Heated barrel

In injection molding, the polymer is fed from a hopper into a heated barrel where it softens and becomes a viscous melt. It is then forced under high pressure into a relatively cold mold cavity where the polymer has sufficient time to solidify. Then the mold is opened and the fabricated part is ejected. The cycle of operation is then repeated. 

Twin-screw extruders that contain two internal rotating screws that press material against heated barrel walls and forces the resulting molten mass through a restriction die which aligns the mass in the direction of... 

The unit can be fed polymer in the particulate solids form or as strips, as in the case of rubber extrusion. The solids (usually in pellet or powder form) in the hopper flow by gravity into the screw channel, where they are conveyed through the solids conveying section. They are compressed by a drag-induced mechanism in the transition section. In other words, melting is accomplished by heat transfer from the heated barrel surface and by mechanical shear heating. 

Using a cold-wall CVD reactor similar to the internally-heated barrel described in Figure 22 of Chapter 1, tungsten silicide was deposited from WF6 and SiH4,5 which is often described by the overall reaction... 

The other configuration of silicon ept reactor that has been successful commercially is the so-called pancake reactor. The basic concept behind the system was shown in Chapter 1, Figure 22. As a production reactor, it is simple, rugged and inexpensive compared to the radiantly-heated barrel just described. The typical graphite wafer holder inside a quartz bell jar is shown in Figure 13. 

Solid polymer in the form of pellet or powder dropped into the screw channel from the hopper is compacted into a solid plug due to the screw rotation and the solid plug is forced to rub on the heated barrel surface. The solid plug is plasticated or melted in screw extruders primarily by the mechanical energy provided by the drive motor rather than the thermal energy from the heated barrel. 

Melting of the solid polymer is the result of heat from two sources conduction from the heated barrel, and friction between the barrel and the polymer. Most of the heat comes from friction between the barrel, the viscous melt film, and the remaining solid polymer. This frictional heating is most efficient when the melt viscosity is high, and when the melt film is thin. Thinness of the melt film depends on designing and maintaining a very small clearance between the screw flights and the barrel surface. 

Extrusion is the process of heating practically only TPs that may be in the shape of powders, beads, flakes, pellets, or combinations of these forms. This plastic enters the extruder s hopper. The extruder utilizes a plasticator [spiral screw that rotates within a heated barrel (cylinder)] to melt the plastic (Chapter 3). The melted plastic is then forced through a die to produce the desired continuous product shape. Figure 5.1 shows a very simplified schematic of the extrusion process. Information on dies (mono-layer and coextruded) used in extrusion is in Chapter 17. 

During the plastication stage, the polymer melt is typically plasticized from solid granules or pellets through the combined effect of heat conduction from the heated barrel and the internal shear heating caused by molecular deformation with the rotation of an internal screw. Screws in injection molding have many... 

The ram extruder for this process is a special unit and can be either horizontally or vertically oriented, which refers to the direction of the ram movement. The extmder consists of a heated barrel where the preform is loaded, and a hydraulic or screw-driven ram. The conductor is drawn by a power system through a hollow mandrel located at the center of the barrel. The mandrel terminates in a wire guide tube. 

Extrusion is a continuous process in which an Archimedes screw, rotating within a heated barrel, melts and pumps the polymer through dies to form continuous shapes such as pipes, profiles or sheets. In addition, extrusion is the basic element in a number of other polymer processing operations. The plastication and pumping functions... 

There are essentially two methods of using the plasticating screw. The first is the screw-plunger system also called the two-stage or screw-pot system. The screw rotates in the heated barrel and consequently plasticizes the polymer material. The plasticized material is then transferred into a second heated cylinder from which it is injected into the mold by a plunger. Figure 11.6 shows the basic features of a screw-pot injection molding machine. 

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