Plastics Technology

FEB 2013

Plastics Technology - Dedicated to improving Plastics Processing.

Issue link: http://pty.epubxp.com/i/105397

Contents of this Issue

Navigation

Page 15 of 59

close up extrusion studies boost Prospects for high-speed extrusion a properly designed high-speed, single-screw extruder (hssse) can significantly boost the processing capability of a small-diameter machine for a wide range of applications. That was the conclusion of a study conducted by Davis-Standard LLC (D-S), Pawcatuck, Conn., and presented at the Society of Plastics Engineers ANTEC 2012 meeting last April by John Christiano, D-S's v.p. of processing technology. D-S unveiled its frst high-speed extruder at NPE2012, which was co-located with ANTEC in Orlando, Fla. High-speed twin-screw extrusion systems have been the norm in compounding since the 1990s, when a handful of machine builders introduced super-fast, high-torque designs. In single-screw extrusion, however, high-speed technology is far more common in Europe than North America. The D-S study investigated the extrusion characteristics of an HSSSE to determine whether it was suited to several different extrusion applications including sheet, fber, and extrusion coating. This article reports on the results related to sheet extrusion, looking at how PP and HIPS grades performed at high speeds. The extruder used for the study was a 2.5-in., 40:1 L/D extruder equipped with a 500-hp motor. Screw speeds up to 1200 rpm were evaluated. The extruder was "highly instrumented," Christiano's paper remarks. Specifcally, D-S's Thermatic Temperature Control system was used to control the barrel temperatures and measure the heat fux in each zone of the extruder. Eight pressure transducers were located along the barrel at approximately 5 D increments to measure axial pressure generation along the screw. A pressure transducer and exposed-junction melt thermocouple were located in the discharge adapter of the extruder in order to evaluate the output and processing thermal stability of the melt. A valved adapter and a 30-in.-wide coathanger die with a 0.050-in. die gap was utilized for all of the sheet trials. A 4 D static mixer was installed between the valved adapter and the die. A two-stage, moderate- to high-intensity screw design was used for processing sheet-grade PP and HIPS. The data recorded at each operating condition included the extruder output, melt temperature, motor power consumption, and pressure at the discharge of the extruder. Pressure readings 14 february 2013 Plastics technology This 3-in., 40:1 high-speed, single-screw extruder was introduced by Davis-Standard at NPE2012 for PS and PP sheet and inline thermoforming. were recorded with a data-acquisition system at a rate of 50 Hz. The processing stability was evaluated for each material by monitoring pressure variation and melt temperature variation vs. time at the discharge of the extruder. The melt quality of the extrudate was inspected visually as it exited the die. The screw speeds examined ranged from 400 to 1200 rpm for each of the materials. The suitability of the HSSSE for each application was judged by comparing its performance with a predefned set of parameters that are commonly used for each application, notably feeding consistency vs. screw speed or linearity of specifc output (kg/hr-rpm) vs. screw speed; acceptable melt temperature range; and required process stability. Process stability was evaluated by monitoring pressure variation, as an indication of output consistency, and melt temperature variation in time, as an indication of thermal homogeneity. Melt quality was inspected visually for defects exiting the die. In the PP sheet test, output rate increased linearly with screw speed. There was a slight increase in the specifc output rate at 800 rpm; however, at 400 and 1200 rpm the value was constant. The relatively constant specifc output throughout the processing range indicates that the solids conveying zone of the screw was operating effciently. The variation in specifc output of less than 1.8% over the desired processing range is well below the required feed-rate

Articles in this issue

Archives of this issue

view archives of Plastics Technology - FEB 2013