Digital ceramic printing on glass

Digital ceramic printing on glass is a technological development used for the application of imagery, pattern or text to the surface of flat glass. Like other printing on glass methods, it uses a form of printmaking. Digital ceramic printing on glass has allowed for new possibilities and improvements in flat glass decoration and treatment[1] such as high levels of customization, translucency and opacity control, light diffusion and transmission, ability to calculate solar heat gain co-efficiency,[1] electrical conductivity, slip resistance, and reduced incidences[spelling?] of bird collision.

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History

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Unlike paper or fabric, glass is nonabsorbent and transparent so applying digital printing technology had to be adapted to overcome the challenges presented by the glass itself. Until 2007 the two main methods of printing on glass were silk screen printing and digital UV printing. Silk screen printing, where the ink is applied directly onto the surface of the glass through a mesh stencil, was patented in 1907. Screen printed transfers, where the image is transferred from a paper onto the glass, was patented in the 1930s by Johnson Mattey. Firing is necessary in both methods in order for the ink to be permanently infused with the glass.[2]

Printing on glass with UV pinning and curable inks came about almost 60 years later. In this method of printing, ultraviolet waves are applied on the inks, drying them to the glass. This method was the first to enable the digital printing on glass of any digital image including multi color and complex images. Since UV curable inks are not fused with the glass the same way ceramic inks are, the printed outcome lacks a level of durability necessary for certain projects, namely external applications for automotive glass and architectural glass.[3]

Process

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Digital printing with ceramic inks, desirable for decorative, functional and environmental purposes, poses a new set of challenges addressed through technological innovations. At the most advanced level, digital glass printers, ceramic inks, and image processing software are fully integrated with one another and each contributes to the overall advancements in the digital printing on glass process.[4] The three part system allows for control and flexibility over the application of the ceramic inks. Transparency and levels of translucency and opacity can be precisely manipulated. There is a high level of control over color matching, and multiple colors can be printed simultaneously. Unlike screen printing, digital ceramic printing on glass does not require screens and the files are stored digitally making printing of all sizes and replacement of any panel simple, in high resolution, full color.[3]

When using ceramic frit based inks the glass is fired or tempered to fuse the inks with the glass. Due to the extreme temperatures of this process there is first a decomposition of organic additives and binders of the ink. Next there is a fusion of the frit to the substrate and pigments followed by the expulsion of voids to give a compacted structure. Lastly there is a formation of a surface with the desire properties. A successful firing of the glass and ceramic ink will result in a bubble free layer of constant thickness and homogeneous pigment dispersion within the glass.

Essential elements

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Digital glass printer

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The digital glass printer is a flatbed digital printer designed with print heads to jet ceramic inks directly onto the glass.[5] The glass remains stationary while only the printer carriage sweeps across the print table. A key feature of the printer is drop fixation in which ink droplets are dried immediately to prevent drop gain. The fixation of the ink enables a single pass of the print carriage even when printing multi layer and multi color files. The drop fixation makes inline double vision printing possible. Double vision is creating a different vision depending on which side of the glass is being viewed it is achieved by printing different graphics one on top of the other. An inline dryer was developed for real time drying to occur and to maximize factory space. A smooth color switching system in included so machine operators can easily shift between print jobs and increase throughput. The high resolution print quality - up to 720 dpi - and the precision of the printers allow glass processors to print anything from fine, sharp, small elements to complex full color images on glasses up to 3.3X18 meters in size.[4]

Digital ceramic inks

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The inks used in digital printing on glass mimic the CMYK color model and are made of ceramic frit and inorganic pigments and elements. The development of inks is a highly controlled production process to remove any variability in the final product. The consistency in a replication of the inks results in high compatibility with the entire color palette of inks. The inks can therefore be digitally mixed and designers will know the precise outcome of the color every time. Printed glass panels can also be replaced when necessary without the risk of the new panels not matching the colors of the existing panels. The inks are also fully integrated with the machine and the image processing software meaning the development and the application of the inks required innovations both in science and technology.[4]

Image processing software

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The image processing software bridges the glass printer and the inks and is also the design tool for preparing the graphic file for printing. The software is more than a photo raster; it calculates ink usage to control levels of translucency and opacity, to control color matching and mixing, and to compensate for different glass sizes and thicknesses. The precision and complexity of the calculations and measurements executed by the software allow designers can achieve their desired outcome. Digital ceramic printing on glass has expanded the options for printing on glass. UV and silk screen printing have limitations that digital printing overcomes. Digitally printed glass can be applied both to the interior and exterior surfaces, the most simple to complex graphic illustrations can be printed in the CMYK color model.[5]

References

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• Printed glass performance [1]
• Digital glass printing [2]
• Digital printing on glass - Bird-Safe Solutions [3]

When screen-printing companies decide to expand their product offerings, an area that many explore is the market for decorated glass and ceramic products (Figure 1). But lack of familiarity with the methods and materials used in glass and ceramic printing usually steers all but the most adventurous printers away from such work. This article will attempt to clear up the confusion related to printing on these substrates by reviewing key attributes of glass and ceramic materials and the inks and printing technologies used to decorate them.

