Three-zone screw

Injection can be done on standard screw-type machines with three zone screws having L/D =... 

The conventional three-zone screw is divided into the following regions feed... 

Standard three-zone-screw (no mixing elements), standard non-return valve open (2.5 to 5.0 mm)... 

A a three-zone screw with feed section of constant flight depth, a compression section (it is the geometry and position of the compression section that are designed to match the melting or softening characteristics of the polymer), and a metering section ... 

The screw configuration is as follows thread depth ratio, approximately 1 2, and L/D ratio, 17/1 to 23/1 (standard three-zone screws feed, transition or middle, and metering or feed zones). Screws are equipped with a nonreturn valve to prevent backflow. Decompression-controlled injection-molding machines have an open nozzle. A short nozzle with a wide bore (3-mm minimum) is recommended in order to minimize pressure loss and heat due to friction. Residence time should be as short as possible, and this is accomplished with barrel temperatures at the lower limits of recommended settings. ... 

Figure 7.8 Screw diameters and screw flight depths of three-zone screws.

Although in general both a single- and a double-screw extruder can be used for SFE experiments we choose a Schwabenthan single-screw (D = 30 mm) extruder, equipped with a hopper, a three zone screw (L = 35 D), an injection unit for the extraction fluid at 20 D from the hopper and a capillary spin opening (0 1.7 mm). 

Blends of PA-6 or PA-66 with PP (e.g. Oigalloy , Elf Atochem, Ultra-mid JQR, BASF) have flow properties similar to PP. This constitutes an improvement over the flow behavior of neat PA. The PA/PP blends are easier to extrude than PA because the melt viscosity is less sensitive to temperature variations. The processing conditions for PP/PA-66 blends are comparable to those normally used to process PA-66. However, it shovild be noted that (hying of these blends is much slower than that for the neat PA. For single-screw extrusion, the barrel should have L/D s 20, fitted with a standard three-zone screw, with compression ratio of 2.5-3. Excessive shearing may cause destruction of morphology, and thus... 

B a three-zone screw with a vented section in which the pressure is dropped so that gases may be exhausted from the melt by a vacuum line, or merely by exposure to the atmosphere (a hole is drilled through the barrel wall) ... 

It is preferable to have a three-zone screw evenly divided into feed, compression and metering zones. However, a higher percentage of feed zone length is required for smaller machines approximately same as the other zones together ... 

Screw design is primarily aimed at achieving high and consistent output rate for a particular product from a particular polymer. A single flighted three zone screw as shown in Figure 7.2 has three main stages ... 

Comparisons were made with a conventional three zone screw with the same feed and metering zone dimensions (Figure 7.2). Any semi-melted granules arriving at the element could not continue into the metering zone until melting was completed. 

Figure 4.30 Graphic relationship between data measured during processing of polypropylene (Stamyian P14E10) on a three-zone screw (D = 50 mm, L/D = 21.5) at various screw rotation speeds under extrusion conditions (counter pressure = 200 bar) and the analytical model [607]...

Three-zone screw Barrier screw Multi-zone screw... 

Melt temperatures for extrusion of HDPE are 200-300 "C and for LDPE are 180-240 C. Higher melt temperatures are needed for extrusion coating (280-310 "C). A three-zone screw with a length-... 

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