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Heat Sealing Coated/Laminated Fabric Seams

Dialectric Welding Vs. Rotary Welding

Generally made from a polyester or nylon internal fabric scrim and covered with a thermoplastic coating, industrial coated fabrics are used today for a vast array of important and interesting products. When manufacturing products from these fabrics, heat sealing machines apply a measured amount of heat and pressure to the seams of pattern pieces to fuse them together, making an airtight and watertight seal.

Thermoplastic coatings, such as polyvinylchloride (PVC), polyurethane (PU), polyethylene (PE) and polypropylene (PP) lend themselves to heat sealing, eliminating the need for sewing, taping, gluing or other more complicated and costly methods. Tents, awnings, tarpaulins, inflatable boats, water toys, geo-membranes, truck covers, farm building covers, fishing waders, environmental and protective garments, and inflatable advertising balloons are just a few of the common products made today from industrial coated fabrics using heat sealing machines.

There are two basic types of heat seal machines in common use today, the dielectric or radio frequency (RF) welder and the rotary heat sealer.

Dielectric welder was developed over 40 years ago. It is a discrete process where a die is lowered onto the pattern pieces which are supported by an underlying base plate and then a timed pulse of radio frequency energy is delivered between the die and the base plate. The energy heats the fabric between the die and base plate enough to temporarily melt the thermoplastic coating, thereby fusing the two pattern pieces together. Once fused, the die is lifted, a new section of fabric is moved into position, and the process is repeated.

The younger of the two processes, rotary welding is more widely used today than dielectric. It is a continuous process where the pattern pieces move continuously through the weld area, generally pulled along by a pair of drive wheels. Heat is delivered from some source, either a heated metal wedge or hot air, immediately before the fabric passes between the drive wheels where the welding pressure is applied.

There are several important differences between the dielectric and rotary welding process:

Rotary Welding is Faster

Rotary welding is much faster than dielectric welding for most products. This difference is greatest where seam length and product size are greater than about a meter and increases accordingly with larger size products and longer seams. Typical welding speeds up to 20 feet (6 meters) per minute and higher are attainable.

Rotary Welding Requires a Skilled Operator

While little training is usually required to operate a dielectric welding machine, rotary heat sealing is a skill, similar to sewing, and a longer training cycle can be expected before the full speed and flexibility of rotary heat sealing can be realized.

Rotary Welders Can Produce 3-D Products

Rotary welders can be used to produce three dimensional finished products (products that do not lie flat) such as garments, inflatable boats, bags, packs and luggage. Due to the flat base plate, dielectric welding is typically limited to products whose seams must lie flat during the sealing process or where the curvature is so small that the pieces can be laid flat.

Note: Some specialized dielectric welders incorporate three dimensional dies and base plates and vacuum systems for holding the pattern pieces in position while the dies are applied. However, because of the high cost of these machines and especially the associated tooling, they are typically used only for certain, very high volume applications.

Rotary Welding is Adaptable

Dielectric welding is good for making many small, flat seamed welds, each exactly like the other. A rotary welder can be used for a variety of seam types and pattern designs with little or no setup change required.

Rotary Welding Requires No Tooling

Other than a selection of appropriate drive wheel widths and air nozzles to match them, rotary welders do not require dies or other tooling. This makes production set up much faster than with dielectric welding, a huge benefit in the product development cycle.

Wedge Welding Vs. Hot Air Welding

There are also important differences between rotary wedge welding and rotary hot air welding. Typically, hot air is a much faster welding process that hot wedge.

A wedge welder uses a small metal wedge to deliver heat to the fabric immediately before it passes between the drive wheels where pressure is applied to seal the fabric together. The advantages of the wedge welder are that it uses less power and is relatively quiet compared to the noise of the airflow from hot air welders.

Hot air welders use a hot air nozzle instead of a metal wedge to deliver heat. This simple difference gives the hot air welder several distinct advantages.

Heat Build-up

Hot air welders are not subject to the heat build-up experienced when stopping and starting a wedge welder. Whenever the seaming process is interrupted or stopped, the heat in the wedge builds up. When the wedge is reintroduced to the fabric for seaming, it can often burn the fabric at the start point.

Uniform Heat Transfer

Contamination on the surface of the wedge can block the transfer of heat from the wedge to the fabric, affecting the uniformity of the weld. This contamination can come from two sources: dirt on the fabric, picked up during the material handling process, or bits of melted coating that adhere to the wedge while welding. Since hot air welders use air rather than metal to deliver heat, they are not subject to this problem.

Surface Irregularities

Irregularities on the surface of the fabric being welded, particularly a cross seam, raise the wedge as it goes over the irregularity. On the back side of the irregularity, this momentarily leaves the rigid metal wedge too far above the bottom piece of fabric to ensure a good weld. Since hot air is not rigid, it can flow over surface irregularities, ensuring a better weld.

These advantages make the hot air welder more versatile than the wedge welder. While the wedge welder is suitable for welding relatively simple products made from less technically advanced fabric that have few or no cross seams, the hot air welder is used in a much broader field of applications with more advanced design and fabrics.


Feel free to contact Seamtek with any other questions about the various welding processes.

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