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With its ever-increasing popularity, heat welding is a growing part of the flooring industry.  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 products are specified for use in medical, technical, and institutional facilities, i.e. laboratories, clean rooms, and operating rooms.  

Materials

There is wide variety of rubber products that can be heat welded; these can be in sheet or tile forms. Rubber, because of its high rubber content, lends itself to the thermo-fusion heat welding process very well.   

Preparation

A lot of the heat welded rubber products, because of their density, 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 per ASTM F-1869 or 75% per ASTM F2170.  This requires moisture testing to ensure success of the installation. The rubber products are very susceptible to telegraphing subfloor irregularities, therefore, every precaution must be taken when preparing the substrate.   

Adhesives – Photo 1

There are two basic types of adhesive used in the installation of rubber products: acrylic and urethane adhesives.  These adhesives must be applied to manufacturer’s recommendations; proper trowel notching and notch types can vary depending upon the product being installed and the proper open time must be followed.  Most manufacturers recommend the adhesive be applied, and allowed to dry overnight prior to the start of heat welding.  

Seaming – Photo 2

The seaming procedure for rubber products is generally the double-cut method.  Seams should be overlapped approximately 1/2 inch or more and both layer should be cut together.  Never butt factory edges. I find 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 key to hand grooving is 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 a large selection 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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Temperature Selection – Photo 4

The temperature selection must be determined. Settings for the temperature will range from 750°F - 850°F (350°C - 400°C)  All heat guns are different so be sure the temperature is set properly by doing a test weld on a scrap piece of material.   There are several factors that will dictate variations of the setting:  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 all of the other factors.

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Heat Welding Process – Photo 5

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 which will cause the rod to be forced into the groove and at the same time there is a slight 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. 

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.

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Skiving (First pass) – Photo 6

Before the rod has cooled (5 – 15 minutes), the material is ready to trim. The first pass is made with a quarter moon skiving knife and a trim plate.  In this photo I was using the 2 ’n 1 weld skive knife with the trim plate attached. 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 and without stopping. 

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Skiving (Second pass) – Photo 7

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 polish is applied, the higher gloss will highlight the irregularities, and it is sure to lead to a complaint. 

Heat welding is growing in popularity as an installation system.  It is an expensive investment 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.