output of this lamp is similar to the spectral output of a standard mercury vapor lamp, its spectral band shifts toward long wavelengths. As a result of this movement, the spectral window partially blocked by the absorption of the colorant is also effective for the light absorption of the photoinitiator, thereby initiating the photochemical reaction.
Many people believe that the demand for low-calorie lamps will increase as people's demand for printing on flexible films grows. The "Excited Atom" lamp received a better recognition because it was considered to be completely heat-free. There are many photoinitiators that work effectively with this type of "excited atom" lamp. Their absorption range is generally 280-310 nm. Experimental data shows. It is best to reduce the normal amount of photoinitiator to obtain the curing effect of the surface and the inner layer.
End-use applications UV flexo printing is now widely used in a variety of printing applications, including packaging. There are different narrow, medium, large format rotary presses and CI printers to produce the final print. The question often asked is: "Can UV flexo be used in food packaging?" "Is it checked by the US Food and Drug Administration?"
The answer to the last question is negative, and the answer to the first question is more complex and requires further discussion.
Regardless of whether it is a rotary printing or a sheet-fed printing, the front side of the printed sheet may always come into contact with the back of the printed sheet. Therefore, the ink may be transferred to the back surface, such as the inner layer of the actual package, resulting in the ink directly or indirectly contacting the food. Whether the ink will come into contact with food depends to a large extent on the type of functional barrier between the food and the residual ink. The purpose of a functional barrier is to prevent the ink from coming into direct contact with the food while also preventing any transfer of material through the barrier.
As we all know, photoinitiators and photochemical by-products can cause odors. These odors will come into contact with food and change the original flavor of food, making it unacceptable. Phenol and other sulfur containing photoinitiators are no longer used because they produce odorous by-products. Even the use of the BDK is denied by the fact that it produces a fruity taste of methyl benzoate through a decomposition method accompanied by thermal decomposition.
Therefore, the ink used for food packaging must fully comply with the following formula:
1. A monomer or oligomer that is very light in nature;
2. High purity ingredients;
3. Light-inducing photoinitiators (eg, phenols, phenol derivatives, and derivatives of sulfur-containing ketones cannot be used);
4. The ink has good curability;
Multi-color and monochrome printing proves that under the ideal conditions, the effect of UV flexo printing can be comparable to that of offset printing. Ideal conditions include the use of the most suitable plate, anilox, chemical composition, and binders to configure the correct ink, press, and operating environment.
Today's photosensitive resin plates can provide very high print quality. There are many coated and non-coated photosensitive resin plates with different hardness readings available. It is best to consult the manufacturer of the media about the choice of media.
As with the plate, there are many options for the anilox roller's engraving mode, number of ink holes, and ink volume. In general, 200-400 lines/2.5-10bcm Anilox roller is suitable for monochrome printing, and 700-1000 lines/0.8-2.5bcm Anilox roller is suitable for multi-color printing.
Although the choice of plate and transfer roller is very important for the print quality, the effect of the viscosity of the ink on the final quality of the print is very important. Whenever possible, higher viscosity inks should be used for four-color printing for minimal dot gain. On the other hand, lower-viscosity inks should be used for printing on the field, because when the pinhole and cellulite phenomena are controlled to a minimum, a solid-looking effect can be obtained.
Health As a general category, acrylate monomers are considered to stimulate the skin. Most acrylics are irritating to the skin because they contain a variety of impurities, including acids. These products are no longer used by the industry. Although no chemicals can prove to be "safe," UV/EB curing monomers are safely used by many people and do not produce any skin-related problems. However no matter how safe the product is considered to be, the supplier always reminds the customer to follow good chemical hygiene practices.
For many years, customers have always relied on the relevant index of skin irritation to decide whether to use or how to use a monomer. Skin irritation is obtained by experimenting on rabbits. However, for the protection of animals, similar routine experiments are their biggest hidden danger. Large chemical companies do not use animals for experiments, and more and more experiments and processes no longer use animals. Our industry has a responsibility to find a way to not only continue to provide customers with credible information on safety and health, but also to meet the humanitarian needs of our society.
UV flexo will continue to grow, mainly
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