The Key Elements of Sublimation Heat Transfer Printing
Courtesy of: Marty Pearson, Advanced Innovative Technologies. Mr Pearson has been working with AIT/Astechnologies/Simplex sublimation printing equimpment since 1974.
Heat Transfer Printing - Sublimation
The use of a heat transfer press for sublimation printing of polyester and other fabrics with “sublimation” dyes is a technology that has been in use for over 30 years. The process is environmentally clean and the vibrant colors and clear images give the process many advantages.
New textile technologies produce polyester fabrics that look and feel like cotton, rayon, and even silks. Also, new technology has produced polyester fabrics for athletic wear that are warmer/cooler than traditional cotton.
New digital technology has been introduced in recent years which allow a design to be created and printed onto a special transfer paper, and then transferred onto the fabric using a heat press. The entire process can be completed within a matter of hours. Samples and short run production can now be printed, sewn and shipped within days instead of weeks.
Definition of "Sublimation" Printing
A process of printing synthetic fabrics and other substrates using special, “subliming” dies which turn from a solid into a gas at a specified time, temperature and pressure. Under pressure these dyes in gas form are absorbed and encapsulated with pinpoint accuracy into the substrate. The gas returns to solid form once encapsulated. The result is a permanent print which will not wear off and is washable and with withstand dry-cleaning.
The Key Elements of Heat Transfer Printing
There are three integral specifications necessary to achieve optimal heat transfer printing - time, temperature and pressure. All are interrelated to each other during the process, i.e. more time - less temperature; more pressure - less time and vice versa. The relationships are not proportional, but do have an effect on the quality of the print.
The period in which the transfer paper and substrate is subjected to heat and pressure. On a continuous, rotary drum machine, this is a function of belt speed and the length of the heated area. On a platen style machine such as the Astex/AIT 354CR, it is the time that the heads are closed on the machine. Twenty to thirty seconds is a typical range for traditional off-set, screen printed and E-Stat printed paper.
The time variance is dependant on the dye formulation and the substrate to be printed. Forty to forty-five seconds is typical on ink jet printed transfer paper. On all Astex/AIT machines, the time is automatically set with a dial on the control panel. As an example, the Astex 7960T rotary drum machine has a belt wrap on the heated drum approximately 24' in length giving it a production speed of 5.93 feet per minute at a 20-second dwell time.
The process by which the transfer paper and substrate is uniformly introduced to the heat source of the machine. Pressure requirements are relatively low (2 to 6 psi) but it is critical that the pressure is evenly distributed.
The Astex/AIT rotary drum heat transfer machines, including model 7300IJ, apply pressure in a conventional, time-proven method of belt tension applied to the contoured heated surface of a heated drum. This requires a special high temperature belt that has characteristics allowing it to conform evenly and firmly to a curved surface.
Astex/AIT platen style machines such as model 165CR use pneumatic, hydraulic or electro-magnetic (see model 1112) systems to apply the pressure. A common misconception is that 80 to 100 psi of pressure is needed for sublimation. This value only represents the amount of air pressure brought to the machine, and is displayed on the air gauge. The actual applied pressure (2 to 6 psi) is a calculation of the cylinder size and plate area.
Traditional sublimation inks used in the apparel and textile industries for the past 30 years have required temperatures in the range of 400°F. These printed products have to perform well in wash and dry-cleaning tests.
The temperature must be maintained in the range of (+/-) 5° F, or there can be a shade differential in the print. The nature of the heat transfer printing process makes this a fairly complicated specification to control. Heat is constantly withdrawn from the heat source and must be replaced with a control standard of +/- 5° F. As a further complication, the heat is constantly moving toward the outside edges of the heat source with or without substrate withdrawal of heat. Astechnologies pioneered the multi-zone heat control system and was the first manufacturer to provide separate control of the heat across the width of the heated drum to achieve absolute security in temperature variance.
Always use AIT temperature test strips to verify actual temperatures and calibrate your machinery.
Added Value Elements
The heat press is the focal point of the heat transfer process. As the last step in the process, the printed product emerges from the heat transfer press. The heat transfer press must provide user friendly and precise control over the functions of time, temperature and pressure in order to produce the very best possible print on a wide variety of substrates.
The key elements of added value in the heat press are productivity, quality, after-the-sale service and price.
The Astex/AIT range of heat transfer printing machines is purposely designed to provide a choice of models, so that our customers can buy no more or no less than what is required for their specific production needs. For example, the Astex 7960T is designed to run at a maximum production speed of 5 feet per minute. This is the model which would normally be preferred and be the best value to a purchaser of a digital printing system like E-Stat which prints four-color transfer paper at 1.5 to 2 feet per minute.
However, conventional, offset, or screen printing sublimation systems have had a need for a more productive machine, for example our Model 7460 (14 feet per minute) or our Model 7072 (21 feet per minute). In these cases, the best 'value' in productivity was a larger, more expensive model. The lowest cost option for rotary heat transfer printing is the AIT model 7240IJ featuring a printing width up to 40'.
Precise control of the functions of time, temperature and pressure at the desired or optimal production speed is required for consistent and repeatable color transfer. All rotary drum heat transfer machines are limited in production speed by the length of the belt contact with the heated drum. AIT offers several models to operate at various production speeds.
Astex/AIT machines have superior control over temperature with our multi-zone heat control system and a proven pressure control system assuring consistent quality.
Astex/AIT machinery is competitive in the initial purchase price with several ranges of productivity. However, the true value of Astex/AIT machines is found in state-of-the-art technology and after-the-sale support. A lower cost machine that does not produce to a desired quality or production speed is not of value. The cost of a production machine sitting idle because of a lack of a part or service support will quickly equal any initial cost savings.
AIT provides customer support through authorized distributors and an internal service department. Our Service Department is supported by a group of trained technicians who are dispatched on a daily basis throughout the world, for technical support as well as the installation of our systems.
After-the-Sale Service – Over 30 years of manufacturing heat transfer machinery assures our customers of value with machine reliability and availability of parts. Technical support is available from an internal service department with an extensive mechanical knowledge of the equipment and the process.
For additional information, see the companion article "The ABC's of Heat Transfer Printing"