Understanding Grooved feed.
With many polymers, grooved-feed extruders produce 20-40% higher output per rpm than the same diameter extruder with a smooth feed bore. A grooved feed section improves solids conveying and increases extruder pumping action, thereby raising output at a given extruder rpm, reducing energy put into the polymer, and lowering melt temperature.
Grooved feeds debuted in Europe in the l950s, so European processors have had plenty of time to adapt the technology to most applications, both where it has advantages over smooth bore and where it doesn't. Where there are no advantages, they have found ways to neutralize grooved-feeding action by heating the grooves, thereby making grooved-feed sections nearly as versatile as smooth.
U.S. extruder suppliers historically took the opposite approach, making smooth feed bores the norm. Grooved-feed extruders were scarce in the U.S. until the 1980s, when HMW-HDPE became popular for blown film. It contained slip and anti-block formulations that wouldn't develop adequate solids conveying with a smooth feed bore. Smaller European grooved-feed extruders (1.5-2.3 in. diam.) then found a market here.
This brought field-proven grooved-feed extruders to the U.S. from suppliers like Hosokawa Alpine American, Kiefel Inc., Reifen-hauser Inc., Windmoeller & Hoelscher Corp., and Kuhne GmbH. But except for HMW-HDPE film, the U.S. practice was still to use a larger-diameter smooth-bore machine to get desired output at desired melt temperature, even if a polymer would perform better with a grooved feed section.
Finding the right groove
The grooved feed section of a 3.5in. extruder may have eight to 18 grooves evenly distributed around the feed bore. In general, higher viscosity resins like HMW-HDPE or polypropylene benefit more from lots of grooves; lower viscosity resins use fewer grooves. Using too many grooves can feed resin to the mixing stage of an extruder too fast and cause melting inefficiencies and mixing problems.
Grooves are typically 0.15-0.3 in. wide and deepest toward the back of the feed area. Starting depth is 0.12-0.37 in. and tapers to zero in three or four diameters past the downstream end of the feed opening. Grooves run parallel to the screw axis. One machine supplier, Battenfeld Gloucester Engineering Co., used to build helical grooves, but several years ago switched to notched axial grooves.
Feed sections are designed with cooling jackets to provide intensive water flow around the feed opening and the extended grooved section. This prevents resins from melting in the grooves. Conversely, to reduce the feeding action of the grooves and slow the pumping of materials like nylon or polycarbonate, heat (200-300 F or higher) can be applied to the grooves. This starts the resin melting, and avoids a dangerous pressure buildup at the end of the grooves.
Another alternative is short grooves that extend only half a diameter into the barrel. For processes that run up to 50% regrind, short grooved-feed sections can improve feeding consistency. But short grooves won't work on HMW-HDPE. And high regrind percentages of any low-bulk-density material (10 lb/cu ft or less) will also feed poorly unless the fluff is compacted before it is added.
Screw choices for grooves
Long grooves are always hardened either by nitriding or, for more abrasive resins, by coating them with tungsten carbide. Shorter groove sections typically aren't hardened. Screws are often hardened with the same substance as the grooves for compatibility and even wear.
It's not common to change the grooved sleeve in order to process different resins. The sleeve is replaced only because of wear--i.e., every 5-10 years. Of course, the screw can be changed to handle different resins with the same grooved feed.
Choice of hardening material depends not only on the abrasiveness of the polymer but on the early screw depth. The original metering-screw designs of the 1960s and '70s sometimes included one or more mixing sections and a uniform channel depth throughout the screw length. These designs saw the highest pressure build-up--and the greatest need for hardening--at the end of the grooves.
Later grooved-feed screw designs from the 1980s included melt-separating barrier sections with deeper screw channels at the end of the grooves to relieve pressure. These may not need tungsten carbide grooves for most polymers. However, those screws should have shallower flight channels than those used with smooth bores, or else output efficiency may drop to the point where the grooves no longer improve feeding.
Grooved-feed screws today are relatively shallow throughout the feed channels to provide more efficient feeding. On a 3.5-in, screw, channels with a grooved feed will usually be 0.25-0.4 in. deep, half the depth of channels with a smooth bore (0.6-0.7 in.). The shallower screw channels for grooved feed keep the resin solid-bed height short and maintain high forward force in the grooves. This also builds up enough pressure at the end of the grooves to achieve adequate compression in the channels entering the heated extruder barrel.
