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Broader range of web thicknesses is made on new OPS line.

Broader Range of Web Thicknesses Is Made on New OPS Line

An oriented polystyrene extrusion line that is capable of producing optical-quality film down to 0.5-mil thickness has been introduced by Marshall & Williams Co. of Providence, R.I. The company, which custom designs OPS systems, says that various machinery improvements have added greatly to the flexibility of OPS lines using vertical roll stacks. According to Robert Simmons, process consultant and sales engineer. "The biggest advantage is that producers can make more and higher quality material in a shorter period of time, because there is more control available."

Until now, claims Simmons, both horizontal lines and vertical three-roll stack lines have had thickness-range limitations in producing OPS film and sheet. Vertical three-roll stack systems have excelled in producing heavier gauge sheet (6 to 24 mil), used in the U.S. for thermoforming thick-wall parts and deep-drawn containers, particularly for fast-food applications. Horizontal lines, which are prevalent in Japan and were used on some of the earlier lines in the U.S., are limited to thin-gauge (1 to 8 mil) OPS film applications.

Now, Simmons claims, it is possible to produce thin film of less than 1 mil on a conventional vertical three-roll stack system. The company claims to have produced 0.5-mil (12-micron) OPS in its lab, and Simmons says that gauges as thin as 0.25 mil (6 microns) are possible. The key to this added flexibility is in the dies, the added temperature and tension control of the film in the machine direction prior to entering the tenter frame, and the increase in overall draw ratio, but possible in older conventional machines, says the company.

INTERCHANGEABLE DIES

By interchanging dies at the extruder, material can be fed into the system at a narrower width, and higher draw ratios reportedly can be achieved. For example, a 60-in. die may be used to feed 52-in.-wide material into the tenter frame to produce 130-in.-wide finished product with a draw ratio of 2-1/2:1. Substituting a 20-in. die at the extruder to feed 18-1/2-in. material would achieve a 7:1 draw ratio to produce the same width OPS film.

A higher degree of control and greater flexibility can also be achieved with the use of additional drives and heat-transfer zones. For orientation of PS to be successful, sheet in the machine-direction orientation (MDO) step has to be kept as close as possible to its glass-transition temperature--240 F--throughout its cross-section. Simmons claims that multiple heat-transfer zones are necessary to cool the sheet from the extruder at a very controlled rate. The number of recommended heat-transfer zones depends on the system, although typically nine to 11 zones are included on newer systems.

Also critical to the company's OPS line is proper tension between the rolls to prevent sag or roll "picking." Tension in the sheet is necessary to overcome the surface adhesion between the roll and the material. Older lines have typically included only four drives, and other rolls were permitted to freewheel, explains Simmons. Although that may have sufficed if the rolls and material in the cross-section were cool enough, it is not sufficient for more flexible systems designed to handle various material thicknesses, he claims. "When you sell a machine to make a wide range of products, it has to provide many thicknesses, and at different thicknesses you have different roll temperatures."

The optimum--and most expensive--alternative to guarantee the right tension is to put a drive on each roll. Yet without going quite that far, a means of changing speed between one roll and another can still be achieved. One way to accomplish this is through a small mechanical adjustment. A simple belt and tapered cone arrangement can modify speed about 3% to produce the required tension, says Simmons. Other rolls, though, may need as much as 5-10% adjustment, which would require the addition of a drive. Newer, more flexible systems may include as many as eight drives. "You can add on another $50,000 an equipment price, and for that additional investment you can handle the entire spectrum of product thickness ranges."

LIMITING FACTORS

Line speeds increase as film is drawn to narrower gauges. Simmons considers a realistic max. takeoff speed for 1-mil film to be about 600 ft/min. Because of the tensile properties of OPS, sub-mil gauges must be run at less than half the extrusion output of 1-mil film, and at a narrower final width, perhaps as narrow as 70-80 in. wide. The reason is to reduce machine-direction stresses put on the film by the downstream equipment after the tenter frame.

Simmons claims that the additional flexibility that has now been achieved with PS also can be attained with polycarbonate and other amorphous resins. "The difference between the essentially amorphous materials like PS and PC and semicrystalline materials like PP and PET is the large crystalline sites that are formed when the plastic is cooled at a slow rate. Because of the low thermal conductivity of most polymers (i.e., 0.068 Btu/hr-sg ft-[degrees]F for PP), the thicker the sheet is cast, the slower it cools. For crystalline materials, this slow cooling causes rigid or nonextensible crystalline sites called spherulites to form throughout the cross-section of the material, which hampers or sometimes eliminates the possibility of orientation. In amorphous materials, the only problem is one of getting the cross-section of material to a uniform orientating temperature, because no large crystalline sites are formed." This fact is said to allow amorphous materials to be made into a much broader range of web thicknesses that is limited by machine design and not by inherent processing problems within the resin.
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Title Annotation:oriented polystyrene
Author:De Gaspari, John
Publication:Plastics Technology
Date:Jun 1, 1990
Words:946
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