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HMW-HDPE blown-film line pushes the speed limit.

Unusually high output rates were demonstrated recently on an HMW-HDPE blown-film line from Kiefel Inc., Foxboro, Mass. The line, on hand at last June's NPE in Chicago, produced film at speeds up to 800 ft/min and outputs as high as 540 lb/hr with a 120-mm die--equivalent to 36 lb/hr/in. of die circumference. The high speeds were demonstrated with commercial HMW-HDPE resins producing 30- to 33-in.-wide layflats in gauges from 10 to 14 microns and at blow-up ratios of 4:1 to 4.4:1. At 10-micron gauge and 30-in. layflat, outputs reached 31 lb/hr/in. of die circumference. Several incremental machinery improvements before and after the die are said to have worked together to provide the high outputs.

Although Kiefel will not guarantee that customers will reach outputs as high as these, the demonstration is nonetheless significant, according to Kiefel president Steven Engel, because it in effect pushes the limits of realistic line speeds that film producers can expect to achieve in commercial installations.

DESIGN IMPROVEMENTS

Key to the higher outputs demonstrated was the Complex 70 SA/21D extruder with grooved feed section and redesigned screw that provides a higher degree of decompression, which is said to result in low, uniform melt temperature. The cooler, more stable melt exiting the die requires less coopling and provides higher outputs and better film strength properties, according to Engel, who adds that uniform melt temperature also contributes to good gauge control. Engel also notes that the current trend toward increased temperature of the cooling water in the barrel feed section contributes to reduced wear of the screw and barrel, which is especially important at high output rates.

Demonstrated line speeds actually surpassed what was previously regarded as the design limit of the 70-mm extruder. Theoretically, says Engel, the limit of a grooved-feed extruder is the point at which the melt temperature begins to rise significantly with increased screw speed. With the redesigned screw, Engel says, melt temperature remain consistently low--from 400 to 410 F--even at outputs over 500 lb/hr.

Another significant factor is the die with newly designed spiral mandrel, which runs at relatively low pressures for HMW-HDPE. At maximum output, the die functioned in the pressure range of 5000-7000 psi--about 1000 psi lower than Kiefel had achieved before.

After the die, the Jet Cool internal bubble stabilizing and cooling system is said to enhance uniform gauge tolerance because the bubble is centered over the air ring even at high outputs and thin film gauges. The Jet Cool system, commercially available for about three years, is said to be unique in that the top of the neck of the bublle actually contacts an inverted cone that acts as a stabilizing device. This, coupled with internal bubble cooling, reportedly provides excellent stability in the neck area, as well as a high degree of cooling in the transition area to maximum bubble diameter. The system's inverted cone, which provides the critical stabilizing effect at high outputs, is adjustable in height and can run a range of neck lengths. It's also reportedly capable of running a range of blow-up ratios.

As with other components in the line, the Jet Cool system has been refined to provide enhanced performance, says Engel. The diameter of the inverted cone has been made smaller to provide greater neck-in and better blow-up ratio, resulting in improved balance of properties in the machine and transverse directions, according to Engel. Air flow has been streamlined, resulting in less air turbulence around the bubble. To reduce friction, the mandrel and lower stabilizing rings have been spiral-cut to reduce the surface area of contact. A cotton lining on the inverted cone reduces the chance of the still-molten neck of the bubble to stick to the aluminum surface of the inverted cone.

Among the other factors contributing to the high line speeds is a cylindrical screenchanger, which has a lower pressure design said to permit higher outputs than disk-type screenchangers. Also, a new Model EC22W air chiller ensures an air temperature of 50-60 F.

ACCEPTABLE FILM QUALITY?

The bottom line, of course, is whether acceptable product quality can be maintained at high line speeds. Engel admits that film properties were sacrificed somewhat at the very highest line speeds. For example, fluttering, because of the high volume of air produced by the air ring, caused gauge control to decline a bit despite good bubble stabilization. In addition, because these speeds were generated for the first time, Engel cannot be sure of what the long-term effects will on the equipment.

Engel points out that acceptable film properties were achieved when the line was backed off a bit from the very highest line speeds, giving some indication of what commercially achievable line speeds are. "Realistically, we have customers now who are running in the rang of the low- to mid-20s [lb/hr/in.]. This latest demonstration pushes the realistic speed higher--to the high 20s and into the 30s."

The upgraded line, which has been sold to a processor, is now producing HMW-HDPE trash-bag film of 24-in. layflat and 6-micon gauge at rates up to 800 ft/min. Approx. price of the line is $450,000 with 70-mm stationary extruder and 120-mm stationary die. (CIRCLE 9)

OTHER INTRODUCTIONS

Apart from the upgraded blown film line, Kiefel recently introduced a number of other products for extrusion:

* New model QAR1600 oscillating hauloff features a design in which the bearings and gears are moved from the center to the outer diameter of the hauloff. The result is said to be enhanced rigidity of the unit that eliminates film wrinkling. The design reportedly is also easier to operate because the center is open, facilitating thread-up of the web over the air turning bars and idler rolls. The hauloff is available in maximum widths from 800 mm to 250 mm, and ranges in price from $310,000 to $225,000. (CIRCLE 10)

* A new rotating 80-mm extruder for all types of PE has a small turning radius and 20:11 L/D, and is availalbe in fully rotating or reversing versions. Cost is about $225,000. (CIRCLE 11)

* New model EC22W air cooler consists of a fully hermetic compressor, water-cooled condenser, evaporator, and temperature controller. The compact unit is said to eliminate the need for a bulky water chiller plus cooling-coil heat exchanger. The unit costs $29,000. (CIRCLE 12)

* A star-seal folder for fully gusseted film or tubing works in-line with an extruder and bag machine. It's said to provide wrinkle-free film at high speeds and to be easily adjusted. It can be mounted on the floor or in the lower. Cost is about $7900. (CIRCLE 13)
COPYRIGHT 1991 Gardner Publications, Inc.
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Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:De Gaspari, John
Publication:Plastics Technology
Article Type:Product Announcement
Date:Nov 1, 1991
Words:1110
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