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Though large amounts of money are spent on producing these images, they can go from "wow" to "uh-oh" if they are not finished correctly.

Finishing comes in many forms, from sewing and taping to heat-sealing and doing nothing. What is best? In the past, sewing and taping were the primary options for finishing printed images. The industry is now seeing heat-sealing and welding as the option that satisfies many of their concerns related to speed, strength, durability and overall aesthetic appeal. For example, if you hung the Mona Lisa with a piece of tape, it would not hold the same appeal as being mounted and framed.

Heat sealing has many advantages. Print shops that primarily heat seal will have low to no consumable costs, depending on how much they use sewing or taping in their product line. With heat sealing, a printer will not have thread exposure and there is no need to match the image. Further, heat sealing carries greater speed and a lower labor cost than taping. A seam that is welded (heat sealed) will last as long as the material itself, and will be stronger than the material. Heat sealing is used in many other applications where seam strength is critical, such as inflatable boats, truck tarpaulins, tents and even military fuel cells.

Using this technology, an imager is able to weld many different fabrics using heat sealing as their operation method. These include vinyl (PVC), vinyl films, polyethylene (PE), rigid extruded products, weldable webbing and digital textiles (with a heat-activated tape), to name a few.

The choices of heat-sealing technologies are many, each with their own advantages, techniques and user preferences. Hot air, hot wedge, thermo-impulse, radio frequency (RF) and ultrasonic welding all use these three basic principles to achieve a welded seam:

Heat is the electrically-produced, high temperature, precisely-controlled parameter that is applied to two or more layers of thermoplastic material that prepares the surfaces for molecular bonding.

Speed is the amount of time during which the heat source is applied to the thermoplastic material. This is controlled by the rate at which the weld head passes through or is applied to the fabric or the amount of time the material is exposed to a heat source.

Pressure is used to compress the heated thermoplastic materials together during the sealing process to complete the molecular bond between two or more surfaces.

Seal Types
Large and grand-format printers use many seal types, depending on the application. The overlap seal is when two panels are bonded together to make smaller prints into larger prints. This seam configuration is most commonly used in grand-format applications. When an image is larger than the printer's width capability, it must be printed on several different panels. After printing the image on multiple panels, they must be aligned and overlap-sealed together.

A butt seam is used primarily for creating backlit applications, ensuring light can come through the print. A butt seal is used when two panels are butted up against each other and seamed together by welding another clear strip of material over the butted seam.

Hems are one of the most common seam types. They close the edges of a banner and finish the edges of a printed piece. To create a hem, the material is folded over and bonded to itself. This seal configuration improves the look of hanging banners and signs. Hems also are used for support when inserting eyelets or grommets into a banner.

The hem-and-rope seam is used when the edges of a banner are folded over and bonded while rope is fed through the welded area. This seal finishes a banner's edges and improves its appearance while providing a way to assist banner hanging. The hem-and-rope seam also adds extra support to a banner's edge for adding eyelets and grommets.

The pole-pocket seam is when the edges of a banner are folded over and welded, leaving a specific-sized pocket through which a hanging pole can pass. This seal configuration is most commonly used for mounting banners on street poles as well as large billboards.

A weldable-webbing seam is when an extra piece of material is welded to a printed application's back side. This seal configuration provides extra support and helps eliminate unwanted grommet tear-outs. This seam type is seen most in high-wind applications. In a beading seal, beading is welded to a printed work's edge. This seal type is used to mount different types of rigid and semi-rigid plastic extrusions for outdoor and indoor framing systems.

Understanding Seaming Technologies
To choose the proper heat-sealing method, you must understand the choices and how they fit into your production processes. Rotary welding, used primarily for perimeter seaming, can be placed in any sized shop. The rotary welder is available via hot air or hot wedge technology. In the printing industry, hot wedge is the predominant technology used.

Hot air can be used to create hems, rope-and-hems, pole pockets, overlaps, butt seals, webbing seal types and more. Hot wedge is used to create hems, rope-and-hems, pole pockets, overlaps, butt seals, webbing and other types of seals.

A rotary wedge welder can easily replace sewing operations and is the fastest welder in the industry.

A stationary wedge welder is designed for high-volume production of hems, pole pockets and rope-and-hems.

