This discussion looks at the consequences of poor temperature control, both in the dryer and during transfer appliction.
By Rick Davis
Last month, we reviewed the parameters for measuring temperature and heat exposure during flash curing and drying in direct garment printing. This month, we'll consider the testing methods and equipment required to assess the temperature that heat transfers are exposed to in production and during final application. Heat-transfer applications bring an additional twist to the process of controlling the heat to which your prints are exposed. This extra concern involves the fact that heat-transfer prints face heat exposure not only in the dryer after being printed, but also when the prints are applied to the garment with a transfer press. One of the biggest differences between measuring temperature in direct printing and in transfer printing is the fact that heat transfers are initially printed and cured on a carrier sheet. The process of properly gelling, and not overcuring, the ink on the transfer paper is a delicate task that requires proper temperature testing and control. Without taking these precautions, you can't guarantee that all the remaining aspects of the transfer process will go according to plan. Gelling the ink film The curing parameters for heat-transfers jobs are different from those of any other garment-printing application. With heat transfers, the printed ink film must be brought up to a "gel" state rather than completely cured in order to adhere well when the transfer is later applied to a garment. The ink film printed with plastisol transfer inks requires exposure to temperature in the 210-250°F (100-120°C) range in order to properly gel. If the ink film is not adequately gelled, it will remain in a sticky state. This can result in finished transfers sticking to one another when stacked. When transfer prints are exposed to insufficient heat, the plasticizer in the ink film will not properly absorb with the ink's PVC resin, which leads to the sticky ink film. At the other end of the spectrum, excessive amounts of heat will also lead to less-than-desirable results. Namely, the high heat will cause too much absorption of the plasticizers within the ink film, which will deter the ink's ability to properly remelt and bond to the fabric during the transfer application. The best means for gelling a plastisol ink film on a transfer-release paper is by slowly heating the ink film. Trying to rush the process will leave you with transfers that won't perform as intended. For example, I have seen printers who tried to employ flash units as in-line heaters to gel the ink film on the release paper. The result was that they ended up with stacks of wasted transfers. To bring transfer prints to their proper gel state, your best bet is to employ a radiant-heat dryer and allow the transfers to reach their gel points over a longer period of time. I suggest 45-60 sec in a dryer, rather than 5-7 sec under a higher-temperature flash unit. Temperature testing You can test the temperature that your transfers are exposed to during drying with heat tapes or more expensive doughnut probes. I've found that heat-sensing tapes work very well and are especially convenient for a production environment. A doughnut probe might provide a more precise reading, but it would require you to hold up production while the probe is sent through the drying chamber. Additionally, you might try using a non-contact pyrometer, but the temperature reading would only show how hot the transfer was upon exiting the dryer, not how high the temperature climbed as the transfer was moving through the dryer. For measuring the output of radiant-heat conveyorized dryers, heat tapes will provide a ballpark value of the heat that the transfer receives. "Ballpark" values are adequate for this type of measurement since no method will tell you the absolute temperature reading in a production environment. As mentioned last month, a heat tape feature heat-sensitive inks that react to specific temperature ranges. These inks appear as a series of small squares on the surface of the tape strip. Each square changes color when a specific temperature is reached (each subsequent square typically shows a 5 or 10°F temperature increase.) By assessing which square was the last to change color, you can determine the approximate temperature reached. Of course, the most definitive test you can perform to verify that heat transfer have been cured adequately is to take a finished transfer and apply it to a garment. If it transfers as expected and bonds well to the garment, the job can be considered successful. To further ensure that your curing parameters were correct, conduct wash tests with transfers you've applied to garments. This will guarantee that they'll withstand laundering by the end customer. Transfer storage Controlling the ambient temperature of the storage environment is another critical concern with heat transfers, especially those featuring multicolor designs. Transfer papers are susceptible to temperature and humidity changes, and any vast variations in these two parameters can have an adverse effect on the color-to-color registration of the transfers. You can take several precautions to minimize the adverse effects of humidity and temperature on your transfer. Storing them in a room with controlled heat and humidity is one of the best ways to go. Typically, you should store transfers at 75-85°F (24-30°C) and 30% humidity to minimize size changes in the transfer paper. As an additional precaution, you can place the transfers into plastic bags as they come out of the dryer. In order to properly monitor and maintain the conditions within this controlled storage environment, you should invest in a good thermometer and hydrometer. The transfer process Assuming you've controlled curing temperatures and arrived at a properly gelled ink film on your transfer, they should now be ready for application to the garment. If your facility performs the actual process of transferring the images onto garments, keep the following suggestions in mind. Many printers make the mistake of assuming that transfers are more forgiving than direct garment printing. They think, "If I'm just applying an 'iron-on' transfer, how hard can it be?" The truth is that if you do not properly control the transfer process, odds are quite good that you will wind up with an inferior product. You need to keep three primary parameters in mind when applying transfers in order to get good adhesion and a durable applied print. These parameters include temperature, time, and pressure. Any unexpected variations in these three variables can adversely affect the quality of the finished product. Most plastisol transfers are applied at temperatures of 370-375°F (188-190°C). At this temperature level, the printed ink will remelt, allowing it to transfer to the garment and bond with the garment fabric. If this temperature level isn't reached during transfer application, the result will either be poor adhesion with cold-peel transfers or poor splitting characteristics with hot-split varieties. The best test method for determining that you have the correct temperature on you heat-transfer press is to use an on-contact pyrometer. These units are placed in contact with the heating element (platen) in order to measure the temperature that the unit delivers. On-contact pyrometers are the preferred measurement tools since they make direct contact with the heating element and lead to accurate temperature readings. Not only can this tool present dependable temperature measurements, it will also identify any temperature variations that may occur across the transfer area of the platen. Non-contact pyrometers are applied in this application from time to time, but they are not as accurate or consistent as the on-contact variety. Whatever the tool you use, if your measurements indicate any temperature variations on the platen, you need to address the problem immediately and discuss it with the transfer-press manufacturer. Conclusion The variables involved with the production of heat transfers are more troublesome than most realize. You must remember that curing transfers after printing is unlike curing direct garment printing, where you want to achieve a complete cure. With transfers, you care only trying to gel the ink to a degree that allows it to withstand handling and results in a durable graphic when it is applied to the final garment. Using temperature-measurement tools and regular monitoring procedures can help you achieve these goals.
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