In the 1950s and '60s, screw channels with grooved feeds were the same depth throughout the length of the screw, whereas screw channels with a smooth bore get shallower as the polymer moves toward the screw tip. Small screws (1.5 in. diam. and smaller), however, generally have the same shallow feed depth whether for grooved or smooth feed sections. Feed depth is as shallow as it is on European grooved-feed designs. This is why traditional smooth-feed screws on small extruders will respond to even short grooves better than larger extruders. Very small extruders (1 in. diam. and under) often require the help of grooves in solids conveying, since pellet size is large compared with screw flight depth.
Mechanical strength of the grooved-feed section must be high enough to handle high pressure on startup or with certain polymers. Pressures at the end of the grooved section can get as high as 15,000-20,000 psi. So equipment should be designed to take up to 30,000 psi, even though today's groove and screw designs try to maintain feed pressures below 10,000 psi.
Pressure at the end of the grooved-feed section is often higher than at the die end, so screw performance is not sensitive to die pressure the way it is in smoothbore extruders. Smooth-feed extruders operate against die pressures of 3000-4000 psi and must raise screw rpm to get the same output as they do at lower die pressure. The energy to pressurize the melt plus the energy from higher screw speeds on a smooth-feed extruder make operation difficult as melt temperature and pressure rise.
Grooved-feed extruders' higher output per RPM requires a larger gear box than smooth-bore machines of similar diameter. It also takes more torque to start up or restart.
Right resins for grooves
Polyolefins are by far the most common polymers successfully processed with cooled grooved feeds. Since the 1980s, polymer developments have tended toward higher melt viscosities. Single-site catalysts make resins with higher molecular weights, improved properties, and much narrower molecular-weight distribution. But these polymers are more challenging to extrude. With many of these newer polymers, smooth-bore extruders struggle to maintain desired melt temperature as screw speed increases to raise output. So grooved feed is being looked at--and sometimes selected--to lower melt temperature.
Processors most likely to benefit from using grooves have dedicated operations using only one or two polyolefins at high output rates. Examples are lines running high-viscosity PE and PP for pipe, sheet, blown film, and blow molding. Materials with high melting points and higher crystallinity don't work as well on grooved-feed extruders. Their compressibility and melting characteristics are different from polyolefins. To run them safely, feeding efficiency of the grooves must be reduced to protect the barrel and grooved section from pressure damage.
Smooth feed is generally better than grooved feed when you need to process a variety of polymers, use low-bulk-density regrind, and vary regrind particle size and/or percentage. Particle size of feed materials affects groove performance, so if no control is ensured over regrind percentages or particle size, a controlling weigh hopper or a melt pump is needed to ensure constant output. Smooth-feed extruders, with relatively deep feed channels, are less affected by regrind particle size and less susceptible to fluctuating regrind percentages. More disciplined operation is therefore required for a grooved-feed set-up.
Grooved-feed extruders perform best with precolored resin. Adding color masterbatch requires a higher level of mixing from the screw and higher melt temperatures, thus offsetting some of the benefits of grooved feed.
Smooth-feed extruders also do better with higher-melt-temperature polymers like nylon, PET, polycarbonate, and fluoropolymers. If these materials are to run on a grooved-feed machine, the grooves are typically heated. This reduces feeding efficiency because it allows some melting in the grooves. Also deeper screw feed channels can be used to reduce the tendency to overfeed the extruder.
Soft, gummy resins such as TP urethanes, other elastomers, and softer metallocene polymers also have difficulties in grooves if not cooled enough. Soft pellets can smear and fill the grooves, reducing forward force and/or causing gels in film.
Vented extrusion is also not a great choice for a grooved-feed extruder, since output is usually limited by the pumping capacity of the second stage. A very efficient first stage may produce enough output to flood vents or cause poor melting at vents. Melt at the vent must be complete enough to allow evacuation (usually under vacuum) of air, moisture, and resin volatiles.
In spite of the potential gains from grooved feed extrusion, most U.S. processors haven't seriously considered this alternative to the smooth feeds they have used for years. However, a careful review of resins, process conditions, and operating procedures may reveal opportunities for improving output and quality with grooves.
Edward Steward is director of process technology for American Kuhne Inc. in Norwich, Conn.
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|Date:||Apr 1, 2000|
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