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The earth rotates at a speed of 0.0007 rpm. Easy to calculate: One revolution per day, divide by 24 to get the number of revolutions/hour, then divide by 60 to get the number of revolutions/minute = rpm. As for the line speed, it depends on where you live. In Northern California, where I am located, at 38 degrees north latitude, we are driving at 730 miles per hour. This is about the speed of sound, but we cannot feel or hear it because the air around us moves just as fast. It is much faster than in any extruder-in a 12-inch/30-cm barrel, running at a speed of 100 rpm, the particles on the barrel wall can still only reach 314 feet per minute, or per hour 0.36 miles. These are not important when running the extruder, but for us engineers, it is very interesting.

However, it is important to understand how screws work. This is a condensed version of the single screw part of my "Plastic Extrusion Operation Manual" (24th edition in 2021).

We express the length of the system as the aspect ratio (L/D). The most common L/D is about 24:1; some are as long as 30:1 or more, and some are as short as 20:1. If heating, melting or mixing is the output limitation, a longer length may mean more output

A standard screw has three areas:

Many screws have a square pitch, which means that the distance from one thread to the next is the same as the diameter. This makes it easy to obtain L/D just by calculating the number of revolutions. The part below the feed port should not be included in the L/D, but many people do count it because it makes the screw appear longer.

The compression ratio of the screw is the ratio of the volume of the first thread to the volume of the last thread, usually between two and four. It is usually regarded as the ratio of the depth of the first and last passages in a constant pitch screw. The compression ratio is useful, but it is an indeterminate number, and the screw cannot be described correctly unless at least one channel depth is known.

The thread width (thickness) is approximately 10% of the barrel diameter. Wider threads waste screw length and generate excessive heat in the gaps with the barrel wall, while narrower threads may allow excessive flow (leakage) in these gaps. To avoid stagnation where the flight meets the roots, the corners are rounded

The screw is usually machinable steel, but the threaded surface closest to the barrel is further processed to delay wear. For light-duty applications, flame hardening is sufficient. The entire screw surface can be hardened by nitriding, but the usual treatment is to add a cemented carbide cap on the surface of these threads.

The barrel is a steel cylinder, usually lined with wear-resistant alloy.

The gap between the screw thread on the new screw and the barrel is between 0.005 and 0.010 inches (0.125 to 0.25 mm), which is smaller for very small screws and larger for very large screws. A tighter fit will make it more expensive to manufacture and generate excess heat. Some wear beyond these values ​​is usually harmless and may even help, so make sure that there is a real problem (such as overheating, because the screw must run faster for the same output) before rebuilding or replacing.

If we know the resistance (pressure at the screw tip), the required output rate, and the material viscosity, the screw can be designed by the computer, but it is still a good idea to "flavor" the computer with some experience before cutting the metal.

Chrome plating the screw may increase root sliding (which is good) and prevent corrosion, especially when leaving the machine, but is unnecessary for most plastics. For highly abrasive materials, the entire screw surface can be hardened. Finally, PVDC and some fluoroplastics require special metals because iron-based materials will corrode and the duration of electroplating is not long enough.

Some screws have a central channel. The entire length of water cooling improves the mixing in the final flight. Oil is used with rigid PVC to keep the screw tip at around 300°F (150°C) so the PVC will not degrade there. The cooling of the screw only under the barrel is done with some plastic to prevent sticking to the screw root in the feed zone.

The Maddock part is a length of about two diameter screws, usually found in the first few turns of the end, with a pair of large grooves (called grooves) instead of threads.

Each entrance groove has a corresponding exit, and there is a barrier ridge between them (see picture below). The gap between the ridge and the barrel is approximately 0.020 to 0.030 inches (0.50 to 0.75 mm). The groove of the first Maddocks was parallel to the screw axis, but now it is more helical.

The melt enters the inlet groove, flows through the barrier ridge, and exits through the outlet. The unmelted particles cannot pass through as a whole, but are sheared and flattened, and finally passed as a melt. In addition, colder melts stay longer in the high shear zone than hot melts, thereby providing better thermal uniformity. It is usually called the Maddock mixing head, but it rarely appears at the end (head) of the screw. It is not so much an agitator as a filter.

