Heat Wares
by Gene Rebeck
Industrial Fabrics Products Review, February 1997

There are at least six types of heat-sealers.  What kind should you get?

   Though they haven't completely replaced sewing in all plastic-based industrial fabrics, heat-sealers or heat-welders -- machines that "melt" and fuse sheets or parts together --  have established themselves firmly in the industry.

   But just because they've become more familiar to industrial fabric end-product manufacturers end-product manufacturers doesn't mean everyone is familiar with them.  Firms looking to invest in heat-sealing equipment can find the array of such machines daunting.  Even those who own heat sealers aren't always aware of all the available options, some of which may be more suitable than their current equipment.

  So, for both newcomers to heat-welders and veterans who want to know more, here's a guide to the types, plus some thoughts on how to make the most of your investment.  (After all, even a used heat-sealer isn't as cheap as, say, a retooled sewing machine).  In this article, we'll focus on sealers that seam-weld industrial fabrics used in manufacturing such traditional "canvas" products as tents, tarps and awnings, as well as items made for the construction, environmental, architecture and safety/protective garment industries

   Types of sealers
   Industry people identify anywhere from four to eight types of heat-sealing equipment, and not everyone agrees how to distinguish the various types.  We identified six, listed here in roughly chronological order (oldest to newest)  What makes them all heat-sealers or heat-welders is the combined use of heat, pressure and time to weld two or more layers of material together.  In many cases, pressure is accomplished using either a pressure bar or a roller than presses the two pieces of material together.  For optimal performance, many bar-type pressure heat-sealers require a cooling cycle under pressure.

   Thermal or hot-die welders.  Perhaps the first heat sealing technology, these machines represent a relatively inexpensive and still widely used form of heat-welding.  The method uses a heated platen to transfer heat through the materials to be fused, resulting in a heat-sealed, finished product.  According to Glenn Lippman of Novaweld, a Denver, Colo. (now Novaseal Corporation, a Boca Raton, FL company), a heat-sealer manufacturer, this process is limited specifically to "thin plastic materials where a cooling cycle is not important, and where external surfaces can be affected by excess melt or deformation associated with heat penetrating from the outside."

   Radio-frequency (RF), high-frequency or dielectric welders.  This type first appeared in the late 1940s.  Traditional "canvas" fabricators (particularly awning and tarp manufacturers) will be most familiar with this versatile type of welder, which uses radio-frequency energy and dielectric heating to create the melting fusing phenomenon.  It is used primarily for seam-welding medium to thicker plastics such as vinyl, although with frequency modification, other materials can be used.  It offers seam widths up to 1/2-inch, in lengths up to five feet.  RF welders typically have an up/down press action; for continuous applications, and indexer can be used.

   Ultrasonic welders.  These machines channel ultrasonic vibrations through a horn or sonotrode, which transmits the mechanical vibration to the workpiece under pressure.  This technique is used for seam-welding thermoplastic sheets and formed parts -- typically, amorphous resins that include polystyrene and polycarbonate.  it can also be used for making a variety of medical garments from such materials as acrylics and polyester (see "Sound Seams" in the February 1996 Review, page 48).  These machines can seam widths up to 1 inch, with lengths up to about 12 inches, and are available with up/down press or rotational heads.

   Impulse or hot-bar welders.  Bob Wasilko, sales manager with heat-sealer manufacturer Thermex/Thermatron, Bayshore, NY, and an industry veteran, describes the electric-resistive heating elements as "similar to those in a toaster."  These elements are mounted on two pressure plates, between which the materials are passed.  The "impulse" is the pulse of electrical current that is sent to the heating elements.

   According to industry experts, these machines typically are used for thin-film plastics (polyethylene, notably) and lightweight nonwovens.  Bag closures represent perhaps the chief application of this species of welder.  Seam widths range from 1/8 to 5/16 inches; allowable material length is up to 10 feet. 

     Hot-air or hot-gas welders.  Another type of welder familiar to canvas shops.  Here's how Thermex/Thermatron's Wasilko describes their operation:  "This method depends on superheated gas being blown between the two surfaces  to be bonded."  What kind of gas?  "Air is the most commonly used." he says, "However, for plastics that are readily subject to oxidative degradation, an inert gas is preferable."  In other words, the hot air or gas melts the materials, typically by being placed in between the two parts; pressure rollers push the materials together.

   Wedge welders.  The newest heat-sealing technology (as of 1997).  Wedge-welding applies the thermal energy directly between the layers of material with direct contact to the outside surfaces.  Wedge welders use what is called a "translational" technique -- the heating "wedge" moves between the sheets of material, which then are welded using a pressured roller.  The thermal energy is in direct contact with the interfacing sheets and does not need to penetrate from the outside.

