Even the most experienced of operators who have used a hot air rotary welder for fabricating products from coated textile fabrics has had the experience of coming up short on one of the two pieces at the end of a seam. In the fabrication industry, this is known as Longitudinal Registration Error (LRE), the tendency for one of the fabric pattern pieces to overtake the other as the seam is being welded.

All rotary heat seal equipment is subject to longitudinal registration error because the elastic and somewhat slippery fabric is continuously pulled along by the powered drive wheels.

In practice, an experienced operator overcomes LRE by pre-marking the fabric with erasable registration marks, making it possible to see slippage and correct it as the seam is being welded. Typically, the operator watches the alignment of registration marks, pulling back somewhat on the piece that overtaking the other as they pass through the weld head. This is a difficult procedure even for experienced operators. Pulling a little too hard can result in slippage between the drive wheel and the fabric, often causing a seam to be ruined.

SmartSeal™ Rotary Hot Air Welders eliminate the pull-back method for seam registration, and reduce or mitigate the need for other LRE corrections, providing a hot air welder that is significantly more accurate and easier to use than any other floor standing hot air welder available today.

The most common sources of LRE are:

• Drive Wheel Chain Tension
• Drive Wheel speed Differences
• Curved Seams
• Fabric Stretch
• Different Fabric Weights
• Fabric Slippage
• Fabric Shrinkage

Drive Wheel Chain Tension

A primary cause of LRE in rotary hot air welders is an actual difference in the rotation of the two drive wheels. Conventional floor standing hot air welders are driven by a motor with several feet of chains and pulleys used to deliver the drive energy to the wheels. Chains stretch over time and need tightening. Such slack chain adjustment typically causes a registration error at the beginning of a seam when the linkage of one or the other wheels is not yet tight and pulling against the fabric. This error then remains in the seam until the end unless it is observed and somehow eliminated by the operator as the seam is being welded. To eliminate the possibility of this type of error, the chain tension in such machines must be checked frequently.

In the SmartSeal™ Rotary Hot Air Welder, each drive wheel is powered by its own high torque stepper motor which is connected to the wheel with a very short timing belt, no more than a few inches long. When the motor is at rest, free wheel motion is typically less than an 1/8 inch. Once the belt tension is set, it seldom needs retensioning.  The timing belts are designed to not stretch at all.

Drive Wheel Speed Differences

To allow for differential wheel speed adjustment the upper wheel in typical rotary hot air welders is usually driven through a variable speed transmission. The variable speed transmission allows the operator to adjust the speed of the top wheel to a setting that is a little faster or slower than the bottom wheel in order to cancel out an LRE that is characteristic of a particular pattern or setup. Unfortunately, the mechanical nature of the variable speed drive used in these machines, is neither accurate nor repeatable and is a source of much of the LRE in the first place.

In the SmartSeal™ Rotary Hot Air Welder, the speed of each drive wheel is controlled by the timing of the pulses generated by the computer. There is no mechanical device between the motor and the drive wheel to cause wheel speed error.

Curved Seams

Although the patterns for a curved seam may be designed and cut perfectly, as the two pattern pieces are steered through the drive wheels, even a small side-to-side steering error can induce LRE. For example, a sustained side-to-side error of 1/4 inch in a gentle curve can produce a longitudinal error of more than an inch over the length of the curve. Essentially one piece of the pattern is taking a shortcut around the curve thereby consuming less fabric.

Better steering accuracy and the use of mechanical guides help to keep the fabric on track, but LRE appears to be an inherent part of the process of welding curves on the rotary hot air welder. The best approach is to use registration marks to know when it occurs and to have some way to cancel the LRE as quickly as possible.

If LRE does appear, SmartSeal™'s foot-pedal control for adjusting top wheel speed relative to bottom wheel speed allows the operator to set the top wheel to rotate slightly faster or slower than the bottom wheel to cancel any observed LRE without letting go of the fabric.

Fabric Stretch

Another source of LRE is the nature of the hot air welding process itself. As two fabric pieces move toward the drive wheels of a hot air welder, they exist as two independent panels of fabric. They each respond to internal and external stress, friction and steering corrections independently. Until the seam is joined, all the unwelded fabric is free to stretch and pull whichever way it will. Even a small but sustained drag of a few ounces on one piece and not the other can result in an accumulated stretch induced LRE of several inches over a few feet of seam, depending of course on the elasticity of the fabric.

This type of LRE can be reduced by careful attention to the setting up the fabric to reduce friction and drag as the fabric passes through the weld head. Unfortunately some patterns are large and bulky with heavy fabric that will resist all efforts to pull it through the head smoothly. In this case the operator must reduce the stress on the fabric as far as possible and expect to make continuous longitudinal corrections while welding the seam.

Tables of various designs can be constructed to deal with this problem.  In some instances, powered conveyors or other powered material handling devices may be needed.  The onboard SmartSeal™ computer can control these powered material handling devices to synchronize operation with the Seamtek welder.  Additionally, Seamtek manufactures welders with additional fabric pullers to help pull large fabric panels through the welder.

As with other types of LRE, the SmartSeal™ Rotary Hot Air Welder provides foot-pedal control of the top wheel speed with respect to the bottom wheel. Without letting go of the fabric the operator can set the top wheel to rotate slightly faster or slower than the bottom wheel to cancel any observed LRE.

Different Fabric Weights

Using powered drive wheels to weld together two fabrics of different weights is like welding two pieces of the same fabric together with two wheels of different diameter. Each piece of fabric is effectively connected to the wheel by friction and becomes part of the wheel as it passes through the weld area, causing each piece of fabric to move through the weld area at a different speed. Although this difference is usually slight, it will accumulate over a longer seam to cause LRE. This type of error can be eliminated by setting a difference in wheel speed to cancel the effect prior to welding.

