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single screw vs twin screw extruder free sample

- Jul 20, 2022 -

single screw vs twin screw extruder free sample

Compared with the same-rotating twin-screw extruder, the counter-rotating twin-screw extruder has higher conveying efficiency, better venting effect and better melting effect.

Although the dispersion and mixing effect of the material is worse than that of the same-rotating twin screw extruder, However, the ability to establish a stable head pressure is stronger, so it is more suitable for direct extrusion of products.

In addition, a counter-rotating twin-screw extruder is used instead of a single-screw extruder, which can eliminate the pre-plasticization and granulation process and directly extrude and process the PVC powder, which reduces the cost of the product.

The counter-rotating twin-screw extruder is mainly used for extruding PVC pipes. Profiles, plates and pellets. Parallel counter-rotating twin-screw extruders have also been applied in large HDHDPE pipe production lines.

But in recent years, there have been examples of the counter-rotating conical twin-screw extruder applied to the extrusion of polyolefin pipes in China.

In recent years, some people in China have tried to use co-rotating twin-screw extruders (including conical twin and flat twin) to extrude products, and some progress has been made.

However, it is more difficult for a co-rotating twin-screw extruder to establish a stable head pressure. It is not as simple and easy as directly using a counter-rotating twin-screw.

single screw vs twin screw extruder free sample

The same process tasks are performed in any TSE. Initially, materials are metered into the extruder feed throat and solids conveying occurs. The materials being processed must pass through a series of pressurized, fully filled mixing regions. The materials are melted via mixing elements and conveyed via flighted elements. Flighted elements discharge the melt through a die or other pressure-generating device into various shapes for downstream processing.

At the heart of any TSE process sections are the rotating screws contained within barrels. Screws are typically segmented and assembled on shafts, in which case the torque transmitted to the shaft is the limiting factor for the amount of power/torque that is available to process materials. Screws for TSEs can also be a one-piece design with significantly higher torque transmittal possibilities.

The OD/ID ratio (outside screw diameter/inside screw diameter) and channel depth are important TSE design parameters as these parameters dictate the available free volume and torque. A graphic illustration of the outer diameter, inner diameter, and flight depth of screw elements is presented in Fig. 6. As the channel depth increases, the ID decreases and results in less attainable shaft torque. An optimum balance between free volume and torque is important as both represent boundary conditions that may limit attainable material throughput rates.

A TSE is generally referred to by the diameter of screws. For instance, a “ZSE-18” model would reflect a TSE with an 18-mm screw outside diameter, or OD, for each screw. In the plastics and food industries, ODs range from 12 to 400+ mm with outputs from 50 g to greater than 50,000 kg/h. Pharmaceutical TSEs are generally in the 60–70-mm class and below, with research and development efforts performed on TSEs with screw ODs in the 10–30-mm range. Flight depths range from as little as 1 to 3 mm on a small lab extruder to approximately 15 mm on a 70-mm class machine. Even a 140-mm class TSE will only have a flight depth in the 25-mm range, hence the term “small mass” continuous mixer with short mass transfer distances.

There are seemingly an infinite number of possible screw variations. There are, however, only three basic types of screw elements: flighted elements, mixing elements, and zoning elements. Flighted elements forward material past barrel ports, through mixers, and out of the extruder through a die. Mixing elements facilitate the mixing of the various components being processed. Zoning elements isolate two operations. Some elements can be multifunctional.

Kneading elements, the most common mixing elements, are strategically placed along the length of the screws (Fig. 8). Wider kneaders cause extensional mixing and planar shear to be imparted into the materials being processed and are more dispersive in nature as compared to narrow kneaders that result in divisions and recombination of melt streams and, therefore, facilitate distributive mixing. Other parameters regulating mixing intensity include the offset angle of the kneaders (30, 60° forward or reverse) or neutral (90°). It should be noted that an innumerable number of mixing elements (i.e., rotors, slotted vanes, and blister rings) are available but that the kneading type elements account for 90%+ of those used in a corotating TSE.