Floor welders also use hot air or hot wedge, and the manufacturer will dictate which heat source to use. Floor welders are used primarily for welding printed panels together to create large images from small prints. Given their size, they also are more difficult to control.

Ultrasonic welders use a different method to generate heat than hot air and hot wedge. A high-frequency sound generator creates heat while the fabric is driven under pressure, creating the welded seam. Ultrasonic welders are most commonly found in the packaging and medical industries where there is a need to weld together delicate textiles.

Vacuum positioning welders create a welded seam using hot air technology. Unlike most welders that operate like a sewing machine, printed material is aligned and locked in place using a vacuum system and clamps while the welding head travels the length of the panel, creating the welded seam. This type of system is utilized in the sign industry. With the addition of a rotary cutter, welding and trimming of printed images can take place at the same station.

A radio frequency (RF) welder puts fabric under pressure and generates heat by passing RF waves through the material. RF welders utilize a stationary or moving head. Both styles can weld only in bar-sized sections, so the head or the fabric must be moved to make longer seams. An RF welder can be used to create overlaps, straight hems, pole pockets and edge reinforcement. RF is able to weld PVC/vinyl, but not polyethylene (PE). It has long cycle times and, therefore, longer welding times.

Impulse welders apply pressure to the seam area, pulse heat to the seam area throughout the heating element and hold the seam area under pressure while it moves through a liquid cooled process. The seam is then released, allowing the operator to move to the next seam area. These systems can finish the edges of signs and banners with a variety of hems and pole pockets, as well as align and adjoin printed images.

They also are able to weld together nonweldable printable fabrics by using a special heat activated tape and are designed with the large-format printer in mind.

Making Your Choice
Different print shops have different needs, from the product types and production amount to the business base size and scale of prints produced. What welding technology or size of welder is needed? Is it worth buying a heat-sealing system or subcontract needed welding services? The following are some questions to consider.

What welder fits my needs?
A print shop that primarily prints images within the size of their large-format printer and needs to finish the edges of signs and banners has a few options. A shop with high-production needs should lean toward a stationary, rotary-wedge welder as the primary option. Depending on what a shop can spend and its production needs, other options could be the impulse welder, floor welder, ultrasonic or RF welder.

A print shop that prints on a grand-format printer and needs to tile images together and run long, straight-edge seams has a few options. The vacuum-positioned, hot air welding system is a great option for finishing these large projects. Other good options, depending on need, would be an RF welder or a floor welder. Many print shops use multiple equipment types.

What welder size do I need?
A few questions to ask yourself when figuring out what size of welder to purchase include: What type and size of seaming jobs do you have and what size of seaming jobs do you expect? About 80 percent of the seam sizes you are creating now or plan to create should be within the welder size you purchase.

Should I buy a welder or outsource my welding needs?
Some companies charge more than a dollar per linear foot to finish perimeter work. Prepare a cost analysis on what you spend subbing out your welding needs, and then use this simple calculation to find a return on investment for a welder. First, take the price of a standard, sit-down wedge welder at roughly $25,000 at the lease term of 60 months, making your monthly payment around $480. Next, take the cost to finish an average banner, approximately $13 (using $1 per linear foot, at 13 linear feet). It would take 9 banners per week, or 37 banners per month to pay off the welder at a 60 month term.

Keep in Mind
Printing your signs is more than half of the final product presented to your customers. However, it is not the entire result. How your finished product looks when it is hanging is critical to the final product's success. If one has the perfect print but wrinkles throughout the seams, then the whole product is diminished. Regardless of the means you choose to finish your banners, signs or grand-format products, consider appearance, efficiency, quality and ease of use for your operators.

Jeff Sponseller has been in the industrial fabrics and graphics business for more than 10 years. As the Executive Vice President of Miller Weldmaster, he has worked with numerous industrial fabrics manufacturers, government contractors and sign industry leaders, and has consulted on numerous projects requiring thermoplastic welding and fabric handling. jsponseller@weldmaster.com

This article appeared in the SGIA Journal, 1st Quarter 2009 Issue and is reprinted with permission. Copyright 2009 Specialty Graphic Imaging Association (www.sgia.org). All Rights Reserved.