Substrates

The best place to begin our discussion is by defining the substrates used in this market. Ceramic materials are hard, brittle, heat- and corrosion-resistant substrates made by shaping and then heating a non-metallic mineral, such as clay, at a high temperature. Enamels, porcelain, and bricks are other examples of materials that are produced by molding or shaping minerals and baking or firing them at high temperatures.

Glass products are typically made by fusing silicates with boric oxide, aluminum oxide, or phosphorus pentoxide at high temperatures. They have highly variable mechanical and optical properties and solidify from the molten state without crystallization into a transparent or translucent form. While glass items are generally hard and brittle, their lack of crystalline structure puts them in the class of amorphous solids (solid materials with a random rather than geometric molecular structure). Glass items that may require printed graphics include windows, mirrors, cooking utensils, bottles, containers, and more.

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The common property of both glass and ceramic materials is that they have to be heated to high temperatures after printing to ensure a durable decoration. The main difference is that formed glass can be melted and reformed, whereas ceramic materials can only be formed and fired once.

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From a printing point of view, several methods are available to decorate glass and ceramics with high-quality images. In this article, the main methods we’ll focus on will be screen printing, pad printing, and digital printing with sublimation inks.

Inks

The printing processes used for glass and ceramic printing rely on a variety of ink systems. Other than sublimation inks, which we’ll address later, most inks fall into one of two families: organic and inorganic. Organic inks are typically used in screen and pad printing, and they consist of organic pigments and resins along with other chemistries that cure over time and rely on temperature or some other form of energy to create a bond with the substrate. Inorganic inks use mineral-based pigments and materials that, once printed, have to be heated and melted at high temperatures in order to combine with the substrate surface and form a permanent bond. In both cases, the inks can either be applied to the substrate via transfers (decals) or printed directly onto the substrate.

Organic inks

Ceramic and glass materials are non-absorbent, which means that the inks need to adhere to the surface. The most effective organic inks are produced as two-component or two-part systems. These inks generally contain resins capable of polymerization that are blended with catalysts to initiate polymerization. Heating the products to a temperature of approximately 390°F (200°C) after printing may accelerate the curing process and improve adhesion. In addition, such heat exposure will typically enhance the mechanical and chemical resistance of the print. After printing, organic ink films will require at least 48 hr to polymerize unless heat is applied.

Different types of two-component organic inks are available, including e-poxy- and polyurethane-based systems. An epoxy system provides better elasticity than a polyurethane system, but the gloss and weather resistance of a polyurethane ink is considered superior. On the other hand, polyurethane formulations will not be as scratch resistant as enamels and will split off relatively easily. Solvents included in both of these organic inks allow the ink-transfer mechanism in pad printing to operate effectively and facilitate easy adjustment of flow characteristics for screen printing.

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An often overlooked parameter when working with these inks is the effect that ambient temperature can have during the curing process. After printing, the ink film must be cured completely before it is exposed to low temperatures. If it faces temperatures below 5°F (-15°C) before it is completely polymerized, the curing process halts and cannot be restarted.

You must take care when mixing the curing catalysts into these inks–the weight ratios of catalyst to base must be correct. Altering the ratio can dramatically affect the adhesion and chemical resistance of the ink after curing. It’s also important to note that these ink systems have a limited pot life: Some will only be usable up to 4 hours.

Because they are not fully resistant to chemical and mechanical wear, organic ink systems are best suited for applications in which chemical exposure and abrasion are not extreme. Prints created with these inks are unlikely to withstand the effects of automatic dishwashing, so using them on dinnerware is not suggested. But they are generally suitable for less demanding applications, such as cosmetic containers.

An emerging class of ink in this category is new, UV-curable formulations. These inks provide many of the same resistance properties as conventional organics but cure much more rapidly when exposed to ultraviolet radiation.

Inorganic inks (ceramic colors)

Ceramic colors, as inorganic ceramic inks are called, are a mixture of pigments (metal oxides and salts) and finely ground glass particles, called frit. These materials are fused to the substrate by calcining (also called “firing”) them at temperatures between 1100-2600°F (600-1450°C). Firing temperatures vary depending on the make-up of the color, the nature of substrate, and other application criteria, but in all cases the temperatures must be carefully controlled to achieve specific colors after firing.

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The reason for the extreme temperatures is because the components of ceramic colors need to be melted so they can fuse to the ceramic surface on which they’re printed. Also, while these inks are called “inorganic,” they do contain small amounts of organic material. The organic components are the materials in which the pigment and frit are suspended to create a printing ink. These organic materials, which are oily in nature, are designed to burn off rapidly during firing without affecting print quality and final color.

When printing onto ceramics, colors can be printed on glaze, in glaze, or under glaze. A wider range of colors is available for on-glaze decorating than for in-glaze or under-glaze jobs. On-glaze colors, which are printed on top of the glaze coating on a ceramic item, are more resistant to chemical and mechanical wear than organic inks, but they still can be affected by automatic dishwashing and other abrasive environments. With items destined for demanding environments, in-glaze inks (which are sandwiched between layers of glaze coating) and under-glaze inks (which are printed prior to the glaze coating) are preferred.

