'Universal' grooved-feed extruder achieves higher throughput.
New data illustrate that grooved-feed sections can achieve high throughput rates with a broad range of resins - not just polyethylenes - that characteristically exhibit poor solids conveyance. This was demonstrated by Filmaster Inc., Fairfield, N.J., on its relatively new all-purpose grooved-feed extruder with a "universal" decompression screw. Using free discharge (no die), the company achieved typical throughputs in the range of 300-420 lb/hr with a variety of resins using a 2 1/2-in., 24:1 L/D machine with 50-hp drive. In a lab environment, these outputs were achieved for the following resins:
*Quantum #NA233 LDPE, 1.4 MI: 360 lb/hr.
*Quantum #NA355 HMW-LDPE, 0.5 MI:380 lb/hr
*Chevron #5754 LDPE, 0.8 MI:300 lb/hr/
*PP, 7.0 MI:360 lb/hr.
*Phillips K-Resin #KR4, 8.0 MI:420 lb/hr.
These materials generally are difficult to process in smooth-bore extruders and run poorly on barrier-type and double-flighted screws, Filmaster says.
Throughputs were achieved with a high level of power efficiency. Rates up to 420 lb/hr were obtained with a 50-hp d-c motor rated at 100-amp continual service.
Power consumption at 140 rpm did not exceed 40 amps. Although this rating was achieved in free discharge from the end of the extruder barrel, the machine would accomplish 75-85% of the free-discharge rate with a die, according to Filmaster sales v.p. Frank Goffreda.
High outputs and consistent melt pressure and temperatures were also achieved under production conditions. A similar machine using a 75-hp d-c motor, rated at 150 amps max. continuous duty, processed 380 lb/hr of Hoechst #9255 HMW-HDPE through an 8-in. spiral die at a head pressure of 5400 psi, with 95-amp power draw and 175-rpm screw speed. Goffreda claims that outputs achieved on the 2 1/2-in. grooved-feed extruder with 50- or 75-hp drive were comparable to a 3 1/2-in. smooth-bore machine with 125-hp drive.
The grooved-feed unit reportedly processes polyolefins at high rates with uniform melt quality and consistent temperature and pressure, and is said to efficiently run high percentages of regrind or scrap. The machine has been shown to be effective in processing material containing up to 70% of fluff or regrind chips up to 0.375-sq.-in. size.
Unlike smooth-bore machines, the force-fed grooved section reportedly achieves 85-95% melting of pellets after 7-9 in. of travel, depending on machine size. Mixing and melt homogenization occur in the remaining barrel length.
Particular attention was paid to the design of the screw in conjunction with the final geometry of the grooves of the feed section. This 2 1/2-in.-diam. screw doesn't employ a constant-lead, square-pitch flight design. Instead, the distance between flights (pitch or lead) is smaller than the barrel diameter. Consequently, there are more screw flights, which is said to give better mixing and homogenizing of the melt in a relatively short barrel length.
Also, the universal screw, which has a 1:1 compression ratio, is said to have nearly as high outputs for HMW-HDPE as screws with negative compression ratios of 3:1 or 4:1. Yet, unlike negative compression screws, the universal screw will also successfully run the broader range of resins. The universal screw is said to produce very low shear after the feed section, and incorporates special segmented flights in the metering and mixing section.
OUTPUT OFFSETS HIGHER COSTS
The higher initial cost, as well as replacement costs of grooved-feed machines, are offset by their better performance than smooth-bore machines, according to Goffreda. Price of a fully equipped Filmaster grooved-feed unit is about $58,000, 30% or so higher than a comparable-size smooth-bore machine, but throughput rate is also 30-40% higher.
Replacement cost of Filmaster's "universal" decompression screw runs about $1200-1800. Grooved-feed barrels, which may have half the service life of their smooth-bore counterparts (depending on application), can be replaced for around $3500. (CIRCLE 57)
PHOTO : OUTPUTS, MELT PRESSURE AND TEMPERATURES OF HOECHST #9255HS RESIN (HDPE)
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|Author:||De Gaspari, John|
|Date:||Feb 1, 1991|
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