   According to Novaweld's Lippman, these sealers are "ideal for seam-welding synthetic materials, including vinyl, vinyl-coated materials, high-density polyethylene, polypropylene and nylon, as well as synthetic nonwoven materials.  Since wedge welders are not significantly affected by dust and dirt, they can easily be used for field applications, where depending on the materials, welds can be achieved at rates up to 60 feet per minute.  For factory applications, production speeds approach several hundred feet per minute with seam widths up to 2 inches."

   Wedge welders are available in hand-held, walk-behind and self-propelled styles.  "The biggest advantages of using wedge-welding technology over other heat-sealing methods is seam width, heat sealing speed, and welded seam aesthetics," Lippman assets.  "Wedge-welding permits welding speeds in excess of several hundred feet per minute for industrial applications, along with widths of 2-plus inches, without damage to the exterior of the materials being welded."

   It's worth noting that these categories aren't quite exhaustive.  Some heat-sealers on the market seem to be in their own categories.  Case in point.  The bar/diaphragm sealers made by GR Systems, Inc., Sarasota, Fla.  In some respects, GR Systems' welders appear to be variation of impulse or hot-bar welders.  But company president David Seaman says his product is distinctive.  Heat is conducted from heated bars through "diaphragm-set" units.  When the material reaches weld temperature, the bars are withdrawn and air is passed through the diaphragm sets to cool the weld.  The result, asserts Seaman, is a smooth-edged seal that doesn't show a seam. 

   Seaman also says that GR Systems can make heating bars wider and longer than those available on impulse and dielectric sealers.

   Shopping tips
   So now do you know what kind of welder you want to buy?  Probably not.  Consider this overview as, at best, just the beginning of our purchasing efforts.  Heat-sealer experts agree that you should do your homework before you buy.

   First, keep in mind, as Gerald Lippman of Thermo-Plastic Heat Sealing Technology, Boca Raton, Fla, notes, that "each type of heat-sealer is used for specific materials.  For heat-sealing vinyl, use RF; for heat-sealing poly materials, use thermo, impulse or wedge welders; for others, as well as for the above materials, use ultrasonics."

   In other words, sealers are not interchangeable.  Says Novaweld's Glenn Lippman:  "The buyer needs to carefully articulate his/her heat-sealing requirements -- in other words, what materials need to be heat-sealed?"

   LeRoy Sperr of R.F. Thermoform Products Inc., Huntsville, Ala, offers this advice for welder shoppers:  "Contact each of the heat-sealer manufacturer's reps and find one you are comfortable with.  There aren't very many differences between the American machines, but there is a vast difference between sales and service representatives.  Talk to people who own machines and ask who they use for service.  With luck, you'll find one nearby, since travel expenses and service time are expensive."

   "Most manufacturers of heat-sealing equipment will provide customers with samples,"  Glenn Lippman adds, "This is one way for customers to overcome any possible fear of making a decision.  Once the customer reviews and tests the sample, I recommend that the customer assess the various methods and make a chart that includes production speed, seam quality, price, delivery, flexibility (die/fixture costs), risk analysis and overall return on investment. 

   "I think the biggest problem facing customers is lack of knowledge -- that is, what methods exist and how they apply," Lippman adds, "I've talked with customers who were in complete shock when they learned that they could get speeds in excess of five times what they are normally used to at 50 percent less money."

   Richard Sperr of Dielectric Tech Corp, Lakeview, Ark., suggests attending seminars and workshops, and lists four "factors" shoppers should keep in mind before deciding on a particular machine:  (1)  Availability of help and information on production problems and after-the-sale service on the machine.  (2)  Size and reliability of the machine's manufacturer.  (3) The rating on the oscillator tube (part of an RF heat sealer than generates current to heat seal the plastic).  "Some [RF] manufacturers use under-rated power tubes, which don't last much past the warranty period," he says.  (4) Don't buy on price, on way or another.  "Most machines are reliable," he says.  "High price does not mean best."

   In doing your shopping, keep in mind that while manufacturers provide standard machines, many also provide custom units.  For instance, Vinyl Welders, a Navarre, Ohio, manufacturer of hot-air welders, regularly designs and builds units for specific applications and companies.  Vinyl Welders is by no means alone in this capability.  Even small fabricators can get longer presser bars and other alterations on standard machines

   For more information

• William Croft, president of Cadillac Products/Clements National Co., a Chicago-based manufacturer of hot-air welders, offers a simple checklist for avoiding common heat-welding problems in "Keeping the Heat On," an article that appeared in the December 1994 Review.  (An extended version of this is available from Cadillac's Chicago office -- contact the company at 708/594-5890, fax 708/594-2481.)
• In the October 1993 Review, Richard Sperr, discusses ways to keep your RF sealer working properly.  He also has a book on these sealers:  Electronic Heat-Sealing:  A User's Guide to Better Products.  It is available from Dielectric Tech. Systems, P.O. Box 451, Mountain Home, AR 72653; fax: 501/431-5269.
• For wedge-welding tips, check out Novaseal's Web site, www.novaseal.com.

Gene Rebeck is editor of the Review.

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