Lead content is a crucial concern with all of these ink systems. Traditionally, lead has been used to enhance the color range and to improve firing characteristics. Legislation now specifies either very minimal levels of allowable lead content or no lead at all. Other heavy metals frequently found in ceramic colors, such as cadmium, must also occur in very low levels.

With inorganic inks, the appearance of the final colors is determined by how the printed ware is fired. Temperature, time, and atmospheric conditions within the kiln all influence the chemical interactions that occur during firing and, consequently, the print’s resultant colors and resistance to wear.

Inorganic inks come in various forms. These include screen- and pad-printable process-color formulations, thermoplastic varieties, and total-transfer inks. Both the screen-printing and the total-transfer systems are known as “cold color” inks, which means they do not have to be heated to become printable; the thermoplastic inks must be heated before they can be applied to the substrate.

Process-color ceramic inks Those of you who are experienced in process-color screen printing know that it is a relatively straightforward method. The process combines yellow, cyan, magenta, and black halftone dots in various arrangements, which are visually blended and perceived as a broad range of colors on the final image.

With ceramic inks, the approach is different. Rather than using halftone dot arrangements to emulate colors, the transparent process-color ceramic inks are overprinted to create new colors. While the overprinting of primary colors–cyan, magenta, and yellow–can be used to create a black appearance, a separate ceramic black ink is generally printed to deliver a richer color. It’s important to note that these inks don’t achieve their true final color after firing, which can complicate accurate color matching.

When firing ceramic process colors, controlling the conditions is crucial. Besides proper temperature, the correct atmosphere must be present within the kiln because it influences the way inorganic pigments react (whether they are oxidized or reduced), which greatly influences the resulting color. Magenta is most sensitive to the firing environment.

Pad printing can also successfully be used with process-color ceramics. Plaques with multicolor images are frequently decorated with this process.

Thermoplastic colors Thermoplastic ink systems are waxy at room temperature and have to be heated up for printing (Figure 2). For pad printing, the ink trough, plate, and occasionally the pad are kept at a temperature of approximately 140°F (60°C). When the pad carrying the ink comes into contact with the cold object to be printed, the ink cools and sticks to the object.

When screen printing with thermoplastic inks, the mesh is made from stainless steel and an electric current is passed through it. This heats up the screen and melts the ink, which then flows through the mesh and solidifies when it makes contact with the cold ceramic or glass. Controlling current flow is critical because too much will overheat the color and burn out the mesh.

In both cases, once printed, the item has to be fired to form a permanent image. This is done in a high-temperature oven called a kiln or lehr.

The main advantage of thermoplastic ink systems is that they facilitate printing multicolor images on the same machine. Cylindrical glass bottles can be printed at rates of up to 100/min and flatware at speed ups up 600 pieces/hr.

Total-transfer printing This technique combines screen printing and pad printing in a single process. The image is first screen printed onto a silicone-rubber blanket. The pad then picks up the screen-printed image from the blanket and transfers it to the ware. The decorated product is then fired in the normal way. The reason for using this process is to enable relatively thick deposits of ink to be printed with the pad-printing process (Figure 3).

Transfers

There are two types of ceramic and glass transfers. The most common is waterslide. Here, the image is printed onto a paper that is treated with a water-soluble coating. Once ceramic inks are printed onto the coated paper, a cover coat is printed over the image. When the printed paper is immersed in water, the soluble base coat dissolves, allowing the ceramic colors that are bound with the cover coat to float free as single film. This film is then positioned on the ware by hand, and the piece is fired in a kiln.

The other variety is called heat-applied transfers. Here, the full image is printed onto a paper or polyester carrier that has a release coat on the surface. A heat-activated adhesive is printed over the image instead of relying on a cover coat. The transfer is pressed onto the glass or ceramic surface by a heated silicone-rubber blanket. Heat activates the adhesive, causing the transfer to stick to the ware. The item is then fired in the normal way.

Sublimation decoration

This is a very popular method of decorating a whole range of items–particularly ceramics. The item first has to be coated with a polyester lacquer. This coating absorbs sublimation inks, which turn from a solid form into a gaseous state as they are applied to the ware under heat and pressure from a heat-transfer press. The main drawback with sublimation decorating is that the image will only last as long as the polyester coating on the substrate.

Sublimation images are often created digitally using sublimation inks on transfer paper. The printed transfer is then applied to the substrate. This process is ideally suited to applications where the number of pieces to decorate are relatively low and permanence is not required. (For more information on digital sublimation transfers, see “Custom Decorating with Digital-Transfer Technology,” Screen Printing, Jan. ’03, page 58.)

Conclusion

Glass and ceramic items have been decorated for hundreds of years. While the range of materials used to embellish them has expanded, the decorating techniques employed today have changed little over the years. The main advancements that have occurred relate to improving the control and precision that glass and ceramic decorators are able to achieve. By familiarizing yourself with these developments, you can make glass and ceramic decorating a valuable addition to your product offerings.

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