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Heat welded seams are consistently increasing in popularity. Heat welding will produce an integral surface via the heat weld system. This integral surface will prevent any type of contaminates from penetrating the material, especially at seams. These types of material are specified for use in medical, technical, and institutional facilities, i.e. laboratories, clean rooms, and operating rooms.   Materials There is wide variety of materials that can be heat welded; these are homogeneous, heterogeneous and felt and glass-backed materials. Sheet vinyl, because of its high vinyl content, lends itself to the thermo-fusion heat welding process very well.   Preparation A lot of the heat-welded materials are vinyl-backed; they are limited to the amount of Moisture Vapor Emissions Rate MVER they can handle. Most of them are at 3.0 pounds per 1,000 s/f per 24 hours. This requires moisture testing to insure success of the installation. The vinyl-backed materials are very susceptible to telegraphing sub floor irregularities; therefore, every precaution must be taken when preparing the substrate.   Adhesives - Photo 1 The most popular adhesive used is the pressure sensitive adhesive. These adhesives must be applied to manufacturer’s recommendations regarding proper trowel notch and open time. Most manufacturers recommend the adhesive to be allowed to dry-to-touch. There are advantages to allowing the adhesives to dry-to-touch. The seam can be heat welded as soon as you are ready to weld. Seams that are installed into a wet adhesive require 10 – 12 hours before the seams can be welded.   Seaming - Photo 2 The seaming procedure used is usually the under scribe method, which is also known as recess scribe or hinge scribe. This is accomplished by trimming approximately 1/2 inch or more from the factory edge of the first sheet, overlap the second sheet about 1/2 to 1 inch, then after the material is placed into the adhesive, the second piece which is overlapped is under scribed with the under scribe set slightly open (about the thickness of a business card). This makes grooving of the seam much easier, plus allows the leading wheel of the power groover a slight opening to follow. Never butt factory edges. I find that many seam complaints are a result of this being done.

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  Grooving - Photo 3 There are two methods used to groove the seams: hand grooving and power grooving. Both are widely used. There is a large assortment of hand groovers available and each installer will have his favorite type. The keys to hand grooving are a sharp tool and practice. A new groover and an inexperienced installer is not a good recipe for success. On larger installations, the power groover is a must, for uniformity and production. The object is to groove the seam with the proper width groove for the welding rod to be used and the proper depth, which is normally 1/2 to 2/3 the thickness of the wear surface. Care should be taken not to cut too deep into the material, as the seam weld integrity will be compromised. After the grooving is completed be sure not to contaminate the freshly cut grooves.   Heat Welding Gun Like hand groovers, there are large selections of heat welding guns to choose from; the hand-held, light duty, to the heavy-duty hand held type, to the automatic self-propelled types. These range in price from hundreds to thousands of dollars. The things to shop for in a heat welder are: one to fit your use requirements, adjustable heat settings, durability, element replacement costs, versatility and a carrying case to protect it. They are a major investment and must be taken care of.  

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  Tip Selection - Photo 4 There are a wide variety of weld tips and styles. Most of them work well and are a matter of installer preference. Next is the selection of tips required for the welding rod that is to be used. There are 4mm and 5mm speed tips for the round and square welding rods, and the 5.7 mm triangular tip for the half round patterned welding rod. Once the proper tip selection is made, the welding process can begin  

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  Temperature Selection - Photo 5 The temperature selection must be determined. It will range from 650º F (340ºC) to 850º F (450ºC). There are several factors that will dictate variations: the floor temperature, (wood floors are warmer than concrete), ambient room temperature, the amperage of the electrical supply and the length and size wire of the extension cord. The selection of the proper setting is one of experience, trial and error, and the speed in which the welder is moving. The best indicator is to take the welding rod and expose it to the direct flow of the hot air. The welding rod should start to liquefy and not burn. Once that setting is determined, the setting is increased or decreased to accommodate the other factors.  

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  Heat Welding Process - Photo 6 After the settings are determined, the welding process begins. When ready be sure there is plenty of extension cord and that there is nothing in your way to impede your movement. Start by pre-heating the groove at the starting point. Insert the rod into the tip and move in and out to preheat the rod, and immediately insert the rod into the groove. Any delays will cause the welding rod to melt in the tip causing further delays by having to stop and clean the molten, burned, welding rod out of the tip and causing you to start all over again. 

Once moving, watch the welding process at the juncture of the welding rod and the material. There must be slight downward pressures on the tip that will cause the rod to be forced into the groove and at the same time there is a widening of the groove and a slight peaking of the material, where the welding rod and groove meet. This peak will be an indicator that all is going at the right pace. This peak will be trimmed off later. This scorching will also aid in determining your speed of welding. If you’re moving too fast the slight peaking will not occur, and if you are too slow the peak of the material at the rod will burn. Be careful not to burn the material, as it is difficult to repair.

Once the welding is complete, turn the hot air setting to zero to allow the heat welder to cool, failure to do this will cause the element to crack and have to be replaced.   Cooling It is imperative to allow the material to properly cool down before the skiving process begins; normally a minimum of 20 minutes is required before the trim pass. This allows the molecular structure of both the sheet vinyl and the welding rod to solidify from the effect of the heat. Seams that are skived too soon will show a severe concave effect that cannot be corrected without re-doing the entire process.  

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  Skiving (First pass) - Photo 7 Once cooled, the material is ready to trim. The first pass is made with a quarter moon skiving knife and a trim plate, the second with the skiving knife only. This allows the rod to be trimmed to a uniform thickness that makes the second pass easier and less concave to the finished seam

Each installer should have their own skiving knife because it must be sharpened to each installer’s likes and habits. The sharpening is done only on the underside of the skiving knife. On the first pass the trim plate will hold the cutting edge of the skiving knife up about 1/32” above the surface of the material; this will also stop the concave effect by allowing the second pass to be more uniform. The first pass should be smooth and without stopping.  

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  Skiving (Second pass) - Photo 8 The second pass is even more important, the angle of attack of the cutting edge of the skiving knife must be constant to ensure a flush trim. A too low angle will allow the knife to climb out of the rod and too high an angle will cause the knife to dig into the material. The flow must be smooth and constant, each time you stop and start there is a “stop/start” mark or small ledge that is left in the surface of the material. Failure to maintain a constant angle of attack or a smooth clean pass will result in an unsightly seam. And once a floor finish is applied, the higher gloss will highlights the irregularities, it is sure to lead to a complaint.

Heat welding is a growing in popularity as an installation system. It is an expensive one for tools and equipment. So before attempting to start heat welding, an installer needs to practice I would say it takes about 100 l/f of practice before you can begin to feel confident with the heat welding process. So practice on a scrap piece of material until you become confident in the performance of the tools and equipment as well as your ability to weld.