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A Primer on Joining Technologies

Information on sewing, RF or HF welding, heated welding and ultrasonic welding.

Big Picture

While you may be familiar with industrial sewing machines used to join printed graphics, you may not be aware of the advantages and disadvantages to the other joining technologies. Here are some basics on each:

Sewing: Joining panels with a sewing machine offers several advantages, including that the seam can be removed (welded seams cannot be removed without destroying the material). Sewing provides a very strong seam, particularly with industrial sewing machines and over-lock sergers, which typically sew and finish a seam and trim off the excess fabric all in one step. Then there’s the speed factor-some offer speeds of more than 11 feet/minute with 40 stitches/inch. Some industrial machines have a walking foot to produce consistent stitches, and double-needle chainstitch machines use multiple spools of thread for large-volume production.

On the downside, however, since sewing makes holes in the material, the resulting seam is generally not watertight and the holes may weaken the fabric. Plus, depending on the project, shops may have to deal with matching thread color and threads that fade under constant sunlight. And while a removable seam is a bonus, how often will your customer want to keep and store a superwide banner?

RF or HF Welding: Radio or High Frequency (RF or HF) welders use radio-frequency energy to generate heat, and then pressure is applied. The energy is generated between two metal bars; this is important because the weld is only applied on the material that touches the bars-limiting the area that can be welded at one time. The speed of RF welding is typically slower than that of heated welding, and RF welders can generally only be used on PVC, PET, and polyurethane (other materials can be RF welded, but only under special conditions). The material also must be very clean for a secure RF weld; dirt is not such a problem, but if a fabric has any salts or iron shavings on it, or is wet, burns or arcs may result.

Heated Welding: Hot-air or wedge rotary welders utilize hot air or a heated metal wedge to heat the fabric. The material is typically pulled through rollers (or the heated head travels along a track) where heat is applied, with the rollers applying the required welding pressure. Heat-based welders can weld PVC, polyurethane, PP, PE, and acrylics (with welding tape). Continuous feeding of materials (with rollers) adds to the speed, and rollers make it easy to weld long, straight seams.

Both types of welders have their pros and cons. OEMs in the hot-air welder camp note that heated welding doesn’t affect solvent-based inks; in addition, hot-air welders report that compared to RF hot-air’s speed is roughly double, it takes less people to operate, and it requires a smaller initial capital expenditure.

Manufacturers of RF welders, however, report that the RF/HF welding process is more controlled and the quality of the seam is better and more reliable (because RF is more of a fusion of materials than a bonding). In addition, they report that RF is easier to use on projects that involve many tiny pieces or complicated angles, because it’s difficult to start and stop with heated rotary welding techniques.

Ultrasonic Welding: Another welding option is ultrasonic welding. Like RF welding, ultrasonic creates heat through friction, but the heat is created between the layers, rather than in the materials themselves; the vibrating tool (die) creates the heat. Ultrasonic can join most plastic materials such as PE, PP, PVC, and PU. The newest of the welding technologies, it’s fast (up to 65 ft/min), but it can prove more costly to weld large surface areas.

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