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I teach a one-day squeeze introductory seminar, and many of my participants are newbies to squeeze. Therefore, I explained the basic principles of thermoplastic screws. This is good for novices, but sometimes I get more experienced squeezers to thank me for providing these basics, they have never learned before. So that's it. I hope you all find it useful.

The screw is a conveyor belt. When it turned, it tried to screw itself back out of the barrel, but the bearings prevented it from coming out from behind. Because every action has a reaction-remember Isaac Newton? -It is also pushing forward in the other direction, which is why the material is pushed out of the mold.

The material needs to soften to pass through the mold. Any thermoplastic will become soft and plastic (plastic) by heating (heat). The feed is sometimes preheated (usually used for drying), but when it moves against the barrel wall and screw surface, it gets most of the heat from internal friction. The gap from the thread to the barrel is where the most heat is generated. Exceptions: some twin-screws, small machines, high-temperature resins and PE coatings, where the barrel heat is also important.

The three-zone principle. The screw starts in the feed zone: a constant depth, which accounts for 15% to 30% of the length. In the middle is the compression zone, the wall is close to the melt/particle mixture, pushing the air backwards and compensating for the sliding and rolling of the particles in the feed zone. This area contains the "barrier" of the barrier screw: a long double channel section that separates the melt from the particles, allowing the particles to rub against each other to generate more heat, instead of swimming in the ever-increasing melt volume and only heat conduction. Finally, at the output is the metering (pumping) zone, the channel depth is again constant at 25% to 50% of the feed depth, usually with filtering and mixing elements (Maddocks, pineapple, pins).

Channel depth, not just their ratio, is crucial. In small machines, the feed must be deep enough to allow smooth feed (at least twice the particle size), but not deep enough to risk breaking the screw shaft. In the metering zone, shallow means better mixing and less output per revolution, while deeper means the opposite, more sensitive to high pressure.

Length: The usual metric is the ratio of length to diameter, or L/D, also written as L:D. Today, 24:1 is the standard, 20:1 is short (know the reason), and 25 to 30 are common. The longer the length, the longer the melting time, which usually increases the output, but requires a higher melting temperature. Longer production lines have been built and vent extrusion is required, but beyond that, the trend is to be larger (cooler) rather than longer.

The ventilated barrel has a hole in the barrel to remove moisture and trapped air (same as powder feed). The screw becomes very deep at that point to avoid pushing the melt out of the vent, applies a vacuum there, and then becomes shallow again to pump out the melt.

The pitch (angle) of the threads is usually square: that is, the distance from one thread to the next is the same as the diameter. If the channel is "unfolded", this corresponds to a helix angle of 17.6°. In many barrier sections and some feeding sections for light-duty feeding, the angle increases.

The flying thickness is approximately 0.1 x diameter. Thicker means more heat development area and less transfer per revolution (usually not needed), while thinner means more backward leakage (less pumping but more mixing).

Hollow screw. Many screws are drilled full length to allow water (to help mixing), oil (to avoid degradation of the tip of rigid PVC) and even air (rare but cheaper) to pass through. Some screws are only drilled a third to prevent sticking to the roots of the feed zone.

The radius of the channel angle. Too small will cause stagnation and potential degradation; too large will waste channel capacity. No one formula is suitable for all situations: it depends on the thermal stability of the material, the flow rate in the channel, the use of cleaning agents and metal adhesion processing aids, and the screw surface material.

Unusual changes include a slotted barrel to increase thrust per revolution (very common for HDPE, where the screw has little or no compression, a mixing device is recommended), and deep circulation of channels in parallel channels to improve mixing and uniformity ( Wave screw).

Material. Most screws are machined steel with a hardened threaded surface, either by welding on caps approximately 0.040 to 0.080 inches (1 to 2 mm) thick, or by nitriding the entire surface. The latter method is cheaper, but can prevent future changes in flight depth; the service life depends on the depth of nitride penetration. Chrome plating is common: the surface of the screw certainly looks better, is said to pass more easily (less frictional heat) and is less likely to degrade. For abrasive and corrosive feeds, more expensive metals can be used.

Computer simulations of screw performance have been widely used and are not new. I demonstrated it on a DEC computer (20 MB hard drive!) at a seminar from 1987 to 1992, after which it became too complicated to take an introductory course. Today’s procedure is good, but success depends on reliable viscosity data as a function of temperature and shear rate. Will I only make screws based on simulations? Do not. Will I make one based on my own experience? Not if I can help. If the line is large enough and I have reliable viscosity data, I want to combine the two.

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 and public and public educator through webinars and seminars. internal. As early as the 1960s, he wrote the first practical extrusion book and the plastic extrusion operation manual updated almost every year, and it is available in Spanish, French and English. Learn more on his website www.griffex.com or send an email to [email protected].

In the fall, Griff will hold one-day hands-on seminars "Introduction to Squeeze" in Chicago on September 19, Los Angeles on November 15 and Houston on December 5. Topics include ten (11) key principles of extrusion, plastic chemistry for non-chemists, review of extrusion hardware, productivity limitations, raw material quality control, simplified rheology, startup and shutdown procedures, and troubleshooting of common extrusion problems. Email him to the address listed above for more information.

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