Extrusion: Learn About Grooved Feeding

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In specific circumstances, Grooved Feed can increase output and reduce melt temperature. But these throats are not for every application.

There are basically two types of single-screw extruders, distinguishable primarily by their feed-throat design.

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Most single-screw extruders have smooth-bore feed throats—basically a smooth pipe surrounding the rotating screw.

Extruders with smooth-bore feed throats are affected a number of ways by the resistance or “head” pressure against which the screw is rotating. As the head pressure increases, a reverse flow (or pressure flow) develops in the screw channels that subtracts from the output (or drag flow) while increasing the melt temperature (because the material is essentially being re-circulated back into the screw).

This can cause significant problems in extrusion processes that have inherently high head-pressure characteristics resulting from a narrow die gap, such as in blown film, extrusion coating, and thin-wall tubing.

With a smooth-bore feed throat, solids transport of material with the rotating screw is highly dependent on the friction of the polymer particles against the barrel wall.

Depending on the polymer and the temperatures, the feed efficiency is typically 15% to 30% based on the channel volume because of “slip” of the polymer particles on the feed-throat wall.

The second type of feed throat has grooves or interruptions machined into the feed-throat wall.

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Extruders with grooved feed throats resist rotation of the solid polymer particles against the feed-throat wall.

This can increase conveying efficiency multiple times. Additionally, because the polymer is much more rapidly compacted into a solid and cannot slip on the feed-throat wall, the screw flights act as a spiral wedge multiplying the pressure development.

As this spiral block of polymer moves down the feed throat, it becomes highly pressurized compared with what occurs in a smooth-bore extruder.

In the absence of any melting, which reduces the pressure development, the pressure tends to rise exponentially with groove length. Groove lengths are usually 4D or less because of the exponential rise in pressure and the difficulty in preventing melting from occurring over longer lengths.

Pressures exceeding 12,000 psi have been achieved at the end of the grooved section, so care must be exercised in design to prevent over-pressurization of the barrel. That can be done modifying the design of the grooves themselves, or by designing a decompression section in the screw.

This pressure offsets, and in many cases can exceed, the pressure at the discharge. The grooved feed throat is intensively cooled so the material won’t prematurely melt and hinder pressure development.

Since the barrel is essentially a pipe, the pressure from the grooved feed throat transfers through the barrel, counteracting the discharge-pressure resistance. If the feed-throat pressure development exceeds the discharge pressure, the output will exceed the drag flow, resulting in further throughput gains.

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Before deciding to move to a grooved feed throat, weigh the advantages against the disadvantages for your process.

The advantages of grooved feed throats:

  • Reduction or elimination of the effect of high head pressure, resulting in higher output.
    • Reduction in melt temperature at same output.
    • Stabilized output.
    • Not very sensitive to barrel temperatures.

The disadvantages of grooved-feed throats:
• Proportionally more drive power required.
• Performance very sensitive to particle characteristics.
• Generally not suitable for most regrind due to plugging of the grooves.
• Does not work on “soft” polymers like many TPEs due to plugging of the grooves.
• Not as good for compounding additives without extensive mixing sections due to higher specific output.
• Not generally suitable for powdered polymers or additives.
• Can have accelerated screw/barrel wear without accurate balanced design due to higher pressures in feed section.
• Can be unstable with “hard” polymers such as polycarbonate.
• Requires higher volume of cold water for cooling.
• Requires a special screw design, unlike those used for smooth bore extruders.

Grooved-feed technology was developed in Germany in the early 1960s and has been more widely used in Europe than in other parts of the world. However, this design is the standard everywhere for much of HDPE pipe and HMW-HDPE blown film production.
Now, grooved-feed designs are more commonplace. Many barrier blown film lines, for example, are equipped entirely with grooved-feed extruders.

Meantime, there also have been many developments in feed-throat “surface improvements.” Though not to be confused with traditional grooved feed throats, these surfaces aim to improve friction at the feed-throat wall by using shallow grooves of various designs. They’ve been shown to significantly improve output with polymers having low feed efficiency, and are effective when processing regrind and fillers. However, where screw design is concerned, they follow the principles of smooth-bore feed throats.

Grooved feed throats have their pros and cons. As a processor, weigh the promise of improved output and lower melt temperature against the disadvantages. Consider them only for situations where they are beneficial overall to the process. Don’t think of them as general-purpose extruders.

Understanding groove 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.

Finding the right groove

The grooved feed section of a 90 mm extruder may have eight to 18 grooves evenly distributed around the feed bore.

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Groove feed sleeves at Raj engineering Works Ahmadabad

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.

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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 (93-148 C 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.

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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 smooth-bore 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.

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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.

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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.

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