The SmartSeal™ Rotary Hot Air Welder allows the operator to preset a differential wheel speed prior to welding the seam, in exact percent units of the base speed setting. Since the wheel speed is controlled by the computer, the differential speed setting is precisely accurate and repeatable - from seam to seam, or month to month.

Fabric Slippage

Yet another source of LRE is the lack of or difference in traction between each piece of fabric and its corresponding drive wheel. As the fabric is welded, the molten weld joint itself is in a fluid state between the two drive wheels and prevents either piece of the pattern from exerting any significant force on the other. All the traction force must come from the wheel/fabric contact area. Normally the drive wheels have enough traction to do the job but if the back stress on the fabric is high, usually caused by the operator attempting to correct for an observed LRE in the seam, and especially if the wheels are hot from a long welding session, one piece of fabric can simply lose traction and stop moving through the weld head. The result is one piece of fabric "piled on" the other in a series of small ridges and folds and an immediate, large LRE in the seam. Again, it is very important to reduce stress on the fabric as its being welded. If stress on the fabric can't be eliminated, then a way must be available to correct the error as soon as it is observed.

Another source of this type of LRE is drive wheels that get too hot during a long welding session, causing the fabric to emit plastic vapor which then collects on the wheels and makes them slippery. In most existing designs, the hot air nozzle is permanently mounted between the drive wheels and a valve is used to send the hot air either through the nozzle while welding or out a waste stack when not. With this arrangement it is necessary to pre-heat the nozzle before welding each seam to avoid a cold weld at the start of the seam while the nozzle heats back up to full temperature. While the nozzle is being pre-heated, which, depending on the time the welder was idle, can take from a few seconds to a minute or more, the hot air blows directly on the lower wheel. After an hour or two of welding it is common for the lower drive wheel in such machines to be too hot to touch.

On these machines, the best solution is to take a break, put a fan or a stream of cool air on the lower wheel and let things cool down for a while. Before welding again, clean any accumulated plasticizer from the drive wheels with alcohol.

On the SmartSeal™ Rotary Hot Air Welder the complete hot air module including the nozzle slides into and out of position for welding. This allows the hot air to flow through the nozzle continuously and eliminates the need for a pre-heat cycle. It also keeps hot air from ever contacting the wheels directly. Even after an extended welding session the drive wheels on the SmartSeal™ Rotary Hot Air Welder seldom rise much above room temperature.

In addition, if LRE is observed in the seam, the SmartSeal™ Rotary Hot Air Welder makes it unnecessary to pull back on the fabric to correct the registration. Instead, the operator uses a foot pedal command to increase or decrease the speed of the top wheel with respect to the bottom wheel. These corrections are applied in small increments over a length of seam to cancel any observed LRE.

Fabric Shrinkage

A particularly damaging source of LRE is shrinkage of the scrim, the woven fabric inside the fabric coating. This shrinkage is caused when too much heat from the welder penetrates the coating, heating the internal fabric which normally has a much lower melting temperature than the coating. Just before melting, the fabric weave tightens, the fabric strands become shorter and the entire fabric shrinks. Unobserved fabric shrinkage causes a poor quality seam, and if one piece shrinks more than the other, it can cause significant LRE.

This type of LRE also shows up in the high frequency welder where the complete cross section of fabric, not just the surface, is heated by the RF energy, much like a microwave oven heats the inside of the food as well as the surface. This will be especially noticeable when the high frequency welder is used with certain fabrics to weld long seams.

This source of LRE can sometimes be eliminated by making sure the hot air nozzle is not directing the hot air flow at one piece of the fabric more than the other. The problem can also be mitigated by careful temperature/speed adjustment to find the best combination for that particular seam and fabric combination. However, some lighter weight fabrics simply have too thin a coating to keep the heat out, making the fabric appear to be unweldable or demanding a product design.

The Smart-Seal Rotary Hot Air Welder provides several ways to reduce fabric shrinkage induced LRE. These include:

• The first step to correct fabric shrinkage is to apply only the required amount of heat to the seam to achieve the desired bond.  Any extra heat only increases the possibility of shrinkage.  This is one reason that digital control is so important.  Beside digital control of the welder, a few other parameters come into play.
   
• Hot Air Nozzle Adjustment: for the hot air module allow easy vertical adjustment to position the weld nozzle in line with the contact area of the two drive wheels. This assures that heat is applied equally to each piece of the pattern.
   
• Test Strip: TEST STRIP operating mode allows the operator to quickly pinpoint the exact wheel speed/air temperature combination for that particular fabric thus producing the fastest weld without burning or shrinking the fabric.
   
• Air Flow: The Seamtek welders offer the ability to control the amount of airflow that comes out of the nozzle.  Sometimes a lower airflow setting can compensate for fabric shrinkage.  Airflow affects seam temperature by controlling the volume of heated air applied to a seam. 
   
• Fabric Pullers:  In some cases, the only solution to eliminate shrinkage is to place a specified amount of tension on the seam until it cools.  To accomplish this, a second set of pinch rollers, called pullers, are located directly behind the weld wheels.  These pullers run a the same speed as the weld wheels or can be digitally adjusted to run slightly faster to create higher fabric tension.

In practice, each of the factors that cause LRE is usually small and controllable, but under some conditions, one or more of them can become significant and combine with others to cause a longitudinal registration error that is unacceptable. This result is wasted material, production delays and a frustrated operator.

The SmartSeal™ Rotary Hot Air Welder was designed with the primary focus of completely eliminating drive wheel speed error as a source of longitudinal registration error and to provide a variety of other means to reduce or cancel out any remaining LRE.

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