MDOs can 'stretch' film yield and boost properties.

About 20 film processors in North America are in on a little secret: Machine-direction orienters, known as MDOs, can do wonders for both yields and product properties from otherwise ordinary blown- or cast-film lines.

MDOs are standard equipment on the front end of biaxially oriented film extrusion lines for PP, PET, and PS. Specialized casting lines for strapping are a fairly common nonbiax application for these devices.

But lately these units have found their way into more mainstream film extrusion jobs, reports Andrew Christie, engineering manager for extrusion systems at Black Clawson Converting Equipment Co., Fulton, N.Y. Christie says his company has supplied eight MDOs in North America - six for cast film, two for blown. Five of these have been installed within the last few years. He estimates there are about 20 MDO installations for blown or cast film in North America all told. Christie reported on MDOs for thin films at Film & Sheet Conference '97, a three-day event held Dec. 9-11 in Somerville, N.J.

Processors that have bought these units say MDOs are responsible for impressive improvements in both yield and properties. Yield is improved because the units allow for downgauging. In one application, a 10-mil film was downgauged to 5 mils through off-line orienting. Christie says downgauging levels of 50% are typical.

Orientation is known to boost barrier properties. In fact, one processor equipped with an MDO said he could use less of the expensive barrier resins as a result. Not surprisingly, film stiffness in the machine direction is another property that MDOs improve. That's an important feature for film used to make stand-up pouches, for instance. It also makes snack-food packages easier to open, since orienting in the machine direction makes the film split the other way.

HOW THEY DO IT

In a typical MDO, film is fed through a series of rolls, where it is sequentially heated, drawn, annealed, and cooled (see illustration). The temperature of the heating station depends on the resin being used and the final properties desired, Christie explains. Generally, the orientation temperature is 60-75% of the distance between the material's glass-transition temperature and melting point.

After preheating, the film is first wrapped around a heated, slow-turning draw roll and then around a faster draw roll. Both rolls are heated to the material's orientation temperature. The fast draw roll is driven at speeds up to 10 times faster than the slow roll. From there, the film passes over a large-diameter annealing roll. Annealing or heat treatment relaxes stresses in the film. The film is then cooled on a final roll.

For blown or cast film, MDOs can be positioned either off line or in line right before the winder.

Christie reports that MDOs can be used on a wide range of materials, including virtually all polyolefins and PET, and are effective on films as thin as a half-mil. Black Clawson sells MDOs in single- and dual-stage models in widths from 80 to 110 in. Dual-stage machines have two stretching stations and are suited for higher-speed lines. A typical price is $500,000.

Other suppliers of MDOs include Marshall & Williams Co., Providence, R.I.; Lenzing of Austria, which is represented in the U.S. by Tex America Inc., Charlotte, N.C.; and Bruckner Machinery Corp., Spartanburg, S.C.