The cross-section of the barrier screw occupies most of the compression zone, and the additional threads form two parallel channels-one for the melt and one for the particles. The gap between the new scraper and the barrel is large enough that the melt formed in the particle channel can flow back into the melt channel, but it is small enough to block about 0.060 inches (1.5 mm) of particles. The particles stay in their main channel, but will discharge excess melt, so when they rub against each other will generate more frictional heat. Therefore, melting is more efficient per revolution. As the material moves down the screw, more melt is produced, so the volume of the melt channel increases. However, since there are fewer unmelted particles, the particle channel becomes smaller until the part finally ends, the particles disappear, and a flight takes the melt away through the metering zone. This barrier is usually combined with the Maddock section in the metering area or other special mixing equipment.

The barrier section in the picture is only 4 diameters long, but has been shortened for clarity; usually the length is at least 10 diameters.

The mixing pin is a stud ring protruding from the root of the screw, used to break the streamlined flow like rocks in a stream, thereby improving mixing. They are usually placed in the last quarter of the screw.

The grooved barrel has axial or spiral grooves in the barrel and is located in a separate water-cooled feeding area to improve the intake of smooth and hard plastics such as high-density polyethylene. Screws with shallower feed and deeper metering zones are required, usually without compression at all. Since the deeper metering zone leads to poor mixing, further hardware is required, namely a powerful mixing part at the output end of the screw or a static mixer at the head.

For venting (two-stage) extrusion, a very long screw is used, because all materials must be melted before venting, and the vent usually accounts for about 70% of the total length. The first part is an ordinary three-zone screw, but then it suddenly becomes deeper, reducing the melt pressure, so a vacuum can be applied through the hole (vent) on the barrel to expel air, moisture or other volatiles. The melt continues to flow downstream, is recompressed, passes through the final metering and mixing section, and then exits through the mold.

Materials such as foaming gas and foaming agent, waste, mixed resin and trace additives can be added through the vent. It is even possible to insert non-thermoplastics, such as glass fibers, which do not have to melt and wear less when added to molten materials instead of mixing with hard solid feed pellets.

In the vented screw, the second stage must take away the substances put into the exhaust zone of the first (rear) stage, and must also resist head resistance. Therefore, the pumping capacity of the second stage must be greater than the pumping capacity of the first stage, which works in the case of zero resistance-otherwise the feed must be controlled separately-to prevent the molten plastic from flowing out of the exhaust port. The usual front-to-back metering depth ratio is between 1.5 and 2.0. However, deep channels cannot pump well under high pressure, so a typical exhaust screw can only work with a maximum resistance (including the screen) of about 2500 psi (17 MPa). In order to obtain higher head resistance, it is necessary to control the feed or gear pump to allow exhaust.

Some or all of the double-threaded screws have two parallel paths. In the metering zone, this helps heat transfer, so it is sometimes used where very high temperatures are required, such as extrusion coatings. The double-threaded feed zone is thought to provide a smoother feed (less pulsation), but it is rarely seen today. All barrier screws are double-threaded in their barrier part, but the two paths are not equal, as described above. In a corrugated screw, the width of the two (or three) paths are equal, but there is a barrier between them low enough to allow the melt to flow through. The depth of the channel increases and decreases in a wave-like manner and is out of phase with each other. Therefore, when a path is shallow, the path through the barrier is deeper, and the melt flows from shallow to deep. After half a lap, the depth is just the opposite. The melt still flows from shallow to deep, so it moves back and forth across the barrier as it moves downstream, which facilitates mixing and stabilizes the flow.

Allan Griff is a senior extrusion engineer. He initially provided technical services for a major resin supplier. Now he has been working independently for many years as a consultant and an expert witness in legal cases, especially as an educator through webinars and seminars. , Both are public and internal, and are now in his new audiovisual version. He wrote "Plastic Extrusion Technology", the first practical extrusion book in the United States, and the "Plastic Extrusion Operation Manual" updated almost every year, available in Spanish, French and English. Learn more on his website www.griffex.com or send an email to [email protected].

Griff said that in the near future, or may never plan to hold live seminars, because his virtual audiovisual seminars are even better than live broadcasts. No need to travel, no need to wait for the live broadcast date, use the same PowerPoint slides, but with audio explanations and written guides. Watch at your own pace; group attendance is offered at a single price, including the right to ask questions and get full answers via email. Call 301/758-7788 or send an email [email protection] for more information.

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