Understanding Glass and Ceramic Decorating
Find out how these applications differ from conventional screen-printing jobs and examine the inks and processing methods.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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.)
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.