The construction industry has long recognized the need for an economical means for handling misalignment between system components and ducting around obstacles where fabricated and fitted ducts are difficult and costly to install. This led to the development of several types of flexible duct material to perform this function in a practical manner. Initially, flexible ducting was constructed of a fabric tube glued over a spiral wire helix on a mandrel. This limited the product to short lengths and long production time to allow the glue to dry. In cases where the glue is applied unevenly, the spiral wire has a tendency to separate from the fabric, allowing the duct to collapse.

This process was improved by development of a method allowing the duct to be formed by winding strips of fabric over the spiral wire with adhesive being applied as the duct is formed. This process is sometimes utilized with a hot melt adhesive applied to the wire and subjected to heat to cause the wire to adhere to the fabric. A method of solvent sealing is also used wherein the wire is encapsulated with a soluble material and a solvent applied to cause it to adhere to the fabric. These methods reduced production time and permitted duct to be formed in longer lengths. With this construction, the application of the adhesive is critical and any variation in the manufacturing process causes weakness in the duct. In many cases a tug on the helix end will cause the entire liner to start unwinding. Strain and pressure can separate the helix from the fabric with the likelihood of rupture or collapse. The recognition of glue and other adhesives as a weakness brought about the development of flexible duct with fabric locked into the helix by mechanical means. This process eliminated adhesives by interlocking the fabric in a formed metal spiral. While this solved the problem of separation, it retained the helix on the inside where it causes turbulence in the air stream and increased pressure drop and acoustic levels, particularly when installed in complex configurations. The fabric on the outside is stretched over the helix and scuffing during handling can easily cause leaks. Nevertheless, all of these products have solved the problems of misalignment and properly installed serve the purpose intended.

While the advantages of mechanical lock were recognized, this method has been more costly and economic pressures have caused other manufacturers to deemphasize this construction in favor of cheaper methods.  Masterduct has resisted this trend by developing the equipment and technique to economically fabricate flexible duct by the mechanical lock method with the metal helix on the outside. This construction gives the product the strength, durability and scuff resistance of sheet metal ductwork while retaining the flexibility of a fabric duct. The unique patented machines that produce the Flexmaster U.S.A.^® HVAC products run at high speeds and the manufacturing process is automated to produce the highest quality product at the lowest possible cost. The mechanical lock construction illustrated below provides a positive seal while solidly positioning the helix as an integral part of the fabric.

The process is compatible with any flexible material from ordinary paper to the most exotic laminated fabrics. With this flexibility, Masterduct researched and tested fabrics offered for the best combination of strength, permeability and flame resistance with properties to resist temperature change, mildew and age hardening. Current production is fabricated with a trilaminate of aluminum foil, fiberglass and aluminized polyester, which has passed rigid testing in all classifications. Testing is continuing, and, as new and better fabrics are developed, they will be considered for use in the Flexmaster U.S.A.^® HVAC product line duct material. During a period of scarcity of insulating materials, the industry generalized its reference to insulation thickness to allow for occasions when designed thicknesses were not available. To meet economic pressures, some manufacturers are still cataloging and marketing material with minimum insulation and C factors as high as .37.  During periods of shortage, the Flexmaster U.S.A.^® HVAC products experienced some variance in insulating materials, but, in all cases, a combination of thickness and density is maintained to provide a C factor of .23 or lower. Masterduct will continue to maintain high standards as set by the industry as conditions change.

A variety of vapor barrier outer jacket material is currently in use. Many manufacturers use a vinyl film which has the disadvantages of age hardening at freezing temperatures and will crack and rupture at extremely low temperatures. After extensive testing, Flexmaster U.S.A.^® HVAC products adopted, as standard, a fire retardant polyethylene material with low permeance and excellent strength at low temperatures passing A.S.T.M D l 790 at - 40F. It is tested and classified by ETL with a flame spread of 20, fuel contributed 0 and smoke developed 35. As an extra cost option Flexmaster U.S.A.^® HVAC products offer a metalized jacket consisting of a fiberglass fabric coated with a elastomer compound and laminated to metalized polyester film. Both jackets have features to recommend them but the primary advantage of the metalized jacket is cosmetic, giving the appearance of an all metal duct when properly installed. Masterduct does not feel this justifies the extra cost and recommends the standard jacket for all normal service but can supply the metalized jacket if the occasion demands.

The outstanding performance of the Flexmaster U.S.A.^® HVAC product line is the result of the unique construction method which forms a rigid aluminum helix on the outside rather than using a cold rolled steel spring wire on the inside. Positioned on the outside, the helix protects the fabric from scuffing. The strength of the formed helix will not allow the duct to oval at bends, to balloon or collapse at recommended positive or negative pressures. The mechanical lock eliminated problems of separation and unwinding and duct ends can be trimmed clean and easily joined to prevent waste. Masterduct is proud of the Flexmaster U.S.A.^® products acceptance in the marketplace and research and development continues to refine equipment and materials to offer superior products at competitive prices. The company is active in all industry programs and is dedicated to maintaining its quality standards for the benefits of, the ARCHITECT, the ENGINEER, the CONTRACTOR and the ultimate end user, the OWNER.

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