Another variable in the geometry of mixing elements is the “lobe count.” The lobe count refers to the number of screw tips/flights that are wiping the barrel wall. The OD/ID ratio and the mode of operation determine the number of lobes that are geometrically possible for a given design. Examples of bilobal and trilobal TSE screw elements are presented in Fig. 9. Most corotating intermeshing TSEs are bilobal due to interference issues, and a corotating intermeshing TSE is limited to two lobes at typical OD/ID ratios (1.4 to 1.7/1). For laboratory applications, a 1.2/1 OD/ID can be specified and three lobes can be used to facilitate low free volumes and high-torque for testing 20 g batches, or less.

There are two main types of mixing: distributive mixing and dispersive mixing. Distributive mixing involves melt division and recombination, while dispersive mixing involves shear and elongational mixing. A graphic illustration of two types of mixing is presented in Fig. 10. Screw designs can be made shear intensive and/or passive, based upon the elements specified in the design. Mixing elements may be dispersive, distributive, or a balance of each/both. Screw elements that accentuate extensional mixing and planar shear effects are dispersive in nature, as compared to elements that facilitate melt divisions/recombinations, which are more distributive and therefore useful for mixing heat and shear sensitive materials. Shear-sensitive APIs are often distributed in a melt and then allowed to dissolve into the polymer matrix before exiting the TSE, whereas more severe dispersive mixing is sometimes needed. Other factors that dictate mixing intensity include, but are not limited to, the screws’ rpm, the gaps between the screws and screw flight/barrel wall, and the “lobe count.”

In a TSE, the materials being processed are bounded by screw flights and barrel walls, often referred to as the melt pool. The materials are separated into small melt pools by screw flights and barrel walls, which is why the TSE is by definition a “small mass” continuous mixer, as compared to the large mass batch mixer described earlier. As shown in Fig. 11, there are five shear regions in the screws for any TSE, regardless of screw rotation or degree of intermesh. The following is a brief description of each shear region.

Screw channel:a low shear region, highly dependent on the degree of screw fill in a starved TSE; shear is significantly lower as compared to the other shear regions.Overflight/tip:a high shear region, independent of the degree of screw fill, located between the screw tip and the barrel wall; the material undergoes significant planar shear effects.Lobal pool:a high shear region, independent of the degree of screw fill, the compression/acceleration entering into the overflight region; the material experiences a particularly effective extensional mixing effect.Apex (top/bottom):a high shear region, independent of the degree of screw fill, where the interaction from the second screw results in compression/decompression/extensional effects associated with pressure fields and directional flow changes that result in increased mixing rates.Intermesh:a high shear region, independent of the degree of screw fill; it is a high-intensity mixing zone between the screws where the screws “wipe” each other.

As shown in Fig. 12, each screw, to a degree, wipes the other in a corotating twin screw extruder. This is termed “self-wiping.” This self-wiping effect sequences unit operations and discharges the small volume of materials in a first-in, first-out sequence with a given residence time distribution (RTD). This results in a uniform deformation/mixing history, and depending on the length and screw design of the TSE, there will be an associated RTD for the process section. The more filled the screws, the tighter the RTD, and the more starved the screws, the wider the RTD. An illustration of the effect of the degree of screw fill on the residence time distribution is presented in Fig. 13.

For simplicity, the four high mass transfer regions shown in Fig. 11 can be viewed as independent of the degree of screw fill, which is why, in a starve-fed machine, when rate is decreased at a given screw rpm, more mixing occurs due to a longer residence time in the mixing zones. Alternatively, as rate increases at a given screw rpm, the low shear channel region plays a larger role and the materials pass over the mixing zones quicker, and therefore, there is shorter exposure time to high shear regions and less lobal mixing events.

The intense mixing associated with the short interscrew mass transfer characteristics inherent with a TSE small mass continuous mixer results in highly efficient distributive and/or dispersive mixing that results in a more uniform product as compared to large mass batch mixers. Entrapped air, moisture, and volatiles are also removed via venting. The short residence time associated