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What is Sublimation?Unlike other decorating methods, where a solid decoration is attached to the surface, a sublimated part is actually dyed. Webster’s dictionary defines sublimation as “to pass directly from the solid to the vapor state.” An example of sublimation is dry ice vaporizing without any liquid. In sublimation printing, heat vaporizes a solid dye (cyan, magenta, yellow or black), and pressure transfers it onto the substrate.
Dye penetration is needed to show the brilliant colors and to keep the dye from rubbing off. A ceramic glaze is too hard for dye penetration; therefore, the ceramic must be coated with a semi-crystalline polymer prior to sublimation. Polymers are different from other materials because they change from being hard and glass-like to being soft and rubbery when heated. Examples of polymers include plastics, rubber and polyester—such as T-shirts.
Sublimation ProblemsProblems encountered in sublimation are usually rooted in the process described in Figure 1. These include abnormally shaped parts, color matching, paper sticking and dishwasher/ultraviolet (UV) resistance.
Abnormally Shaped Parts. The uneven surfaces of ceramic parts can cause problems with sublimation. An uneven surface prevents even application of pressure, which is necessary to direct the gas to the part. Absence of pressure and direction causes blurring and light/blank spots. Creative methods of applying pressure—such as wrapping an elastic band around the part—or increasing the heat can compensate for these irregularities. Regular part measurements, however, can identify problem parts before they are decorated.
Color Matching. Color is a result of dye penetration into the coating. The amount of penetration can vary with the coating characteristics. A coating with a lower glass transition temperature would be softer at a 400°F sublimation temperature, allowing greater dye penetration and showing darker colors, but some sharpness would also be compromised. Heat and pressure also affect dye penetration, and can be adjusted for the coating. A quality program with color testing is critical to isolating the cause of a color matching problem.
Paper Sticking. Referring back to Figure 1, the coating becomes soft when heated. If the coating becomes too soft, either because of excessive heat or a lower glass transition temperature of the coating, the pressure may force the ink, and the paper, into the coating. Upon cooling, the transfer paper will be frozen into the coating and hard to remove. Removal of the transfer paper while the part is still hot is one way to prevent the sticking, as is reducing the pressure. However, if this problem is experienced as part of a standard testing procedure, where heat, pressure and paper are constant, it could be an indication of coating or equipment problems.
UV Resistance. Resistance to sunlight radiation is a function of the coating chemistry. Coating suppliers have developed chemistries that resist UV degradation in other industries, such as outdoor paints and automotive applications. But for sublimation, each coating must be custom-formulated to meet the sublimation decorator’s needs. In some cases, there may be a trade-off—imparting high UV resistance may compromise the coating strength, for example, or may affect another coating property. Developing a UV resistant sublimation coating requires a joint effort from both the sublimation decorator and the supplier.
Dishwasher Resistance. Dishwasher fading happens when the coating returns above the glass transition temperature. A higher glass transition temperature keeps a coating hard and glassy in the dishwasher, preventing the ink from escaping. Another cause of dishwasher fading is thin coating, which, although less expensive, allows water penetration to wash the dyes away. The American Society for Testing and Materials has a standard procedure (D2248-93) for evaluating dishwasher/detergent resistance of coatings. Ensuring dishwasher resistance requires communication with the part supplier about the glass transition temperature, coating thickness, and compliance to the established standard.
Developing a Quality ProgramThe most effective method of permanently eliminating sublimation problems is by identifying their root cause. What kind of problem is occurring? Where does it occur? Where is it most visible? And, most important, why is it occurring? Documentation of regular testing and good communication with your supplier can localize problems to their fundamental causes. Often, time or financial constraints necessitate quick fixes, but these methods rarely lead to consistent quality.
Standardized part testing can prevent nonconforming decorations from being produced. Testing must employ the same process conditions for the results to be credible. For example, suppose every batch of coated parts was tested for paper sticking using the same sublimation time, temperature and pressure. If the results were to suddenly change, but not the conditions, the cause can be found and fixed before bad decorations are produced. Test results can be organized using software like MS Excel, Access or by hand. Well presented standardized test data facilitates effective problem solving.
- Coating Quality/Consistency: Visually inspect parts, looking for voids or other defects. Figure 3 plots a percentage of parts with defects using Minitab Statistical Software. Points beyond the red lines indicate a higher than "statistically normal" number of defects.
- Color Matching/Paper Sticking: Print an image that contains the four primary sublimation inks (cyan, magenta, yellow and black). Save the samples and compare both color and ease of transfer release to future supplier shipments.
- Part Size: Ceramic parts often show dimension variation and should be measured before sublimation to prevent problems with uneven surfaces. Data can be plotted with Minitab Statistical Software, as shown in Figure 4. Points beyond the red lines indicate a higher than "statistically normal" amount of variation.
- Heat Press: After heating for sublimation, measure the part temperature. If equipment is not available for a direct measurement, immerse the part in water and measure the time required to reach a certain water temperature. Note that the same amount of water in the same container is needed to maintain accuracy. Data can be plotted as in the previous tests, and variations can indicate equipment problems. Additionally, a stopwatch can be used to ensure consistent time for the heat press to reach the required temperature.
Ensuring Quality Sublimation ProductsSublimation is a convenient method of printing photo quality images on ceramic parts. The process involves using heat and pressure to vaporize and direct a solid dye onto a coated ceramic part. The polymer coating softens to receive the dye and display the colors. As it cools and hardens, the coating stabilizes the dye permanently into the ceramic. Problems with sublimation are rooted in this process. By documenting pre-decorating inspection results, and communicating them to coated part suppliers, customers and suppliers can ensure quality sublimation products.
For Further Reading1. Chavan, R. B., and Lenger, Hanif, “Sublimation Printing of Polyester/Cotton Blends,” Textile Research Journal, Vol. 58 (Jan. 1988), pp. 51-56.
2. Moen, Nolan, Provost, Quality Improvement Through Planned Experimentation, New Work, McGraw Hill and Companies, 1991.
3. Schoff, Clifford, “Surface Defects: Diagnosis and Cure,” Journal of Coating Technology, Vol. 71 , (Jan. 1999), pp. 57-73.
4. Campbell, D. and White, J. R., Polymer Characterization: Physical Techniques, New York, Chapman & Hall, 1989.