Learn about heat-transfer presses for textile appliations.
By Lori Leaman
Heat transfers are once again drawing attention from screen printers. After portability brought them to center stage in the '70s, they fell out of grace during the '80s due to poor image quality, lack of durability, and a heavy hand that made heat-transferred apparel seem like armor. But over the last decade, ink manufacturers have been working hard on new hot-split plastisols and specialty transfer inks, which have made transferred images virtually indistinguishable from designs directly screen printed onto garments. The rebirth of heat transfers has also been driven by the introduction of assorted digital-imaging systems over the last seven years or so. Today, many shops are turning to digital heat-transfer production to satisfy prototyping needs and produce short-run custom orders. These developments have been matched on the equipment side with sophisticated heat-transfer machines for applying the images to garments. If you're presently involved in heat-transfer production and application, or are thinking about investing in the technology, the current selection of heat-transfer presses deserves a closer look. Press types In basic terms, a heat-transfer press sandwiches a garment and a heat-transfer image between a heated upper platen and a lower platen that is usually coated with temperature-resistant silicone or Teflon. During operation, the press delivers heat and pressure to the garment/transfer combination, and, after a period of time, is raised again. Then the garment is removed, the transfer paper is pulled away from the garment, and the image remains affixed to the fabric. Heat-transfer presses are available in a broad range of configurations. But most fall into one of four categories: clamshell, swing away, platen slide, or shuttle. Clamshell The most popular transfer-press design, the clamshell is well suited for working in limited space (Figure 1A). All clamshells feature a heated upper platen that is hinged in some manner at its rear side and can be raised away from a stationary lower platen to allow garment and transfer loading. After loading, the upper platen is lowered onto the garment, applying heat and pressure for a specific amount of time. Clamshells are offered in both manual and semi-automatic varieties. On manual models, you pull down the handle to lower the upper platen and lock the heat platen in place, then set the dwell time. At cycle completion, you are alerted by a bell timer or buzzer and then manually raise the top platen by pulling up on the handle. On automatic models (which are driven pneumatically), you also pull the handle down to lower the heat platen. However, air-pressure assists with the downward motion, making it much easier to lower than all-manual models. Another difference is that at the end of the cycle, the press opens automatically. Swing away This model features a heated upper platen that automatically raises parallel from the lower platen after the transfer cycle and rotates out of the way so that the next garment and transfer can be loaded (Figure 1B). Swing-away presses minimize the risk of burns to the operator, which is an especially useful feature for jobs that require the operator to manually line up diecut letters or numbers for application. Platen slide On this model, the lower platen automatically moves down and slides forward from beneath the stationary heated platen after the transfer cycle (Figure 1C). After a garment is loaded and a transfer is positioned over it, you start the transfer cycle, causing the lower platen to retract. This press type also reduces the chance of operator injury from contact with the heated platen. Most platen slide systems are pneumatic, and after the garment and transfer are loaded, operators simply push a button or depress a foot pedal to start the transfer cycle. Shuttle For more efficient production, twin-platen shuttle systems are available (Figure 1D). Similar to platen-slide systems, these presses feature lower platens that move in and out from beneath the heated platen (on a belt or similar indexing mechanism). But they include two lower platens so that while one is being loaded, the other is undergoing the transfer cycle. High-productivity transfer presses Besides the four categories listed previously, you could also add a fifth category for high-production machines. While many of these devices may incorporate features similar to standard press types, they use a variety of methods to deliver garments and other textiles for transfer application. Among these systems, you'll find carousel units, continuous rotary presses, and roll-to-roll devices. Carousel models are offered with up to six lower and three heated upper platens and can be operated by up to three users simultaneously. On a one-operator system, a garment is loaded onto the lower platen, then rotated under the heated head while another platen rotates out toward the operator. While one garment is undergoing the transfer process, another is being loaded onto the second platen. Two- and three-user systems operate under the same principle, but the print heads and loading stations are staggered so that the garments/transfers one user loads are removed by the next user in the circuit. Rotary conveyor heat-transfer presses are another option. These systems feature a conveyor belt on which the operator loads the garment/transfer combination. The conveyor moves the materials into the unit, where they are delivered around a cylindrical, heated drum and driven between rollers that apply pressure to fuse the image to the garment. Finished garments are delivered through an exit conveyor. Similar to the rotary conveyor, roll-to-roll transfer presses are also available. Rather than transferring images to individual garments, these systems are used for transferring to bolt fabrics and other roll materials. The transfer images themselves are also on continuous rolls. During operation, both rolls are fed into the unit with the transfer image against the material surface. The materials thread their way through a series of rollers that exert pressure and force the materials against a heated drum, which transfers the images to the final material. When the materials exit the unit, they are taken up by separate spindles and rerolled for easy transport. On some units, the imaged material is cut or finished in some other way. Many models also remove the used transfer paper from the imaged material automatically after transfer application and roll it onto a separate spindle or into a waste bin. Roll-to-roll systems are especially popular for transferring images produced digitally on electrostatic printers. Selection criteria Regardless of the press type or features it incorporates, you have to consider several performance and design characteristics before deciding on a particular model. The main areas you need to investigate include temperature, pressure, dwell time, safety features, and ease of maintenance. Temperature Most transfer presses are designed to provide temperatures ranging from 250-450°F (120-230°C). This range is compatible with virtually all transfer materials, such as inks (hot-split plastisol, puff, suede, etc.), specialty materials (metallic foils), and transfer papers. These materials generally are designed for applications between 350- 400°F (175-205°C). So to accommodate the widest range of jobs, most press manufacturers recommend keeping heat-transfer presses set at approximately 375°F (190°C). The majority of heat transfers produced on digital printers with special transfer papers usually affix to fabrics under these temperatures as well (see "Working with Digital Transfers"). To monitor temperature for job-to-job