With the ever-increasing popularity in heat-welded seams, now is a good time to revisit this topic. Heat welding will produce an integral surface via the heat-weld system. The integral surface will prevent any type of contaminants from penetrating the material, especially at the seams. These types of material are specified for use in medical, technical, and institutional facilities; i.e. laboratories, clean rooms, operating rooms and so on.

There is a wide variety of materials that can be heat welded, including homogeneous, heterogeneous and felt and glass-backed materials. Sheet vinyl, because of its high vinyl content, lends itself very well to the thermo-fusion heat welding process.

Many heat-welded materials are vinyl-backed and are limited to the amount of moisture they can handle, measured via the Moisture Vapor Emissions Rate (MVER). Most are at the 3.0 pounds per 1,000 s/f per 24 hours level.

Moisture testing is required to ensure success of the installation. Vinyl-backed materials are very susceptible to telegraphing subfloor irregularities; therefore, every precaution must be taken when preparing the substrate. 

Photo 1: Adhesives. The most popular adhesive used is the pressure-sensitive adhesive. These adhesives must be applied following manufacturer’s recommendations, including proper trowel notch and open time. Most manufacturers recommend the adhesive to be allowed to dry-to-touch. The main advantage of this is that the seam can be heat welded as soon as you are ready. Seams that are installed into a wet adhesive require 10 to 12 hours before they can be welded.

Photo 2: Seaming.The seaming procedure usually follows the under scribe method (also known as recess scribe or hinge scribe). This is accomplished by trimming approximately 1/2” or more from the factory edge of the first sheet and overlapping the second sheet about 1/2” to 1”.

Then, after the material is placed into the adhesive, the second piece which is overlapped is under scribed with the tool set slightly open (about the thickness of a business card). This makes grooving of the seam much easier, and allows the leading wheel of the power groover a slight opening to follow. Never butt factory edges. Most seam complaints I hear are because of this.

Photo 3: Grooving. There are two methods used to groove the seams: Hand grooving and power grooving. Both are widely used. A large assortment of hand groovers is available and each installer will have his favorite type. The key to hand grooving is a sharp tool and practice.  A new groover and an inexperienced installer are 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 and the proper depth, which is normally one-half to two-thirds 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.Heat-welding guns come in all shapes and sizes, from the handheld light-duty models, to heavy-duty handheld types, to the automatic self-propelled machines. These range in price from hundreds to thousands of dollars.

Welders are a major investment, so when shopping for one make sure the tool fits your use requirements, features adjustable heat settings and is durable. Also consider the element replacement costs, versatility of the unit and buying a carrying case to protect it.

Photo 4: Tip selection. Weld tips and styles vary. Most of them work well depending on the installation requirements, and are a matter of installer preference. The 4mm and 5mm speed tips are designed for round and square welding rods, while the 5.7 mm triangular tip is used for the half-round patterned welding rod. Once the proper tip selection is made, the welding process can begin.

Photo 5: Choosing the temperature. Temperature selection will range from 650º F (340ºC) to 850º F (450ºC). There are several factors that will dictate variations, including the floor temperature (wood floors are warmer than concrete), ambient room temperature, 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 can be increased or decreased to accommodate the other factors.

Photo 6: Heat-welding process. After the settings are determined, the welding process can begin. Be sure there is plenty of extension cord and nothing is in your way that could 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, then 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 starting all over again.

Once moving, watch the welding process at the juncture of the welding rod and the material. There must be slight downward pressure on the tip, which will cause the rod to be forced into the groove. At the same time there should be 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, and will be trimmed off later.

The scorching will also aid in determining your speed of welding. If you’re moving too fast the slight peaking will not occur; 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.

Cooling. 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, which will then have to be replaced.

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, which cannot be corrected without redoing the entire process.

Photo 7: Skiving (first pass). 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, which 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, without stopping.

Photo 8: Skiving (second pass). The second pass is even more important. The angle of attack of the skiving knife’s cutting edge must be constant to ensure a flush trim. A too-low angle will allow the knife to climb out of the rod; 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. Once a floor finish is applied, the higher gloss will highlight the irregularities, which is sure to lead to a complaint.

Heat welding is growing in in the field of installation, but it can also be expensive to acquire the necessary tools and equipment. So before attempting to start heat welding, take the time to practice. Practice on a scrap piece of material, slowly become confident in the performance of the tools and equipment, and you