consistency, heat-transfer presses offer two types of temperature controls: solid state and thermostatic. Solid-state controls continually monitor and regulate platen heat to provide a constant temperature level. Generally, these controls also provide a digital readout of the actual platen temperature. Thermostatic controls are thermometer based and found on less expensive systems. In many cases, they rely on the operator to monitor temperature on a gauge and physically adjust the control to maintain a consistent temperature. Keep in mind that a thermostatic system responds more slowly to temperature changes, so the temperature displayed may not always be accurate. Related to temperature is the issue of image size in relation to platen size. Heat-transfer presses are typically much cooler than the desired temperature near the platen edges. To avoid the possibility that portions of your image won't transfer because of problems from these "cold zones," the upper platen should be a few inches larger on all sides than the largest heat transfer you plan to apply. You also want a uniform temperature across the surface of the platen. Pressure Some level of pressure is required to ensure that the image transfers and forms a strong bond with the garment surface. But if too much pressure is applied, you may scorch the garment or force the image too far into the fabric, reducing the image's color quality and sharpness. Insufficient pressure can result in gaps in the transferred image or unpredictable color variations (if only a thin layer of the ink transfers to the garment). All transfer presses provide some method of adjusting platen pressure. On manual units, you apply pressure during the transfer cycle. To increase pressure, you simply hold the top platen down with more force. On automated machines (both mechanical and pneumatic), you preset the desired pressure during the transfer cycle with a control knob or digital control panel. Additionally, some pneumatic models (which require an air compressor for operation) automatically adjust pressure for different fabric thicknesses. Note that the platens on the press should be flat and parallel when they come together for transfer application. This ensures that you'll get an equal pressure level across the entire transfer. Dwell time Proper temperature and pressure are critical for successful image transfer, but only if the time period over which they are applied is right for the inks or transfer papers you're using (material suppliers can provide details about proper temperature / pressure/ dwell-time requirements for their products). On manually automated presses, a simple bell timer is often used to gauge whether the transfer has been applied long enough. Some units features more sophisticated timers that automatically reset themselves when platens are separated between transfer cycles. And on automated systems, you set the dwell time at the start of the run and it is automatically applied and reset by the press for each transfer cycle. Safety Because transfer presses operate with high temperatures, protecting the operator from contact with the heated platen is an important consideration. As mentioned previously, some press designs (e.g., swing away and platen slide) help to avoid operator contact with the hot platen either by moving it out of the way or repositioning the lower platen between transfer cycles so that it can be loaded without bringing the operator into contact with the heated platen. Additionally, some models require two-handed operation to prevent a free hand from slipping between the platens and/or have an emergency release button to quickly separate the platens in case of a problem. Maintenance The final areas to consider involve parts replacement and maintenance. Depending on the unit you purchase, components such as the heating platen, fuses, silicone-coated lower platen, or timer may occasionally need to be replaced. Make sure that the manufacturer has these components readily available for quick delivery and that they are easy to access, remove, and reinstall with basic tools. Other options For true versatility, it's hard to beat combination transfer presses. Most of these models are based on the basic clam-shell design and incorporate interchangeable platens that allow you to apply transfers to T-shirts, caps, and other garments. Some of the available attachments also allow you transfer designs to non-garment items, including mugs, puzzles, plaques, and more. At the top of the heat-transfer equipment ladder, you'll also find specialty presses in a number of configurations that are designed to serve multiple functions. In addition to transferring images to garments, these presses provide very high pressure levels that allow you to emboss and perform simple diecutting functions. One unit is even designed to be positioned at a print station on an automatic garment press, allowing the user to heat apply graphic images or metallic foils as garments are being directly screen-printed . Shopping smart Price will surely influence your decision to purchase a particular heat transfer press. But you should also think about how much space you have available and how many transfers you expect to apply per hour, shift, or week. Also consider whether you want to focus strictly on garments, apply transfers to other items (caps, mugs, etc.), or whether you need additional decorating capabilities, such as embossing. As you consider various models, look for a machine that will accommodate your future needs as well as satisfy your current demands. The machine you select should be durable and provide user-friendly controls. Additionally, you want a transfer press that provides accurate and consistent temperatures and pressure levels with gauges or digital displays that allow you to verify that it is operating properly. For consistency, you may want to consider units with automatic reset features and programmable job settings. Finally, make sure the system you purchase has a sound manufacturer's warranty. Working with Digital Transfers Since the early '90s, several options have emerged for producing heat transfers on digital color printers, including thermal wax, sublimation, inkjet printers, as well as color laser copiers (CLCs). The devices range in size from small, desktop units to more advanced wide-format machines. Each of these printers deposits CMYK inks or toners onto transfer paper. Thermal-wax printers use a ribbon coated with a wax-like ink. Sublimation printers use a sublimation-dye-coated ribbon (sublimation dyes turn into a gas when applied and penetrate and bond with materials containing or coated with polyester). Inkjets use a dye-based ink or a sublimation dye. And color laser copiers (CLCs) use a conventional toner or sublimation toner. Depending on the specific device and the application, the transfer paper used may be coated or uncoated. With coated varieties, the coating transfers with the image when it's heat applied to the garment, essentially sandwiching the image between it and the garment surface. Regardless of system type, if you're producing transfers digitally, you need to make sure that the transfer press will support the temperature and pressure requirements of the ink and/or coating on the papers you use. The time, temperature, and pressure controls found on most transfer presses make them compatible with the vast majority of digital inks, toners, and transfer papers. Certain materials and applications may require more pressure than what is normally applied, but most heat-transfer presses can provide this additional pressure. Follow the recommendations of your digital ink or transfer paper supplier to avoid problems.
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